Oxidation of Malonic Acid Derivatives by Manganese(III) Acetate. Aromatic Malonylation Reaction. Scope and Limitations

Article abstract of DOI:10.1021/jo00272a046

The oxidation of malonic acid derivatives RCH(COOR₁)COOR₂ (R₁ = or <*> R₂ = H, Me, Et; R = H, Me, Et, n-Bu, i-Pr, C6H5, 4-OMeC6H4) by anhydrous or dihydrated manganese(III) acetate was studied in acetic acid in the presence of aromatic substrates at 20-80 deg C, generally with stoichiometric amounts of reagents.Electron-rich aromatics (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 oxidation of the malonic acid derivatives (aryl malonates, tartronates, etc., or dimerization and disproportionation products) were observed.The selectivity and the yield of aromatic substitution by the malonyl group was found to be affected by the electron density of the aromatic ring, the steric inhibition of substituents in the Mn(III) oxidation of the malonic acid derivative, the oxidizability of malonyl radical by Mn(III), the base (acetate ions or water) eventually present in the medium, and the further easy oxidation of the primary aryl malonate product, when unsubstituted dialkylmalonates 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 oxidation, dimerization (or disproportionation), and reversible addition to the aromatics.