PHOTOISOMERIZATIONS OF PROTONATED PHENOLS - EXTENSION OF THE PHOTOCHEMISTRY OF CYCLOHEXA-2,5-DIENONES

Protonated methyl-substituted phenols have been found to undergo a variety of photoisomerizations when irradiated at a low temperature in FS03H. The products obtained depend upon the wavelength of the light used. The general sequence of reactions was shown to be the photochemical transformation of the para-protonated form of a phenol (Xmax~ 310 nm) to the corresponding protonated bicyclo[3.1.0]hex-3-en-2-one, a reaction which is directly comparable to the photoisomerization observed with cross-conjugated cyclohexadienones. Several bicyclo[3.1.0]hex-3-en-2-ones were recovered by neutralization of the acid solutions. The second step in the overall photoisomerization sequence involved the transformation of the protonated bicyclohexenones (Xmax ~ 330 nm) to protonated phenols. The extent of the photoreaction of the starting protonated phenol was governed by the wavelength of the light used for the irradiation; light of wavelength 300 nm drove only the first step of the sequence, while with broad band irradiation (X > 320 nm) both steps occurred. One exception to this general behavior was found with protonated 2, 4, 6-trimethylphenol. In this case a meta-protonated isomer (Xmax ~ 373 nm) was converted to protonated l, 3, 5-trimethylbicyclohex-3-en-2-one on irradiation with light of wavelength greater than 360 nm. With shorter wavelength light, photoisomerizations of the para-protonated form of 2, 4, 6-trimethylphenol were also observed.While the photochemical reactions of a large variety of cross-conjugated cyclohexadienones have been examined, 1 these studies have not been extended to molecules having less than two substituents at C4. The reason for this apparent neglect is not hard to find. Such compounds are difficult to prepare and readily revert to their more stable phenolic tautomers, eq 1. The photochemical behavior. © 1979, American Chemical Society. All rights reserved.