Abstract
Metal-catalyzed reactions play a vital part to construct a variety of pharmaceutically important scaffolds from past few decades. To carry out these reactions under mild conditions with low-cost easily available precursors, various new methodologies have been reported day by day. Sandmeyer reaction is one of these, first discovered by Sandmeyer in 1884. It is a well-known reaction mainly used for the conversion of an aryl amine to an aryl halide in the presence of Cu(I) halide via formation of diazonium salt intermediate. This reaction can be processed with or without copper catalysts for the formation of C–X (X = Cl, Br, I, etc.), C-CF3/CF2, C–CN, C–S, etc., linkages. As a result, corresponding aryl halides, trifluoromethylated compounds, aryl nitriles and aryl thioethers can be obtained which are effectively used for the construction of biologically active compounds. This review article discloses various literature reports about Sandmeyer-related transformations developed during 2000–2021 which give different ideas to synthetic chemists about further development of new and efficient protocols for Sandmeyer reaction.
Graphical abstract
An updated compilation of new approaches for Sandmeyer reaction is described in this review to construct a variety of carbon-halogen, carbon-phosphorous, carbon-sulfur, carbon-boron etc. linkages.
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Introduction
Aromatic diazonium salts, discovered by Griefs in 1858 [1], have wide spread applications in organic synthesis as well as at industrial level. They are frequently used for the preparation of organic nanocompounds and grafted a variety of organic molecules on metallic surfaces [2]. Furthermore, Meerwein arylation [3, 4], Balz–Schiemann [5, 6] and various metal-catalyzed reactions [7, 8] involve diazonium salts as starting precursors for the production of various halides, phenols, cyanides, azides and alkenes [9] which serve as effective intermediates for the synthesis of important molecules [10,11,12,13]. Sandmeyer reaction is one of them, in which diazonium salts are used for the construction of carbon–halogen, carbon–phosphorous, carbon–sulfur, carbon–selenium, carbon–boron bond formation. Moreover, various trifluoromethylated compounds as well as a number of pharmaceutically important drugs can be synthesized via Sandmeyer approach [14].
Literature study reveals the importance of metal-catalyzed cross-coupling reactions which are extensively used in organic synthesis and in pharmaceutical industry. These reactions are carried out by the treatment of various organic halides with a suitable coupling partner using a variety of catalysts and ligands [15,16,17,18,19]. Sandmeyer approach, first discovered in 1884 by Sandmeyer [20, 21], is one of these metal-catalyzed reactions which effectively converts benzenediazonium salts into bromo-, chlorobenzene, benzonitriles, etc., in the presence of different copper catalysts. Since 1884, different methods have been discovered to improve the efficacy of Sandmeyer reaction as an innovation of organic phase diazotization process reported by Doyle et al. in 1977 [22, 23] and the effective utilization of these diazonium salts in a variety of metal-catalyzed reactions (Kikukawa and Matsuda, 1977) [24, 25], etc. Although the mechanism of this reaction is not yet clear however, a general mechanism reported by Waters [26] and later on by Kochi [27] is highlighted in Scheme 1 according to which diazonium salt readily undergoes homolytic dediazoniation in the presence of copper salt, resultantly affording aryl radical. This radical upon treatment with reactive species gives desired product with regeneration of copper(I) species.
General mechanism of Sandmeyer reaction
Owing to the extensive applications of Sandmeyer reaction, herein we describe an updated compilation of new approaches reported during 2000–2021 for Sandmeyer reaction.
Review of literature: applications of Sandmeyer reaction
Formation of carbon–halogen linkage
Chlorination via Sandmeyer reaction
Marco-Contelles and colleagues reported copper-catalyzed Sandmeyer reaction of N-(prop-2-yn-1-ylamino) pyridines in the presence of organic nitrites afforded various bicyclic chlorinated pyridones [28]. For example, reaction of pyridine 3 in combination with isopentyl nitrite and cupric chloride gave pyridone 4 in 62% yield. Temperature was maintained at 65 °C to achieve maximum conversion in acetonitrile solvent. Mechanism of this reaction first involved the coordination of nitrosyl complex of cupric chloride with alkyne group followed by the breakage of RO–NO bond releasing alkoxy radical that captured hydrogen from NH2 group to give ROH molecule. On the other side, breakage of NH2 bond started the free radical chain reaction resultantly affording E-exo-chloromethylene intermediate which after hydrolysis gave targeted pyridone 4 (Scheme 2).
Copper-catalyzed Sandmeyer reaction for the conversion of N-(prop-2-yn-1-ylamino) pyridine 3 to pyridone 4
A single report considering the importance and synthesis of monofluorinated polychlorinated biphenyls has been disclosed by Sott et al. [29] in which Suzuki and Sandmeyer reactions are the key steps. Table 1 presents some of these reactions performed under optimized conditions. Suzuki coupling between arylboronic acid 5 and 2,3,5,6-tetrachloro-bromoaniline (6) was processed using 3% Pd(PPh3)4 as catalyst, sodium carbonate as base and toluene as solvent. As a result, biphenyl product 7 was obtained which subsequently subjected to Sandmeyer reaction in the presence of t-BuONO and CuCl2 to obtain required PCB (polychlorinated biphenyls) 8 in 4% overall yield. Likewise, other two reactions were also performed; however, deamination in the absence of CuCl2 gave desired PCB 11 and 12 in 26% and 7% overall yields, respectively, which could be used as analytical standards for PCB analysis.
An approach for the synthesis of tetrasubstituted pyrazole derivatives and their fungicidal properties against Uncinula necator was investigated by Dumeunier et al. [30]. Their methodology started from the reaction of acetonitrile 13 with 4-chloro benzaldehyde (14) to afford 2,3-diarylacrylonitrile 15 which was subjected to ring closure reaction with hydrazine. Resultantly, pyrazoline derivative 16 was obtained which readily converted into desired pyrazole 17 by applying standard protocol of Sandmeyer reaction (HCl, NaNO2, CuCl) (Scheme 3).
Synthesis of pyrazole 17 by applying standard protocol of Sandmeyer reaction
Hughes et al. [31] proposed a strategy for the synthesis of a variety of 4-aryl-5-cyano-2-aminopyrimidines which are effectively used as VEGF (vascular endothelial growth factor)-R2 kinase inhibitors. These inhibitors stop angiogenesis process successfully, resultantly inhibiting the growth of tumor cells. The synthetic protocol of these inhibitors started from the reaction of aryl methyl ester 18 with lithium salt of the acetonitrile to obtain corresponding α-cyanoketone which on treatment with N,N-dimethylformamide diethyl acetal afforded vinylogous amide. In the next step, pyrimidine ring 19 was obtained by the reaction of vinylogous amide with guanidinium salt. Sandmeyer reaction of pyrimidine 19 in the presence of antimony trichloride and tert-butyl nitrite gave 2-chloropyrimidine derivative 20 which was efficiently converted into targeted 4-aryl-5-cyano-2-aminopyrimidine derivative 21 by the displacement of chloro group with a variety of aliphatic amines in 73–95% yield range (Scheme 4).
Sandmeyer reaction for the preparation of aminopyrimidines derivatives
A quite efficient and simple way to synthesize different arylpiperazines involving Sandmeyer reaction as key step was demonstrated by Rancati et al. [32]. A reference pathway is described in Scheme 5 which started from the nitration of 1,4-benzodioxin-5-carboxylic acid 22 followed by catalytic hydrogenation provided hydrochloride form of amino derivative 23 in quantitative yield. This benzodioxine was subjected to Sandmeyer reaction in the presence of sodium nitrite and CuCl to obtain halogenated benzodioxine 24 quantitatively. Later, conversion of carboxylic acid group of this chloro residue (24) to acyl azide followed by decomposition process afforded 5-amino derivative (22% yield) which upon treatment with basic alumina supported amine 25 and subsequent reaction with Boc anhydride gave desired piperazine derivative 26 in 62% yield (over 2 steps).
