Photohydration reaction of 1-Aryl-5,5-dimethyl-1,3-hexadiynes

Abstract

The photohydration reaction of 1-phenyl-5,5-dimethyl-1,3-hexadiyne (PDHD) in 10% aq. sulfuric acid gives two acetylenyl ketones and one allenyl ketone compound. The effect of 9-fluorenone-1-carboxylic acid on photohydration of PDHD shows the curved plot in Stern-Volmer plot indicating that the three photohydration products are formed via both singlet and triplet excited states. The plot of quantum yields versus medium acidity show a sigmoidal curve suggesting that the acetylenyl ketone compounds are produced by the mechanism in which the protonation step is the rate-limiting step. On the other hand, the allenyl ketone compound shows the maximum efficiency at $H_0=-0.7$ as medium acidity is increased. The results suggest the synchronous addition of water with protonation. Irradiation of 1-(p-nitrophenyl)-5,5-dimethyl-1,3-hexadiyne (p-NDHD) in aqueous 10% sulfuric acid yields two conjugated allenyl ketones and two β-dicarbonyl compounds. It is found from the kinetic studies that the p-NDHD is initially converted into two allenyl ketones and two β-dicarbonyl compounds are produced as secondary photoproducts. Treatment of the primary photoproducts with 10 % sulfuric acid yields the secondary products very rapidly, suggesting that the secondary hydration is a thermal process. The fast thermal hydration of allenyl ketones is probably due to the fast 1,4-addition of water as in α,β-unsaturated ketones. The oxygen effects on the photohydration of p-NDHD and linear Stern-Volmer plot on 9-fluorenone-1-carboxylic acid quenching strongly support that the triplet excited states of p-NDHD are involved in the photohydration mechanism. The maximum efficiency at $H_0=-1.0$ is observed in the photohydration quantum yield measurements as a function of acidity for p-NDHD. The suggested mechanism involves the nucleophilic attack of water, synchronous with proton transfer. Irradiation of 1-(m-nitrophenyl) -5,5-dimethyl-1,3-hexadiyne (m-NDHD) in 5% sulfuric acid gives two hydration p...