Origin of Regiochemical Control in Rh(III)/Rh(V)-Catalyzed Reactions of Unsaturated Oximes and Alkenes to Form Pyrdines

The Rh(III)-catalyzed reactions of alpha,beta-unsaturated oximes with alkenes are versatile methods for the synthesis of pyridines. Density functional theory (DFT) calculations reported here reveal the detailed mechanism and origins of selectivity in this reaction. The Rh(III)/Rh(V)/Rh(I) catalytic cycle was found to be more favorable than the previously proposed Rh(III)/Rh(I)/Rh(III) catalytic cycle. The Rh(III)/Rh(V)/Rh(I) catalytic cycle involves C-H activation, alkene insertion, deprotonation, oxime migratory oxidative addition, nitrene insertion, 1,5-hydrogen shift, and beta-hydride elimination to give the pyridine product and form a Rh(I) species. Subsequent oxidation by Ag+ regenerates the Rh(III) catalyst. Reductive elimination from a alkyl-Rh(III) species is predicted to be difficult, so that the Rh(III)/Rh(I)/Rh(III) catalytic cycle can be excluded. The reactivities of oxime ethers and oxime esters are compared. The oxime ester acts as both a directing group and an internal oxidant. In this reaction, the N-O bond is activated by the pivalate, and migratory oxidative addition onto the Rh(III) species generates the corresponding Rh(V) nitrene complex. However, in the absence of the pivalate on the oxime ether, the activation energy for oxidative addition is much higher. The reactivity was analyzed by NPA charge calculations, comparison of the N-O bond orders, and the bond dissociation energies. The calculations also explain the regioselectivity of alkene insertion, which is shown to be an electronic effect rather than a steric effect.