Markovnikov photohydration

Their detailed mechanistic studies revealed that the efficiency of photohydration is independent of the solution pH in the 0 to 7 range. Because the pK of 30 is expected to lie within this range, it was concluded that the photo hydration did not occur via initial protonation of the acetylene by aqueous acid, but instead via initial ESIPT from the phenol to generate quinone methide (35) (or 36 from 33). This quinone methide is trapped by water to give the observed Markovnikov photohydration product. This was the first clear demonstration of ESIPT leading to an irreversible photochemical reaction. 

Acid-catalyzed photohydration of styrenes19 and additions to cyclohexenes20 leading exclusively to the Markovnikov products are also possible. Sensitized photoaddition, in contrast, results in products from anti-Markovnikov addition. The process is a photoinduced electron transfer21 taking place usually in polar solvents.22,23 Enantiodifferentiating addition in nonpolar solvents has been reported.24 The addition of MeOH could be carried out in a stereoselective manner to achieve solvent-dependent product distribution 25... 

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. 

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. 

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