Competing Mechanisms, Substituent Effects, and Regioselectivities of Nickel-Catalyzed [2 + 2 + 2] Cycloaddition between Carboryne and Alkynes: A DFT Study
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- 作者：Wei-Hua Mu ; Shu-Ya Xia ; Ji-Xiang Li ; De-Cai Fang ; Gang Wei ; Gregory A Chass
- 刊名：Journal of Organic Chemistry
- 出版年：2015
- 出版时间：September 18, 2015
- 年：2015
- 卷：80
- 期：18
- 页码：9108-9117
- 全文大小：739K
- ISSN：1520-6904
文摘
Competing reaction mechanisms, substituent effects, and regioselectivities of Ni(PPh3)2-catalyzed [2 + 2 + 2] carboryne鈥揳lkyne cycloadditions were characterized by density functional theory using the real chemical systems and solvent effects considered. A putative mechanism involving the following steps was characterized: (1) exothermic carboryne鈥揷atalyst complexation and nucleophilic attack by the first alkyne; (2) insertion of the second alkyne, the rate-determining step (RDS) in all four reactions studied; (3) isomerization of reactant-bound complexes; and (4) product elimination and catalyst regeneration. The RDS in three reactions is mediated by free energy barriers of 27.2, 31.1, and 36.6 kcal路mol鈥?, representative of the corresponding experimental yields of 67, 54, and 33%, respectively. A fourth reaction with 0% experimental yield showed representative RDS free energy barriers of 60.4 kcal路mol鈥?, which are difficult to surmount even at 90 掳C. Alternative pathways leading to differing isomers were similarly characterized and successfully reproduced experimentally determined product regioselectivities. Kinetic data derived from free energy barriers are in quantitative agreement (< 卤 0.75鈥?.0 kcal路mol鈥?) of the experimental times, affirming the theoretical results as representative of the real chemical transformations. Complementary determinations show the use of truncated models (Ni(PMe3)2, Ni(PH3)2) causes the RDS to vary from step 2 (alkyne insertion) to step 1 (alkyne attack), highlighting the need to employ real chemical systems in modeling these reactions.