Hydrocyanation and Cyanosilylation

Mono- or di-lithium salts of (7 )-BINOL give high yields and good ees in cyanations of aromatic aldehydes.262 Formation of an aqua (or alcohol) complex of the catalyst gives higher and reversed ee, and non-linear effects in some cases. 

Asymmetric cyanohydrin synthesis from aldehydes using trimethylsilyl cyanide (TMSCN) has been carried out using a chiral Al(salen) complex-triphenylphosphine Lewis acid-base combination.263 

An enantiopure cyanohydrin of 2-p-tolylsulfinylbenzaldehyde has been prepared (g) using various metallic cyanating agents in the presence of Yb(in) or Y(III) triflate.264 Sulfoxide coordination with the metal is implicated, supported by DFT calculations. 

Aldehydes, RCHO, have been cyanoethoxycarbonylated with ethyl cyanoformate, to give cyanohydrin-related products, RCH (CN)0C02Et, using a heterobimetallic chiral catalyst to achieve high ees.265 Kinetic studies have probed the roles of achiral additives in raising the yield and ee. 

Lithium chloride is a convenient catalyst for cyanosilylation of a range of ketones and aldehydes by trialkylsilyl cyanide, under solvent-free conditions the silylated cyanohydrin product can be directly distilled out.266 As little as a microequivalent of catalyst proved effective. Evidence for nucleophilic chloride, generating a pentavalent silicon (65) as reactive species, is presented. 

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Recently, hydrocyanation and cyanosilylation reactions with other type of chiral aluminum complexes were reported. In 1999, Shibasaki and Kanai reported enantioselective cyanosilylation of aldehydes catalyzed by Lewis acid-Lewis base bifunctional catalyst (64a) [56, 57]. In this catalyst, aluminum center works as a Lewis acid to activate the carbonyl group, and the oxygen atom of the phosphine oxide works as a Lewis base to activate TMSCN. Asymmetric induction was explained by the proposed transition state model having the external phosphine oxide coordination to aluminum center, thus giving rise to pentavalent aluminum... 

Interestingly, completely different types of organocatalyst have been found to have catalytic hydrocyanation properties. Among these molecules are chiral diketo-piperazine [4, 5], a bicydic guanidine [6], and imine-containing urea and thiourea derivatives [7-13]. All these molecules contain an imino bond which seems to be beneficial for catalyzing the hydrocyanation process. Chiral N-oxides also promote the cyanosilylation of aldimines, although stoichiometric amounts of the oxides are required [14]. 

Finally, carbohydrate ligands of enantioselective catalysts have been described for a limited number of reactions. Bis-phosphites of carbohydrates have been reported as ligands of efficient catalysts in enantioselective hydrogenations [182] and hydrocyanations [183], and a bifunctional dihydroglucal-based catalyst was recently found to effect asymmetric cyanosilylations of ketones [184]. Carbohydrate-derived titanocenes have been used in the enantioselective catalysis of reactions of diethyl zinc with carbonyl compounds [113]. Oxazolinones of amino sugars have been shown to be efficient catalysts in enantioselective palladium(0)-catalyzed allylation reactions of C-nucleophiles [185]. 

A review of asymmetric hydrocyanation, cyanosilylation, and hydrophosphonylation of carbonyl and imino groups covers catalyses by metals species, organics, and enzymes. ... 

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