Mechanistic Investigation of the Asymmetric Addition of Trimethylsilyl Cyanide to Aldehydes Catalysed by Dinuclear Chiral (Salen)titanium Complexes

Abstract

Keywords: Asymmetric catalysis / Cyanohydrins / Titanium / AldehydesTitanium complexes 1 derived from chiral salen ligands are highly active precatalysts for the asymmetric addition of trimethylsilyl cyanide to aldehydes and ketones. Based on spectroscopic studies and the identification of adducts between complexes 1 and carbonyl compounds or trimethylsilyl

Asymmetric Addition of Cyanide to β‐Nitroalkenes Catalysed by Chiral Salen Complexes of Titanium(IV) and Vanadium(V)

“…Extensive mechanistic work27–36 on the use of 3 and 4 a as catalysts for asymmetric cyanohydrin synthesis has resulted in the transition-state model shown in Figure 2,27, 33 which shows that (for complexes derived from the ( R , R )-salen ligand) cyanide addition occurs selectively on the re face of the coordinated aldehyde to lead to the ( S )-cyanohydrin trimethylsilyl ether. In contrast, the results presented in this work show that the same complexes will catalyse the formation of ( S )-nitronitriles, which requires cyanide addition to occur on the si face of the coordinated alkene.…”

“…Subsequently, we were able to prepare, isolate and structurally characterise the titanium(salen) dichloride complex 2 , which was a much more active catalyst for asymmetric cyanohydrin synthesis 26. Mechanistic studies27 showed that the active species in both of these systems was actually the bi-metallic complex 3 and just 0.1 mol % of 3 was able to catalyse the asymmetric addition of trimethylsilyl cyanide to aldehydes in less than one hour at room temperature 28, 29. Based on the mechanistic information obtained with titanium complexes,27, 30 we were able to develop vanadium(V)(salen) complexes 4 as even more enantioselective catalysts for the asymmetric addition of trimethylsilyl cyanide to aldehydes 29–35.…”

“…Mechanistic studies27 showed that the active species in both of these systems was actually the bi-metallic complex 3 and just 0.1 mol % of 3 was able to catalyse the asymmetric addition of trimethylsilyl cyanide to aldehydes in less than one hour at room temperature 28, 29. Based on the mechanistic information obtained with titanium complexes,27, 30 we were able to develop vanadium(V)(salen) complexes 4 as even more enantioselective catalysts for the asymmetric addition of trimethylsilyl cyanide to aldehydes 29–35. In view of this precedent, we decided to investigate whether structurally well-defined metal(salen) complexes 3 and 4 might form highly active and enantioselective catalysts for the asymmetric addition of trimethylsilyl cyanide to nitroalkenes and in this paper we report the results of this work.…”

Industrial Methods for the Production of Optically Active Intermediates

“…Recently new (salen)titanium and (salen)vanadium complexes 43 (CACHy-catalysts), [56] which mediate the asymmet- ric addition of trimethylsilyl cyanide to aromatic and aliphatic aldehydes 44 [57] and ketones [58] , have been described for the preparation of substituted a-hydroxycarboxylic acids (Scheme 19, Table 1). This method has been extended to the use of potassium cyanide as a cheaper nucleophile.…”

Ultrasound‐mediated synthesis of camphoric acid‐based chiral salens for the enantioselective trimethylsilylcyanation of aldehydes

“…The studies of Belokon and North support the existence of a dimeric titanium species, with the two titanium atoms connected through two oxygen bridges. 5,15,[30][31][32] The involvement of a monomeric titanium species is suggested by others 10,[33][34][35][36] and is said to be favored when there are large substituents, such as t-butyl, on the aldehyde moiety. It has been referred that the catalyst structure (monomeric, dimeric, or a mixture of both) is probably related to some reaction conditions such as molar ratios, solvents, and ligand structures.…”