Photoreaction of 1,4-Disubstituted-1,3-Butadiyne with Alcohol

Irradiation of 1,4-diphenyl-1,3-butadiyne and 5,5-dimethyl-1-phenyl-1,3-hexadiyne with methanol yields 1:1 polar addition products, [E]- and [Z]-1,4-diphenyl-1-methoxy-1-buten-3-yne and [E]- and [Z]-5,5-dimethyl-1-methoxy-1-phenyl-1-hexen-3-yne, respectively. These geometrical isomers were converted into each other reaching the photostationary state on irradiation with 300 nm UV light. The photoaddition reaction of 1,4-diphenyl-1,3-butadiyne and 5,5-dimethyl-1-phenyl-1,3-hexadiyne with methanol seems to proceed from the zwitterionic lowest excited state.