Hydroxybenzyl alcohol photodehydration

Wan s group showed that the observed photodehydration of hydroxybenzyl alcohols can be extended to several other chromophores as well, giving rise to many new types of quinone methides. For example, he has shown that a variety of biphenyl quinone methides can be photogenerated from the appropriate biaryl hydroxybenzyl alcohols.32,33 Isomeric biaryls 27-29 each have the benzylic moiety on the ring that does not contain the phenol, yet all were found to efficiently give rise to the corresponding quinone methides (30-32) (Eqs. [1.4—1.6]). Quinone methides 31 and 32 were detected via LFP and showed absorption maxima of 570 and 525 nm, respectively (in 100% water, Table 1.2). Quinone methide 30 was too short lived to be detected by LFP, but was implicated by formation of product 33 that would arise from electrocyclic ring closure of 30 (Eq. 1.4). 

Wan also studied hydroxybenzyl alcohols based on the naphthalene chromo-phore.40 Naphthols 50 and 51 were both examined for their ability to photogenerate naphthoquinone methides 52 and 53, respectively (Eqs. 1.12 and 1.13). While 50 underwent very efficient photosolvolysis, presumably via naphthoquinone methide 52, 51 was essentially unreactive when exposed to light. The inability of 51 to photogenerate 53 is a rare example where the generality of the photodehydration of benzyl alcohols fails. LFP of 50 yielded a very strong visible absorption (A x = 410 and 700 nm) that decayed in aqueous acetonitrile with a lifetime of 34 ps (Fig. 1.2). This transient was assigned to naphthoquinone methide 52 due in part to its efficient quenching when the nucleophilic ethanolamine was added. 

Through these works, Wan has conclusively demonstrated that the photodehydration of hydroxybenzyl alcohols is a general reaction, and a wide variety of quinone methides can be photogenerated and detected using this method. Quinone methide photogeneration via this method has been shown to have importance in the photochemistry of Vitamin B641,42 and in model lignins 43... 

The propensity of hydroxy-substituted biphenyls to undergo photodehydration in a manner like the hydroxybenzyl alcohols above (to give biphenyl-type quinone methides) was discovered during an investigation into the photoisomerization of xanthene (91) to 6H-dibenzo[fc,d]pyran (92) °... 

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