Stereoselective Synthesis of Both Enantiomers of Axially Chiral Biaryls Utilizing Planar Chiral Tricarbonyl(arene)chromium Complexes

Abstract

Tricarbonyl(2,6-disubstituted 1-bromobenzene)chromium complexes were treated with ortho-substituted arylboronic acids in the presence of Pd(0) catalyst to give mono Cr(CO)3 complexes of biphenyl compounds with complementary axial chirality, with extremely high stereoselectively depending upon the steric bulkiness of the ortho substituents. Cross-coupling of o-alkyl- or hydroxymethyl-substituted phenylboronic acids with (arene)chromium complexes diastereoselectively gave Cr(CO)3-complexed biaryls in which the ortho substituents are in a syn-orientation to the tricarbonylchromium fragment. With o-formyl phenylboronic acids, diastereoisomeric anti-coupling products were stereoselectively obtained. The kinetically controlled coupling products were easily isomerized to thermodynamically more stable mono-Cr(CO)3-complexed biaryls by modification of the o-substituents to less hindered ones, or the thermal conditions assisted the axial isomerization. The overall process can be considered to be an enantioselective preparation of both axially chiral biaryls starting from a single planar chiral (arene)chromium complex.