Thermal ring-splitting reactions of diarylcyclobutanes: Significance of steric effects on orbital interactions in transition states and biradical intermediates.

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Regiochemistry and reactivities in the thermal ring-splitting reactions of diarylcyclobutanes (1–5) have been studied and shown to depend on the stable conformations and rotational mobilities of the aryl substituents. The reactions of 1 and 2 result in a regiospecific symmetric cleavage to give indene or styrene along with significant isomerization of 2 to 3. In the cases of 3–5 both the symmetric and unsymmetric cleavages competitively occur with decreasing symmetric-to-unsymmetric ratios with an increase in methyl substitution. The olefin products from 4 are mixtures of cis- and trans-2-butene, cis- and trans-β-methylstyrene, and trans-stilbene. Thermochemical analyses combined with product analyses indicate that the symmetric cleavage of 1 and the unsymmetric cleavage of 3 proceed with a concerted mechanism, whereas 1,4-biradicals are involved in the other reactions. Structure-reactivity relationships of the present reactions are discussed in terms of mixing of the σ* character in a bonding MO by specific π–σ* interactions, depending on the conformational situations of the aryl groups and in terms of the steric effects which destabilize 1,4-biradicals as well as transition states of the biradical fragmentation to the olefins.
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