Dedicated to Professor Dieter Seebach on the occasion of his 65th birthdayThe utility of the chiral [Ti(m-O)(salen)] 2 complexes (R)-and (S)-1 (H 2 salen was prepared from (R,R)-or (S,S)-cyclohexane-1,2-diamine and 3,5-di(tert-butyl)-2-hydroxybenzaldehyde) as catalysts for the asymmetric addition of KCN and Ac 2 O to aldehydes to produce O-acetylcyanohydrins was investigated. It was shown that the complexes were active at a substrate/catalyst ratio of 100 : 1 and produced the O-protected cyanohydrins with ee in the range of 60 ± 92% at À 408. Other complexes, [Ti 2 (AcO) 2 (m-O)(salen) 2 ] ((R)-4) and [Ti(CF 3 COO) 2 (salen)] ((R)-5), were prepared from (R)-1 by treatment with different amounts of Ac 2 O and (CF 3 CO) 2 O, and their catalytic activities were tested under the same conditions. The efficiency of (R)-4 was found to be even greater than that of (R)-1, whereas (R)-5 was inactive. The synthesis of the corresponding salen complexes of V IV and V V ,, was elaborated, and their X-ray crystal structures were determined. The efficiency of (R)-3 was sufficient to produce O-acetyl derivatives of aromatic cyanohydrins with ee in the range of 80 ± 91% at À 408.Introduction. ± As enantiomerically pure cyanohydrins are versatile intermediates in organic synthesis, many synthetic approaches to their syntheses are being vigorously pursued [1]. The catalytic ways of making this class of compounds rely upon the asymmetric addition of a cyanide source to the carbonyl group of aldehydes, as catalyzed by enzymes [2] or purely chemical chiral catalysts [3]. Enantiomerically enriched O-protected cyanohydrins are customarily made by the reaction of aldehydes with Me 3 SiCN usually catalyzed by chiral Lewis acids [1]. We recently reported an efficient catalysis of this reaction by the chiral binuclear [Ti IV (salen)] complex 1 (Fig. 1), active at a ratio of substrate/catalyst as high as 1000 : 1 and promoting the addition at room temperature with ee in the range of 80 ± 92% [4]. Very efficient catalysts based on bifunctional complexes of Al III and Ti IV have also been developed by Shibasaki and co-workers, giving O-(trimethylsilyl) derivatives of cyanohydrins with ee as high as 90 ± 99% at À 428 [5].Unfortunately, Me 3 SiCN is an expensive material, and HCN is extremely toxic. Evidently, there is a need to find cheaper and safer initial materials for the synthesis of enantiomerically pure O-protected cyanohydrins. This paper reports the asymmetric synthesis of O-acetylcyanohydrins by the reaction of KCN, acetic anhydride (Ac 2 O),

Stereochemically inert cationic cobalt(III) complexes were shown to be one-component catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide at 50 °C and 5 MPa carbon dioxide pressure. The optimal catalyst possessed an iodide counter anion and could be recycled. A catalytic cycle is proposed in which the ligand of the cobalt complexes acts as a hydrogen-bond donor, activating the epoxide towards ring opening by the halide anion and activating the carbon dioxide for subsequent reaction with the halo-alkoxide. No kinetic resolution was observed when terminal epoxides were used as substrates, but chalcone oxide underwent kinetic resolution.

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A (salen)titanium catalyst has been found to induce the asymmetric addition of potassium cyanide and acetic anhydride to aldehydes, giving enantiomerically enriched cyanohydrin esters with up to 92% enantiomeric excess using just 1 mol% of the catalyst. This is the first report of the asymmetric synthesis of cyanohydrin derivatives using a cyanide source which is non-volatile and inexpensive.

The wide potential of molecules with multiple (multidentate) Lewis acid sites that bind and chemically activate small organic molecules has been recognized for some time.[1] Quite recently, several groups made significant progress towards the design and use of chiral homo-and heterobimetallic complexes for asymmetric catalysis, with particularly important contributions reported by Shibasaki et al. [2] and Trost et al.[3]The former group developed an effective chiral bimetallic catalyst based on an alkali metal/lanthanoid complex for asymmetric Michael addition reactions, whereas the latter group elaborated on dinuclear zinc complexes of a chiral pentadentate ligand for asymmetric Henry and aldol condensation reactions. In the case of the asymmetric opening of epoxides catalyzed by chiral [(salen)Co] complexes (salen = N,N-bis(salicylaldehydo)ethylenediamine), [4] the reaction involved two separate chiral salen-metal species in the ratelimiting step of the reaction, and subsequently, very efficient oligomeric catalysts were developed based on this principle. [5] It is reasonable to propose that many other established catalysts may operate via unrecognized polymetallic intermediates lying on the reaction coordinate; a large number of nonlinear effects observed in asymmetric catalytic reactions seem to indicate such a possibility. [6] Homo-or heterobimetallic complexes have enormous potential to revolutionize asymmetric catalysis. They can activate both components of a bimolecular reaction simultaneously, overcome entropy barriers associated with bringing the two reagents together, minimize the energy barrier that arises from solvent-shell rearrangements during the reaction, and recognize prochiral faces or groups within the reagents through predetermination of the reaction trajectory.Belokon, North and co-workers have developed an efficient catalyst based on dinuclear, chiral [{(salen)Ti IV -(m-O)} 2 ] precatalysts for the asymmetric addition of trimethylsilyl cyanide to aldehydes. [7] Kinetic studies indicated that two Ti IV complexes were involved in the rate-limiting step of this reaction.[+ EtOSO 3 À complexes were shown to be even more enantioselective in the same reaction, although the rate was almost two orders of magnitude slower than that with the analogous Ti IV catalysts.[9] Kinetic studies indicated that two cationic V V complexes were again involved in the ratelimiting step of the reaction.[9a]Based on these results, we thought that a mixed-metal complex derived from Ti IV and V V might form an even better catalyst, which would exhibit the asymmetric induction associated with V V complexes and the faster rates of reaction associated with Ti IV catalysts. Furthermore, we considered complexes derived from enantiomeric ligands to study the interactions between the complexes, as mixed species could give rise to anomalies with respect to the independent metal catalysts. Herein, we report the performance of mixtures ofÀ in the asymmetric addition of trimethylsilyl cyanide to benzaldehyde at different mol...