Intramolecular catalysis in the enolisation of β-piperidinopropiophenone and its methiodide derivative

Abstract

Measurements have been made on the rates of iodination of β-piperidinopropiophenone and its methiodide derivative, and on the rates of bromination of β-piperidinopropiophenone. The rates are independent of halogen concentration, except at low pH and high iodide concentrations when the iodination reactions are reversible, and represent the rate of ionisation or enolisation of the substrates. The reactions are base-catalysed and catalytic constants for a number of bases have been determined. The rates of ionisation of the protonated and N-methylated aminoketones are closely similar, and are up to 4 000 times larger than those of corresponding reactions of the neutral aminoketone and acetophenone. The close similarity between the reactivities of the protonated and N-methylated aminoketone suggests that the effect is due to an intramolecular electrostatic stabilisation of the negative charge developing on the carbonyl oxygen in the transition state. The results are discussed in relation to those of earlier studies on related systems.