Water plays a crucial role: Small molecule catalyzed C–C/C–X bond forming reactions using organosilicon reagents under “wet” conditions
Introduction
Asymmetric catalysis using organosilicon reagents [1,2], which enables the formation of carbon–carbon (C–C) or carbon–heteroatom (C–X) bonds, represented by the Mukaiyama aldol reaction [3] and its related transformations has been attracting considerable research attention since the past few decades. Due to their excellent efficiency, catalytic methods using relatively stable and readily available organosilicon reagents have been used in diverse practical applications toward the synthesis of various optically enriched pharmaceuticals and natural products [4]. Mainly, silyl enol ethers [5], silyl ketene (thio)acetals [6], trialkyl silyl cyanides [7], and trialkyl (or alkoxy) allylsilanes [8] were used as the silylated nucleophilic partners in these reactions. Various electrophiles were employed in a diverse range of reactions, including the aldol reaction (with carbonyl groups: aldehydes or ketones), Michael addition reactions, Mannich reactions (with N-protected imines), and α-alkylation reactions. A wide variety of catalytic systems have been applied to promote these addition reactions, such as chiral metallic Lewis acids containing tin, titanium, aluminum, zinc, and other metals [3,9]. A useful complement to these advancements is the use of organocatalysts, which do not include metallic species. They have also been widely investigated because of their excellent compatibility, low toxicity, simplicity, and high stability [10]. In addition to the nucleophiles which were employed to install esters or ketones, organosilicon reagents typically used in cyanation, allylation, and alkylation reactions have also been investigated using a diverse range of organocatalysts [11].
Due to the worldwide pursuit of sustainable science and technology, the modern chemical society is devoted to the development of innovative synthetic methodologies in an environmentally friendly and streamlined manner [12]. In this context, water has been recognized as an essential ingredient for organic synthesis because of its abundance, cost-effectiveness, environmental suitability, non-toxicity, and incombustibility [13]. However, water has been rarely used in conventional catalytic organic reactions, due to it may interfere with the transition state of a catalytic activation process, and diminishes the inherent activity and/or stereoselectivity. Despite these stereotypes, however, Breslow reported a seminal rate amplification phenomenon in the Diels-Alder reaction using water as the reaction medium, compared a conventional organic solvent-based system [14]. Later, Sharpless et al. observed a significant rate acceleration effect in the [2σ+2σ+2π] cycloaddition of diethyl azodicarboxylate and quadricyclane using water as the reaction medium, which leads to the introduction of the concept “on-water” chemistry [15]. These groundbreaking discoveries have attracted tremendous attention from synthetic chemists [16]. The continuous efforts toward further understanding of these unique reaction systems involving water are reflected by several recent studies, such as using “aqueous” [[17], [17](a)], “in water” [17b], “in the presence of water” [17c], “at water” [17d], and other strategies [17e]. More recent findings have emphasized that an organic reaction can be dramatically accelerated using an “on-droplet (microdroplets of ca. 5 μm diameter)” [18]. Besides, one equivalent of water in a reaction system is enough to show a significant acceleration effect similar to the “on water”-like effect [19]. More interestingly, when it comes to the asymmetric organocatalysis, not only reactivity enhancement but also remarkable amplification of stereoselectivity was observed in “confined water cages” [[20], [20](a)]. Nonpolar compounds such as hydrocarbons/silicons can exist differently rather than ionic salts or polar organic molecules under aqueous environment (Scheme 1, bottom). The hydrophobic solvation, the clathrate of water creates around the nonpolar solute efficiently build the cage in the organic solutes, benefiting from aqueous–nonaqueous van der Waals interactions [20b]. Though the reason is not clearly understood by far, these ongoing active studies and discussions pursue better understandings of the decisive role of water in the catalytic organic syntheses.
