Mechanism of Aldehyde-Selective Wacker-Type Oxidation of Unbiased Alkenes with a Nitrite Co-Catalyst

Traditional Wacker-type oxidations of unbiased alkenes produce ketones as major products. Recently, Grubbs' group reported a Wacker-type oxidation system in which aldehydes (rather than ketones) have been generated predominantly in the presence of a nitrite co-catalyst. To elucidate the mechanistic origin of the aldehyde selectivity, density functional theory (DFT) studies have been conducted in this study. Two oxymetalation pathways, i.e., syn addition and anti addition pathways, were considered for various possible active species including monomeric Pd, bimetallic Pd–Pd, heterobimetallic Pd–Cu, and heterobimetallic Pd–Ag complexes. It is found that syn addition is kinetically more favored than anti addition in general. Meanwhile, the most feasible oxymetalation processes occur on the heterobimetallic Pd–Cu complexes. Investigations on the subsequent aldehyde formation process show that 1,2-H shift mechanism on tBuOH-ligated Pd–Cu complexes is superior to the betaH-elimination mechanism. Besides, the 1,2-H shift is the rate- and regioselectivity-determining step of the whole catalytic cycle. The analysis on spin density population indicates that the tBuOH-ligated Pd–Cu complex promotes a radical 1,2-H shift on the oxygenated alkene. The longer Pd–C(alkene) distance facilitates the aldehyde-selective pathway (relative to the ketone-selective pathway) due to the stronger stability of the secondary carbon radical and the smaller distortion energy therein.