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A comparison of the Cope rearrangements of cis-1,2-divinyleyelopropane, cis-2,3-divinylaziridine, cis-2,3-divinyloxirane, cis-2,3-divinylphosphirane, and cis-2,3-divinylthiirane: A DFT study

Transition structures, energetics, and nucleus-independent chemical shifts (NICS) for Cope rearrangements of cis-2,3-divinylaziridine (IN), cis-2,3-divinyloxirane (10), cis-2,3-divinylphosphirane JP), and cis-2,3-divinylthiirane (IS), leading to 4,5-dihydro-1H-azepine (3N), 4,5-dihydrooxepine (30), 4,5-dihydro-1H-phosphepine (W), and 4,5-dihydrothiepine (3S), respectively, are reported at the (U)B3LYP/6-31G* level and compared to those of cis-1,2-divinylcyclopropane (1C). The minimum energy path for all rearrangements proceeds through an endo-boatlike, aromatic transition structure. The predicted activation barriers increase in the order of 1C < IN < 10 < 1P < IS, which agrees qualitatively with the decreasing ring strain order of reference compounds (cyclopropane > aziridine > oxirane > phosphirane > thiirane). The exothermicities for these rearrangements decrease in the order of IN > 10 > 1C > 1P > IS. If the place of 1C in this sequence is ignored, the decreasing reaction exothermicity order correlates well with the increasing activation barrier order and with decreasing strain order of reference compounds. NICS values calculated for transition structures are typical of highly aromatic transition structures of thermally allowed pericyclic reactions.