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

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 H(2)QE(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)(center dot-), TME center dot+] as the primary intermediate. We found that the yields and selectivity of these photoproducts are changed upon electronic interactions of QE(n)(center dot-) with the metal salts. The photochemical reaction in the absence of metal salts provides H(2)QE(n) as its major product, whereas QE(3), having the long sidearm, dominantly produces 2aE(3) at the expense of 1aE(3), 1bE(3), and H(2)QE(3) when it forms a size-favorable host-guest complex with divalent Ca2+. In contrast, QEn 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.