Multinuclear Zn(II)-arylhydrazone complexes as catalysts for cyanosilylation of aldehydes☆

https://doi.org/10.1016/j.jorganchem.2020.121171Get rights and content

Highlights

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    Multinuclear zinc(II) complexes with arylhydrazones of active methylene compounds.

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    A tetranuclear zinc(II) complex shows high catalytic activity in cyanosilylation of aldehydes.

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    The yield of cyanohydrin trimethylsilyl ethers depends on the amounts of catalyst and nature of solvents.

Abstract

Three known multinuclear Zn(II)-arylhydrazone complexes, [Zn{(CH3)2SO}(H2O)(L1)] (1), [Zn2(CH3OH)2(μ-L2)2] (2) and [Zn4(μ-OH)2(1κO:2κO-HL3)4(κO-HL3)2(H2O)4] (3) were prepared upon reaction of ZnCl2 or Zn(CH3COO)2·2H2O with 3-(2-(2-hydroxy-4-nitrophenyl) hydrazineylidene) pentane-2,4-dione (H2L1), 3-(2-(2-hydroxyphenyl)hydrazineylidene)pentane-2,4-dione (H2L2) and 2-(2-(2,4-dioxopentan-3-ylidene)hydrazineyl)benzoic acid (H2L3), respectively, in methanol solution. Compounds 1–3 were tested as catalysts for the cyanosilylation reaction of a diversity of both aliphatic and aromatic aldehydes with trimethylsilyl cyanide yielding the corresponding cyanohydrin trimethylsilyl ethers in high yields (72–98%) in methanol and at room temperature.

Introduction

Cyanohydrins are well-known synthetic intermediates for the synthesis of α-hydroxy acids, α-hydroxyketones, α-aminonitriles, β-hydroxyamines, β-aminoalcohols, etc., which are also found as components of pharmaceuticals [1,2]. In order to avoid the volatile and extremely toxic HCN in the synthesis of cyanohydrins, the cyanation of aldehydes or ketones with trimethylsilyl cyanide (TMSCN) as the cyanide source, is a much-explored reaction (Scheme 1).

In the absence of any catalyst, the reaction of benzaldehyde with TMSCN shows a limited conversion of 13 or 18% after 3 or 14h [3,4], respectively, at room temperature. Thus, a great number of organo- and metal catalysts have been applied in order to improve the reaction yield, for example, 1,1,3,3-tetramethylguanidine [5], P(RNCH2CH2)N [6], N-methylmorpholine N-oxide [7], N-heterocyclic carbenes [8,9], tetraethylammonium 2-(N-hydroxycarbamoyl)benzoate [10], LiCl and LiClO4 [[11], [12], [13]], MgBr2·Et2O [14], Mg–Li bimetallic complex [15], Yb(CN)3 [16], Yb(OTf)3 [17], Cu(OTf)2 [18], ZnI2 [19], KCN:18-crown-6 [20], R2SnCl2 an Sn-montmorillonite [21,22], BF3 [23], VO(OTf)2 [24], InBr3 [25], FeCl3 [26], Zr(KPO4)2 [27], NbF5 [28], Fe(Cp)2PF6 [29], Cu(II)-arylhydrazones [[30], [31], [32]], lead(II)-3-aminopyrazine-2-carboxylate [33], lanthanide-containing polyoxometalates intercalated layered double hydroxides [34], supported ionic liquid [35], metal organic frameworks (MOF’s) [[36], [37], [38], [39], [40], [41]], etc. Most of these catalysts require harmful solvents, heating, prolonged reaction times, provide rather modest yields, or recover and reuse of catalysts are difficult. Therefore, the search the efficient catalyst for cyanosilylation of aliphatic and aromatic aldehydes with TMSCN in high yields, under mild reaction conditions, still remains a challenge in the field of current organic synthesis.

As other transition metals, zinc complexes have also been used as catalysts in organic synthesis [42]. According to the mechanism of Zn(II)-catalyzed transformation of aldehydes or ketones, the coordination of the oxygen atom of carbonyl group to metal centre increases the electrophilic character of carbon atom at the Cdouble bondO group of aldehyde or ketone towards the nucleophilic attack by the second substrate [43]. Thus, Zn(II) complexes are able to activate Cdouble bondO bonds in carbonyl compounds as alternative catalysts instead of expensive Zr, In, V, Ln, etc. based metal catalysts. Inspired by the previously obtained positive results with Zn(II)-arylhydrazone complexes in the Henry reaction by our group [44], herein we expand the catalytic potential of the complexes [Zn{(CH3)2SO}(H2O)(L1)] (1), [Zn2(CH3OH)2(μ-L2)2] (2) and [Zn4(μ-OH)2(1κO:2κO-HL3)4(κO-HL3)2(H2O)4] (3) towards the cyanosilylation of aliphatic and aromatic aldehydes with TMSCN under mild conditions.

Section snippets

Materials and instrumentation

All the chemicals were ordered from commercial sources (Aldrich) and used as received. H2L1−3 and 1–3 were synthesized according to the reported procedures [42,43]. The 1H NMR spectra were recorded at room temperature on a Bruker Avance II + 300 (UltraShield™ Magnet) spectrometer operating at 300.130 MHz for proton. The chemical shift is described in ppm using tetramethylsilane as the internal reference.

Catalytic activity studies

In a typical cyanosilylation procedure, the solvent (CH2Cl2, THF or anhydrous MeOH; 2 mL)

Results and discussion

Our initial experiments were performed using 1–3 as catalysts (Scheme 2), benzaldehyde and TMSCN as model coupling partners in protic (methanol) and aprotic (1,2-dichloromethane and THF) solvents. The results presented in Table 1 indicate the significant influence of the solvent polarity on the yield of 2-phenyl-2-((trimethylsilyl)oxy)acetonitrile. Thus, the reaction is much efficient in the protic MeOH solvent, moreover, 1–3 show a much high solubility than in CH2Cl2 or THF (entries 1–9,

Conclusions

Three previously reported Zn(II)-arylhydrazone complexes [44,45] have been prepared and used as catalysts in the cyanosilylation of both aromatic and aliphatic aldehydes with TMSCN. On account of the acidic proton(s) of the bridged hydroxyl group(s), the catalytic activity of complex 3 (tetranuclear) is higher than those of 1 (mononuclear) and 2 (dinuclear) in methanol at room temperature, producing cyanohydrin trimethylsilyl ethers in high yields (72–98%) in 3 h. As substrates, both aliphatic

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by the Fundação para a Ciência e Tecnologia (FCT), project UIDB/00100/2020 of Centro de Química Estrutural . KTM acknowledges the FCT and Instituto Superior Técnico (DL 57/2016 and L 57/2017 Program, Contract no: IST-ID/85/2018). Authors are thankful to the Portuguese NMR Network (IST-UL Centre) for access to the NMR facility and the IST Node of the Portuguese Network of mass-spectrometry for the ESI-MS measurements. This work also was supported by the ’’RUDN University

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    This paper is dedicated to the 65th anniversary of INEOS and the 120th anniversary academician Alexander N. Nesmeyanov

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