Semiempirical (AM1 and PM3) and density functional theory (DFT) calculations were performed on about 50 porphyrin isomers with 25 each of 1,2 (syn) and 1,3 (anti) tautomeric forms. The corresponding oxa-and thia-core-modified analogues were also computed. The variations of relative energies and stabilities of the core-modified analogues were compared with parent porphyrin 1 and the corresponding oxa-and thia-analogues. The trends in relative energies are not significantly changed while going from parent system to oxa-and thia-core-modified porphyrins in case of both syn and anti tautomers. Isomers of types [2⋅2⋅0⋅0], [3⋅0⋅1⋅0], [3⋅1⋅0⋅0], and [4⋅0⋅0⋅0] are destabilized due to the absence of methine bridge, which results in angle strain for tetrapyrroles. Isomers having [2⋅1⋅1⋅0], [2⋅1⋅0⋅1], [2⋅0⋅2⋅0] and [2⋅2⋅0⋅0] connectivity, the Z isomers, are more stable compared to the corresponding E isomers in both syn and anti forms of parent and core-modified analogues.
epoc ABSTRACT: B3LYP/6-31G* calculcations were performed to model the Diels-Alder reaction of cyclobutano-, cyclobuteno-and benzo[b]-fused cyclic five-membered ring dienes with two prototypical dienophiles, ethylene and acetylene. The cyclobuteno[b]-fused dienes, which loose the unfavorable antiaromaticity upon cycloaddition, are the most reactive; in contrast, the benzo[b]-fused dienes, which encounter disruption of aromaticity in the course of the reaction, are the least reactive. Synchronicity calculations based on bond order analysis indicate that the reactions of benzo[b]-annelated dienes show significant deviations from synchronicity, indicating highly unsymmetrical transition states. Deformation energies were employed to corroborate the computed activation energies. Although there are some deviations in the quantitative results between ethylene and acetylene reactions, the trends are essentially the same with all the dienes considered, with acetylene showing a slightly higher reactivity. The comparison with the corresponding [c]-fused dienes further strengthens the argument that product stability controls the reactivity in these reactions, as exemplified by excellent linear fits between the activation and reaction energies. a Gibbs free energies of reactions at 398 K (ÁG r398 K ) are given only for ethylene as dienophile. Entropies of reactions are in eu and all other values are in kcal mol À1 . The values in parentheses in bold are for acetylene as dienophile.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.