Uncatalyzed syn-anti Isomerization of Imines, Oxime Ethers, and Haloimines

Abstract

The imines (p-CH3OC6H42,C=NC6H4X-p with X = N(CH3)2, CH3, H, and COOC2H5 have been found to have nmr spectra near room temperature characteristic of structures with the C6H4X cis to one of the p-methoxyphenyl rings and with interconversion of stereoisomers slow on the nmr time scale. When the samples are heated the spectra change in the way to be expected if isomerization becomes rapid. The coalescence temperatures of the methoxyl absorptions and first-order rate constants (sec-1) at those temperatures are found to be 78.8°, 5.8; 69.5°, 8.7; 62.2°, 10.9; and 29.8°, 12.4. Evidence that the nmr is indeed measuring the rate of the change assumed is provided by studies of the unsymmetrically substituted imines p-CH3OC6H4(C6H5)C=NC6H4X-p (with X = N(CH3)2, CH3, and CI) whose infrared spectra suggest that they crystallize preferentially in a single stereoisomeric form but isomerize very rapidly in solution even in nonpolar solvents to a mixture with an equilibrium constant near 1. The rate of isomerization in carbon tetrachloride of trans-N-[p-dimethylaminophenyl]-p-methoxybenzophenone imine in the range -7.2 to 12.5° measured by the change in ultraviolet spectrum leads to a value extrapolated to 62° of 1.0 sec-1 in reasonable agreeement with the value estimated from the nmr results jut discussed. The o-methyl ethers of cis- and trans-p-chlorobenzophenone oxime have been found to have quite unexpected configurational stability; the cis isomer in decane is not appreciably isomerized after 170 hr at 230°. cis- and trans-p-chlorobenzophenone N-chlorimine have been prepared and consideration of their infrared spectra suggests that the configurational assignments previously made on the basis of dipole moments should perhaps be reversed. These co pounds and cis-p-chlorobenzophenone N-bromimine show configurational stability intermediate between the N-alkyl- or aryl imines and the oximes but their isomerization is complicated by the incursion of a free-radical reaction involving breaking of the N-halogen bond. Examination of the aromatic proton region of the nmr spectra of the triarylimines prepared in the course of this work leads to the conclusion that the aromatic protons of one aromatic ring are shifted to higher field by a ring cis to the first; the shift of the inner protons (ortho to the point of attachment of the ring to the imine functional group) amounts to 0.4-0.6 ppm (relative to the trans arrangement) whereas the shift of the more remote protons (meta to the point of attachment) is only 0.04-0.16 ppm. These shifts are comparable in direction and magnitude to shifts found in other systems. It is proposed from a consideration of the aromatic region near 830 cm-1 in the infrared spectra that the absorption associated with the out-of plane bending motion of the ring protons of a para-disubstituted phenyl ring attached to a doubly bound atom occurs at a frequency of from 4 to 10 cm-1 higher in that isomer with an adjacent cis methyl, methoxyl, or halogen (relative to the trans arrangement). Possible limitations to this generalization are cited. A comparison of the ultraviolet spectra of the para-substituted benzaldehyde methylimines (XC6H4CH=NCH3 with X = H, CI, NO2) with those of the unsymmetrically substituted benzophenone methylimines (XC6H5(C6H4)C=NCH3) leads, if it is assumed that the benzaldehyde imines are predominately trans, to the observation that replacement of the aldehydic hydrogen by an aryl group results in a rather small shift (≤ 8 mu) in the absorption maximum and provides further support for the stereochemical assignments. © 1966, American Chemical Society. All rights reserved.