STRUCTURAL STUDIES BY NUCLEAR MAGNETIC RESONANCE .22. CONFORMATIONAL ANALYSIS OF GLYCIDALDEHYDE PHENOXYACETALDEHYDE METHOXYACETALDEHYDE AND METHYLMERCAPTOACETALDEHYDE

The vicinal spin-spin coupling constants between aldehydic and α-protons of methoxyacetaldehyde, phenoxyacetaldehyde, methylmercaptoacetaldehyde, glycidaldehyde, and cyclopropanecarboxaldehyde were studied at 60 Mc as a function of temperature and solvent. The following conclusions were drawn. (1) A threefold barrier to rotation about the carbon-carbon bond best fits the data from the first three compounds. No unambiguous decision could be made regarding the analogous barrier to rotation of the last two compounds. (2) The most stable rotamer of methoxyacetaldehyde and phenoxyacetaldehyde is the one where the C-X bond eclipses the carbonyl group, and the most stable rotamer of the other three compounds is the one where the C-H bind eclipses the carbonyl. (3) The free energy and enthalpy values for I→ II (X = OCH3, OC6H5) are strongly solvent dependent, being much more negative in solvents of high dielectric constant. Those of methylmercaptoacetaldehyde and glycidaldehyde are solvent insensitive. (4) Whereas the cyclopropyl group acts as an electron donor, the oxirane group acts as an electron withdrawer. This difference notwithstanding, the oxirane resembles more the cyclopropyl than it does the methoxy. © 1969, American Chemical Society. All rights reserved.