The DFT Quest for Possible Reaction Pathways, Catalytic Species, and Regioselectivity in the InCl-Catalyzed Cycloaddition of -Tosyl Formaldimine with Olefins or Allenes.

The present work focuses on a theoretical investigation of the plausible mechanism, determination of catalytically active species, and understanding of the regioselectivity in the InCl-catalyzed cycloaddition of -tosyl formaldimine with alkenes or allenes. InCl and InCl coordinated by dichloroethane (InCl-DCE) were investigated as model catalytic systems. DFT data supported that InCl-DCE represent the plausible in situ generated catalytic species. The catalytic cycle starts from the coordination of -tosyl formaldimine to InCl-DCE, generating an In-complexed iminium intermediate. This then undergoes intermolecular reaction (aza-Prins) with alkene substrate to form a carbocation intermediate, which is chemoselectively attacked by the second -tosyl formaldimine molecule to form a formaldiminium intermediate. In a final step, this intermediate undergoes the ring closure, leading to hexahydropyrimidine along with the regeneration of catalyst. In addition, our DFT results indicate that -tosyl formaldimine not only acts as a reactant but also accelerates the 1,3-H-shift as a proton acceptor, giving an experimentally observed allylamide product. Also, the "iminium/alkene/imine" path was supported by calculation results for diastereoselective [2 + 2 + 2] reaction using an internal alkene. Finally, the regioselectivity of the InCl-catalyzed cycloaddition using allenes along with -tosyl formaldimine was also analyzed.