Geminal bond participation and torquoselectivity in cheletropic reactions

We applied the geminal bond participation theory to cheletropic reactions. The interaction between σ-orbital of the inner bond and the π*-orbital of the NN bond was predicted to be bonding, while that involving the outer bond should be antibonding. The difference in the bonding/antibonding character of these interactions is due to the cyclic orbital interaction between the π*-orbital of the NN bond and the geminal bonds at the reaction center, which is influenced by the phase continuity between the π*–σ–σ orbitals. This prediction was confirmed by the bond model analysis of model compound 1a. These results suggest that reactivity could be enhanced by substituting the inwardly rotating σ-bond by a more electron-donating one, i.e. inward rotation of a substituent with a more electropositive atom. Theoretical calculations were performed for systematically substituted substrates. The prediction was confirmed except in the case of 1g, which was affected by the inductive effect of oxygen. Silyl/methyl-substituted 1e was subjected to the bond model analysis to confirm that the preferred inward rotation of the silyl group of 1e results from the geminal bond participation and little affected by steric effects.