Phenylacetylene photohydration

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

Chiang, Y., Kresge, A. J., Capponi, M., Wirz, J., Direct Observation of Acetophenone Enol Formed by Photohydration of Phenylacetylene, Helv. Chim. Acta 1986, 69, 1331 1332. 

A study of the photohydration of the alkynes 11 and 12 in sulphuric acid has shown that only the normal Markownikov products, in this instance in the keto form 13, are formed. The mechanism, as proposed by Yates and collaborators involves a rate-limiting protonation of the Si state of the alkyne. This leads to the cation 14, solvation of which affords the enol of the final product. Associated with this, and providing spectroscopic substantiation, is the report of the direct observation of the enol of acetophenone during a study of the photohydration of phenylacetylene The introduction of nitro substituents as in 15 leads to abnormal Markownikov addition products. Excitation in these systems results in a reversal of polarity in the excited state, consistent with the... 

One method of generating enols involves photohydration of phenylacetylene, while another method utilizes the Norrish Type n cleavage of y-hydroxybutyrophenone. Photochemical reactions will be discussed in Chapter 12. 

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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