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

Abstract

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.

N-Heterocyclic carbenes as privileged ligands for nickel-catalysed alkene functionalisation

“…The authors performed detailed DFT first-principles calculations to elucidate the mechanism and the origin of the enantioselectivity. Supported with the results from previously done calculations, 77 their calculations also suggest a reaction pathway involving a ligand-to-ligand hydrogen transfer (LLHT) mechanism, contrary to the stepwise oxidative addition-hydronickelation pathway as suggested by Hiyama and co-workers. 74 LLHT involving the ortho or meta-C-H bonds was also less feasible due to a higher energy barrier, Scheme 19 Ni/IMes-catalysed hydroheteroarylation of vinylarenes.…”

“…Hetero-substituted 1,3-dienes (75) without bulky R1 or R2 substituents proved to be compatible, delivering the product in moderate yield and excellent ee. N-or O-substituted terminal olefins were also efficient (76,77). The reaction is also highly chemoselective for the insertion of the less hindered terminal alkenes over internal allylic ethers as exemplified with 77.…”

Metallated dihydropyridinates: prospects in hydride transfer and (electro)catalysis

“…Proposed mechanisms for the pyridine alkylation reaction have included Ni(0) insertion into the py C 4 bond or a Ni-supported, direct HT from the py C 4 to the alkene ( Scheme 14 , bottom). 119 Recent computational and experimental work support the latter pathway. These results suggest that bimetallic pyridine systems as shown in Scheme 14 , can support new HT reaction pathways, which incorporate reaction processes supported by transition metals.…”