Photogeneration and chemistry of quinone methides from hydroxybenzyl alcohols




Diao, Li

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The photosolvolysis of a series of hydroxy-substituted benzyl alcohols (ArCH₂OH) has been studied. Photomethanolysis of these alcohols showed exceptionally higher efficiencies for methyl ether formation (in 1:1 H₂O-MeOH) than the corresponding methoxybenzyl alcohols. UV-Vis absorption spectra of photogenerated transients were recorded in aqueous solution and had similar appearance to the carbocations that are observable from the methoxybenzyl alcohols, but with much longer lifetimes. These transients were also observable in neat organic solution for the ortho isomers. The yields of all these transients increased with increasing water content. The highest yields were observed in basic aqueous solution when pH > pKₐ(S₀) (of the phenol moiety). In addition, photolysis of the appropriate coumaranones gave the same transient absorption as that from the corresponding o-hydroxybenzyl alcohols. Since photolysis of coumaranones are known to give o-quinone methide, (o-QM) intermediates (via loss of CO), the transients observed for the o-hydroxybenzyl alcohols are assigned to o-QMs. The transients observed for the m and p-isomers are assigned to the corresponding m and p-QMs. The quantum efficiencies for QMs generation from hydroxybenzyl alcohols are in the order as o > m >> p, in agreement with Zimmerman's orthoctivation theory. The mechanism proposed at pH < pKₐ(S₀) involves adiabatic deprotonation of the ArOH moiety in the first excited singlet state (S₁) followed by heterolytic cleavage of the C-O bond of the hydroxymethyl group. In basic media, direct excitation of the phenolate ion results in the loss of hydroxide ion to generate QMs. Subsequent nucleophilic trapping of these QMs by solvent results in the observed solvolysis product. o-QMs were also generated in neat organic solution. The proposed mechanism involves adiabatic deprotonation of phenol moiety facilitated by either intermolecular or intramolecular hydrogen bonding with a benzylic OH group. This Thesis has demonstrated that a simple and general method is available for the photogeneration of all the QM isomers. Notably, the method is applicable to m-QMs, which have previously required more elaborate methods for their generation. A polymerization reaction of m-QMs has been discovered in basic media during investigations of their chemistry.



Quinone, Photochemistry