Diastereoselective Reductions with Zinc Powder in a Mixture of NaOH aq. and MeOH for Syntheses of (±)‐1,2‐Diarylethylenediamines

Abstract

AbstractA practical diastereoselective reduction of spiroisoimidazole to afford rac‐1,2‐diarylethylenediamines was developed. Zinc was used as a reducing agent in a mixture of basic H2O and an alcoholic cosolvent under open air. The choices of NaOH as the base and MeOH as the cosolvent were key factors that heightened selectivity, and under these optimized conditions, the yield of rac‐1,2‐diphenylethylenediamine (DPEN) was > 11‐fold that of meso isomer. The reaction was amenable to a 0.10 mol scale, which resulted in the isolation of 16.9 g of rac‐DPEN. Control experiments suggested that the reduction with zinc followed a different pathway from that of a known Birch reduction.magnified image