Mechanism and Origins of the Chemo- and Regioselectivities in Nickel-Catalyzed Intermolecular Cycloadditions of Benzocyclobutenones with 1,3-Dienes

5 years ago

Mechanism and Origins of the Chemo- and Regioselectivities in Nickel-Catalyzed Intermolecular Cycloadditions of Benzocyclobutenones with 1,3-Dienes

Genping Huang, Zhong-Liang Wang, Hongyan Zou

The nickel-catalyzed intermolecular cycloadditions of benzocyclobutenones with 1,3-dienes developed by Martin and co-workers are featured with the exclusive proximal C−C bond cleavage and a high chemoselectivity of the [4+4] over the [4+2] cycloaddition. In this report, the detailed reaction mechanism and the origins of the selectivities were investigated by means of density functional theory calculations. The results show that the reaction is initiated by a C−C oxidative addition of the benzocyclobutenone to form the five-membered nickelacycles. A subsequent exo 1,4-insertion/C−C reductive elimination and an endo 1,4-insertion/C−C reductive elimination lead to the [4+4] and [4+2] cycloaddition products, respectively. The 1,4-insertion of the 1,3-diene into the Ni−C bond was calculated to be the rate- and selectivity-determining step of the reaction. The calculations reproduced quite well the experimentally observed exclusive proximal C−C bond cleavage and the high chemoselectivity of the [4+4] over the [4+2] cycloaddition. In particular, it was found that the steric repulsion between the phosphine ligand and the α-substituent of the benzocyclobutenone has a dramatic impact on the 1,4-insertion, which enables the experimentally observed selectivities. Different ways: DFT calculations were performed to investigate the nickel-catalyzed intermolecular cycloadditions of benzocyclobutenones with 1,3-dienes (see scheme). The steric repulsion between the phosphine ligand and the α-substituent of the benzocyclobutenone has a dramatic impact on the 1,4-insertion, which enables the observed exclusive proximal C−C bond cleavage and the high chemoselectivity for the [4+4] cycloaddition.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201702316