A sugar-derived ligand for room temperature aerial oxidation or non-aqueous Markovnikov hydration of styrenes using a preformed or in situ generated Co complex

Abstract

The oxidation of styrenes to acetophenones is an industrially relevant transformation that has been traditionally conducted with the Wacker-type reaction using pure oxygen at high temperature and pressure using Pd–Cu catalysts. Herein, we report a Co(II) complex of a salen appended L-diaminoribose-derived ligand that is catalytically active for the room temperature, selective aerial oxidation of styrenes to acetophenones. Further, the in situ generated Co(II) complex (from a mixture of the salen appended L-diaminoribose-derived ligand and a Co(II) salt) is also found to enable the catalysis. The oxidation is efficiently conducted in the presence of Et₃SiH as a hydrogen atom transfer (HAT) agent and provides very high isolated yields (71–95%) of the acetophenones. Further, the HAT mediated transformation with NaBH₄ also enables the non-aqueous, Markovnikov hydration of styrenes, providing the corresponding benzylic alcohols, exclusively as a single product in over 72–97% isolated yields via an oxidation–reduction mechanism. The methodologies provided very high exclusive yields of the products and were compatible with various substitutions on the aryl ring of the styrene. Detailed experimental and computational studies revealed a structural feature of the ligand that enabled the facile formation and stabilization of a pseudo-octahedral Co(III) intermediate that facilitated the oxidation reaction.