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

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

ConspectusWell-controlled olefin insertion is critical for achieving catalytic and productive bulk and fine-chemical synthesis. Developing efficient and selective methods for meeting diverse insertion demands is extremely noteworthy, as it supports numerous transformations. The challenges are related to improving catalyst performance and selectivity control and uniting previously unreactive substrate pairs to achieve higher molecular structural complexity and utility. Nickel catalysts have received persistent attention in higher substituted olefin synthesis and polymerization, and numerous new strategies have been established to fulfill the ever-changing demands. This Account focuses on the recent progress based on N-heterocyclic carbene (NHC) ligands and nickel catalysts in our laboratory in using simple terminal olefins as olefin donors or acceptors.It begins with a brief history of olefin codimerization and the major advances in hydrovinylation achieved by other research groups using ethylene as an olefin donor. It then describes problems related to the reductive elimination that can occur when both the hydrometalated alkene and NHC are on the catalyst. It emphasizes the impact of NHC catalyst generation methods on the competing reactivity. Next, it explains the principal challenges and great opportunities in using our method (with α-olefins as olefin donors and alkenyl sources) to replace intermolecular reductive hydroalkenylation reactions (which require rare and more expensive alkenyl halides and boronic acids as reactants, alongside a stoichiometric amount of metallic reagents). The Account then illustrates the potential uses of our method for solving challenging organic synthesis problems using tailor-made (NHC)Ni(II) catalysts to allow redox-neutral catalytic cycles based on high chemo- and regioselective cross-insertion controls. It shows that upon optimal steric and electronic cooperation between the NHC, olefin donor, and olefin acceptor, regiodivergent insertion and convergent synthesis can be achieved easily.In the course of our work, we uncovered several unique insights into regulating (anti-)Markovnikov hydronickelation, carbonickelation, hydrocarbonation, ring closure, 1,3-allyl shift, isomerization, and catalyst regeneration under green, neutral, and mild-temperature conditions. These insights are also outlined here, along with theoretical calculations that offer additional understandings of the insertion reactivity and selectivity differences observed between the NHC and the highly related phosphorus-based Ni(II) hydride-catalyzed cross-hydroalkenylation and cycloisomerization systems.Compared to traditional olefin and cyclic structure synthesis technology, such as olefin cross-metathesis, enyne cyclization, and cross-coupling reactions, the new catalyst systems often offer previously inaccessible product structural characteristics, substrate scope, and outcomes. In particular, the method is effective for the catalytic synthesis of unsymmetrical and functionalized 1,1-disubstituted olefins (a.k.a. gem-olefins), 1,4-dienes (a.k.a. skipped dienes), conjugated dienes, endo- and exocyclic olefins, fused and spiro rings, and aromatic products. These syntheses are variously achieved by cross-hydroalkenylation, insertion-induced rearrangement, cycloadditions, and other approaches inspired by our investigations and detailed in this Account. Cross-hydroalkenylation can be achieved with high enantioselectivity by application of carefully designed and structurally flexible C₁ and C₂ chiral NHC ligands, yielding a pool of chiral branched alkenes and 1,4-dienes directly from simple chemical feedstocks used in industry. This Account will draw further attention to green alkenylation and the related development of redox-neutral catalytic cycles.

Nickel-catalyzed divergent Mizoroki-Heck reaction of 1,3-dienes

Developing efficient strategies to realize divergent arylation of dienes has been a long-standing synthetic challenge. Herein, a nickel catalyzed divergent Mizoroki-Heck reaction of 1,3-dienes has been demonstrated through the regulation of ligands and additives. In the presence of Mn/NEt₃, the Mizoroki-Heck reaction of dienes delivers linear products under Ni(dppe)Cl₂ catalysis in high regio- and stereoselectivities. With the help of catalytic amount of organoboron and NaF, the use of bulky ligand IPr diverts the selectivity from linear products to branched products. Highly aryl-substituted compounds can be transformed from dispersive Mizoroki-Heck products programmatically. Preliminary experimental studies are carried out to elucidate the role of additives.

Origins of Stereospecificity and Divergent Reactivity of Pd-Catalyzed Cross Coupling with α,α-Disubstituted Alkenyl Hydrazones

This article presents an exploration of stereospecificity and divergent reactivity of Pd-catalyzed α,α-disubstituted alkenyl hydrazones to synthesize 1,4-dienes in the Z configuration and vinylcyclopropane. We calculated the energy profiles of four α,α-disubstituted alkenyl hydrazones. The results show that the energy profiles of the whole catalytic cycle are basically the same before the syn-carbopalladation step. Subsequent syn-β-C elimination yields skipping dienes, or direct β-H elimination yields vinylcyclopropane. Current theoretical calculations reveal that the stereospecificity and the divergent reactivity of reactions result from the competition between syn-β-C elimination and β-H elimination. The C-C bond rotation and subsequent syn-β-C elimination step control the stereospecificity of the reaction by changing the olefin stereostructure from E to Z configuration. The steric factor of α-substituted groups mediates the transformation between syn-β-C elimination and β-H elimination. The results are of great significance for the scientific design of substrates to achieve accurate synthesis of target products.

