Control of reaction pathways in the photochemical reaction of a quinone with tetramethylethylene by metal binding

Abstract

The present study reports a novel supramolecular photochemical reaction that focuses on the direct electronic interactions between a host reaction substrate and guest metal salts. The reaction pathways in the photochemical reactions of quinone derivatives bearing a methoxy group and a long oligoether sidearm QE_n (n = 0 and 3) with tetramethylethylene (TME) are changed upon noncovalent complexations of the host reactant with alkali and alkaline earth metal ions and a transition metal salt. The photochemical reaction of QE_n with TME provides a mixture of [2 + 2] cycloadducts 1aE_n and 1bE_n, hydroquinone H₂QE_n, and monoallyl ether adducts of hydroquinones 2aE_n and 2bE_n. The photochemical reaction proceeds by the photoinduced electron transfer mechanism, where photoirradiation brings about formation of a radical ion pair [QE_n˙⁻, TME˙⁺] as the primary intermediate. We found that the yields and selectivity of these photoproducts are changed upon electronic interactions of QE_n˙⁻ with the metal salts. The photochemical reaction in the absence of metal salts provides H₂QE_n as its major product, whereas QE₃, having the long sidearm, dominantly produces 2aE₃ at the expense of 1aE₃, 1bE₃, and H₂QE₃ when it forms a size-favorable host–guest complex with divalent Ca²⁺. In contrast, QE_n selectively provides oxetanes 1aE_n and 1bE_n in the presence of Pd(OAc)₂, which can form complexes with the quinone through metal–olefin and coordination interactions in the ground and photoexcited states of the quinone.