Surpassing Torquoelectronic Effects in Conrotatory Ring Closures: Origins of Stereocontrol in Intramolecular Ketenimine-Imine [2+2] Cycloadditions

Unexpected and almost perfect stereochemical control is obtained in the title reactions [Eq. (a)], although torquoelectronic theory predicts negligible stereoselection. According to ab initio and DFT calculations, this stereocontrol is determined by the geometry of the first transition state of the stepwise mechanism. In good agreement with the model proposed, the presence of an additional methylene group attached to the iminic nitrogen atom promotes a significant loss of stereocontrol.