A New Strategy for Designing Non‐ C 2 ‐Symmetric Monometallic Bifunctional Catalysts and Their Application in Enantioselective Cyanation of Aldehydes

• Published in: Chemistry : a European journal 2008-02, Vol.14 (7), p.2223-2231
• Main Authors: Yang, Fei, Wei, Siping, Chen, Chien‐An, Xi, Peihua, Yang, Li, Lan, Jingbo, Gau, Han‐Mou, You, Jingsong
• Format: Article
• Language: English
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200701660

A monometallic bifunctional catalyst, in which only one imidazolyl moiety is directly attached at the 3‐position of a binaphthol moiety, has been developed. The ligand ( R )‐ 1 , which lacks C 2 ‐symmetry and flexible linkers, in combination with Ti(O i Pr) 4 , has been demonstrated to promote the enantioselective cyanation of aldehydes with trimethylsilylcyanide (TMSCN), giving excellent enantioselectivities of up to 98 % ee and high yields of up to 99 %. The use of this bifunctional catalytic system obviates the need for additives and is extremely simple as the reagents are added in one portion at the beginning of the reaction. The protocol has been found to tolerate a relatively wide range of aldehydes when 10 mol % of the ( R )‐ 1 /Ti(O i Pr) 4 complex is deployed in CH 2 Cl 2 at −40 °C, the conditions which proved most practical and effective. The asymmetric cyanations also proceeded with lower catalyst loadings (5 mol %, or even 2 mol %), still giving satisfactory enantiomeric excesses and yields. Interestingly, the use of freshly distilled TMSCN dried over CaH 2 gave a low enantioselectivity and only a moderate yield of the adduct as compared with direct use of the commercial reagent. The results of 13 C NMR spectroscopic studies implicate HCN as the actual reactive nucleophile.