Bergman reaction, computational study

Extensive mechanistic studies of this cyclization reaction were carried out by Myers et al. and extended with theoretical work by Squire s et al. It is known that, in contrast to the Bergman cyclization of the ene-diyne (Chapter 4.2), this transformation proceeds as an exothermic process determined by the increased stability of a benzyl radical versus a phenyl radical. The barrier for cyclization from substrate to a diradical product is low and can further be reduced by an appropriate substitution at the allenic terminus of the substrate. The dichotomous (polar and free radical) reactivity is observed on pyrolysis in the presence of polar reactants. Both radical and polar products arise from a common intermediate, which is described as a polar diradical, a linear combination of limiting structure 7 and zwitterion 11. According to Squires, polar diradical singlet species are involved. Based on computational studies supported by experimental product distribution studies, it has been proposed that both the diradical 7 and... 

Quantum chemical studies of cyclizations of enediynes and enyneallenes have been reviewed.180 The intermediates are computationally tractable as a result of the unrestricted broken spin symmetry (UBS) approach using GGA functionals for the description of open-shell biradicals. The intermediacy of biradicals in Cope-type rearrangements, to which the Bergman and Myers-Saito reactions belong, are shown to be predictable using a very simple rule biradicals are likely to be intermediates if they are stabilized either by allyl resonance or by aromaticity. 

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