Effect of formal CH/P exchange on tandem Cope rearrangement and [2 + 2] cycloaddition of cis-1,2-diethylnylcyclopropane and its mono-hetero analogues

Effect of formal one and two CH/P exchange(s) on tandem Cope rearrangement and [2 + 2] cycloaddition of cis-1,2-diethylnylcyclopropane and its mono-hetero analogues has been investigated theoretically in the gas phase at the DFT (B3LYP/6-31+G(d)) level. The activation free energy for the first step, i.e., Cope rearrangement decreases successively on replacing one and two CH moieties of cis-1,2-diethylnylcyclopropane and its mono-hetero analogues with P atom(s), which could be rationalized on the basis of weaker C = P π bond than C = C π bond. However, this trend is not maintained in the second step, i.e. [2 + 2] cycloaddition of the initially formed intermediate; instead, the activation free energy for monophospha- and diphospha- analogues is slightly higher, which may be attributed to the lesser stability of monophospha- and diphosphacyclobutene rings than cyclobutene. The CASSCF calculations indicate that except the intermediates generated from the Cope rearrangement of cis-diethynylthiirane and cis-bis(phosphaethynyl)cyclopropane which are expected to follow concerted mechanism for the [2 + 2] cycloaddition, in all other cases, apparently, there is no preference for the concerted or stepwise mechanism. Inclusion of the dispersion correction in the DFT functional does not cause significant changes in energetics.