Quinone methides photochemical generation

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. 

Nakatani, K. Higashida, N. Saito, I. Highly efhcient photochemical generation of o-quinone methide from Mannich bases of phenol derivatives. Tetrahedron Lett. 1997, 38, 5005-5008. 

The combination of neutral non-aromatic and zwitterionic aromatic contributing valence bond structures confers a distinctive chemical reactivity to quinone methides, which has attracted the interest of a tremendous number of chemist and biochemists. This chapter reviews reactions that generate quinone methides, and the results of mechanistic studies of the breakdown of quinone methides in nucleophilic substitution reactions. The following pathways for the formation of quinone methides are discussed (a) photochemical reactions (b) thermal heterolytic bond... 

This chapter will focus on o- and p-quinone methides and will be divided into two parts. The first will present methods for generating quinone methides in photochemical and solvolysis reactions and will emphasize how the structure and stability of quinone methides dictates the pathways for their formation. The second section will discuss the results of experiments to characterize the reactivity of quinone methides with nucleophilic reagents, and the broader implications of these results. The scope of this presentation will reflect our interests, and will focus on studies carried out at Buffalo. We considered briefly writing a comprehensive chapter on quinone methides, but abandoned this idea when we learned of Steven Rokita s plans to edit a 12-chapter text, which presents an extremely comprehensive coverage of the chemistry and biochemistry of quinone methides.9... 

Absorption of a photon in the UV spectral region may lead to generation of electrophilic species by fast heterolytic bond cleavage at the photochemically excited state.10 Quinone methides are readily accessible through reactions of such photochemical excited states.11,12 This section outlines photochemical pathways for the generation of quinone methides. 

The various methods of generating o-quinone methides,4-5 including the thermal or (Lewis) acid-catalyzed elimination of a phenol Mannich base,149 150-160-161163 the thermal or (Lewis) acid-catalyzed dehydration of an o-hydroxybenzyl alcohol (ether),147-149-151-153-156-157-162-163-165-168 171-175-178-183 the thermal 1,5-hydride shift of an o-hydroxy styrene,171-173 175 178-183 the thermal dissociation of the corresponding spirochromane dimer,158 163-164,166 oxidation of substituted o-alkylphenols,152-170 and the thermal or photochemical-promoted cheletropic extrusion154-155 159 of carbon monoxide, carbon dioxide, or sulfur dioxide (Scheme 7-III), as well as their subsequent in situ participation in regiospecific, intermolecular [4 + 2] cycloadditions with simple olefins and acetylenes,147 149-151 152 153159 162-164... 

Scheme 7.57 Photochemical and thermal generation of o-quinone methide (o-QM).

Their detailed mechanistic studies revealed that the efficiency of photohydration is independent of the solution pH in the 0 to 7 range. Because the pK of 30 is expected to lie within this range, it was concluded that the photo hydration did not occur via initial protonation of the acetylene by aqueous acid, but instead via initial ESIPT from the phenol to generate quinone methide (35) (or 36 from 33). This quinone methide is trapped by water to give the observed Markovnikov photohydration product. This was the first clear demonstration of ESIPT leading to an irreversible photochemical reaction. 

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