Theoretical Investigations toward the Asymmetric Insertion Reaction of Diazoester with Aldehyde Catalyzed by N-Protonated Chiral Oxazaborolidine: Mechanisms and Stereoselectivity.

Abstract

In recent years, the N-protonated chiral oxazaborolidine has been utilized as the Lewis acid catalyst for the asymmetric insertion reaction, which is one of the most challenging topics in current organic chemistry. Nevertheless, the reaction mechanism, stereoselectivity, and regioselectivity of this novel insertion reaction are still unsettled to date. In this present work, the density functional theory (DFT) investigation has been performed to interrogate the mechanisms and stereoselectivities of the formal C-C/H insertion reaction between benzaldehyde and methyl α-benzyl diazoester catalyzed by the N-protonated chiral oxazaborolidine. For the reaction channel to produce the R-configured C-C insertion product as the predominant isomer, the catalytic cycle can be characterized by four steps: (i) the complexation of the aldehyde with catalyst, (ii) addition of the other reactant methyl α-benzyl diazoester, (iii) the removal of nitrogen concerted with the migration of phenyl group or hydrogen, and (iv) the dissociation of catalyst from the products. Our computational results show that the carbon-carbon bond formation step is the stereoselectivity determining step, and the reaction pathways associated with [1, 2]-phenyl group migration occur preferentially to those pathways associated with [1, 2]-hydrogen migration. The pathway leading to the R-configured product is the most favorable pathway among the possible stereoselective pathways. All these calculated outcomes align well with the experimental observations. The novel mechanistic insights should be valuable for understanding this kind of reaction.