Preparation of arylpiperazines using Sandmeyer reaction as key step
Research group of Han disclosed the synthetic route of thienopyrimidine analogs which were found to be effective FLT3 inhibitors [33]. Standard conditions to carry out this protocol started from the Knoevenagel condensation of 2-acetylthiophene 27 with malononitrile 28 followed by the treatment with elemental sulfur to obtain corresponding thiophene, ready to produce thienopyrimidine 29 after treatment with formamide. Next, Sandmeyer reaction of thienopyrimidine 29 proceeded well at 70 °C in the presence of t-BuONO and CuCl2. THF/CH3CN was used as solvents to carry out maximum conversion. Later on, reaction of chloride 30 with hydrazine monohydrate and furan 31 afforded required thienopyrimidine 32 efficiently. By using same reaction pathway, a variety of thienopyrimidine derivatives were obtained in good yield range (Scheme 6).
Synthetic route for thienopyrimidine analogs
Ding et al. [34] presented a valuable approach to obtain biologically active isatin derivatives which play an important role in pharmaceutical industry due to their excellent antitumor properties against a variety of cell lines (K562, HepG2, HT-29, HL60, etc.). Focusing the synthesis of isatin derivatives, a quite simple and easy pathway is outlined in Scheme 7 involving the condensation of aniline 33 with hydroxylamine hydrochloride at 90 °C and chloral hydrate in 2 mol/L of HCl and water solution to afford oxime 34. Cyclization of this oxime (34) in the presence of sulfuric acid with subsequent bromination reaction afforded required isatin 36 in 86% yield.
Synthesis of isatin 36 from oxime 34
Player et al. [35] reported the synthesis of 2-(2-chloro-6-fluorophenyl)acetamides and proved their potential applicability as thrombin inhibitors. A reference synthetic protocol is highlighted in Scheme 8 which started from the reaction of nitrobenzene 37 with diethyl malonate followed by decarboxylation (in the presence of LiCl) and aromatic nucleophilic substitution reaction (with amine 38) provided aryl fluoride 39. Protection of -NH group of compound 39 with subsequent reduction (Zn, AcOH) and Sandmeyer reaction (tert-butyl nitrite, CuCl2, CH3CN) provided derivative 40. Later on, deprotection of the -NH and ester groups followed by the reaction with O-guanidine segment afforded desired 2-(2-chloro-6-fluorophenyl)acetamide 41.
Preparation of 2-(2-chloro-6-fluorophenyl)acetamide 41 as thrombin inhibitor
Bromination via Sandmeyer reaction
Lobana and colleagues reported a first example of Sandmeyer reaction for the conversion of 2-mercapto-1-methyl-imidazoline to 2-bromo-1-methyl-imidazole at ambient temperature [36]. Copper(I) bromide was selected as suitable catalyst for this purpose, and the reaction was carried out in CH3CN-CHCl3 mixture. Plausible mechanism for this conversion is presented in Scheme 9 which started from the oxidation of Cu(I) to Cu(II) ion. This copper ion further used to oxidize thio moiety 42, resultantly produced disulfide imidazoline 44 which in the last step was converted into required 2-bromo-1-methyl-imidazole 45. This brominated imidazole (45) coordinated with bromide ions in the presence of Cu2+ to obtain tetranuclear complex [Cu4(η1-N-(N2C4H5Br)4(μ4-O)(μ-Br)6].
Mechanism for the conversion of 2-mercapto-1-methyl-imidazoline 42 to 2-bromo-1-methyl-imidazole 45
The research group of Laali performed Sandmeyer reaction for the bromodediazotization of the diazonium salt 46 [37]. Reaction performed under nitrogen atmosphere using [BMIM][PF6] ionic liquid, as solvent. Copper(I) bromide was used as bromine source, and temperature was maintained at 65–75 °C. Resultantly, halogenated products were obtained and their formation ratio strictly depended upon the nature of the substituents of the diazonium salts. As depicted in Scheme 10, diazonium salts having electron-donating substituents gave mainly Schiemann product; however, electron-withdrawing substituents afforded Sandmeyer product predominantly along with the formation of hydrodediazoniation product.
Sandmeyer reaction for the bromodediazotization of the diazonium salt 46
Evans and coworkers described an impressive reaction pathway to synthesize 5-amino-3-aryl-1-(tert-butyl)-1Hpyrazole-4-carboxamides in good yield range [38]. Reaction of potassium tricyanomethanide (50) with tert-butylhydrazine (51) in a mixture of HCl and water gave 41% yield of pyrazole 52 which was successfully subjected to Sandmeyer reaction. This reaction worked very well by using t-BuONO and CuBr2 in acetonitrile solvent. As a result, corresponding 3-bromo regioisomer 53 was afforded in 59% yield. Later on, hydrolysis of cyano group of compound 53 followed by Suzuki–Miyaura reaction gave desired pyrazole-4-carboxamide 56 in 87% yield. By using similar reaction approach, a variety of targeted compounds were obtained in 25–87% yield range (Scheme 11).
Synthesis of pyrazoles by adopting Sandmeyer and Suzuki–Miyaura approaches
Özkan et al. [39] published a report on the facile synthesis of bromobenzenes by using Sandmeyer approach. In their methodology, a quick reaction of aniline with concentrated HCl produced corresponding anilinium salt which was diazotized in the presence of ethyl nitrite. In the next step, this diazonium salt was treated with bromine radical, obtained by the reaction of molecular bromine with ammonium persulfate. As a result, desired substituted bromobenzene was afforded in moderate to good yield range (55–80%). A reference example is highlighted in Scheme 12.
Facile synthesis of bromobenzene 60 by using Sandmeyer protocol
Research group of Schäfer reported a simple, efficient and cost-effective synthetic pathway for ethyl 5-(2,4-difluorophenyl)-1,3,4-thiadiazole-2-carboxylate (64) including Sandmeyer bromination and Suzuki–Miyaura couplings as key steps [40]. Reactions performed at gram scale and kilogram scale level under mild conditions. An outline of these reactions is presented in Scheme 13 which started first from the conversion of ethyl 5-amino-1,3,4-thiadiazole-2-carboxylate (61) into bromo thiadiazole 62 in 71% yield. Reaction processed at room temperature in the presence of tert-butyl nitrite and 1.5 equivalents of copper bromide using acetonitrile as an effective reaction media. Later, compound 62 was subjected to Suzuki–Miyaura reaction using boronic acid 63 as another coupling partner. This palladium-catalyzed reaction along with xanthphos ligand afforded desired cross-coupled product 64 in 85% yield.
Synthesis of ethyl 5-(2,4-difluorophenyl)-1,3,4-thiadiazole-2-carboxylate (64) via Sandmeyer bromination and Suzuki–Miyaura couplings
A variety of pharmaceutical agents having cyclopropylpyridine scaffold are good inhibitors of interleukin receptor-associated kinases, PDE4 enzyme inhibitors and have been widely used to synthesize canabinoid receptor 2 agonists. Considering their importance, Striela et al. [41] prepared bromocyclopropylpyridines by the reaction of aminocyclopropylpyridines (obtained via Suzuki reaction of aminobromopyridines) with amyl nitrite through Sandmeyer approach. Reaction proceeded at room temperature in dibromomethane solvent using 0.5 equivalent CuBr2 to obtain good yield range.
