Theoretical study of the substituent effect of hydroxy group on tandem Cope rearrangement and [2+2] cycloaddition in cis-1,2-diethynylcyclopropane and its mono-hetero analogues

The tandem Cope rearrangement and [2+2] cycloaddition of cis-1,2-diethynyl-1,2-dihydroxycyclopropane and its mono-hetero analogues have been investigated at the B3LYP/6-31+G* level. The presence of the hydroxy group lowers the activation enthalpies for the Cope rearrangement, whereas activation enthalpies for the [2+2] cycloaddition are raised as compared to those for their non-hydroxy derivatives. The NBO analysis indicates that in the transition structure involved in the Cope rearrangement, lone pairs of the oxygen atoms of the hydroxy groups are transferred into the sigma* C-C bond undergoing migration, as a result of which it is weakened. On the other hand, the lone pairs of the oxygen atoms interact with the pi* C=C orbitals of the bis-allenic systems in the intermediate thereby stabilizing it and, thus, suppressing its driving ability for the [2+2] cycloaddition. In the products so formed, 6 pi electrons are delocalized conferring stability on them, which is further augmented by extended conjugation with the hydroxy groups. Due to high stability of these products, activation barrier for the change of enol into ketone is very high.