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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 degrees 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 tau1/2 = 34.9 min in 30% phosphate buffer in CH3CN at 20 degrees C.

Visible light irradiation of 5-acyloxymethyl- and 5-phenoxymethyl-2-pyrrolidino-1,4-benzoquinones effects photoisomerization to labile oxazolidines, which undergo elimination of carboxylate or phenolate leaving groups in high yields to generate trappable o-quinone methide intermediates. [reaction: see text]

l-@-cyanophenyl)-2-(pentamethyldisilanyl)cyclopentene (1) exhibits a fluorescence at h > 400 nm, which strongly shifts to the red with increasing polarity of the solvent. The corresponding solvatochromic plot is indicative of an emissive, lowest energy charge-transfer (CT) state. Stern-Volmer quenching of the fluorescence by dilute methanol in pentane gives a linear plot with a unit intercept and a slope kQrct = 0.70. Quenching is accompanied by formation of (arylcyclopenteny1)dimethylsilane 7, the major product observed upon photolysis of 1 in the presence of methanol. This is evidenced by the linear double-reciprocal plot of quantum yields of 7 versus methanol concentration, which gives a kQTct value in agreement with fluorescence quenching. In addition to nucleophilic attack by methanol at the terminal trimethylsilyl group to give 7, two minor photoproducts, (arylcyclopenteny1)trimethylsilane 6 and a tricyclic product 8 are observed, which are attributable to silylene extrusion and to homolytic S i S i bond cleavage, the latter process giving a silyl radical pair that cyclizes onto the ortho position of the aryl group with subsequent disproportionation. The Stern-Volmer plot of @'/a versus [MeOH] for 8 is curved, suggesting that both the locally excited (LE) and CT states are involved in the formation of this product. In contrast, quantum yields of 6 do not vary with alcohol concentration, consistent with the LE state as the origin of this product.