Conformational Preferences in Hexahydro-2H-azepin-2-ones (Caprolactams)

Abstract

It was predicted and found that caprolactams are similar to cyclohexanes in conformational preferences. The compounds investigated were 7-methyl- and 3- and 7-(1,1-dimethylethyl)hexahydro-2H-azepin-2-ones. The 600-MHz NMR spectra showed discrete sets of axial and equatorial hydrogens, with the equatorial hydrogens lying «downfield» as in cyclohexane. The boat forms were surprisingly low in energy. The chair form with an axial substituent is predicted to have the highest energy of the four main conformers. Agreement between calculated and observed coupling constants is acceptable for tert-butyl-substituted caprolactams, but not for methyl (the energy of the axial methyl substituent appears to be too high in MM2 and especially in MM3). Some of these compounds show abnormally small coupling constants for hydrogens near the tert-butyl group. Calculations suggest that the effect is related to bond lengthening