Oxidation of diethyl 3-phenylpropylmalonate: a mechanistic probe for alternative benzylic and α-carbonylalkyl carbon-hydrogen bond activation

Oxidn. of PhCHRCH2CR1(CO2Et)2 [I; R = R1 = H (II)], by metal salts [Mn(III), Cd(IV), Fe(III), Co(III), Pb(IV)] and nickel peroxide affords the substitution products at the malonic C-H bond [compds. III and/or I (R = H)] with high selectivity. Peroxides, pos. halogen oxidants, and oxygen [under catalysis by Mn(III)] oxidized II at the benzylic position. The kinetics of the oxidn. of II by Mn(III) acetate, Ce(IV) ammonium nitrate and Fe(III) perchlorate have been investigated under conditions of selective formation of III and the activation parameters were deduced. The influence of NaOAc in acetic acid and CF3CO2H in acetonitrile on the kinetics of the oxidn. of II by Mn(III) acetate was also studied. The different selectivity obsd. is attributed to a different mechanism of C-H bond activation: oxidative deprotonation at the carbonyl group with metals and hydrogen abstraction with electrophilic radicals. The mechanism of the oxidn. at the malonic group is discusse