Electron spin resonance studies. Part 68. Addition versus overall one-electron abstraction in the oxidation of alkenes and dienes by SO4–˙, Cl2–˙, and ˙OH in acidic aqueous solution

Abstract

E.s.r. spectroscopy has been employed to characterize radicals formed by the reaction of alkenes, dienes, and some substituted derivatives with SO4–˙(generated photolytically), Cl2–˙(from Cl– and SO4–˙ or ˙OH, and ˙OH (from TiIII–H2O2 in a flow system). For some substrates (e.g. HOCH2CHCH2, butadiene) the appropriate SO4–˙ and chlorine-atom adducts are detected, whereas for others (e.g. cyclopentadiene) only hydroxyl adducts are characterized. It is suggested that the reaction mechanism involves formation of the appropriate chlorine-atom and sulphate adducts, with subsequent rapid hydrolysis: this may involve production and rapid hydration of an incipient radical-cation. It is found that Cl2–˙ is more effective than SO4–˙ in achieving overall one-electron oxidation. The regioselectivities of ˙OH attack on alkenes and of the hydration of alkene radical-cations are contrasted and rationalized. Finally, cyclization reactions of pent-4-en-1-ol (with Cl2–˙, ˙OH–H+) and hex-4-en-1-ol (CI2–˙, ˙OH–H+, SO4–˙) are rationalized in terms of rapid internal nucleophilic attack on a transient alkene radical-cation.