Understanding the role of ring strain in beta-alkyl migration at Mg and Zn centres

The activation of C-C sigma-bonds within strained three- and four-membered hydrocarbons at electrophilic Mg and Zn centres is reported. This was achieved in a two-step process involving (i) hydrometallation of a methylidene cycloalkane followed by (ii) intramolecular C-C bond activation. While hydrometallation of methylidene cyclopropane, cyclobutane, cyclopentane and cyclohexane occurs for both Mg and Zn reagents, the C-C bond activation step is sensitive to ring size. For Mg, both cyclopropane and cyclobutane rings participate in C-C bond activation. For Zn, only the smallest cyclopropane ring reacts. These findings were used to expand the scope of catalytic hydrosilylation of C-C sigma-bonds to include cyclobutane rings. The mechanism of C-C sigma-bond activation was investigated through kinetic analysis (Eyring), spectroscopic observation of intermediates, and a comprehensive series of DFT calculations, including activation strain analysis. Based on our current understanding, C-C bond activation is proposed to occur by a beta-alkyl migration step. beta-Alkyl migration is more facile for more strained rings and occurs with lower barriers for Mg compared to Zn. Relief of ring strain is a key factor in determining the thermodynamics of C-C bond activation, but not in stabilising the transition state for beta-alkyl migration. Rather, we ascribe the differences in reactivity to the stabilising interaction between the metal centre and the hydrocarbon ring-system, with the smaller rings and more electropositive metal (Mg) leading to a smaller destabilisation interaction energy as the transition state is approached. Our findings represent the first example of C-C bond activation at Zn and provide detailed new insight into the factors at play in beta-alkyl migration at main group centres.