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

Abstract

The [LNiH] + -catalyzed hydroalkenylation between styrene and α-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, while 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 α-olefin substrates; (c) typical side reactions with α-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 hetero-hydroalkenylation 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 hetero-dimerization, the insertion of the secondary styrene breaks its π-conjugation, making the insertion of styrene about 2 kcal/mol less favorable than that of alkylsubstituted alkenes. In addition, the interaction between nickel and phenyl group of styrene stabilizes the resting state and inhibits the side reactions with α-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.

Atroposelective Synthesis of Axially Chiral Styrenes via an Asymmetric C–H Functionalization Strategy

“…58 This alkene insertion generates the alkyl-palladium species 28. 28 isomerizes to the more stable 29 with pyridine coordination, followed by the pyridine-assisted b-hydride transfer, [59][60][61] through TS30. Subsequent proton transfer from pyridinium to carboxylate via TS32 produces the product-coordinated complex 33.…”

Computational Insights into the Reaction Mechanisms of Nickel‐Catalyzed Hydrofunctionalizations and Nickel‐Dependent Enzymes

“…Ho and co‐workers reported the in situ generated catalyst, that is, [(NHC)NiH] + (NHC=N‐heterocyclic carbene), promoted the hydrovinylation of vinylarenes with α‐olefins to obtain the tail‐to‐tail 1,1‐disubstituted alkenes with excellent regio‐ and chemoselectivities (Scheme ) . Houk and co‐workers performed DFT (SMD M06//B3LYP method) calculations to investigate the mechanism and origin of the selectivity for the (NHC)Ni II ‐hydride catalyzed hydrovinylation of styrenes with α‐olefins . The proposed mechanism shown in Scheme is supported by their DFT computations.…”

[(NHC)NiIIH]‐Catalyzed Cross‐Hydroalkenylation of Cyclopropenes with Alkynes: Cyclopentadiene Synthesis by [(NHC)NiII]‐Assisted C−C Rearrangement

“…Here, a chemoselective insertion of 1 occurs (favored by ring‐strain release), followed by a fast regioselective insertion of 2 , rather than either a homo‐insertion of 1 (by steric interaction of 1 with bulky NHCs as depicted in Table ) or premature ring‐opening in Pd. After a (NHC)Ni II assisted vinylcyclopropane rearrangement at just 30 °C, as compared to Scheme c (alkenyl‐Ni II activation), (NHC)Ni II ‐allyl 1,3‐shift and catalyst regeneration by a β‐hydride elimination/transfer,– the desired product 3 is obtained. In this way, this HARC occurs without slow addition of the substrates, and fairly free from non‐selective oligomerization of either 1 or 2 .…”