Photogeneration and chemistry of o-quinone methides
Date
1994
Authors
Yang, Cheng
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Abstract
The photochemistry of several o-hydroxybenzyl alcohols has been studied in aqueous alcohol solution. The aim of this investigation is to achieve a comprehensive understanding of the photochemical behaviour of these substrates in aqueous solution, which may involve synthetically and biochemically important o-quinone methides (o-QM) as critical intermediates.
In general, photolysis of these compounds in aqueous alcohol solutions results in an efficient photosolvolysis to give the corresponding ethers. Photolysis of the substrates in the presence of electron-rich dienophiles results in a regiospecific [4 + 2] Diels-Alder type addition, to give the corresponding 2-substituted chroman derivatives in good yields, implicating the involvement of o-QM intermediates. Product quantum yield measurements indicate that the formation of o-QM's is very efficient. It is estimated that Φₚ = 0.29 for formation of the parent o-QM (1) and Φₚ = 0.58 for formation of the phenyl substituted o-QM (113), from the corresponding benzyl alcohols in 50% CH₃CN-H₂0. The mechanism of reaction is believed to involve adiabatic deprotonation of the phenol form of the substrate upon excitation to S1, to generate the phenolate ion in the singlet excited state, which subsequently reacts by dehydroxylation, to give the reacting transoid-o-QM. The transiod-o-QM is then trapped by alcohol to give a Michael type addition product, or trapped by H₂0 to regenerate the starting material. It will also react with electron-rich dienophiles in a formal endo addition manner to give the corresponding [4 + 2] cycloaddition products.
The photosolvolytic behaviour of o-aminobenzyl alcohol has also been studied in aqueous solution over a range of pH and in moderately concentrated aqueous H₂SO4. Although reactive at all pH's studied, clean solvolytic reaction was observed only when pH < pKₐ. The reaction in acidic medium is believed to proceed via adiabatic deprotonation of the protonated substrate in S₁ to give the excited free amine, which subsequently dehydroxylates to give protonated o-QMI (or dehydrates to give free o-QMI) as the reactive intermediate. Nucleophilic attack by added ROH gives the photosolvolytic product.