Photochemical Reactions. Part 64 [1]. The photochemistry of 3‐alkyl‐cyclopent‐2‐en‐ones

On ultraviolet irradiation in toluene, cyclopent‐2‐en‐one (1a) and 3‐methylcyclopent‐2‐en‐one (1b) undergo dimerisation (→ 3a, b, 4b), whereas 3‐t‐butyl‐cyclopent‐2‐en‐one (1e) and the bicyclo [3.2.1]octenone 2a only react with the solvent (→ 5e–7e, 8–10). The 3‐ethyl‐(1c) and 3‐isopropyl‐cyclopent‐2‐en‐ones (1d) afford by dimerisation products 3c, d, 4c and by reaction with the solvent 5c, d, 6c, d, 7d. The β‐methoxy‐enones 1f and 2b are unreactive under comparable irradiation conditions. The head‐to‐head cyclobutane dimer 3b (HH) by separate irradiation at < 3400 Å in toluene, is reconverted to 1b. By similar irradiation each of the head‐to‐tail dimers 3b (anti‐ and syn‐HT) and also the unsaturated dimer 4b give a new isomer of unknown structure besides small amounts of monomer 1b and relatively large amounts of insoluble material. The photoreactions of 1b‐e can be quenched by naphthalene. Stern‐Volmer plots for the quenching of the dimers of 1b, 1c, and 1d, and of 3‐t‐butylcyclopentanone (5e) and the dihydro dimer 7e are linear and within each experiment, the slopes satisfactorily coincide, whereas the slope for the α‐benzyl ketone 6c is distinctly greater than that for the dimers 3c. A similar differentiation, although less pronounced, is found between the benzyl ketones 6d and 6e and the respective accompanying products.