Reactivity Tuning for Radical-Radical Cross-Coupling via Selective Photocatalytic Energy Transfer: Access to Amine Building Blocks

Abstract

Reductive N-O bond cleavage has been widely explored for providing either N or O radical species for various coupling processes. Despite significant advances, this photoredox pathway is less appealing due to poor atom economy owing to the loss of one fragment during the transformation. In this regard, the homolytic N-O bond cleavage by an energy-transfer pathway to provide two key radicals would be highly desirable for overcoming the limitations of the use of one fragment. We report an exclusive energy-transfer approach for the development of a challenging radical-radical C(sp3)-N cross-coupling process by reactivity-tuning of the catalytic system. The homolytic N-O bond cleavage of oxime esters in the presence of an Ir complex produces acyloxy and iminyl radicals, which undergo decarboxylative cross-coupling to yield valuable imines (typically 0.25 mol % cat. and 1 h reaction time). Extensive photophysical and electrochemical measurements, as well as density functional theory studies, were carried out to probe the mechanism and the operation of a Dexter-type energy-transfer pathway was revealed. The choice of solvent (EtOAc) and reaction concentration were critical for achieving the selectivity and reactivity in this cross-coupling process. The synthetic utility of this method was explored by studying highly functionalized oxime esters, including derivatives of biologically active natural products and drug molecules. Furthermore, in situ transformations of the imine products into pharmaceutically important amines were also demonstrated, showcasing the utility of the imine products as valuable amine building blocks. Copyright © 2019 American Chemical Society.