Spectroscopy and photochemistry of a series of alkyl-substituted o- and p-benzoquinones and their mono-oxime O-methyl ethers

Article abstract of DOI:10.1002/recl.19921111005

Following our previous reports on the spectroscopy and photochemistry of methyl ethers of cyclic α-oxo oximes, we have now studied a number of o- and p-benzoquinone mono-oxime O-methyl ethers and, for comparison, some corresponding o- and p-benzoquinones.The ground-state electronic structure of o-benzoquinones is described satisfactorily by the sum of the two propenal fragments.The structure of the corresponding (E)- and (Z)-mono-oxime ethers can be described similarly as the sum of the propenal and (E)- or (Z)-(methoxyimino)propene fragments.This method fails whenused for the construction of the ground-state electronic structure of the isomeric p-benzoquinones and their mono-oxime ethers.The UV spectra of o-benzoquinone mono-oxime ethers are quite different from those of the parent o-benzoquinones in that the free lying n⁺?^* absorption of the latter is, for the former, shifted to lower wavelength and apparently hidden under the long-wavelength ??^* absorptions.For the p-benzoquinone mono-oxxime O-methyl ether 12, however, the long-wavelength n?^* absorption remains visible.Direct irradiation of the tert-butyl-substituted o-benzoquinone mono-oxime O-methyl ethers (E)-5 - (E)-7 dissolved in acetonitrile at λ 350 nm leads (in addition to E-Z isomerization) to the formation of the (E)-azobenzenes 16-19 by dimerization of the respective phenylnitrenes.However, in addition, the 2,4-di-tert-butyl derivative (E)-7 undergoes mainly γ-hydrogen abstraction, probably from the S¹(n?^*) state, leading to subsequent cyclization to the relatively stable dienol (E)-14, which eventually isomerizes thermally to the enone (E)-15.Evidence is presented that (Z)-7 exhibits only Z-E photoisomerization and no photocyclization, as has been observed previously with the saturated six-membered ring analogue.Direct irradiation of the corresponding 3,5-di-tert-butyl-o-benzoquinone (3) at 350 nm leads to decarbonylation with formation of cyclopentadienone 13.Sensitized irradiation of mono-oxime O-methyl ether (E)-7, using 1-acetylnaphthalene and 1-fluorenone as triplet sensitizers and acetonitrile as solvent, leads to E-Z isomerization only with eventual photostationary state / ratios of 0.78 and 0.88, respectively.The p-benzoquinone mono-oxime O-methyl ethers 11 and 12 are photostable against 254, 300 and 350 nm radiation; however, any occuring E-Z isomerization escapes detection for symmetry arguments.