Synthesis and Electron Spin Resonance Studies of Cu(II) Complexes with Schiff Base Macroligands

Papaspyrou, Athanasia; Spyropoulou, Konstantina; Paraskevas, M.; Paraskevas, S. M.

doi:10.1080/15533170802293386

Cited by 2 publications

(1 citation statement)

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“…Experimentally, pyrrole is reported to undergo DA reaction with maleic anhydride to furnish a mixture (composition not determined) of the adducts, 38 and both furan 39 and thiophene 20 the exoadducts largely. Since the DA adduct formed from selenophene undergoes rapid extrusion of selenium, it prevents comment on the endo/exo selectivity.…”

Section: Resultsmentioning

confidence: 99%

A Computational Study of the Relative Aromaticity of Pyrrole, Furan, Thiophene and Selenophene, and Their Diels-Alder Stereoselectivity

Yadav¹

2020

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The collinearity of terminal p orbitals of a diene with that of a dienophile is required for an effective overlap to result in s bond formation during the Diels-Alder reaction. The ease of the DA reaction of a cyclic diene with a given dienophile, therefore, must also depend on the distance between the termini of the diene. A distance larger than the unsaturated bond of the dienophile is expected to raise the energy of activation. This scenario has been amply demonstrated from the study of reactions of several dienes, some designed to serve the purpose, with different dienophiles. The five-ring heterocycles pyrrole, furan, thiophene and selenophene possess varying aromatic character for the varied resonance participation of the heteroatom lone pair with ring p bonds. The aromaticity decreases in the same order due to: (a) the increasing sC-X (X = heteroatom) bond length lifts the bond uniformity required for ring current, hence aromaticity, such as in benzene and (b) size-mismatch of the interacting lone pair orbital and the ring p orbitals, especially in thiophene and selenophene, both allowing poor overlap in the ground state structures. It is demonstrated that increase alone in the activation energies of the DA reactions of pyrrole, furan, thiophene and selenophene cannot be considered a measure of relative aromaticity as often done and even theoretically attempted in many ways to prove just that. The separation of the termini of the diene has a much larger role in the determination of activation energy, especially in thiophene and selenophene, than their aromaticity profile. There cannot be a measure better than the relative intensity of heteroatom lone pair overlap with ring p bonds, giving rise to a six-electron like system in following Hückel’s 4n+2 rule, to assess the relative aromaticity.

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Section: Resultsmentioning

confidence: 99%