Key Mechanistic Features of Ni-Catalyzed C鈥揌/C鈥揙 Biaryl Coupling of Azoles and Naphthalen-2-yl Pivalates

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• 作者：Huiying Xu ; Kei Muto ; Junichiro Yamaguchi ; Cunyuan Zhao ; Kenichiro Itami ; Djamaladdin G. Musaev
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：October 22, 2014
• 年：2014
• 卷：136
• 期：42
• 页码：14834-14844
• 全文大小：733K
• ISSN：1520-5126

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.}