Structure-reactivity correlations of the abnormal Beckmann reaction of dihydrolevoglucosenone oxime.

Amani Alhifthi,Lars Goerigk,Benjamin L Harris,Spencer J Williams,Jonathan M White

doi:10.1039/c7ob02499a

Amani Alhifthi, Lars Goerigk + Show 3 more

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https://doi.org/10.1039/c7ob02499a

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Abstract

A structural, spectroscopic and computational study of a series of oximes was undertaken to investigate how geometric and structural changes relevant to the reaction coordinate for the Beckmann reaction (normal Beckmann) and Beckmann fragmentation (abnormal Beckmann) manifest in the ground state. X-ray structures of a range of oximes derived from dihydrolevoglucosan (Cyrene™; which undergoes the abnormal Beckmann reaction), in which the oxygen substituent was systematically varied were determined. As the electron demand of the OR group increased, the major structural changes included lengthening of the N-OR bond distance, and a decrease in the magnitude of the C2[double bond, length as m-dash]N-O bond angle, consistent with the changes seen for cyclohexanone oximes, which undergo the normal Beckmann reaction. However, unique to the Cyrene oximes, an increase in the length of the fissile C1-C2 bond was observed, which correlated with a decrease in the 13C-13C 1-bond coupling constants as the electron demand of the OR substituent increased. Computational studies of Cyrene and cyclohexanone oximes using Natural Bond Orbital analysis support an electronic structure involving n(O) → σ*C1-C2 and σC1-C2 → σ*N-O localized orbital interactions.

Highlights

Ketoximes undergo rearrangement reactions involving cleavage of the carbon–carbon bond adjacent to the imino carbon.1 These reactions include the Beckmann rearrangement (‘normal’ Beckmann reaction),2–4 leading to amides, and the Beckmann fragmentation (‘abnormal’ Beckmann reaction),5,6 leading to nitriles
A similar plot for the 2,2-dimethylcyclohexanone oxime analogues, which are prone to abnormal Beckmann reaction, revealed structural effects that varied in a similar fashion but which were weaker in magnitude
The structure–reactivity correlations of Cyrene oxime and derivatives show changes in bond-length and angles that are consistent with the early stages of the abnormal Beckmann reaction: lengthening of the fissile N–OR bond, and contraction of the CvN–O and C1–C2vN bond angles

Summary

Introduction

Ketoximes undergo rearrangement reactions involving cleavage of the carbon–carbon bond adjacent to the imino carbon. These reactions include the Beckmann rearrangement (‘normal’ Beckmann reaction), leading to amides, and the Beckmann fragmentation (‘abnormal’ Beckmann reaction), leading to nitriles. The abnormal Beckmann reaction occurs in cases where the adjacent carbon can stabilize positive charge; a carbocation intermediate is formed that undergoes heterolytic fragmentation of the carbon–carbon bond anti to the leaving group to yield a nitrile.. Structure–reactivity correlations of cyclohexanone oxime analogues that varied in the nature of the group OR revealed structural changes consistent with the manifestation of early stages of the normal Beckmann reaction.. Structure–reactivity correlations of cyclohexanone oxime analogues that varied in the nature of the group OR revealed structural changes consistent with the manifestation of early stages of the normal Beckmann reaction.20 These included an increasing N–OR bond distance and a closing of the N–C1–C2 bond angle, consistent with the early stages of bond breaking and of migration of the antiperiplanar carbon onto nitrogen. A similar plot for the 2,2-dimethylcyclohexanone oxime analogues, which are prone to abnormal Beckmann reaction, revealed structural effects that varied in a similar fashion but which were weaker in magnitude

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