Methyl vs. methylene condensation of aromatic aldehydes with 1,1,1-trifluoroacetylacetone
From the slow Knoevenagel condensations of 1,1,1-trifluoroacetylacetone (4) with aromatic aldehydes in benzene in the presence of piperidine–AcOH, low yields of three products were isolated: the enol form of the methyl-condensation product ArCHCHCOCHC(OH)CF3(5), the methylene-condensation product ArCHC(COCH3)COCF3(6), and the cleavage product ArCHCHCOCF3(7). The effects of the bulk and the electron donating or withdrawing characteristics of the aldehyde component on the condensation at the CH3vs. the CH2 group were investigated. Compound (5) is the major isolated product in the reaction of 2,4,6-trimethylbenzaldehyde. The low reactivity and the dependence of the Me vs. CH2, selectivity of the diketone (4) on the structure of the aldehyde are ascribed to an initial formation of the enol form of the hydroxy amine CF3C(OH)(NC5H10)CHC(OH)Me (12) formed by the addition of piperidine to (4); (12) is the main product after short reaction times. The acidity of the CH2 group of compound (12) is lower than that of trifluoroacetylacetone (4) and the approach of the aldehyde to the derived anion is more hindered than to the anion derived from Me ionization. The spectral properties of the products are discussed.