In Bartlett and co-workers original paper on the thermal cycloaddition of 1,1dichloro-2,2-difluoroethylene to the isomeric 2,4-hexadienes, 5-7, the values of p and q (see Supporting Information for definitions of p and q) were similar for the cis-1-propenyl and trans-1-propenyl (R in Bartlett's notation ) biradical intermediates. The parameters p and q are ratios of cis and trans cyclobutyl products starting from the cis and trans olefins and are also ratios of various rate constants for interconversion of the cis and trans rotamers (k 1 and k -1 ) and for ring closures (k 2 and k 3 ), p = k 2 (k -1 + k 3 )/k 1 k 3 , q = k 3 (k 1 + k 2 )/k -1 k 2 (see Supporting Information for explicit definitions). Bartlett and co-workers calculated average vales of p and q and thus