Syn-anti isomerism in an N-benzenesulfonylimine. Mechanism of stereomutation at the carbon-nitrogen double bond

Abstract

Chemical-shift nonequivalence in the room-temperature spectrum of (V-(l,2,3-trimethyl-2-butenylidene)benzenesulfonamide (1) indicates the presence of two isomers in solution resulting from syn-anti isomerism. Using nuclear magnetic resonance spectroscopy the ratio of the isomers in solution at room temperature was determined to be 1.7, with the low-field isomer predominating. However, crystallization of 1 yields the minor upfield isomer exclusively. Equilibrium studies indicate that entropy, rather than enthalpy, is the predominant factor determining the stabilities of the two isomers at temperatures above 150°K. The activation parameters for stereomutation of 1 have been determined using complete line-shape analysis (CLS) and by a combination of rates determined by CLS with rates determined by equilibration at low temperatures. The comparison of Arrhenius data obtained using both procedures indicates that the exclusive use of CLS may prove to be unreliable but that when used in conjunction with another method, accurate results can be obtained. In addition, the free energy of activation at the coalescence temperature obtained from CLS was compared with that calculated using the approximate equation k = irSvjy/l. The two were found to be essentially the same. The mechanisms that have been proposed for synanti isomerism in imines, nitrogen inversion, torsion about the carbon-nitrogen double bond, and a mechanism intermediate between these two extremes, have been critically discussed. The JV-benzenesulfonyl group has been found to be more effective at lowering the barrier to syn-anti isomerism than an JV-phenyl substituent. This result indicates that the mechanism of isomerization cannot be pure inversion, but that the mechanism has some torsional character.he stereochemical properties of the carbon-nitrogen double bond have been the subject of numerous investigations in recent years.3-10 Of particular concern has been the question of the mechanism of stereomutation in these compounds. In some respects, this controversy has paralleled that concerning the mechanism of conformational interchange in acyclic trisubstituted nitrogen compounds which bear heteroatoms bonded to nitrogen. The identity of the slow conformational change in such compounds including amino-

Thermal Ring Expansion of 2‐Sulfonylimidoyl‐1‐phthalimidoaziridines into N‐Sulfonylimidazoles

“…Typically, N ‐sulfonylimines have a low barrier of ( E/Z )‐isomerization about the C=N bond, which results in the broadening of some signals in their NMR spectra at room temperature. Room temperature 1 H NMR spectra of imines 2l and 2m with the ( E )‐configuration of the C=C bond feature signals of all aromatic protons as one strongly broadened multiplet at δ = 6.68–8.08 ppm.…”

An Explanation about the Use of (S)-Citronellal as a Chiral Derivatizing Agent (CDA) in 1H and 13C NMR for Sec-Butylamine, Methylbenzylamine, and Amphetamine: A Theoretical-Experimental Study

“…The conformational search (GMMX) was performed with PCModel version 7.0 [22]; the parameters of our previous work were used. Because imines preferentially form the E -isomer due to lower steric hindrance, the torsional angle defined by the C–C=N–C atoms of compounds 5 , 6 , and 7 was restricted to limit the conformational search to this isomer [23,24,25].…”

E/Z‐Photoisomerization of N,N′‐Bis(4‐dimethylaminobenzylidene)1,2‐diaminoethane and N,N′‐Bis(4‐dimethylaminobenzylidene)1,3‐diaminopropane in Chloroform

“…Caged compounds which can be removed by photo irradiation have been attached to DNA, RNA and proteins for initiation of biological phenomena 1 . The therma1 [2][3][4] and photo [5][6][7] interconversions of the syn and anti isomers of imines are a subject of long-standing interest. The mechanism for the thermal interconversion of imine diastereomers is currently the subject of considerable debate [8][9][10] and has been considered in terms of either a planar inversion mechanism or a rotation mechanism.…”