A competent method for the synthesis of aryl bromides involved the reaction of arenediazonium salts with KBr, resultantly affording a variety of aryl bromides in an excellent yield range. This Sandmeyer reaction was carried out at 20–25 °C in acetonitrile solvent. Maximum conversion was achieved by using equimolar (10 mol%) catalytic mixture of CuBr and CuBr2 along with dibenzo-18-crown-6 as a phase transfer catalyst and 1,10-phenanthroline (phen) as ligand. This protocol covers a wide substrate scope by allowing the preparation of different electron-donating and withdrawing groups containing aryl bromides and dibromides in 56–99% yield range [42].
Siméon et al. [43] reported halogenation reactions of 2-amino-1,3-thiazoles in the presence of CuBr/CuBr2 for the preparation of monohalo and dihalo 1,3-thiazole derivatives. Temperature played a vital part to achieve required products in reasonable yield. For instance, reaction of 2-aminothiazole 65 in the presence of CuBr, n-butyl nitrite and acetonitrile gave desired monohalogenated product 66 in 46% yield. This reaction was completed at 60 °C within 15 min. However, when the same reaction was performed first at 40 °C then at 25 °C (for 15–120 min) and 65 °C (for 15 min) using CuBr2 as catalyst, dihalo product 67 was obtained in 79% yield. On the other hand, in the absence of n-butyl nitrite 2-aminothiazole 65 gave halogenated product 68 at room temperature within 10 h in 94% yield. In this methodology, all reactions were performed in a regioselective fashion under mild conditions which led to the formation of a variety of novel iodo, bromo and chloro derivatives (Scheme 14).
Thiazole derivatives 66–68 obtained by Sandmeyer reaction
Iodination via Sandmeyer reaction
Synthesis of a variety of hydroxycoumarin and pyranocoumarin derivatives and evaluation of their anti-proliferative activity was reported by Mao et al. [44]. 3,5-Dimethoxyaniline (69), starting precursor of this methodology first underwent Sandmeyer reaction in the presence of NaNO2, H2SO4 and KI. Resultantly, iodine-substituted methoxy ether 70 was obtained in 75% yield which demethylated followed by the reaction with β-ketoester afforded iodo-substituted 5-hydroxycoumarin 71. Conversion of this coumarin (71) to chromene 72 by annulation with 3-methylbut-2-enal was achieved in 79–82% yield range. After that, these coumarin and chromene derivatives were successfully subjected to palladium-catalyzed Suzuki cross-coupling reaction using different arylboronic acids to obtain desired hydroxycoumarin and pyranocoumarin derivatives in good to excellent yield range (Scheme 15).
Sandmeyer reaction of 3,5-dimethoxyaniline (69) for iodine-substituted methoxy ether 70
Another application of Sandmeyer reaction was reported by Kim et al. [45] where they described [5,5]-sigmatropic rearrangement reactions of N,N’-diaryl hydrazides, resultantly affording 4,4′-diamino-biphenyls (benzidines). Their methodology started from the copper-catalyzed coupling of bis(m-bromophenyl) ethers 73 followed by cyclization reaction in the presence of palladium catalyst furnished corresponding diaryl hydrazides which were then subjected to benzidine rearrangement in the presence of aq. HCl. As a result, benzidines 74 were obtained whose structures were confirmed by treating one of the derivatives with sodium nitrite and KI. As expected, corresponding diiodide 76 was obtained which confirmed the structural integrity of benzidines 74. Later, these benzidines 74 were readily converted into desired acetamides 75 (45–47%) via passing through acetylation process (Scheme 16).
Sandmeyer approach for the confirmation of benzidine 74 structure
Owing to the wide spread applications of conjugated compounds in optoelectronic devices due to their charge transfer and luminescent properties, synthesis of newly functionalized conjugated polymers and oligomers has fascinated a great deal of consideration. [2.2] Paracyclophane skeleton plays a main role in this regard and has been synthesized by different era of chemists due to its exclusive conjugated system. To carry out this research work, Gon et al. [46] synthesized tetrasubstituted [2.2]-paracyclophane core which involved Sonogashira-Hagihara coupling (PdCl2(PPh3)2, PPh3, CuI, Et3N, THF) and Sandmeyer reactions (H2SO4, NaNO2, KI) as key steps. Results declared that targeted derivatives showed good optical properties due to their larger molar extinction coefficient and photoluminescence quantum efficiency.
Miscellaneous
Liu et al. [47] proposed a complementary electrochemical method for Sandmeyer halogenation in which graphite can be used as cathode material which is an inexpensive metal as compared to platinum. This electrochemical reaction generated a variety of aryl halides by treating diazonium salts with different halogen sources such as CBrCl3, CH2I2, LiCl, CCl4, NaBr, NBS. Reaction processed at 20 °C in 5:1 mixture of MeOH/DMF using Bu4NClO4 as an electrolyte. Moreover, this reaction can also be performed in a one-pot fashion by obtaining diazonium salt from different anilines in the presence of tert-butyl nitrite followed by halogenation under optimized conditions provided required aryl halides.
The efficient synthesis of novel benzo-substituted phthalazines was reported by Tsoungas and Searcey [48]. Their synthetic pathway started from the catalytic hydrogenation of aldehyde 77 to obtain alcohol 78 in 74% yield which was then subjected to Sandmeyer reaction in the presence of sodium nitrite and trimethysilyl bromide. As a result, diazonium salt 79 was formed which readily converted into compound 80. After deprotection 58% yield of free alcohol 81 (from compound 78) was obtained that further underwent halogen lithium exchange process followed by oxidation (PCC, DCM) and cyclization (N2H4, EtOH) reactions to provide targeted phthalazine 82 in 82% yield (Scheme 17).
Synthesis of benzo-substituted phthalazine 82 via Sandmeyer reaction
An alternate route to obtain phthalazine 82, started from the reduction of aldehyde 77 followed by diazotization and Sandmeyer reaction (t-BuONO, CuBr, HBr), provided aldehyde 85 in 45% yield. This aldehyde after protection ((CH2OH)2, TSA) followed by lithium halogen exchange process gave resulting intermediate in 76% yield (over 2 steps). Deprotection in the presence of 3 N HCl and cyclization of 4-methoxyphthalaldehyde with N2H4 provided required phthalazine 82 in 82% yield (Scheme 18).
An alternate route for phthalazine 82 by diazotization and Sandmeyer reaction (t-BuONO, CuBr, HBr)
Buchtík et al. [49] reported a simple experimental procedure for the synthesis of polynuclear heterocyclic molecules based on 5-phenyl-6-azauracil scaffold. For this purpose, 3-[3-(6-azauracil-5-yl)-2-aminophenyl]-1,2-dihydro-quinoxaline-2-one (86) was used as starting precursor which first converted into diazonium salt that further produced a variety of heterocyclic N–H acids in good yield range. Two reference compounds, prepared via Sandmeyer reaction, are highlighted in Scheme 19. Reaction proceeded well when 6-azauracil 86 was reacted with sodium nitrite followed by the treatment with CuCl or CuBr in the presence of HCl/HBr provided 2-chloro (87) and 2-bromo (88) derivatives in 57 and 80% yield, respectively.
Synthesis of polynuclear heterocyclic molecules based on 5-phenyl-6-azauracil scaffolds
Formation of carbon–CF2/CF3/C2F5 linkage
Highlighting the medicinal importance of organofluorine compounds, research group of Zheng synthesized trifluoromethylated arenes via Sandmeyer trifluoromethylation process [50]. Simple and mild reaction conditions, easy availability of the reagents and wide functional groups tolerance are the prominent features of this methodology. The reaction proceeded first from the formation of diazonium salt 90 which subsequently treated with Langlois’ reagent and CuCl in the presence of sodium bicarbonate (additive). Maximum conversion was achieved within 20 h by carrying out reaction at room temperature in acetonitrile solvent. Proposed mechanism of this reaction is presented in Scheme 20 which started from the conversion of diazonium salt to diazo radical via Cu(I)-mediated single-electron transfer process. This azo radical was further transformed into aryl radical 92 by releasing nitrogen gas. On the other side, Langlois’ reagent upon treatment with TBHP produced trifluoromethyl radical whose reaction with CuCl generated Cu(II)CF3 species that was used to insert CF3 group in aryl radical 92 in the last step.
