Transition metal-catalyzed hydroalkenylation is widely applied in organic synthesis to construct carbon–carbon bonds and synthesize substituted alkenes. In this work, the mechanism and regioselectivities of [(NHC)NiH]+-catalyzed intramolecular cycloisomerization of dienes are studied by density functional theory (DFT) calculations. Through an initial hydride insertion, [(NHC)NiH]+ reacts with the diene substrate to generate the alkyl nickel species. This alkyl nickel species is the resting state of the catalytic cycle, and the hydroalkenylation reaction involves sequential olefin insertion and β-hydride transfer. The selectivity between the possible cycloisomerization products is determined by both the olefin insertion and β-hydride transfer steps. The olefin insertion favors the exo-cyclization due to the high ring strain of the insertion transition states for the endo-cyclization. In the β-hydride transfer step, the hydride transfers to the internal olefin position selectively to avoid the steric repulsions between the bulky NHC ligand and the alkyl substituent of the olefin substrate. This selectivity of β-hydride transfer leads to the n-exo cyclization instead of the (n − 1)-exo cyclization. These mechanistic insights will shed light on the future development of transition metal-catalyzed hydroalkenylation reactions.

AbstractHighly selective cross-hydroalkenylations of endocyclic 1,3-dienes at the least substituted site with α-olefins were achieved with a set of neutral (NHC)NiIIH(OTf) catalysts and cationic NiII catalysts with a novel NHC ligand. Under heteroatom assistance, skipped dienes were obtained in good yields, often from equal amounts of the two substrates and at a catalyst loading of 2–5 mol %. Rare 4,3-product selectivity (i.e., with the H atom at C4 and the alkenyl group at C3 of the diene) was observed, which is different from the selectivity of known dimerizations of α-olefins with both acyclic Co and Fe systems. The influence of the various substituents on the NHC, 1,3-diene, and α-olefin on the chemo-, regio-, and diastereoselectivity was studied. High levels of chirality transfer were observed with chiral cyclohexadiene derivatives.

AbstractHighly selective cross-hydroalkenylations of endocyclic 1,3-dienes at the least substituted site with α-olefins were achieved with a set of neutral (NHC)NiIIH(OTf) catalysts and cationic NiII catalysts with a novel NHC ligand. Under heteroatom assistance, skipped dienes were obtained in good yields, often from equal amounts of the two substrates and at a catalyst loading of 2–5 mol %. Rare 4,3-product selectivity (i.e., with the H atom at C4 and the alkenyl group at C3 of the diene) was observed, which is different from the selectivity of known dimerizations of α-olefins with both acyclic Co and Fe systems. The influence of the various substituents on the NHC, 1,3-diene, and α-olefin on the chemo-, regio-, and diastereoselectivity was studied. High levels of chirality transfer were observed with chiral cyclohexadiene derivatives.

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, 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 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 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 π-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 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.Keywords: density functional theory; hydrovinylation; NHC ligand; regioselectivity; β-hydride transfer

This paper describes a catalytic and selective synthesis of substituted 1,3-cyclohexadiene fused with xH-pyran and furan. By using terminal enynes as substrates, NHC–Ni(0) as catalyst, and an ether as spacer between the two unsaturated hydrocarbon termini, the system competed with the terminal alkyne cyclotrimerization and yielded the desired [2+2+2] cycloaddition.

This paper describes a new approach in transition-metal-catalyzed unsymmetric cycloisomerization for medium-sized heterocycles. The steric and electronic effects of an NHC–NiH catalyst and γ-heteroatom chelation were used together as a basis for 1,n-diene termini differentiation and for nγ-exo-trig (head-to-tail) product selectivity. Heterocycles bearing an exocyclic methylene such as oxepines, thiepines, siloxepines, and oxocanes were synthesized from the corresponding 1,n-dienes by a fine-tuning of the NHC properties. The implication of the underlying hypothesis was further demonstrated in a competition experiment in which strained oxepines were formed preferentially over other competing oxa-/carbocycles. Under more forcing physical conditions and the use of a suitable NHC ligand, the exocyclic methylene products were isomerized further into endocyclic olefin products regioselectively in one pot.