Photodecarboxylation of benzoyl-substituted biphenylacetic acids and photo-retro-Aldol reaction of related compounds in aqueous solution. Acid and base catalysis of reaction

To further explore the structural effects on efficiency of photodecarboxylation of Ketoprofen-type molecules the photodecarboxylation of two “extended” benzoyl-substituted acetic acids, 3′-benzoyl and 4′-benzoyl-4-biphenylacetic acids (2 and 3, respectively) have been studied in aqueous solution. In these systems, the benzoyl or benzophenone activating chromophore is distal to the phenylacetic acid moiety, separated by a phenyl spacer. Both of these compounds were found to undergo efficient (Φ = 0.3–0.7) photodecarboxylation in solutions of various pHs, giving rise to very broad long wavelength transient absorptions in laser flash photolysis studies (at pH > pKa) that have been assigned as the corresponding highly conjugated benzylic carbanions/enolates. The photodecarboxylation of the corresponding parent benzophenone systems was also studied as a function of pH for comparison as are two photo-retro-Aldol reactions of benzophenone alcohols. Photo-retro-Aldol reaction requires base-catalysis but surprisingly, photodecarboxylation can also be mediated in acidic pH for some of the studied phenylacetic acids. The acid-catalyzed pathway requires prior protonation of the photoexcited benzophenone oxygen and has precedent in the acid catalyzed photohydration and intramolecular photoredox chemistry of benzophenones. These and other new mechanistic insights discovered in this work show that the benzophenone chromophore can activate decarboxylation over much longer molecular distances as well as induce new types of ionic chemistry.