Photochemical and Thermal Hydrations of Aromatic Allenes. Evidence for Allyl and Vinyl Cation Intermediates

Article abstract of DOI:10.1021/jo00183a005

The photohydration of aromatic allenes has been studied in water and in dilute aqueous sulfuric acid (0-25percent H2SO4).It was found that phenylallene (2a) and α-methylphenylallene (2b) undergo photohydration to give the corresponding cinnamyl alcohol derivatives.The proposed mechanism involves protonation of the central carbon of phenylallenes in their singlet excited state (S1).In contrast, (p-nitrophenyl)allene (2g) and (m-nitrophenyl)allene (2h) gave the corresponding (nitrophenyl)acetones.The mechanism for the formation of these ketones is believed to be due to initial protonation of the α- or γ-carbon atom of the (nitrophenyl)allene in the triplet excited state (T1).Phenylallenes with CN, CF3, and F substituents failed to photohydrate.The results of these photohydrations were compared with those of the much slower thermal hydrations in 70-83percent H2SO4.Phenylallenes 2a and 2b cyclized to the corresponding indene derivatives in 70percent H2SO4.Phenylallenes with NO2-substituents (2g-2h) underwent hydration to give the corresponding (nitrophenyl)acetones (expected products) and (nitrophenyl)-1-propanones (unexpected products) in 83percent H2SO4.Similar results were obtained with (m-cyanophenyl)allene.The formation of both types of ketone is discussed in terms of the intermediacy of a common vinyl cation intermediate.