Structure Correlation Study of the Beckmann Rearrangement: X-ray Structural Analysis and 13C–13C 1-Bond Coupling Constant Study of a Range of Cyclohexanone Oxime Derivatives

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The X-ray structures of a range of oxime derivatives (1 and 4), of cyclohexanone and 4-tert-butylcyclohexanone, where the electron demand of the oxygenated substituent on the oxime nitrogen (OR) is systematically varied were determined. It was established that as the OR group becomes more electron demanding, then the N–OR bond distance increases, consistent with the early stages of bond breakage. Concomitant with this structural effect was a noticeable closing up of the N1–C1–C2 bond angle, consistent with the early stages of migration of the antiperiplanar carbon onto the nitrogen substituent. These structural effects are consistent with the manifestation of the early stages of the Beckmann rearrangement in the ground state structures of these oxime derivatives. The carbon–carbon bond distances of the participating carbons in this rearrangement, however, did not vary in a systematic way with the electron demand of the OR substituent, suggesting that the structural effects are too small to be detected using X-ray crystallography. However, the 13C–13C 1-bond coupling constants, which are sensitive to the effects of hyperconjugation, were shown to vary in a systematic way with the electron demand of the OR substituent. Structural effects in the oxime 5 derivatives of 2,2-dimethylcyclohexanone, a substrate that is prone to Beckmann fragmentation rather than Beckmann rearrangement, were similar but smaller in magnitude.