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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),

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The asymmetric synthesis of a range of axially chiral 2-arylpyridines by a cobalt-catalyzed [2 + 2 + 2] cycloaddition reaction is described. The use of a planar chiral (1-neomenthylindenyl)cobalt(COD) complex under photochemical conditions is the key for reacting the 1-naphthyldiynes with a range of differently functionalized nitriles, giving the enantiomeric atropoisomers with high chemical yields and enantiomeric excesses of up to 94% ee.

A novel and efficient, two-step route to axially chiral biaryls is demonstrated. In a direct asymmetric cross-cyclotrimerization in the presence of a chiral cobalt(I) catalyst, axially chiral biaryls bearing phosphoryl moieties have been prepared, and through indirect evidence the authors have been able to clarify the origin of the stereochemical induction and the nature of the central intermediate in the catalytic cycle. By subsequent reduction of the phosphoryl moiety to the corresponding phosphine, a very efficient and atom-economical approach to chiral systems has been developed. These chiral systems clearly have great potential use as axially chiral monodentate P- or bidentate P,O-ligands, as has been demonstrated by the employment of the novel NAPHEP as a new monodentate acting ligand in an asymmetric hydrosilylation reaction.

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 objective of this review is to summarize developments in the recently discovered area of palladium‐catalyzed asymmetric Michael additions of organometallic reagents to activated electron‐deficient olefins in its practical and theoretical aspects. The methodology has been rapidly developed in the last five years as a useful alternative approach to complement the already existing copper‐ and rhodium‐catalyzed variants, especially in the area of asymmetric aryl transfer to typical Michael acceptors. Although very difficult to control because of numerous side reactions, this palladium catalysis is very promising for applications both in academia and in industry. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Several optically active (Cp R )Co(L) (L ) 1,5-cyclooctadiene, norbornadiene) complexes were prepared. The complex (-)-(menthylCp)Co(COD) (1) was synthesized by the direct treatment of (-)-menthylcyclopentadiene with highly reactive "atomic" cobalt in the presence of COD.The other examples, (+)-pR-(1-neomenthylindenyl)Co(COD) (2) and (+)-pR-(1-neomenthylindenyl)Co(NBD) (3) were prepared by deprotonation of (-)-3-neomenthylindene with BuLi and the metathetic reaction of the corresponding lithium salt with tris(triphenylphosphine)cobalt(I) chloride, followed by replacement of PPh 3 with COD or NBD, respectively. The diastereoselectivity of the complexation was as high as 74.5% de, and the major diastereomers were separated chromatographically. Optically pure complex (-)-pS-(1-neomenthylindenyl)-Co(COD) (4) was prepared similarly, starting from (+)-3-neomenthylindene, and the diastereomeric excess of the major diastereomer was 74.4% de. The (+)-neomenthylfluorene 5 was prepared by the alkylation of fluorenyllithium with (-)-menthyltosylate, and the complex (+)-(pseudo-neomenthylfluorenyl)Co(COD) (6) was synthesized by the metathetic reaction of the corresponding lithium salt with (PPh 3 ) 3 CoCl and COD, in which an inversion of the cyclohexane ring of the terpene moiety was observed upon complexation. The structures of the compounds 1, 2, 3, 4, 5, and 6 were determined by single-crystal X-ray diffractometry. Structural features of the complexes are also discussed on the basis of 1 H, 13 C NMR, and MS spectra.