Catalysis

Understanding the Role of Ring Strain in beta-Alkyl Migration at Mg and Zn Centres

Authors

Abstract

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 -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 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 less 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.

Content

Thumbnail image of _CCbreak_Nov10.pdf

Supplementary material

Thumbnail image of _SI_CCbreak_Oct14.pdf
Supporting information
Details of experiments, calculations, and characterisation data.
Thumbnail image of CCbreak.cif
cif
cif file for XRD data