Liquid-phase photolysis. Part X. Formation of spiro-oxetans by photoaddition of olefins to p-benzoquinone
Irradiation of solutions of p-benzoquinone and olefins in benzene gives spiro-oxetans by 1,2-addition of a carbonyl group of the quinone. The following olefins react in this way: cyclohexene, cyclo-octene, cyclo-octa-1,5-diene, cyclo-octatetraene, bicycloheptadiene, oct-1-ene, and oct-2-ene. The adducts readily undergo acid-catalysed rearrangement to hydroxycoumarans, whilst catalytic hydrogenation leads to p-(2-hydroxyalkyl)phenols, and the starting materials are re-formed on pyrolysis, probably through the hydroxycoumarans. They can function as thermal dienophiles, but showed no tendency to undergo photoaddition of a second molecule of olefin.
Quantum yield determinations have shown that the order of photochemical efficiency for formation of the spirooxetan from cyclo-octene through the three relevant electronic transitions in p-benzoquinone is n→π*π→π*π→π**. It is concluded that an olefin →p-benzoquinone charge-transfer transition is not significantly involved in the photoaddition process, although there is evidence that polarisation complexing between the olefin and p-benzoquinone is important.