Photolysis of Aromatic Ethers

H e i n z P e t e r S c h u c h m a n n a n d C l e m e n s v o n S o n n t a g * Institut für Strahlenchemie im Max-Planck-Institut für Kohlenforschung, Stiftstraße 34-36, D-4330 Mülheim a. d. Ruhr Dedicated, to Professer Oskar E. Polansky on the occasion of his 60th anniversary Z. Naturforsch. 84b, 1002-1009 (1979); received February 23, 1979 !H NMR, Photo-CIDNP, Photolysis of Aryl Ethers, Keto-Enol Tautomers, Quantum Yields The photolysis of twelve aromatic ethers in solution has been studied by CIDNP and product analysis. The primary step is the scission of the C-O bond in the excited singlet state. The partners of the resulting radical pair (Ph-O" and R1) undergo three major reactions: (i) geminate recombination and out-of-cage combination resulting in the reformation of the starting material, (ii) geminate recombination and out-of-cage combination resulting in cyclohexa-2,5dienones and cyclohexa-2,4-dienones which are isomers of the starting material, (iii) H-abstraction from the solvent by the more reactive ones of the two radicals (mainly RO") and combination of the less reactive ones {e.g. benzyl) in the bulk solution. The quantum yield of photodecomposition is low for diphenyl ether ( 0 ~ 0.002) but approaches unity (0.3-0.9) in benzyl aryl ethers where the benzyl-oxygen bond is cleaved. If the aroxyl radicals are substituted in ortho (para) position the resulting ortho (para)disubstituted cyclohexadienones are stable. Otherwise they swiftly rearrange into the phenols. The cyclohexa-2,4-dienones which are the precursors of the ortho-substituted phenols isomerize faster than the cyclohexa-2,5-dienones that give the para-substituted phenols. The rates of these rearrangement reactions are enhanced by acid catalysis.