Quinone methides hydration reaction

The oxidation by strains of Pseudomonas putida of the methyl group in arenes containing a hydroxyl group in the para position is, however, carried out by a different mechanism. The initial step is dehydrogenation to a quinone methide followed by hydration (hydroxylation) to the benzyl alcohol (Hopper 1976) (Figure 3.7). The reaction with 4-ethylphenol is partially stereospecific (Mclntire et al. 1984), and the enzymes that catalyze the first two steps are flavocytochromes (Mclntire et al. 1985). The role of formal hydroxylation in the degradation of azaarenes is discussed in the section on oxidoreductases (hydroxylases). 

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... 

Cholest-4-en-3/3-ol and its 5a-A6- and 5a-A7-isomers gave complex mixtures of oxidation products on treatment with 60Co y-radiation in air. Reaction paths are discussed and compared with those occurring in photosensitized and free-radical oxidations.119 1-Methyl-11-oxo-oestrone (107) is converted by DDQ in 1% aqueous dioxan into the 9/3-hydroxy-derivative (108), apparently by hydration of an intermediate quinone methide.120... 

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. 

Chiang, Y. Kresge, J. Zhu, Y. Flash photolytic generation of o-quinone a-phenylmethide and o-quinone oc-(p-anisyl)methide in aqueous solution and investigation of their reactions in that medium. Saturation of acid-catalyzed hydration. J. Am. Chem. Soc. 2002, 124, 717-722. 

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