Lithium salts of chiral metallocomplex anions as catalysts for asymmetric trimethylsilylcyanation of aldehydes

Abstract

A series of the anionic Co III complexes based on optically active amino acids containing the lithium cation in the external sphere of the complex was synthesized. The synthesized com pounds were used as catalysts in the asymmetric addition of trimethylsilyl cyanide to aldehydes. The influence of the temperature, catalyst concentration, and modification of the chiral anion structure on the enantioselectivity of catalysis was studied.In vast majority of the cases, metal complexes with chirality in the cationic part are used in asymmetric catal ysis. 1 An alternative strategy, which involved the com pounds bearing the chiral anion responsible for stereodif ferentiation as catalysts, is very rare. 2 A specific feature of these catalytic systems is that the negative charge is delo calized inside rather bulky fragment and, hence, the charge of the outer sphere cation is compensated to a less extent. Therefore, the cation is a stronger Lewis acid than the same ion chelated by chiral ligands. Thus, even alkali metal cations can be "transformed" into efficient Lewis acids.Only several examples of asymmetric reactions catalyzed by the ion pairs [alkali metal cation] + [chiral anion] -were described. 3-7 A family of heterobimetallic cata lysts representing rare earth metal complexes with chiral binaphthol and alkali metal cations in the outer sphere of the complex has previously 3 been synthesized. They were used in the aldol reaction and the Michael asym metric addition. Binaphthol monolithium salt was used in the asymmetric trimethylsilylcyanation of aldehydes. 4The influence of water additives on the enantioselectivity of this reaction was studied. 5 The cyanosilylation of ketones in the presence of lithium salts of phosphoric ac ids based on binaphthol was studied. 6 3,3´ Dichlorobi naphthol dilithium salt was used in the asymmetric aldol reaction. 7 It is considered that among alkali metals the lithium cation possesses the highest Lewis acidity. 8 It was shown 9 that a solution of lithium salt of permethylated icosahe dral monocarbadodecaborate LiCB 11 Me 12 in benzene is an efficient catalyst for pericyclic reactions. It seems prac tically important to use LiCB 11 Me 12 as a catalyst of alk ene radical polymerization. 10 In the previous work, 11 we used the chiral anionic com plexes of Λ and Δ bis[N salicylidene (S) aminoacidato] cobaltates 12 with potassium, sodium, and silver cations as Lewis acids in the asymmetric trimethylsilylcyanation of aldehydes and in the Mukayama reaction. We assumed that the rate of benzaldehyde trimethylsilylcyanation should increase with stronger Lewis acids, viz., complexes containing the lithium ion in the outer sphere. Indeed, it turned out that the half conversion time of the addition of trimethylsilyl cyanide (TMSCN) to benzaldehyde (Scheme 1) catalyzed by the complex Li + [Δ 1] -(the struc ture of the complex is shown in Scheme 2) is 1.3 min and the enantioselectivity reaches 23%, whereas for the com plexes Na + [Δ 1] -and K + [Δ 1] -the half conversion peri od is 5 and 6 min, res...