Transient quinone methide intermediate

Burnham, K. S. Schuster, G. B. A search for chiral photochromic optical triggers for liquid crystals photoracemization of l,l -binaphthylpyran through a transient biaryl quinone methide intermediate. J. Am. Chem. Soc. 1998, 120, 12619-12625. 

The prototype o-quinone methide (o-QM) and / -quinone methide (p-QM) are reactive intermediates. In fact, they have only been detected spectroscopically at low temperatures (10 K) in an argon matrix,1 or as a transient species by laser flash photolysis.2 Such a reactivity is mainly due to their electrophilic nature, which is remarkable in comparison to that of other neutral electrophiles. In fact, QMs are excellent Michael acceptors, and nucleophiles add very fast under mild conditions at the QM exocyclic methylene group to form benzylic adducts, according to Scheme 2.1.2a 3... 

FIGURE 6.18 Oxidation of ortAo-methylphenols to the corresponding ortho-quinone methide via transient zwitterionic intermediates that are stabilized by forming a complex 19 with the 2,5-dihydroxy[l,4]benzoquinone-derived bis(sulfonium ylide). 

Wan and his co-workers have reported the photohydration and photosolvolysis of the alkenes (45). The reactions involve the intermediacy of quinone-methide type intermediates. Flash photolysis of coniferyl alcohol (46) and isoeugenol (47) in acetonitrile has supplied evidence for the formation of the corresponding radical cations. These transient species react readily with water and other hydroxylic solvents to afford 4-vinylphenoxyl radicals. ... 

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