title: [2n2π + 2n2π] cycloadditions: an alternative to forbidden [4π + 4π] processes. The case of nitrone dimerization.
abstract: A theoretical study based on (U)M06-2X/cc-pVTZ calculations has been used to investigate the [3 + 3] thermal dimerization of nitrones to 1,4,2,5-dioxadiazinanes in both the gas phase and in dichloromethane solution. Calculations suggest that dimerization of nitrones takes place through a concerted mechanism involving a formal disallowed [4π + 4π] cycloaddition with a free energy barrier of 30.8 kcal mol(-1). The corresponding diradical and zwitterionic stepwise mechanisms have also been studied, but the located transition structures are kinetically disfavoured. An alternative mechanism through a five-membered ring intermediate formed by a classical [3 + 2] dipolar cycloaddition can also be discarded. The five-membered ring intermediate is unstable to cycloreversion and its isomerization to the final dioxadiazinane involves a high free energy barrier (68.6 kcal mol(-1)). Calculations also show that the dimerization process is slower in dichloromethane than in the gas phase owing to the larger polarity of nitrones and that inclusion of diffuse functions at the studied level does not modify the observed results. The apparently disfavoured [3 + 3] dimerization of nitrones can actually be explained as a bispseudopericyclic [2n2π + 2n2π] process in which the favourable FO interactions between the nitrone oxygen and the C=N π* bypass the WH-forbidden process.
authors.affiliation: Laboratorio de Síntesis Asimétrica, Departamento de Síntesis y Estructura de Biomoléculas, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Universidad de Zaragoza, CSIC, E-50009 Zaragoza, Aragón, Spain. pmerino@unizar.es.
authors.forename: David
authors.initials: D
authors.lastname: Roca-López
authors.forename: Tomás
authors.initials: T
authors.lastname: Tejero
authors.forename: Pierluigi
authors.initials: P
authors.lastname: Caramella
authors.forename: Pedro
authors.initials: P
authors.lastname: Merino
mesh_terms: D003500:Cyclization; D019281:Dimerization; D015394:Molecular Structure; D009589:Nitrogen Oxides / Q000138:chemical synthesis* / Q000737:chemistry; D011789:Quantum Theory
chemical_list: D009589:Nitrogen Oxides; C015388:nitrones