Theoretical investigation on SnCl₄-catalyzed tandem dimerization/oxy-2-azonia-Cope rearrangements between β,γ-unsaturated ketones and imines

The mechanism of the Lewis acid-catalyzed oxy-2-azonia-Cope rearrangement between β,γ-unsaturated ketones and imines leading to the formation of homoallylic amides and lactams has been theoretically studied using the B3LYP density functional theory methods enhanced with a polarized continuum solvation model. It was predicted that the SnCl₄-catalyzed tandem dimerization/oxy-2-azonia-Cope rearrangement mechanism is highly preferred over the uncatalyzed version as well as the plausible tandem dimerization/Prins rearrangement mechanism. A two-step pathway was found for the overall reaction, involving the initial nucleophilic dimerization followed by the [3,3]-sigmatropic rearrangement. The latter phase was considered to be the rate-limiting step. Particularly, the transition states account for the experimentally observed stereoselectivities and Z/E selectivities. The high stereoselectivity and Z/E selectivity for the chiral cyclic substrates can be attributed to the relative conformational stabilities of TSs. Moreover, distortion–interaction analysis has been performed in an attempt to quantify the various contributions to the reaction transition states, and it revealed that interaction energy E_int ^II and distortion energy ∆E_d ^I associated with the formation of the 2COM2 complex are the determining factors to define the Z/E selectivities for nine- and ten-membered ring pathway, respectively. Investigation on the ethyleneimine-involved reaction predicts a relatively very low barrier in the pathway; thus, the sequence might be a useful strategy for synthesis of macrolactams.