Sandmeyer trifluoromethylation approach in the presence of Langlois’ reagent
Danoun et al. [51] designed convenient, competent and inexpensive practical procedures for the trifluoromethylthiolation of arenediazonium salts via Sandmeyer reaction. Optimized parameters of this reaction involved TMSCF3, CuSCN, Cs2CO3 and sodium thiocyanate as sulfur source. Reaction worked very well at room temperature in acetonitrile solvent to obtain 23–98% yield range. Mechanism of this reaction is highlighted in Scheme 21.
Trifluoromethylation in the presence TMSCF3 and CuSCN
Later on, the same research group reported either one pot or sequential diazotization and trifluoromethylation as presented in Scheme 22 [52]. In method A, 4-methoxyaniline (98) was first diazotized in the presence of equimolar amount (2 equiv.) of t-BuONO and HBF4 to produce diazonium salt 99 which was then dissolved in acetonitrile mixture containing TMSCF3, copper(I) thiocyanate and cesium carbonate. As a result, 81% yield of the targeted product 100 was achieved at room temperature. On the other side, in a one-pot procedure a reaction mixture containing diazotized aniline was added to a suspension of TMSCF3, copper(I) thiocyanate and cesium carbonate, resultantly afforded benzotrifluoride 100 in 85% yield. Yields of targeted derivatives were almost comparable of both pathways or sometimes higher in the later one. Wide functional group tolerance such as ether, ester, ketone and cyano groups and good yield range broadened the scope of this methodology.
Sequential (Method A) and one-pot (Method B) diazotization and trifluoromethylation to obtain benzotrifluoride 100
Another application of Sandmeyer reaction for the trifluoromethylation of arenediazonium tetrafluoroborates was disclosed by Danoun et al. [53]. Plausible mechanism of this reaction started first by the reaction of copper(I) thiocyanate with trimethylsilyl cyanide in the presence of cesium carbonate; as a result, trifluoromethyl copper(I) species was generated which reacted with diazonium salt to obtain corresponding benzotrifluoride as described in Scheme 23. The methodology covers wide substrate scope giving rise to 40–98% yield range.
Plausible mechanism for the conversion of diazonium salt 99 to benzotrifluoride 100
Goossen et al [54] provided detailed investigation of novel copper-catalyzed Sandmeyer reaction in which Ruppert-Prakash trifluoromethylating reagent produced a variety of trifluoromethylated arenes in good yield range without formation of CuCF3 species. They began their investigation by treating 4-methoxyaniline with tert-butyl nitrite for the formation of diazonium salt. To select suitable acid for this conversion, the performance of eight acids (p-toluenesulfonic acid (p-TSA), p-TSA.H2O, trifluoroacetic acid (TFA), ethereal.HCl, acetic acid, methanesulfonic acid (MSA), trichloroacetic acid (TCA), benzenesulfonic acid (BSA)) was observed and concluded that p-TSA gave maximum yield (85%). In addition to this, TMSCF3, CuSCN, Cs2CO3 and room temperature were the other parameters to carry out trifluoromethylation and trifluoromethylthiolation in 41–98% and 32–70% yield range, respectively.
A new synthetic approach of trifluoromethylated arenes via copper-catalyzed Sandmeyer reaction in the presence of Umemoto’s reagent was established by Dai et al. [55]. The potential applicability of Umemoto’s reagent in combination with copper powder was proven by carrying out reaction using a variety of aryl amines; as a result, desired trifluoromethylated arenes were obtained in moderate to good yield range. Two equivalents Cu powder, 1.5 equivalents Umemoto’s reagent 102, 3 equivalents of isoamyl nitrite and acetonitrile solvent are the optimized parameters of this methodology (Scheme 24).
Sandmeyer reaction in the presence of Umemoto’s reagent
Matheis et al. [56] performed direct, simple and selective Sandmeyer reaction of diazonium salt 106 by using difluoromethyl-copper complex as difluoromethylating reagent. This complex can be formed by treating 2.5 equivalents of TMS-CF2H with 1 equivalent copper thiocyanate and 3 equivalents cesium fluoride in DMF solution. This successful difluoromethylation process tolerated a wide variety of functional groups by giving 34–86% yield range. It was observed that both electron-donating and withdrawing substituents afforded almost high yields. However, by comparing ortho, meta and para positions functional groups, ortho-substituted substrates gave lower yields (38–73%) as compared to para-substituted substrates (69–81%). In case of heteroarene diazonium salts such as quinolone, carbazole and indole derivatives, a reasonable yield range (35–78%) was also observed (Scheme 25).
Difluoromethylation in the presence of difluoromethyl-copper complex [Cu-CF2H]
In 2014, Wang et al. [57] have made a novel contribution toward Sandmeyer-type trifluoromethylation reaction by using trifluoromethyl-silver complex as CF3 source. This methylating reagent was prepared by treating silver fluoride with TMSCF3 in propionitrile. Temperature was first maintained at − 78 °C and then gradually raised to 25 °C. As a result, desired AgCF3 complex obtained which was subjected to Sandmeyer process. This process started from the diazotization of the different aromatic amines via standard protocol (HCl, t-BuONO) followed by the oxidative addition of the trifluoromethyl-silver complex to diazonium salt, resultantly afforded high-valent silver intermediate which readily underwent reductive elimination reaction to obtain targeted trifluoromethylated product in 10–97% yield range.
Later on, in 2015 Wu et al. [58] proposed one-pot difluoromethylthiolation approach via Sandmeyer reaction under mild conditions. Optimized parameters to carry out this conversion including [Cu(CH3CN)4]PF6 as copper salt, 2,2’-bipyridine as ligand, acetonitrile as solvent and [(SIPr)Ag(SCF2H)] as difluoromethylating reagent (Fig. 1) which could be easily prepared by treating [(SIPr)Ag(CF2H)] with sulfur in THF solvent. This methodology gave wide substrate scope by giving 32–92% yield range of difluoromethylthiolated heteroarenes within 24 h by maintaining temperature at 50 °C.
[(SIPr)Ag(SCF2H)] as difluoromethylating reagent prepared from [(SIPr)Ag(CF2H)]
In 2019, Hu and et al. [59] reported for the first time pentafluoroethylation reaction by utilizing Sandmeyer approach. In their methodology, CuC2F5 was used as pentafluoroethylating reagent which obtained in a sequence manner first by treating TMSCF3 with sodium iodide in THF solvent; as a result, tetrafluoroethylene species was attained. This species further reacted with cesium fluoride in the presence of copper thiocyanate at 70 °C to afford targeted CuC2F5. In the last step, diazonium salt 110 was treated with CuC2F5 in acetonitrile solvent; resultantly, desired product 111 was obtained in 93% yield. This protocol covered wide substrate scope by giving targeted derivatives in 51–93% yield range within short reaction time (Scheme 26).
Pentafluoroethylation via Sandmeyer reaction
Hong et al. [60] examined the use of Togni’s reagent in one-pot Sandmeyer trifluoromethylation reaction. Their pathway started from the diazotization of the aromatic amines in the presence of HCl and t-BuONO. Then this salt was treated with Togni’s reagent II and copper salt, Cu(MeCN)4BF4 at 45 °C. Sodium bicarbonate was used as base in dichloroethane solvent. As a result, corresponding tifluoromethylated analogs were obtained in 42–90% yield range. A plausible mechanism is highlighted in Scheme 27 according to which Togni’s reagent was used to produce CF3 radical via copper(I)-mediated single-electron transfer (SET) approach.
