L-Phenylalanine derived tripodal vanadium complexes as catalysts for the asymmetric reductive coupling of benzaldehyde

Abstract

Two tripodal vanadium complexes derived from modified l-phenylalanine were prepared and characterised. Both compounds were tested for their catalytic activity in the reductive coupling of benzaldehyde. Overall, the complexes are capable of catalysing the reductive coupling of benzaldehyde in the presence of metallic zinc as co-reductant and alkylammonium or alkylpyridinium acetate salts in ethanol under mild aerobic conditions. While benzyl alcohol is generally the major product, the yield of hydrobenzoin reaches ca. 38% under these conditions. Enantioselectivities reach 39% and there is a preferential formation of the trans diastereoisomer of hydrobenzoin. Attempts to study some aspects of the underlying mechanism were made. It was found that metallic zinc is capable of reducing the complexes to VIII species, which are likely to be the active catalytic species. The methodology described may set up a basis for the development of catalytic systems for the asymmetric synthesis of hydrobenzoins under mild conditions, not requiring chlorosilanes as electrophilic reagents for the regeneration of the catalyst.