Dynamic processes in enol form of ethyl pivaloylpyruvate

Abstract

For symmetric (X = Y) B-diketones, up to 130~ PMR spectra are observed which are characteristic of a chelate ring with the C2o symmetry [2, 3]. This indicates either rapid migration of the enol proton in a double well potential, or in the existence of a symmetric hydrogen bond. The data of microwave [4] and X-ray photoelectron spectroscopy [2], and also the positive value of the difference in the chemical shifts of enol proton and deutron [5, 6] indicate the presence in these compounds of a double well potential of the hydrogen bond. The proton migration in a double-well symmetric potential can occur by either an activational (a proton jump), or by a tunneling (delocalization. of the proton along the hydrogen bond) mechanism. The activation mechanism of the proton migration was widely used to explain the influence of different structural factors and temperature on the spectral parameters of NMR [3, 7], and also for interpreting the IR spectra of B-dicarbonyl compounds [8]. However, the presence in the enol forms of symmetric B-diketones of anomalously high AD(OH) and secondary AD(c) isotope effects [AD(OH) is the difference in magnetic screening of a proton and deutron in a hydrogen bridge, AD(c) is the change in chemical shifts of carbon nuclei when a bridge proton is exchanged for a deutron] [5], and also the similarity in the structure of these compounds to a quasisymmetric structure C [9, I0] indicate a tunnelling mechanism of proton (deutron) migration. An analysis of the data on microwave [4] and electronic spectra [ii] of malondialdehyde (X = Y = Z = H) also leads to the conclusion on a tunnelling mechanism of proton migration in this compound.