Sandmeyer trifluoromethylation reaction in the presence of Togni’s reagent II
Some other reports on Sandmeyer-type fluoromethylation are presented in Table 2.
Formation of carbon–CN linkage
In 2014, Xu et al. [63] reported Cu2O-catalyzed Sandmeyer reaction of arenediazonium tetrafluoroborates with TMSCN. The reaction worked very well and gave maximum yield with 0.4 equivalent catalyst loading that was not even significantly increased by using 1 equivalent of Cu2O in acetonitrile solvent. Temperature was maintained at 55 °C to obtain targeted products in 38–92% yield range within 10 h. This ligand and halogen-free protocol provided many benefits over classic Sandmeyer reaction as nontoxic, mild reaction conditions, low catalyst loading and wide substrate scope are the salient features of this methodology.
Later on, this research group presented another nontoxic palladium-catalyzed cyanation via Sandmeyer approach in which acetonitrile was used as nonmetallic CN source [64]. Reaction processed under ambient air in the presence of 0.1 equivalent of PdCl2 and 1 equivalent of Ag2O (additive) at 55 °C. As a result, 30–64% yield range was obtained of the targeted derivatives. A plausible mechanism is highlighted in Scheme 28 which started from the reduction of divalent palladium to zero-valent palladium. In the next oxidative step, this zero-valent palladium added to ArN2+BF4− to obtain Ar-Pd species (A) followed by the cleavage of CH3-CN bond in the presence of Ag2O gave intermediate (C). Reductive elimination was the last step which provided aromatic nitrile along with zero-valent palladium.
Plausible mechanism for palladium-catalyzed cyanation via Sandmeyer approach
In order to develop new and efficient conditions for Sandmeyer cyanation, Barbero et al. [65] utilized arene and heteroarenediazonium o-benzenedisulfonimides as starting precursors and tetrabutyl ammonium cyanide as CN source. Reaction was carried out at room temperature in acetonitrile solvent. This approach under mild reaction conditions gave targeted compounds in 34–92% yield range. A reference example is presented in Scheme 29 which highlighted the mechanism of this copper-free protocol started by the transfer of electron from anionic part of the salt 119 toward cation. As a result, resonance-stabilized complex 119a was formed which reacted with CN− part of tetrabutyl ammonium cyanide to provide corresponding aryl nitrile 120.
Sandmeyer cyanation, using arenediazonium o-benzenedisulfonimide 119 as starting precursor and tetrabutyl ammonium cyanide as CN source
Da Silva et al. [66] designed an effective approach for the synthesis of 2-chloro-3-carbonitrile analogs which are well-known intermediates and can be transformed into a variety of useful and biologically important heterocyclic molecules, for example the highly polyfunctionalized 4H-pyran, oxazolo, pyrazolo, 1,4-dihydropyridine or pyridines derivatives. The authors used 2-amino-3-carbonitriles as starting precursors to obtain corresponding 2-chloro-3-carbonitriles via Sandmeyer approach. The reaction was catalyzed by 1.5 equivalents of CuCl2 in acetonitrile solvent. After the addition of isoamyl nitrile, temperature was maintained at 65 °C to obtain targeted derivatives in 10–69% yield range (Fig. 2).
2-Chloro-3-carbonitrile analogs (122–132) prepared via Sandmeyer reaction
For the construction of medicinally important indole-1,2,4-benzotriazine derivatives, Sandmeyer reaction seems to be a suitable methodology as elaborated by Xu et al. [67]. Their protocol started by the SNAr reaction of the indole 133 with 2-nitrophenyl halide 134 in the presence of cesium carbonate to obtain respective indole derivative 135 which was subsequently reduced with stannous chloride dihydrate. As a result, indole 136 obtained (45% yield) that was cyclized by using tert-butyl nitrite via a modified Sandmeyer reaction to afford targeted indole-1,2,4-benzotriazine 137 (24% yield) which proved to be a promising lead compound against a variety of phytopathogenic fungi (Scheme 30).
Preparation of indole-1,2,4-benzotriazine derivative 137 involving Sandmeyer reaction as a key step
The research group of Beletskaya reported copper-catalyzed Sandmeyer cyanation approach with a variety of diazonium salts [68]. Reaction proceeded very well using potassium cyanide as CN source, equimolar amount (10 mol%) of CuCN as catalyst, 1,10-phenantroline as ligand, dibenzo-18-crown-6 as phase transfer catalyst and Cu(BF4)2 as co-catalyst. Maximum yield range (52–93%) was obtained by carrying out reaction at room temperature in acetonitrile solvent.
Formation of carbon–sulfur linkage
Sulfonyl fluorides have gained tremendous interest in synthetic organic chemistry due to their unique characteristics such as stability, reactivity pattern and proton-mediated reactivity. They are extensively used for the construction of a variety of pharmacologically important scaffolds. Considering their importance, Lin et al. developed an efficient, cost-effective and copper-free methodology for the synthesis of arenesulfonyl fluorides via Sandmeyer approach. In their protocol, different arenediazonium salts having electron-donating and withdrawing substituents were treated with N-fluorobenzenesulfonimide (NFSI), a fluorine source and K2S2O5 which plays dual role as a reductant and a sulfonyl source simultaneously. Reaction conducted very well under argon atmosphere in a mixture of acetonitrile, water and acetic acid (co-solvent). Maximum conversion was attained within 6 h at room temperature.
Diaryl sulfones exhibit a wide range of biological activities; for example, they act as anticancer, antifungal, antibacterial agents and are efficient thymidylate synthase and HIV-1 reverse transcriptase inhibitors. Besides this, they play a vital role as synthetic intermediates in organic chemistry. Despite the discovery of various synthetic methods for diaryl sulfones, search of new and efficient conditions is still under process. In this regard, Yang et al. [70] reported a competent copper-catalyzed Sandmeyer approach for the synthesis of diaryl sulfones. For example, reaction of aryl amine 89 with arylsulfinic acid 138 in the presence of equimolar amounts (3 equivalents) of copper powder and isoamyl nitrite (diazotizating agent) gave 82% yield of the corresponding product 139. Reaction processed under nitrogen atmosphere by maintaining temperature at 0–25 °C in acetonitrile solvent. This approach depicted a wide substrate scope, allowing the preparation of a variety of diaryl sulfones in 47–82% yield range (Scheme 31).
Copper-catalyzed Sandmeyer approach for the synthesis of diaryl sulfones
Research group of Goossen in 2015 reported difluoromethylthiolation of arene diazonium salts in 61–95% yield range [71]. Standard parameters to make this conversion effective included following steps: First a solution of diazonium salt 99 in acetonitrile solvent was mixed in sodium thiocyanate, cesium carbonate and copper thiocyanate mixture. Then cesium fluoride, copper thiocyanate and TMS-CF2H in DMF were added in the reaction mixture; as a result, desired product 140 was obtained in 95% yield within 12 h by carrying out reaction at room temperature.
Later on, same research group presented trifluoromethylthiolation and pentafluoroethylthiolation by applying Sandmeyer conditions as described in Scheme 32. Reaction of 4-methoxybenzenediazonium tetrafluoroborate (99) was performed with 1.8 equivalents of Me4NSCF3 in the presence of copper thiocyanate. Resultantly, trifluoromethyl thioether 141 was obtained in 97% yield [72]. However, when the same reaction was performed in the presence of 10 mol% elemental copper using Me4NSC2F5 as SC2F5 source, pentafluoroethyl thioether 142 was obtained in 98% yield. This methodology under mild reaction conditions tolerates a variety of functional groups by giving 61–99% yield range [73].
