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 anal. have provided a detailed insight into the reaction. Progress kinetic anal. demonstrated that neither catalyst decompn. 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 expts. with deuterated solvent ([D7]​DMF) and deuterated base ([D15]​Et3N) supported the role of the amine as a reductant in the reaction. The palladium complex [Pd0(NHC)​(1)​] has been identified as the resting state. The kinetic expts. by stopped-​flow UV​/Vis also revealed that the presence of the second substrate, benzylideneacetone 1, slows down the oxidative addn. of 4-​iodoanisole through its competing coordination to the palladium center. The kinetic and mechanistic studies indicated that the oxidative addn. of the aryl iodide is the rate-​detg. step. Various scenarios for the oxidative addn. step have been analyzed by using DFT calcns. (bp86​/def2-​TZVP) that supported the inhibiting effect of substrate 1 by formation of resting state [Pd0(NHC)​(1)​] species at the cost of further increase in the energy barrier of the oxidative addn. step.