文摘
The mechanism of the Ni-dcype-catalyzed C鈥揌/C鈥揙 coupling of benzoxazole and naphthalen-2-yl pivalate was studied. Special attention was devoted to the base effect in the C鈥揙 oxidative addition and C鈥揌 activation steps as well as the C鈥揌 substrate effect in the C鈥揌 activation step. No base effect in the C(aryl)鈥揙 oxidative addition to Ni-dcype was found, but the nature of the base and C鈥揌 substrate plays a crucial role in the following C鈥揌 activation. In the absence of base, the azole C鈥揌 activation initiated by the C鈥揙 oxidative addition product Ni(dcype)(Naph)(PivO), 1B, proceeds via 螖G = 34.7 kcal/mol barrier. Addition of Cs2CO3 base to the reaction mixture forms the Ni(dcype)(Naph)[PivOCs路CsCO3], 3_Cs_clus, cluster complex rather than undergoing PivO鈥?/sup> 鈫?CsCO3鈥?/sup> ligand exchange. Coordination of azole to the resulting 3_Cs_clus complex forms intermediate with a weak Cs鈥揾eteroatom(azole) bond, the existence of which increases acidity of the activated C鈥揌 bond and reduces C鈥揌 activation barrier. This conclusion from computation is consistent with experiments showing that the addition of Cs2CO3 to the reaction mixture of 1B and benzoxazole increases yield of C鈥揌/C鈥揙 coupling from 32% to 67% and makes the reaction faster by 3-fold. This emerging mechanistic knowledge was validated by further exploring base and C鈥揌 substrate effects via replacing Cs2CO3 with K2CO3 and benzoxazole (1a) with 1H-benzo[d]imidazole (1b) or quinazoline (1c). We proposed the modified catalytic cycle for the Ni(cod)(dcype)-catalyzed C鈥揌/C鈥揙 coupling of benzoxazole and naphthalen-2-yl pivalate.