Stereoselective Synthesis of Axially Chiral Biaryls Utilizing Planar Chiral (Arene)chromium Complexes

Axially chiral biaryl compounds were stereoselectively prepared by utilizing planar chiral (arene)chromium complexes. The preparation of axially chiral mono-Cr(CO)₃-complexed biaryls was carried out using the following two methods: (1) cross-coupling of (aryl halide)chromium complexes with arylmetals and (2) nucleophilic displacement of a methoxy group of planar chiral (ortho-methoxybenzoate)chromium complexes with aryl Grignard reagents. Tricarbonyl(2,6-disubstituted 1-bromobenzene)chromium chomplexes were treated with ortho-substituted arylboronic acids in the presence of a Pd(0) catalyst in refluxing aqueous MeOH to give axially chiral mono-Cr(CO)₃ complexes of biaryl compounds with very high selectivity. The diastereoselectivity was largely dependent on the steric bulkiness of the ortho substituents of the arylboronic acids and on the reaction conditions. Thus, cross-coupling of arylboronic acids having a sterically demanding ortho-substituent with (aryl halide)chromium complexes gave kinetically controlled Cr(CO)₃-complexed biaryls in which the ortho-substituents are oriented syn to the tricarbonylchromium fragment. On the other hand, with ortho-formyl arylboronic acids, anti-coupling products were stereoselectively obtained. The syn-coupling products obtained under kinetic reaction control were easily isomerized to the thermodynamically more stable anti-mono-Cr(CO)₃-complexed biaryls under thermal reaction conditions, or by modification of the ortho-substituents to less hindered ones. The overall process gives a promising approach for the preparation of both atropisomers stereoselectively starting from a single planar chiral (arene)chromium complex. Furthermore, nucleophilic reaction of aryl Grignard reagents with the planar chiral (ortho-methoxybenzoate)chromium complexes gave thermodynamically stable anti-orientated Cr(CO)₃-complexed biaryls with high selectivity. Further extensions of these methodologies to the synthesis of (-)-steganone and O,O′-dimethylkorupensamine A are also described.

axial chirality - (arene)chromium complex - planar chirality - Suzuki cross-coupling reaction - nucleophilic reaction