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 products (allenyl ketone and β-dicarbonyl compound). The photohydration of pure allenyl ketone compound gives the β-dicarbonyl compound, supporting that allenyl ketone compound is a primary photoproduct which yields β-dicarbonyl compound on further hydration. The photohydration of m-NDHD at 5℃ gives only a primary product, suggesting that the secondary hydration reaction is a thermal process.
The oxygen effect on the photohydration of m-NDHD indicates that the triplet excited states are involved in the rate-limiting step of photohydration of m-NDHD. The photohydration quantum yields as a fuction of acidity show the maximum efficiency at $H_0=-0.6$, indicating that the mechanism is the same as the phohydration of p-NDHD.
The photohydration of m-NDHD gives only the protonation product at $C_1$ carbon, and allenyl ketone is obtained as the hydration product. On the other hand, the thermal hydration of m-NDHD gives the acetylenyl ketone as the primary hydration product which is protonated at $C_4$ carbon. These results may be due to the enhanced electron withdrawing effect in the excited state of m-NDHD.
In comparison of p-NDHD with m-NDHD, photohydration of p-NDHD gives two photoproducts while only one photoproduct is obtained in m-NDHD. The results indicate that the electron withdrawing effect of m-NDHD is larger than that of p-NDHD. The effect is called as the meta effect.
Photoreaction of o-NDHD gives no photohydration product but the photocyclization product. Oxygen does not have any effect on the photoreaction of o-NDHD indicating that photocyclization proceeds via the singlet excited state. The UV spectral change as a function of irradiation time was monitored, and the logarithm of absorbance of remaining o-NDHD versus irradiation time shows linear plot. The result indicates that the reaction is a first-order reaction. A plausible mechanism involving radical intermediates is proposed.
