Mechanistic Investigation of Oxidative Mannich Reaction with tert-Butyl Hydroperoxide. The Role of Transition Metal Salt

A general mechanism is proposed for transition metal-catalyzed oxidative Mannich reactions of N,N-dialkylanilines with tert-butyl hydroperoxide (TBHP) as the oxidant. The mechanism consists of a rate-determining single electron transfer (SET) that is uniform from 4-methoxy- to 4-cyano-N,N-dimethylanilines. The tert-butylperoxy radical is the major oxidant in the rate-determining SET step that is followed by competing backward SET and irreversible heterolytic cleavage of the carbon-hydrogen bond at the a-position to nitrogen. A second SET completes the conversion of N,N-dimethylaniline to an iminium ion that is subsequently trapped by the nucleophilic solvent or the oxidant prior to formation of the Mannich adduct. The general role of Rh-2(cap)(4), RuCl2(PPh3)(3), CuBr, FeCl3, and Co(OAc)(2) in N,N-dialkylaniline oxidations by T-HYDRO is to initiate the conversion of TBHP to tert-butylperoxy radicals. A second pathway, involving O-2 as the oxidant, exists for copper, iron, and cobalt salts. Results from linear free-energy relationship (LFER) analyses, kinetic and product isotope effects (KIE and PIE), and radical trap experiments of N,N-dimethylaniline oxidation by T-HYDRO in the presence of transition metal catalysts are discussed. Kinetic studies of the oxidative Mannich reaction in methanol and toluene are also reported.