[Ni(Np#)(η5-Cp)Cl]: Flexible, Sterically Bulky, Well-Defined, Highly Reactive Complex for Nickel-Catalyzed Cross-Coupling

Ni-NHCs (NHC = N-heterocyclic carbene) have become an increasingly important class of complexes in catalysis and organometallic chemistry owing to the beneficial features of nickel as an abundant 3d metal. However, the development of well-defined and air-stable Ni-NHC complexes for cross-coupling has been more challenging than with Pd-NHC catalysis because of less defined reactivity trends of NHC ancillary ligands coordinated to Ni. Herein, we report the synthesis and catalytic activity of well-defined [Ni(NHC)(η5-Cp)Cl] complexes bearing recently commercialized IPr# family of ligands (Sigma Aldrich) and versatile cyclopentadienyl throw-away ligand. The NHC ligands, IPr#, Np# and BIAN-IPr#, are prepared by robust and modular peralkylation of anilines. Most crucially, we identified [Ni(Np#)(η5-Cp)Cl] as a highly reactive [Ni(NHC)(η5-Cp)Cl] complex, with the reactivity outperforming the classical [Ni(IPr)(η5-Cp)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). These [Ni(NHC)(η5-Cp)Cl] precatalysts were employed in the Suzuki and Kumada cross-coupling of aryl chlorides and aryl bromides. Computational studies were conducted to determine steric effect and bond order analysis. Considering the attractive features of well-defined Ni-NHCs, we anticipate that this class of bulky and flexible Ni-NHC catalysts will find broad application in organic synthesis and catalysis.

DFT insight into asymmetric alkyl-alkyl bond formation via nickel-catalysed enantioconvergent reductive coupling of racemic electrophiles with olefins

A DFT study has been conducted to understand the asymmetric alkyl–alkyl bond formation through nickel-catalysed reductive coupling of racemic alkyl bromide with olefin in the presence of hydrosilane and K₃PO₄. The key findings of the study include: (i) under the reductive experimental conditions, the Ni(ii) precursor is easily activated/reduced to Ni(0) species which can serve as an active species to start a Ni(0)/Ni(ii) catalytic cycle. (ii) Alternatively, the reaction may proceed via a Ni(i)/Ni(ii)/Ni(iii) catalytic cycle starting with a Ni(i) species such as Ni(i)–Br. The generation of a Ni(i) active species via comproportionation of Ni(ii) and Ni(0) species is highly unlikely, because the necessary Ni(0) species is strongly stabilized by olefin. Alternatively, a cage effect enabled generation of a Ni(i) active catalyst from the Ni(ii) species involved in the Ni(0)/Ni(ii) cycle was proposed to be a viable mechanism. (iii) In both catalytic cycles, K₃PO₄ greatly facilitates the hydrosilane hydride transfer for reducing olefin to an alkyl coupling partner. The reduction proceeds by converting a Ni–Br bond to a Ni–H bond via hydrosilane hydride transfer to a Ni–alkyl bond via olefin insertion. On the basis of two catalytic cycles, the origins for enantioconvergence and enantioselectivity control were discussed.

The enantioconvergent alkyl–alkyl coupling involves two competitive catalytic cycles with nickel(0) and nickel(i) active catalysts, respectively. K₃PO₄ plays a crucial role to enable the hydride transfer from hydrosilane to nickel–bromine species.

Probing the origin of the stereoselectivity and enantioselectivity of cobalt-catalyzed [2 + 2] cyclization of ethylene and enynes

Theoretical calculations reveal the origin of the stereoselectivity and enantioselectivity of cobalt-catalyzed [2 + 2] cyclization of ethylene and enynes.

Understanding the insertion reactions of CO2, aldehyde and alkene into Cu–X (X = B, C, O) bonds

The relative preference of different unsaturated organic molecules (CO2, PhCHO and styrene) for insertion into different Cu–X (X = B, C, O) bonds has been systematically examined with the aid of DFT calculations.

Insights into the Regioselectivity of Hydroheteroarylation of Allylbenzene with Pyridine Catalyzed by Ni/AlMe3 with N-Heterocyclic Carbene: The Concerted Hydrogen Transfer Mechanism

The hydroheteroarylation of allylbenzene with pyridine as catalyzed by Ni/AlMe₃ and a N-heterocyclic carbene ligand has recently been established. Density functional calculations revealed that the common stepwise pathway, which involves the C-H oxidative addition of pyridine-AlMe₃ before the migratory insertion of allylbenzene, is unlikely as the migratory insertion needs to overcome a prohibitively high energy barrier. In contrast, the ligand-to-ligand hydrogen transfer pathway is more favorable in which the hydrogen is transferred directly from the para-position of pyridine-AlMe₃ to C2 of allylbenzene. Our distortion-interaction analysis and natural bond orbital analysis indicate that the interaction energy is strongly correlated with the extent of the charge transfer from the alkene (hydrogen acceptor) to the pyridine-AlMe₃ (hydrogen donor), which dictates the selectivity of the H-transfer to the C2 position of allylbenzene. Then, the subsequent C-C reductive elimination of the regioselective linear product is facilitated by the steric hindrance of the IPr ligand. Understanding these key factors affecting the product regioselectivity is important to the development of catalysts for hydroheteroarylation of alkenes.

