Ruthenium-catalyzed ester reductions applied to pharmaceutical intermediates

Ruthenium pincer complexes were synthesized and used for catalytic ester reductions under mild conditions (∼5 bar of hydrogen). An experimental design approach was used to optimize the conditions for yield, purity, and robustness. Evidence for the catalytically active ruthenium dihydride species is presented. Observed intermediates and side products, as well as time-course data, were used to build mechanistic insight. The optimized procedure was further demonstrated through scaled-up reductions of two pharmaceutically relevant esters, both in batch and continuous flow.