Biradical intermediate: Cope

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. 

From a theoretical point of view, in multi-bond reactions it is essential to use a wavefunction where the possibility of biradical and aromatic transition states can be treated with a balanced level of accuracy. Thus the MC-SCF results of Morokuma et al [21] on the Cope rearrangement of the "model" reaction of 1,5 hexadiene are very convincing and provide reliable evidence that the lowest energy pathway for the "model" reaction is the synchronous one with the biradical intermediate lying 22 Kcal mole higher in energy than the synchronous transition state. 

While the Cope rearrangement is considered to be a concerted [3,3]-sigmatropic rearrangement, it displays a unique response to attached substituents, which suggest the involvement of biradical intermediates. 

Ultimately, these experimental studies cannot completely rule out biradical intermediates in the Cope rearrangement, especially for the heavily substituted systems. In the end, only theory can make a definitive statement about the structure of a transition state. The interplay between theory and experiment in the Cope rearrangement and other pericyclic processes has at times been contentious. In addition, differing theoretical models have often made diametrically opposed predictions (see the Going Deeper highlight on page 900). This further fueled the debate over the true nature of pericyclic transition states. 

The work with Porter on cyclic azo compounds coupled Bartlett s free radical studies into his ambitious effort to delineate the border between concerted and biradical cycloaddition mechanisms. Few demonstrations in chemistry have matched the clarity of his 1963 stereochemical proof with Montgomery that the 2 -f 2 cycloaddition of dichlorodifiuoro-ethylene to 2,4-hexadiene involves a biradical intermediate. Through the following decade Bartlett led the biradical parade. His long series of investigations on cycloaddition became particularly important in demonstrating nature s resourcefulness in coping with the supposedly inviolable constraints of orbital symmetry. 

We have also recently shown that Bergman (Scheme 8.7) and related reactions (not shown) [84, 85] of polyunsaturated hydrocarbons, with a 1,3,5-hexatriene skeleton form a branch inside a larger Cope reaction family characterized by a common 1,5-hexadiene structural unit. The examination of this whole family of reactions allowed us to derive a very simple rule for involvement of transient biradicals in Cope-like reactions of hydrocarbons A non-concerted reaction takes place when biradical intermediates are stabilized either by aUyl or aromatic resonance [84, 85]. 

The same is true for Cope rearrangements in general. Most substituents at C-1 and C-3 accelerate the reaction, and with more than one of them, their effects are cooperative. Substituents at C-2, however, shift the balance of the transition structure towards a biradical-like intermediate in which the new a bond is formed ahead of the old one breaking. Substituent effects at this site are not cooperative with substituent effects at C-l and C-3, because they change the nature of the transition structure rather than contribute to it in the same way. 

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