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 Ti^III–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.