Di-, trifluoromethylthiolation and pentafluoroethylthiolation via Sandmeyer approach
Zhang et al. [74] adopted a different approach for trifluoromethylation of arenediazonium tetrafluoroborates in the presence of Langlois reagent (NaSO2CF3). Reaction worked very well at 45 °C using t-butyl hydroperoxide as oxidant, CuBF4(CH3CN)4 as copper salt and 2,2′;6′,2′′-terpyridine (tpy) as ligand. As a result, 30–63% yield range of the corresponding trifluoromethylated derivatives was obtained in a mixture of acetonitrile/water solvent. Similarly, the authors reported trifluoromethanesulfonylation of arenediazonium tetrafluoroborates in 45–90% yield range in the presence of NaSO2CF3 and 10 mol% Cu2O. To carry out maximum conversion at room temperature, DMSO was selected as suitable solvent.
Organothiophosphates are worthy of attention due to their outstanding insecticidal, antiviral and enzyme inhibition properties against acetylcholinesterase (AChE) enzyme. Their wide spread contribution in pharmaceutical chemistry encouraged Kovacs et al. [75] to disclose different phosphorothiolation methods mainly including phosphorothiolation of arenediazonium salts via Sandmeyer approach. In their methodology, the highlighting reagent used for phosphorothiolation was 144 which when reacted with diazonium salt 143 under standard Sandmeyer conditions (CuSCN (20 mol%), acetonitrile, r.t.), aand phosphorothiolated arene 145 was obtained in 95% yield. The reaction covered a wide variety of substrates by giving targeted products in 47–95% yield range within 16 h (Scheme 33).
Phosphorothiolation of arenediazonium salt 143 via Sandmeyer approach
Ou et al. [76] in 2019 published a convenient procedure for the construction of (diethylphosphono)-difluoromethyl thioethers proceeding via two steps: Sandmeyer thiocyanation and subsequent fluoroalkylation reaction. Optimal reagents for thiocyanation included copper thiocyanate, cesium carbonate, sodium thiocyanate (sulfur source) and acetonitrile solvent. As a result, corresponding thiocyanate derivative was obtained which was subjected to fluoroalkylation reaction using TMS-CF2PO(OEt)2 as the difluoroalkyl source. Plausible mechanism of this reaction is presented in Scheme 34 which started from the generation of CuI(SCN)2 species by the reaction of CuSCN with NaSCN. In the next step, diazonium salt 146 was treated with CuI(SCN)2 to obtain respective thiocyanate 148 via Sandmeyer approach. On the other side, conversion of TMS-CF2PO(OEt)2 to Cu-CF2PO(OEt)2 in the presence of base gave species (A) which was in the final step reacted with thiocyanate 148 to afford targeted product 147.
Mechanism of sequential Sandmeyer thiocyanation and fluoroalkylation reactions
Very recently, research group of Qing reported a similar methodology for fluorosulfonylation of arenediazonium tetrafluoroborates [77]. However, they used Na2S2O5 as sulfonyl source instead of K2S2O5. The other optimized parameters were N-fluorobenzenesulfonimide (NFSI), 2/0.1 mixture of acetonitrile/water, 60 °C temperature under nitrogen atmosphere. As a result, 43–81% yield range was obtained within 6 h. A plausible mechanism is highlighted in Scheme 35. Single-electron transfer reduction of aryldiazonium salt 149 provided aryl radical 114 which upon reaction with SO2 (generated from Na2S2O5) gave arenesulfonyl radical 150. In the last step, transfer of fluorine atom from N-fluorobenzenesulfonimide afforded targeted arenesulfonyl fluoride 151.
source and N-fluorobenzenesulfonimide (NFSI) as fluorine source
Fluorosulfonylation of arenediazonium tetrafluoroborate 149 using Na2S2O5 as sulfonyl
In another report, various diazonium salts were subjected to fluorosulfonylation reaction by passing through Sandmeyer approach. For this purpose, Na2S2O5 was used as sulfur dioxide source and Selectfluor 152 as fluorine source in methanol solvent. Temperature was maintained at 70 °C to attain maximum yield (85%) of sulfonyl fluoride 153. This methodology was applied on a variety of substrates; resultantly moderate to good yield range was obtained (Scheme 36) [78].
Fluorosulfonylation reaction using Selectfluor 152 as fluorine source
Tarkhanova et al. [79] demonstrated that copper catalysts incorporating ionic liquid on Silochrom support efficiently catalyzed Sandmeyer reaction. A highlighted example for the thiocyanation of 3-methyl-4-nitrophenyldiazonium tetrafluoroborate (154) is presented in Scheme 37. For this purpose, potassium thiocyanate was used as nucleophile and reaction was catalyzed with CuCl-Et3PrNCl in acetonitrile solvent. 96% Yield of the required product 155 was obtained by carrying out reaction at room temperature. Same methodology was adopted to attain 87–97% yield range of a variety of aryl bromides.
Sandmeyer reaction catalyzed by copper incorporating ionic liquid on Silochrom support
Formation of carbon–tin linkage
Organotin reagents are highly important due to their usage in the synthesis of various C–N, C–F and C–OCF3 bond formation reactions. They are also used for the generation of C–C bond by highly famous Stille cross-coupling reaction whose wide applications in organic synthesis create a dire need to develop new and efficient synthetic methods for aryl stannane compounds. On account of this, Qiu et al. [80] developed a Sandmeyer-type stannylation approach for the preparation of aryl trimethylstannanes. A highlighted example is presented in Scheme 38 in which amine 57 was treated with tert-butyl nitrite and (SnMe3)2 in dichloroethane solvent at 0 °C. Screening a variety of acidic additives such as TsOH, BF3.OEt2, AlCl3 and AcOH which found to be helpful for diazotization process, maximum yield was obtained with p-toluenesulfonic acid. Overall, moderate to good yield range (36–86%) of the targeted products was obtained which were most likely subjected to different cross-coupling reactions without purification such as Stille reaction and can also be used for the synthesis of different pharmaceutical agents.
Sandmeyer-type stannylation approach for the preparation of aryl trimethylstannanes
Later on, the same research group developed another methodology for stannylation of aromatic amines, simultaneously generating trimethylstannyl arylboronate analogs which were effectively subjected to Stille and Suzuki–Miyaura cross-coupling reactions [81]. For example, conversion of p-nitroaniline 57 into arylboronate 157 was accomplished in the presence of tert-butyl nitrite, bis(pinacolato)diboron, benzoyl peroxide and acetonitrile solvent under metal-free conditions. Then reduction of nitro group to NH2 group via palladium-catalyzed reaction generated boron-substituted aniline 158 in 98% yield which further converted into corresponding stannylation analog 159 under optimized Sandmeyer-type stannylation reaction conditions (t-BuONO, (SnMe3)2, TsOH, DCE) (Scheme 39).
Synthesis of trimethylstannyl arylboronate analog 159
Formation of carbon–phosphorous linkage
In an effort to search diversified methods for aryl phosphonates which play a vital role in material science, organic and medicinal chemistry, Wang and their colleagues studied the applicability of Sandmeyer phosphorylation reaction for the synthesis of these aryl phosphonates [82]. They began their investigation by the reaction of ethyl 4-aminobenzoate with tert-butyl nitrite under different conditions. First of all, trimethyl phosphite, triethyl phosphite and triphenyl phosphite were screened as phosphorous source and results supported the use of triphenyl phosphite which gave significant yield (78%) as compared to the other phosphites. Reaction processed at 0 °C using TsOH.H2O as an additive which was selected among different acids such as HCl, TsOH.H2O, H2SO4 and BF3.OEt2. Reaction completed within 8 h in acetonitrile solvent by giving 25–99% yield range.
