Molecular structures of diarylcyclobutanes associated with reactivities in ring-cleavage reactions: Implications of conformationally controlled through-bond coupling.

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Molecular structures of diarylcyclobutanes (1–6) have been determined by single-crystal X-ray analyses. Although the distances and angles of the cyclobutane rings are relatively normal, the torsional angles between the aryl ring and the plane involving the ipso carbon and the aryl-substituted carbons of cyclobutane ring are close to 90° in cases of 1–4, but very small (2.3–11.3°) in cases of 5 and 6; conformations of the aryl substituents are highly restricted by the rigid tricyclic structure or by nonbonding interactions between the vicinal substituents in the cis configuration. Restricted conformations are closely related to the different reactivities of the cyclobutane in photochemical, thermal, and radiation-induced ring-splitting reactions. The structure-reactivity relationships are discussed in terms of the conformation-controlled π–σ orbital interactions associated with the capabilities of the π electron systems in through-bond coupling.
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