The mechanism and regioselectivities of (NHC)nickel(ii)hydride-catalyzed cycloisomerization of dienes: a computational study

Abstract

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.

Computational insights into the [Ni]H-catalyzed remote C(sp3)–H hydroamidation of alkenes: mechanism and insertion selectivity

“…The B3LYP functional has a proven track record of predicting structures accurately for nickel-catalyzed hydrofunctionalization reactions. 42–46 The transition states were confirmed by the presence of a negative frequency. To improve the energetics of the reported structures, solvation single-point calculations in tetrahydrofuran were performed using the SMD model at the M06 level of theory in conjunction with the basis sets of SDD (with ECP) for the Ni center and 6-311++G(d,p) for the other atoms.…”

(NHC)Ni(II)-Directed Insertions and Higher Substituted Olefin Synthesis from Simple Olefins

“…13 DFT studies further highlighted the ring-closing selectivity's origins and the differences between inter-and intramolecular HA (Figure 11). 44 First, the Markovnikov selectivity of the olefin acceptor is controlled by the β-H transfer step, in which the steric repulsions between the bulky NHC and the diene substituent on the tether eventually favor the desired nmembered-ring formation. Second, Markovnikov olefin donor addition selectivity is favored in both cycloisomerization and cross-HA, but for different reasons.…”

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

“…(alkenyl-Ni II activation), [11b, 17, 26] (NHC)Ni II -allyl 1,3shift [27] and catalyst regeneration by a b-hydride elimination/transfer, [27c-e] the desired product 3 is obtained. In this way,t his HARC occurs without slow addition of the substrates,a nd fairly free from non-selective oligomerization of either 1 or 2. d-labelling experiments support the hypothesis in part:i ) < 4% De xchange was observed when [D 1 ]2a was used and produced the corresponding dlabelled 3aa successfully (Scheme 3a).…”