Computational Investigation of Nickel-Mediated B–H Activation and Regioselective Cage B–C(sp2) Coupling of o-Carborane

摘要

Density functional theory (DFT) methods including LC-ωPBE, CAM-B3LYP, B3LYP, and B3LYP-D3, combined with double Zeta all-electron DZVP basis set, have been employed to conduct computational investigations on nickel-mediated reaction of o-carboranylzirconacycle, n-hexene, and 2-bromophenyltrimethylsilylacetylene in toluene solution. A multistep mechanism leading to the C,C,B-substituted carborane-fused tricyclics, including (1) sequential insertion of alkene and alkyne into Ni−C bonds; (2) double 1,2-migration of the TMS group; (3) B−H activation assisted by Cs2CO3 additive; and (4) reduction cage B−C (sp2) coupling, was proposed. Among these steps, the B−H activation of o-carborane was located as rate-determining step (RDS). With assistance of Cs2CO3 additive (replaced by K2CO3 in simulation), the RDS free-energy barrier at PCM-LC-ωPBE/DZVP level was calculated to be 23.1−23.9 kcal·mol−1, transferring to a half-life of 3.9−15.1 h at 298 K. The predicted half-life coincides well with 80% experimental yields of C,C,B-substituted carborane-fused tricyclics after 12 h. Kinetic data obtained by employing LC-ωPBE method also reproduced the experimental diastereoselective ratio well. Various B−H activation pathways with and without Cs2CO3 additive were taken into consideration, which illustrates Cs2CO3 as an essential guarantee for smooth occurrence of this reaction at room temperature.