Oxidation of malonic acid derivatives by manganese(III) acetate. Aromatic malonylation reaction. Scope and limitations

The oxidn. of malonic acid derivs. RCH(COOR1)COOR2 (R1, R2 = H, Me, Et; R = H, Me, Et, Bu, CHMe2, C6H5, 4-MeOC6H4) by anhyd. or dihydrate manganese(III) acetate was studied in acetic acid in the presence of arom. substrates at 20-80°, generally with stoichiometric amts. of reagents. Electron-rich aroms. [ionization potential (IP) > 7.5 eV] underwent nuclear acetoxylation or quinone formation, the process being exclusive with anthracene and competitive with nuclear malonylation for 1- and 2-methoxynaphthalene. With other less electron-rich substrates (IP > 8.5 eV) only the products coming from the oxidn. of the malonic acid derivs. (aryl malonates, tartronates, etc., or dimerization and disproportionation products) were obsd. The selectivity and the yield of arom. substitution by the malonyl group was found to be affected by the electron d. of the arom. ring, the steric inhibition of substituents in the Mn(III) oxidn. of the malonic acid deriv., the oxidizability of malonyl radical by Mn(III), the base (acetate ions or water) eventually present in the medium, and the further easy oxidn. of the primary aryl malonate product, when unsubstituted dialkyl malonates or malonic acid were used. A mechanism is suggested in which inner-sphere electron transfer from Mn(III)-malonate complex affords Mn(II) malonyl radicals that are partitioned between oxidn., dimerization (or disproportionations), and reversible addn. to the aroms