Mechanism and origins of stereo- and enantioselectivities of palladium-catalyzed hydroamination of racemic internal allenes via dynamic kinetic resolution: a computational study

DFT calculations were performed to investigate the Pd-catalyzed hydroamination of racemic internal allenes with pyrazoles.

How does the nickel catalyst control the doubly enantioconvergent coupling of racemic alkyl nucleophiles and electrophiles? The rebound mechanism

DFT mechanistic study unveils that the rebound mechanism is the key to the nickel-catalyzed doubly enantioconvergent C(sp³)–C(sp³) coupling of racemic alkyl nucleophiles and electrophiles.

Theoretical elucidation of the multi-functional synthetic methodology for switchable Ni(0)-catalyzed C–H allylations, alkenylations and dienylations with allenes

The mechanisms and origins of switchable Ni(0)-catalyzed C–H allylations, alkenylations and dienylations with allenes are theoretically elucidated.

Enantioselective Aluminum-Free Alkene Hydroarylations through C-H Activation by a Chiral Nickel/JoSPOphos Manifold

Highly enantioselective nickel-catalyzed alkene endo-hydroarylations were accomplished with full selectivity by organometallic C-H activation. The asymmetric assembly of chiral six-membered scaffolds proved viable in the absence of pyrophoric organoaluminum reagents within an unprecedented nickel/JoSPOphos manifold.

(NHC)NiH-Catalyzed Regiodivergent Cross-Hydroalkenylation of Vinyl Ethers with α-Olefins: Syntheses of 1,2- and 1,3-Disubstituted Allyl Ethers

Cross-hydroalkenylation of a vinyl ether (1) with an α-olefin (2) was first achieved by a set of [NHC-Ni(allyl)]BAr^F (NHC=N-heterocyclic carbene) catalysts. Both 1,2- and 1,3-disubstituted allyl ethers were obtained, highly selectively, by using NHCs of different sizes. In contrast, the chemoselectivity (i.e., 1 as acceptor and 2 as donor) was controlled mostly by electronic effects through the catalyst-substrate interaction. Sterically bulkier alkenes (2) were used as preferred donors compared to smaller alkenes. This electronic effect also served as a basis for the first tail-to-head cross-hydroalkenylations of 1 with either a vinyl silane or boronic ester.

Mechanism, selectivity, and reactivity of iridium- and rhodium-catalyzed intermolecular ketone α-alkylation with unactivated olefinsviaan enamide directing strategy

DFT calculations were performed to investigate the title reaction, focusing on detailed reaction mechanism and origins of selectivity and reactivity.

Mechanism and origins of chemo- and regioselectivities of (NHC)NiH-catalyzed cross-hydroalkenylation of vinyl ethers with α-olefins: a computational study

Density functional theory calculations were performed to investigate the (NHC)NiH-catalyzed cross-hydroalkenylation of vinyl ethers with α-olefins.

Mechanism and origins of the directing group-controlled endo- versus exo-selectivity of iridium-catalysed intramolecular hydroalkenylation of 1,1-disubstituted alkenes

The reaction mechanism and the important role of directing groups in determining selectivity have been investigated by means of DFT calculations.

Factors Controlling the Reactivity and Chemoselectivity of Resonance Destabilized Amides in Ni-Catalyzed Decarbonylative and Nondecarbonylative Suzuki-Miyaura Coupling

N-Glutarimide amides have recently emerged as an exceptional group of compounds with unusually high reactivity in amide C-N bond activation. To understand the key factors that control the remarkable reactivity of these resonance destabilized amides, we explored the Ni-catalyzed decarbonylative and nondecarbonylative Suzuki-Miyaura coupling with N-glutarimide amides through density functional theory calculations. Two leading effects are responsible for the C-N cleavage activity of N-glutarimide amides, the coordinating N-substituents and the geometric twisting. The carbonyl substituent of the N-glutarimide amides provides crucial nickel-oxygen interaction, which essentially acts as a directing group to facilitate the formation of the reactive intermediate for the amide C-N bond cleavage. The geometric twisting weakens the resonance stability by removing the acyl-nitrogen conjugation, which lowers the energy penalty for the C-N bond stretch during oxidative addition. For the chemoselectivity of decarbonylation versus carbonyl retention, we found that the C-C reductive elimination for ketone formation is kinetically faster than that for biaryl formation, while ketone is thermodynamically less stable with respect to the decarbonylated biaryls. The computations also suggest that the nickel catalyst is able to promote the decarbonylation of biaryl ketones via an unexpected C-C bond activation.

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

Computation elucidates the mechanism and origins of regioselectivity of (NHC)Ni(ii)hydride-catalyzed cycloisomerization of dienes.

Mechanism of iridium-catalysed branched-selective hydroarylation of vinyl ethers: a computational study

An unconventional modified Chalk–Harrod-type mechanism was revealed by DFT calculations for the iridium-catalysed branched-selective hydroarylation of vinyl ethers.