A theoretical study on intermolecular [2+2] radical cation cycloaddition reactions and the competition between concerted and stepwise mechanisms

Highlight
• Four possible reaction paths, leading to different cyclobutane isomers (regioselectivity and stereoselectivity), were calculated.
• The origins leading to regioselectivity and stereoselectivity were revealed.
• Two competitive reaction mechanisms (stepwise and concerted) have been discovered.
• The reasons resulting in different mechanisms were analyzed.

DFT (U)B3LYP and (U)M06-2X calculations with 6-31G(d,p) basis sets have been carried out to investigate the influences of the substituents on the regio- and stereo selectivities for the intermolecular [2+2] radical cation cycloaddition reactions between an aliphatic cyclic enol ether and olefins. Four possible pathways for each reaction were explored. The advantageous pathways of the cycloadditions were certified by the free energy barriers and reaction free energies. The structural features which lead to the products with regio- and stereoselectivities were uncovered. The computational results indicated that there are two different mechanisms for these cycloaddition reactions. One is a stepwise mechanism with two transition states along the reaction path while the other is a concerted mechanism with only one transition state. The stepwise mechanism has a much lower barrier. The factors affecting the cycloaddition mechanism have been discussed.

Figure optionsDownload high-quality image (63 K)Download as PowerPoint slide