(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.

Our Voyage from Catalytic Cross-Hydroalkenylation to Transfer-Dehydroaromatization of Cyclic π-Systems: Reactivity and Selectivity Changes Enabled by NHC-Ni and NHC-Pd Hydride Equivalents

N-Heterocyclic carbene ligated transition-metal catalysts often show interesting properties and reactivity as compared to conventional ligand systems. In (NHC)Ni and (NHC)Pd hydrides, a dramatic reactivity changed from cross-hydroalkenylation to transfer-dehydroaromatization was observed under optimized conditions. This account summarizes our recent efforts and stories behind this serendipitous discovery. The mechanistic studies revealed that the keys to divert the desired reactivity are the differences in the olefin insertion selectivity and the hydrometallated species reactivity.

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.

Highly Enantioselective Nickel-Catalyzed Intramolecular Hydroalkenylation of N- and O-Tethered 1,6-Dienes To Form Six-Membered Heterocycles

A highly enantioselective nickel-catalyzed intramolecular hydroalkenylation of N- or O-tethered 1,6-dienes was developed by using monodentate chiral spiro phosphoramidite ligands. The reaction provides an efficient and straightforward method for preparing very useful six-membered N- and O-heterocycles with high regioselectivity as well as excellent stereoselectivity from easily accessible starting materials under mild reaction conditions. The chiral spiro nickel catalyst developed in this study represents one of the few catalysts for highly enantioselective cyclization of unconjugated dienes.

(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 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.