Computational Exploration of Mechanism and Selectivities of (NHC)Nickel(II)hydride-Catalyzed Hydroalkenylations of Styrene with alpha-Olefins

The [LNiH](+)-catalyzed hydroalkenylation between styrene and alpha-olefins gives distinctive chemo- and regioselectivities with N-heterocyclic carbene (L = NHC) ligands: (a) the reaction with NHC ligands produces the branched tail-to-tail products, whereas the reaction with phosphine ligands (L = PR3) favors the tail-to-head regio-isomers; (b) the reaction stops at heterodimerization with no further oligomerization even with excess alpha-olefin substrates; (c) typical side reactions with alpha-olefins, such as isomerization to internal olefins or polymerization, are either significantly diminished or eliminated. To understand the operating mechanism and origins of selectivities, density functional theory (DFT) calculations were performed, and several additional experiments were conducted. The olefin insertion step is found to determine both the regioselectivity and chemoselectivity, leading to the tail-to-tail heterohydroalkenylation product. With a small NHC ligand (1,3-dimethylimidazol-2-ylidene), the intrinsic electronic effects of ligand favor the tail-to-head regioisomer by about 1 kcal/mol in the olefin insertion step. With bulky NHC ligands (1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene or SIPr), the steric repulsions between the ligand and the substituent of the inserting alkene override the intrinsic electronic preference, making the tail-to-tail regioisomer favored (about 3 kcal/mol with both ligands). In the competition between homo- and heterodimerization, the insertion of the secondary styrene breaks its pi-conjugation, making the insertion of styrene about 2 kcal/mol less favorable than that of alkyl-substituted alkenes. In addition, the interaction between nickel and phenyl group of styrene stabilizes the resting state and inhibits the side reactions with alpha-olefins, suggesting that styrene, or similar aryl olefins, is not only a substrate, but also an inhibitor for side reactions. This unique effect of styrene is verified by control experiments.