Photoactivation of Amino-Substituted 1,4-Benzoquinones for Release of Carboxylate and Phenolate Leaving Groups Using Visible Light

Upon exposure to visible light, 2-pyrrolidino-substituted 3,6-dimethyl-1,4-benzoquinones photocyclize
to give benzoxazolines with quantum yields of 0.07−0.10 in CH2Cl2, 0.02−0.04 in CH3CN, and <0.01
in 30% aq CH3CN. With carboxylate or phenolate leaving groups incorporated via coupling to a
5-hydroxymethyl group of the quinones, the photocyclizations give benzoxazolines that eliminate the
leaving groups in a dark reaction. Lifetimes for elimination of 4-YC6H4OH in 30% phosphate buffer in
CD3CN (pD 7) at 17 °C are 13.1, 0.54, and 0.13 h for Y = H, CF3, and CN, respectively, and the linear
equation log k (h-1) = 0.998(−pKa) + 8.80 gives a best fit to the data. Carboxylate leaving groups are
rapidly eliminated upon photolysis of the quinones in aq CH3CN to produce an o-quinone methide
intermediate that is trapped by 4 + 2 cycloaddition with unreacted starting material or with added
3-(dimethylamino)-5,5-dimethyl-2-cyclohexen-1-one. The ortho-quinone methide is observed at 339 and
455 nm by conventional absorption spectroscopy and gives a pseudo-first-order fit of the decay kinetics
with τ1/2 = 34.9 min in 30% phosphate buffer in CH3CN at 20 °C.