Preparation and lithiation of optically active 2,2'-dihalo-1,1'-binaphthyls. A general strategy for obtaining chiral, bidentate ligands for use in asymmetric synthesis

Article abstract of DOI:10.1021/ja00329a014

Chiral, bidentate ligands based on the 1,1'-binaphthyl system have achieved remarkable success in certain asymmetric syntheses that have been reported in the literature. Enantiomeric excesses are often above 90% and may approach 99% or better. In the present paper, it is demonstrated that 2,2'-dibromo-1,1'-binaphthyl and the corresponding diodide can be obtained in preparative quantities with optical purities in excess of 98%. The compounds can be mono- or dilithiated, and the lithio intermediates are configurationally stable from -131°C up to at least -44°C. Yields of the dilithio intermediate are 95% or higher, and the monolithio intermediates can be formed in chemical yields of 80-85%. The discovery provides an efficient, general route for obtaining symmetrical and unsymmetrical chiral, bidentate ligands based on the binaphthyl system. These derivatives are expected to have broad applications in mechanistic studies of asymmetric reactions and in practical synthetic procedures. For example, (R)- and (S)-BINAP (2,2'-bis(diphenylphosphino)-1,1'-binaphthyl), which are reported to give enantiomeric excesses approaching 100% in certain rhodium-catalyzed hydrogenations, have been synthesized from 2-naphthol in overall yields ranging from 16% to 34%.