Research interest in organic synthesis under “wet” conditions has also been transferred to catalytic organosilicon chemistry. Some organosilicon reagents may be unstable in the presence of moisture, and active starting materials such as TMS-Nu (where TMS = SiMe3, Nu = anionic nucleophile such as CN−) is readily hydrolyzed to provide trimethylsilanol (TMS-OH) and hexamethyldisiloxane (TMS-O-TMS) with a protonated nucleophile (H-Nu) [21]. Despite this bias, several research groups have successfully reported results regarding metal Lewis acid-catalyzed addition reactions performed in water [22]. Nonetheless, a combined article regarding organocatalytic C–C/C–X bonds [23] forming reactions using organosilicon reagents under aqueous conditions, as known as “wet” conditions, has not been reported to date. Herein we report a review on the constructive effects of water in the reactions as mentioned above (Scheme 1, top), mainly focusing on the rate acceleration, superior chemical yield, and/or selectivity enhancements observed in these reactions when compared to the use of conventional organic solvents. Some recent related examples regarding metal-free, water promoted catalyst-like activation processes will also be introduced (46 examples).
Section snippets
Mukaiyama aldol reactions under “wet” conditions
In 2001, Kobayashi and co-workers reported a highly stereoselective boron catalyzed Mukaiyama-aldol reaction between various aldehydes and silyl enol ethers in water (Scheme 2) [24]. Diphenyl borinic acid (Ph2B(OH)) was used as a catalyst, and a sub-stoichiometric amount of sodium dodecyl sulfate (SDS) was utilized as a surfactant. A catalytic amount of benzoic acid played an essential role in the reaction, where only a trace amount of the target product was observed in the absence of this
Michael addition reactions under “wet” conditions
The Corey group reported an asymmetric Mukaiyama Michael reaction between a range of silyl enol ethers and chalcones using a phase-transfer catalyst (PTC, C-5) (Scheme 13) [36]. The reaction occurred in a biphasic mixture PhMe/KOH(aq.). A cinchonidine-derived quaternary ammonium salt was used as the PTC in the biphasic reaction mixture. Under the optimized conditions, a library of chalcones underwent the reaction with a variety of silyl enol ethers, and their corresponding Michael adducts were
Mannich reactions under “wet” conditions
The Kobayashi group demonstrated an efficient and mild Brønsted acid–surfactant catalyst (dodecylbenzenesulfonic acid: DBSA) promoted three-component Mukaiyama Mannich reaction in water (Scheme 31) [55]. Under the optimized conditions, a library of aldehydes and silyl enol ethers underwent the Mukaiyama Mannich reaction with (2-methoxy)aniline in water to furnish β-amino carbonyl compounds in good to excellent yields. Interestingly, unconventional substrates such as formaldehyde and α,β
α-Alkylation reactions under wet condition
MacMillan and co-workers disclosed an asymmetric organocatalytic singly occupied molecular orbital (SOMO) approach toward the α-enolation of aldehydes with silyl enol ethers (Scheme 40) [64]. Chiral imidazolidinone C-6 was used as a catalyst in this transformation. To achieve high levels of enantioselectivity and reactivity, ceric ammonium nitrate (CAN, 2 equiv.) as oxidant and di-tert-butyl peroxide (DTBP, 2 equiv.) were necessary ingredients. When the concentration of water and that of
Miscellaneous reactions
Akiyama et al. reported an HBF4 (in MeOH, 10 mol%) catalyzed three-component aza-Diels-Alder reaction conducted in aqueous media (10 equiv. of water) (Scheme 44) [68]. As reactants, anilines, aldehydes, and Danishefsky’s diene were employed to provide the corresponding adducts in good to excellent chemical yields. Furthermore, this Brønsted acid-catalyzed reaction was also performed in water without using any organic additive, in the presence of SDS as a surfactant (0.4 equiv.).
In 2003, the
Conclusion and outlook
In this review, we have shown a wide variety of addition reactions using organosilicon reagents under the aqueous conditions. Small molecules such as synthetic organocatalysts, to some extent, main group catalysts were successfully employed. Organosilicon reagents are activated by (Brønsted/Lewis) acid or base catalysts, which incorporate chiral backbones such as binaphthols, cinchona alkaloids, imidazolidinone, pyrrolidines, and diamine moieties. Catalytic methods such as Mukaiyama aldol
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The generous supports of the Ministry of Science, ICT and Future Planning of Korea (NRF-2019R1F1A1046943, 2019R1A6A1A10073079, and NRF-2020R1C1C1006440) are gratefully acknowledged. P.G. acknowledges Korea Research Fellowship (KRF) program (NRF-2019H1D3A1A01102782).
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Both authors contributed equally.