Photohydration of Aromatic Alkenes. Catalytic Phenomena and Structure-Reactivity Studies

Article abstract of DOI:10.1021/jo00154a024

The photohydrations of aromatic alkenes 3-14 have been studied in aqueous sulfuric acid.Aromatic alkenes with substituents that have electron donating effects equal to or greater than that of hydrogen (3-7 and 14) undergo photohydration via S₁ if other photoprocesses do not compete favorably.The electron-donating or -withdrawing abilities of substituents in S₁ do not necessarily reflect their ground-state ? values.For example, the meta-fluoro group is strongly electron withdrawing (?⁺ = 0.35) in the ground state, but evidence presented in this work suggests it is much less electron withdrawing (or in fact even becomes electron donating) in S₁.The opposite appears to be true for the para-fluoro substituent.It is shown that photohydration can be water, hydronium ion, or general acid catalyzed.The observation of general acid catalysis supports the proposed mechanism involving rate-limiting proton transfer to S₁.The individual catalytic rate constants for photoprotonation obey the Broensted catalysis law, with an α value of ca. 0.15, suggesting early transition states in these photoprotonations.