New developments in enantioselective hydrogenation

Abstract

A broad variety of atropisomeric biphenyl diphosphines have been synthesized via optically active 6,6'-dimethoxybiphenyl-2,2'-diyl-bis(diphenyl phosphonates), -bis(phosphonic dichlorides), -bis(phosphonous dichlorides) and -bis(primary diphosphines). The usefulness of diphosphine variety in enantioselective hydrogenation was demonstrated by application to substrates of unconventional and unprecedented type such as an α-pyridyl ketone, γ-keto olefins, an α-pyrone and an enol ether of a β-keto lactam. Enantioselectivities and rates in these hydrogenations were markedly influenced by the electronic and steric properties of the diphosphine ligands. Of particular utility were the biphenyl diphosphines with α-furyl and 3,5-di-t-butylphenyl groups on the P atoms. A synthesis of a tetrasulfonated biphenyl diphosphine carrying the sulfonato substituents in the para position of the P-phenyl rings has also been developed. Ruthenium catalysts derived from this ligand proved highly efficient for several enantioselective hydrogenations in aqueous systems.