Hydroxyl-radical-induced oxidation of cyclohexa-1,4-diene by O2 in aqueous solution. A pulse radiolysis and product study

Abstract

Hydroxyl radicals have been generated radiolytically in N₂O/O₂(4 : 1 )-saturated aqueous solutions. They react with cyclohexa-1,4-diene by H-abstraction and by addition to a double bond yielding equal amounts of cyclohexadienyl 1 and 6-hydroxycyclohex-3-enyl radicals 2. As shown by pulse radiolysis, 1 and 2 react with O₂ with an overall rate constant of k= 1.2 × 10⁹ dm³ mol^–1 s^–1. Radical 1 yields two isomeric peroxyl radicals: 4(cyclohexa-1,3-diene structure) and 5(cyclohexa-1,4-diene structure). Since the C–H ⋯˙O–O distance in 4 is only ca. 1.8 Å, it is suggested that 4 is the radical which rapidly eliminates HO₂˙(k× 10⁵ S^–1 as measured by pulse radiolysis) yielding benzene (ca. 60% of 4+5). The peroxyl radical 5 which has a longer C–H ⋯˙O–O distance (ca. 3.8 Å) might not undergo this reaction. Instead it undergoes intramolecular cyclization thereby forming endoperoxidic intermediates which are believed to be the precursors of products which upon reduction with NaBH₄ yield trihydroxycyclohexenes.

In the presence of superoxide dismutase to remove O₂^.– radicals and at low cyclohexa-1,4-diene concentration (4 × 10 ^–4 mol dm^–3), the 6-hydroxycyclohex-3-en-yl-peroxyl radicals 6 and 7 derived from 2 decay bimolecularly yielding equal amounts of 4,5-dihydroxycyclohexene, 6-hydroxycyclohex-3-enone and oxygen (k= 1.3 × 10⁸ dm³ mol^–1 S^–1). At high cyclohexa-1,4-diene concentration (10^–2 mol dm^–3)6 and 7 abstract H-atoms from the substrate (k= 820 dm³ mol^–1 S^–1) yielding the corresponding 6-hydroxycyclohex-3-enyl hydroperoxides. In competition with these bimolecular reactions, 6 and 7 also undergo intramolecular cyclization similar to radical 5. In subsequent reactions endoperoxidic products are formed which have been identified as tetrahydroxy-cyclohexanes after reduction with NaBH₄.

A number of fragment products with one to three carbon atoms have also been found which may result from the degradation of some of the endoperoxidic intermediates derived from the radicals 5, 6 and 7

A material balance is presented which accounts for the oxygen consumed in this system.