Oxidation of chiral 1,4-cyclohexadienes: A comparison of chemically and photochemically generated singlet oxygen O-1(2) ((1)Delta(g))

Chiral alkyl-substituted 2,5-cyclohexadiene-1-carboxylic acids la-e have be en oxidized in water and in methanol with singlet oxygen, O-1(2) ((1)Delta( g)), generated either photochemically or chemically from the catalytic syst em hydrogen peroxide/sodium molybdate. These methods were compared in terms of chemo-, regio- and diastereoselectivities and the chemical (k(r)) and p hysical (k(q)) quenching rate constants of O-1(2) were determined. The rati o of the cis and trans isomers of the hydroperoxides 2a-c is not influenced by the source of O-1(2), but, on the other hand, it depends slightly on th e solvent and greatly on the steric hindrance of the substituents linked to the chiral carbon. The results may be interpreted on the basis of the succ essive formation of an exciplex and a perepoxide that evolves either by giv ing the final allylic hydroperoxide or by dissociating into the starting su bstrate and singlet or triplet oxygen.