Palladium(0)/NHC-Catalyzed Reductive Heck Reaction of Enones: A Detailed Mechanistic Study

We have studied the mechanism of the palladium-catalyzed reductive Heck reaction of para-substituted enones with 4-iodoanisole by using N, N-diisopropylethylamine (DIPEA) as the reductant. Kinetic studies and in situ spectroscopic analysis have provided a detailed insight into the reaction. Progress kinetic analysis demonstrated that neither catalyst decomposition nor product inhibition occurred during the catalysis. The reaction is first order in the palladium and aryl iodide, and zero order in the activated alkene, N-heterocyclic carbene (NHC) ligand, and DIPEA. The experiments with deuterated solvent ([D-7] DMF) and deuterated base ([D-15]Et3N) supported the role of the amine as a reductant in the reaction. The palladium complex [Pd-0(NHC)(1)] has been identified as the resting state. The kinetic experiments by stopped-flow UV/Vis also revealed that the presence of the second substrate, benzylideneacetone 1, slows down the oxidative addition of 4-iodoanisole through its competing coordination to the palladium center. The kinetic and mechanistic studies indicated that the oxidative addition of the aryl iodide is the rate-determining step. Various scenarios for the oxidative addition step have been analyzed by using DFT calculations (bp86/def2-TZVP) that supported the inhibiting effect of substrate 1 by formation of resting state [Pd-0(NHC)(1)] species at the cost of further increase in the energy barrier of the oxidative addition step.