Magnesium-promoted reduction of branched alkyl phenyl ketones in the presence of ethyl trifluoroacetate and chlorotrimethylsilane in N,N-dimethylformamide (DMF), followed by air oxidation, and the subsequent treatment of tetrabutylammonium fluoride brought about selective and efficient formation of the corresponding aromatic para-substituted diketones possessing a trifluoroacetyl group in good yields through abnormal addition toward the para positions of the aromatic ketones in good yields.