Photogenerate quinone methides

Much attention has been devoted to the development of methods to generate quinone methides photochemically,1,19-20 since this provides temporal and spatial control over their formation (and subsequent reaction). In addition, the ability to photogenerate quinone methides enables their study using time-resolved absorption techniques (such as nanosecond laser flash photolysis (LFP)).21 This chapter covers the most important methods for the photogeneration of ortho-, meta-, and para-quinone methides. In addition, spectral and reactivity data are discussed for quinone methides that are characterized by LFP. 

The ability to photogenerate quinone methides via ESIPT from a phenol OH to an aromatic carbon atom was explored for a wider number of substrates by Wan s... 

Colloredo-Melz, S. Dorr, R. T. Verga, D. Freccero, M. Photogenerated quinone methides as useful intermediates in the synthesis of chiral BINOL ligands. J. Org. Chem. 2006, 71, 3889-3895. 

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 s group investigated a number of phenyl-substituted hydroxybiphenylbenzyl alcohols in the hope that the a-phenyl quinone methides photogenerated from them might show enhanced absorption and lifetimes, and thus be easier to characterize by LFP.37,38 They were successfully able to photogenerate and characterize quinone... 

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... 

Using the same reaction, Cole and Wan later photogenerated mcta-quinone methides 110-112 from the appropriate m-hydroxy-a-phenylstyrenes.78 The three were characterized by LFP, with 110 and 111 showing strong and sharp absorptions at 430 and 450 nm, respectively, similar to those observed for 104 and 105. The dimethoxy-susbstituted 112 showed a much broader absorption centered at 420 nm. Addition of the electron-donating methoxy groups stabilized the quinone methides the lifetimes of 110-112 in 1 1 aqueous acetonitrile were 5-200 times longer than that of 104. 

Shi, Y. Wan, P. Solvolysis and ring closure of quinone methides photogenerated from biaryl systems. Can. J. Chem. 2005, 83, 1306-1323. 

Xu, M. Lukeman, M. Wan, P. Photogeneration and chemistry of biphenyl quinone methides from hydroxybiphenyl methanols. Photochem. Photobiol. 2006, 82, 50-56. 

Brousmiche, D. W. Wan, P. Photogeneration of an o-quinone methide from pyridoxine (vitamin B6) in aqueous solution. J. Chem. Soc., Chem. Commun. 1998, 491 -92. 

Nakantani et al. have since shown that o-quinone methides can also be readily photogenerated from Mannich bases of o-substituted phenols for example. 

Brousmiche, D. and Wan, R, Photogeneration of quinone methide-type intermediates from pyri-doxine and derivatives, J. Photochem. Photobiol. A Chem., 149, 71, 2002. 

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