Contra-Thermodynamic Positional Isomerization of Olefins

A light-driven method for the contra-thermodynamic positional isomerization of olefins is described. In this work, stepwise PCET activation of a more substituted and more thermodynami-cally stable olefin substrate is mediated by an excited-state oxi-dant and a Brønsted base to afford an allylic radical that is cap-tured by a Cr(II) co-catalyst to furnish an allylchromium(III) intermediate. In situ protodemetalation of this allylchromium complex by methanol is highly regioselective and affords an isomerized and less thermodynamically stable alkene product. The higher oxidation potential of the less substituted olefin isomer renders it inert to further oxidation by the excited-state oxidant, enabling it to accumulate in solution over the course of the reaction. A broad range of isopropylidene substrates is ac-commodated, including enol ethers, enamides, styrenes, 1,3-dienes, and tetrasubstituted alkyl olefins. Mechanistic investiga-tions of the protodemetalation step are also presented.