Formation of carbon–selenium linkage
Matheis et al. [72] successfully utilized Sandmeyer approach for the preparation of trifluoromethylselenoethers under mild reaction conditions. For this purpose, Me4NSeCF3 was used as SeCF3 source which efficiently converted diazonium salt 99 to its corresponding selenoether 160 in 98% yield. Reaction processed at room temperature using CuSCN as copper salt in acetonitrile solvent. In order to evaluate substrate scope, maximum conversion was achieved within 1 h by giving 69–98% yield range of the desired trifluoromethylselenoethers. Similarly, a straightforward approach for the insertion of trifluoromethylseleno group into aromatic amines was reported by Nikolaienko and Rueping in 2016 [83]. Reaction was accomplished via two steps, first converting diazonium salt 99 to its respective selenocyanate in the presence of CuCl/CuCl2 catalytic system. 1,10-Phenanthroline was used as additive, cesium carbonate as base and KSeCN as SeCN source in acetonitrile solvent. In the second step, this selenocyanate was treated with TMS-CF3 for the insertion of CF3 group. Reaction was completed within 12 h at room temperature and afforded corresponding trifluoromethylselenoether 160 in 74% yield. In this methodology, arenediazonium salts having both electron-donating and withdrawing substituents were readily converted into desired selenoethers in 40–88% yield range (Scheme 40).
Sandmeyer approach for the preparation of trifluoromethylselenoethers
Formation of carbon–boron linkage
Focusing the green synthetic routes, Zhang et al. [84] published their report on Sandmeyer-type borylation for the synthesis of arylboronate esters. They began their investigation by treating 4-carboxylicphenyldiazonium tetrafluoroborate with bis(pinacolato)diboron as exemplary substrates. Reaction processed at room temperature in a mixture of solvents such as acetone–water, acetonitrile–water, dimethoxyethane–water, dioxane–water, water and acetone. Results revealed that 2:1 mixture of acetonitrile–water gave targeted arylboronate ester in 80% yield. Maximum conversion was achieved in the presence of 5 mol% CuBr which selected by observing the catalytic behavior of Cu(OAc)2, FeCl3, Co(Ac)2, CeCl2 and CuBr. 24–85% Yield range of different arylboronate esters was obtained within 3–6 h.
In continuation of this research work, Qiu et al. [85] in 2013 reported Sandmeyer-type borylation approach under metal-free condition. In this method, a variety of aryl amines were first diazotized in the presence of tert-butyl nitrite followed by the addition of bis(pinacolato)diboron in acetonitrile solvent afforded the desired pinacol arylboronates in 14–77% yield range by maintaining temperature at 80 °C. These aryl boronates further used for Suzuki–Miyaura cross-coupling reaction to obtain a variety of biaryl compounds in moderate to high yield range which proved the efficacy of this protocol.
Some other reports on Sandmeyer-type borylation are presented in Table 3.
Applications of Sandmeyer reaction for the synthesis of medicinally important compounds
Natural bioactive molecules
Synthesis of alkaloid ficuseptine
In 2002, Bracher and Daab developed an efficient protocol for the synthesis of alkaloid ficuseptine 167 (a good antifungal and antibacterial agent) which involved Suzuki, Sonogashira and Sandmeyer reactions as key steps [91]. Their methodology started from the Suzuki cross-coupling of bis(arylation) of 3,5-dibromopyridine (164) with 4-methoxyphenylboronic acid under optimized conditions resultantly afforded corresponding arylated compound 165 in 71% yield which was subsequently subjected to Sandmeyer reaction in the presence of t-BuONO, CH2I2 and I2. Consequently, iodopyridine 166 was obtained in 61% yield. In the next step, by applying Sonogashira reaction conditions followed by palladium-catalyzed hydrogenation and cyclization reactions afforded desired indolizidinium alkaloid 167 in 79% yield (Scheme 41).
Synthesis of alkaloid ficuseptine 167 using Suzuki, Sandmeyer and Sonogashira reactions as key steps
Synthesis of indolizidinium alkaloid ipalbidinium
Later on, the same research group utilized their previously reported methodology for the synthesis of indolizidinium alkaloid ipalbidinium 171 and quinolizidinium alkaloid clathryimine B [92]. Their recommended approach is described in Scheme 42 which started from the Suzuki reaction of the bromopyridine 168 with boronic acid under standard conditions. As a result, corresponding arylpyridine 169 was obtained in 72% yield which subsequently subjected to Sandmeyer reaction in the presence of t-BuONO, CH2I2, CuI and I2, resultantly affording iodo compound 170 in 82% yield. Further, Sonogashira coupling followed by catalytic hydrogenation and cyclization reactions which after cleavage of the OMe group using BBr3 afforded desired ipalbidinium 171 in 19% yield. Likewise, clathryimine B was also prepared by using the same reaction conditions of these key steps.
Synthetic pathway for indolizidinium alkaloid ipalbidinium 171
Synthesis of curcuphenol
Studies on biological activities of curcuphenol reveal that it can be used as antibacterial, antifungal, antimalarial as well as anticancer agent. Besides this, it plays a great role to inhibit proton-potassium ATPase enzyme, resultantly preventing/curing different stomach diseases such as gastroesophageal reflux disease and peptic ulcer disease. Remarkable pharmacological significance of curcuphenol has attracted researchers to develop various techniques for the efficient synthesis of this scaffold. On account of this, Kim and their colleagues proposed an enantioselective synthesis of ( +)-curcuphenol via Sandmeyer and Negishi cross-coupling reactions [93]. Their pathway started from the easily available starting precursor, m-anisidine (172) which was protected using benzyl bromide and potassium carbonate base. Insertion of aldehyde into this moiety using crotonaldehyde 173 in the presence of imidazolidinone catalyst 174 provided corresponding aldehyde derivative in 90% yield. After reduction of this aldehyde with NaBH4 followed by deprotection, reaction provided compound 175 in 90% yield. Compound 175 was then converted to derivative 176 by passing through Sandmeyer reaction. Optimized reagents for this conversion were sodium nitrite and copper bromide which gave resultant analog 176 in 55% yield. Next step was Negishi coupling for the replacement of bromo group to methyl group, and the resultant compound was treated with triphenylphosphine, imidazole and iodine to obtain iodo compound 177 in 84% yield. Last step was the reaction of compound 177 with 2-methyl-1-propenylmagnesium bromide (178) followed by the cleavage of the methyl ether functionality afforded (S)-( +)-curcuphenol (179) in 92% yield (Scheme 43).
Synthesis of ( +)-curcuphenol 179 via Sandmeyer and Negishi cross-coupling reactions
Synthetic bioactive molecules
Synthesis of agomelatine
Kandagatla et al. [94] established a convenient and simple approach for the synthesis of agomelatine (an antidepressant drug) including easily available starting precursors. For this purpose, 8-aminonaphthalen-2-ol (180) was first subjected to Sandmeyer reaction. Diazotization of 180 in the presence of NaNO2 followed by halogenation with KI provided 8-iodo-β-naphthol (181) in 73% yield. After that protection of OH group using dimethyl sulfate with subsequent formylation reaction afforded 7-methoxy-1-naphthaldehyde (182) in 70% yield. Reaction of this aldehyde (182) with nitromethane followed by catalytic hydrogenation and acetylation provided targeted agomelatine 183 in 95% yield (Scheme 44).
