Computational insights into different regioselectivities in the Ir-porphyrin-catalyzed C–H insertion reaction of quinoid carbene

Abstract

The mechanisms and regioselectivities of the Ir-porphyrin-catalyzed C–H insertion reaction of quinoid carbene (QC) were investigated by density functional theory (DFT) calculations. The competing catalytic cycles were identified as the hydrogen-atom abstraction by quinoid oxygen (HAA-O) and hydrogen-atom abstraction by carbene carbon (HAA-C) catalytic cycles. The HAA-O catalytic cycle involves an abstraction of the hydrogen atom by quinoid oxygen of the Ir-porphyrin-quinoid carbene (Ir-QC) species and a radical rebound step. The HAA-C catalytic cycle involves an abstraction of the hydrogen atom by carbene carbon of Ir-QC, a radical rebound step and a keto–enol tautomerization step. The 1-methyl-1,3-cyclohexadiene 1 with lower bond dissociation energy (BDE) of the secondary C–H bond prefers to follow the HAA-O/rebound pathway. DFT calculations revealed that the HAA-O step is the rate-limiting step in the carbene insertion reaction with substrate 1. The secondary C–H arylated product is the major product and this is due to the relatively lower BDE of the secondary C–H bond compared to the primary C–H bond. By contrast, the primary C–H arylated product is the major product in the case of 1-methylcyclohexene 4 with higher BDE of the secondary C–H bond, which significantly increases the barrier of the HAA-O step. Therefore the carbene insertion reaction to 4 prefers to follow the HAA-C/rebound/keto–enol tautomerization pathway. The sluggish carbene insertion to the secondary C–H bond of 4 may be arising from the formation of a very stable ketone intermediate which impeded the secondary C–H bond arylation reaction.