Photochemical Reactions of Chloranil with Cyclooctene, 1,5-Cyclooctadiene, and Cyclohexene Revisited

Depending on the reaction conditions, excited chloranil ((3)CA) and cis-cyclooctene (cis-CO) furnished three types of products, which all contain carbocyclic four-membered rings. The illumination of a solution in benzene by light bulbs gave rise to the all-cis 1:1 cycloadduct (11a) in 76% yield. On its part, this compound reacted with cis-CO under the action of better sources for UV light with the formation of three isomeric 1:2 cycloadducts. Two of them underwent a photochemical dechlorination leading to a cyclobutene derivative. Albeit rather sluggishly, (3)CA and trans-cyclooctene (trans-CO) brought about an unsymmetrical 1:1 and a 1:2 cycloadduct. By contrast, excited 11a reacted smoothly with trans-CO and afforded a 72% yield of the unsymmetrical 1:2 cycloadduct that resulted from (3)CA and cis-CO in addition to two symmetrical 1:2 cycloadducts. Also, excited 11a and cyclohexene (CH) furnished a bis(cyclobutane) derivative, a significant portion of which was subject to a dechlorination to give a cyclobutene derivative. cis,cis-1,5-Cyclooctadiene (COD) and CH were transformed by (3)CA to 1:1 and 1:2 cycloadducts, analogous to cis- and trans-CO. In addition to this, (3)CA suffered photoreduction in the presence of COD and CH. The [2+2] cycloaddition onto a carbonyl group of (3)CA with formation of a spirooxetane occurred to a small extent only with COD, but not at all with cis- and trans-CO and CH.