Nitroalkylation of aromatic hydrocarbons promoted by manganese(III) acetate

Additional mechanistic and synthetic details concerning the formation of nitroalkylated aromatic products from the reaction of nitroalkanes with manganese(III) acetate and an aromatic compounds are presented. A large isotope effect (k/sub H//k/sub D/ = 4.02-4.20) was found for both manganese(III)-and cerium(IV)-promoted nitromethylations with nitromethane-d3 while no isotope effect (k/sub H//k/sub D/ = 1.05) was observed with benzene-d6. This indicated that deprotonation (most likely from an aci radical cation) to the nitromethyl radical is the slow step while subsequent rearomatization of a sigma-radical complex occurs rapidly. Somewhat more convenient methods of activating the manganese(III) acetate promotor were found. However, attempts to find cooxidants or other additives which could improve this process met with only limited success. The reaction of either toluene or benzene with nitroethane or the nitropropanes and manganese(III) acetate in refluxing acetic acid gave much poorer fields (3 to 8%) of the corresponding nitroalkylated products. Incorporation of sodium acetate into these systems completely elimiated the aromatic substituttion products and formed α-nitroalkyl acetates instead. Implications of this finding on the mechanism for the generation of nitroalkyl radicals are discussed