Abstract
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 when used 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^+^ π* absorption of the latter is, for the former, shifted to lower wavelength and apparently hidden under the long‐wavelength ππ* absorptions. For the p‐benzoquinone mono‐oxime O‐methyl ether 12, however, the long‐wavelength __n__π* 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 S^1^(__n__π*) 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 photo‐stationary state [E]/[Z] 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 occurring E‐Z isomerization escapes detection for symmetry arguments.