Styrenes photohydration

The rate of acid-catalyzed hydration of styrenes can be enhanced photochemically (Scheme 2).293,294 Except for nitro-substituted styrenes, such photohydrations undergo exclusive Markovnikov addition. 

Analogous to the photohydration of styrenes, the acid-catalyzed hydration of phenylalkynes is also possible photochemically.293 294 307 Except for nitro-substituted arylalkynes, the reaction follows Mar-kovnikov s rule. This reaction has thus far received limited synthetic attention. 

Addition Beactions.- The photohydration of substituted styrenes and phenyl acetylenes has been studied in acid nediun. - Diyne (1) is also photochenically reactivetirradiation at 300 nm) with methanol affording the addition product (2) as a mixture of Z and E isomers. The isomeric products (2) are photochemically labile and are interconverted on prolonged irradiation affordina a photostationary mixture. ... 

McEwen, J., Yates, K., Photohydration of Styrenes and Phenylacetylenes General Acid Catalysis and Brpnsted Relationships, J. Am. Chem. Soc. 1987, 109, 5800 5808. 

For a review, see Wan, P. and Yates, K., Photogenerated carbonium ions and vinyl cations in aqueous solution. Rev. Chem. Intermed., 5, 157, 1984 See also McEwen, J. and Yates, K., Photohydration of styrenes and phenylacetylenes. General acid catalysis and Bronsted relationships, /. Am. Chem. Soc., 109, 5800, 1987 3-Nitrostyrenes undergo anti-Markovnikov addition Wan, R, Davis, M.J. and Teo, M.-A., Photoaddition of water and alcohols to 3-nitrostyrenes. Structure-reactivity and solvent effects, /. Org. Chem., 54,1354,1989. 

Later work by Fischer and Wan > demonstrated that meta (37 and 38) and para (39) hydroxy-substituted styrenes are also active towards photohydration ( = 0.22, 0.24 and 0.1, respectively). Evidence for formation of quinone methide intermediates was provided by nanosecond LFP. Both steady-state and time-resolved fluorescence quenching of 37 by water showed a cubic dependence on water concentration. This was attributed to an initial ESIPT process in which a three-water-molecule bridge mediates a proton transfer from the phenol to the basic alkene. 

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