Ruthenium-catalyzed oxidative kinetic resolution of unactivated and activated secondary alcohols with air as the hydrogen acceptor at room temperature

Enantiopure alcohols are versatile building blocks for asymmetric synthesis and the kinetic resolution (KR) of racemic alcohols is a reliable method for preparing them. Although many KR methods have been developed, oxidative kinetic resolution (OKR), in which dioxygen is used as the hydrogen acceptor, is the most atom-efficient. Dioxygen is ubiquitous in air, which is abundant and safe to handle. Therefore, OKR with air has been intensively investigated and the OKR of benzylic alcohols was recently achieved by using an Ir catalyst without any adjuvant. However, the OKR of unactivated alcohols remains a challenge. An [(aqua)Ru(salen)] catalyzed OKR with air as the hydrogen acceptor was developed, in which the aqua ligand is exchanged with alcohol and the Ru complex undergoes single electron transfer to dioxygen and subsequent alcohol oxidation. This OKR can be applied without any adjuvant to activated and unactivated alcohols with good to high enantioselectivity. The unique influence of substrate inhibition on the enantioselectivity of the OKR is also described. Alcohol resolution: An (aqua)ruthenium salen complex catalyzes the efficient oxidative kinetic resolution of both activated and unactivated secondary alcohols with air as the hydrogen acceptor at room temperature. The reaction is compatible with various functional groups, including halogen, ether, silyl ether, and ester groups. The reaction rate is lower at higher substrate concentrations as a result of substrate inhibition.