Mechanistic Study on Ligand-Controlled Rh(I)-Catalyzed Coupling Reaction of Alkene-Benzocyclobutenone

Recently, Dong’s group [ Angew. Chem., Int. Ed. 2012, 51, 7567–7571; Angew. Chem., Int. Ed. 2014, 53, 1891–1895] reported the ligand-controlled selectivity of Rh-catalyzed intramolecular coupling reaction of alkene-benzocyclobutenone: the direct coupling product (i.e., fused-rings) was formed in the DPPB-assisted system (DPPB = PPh 2 (CH 2 ) 4 PPh 2 ), while the decarbonylative coupling product (i.e., spirocycles) was generated in the P(C 6 F 5 ) 3 -assited system. To explain this interesting selectivity, density functional theory (DFT) calculations have been carried out in the present study. It was found that the direct and decarbonylative couplings experience the same C(acyl)–C(sp 2 ) activation and alkene insertion steps. The following C–C reductive elimination or β-H elimination–decarbonylation–reductive elimination leads to the direct or decarbonylative coupling reaction, respectively. The coordination features of different ligands were found to significantly influence C–C reductive elimination and decarbonylation step. The requisite phosphine dissociation of DPPB ligand from Rh center for the decarbonylation step is disfavored, and thus, the reductive elimination and direct coupling reaction are favored therein. By contrast, a free coordination site is available on the Rh center in the P(C 6 F 5 ) 3 -assisted system, facilitating the decarbonylation process together with the generation of related decarbonylative coupling product.