Competing Mechanisms, Substituent Effects, and Regioselectivities of Nickel-Catalyzed [2 + 2 + 2] Cycloaddition between Carboryne and Alkynes: A DFT Study.
Competing reaction mechanisms, substituent effects, and regioselectivities of Ni(PPh3)2-catalyzed [2 + 2 + 2] carboryne-alkyne 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-catalyst 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·mol(-1), 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·mol(-1), which are difficult to surmount even at 90 °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 (< ± 0.75-3.0 kcal·mol(-1)) 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.
Faculty of Chemistry and Chemical Engineering, Yunnan Normal University , Kunming 650092, Yunnan, China.
Wei-Hua
WH
Mu
Faculty of Chemistry and Chemical Engineering, Yunnan Normal University , Kunming 650092, Yunnan, China.
Shu-Ya
SY
Xia
Faculty of Chemistry and Chemical Engineering, Yunnan Normal University , Kunming 650092, Yunnan, China.
Ji-Xiang
JX
Li
Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University , Beijing 100875, China.
De-Cai
DC
Fang
CSIRO Manufacturing Flagship , Bradfield Road, West Lindfield, P.O. Box 218, Lindfield, NSW 2070, Australia.
Gang
G
Wei
School of Biological and Chemical Sciences, Queen Mary University of London , London E1 4NS, U.K.
Gregory A
GA
Chass