Cycloadditions of o-thioquinones (o-TQs) with 1,3- dienes could proceed via either a [ 2 + 4] or a [ 4 + 2] mechanism. Under kinetic control and with acyclic dienes the reaction affords the spiro cycloadducts 5 deriving from the [ 2 + 4] path as the main products. Under thermodynamic control, or with cyclic dienes, the o-TQs behave as heterodienes to give the benzoxathiin derivatives 4, in most cases with complete regioselectivity. In the present computational study, DFT calculations were performed in order to achieve a deep understanding of both [ 2 + 4] and [ 4 + 2] paths. The reactions of three o-TQs with six 1,3- dienes were thoroughly investigated at the B3LYP/ TZVP// B3LYP6-31G* level, and the two reaction mechanisms were then compared, evidencing that [ 2 + 4] cycloadditions are kinetically favored, strongly asynchronous, or even unconcerted, while [ 4 + 2] reactions are thermodynamically favored, quite asynchronous, but undoubtedly concerted. Moreover, the observed regioselectivity was rationalized by mean of the FMO theory and by comparison of the activation energies for different pathways.

[2+4] and [4+2] cycloadditions of o-thioquinones with 1,3-dienes : a computational study / A. Contini, S. Leone, S. Menichetti, C. Viglianisi, P. Trimarco. - In: JOURNAL OF ORGANIC CHEMISTRY. - ISSN 0022-3263. - 71:15(2006 Jul 21), pp. 5507-5514.

[2+4] and [4+2] cycloadditions of o-thioquinones with 1,3-dienes : a computational study

A. Contini
Primo
;
S. Leone
Secondo
;
P. Trimarco
Ultimo
2006-07-21

Abstract

Cycloadditions of o-thioquinones (o-TQs) with 1,3- dienes could proceed via either a [ 2 + 4] or a [ 4 + 2] mechanism. Under kinetic control and with acyclic dienes the reaction affords the spiro cycloadducts 5 deriving from the [ 2 + 4] path as the main products. Under thermodynamic control, or with cyclic dienes, the o-TQs behave as heterodienes to give the benzoxathiin derivatives 4, in most cases with complete regioselectivity. In the present computational study, DFT calculations were performed in order to achieve a deep understanding of both [ 2 + 4] and [ 4 + 2] paths. The reactions of three o-TQs with six 1,3- dienes were thoroughly investigated at the B3LYP/ TZVP// B3LYP6-31G* level, and the two reaction mechanisms were then compared, evidencing that [ 2 + 4] cycloadditions are kinetically favored, strongly asynchronous, or even unconcerted, while [ 4 + 2] reactions are thermodynamically favored, quite asynchronous, but undoubtedly concerted. Moreover, the observed regioselectivity was rationalized by mean of the FMO theory and by comparison of the activation energies for different pathways.
diels-alder reactions; 4+2 cycloaddition reactions; faced antioxidant activity; Gaussian-basis sets; efficient synthesis; substituted ethenes; density; dienes; DFT; stereoselectivity
Settore CHIM/06 - Chimica Organica
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/2434/24235
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