Topochemical Photodimerization of 4‐(3‐Oxo‐3‐phenyl‐l‐propenyl)benzoic Acid and Its Esters

The photodimerization behaviors of 4‐(3‐oxo‐3‐phenyl‐1‐propenyl)benzoic acid (1) and its esters (2–7) in the crystalline state are investigated. These crystals, except that of the phenyl ester (7), dimerize photochemically to yield the corresponding cyclobutane derivatives in a typical topochemical fashion. Head‐to‐head type cyclobutane derivatives are obtained from crystals of the acid and ethyl ester (1 and 3), whereas head‐to‐tail type derivatives are formed from the other ester crystals (2, 4, 5, and 6). The temperature dependences of these reaction behaviors and the configurations of the photoproducts are correlated with the molecular arrangement in the reactant crystals. Reversible or irreversible behavior of as‐formed dimer crystals, derived from 2 or 4 crystals, is interpreted in terms of accumulated strain energy in the course of the topochemical process.