Synthetic pathway for agomelatine 183, an antidepressant drug
Synthesis of ceritinib
For the treatment of lung cancer, crizotinib, a tyrosinase kinase inhibitor, was effectively used in the past few years. However, resistance to this drug creates an option to develop new potent anticancer drug alternate to crizotinib. Ceritinib (LDK378) is another new and efficient anaplastic lymphoma kinase inhibitor which is efficiently used to treat cancer with greater potency. Considering its importance, Liu et al. [95] described a cost-effective route for the synthesis of ceritinib (LDK378) which started from easily available starting substrates under mild reaction conditions. In their methodology, first 1,3-dichloro-4-nitropyridine (184) was coupled with 2-(propane-2-sulfonyl)-phenylamine (185) in the presence of chloroform; resultantly, corresponding compound 186 (95%) was obtained by the displacement of 3-Cl of 184 with NH2 group of 185. In the next step, 1-Cl group of compound 186 was displaced with NH2 group of Boc protected compound 187 in acetonitrile solvent to obtain coupled product 188 in 94% yield. Later, catalytic hydrogenation followed by Sandmeyer reaction in the presence of isobutyl nitrite, HCl and cuprous compound gave desired ceritinib (LDK378) 190 in 67% yield (Scheme 45).
Synthesis of ceritinib (LDK378) 190, an anaplastic lymphoma kinase inhibitor
Synthesis of favipiravir
Favipiravir, a pyrazine ring containing compound, mainly acts as anti-influenza drug. It can also be used to treat a variety of viral strains (alphavirus, ebola virus, bunyavirus, and flavivirus) by disrupting the activity of RNA polymerase of virus. It is wide spread biological functions in antiviral and antiparasitic field encourage researchers to develop efficient and low-cost pathways for the synthesis of favipiravir in good yield. On account of this, Guo et al. [96] reported a mild and simple protocol starting from the chlorination of commercially available 2-aminopyrazine 191 in the presence of 1.1 equivalent of TSA (N-chloro-N-methoxy-4-methylbenzenesulfonamide) 192. As a result, corresponding chlorinated product 193 was obtained in 80% yield which subsequently subjected to bromination and palladium-catalyzed cyanation reactions using NBS and NaCN, respectively, to obtain compound 195. 2-Amino group of compound 195 was further replaced with chloro group by adopting Sandmeyer approach which afforded good yield of the corresponding derivative 196 with simple workup procedure. This reaction was carried out at room temperature by using t-butyl nitrite, TiCl4 and DCM as optimized reagents. After that, fluorination of compound 196 followed by acid mediated nitrile hydration and reaction with sodium bicarbonate provided desired favipiravir 197 in 82% yield (Scheme 46).
Preparation method for favipiravir 197, an anti-influenza drug
Miscellaneous
Tsintsadze et al. [97] reported a direct, simple and selective approach for the synthesis of benzo[b]thiophenoindoles. For this purpose, 3-aminodibenzothiophene 198 was used as starting precursor which in the presence of chloral hydrate and hydroxylamine hydrochloride and subsequent reaction with sulfuric acid afforded a mixture of isatins 199 and 200 via Sandmeyer pathway. Alkaline solution of these compounds (199 and 200) was then treated with acetic acid followed by acidification with HCl provided separated compounds 199 and 200 in 60% and 25% yields, respectively. Later, reduction of compound 199 with diborane in the presence of THF produced compound 201 in 70% yield. However, by using LiAlH4/NaBH4 as reducing agents, a mixture of corresponding benzo[b]thiophenoindoles 201 and 202 were obtained. On the other side, by applying same reaction conditions, compound 200 was reduced to benzo[b]thiophenoindoles 203 and a mixture of 203 and compound 204 as depicted in Scheme 47. Later on, by using same reaction parameters Khoshtariya et al. in 2004 [98] and 2007 [99] synthesized 2,3-dioxo-2,3-dihydrobenzo[b]furoindoles and dioxodihydro-1H-benzo[b]furoindole, respectively.
3-Aminodibenzothiophene 198, a starting precursor for the synthesis of benzo[b]thiophenoindoles
To synthesize a variety of aryl azides through green approach, Zarchi and Ebrahimi utilized polymer-supported nitrite ion ((poly(4-vinylpyridine)-supported nitrite ion, [P4-VP]NO2) for diazotization process [100]. First, reaction of aromatic amines in the presence of NaNO2, [P4-VP]NO2 and H2SO4 provided corresponding diazonium salts which on reaction with NaN3 afforded targeted aryl azides in 70–90% yield range. Later, in 2014 the authors used (poly(4-vinylpyridine)-supported ethyl bromide ([P4-VP]Et-Br) for the diazotization-bromination of a variety of aromatic amines [101]. Reaction worked very well in the presence of CuBr and afforded 40–94% yield range. Simple recovery and reusability of polymeric reagent with good functional groups tolerance proved the efficacy of this Sandmeyer protocol.
To highlight the importance of gold catalysis which is effectively used to activate carbon–carbon multiple bonds, Peng et al. [102] reported gold-catalyzed Sandmeyer reaction for the formation of C–Br, C–S and C–P linkages. For C–Br bond formation, different aryl diazonium salts were reacted with sodium bromide and 3% PPh3AuCl in acetonitrile solvent. As a result, desired derivatives were attained in 57–88% yield range at 50 °C. On the other hand, C–S cross-coupling reaction was accomplished by the treatment of aryldiazonium salt with (S)-methyl 2-((tert-butoxycarbonyl)amino)-3-mercaptopropanoate (a cysteine derivative used as sulfur nucleophile) in acetonitrile solvent. 3 Mol% catalyst loading using sodium carbonate as base completed this reaction within 3 h at room temperature. Furthermore, reaction of diazonium salts with HP(O)(OEt)2 (a phosphorous source) in the presence of 5 mol% PPh3AuCl gave desired derivatives in 51–87% yield range. Reaction accomplished within 5 h with the help of 3-chloropyridine additive at 50 °C.
Owing to the synthetic as well as pharmacological importance of 1,2-diamines, Gan et al. [103] performed a coupling reaction of ketimine 205 with N-Boc aldimine 206 by using 5 mol% mesitylcopper as catalyst and (R,RP)-TANIAPHOS as ligand in dimethoxyethane. As a result, 1,2-diamine compound 207 was obtained in 79% yield with 97% ee. After that, removal of ketimine moiety in acidic condition followed by protection of amino group with tosyl chloride and reduction of nitro group provided arylamine 208 which readily underwent Sandmeyer reaction to obtain 1,2-diaryldiamine 209, phenol 210, arylbromide 211, trifluoromethylated compound 212 and arylboronate 213 in 80%, 73%, 75%, 54% and 66% yields, respectively, under different reaction conditions as depicted in Scheme 48.
Sandmeyer reaction for the synthesis of 1,2-diamines (209–213)
Conclusion
In conclusion, we have collected a number of Sandmeyer-type approaches with plausible mechanisms published during 2000–2021. This review has witnessed that significant efforts have been made for the conversion of aromatic amino group to boryl, stannyl, phosphoryl, and trifluoromethyl groups by adopting Sandmeyer protocol with or without copper catalysts. However, aryl halides and trifluoromethylated compounds were the most prevalent choices prepared via Sandmeyer reaction. These Sandmeyer-type conversions processed under mild reaction conditions using easily available starting materials proved to be helpful for synthesizing various biologically active compounds. Although many developments regarding Sandmeyer reaction have been made in the recent past yet a lot of improvements are required to address limitations and hindrances involved in its industrial scale use such as excessive use of metals and restricted choice of reagents in diazotization step. On further detailed mechanistic investigation of Sandmeyer reaction, adopting green synthetic methodologies, implementing electrochemical and photocatalytic approaches, incorporating simple reaction methods with minimum use of expensive metals, the researchers may be able to prepare novel biologically active molecules through Sandmeyer transformation in near future.
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Akhtar, R., Zahoor, A.F., Rasool, N. et al. Recent trends in the chemistry of Sandmeyer reaction: a review. Mol Divers 26, 1837–1873 (2022). https://doi.org/10.1007/s11030-021-10295-3
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DOI: https://doi.org/10.1007/s11030-021-10295-3