Manganese(III) acetate-induced cyclization of α-(arylalkyl) and α-(aryloxy)alkyl β-dicarbonyl derivatives

The oxidation of the title compounds by manganese(III) acetate in acetic acid at 50-80°C affords products of intramolecular aromatic alkylation. High yield and selectivity are observed in six-membered ring closures, whereas five- and seven-membered ring closures are associated with side products of dimerization and/or hydrogen abstraction. The aromatic substitution is favored in all cases by a high electron density of the aromatic carbon atom a to the carbonylalkyl substituent. The activation parameters for the oxidative cyclization of diethyl α-(2-naphthoxyethyl)malonate were determined (ΔH‡ = 25.2 ± 0.3 kcal/mol, ΔS‡ = 2.1 ± 0.5 cal/K mol). The reaction is interpreted to proceed through electrophilic Mn(III)-complexed β-dicarbonylalkyl radicals, formed by reversible inner-sphere electron transfer of Mn(III) carbonyl complexes, which add reversibly to the aromatic ring