Conjugation in Azines. Stereochemical Analysis of Benzoylformate Azines in the Solid State, in Solution, and in the Gas Phase
The stereochemistry of benzoylformate azines 3 was studied in the solid state (X-rays, IR), in solution (1H-, 13C-, and 15N-NMR), and in the gas phase (ab initio theory). Benzoylformate azines are excellent systems to study phenyl conjugation because the ester substituent is electron-withdrawing and in a conformation that impedes π backdonation thereby causing the azine-C to be highly electron-deficient and an excellent potential acceptor for π-donation. The X-ray structure of ethyl benzoylformate azine (3a) is reported and compared to the crystallographic record. Azines 3 assume the N-N s-trans conformation and the Z, Z configuration. The phenyl groups are in the molecular plane of the Ci-symmetric azine while the carboxyl groups lie in planes that are nearly perpendicular. The solid-state structures are compared to optimized ab initio structures of geometrical isomers of 3. RHF/6-31G*//RHF/3-21G energies indicate the ordering Z, Z > E, Z > E, E for the thermodynamic preferences and suggest that 3 prefers the same stereochemistry in the gas phase as in the solid state. Phenyl conjugation is shown to be of little importance. 1H-, 13C-, and 15N-NMR spectroscopic analyses indicate the presence of one isomer in solution consistent with the stereochemical preferences found in the solid state and in vacuum. Infrared (calcd and exptl) and Raman (calcd) spectra show features characteristic of azines, but only the latter might be useful in stereochemical analyses. The stereochemical preferences differ markedly from those of the azines of esters, and an explanation is proposed.
R. Glaser et al., "Conjugation in Azines. Stereochemical Analysis of Benzoylformate Azines in the Solid State, in Solution, and in the Gas Phase," Journal of Organic Chemistry, vol. 58, no. 26, pp. 7446-7455, American Chemical Society (ACS), Dec 1993.
The definitive version is available at https://doi.org/10.1021/jo00078a025
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