Bimetallic Nickel Aluminun Mediated Para-Selective Alkenylation of Pyridine: Direct Observation of η²,η¹-Pyridine Ni(0)−Al(III) Intermediates Prior to C−H Bond Activation

Abstract

We have presented new amino-NHC Ni-Al complex mediated para C-H bond activation for pyridine and quinolin, and isolated for the first time the intermediate structure of a bimetallic eta(2),eta(1)-pyridine nickel aluminum complex prior to its C-H activation, which serves as key evidence for bimetallic catalysis.

Mechanistic Study of a Switch in the Regioselectivity of Hydroheteroarylation of Styrene Catalyzed by Bimetallic Ni–Al through CH Activation

“…A positive KIE of 4.61 was also observed for the reaction performed in the absence of AlMe 3 . These KIE outcomes are in striking contrast to our previous CÀH bond activation studies on pyridine, which displayed only a small primary KIE of 1.25, [4] highlighting the requirement of p-coordination of pyridine prior to the CÀH bond breaking.…”

“…[3c] Similarly, our group, [4] as well as those of Nakao and Hiyama [5] and Kanai, [6] have observed the beneficial effect of added AlMe 3 in promoting nickel-mediated CÀH bond activation of heteroarenes. More recently, we reported a switch in the regioselectivity for CÀH functionalization of benzimidazole derivatives with styrene and allylbenzene to afford either linear or branched products when using AlMe 3 .…”

“…In our previous studies on Ni-mediated CÀH bond activation of pyridine, we found that AlMe 3 not only activates the CÀH bond towards functionalization, but also invokes a steric environment promoting para selectivity. [4] Attempts to isolate and characterize the intermediate resembling complex 4 b bearing adduct 3 have thus far not been fruitful. We therefore undertook a series of indirect experiments to confirm our hypothesis of steric-based linear selectivity induced by AlMe 3 as a chemical switch.…”

Transition-Metal-Catalyzed Site-Selective C–H Functionalization of Quinolines beyond C2 Selectivity

“…21 Ong also reported the C4 alkenylation of azines with alkynes through Ni-Al cooperative catalysis (Scheme 8). 22 A Ni(0)-NHC complex bound to the pyridine-AlMe 3 adduct at the C3-C4 positions was isolated and characterized by single-crystal X-ray diffraction. This result indicated that the catalytic cycle was initiated by π coordination of Al-bound azines toward the Ni center.…”

“…C4 alkenylation of quinolines through Ni-Al cooperative catalysis. 22 Matsunaga and Kanai reported that the Co-catalyzed C4 alkylation of quinolines with styrenes gave 4-alkylquinolines with branched selectivity (Scheme 9). 23 The catalyst system using Co(OAc) 2 in combination with nBuLi and pyridine, which acted as reducing reagents to generate in situ active Co-H species, was optimized for quinoline substrates.…”