30. Light-driven deracemization enabled by excited-state electron transfer

Nick Y. Shin, Jonathan M. Ryss, Xin Zhang, Scott J. Miller, Robert R. Knowles:

Science 2019, 366, 364-369. DOI: 10.1126/science.aay2204

https://science.sciencemag.org/content/366/6463/364

ABSTRACT: Deracemization is an attractive strategy for asymmetric synthesis, but intrinsic energetic challenges have limited its development. Here, we report a deracemization method in which amine derivatives undergo spontaneous optical enrichment upon exposure to visible light in the presence of three distinct molecular catalysts. Initiated by an excited-state iridium chromophore, this reaction proceeds through a sequence of favorable electron, proton, and hydrogen-atom transfer steps that serve to break and reform a stereogenic C–H bond. The enantioselectivity in these reactions is jointly determined by two independent stereoselective steps that occur in sequence within the catalytic cycle, giving rise to a composite selectivity that is higher than that of either step individually. These reactions represent a distinct approach to creating out-of-equilibrium product distributions between substrate enantiomers using excited-state redox events.