Photochemical cleavage reactions of 2-amino-1,4-benzoquinones and alpha-keto amides
Irradiation of 2-amino-1,4-benzoquinones with leaving groups can expel the leaving group (carboxylates and phenolates) upon exposure to visible light. 2-Pyrrolidino-substituted 3,6-dimethyl-1,4-benzoquinones photocyclize to give benzoxazolines quantitatively with quantum yields of 0.09-0.10 in CH 2 Cl 2 , 0.02-0.04 in CH 3 CN, and <0.01 in 30% aq CH 3 CN. Lifetimes for expulsion of various 4-YC 6 H 4 OH in 30% phosphate buffer in CD 3 CN (pD 7) at 17°C are 13.1 h, 0.54 h, and 0.13 h for Y = H, CF 3 , CN, respectively, and log k vs. -pKa is linear. From the plot lifetimes for expulsion of carboxylic acids from the more labile benzoxazolines are estimated to be 360 ms, 89 ms, and 19 ms for CH 3 CO 2 H, PhCO 2 H, and 4-CNC 6 H 4 CO 2 H, respectively. Upon exposure to visible light 2-diethylamino-1,4-benzoquinones expel the leaving groups (carboxylates) with low quantum yields. The photochemical cleavage and release of para -substituted phenols was investigated with α-keto amides. In aqueous media α-keto amides, 4-YC 6 H 4 OCH 2 COCON(R)CH(R')CH 3 (R = Et, R' = H; R = iPr, R' = Me), photochemically expel of 4-YC 6 H 4 OH (Y = CN, CF 3 , H) with formation of 5-methyleneoxazolidin-4-ones. Quantum yields range from 0.2-0.3. The studies support a mechanism involving transfer of hydrogen from an N-alkyl group to the keto oxygen to produce zwitterionic intermediates that eliminate the para -substituted phenolate leaving groups. The quantum yields for photoelimination decrease in CH 3 CN, CH 2 Cl 2 , or C 6 H 6 due to competing cyclization of to give oxazolidin-4-one products which retain the leaving group 4-YC 6 H 4 O - (Y = H, CN).
"Photochemical cleavage reactions of 2-amino-1,4-benzoquinones and alpha-keto amides"
(January 1, 2006).
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