Pathways for Concerted [2+2] Cycloaddition to Cumulenes

A computational search has revealed concerted pathways for [2 + 2] cycloaddition of ethylene to all 10 of the cumulenes with the formula X=C=Y, where X, Y = C, N, O, and S. Four different concerted pathways have been found, three of them pseudopericyclic plus another based on sp-hybridized carbon. In the case of 2 of the 16 possible cycloadditions, a pair of novel three-membered ring intermediates has been discovered. As simple model reactions, cycloaddition of ethylene to formaldehyde, thioformaldehyde, and formaldimine is also described.