In all cases, C-C -bond activation was calculated to proceed via a concerted intramolecular transition state TS-2 best described as a -alkyl migration step (Figure , Table The computational model can be used to explain the effects of both ring-size and influence of metal on the -alkyl migration step. The activation barriers for -alkyl migration increase with increasing ring size of cycloalkane ring i.e. 3 < 4 < 5 < 6. In all cases these barriers are higher for Zn then they are for Mg. Hence, while -alkyl migration for 3-membered ring systems is just about accessible for both metals (Mg, G ‡ = 19 kcal mol -1 ; Zn, G ‡ = 35.2 kcal mol -1 ), for 4-membered rings only the Mg analogue is predicted to facilitate -alkyl migration under the experimental conditions (Mg, G ‡ = 28.0 kcal mol - 1 ; Mg, G ‡ = 44.4 kcal mol -1 ). For both metals, -alkyl migration of substrates containing five-and sixmembered systems by TS-2 have energy barriers that are inaccessible under the reaction conditions (G ‡ > 40 kcal mol -1 ). Furthermore, the overall ring opening process for larger ring sizes are calculated to be endergonic, so would be expected to be reversible even if the activation barriers were traversed.