Understanding the role of ring strain in β-alkyl migration at Mg and Zn centres†
The activation of C–C σ-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 σ-bonds to include cyclobutane rings. The mechanism of C–C σ-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 β-alkyl migration step. β-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 β-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 β-alkyl migration at main group centres.