Photohydration of Aromatic Alkenes and Alkynes

Article abstract of DOI:10.1021/ja00373a029

The photohydrations of aromatic alkenes and alkynes 2-12 have been studied in aqueous sulfuric acid.For the nonnitro-substituted substrates 2-8, the products are the compounds obtained via Markovnikov addition of water to the alkene or alkyne moiety.Nitro-substituted styrenes 11 and 12 and phenylacetylenes 9 and 10 gave anti-Markovnikov addition products.Product quantum yields (Ψ_pdt) generally show a strongly sigmoidal dependence on the activity of the medium, with the exception of the nitrostyrenes, where no such dependence was observed.For compounds 2-8, quenching of fluorescence with increasing acidity was observed, and the proposed photohydration mechanism involves a rate-limiting protonation step on the first excited singlet state (S₁), while for the nitrostyrenes, a rate-limiting attack of water on T₁ is consistent with the available data.A mechanism involving a concerted addition of H3O⁺ to T₁ is proposed for the photohydration of (nitrophenyl)acetylenes 9 and 10.The solvent isotope effect on the product quantum yield (Ψ_H/Ψ_D) is consistent with these proposals.Photoprotonation rate constants (k_H) have been obtained - via transient and steady state measurements - for substrates 2, 4-6, and 8.Alkenes are approximately ane order of magnitude less reactive than alkynes to photoprotonation, both of which are 10¹¹ - 10¹⁴ times more reactive than the corresponding ground-state molecules.