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Issue 3, 2014
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[2n2π + 2n2π] Cycloadditions: an alternative to forbidden [4π + 4π] processes. The case of nitrone dimerization

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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[double bond, length as m-dash]N π* bypass the WH-forbidden process.

Graphical abstract: [2n2π + 2n2π] Cycloadditions: an alternative to forbidden [4π + 4π] processes. The case of nitrone dimerization

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Publication details

The article was received on 07 Oct 2013, accepted on 05 Nov 2013 and first published on 07 Nov 2013


Article type: Paper
DOI: 10.1039/C3OB42014K
Citation: Org. Biomol. Chem., 2014,12, 517-525
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    [2n2π + 2n2π] Cycloadditions: an alternative to forbidden [4π + 4π] processes. The case of nitrone dimerization

    D. Roca-López, T. Tejero, P. Caramella and P. Merino, Org. Biomol. Chem., 2014, 12, 517
    DOI: 10.1039/C3OB42014K

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