Stereospecificity of shielding constants of carbon-13 nuclei in 13C NMR spectra of oximes of hetarenecarbaldehydes and alkyl heteryl ketones

Abstract

It has been discovered that the chemical shifts of carbon atoms in 13 C NMR spectra of oximes having pyrrolyl, furyl, benzofuryl, thienyl, and pyridyl rings as substituents are changed systematically on going from the E-to the Z-isomer. This makes it possible to use the indicated chemical shifts for establishing the configuration of oximes with heterocyclic substituents and studying the special features of their electronic structure.Keywords: oximes with heterocyclic substituents, 13 C NMR spectra, stereospecificity of shielding constants.The stereospecificity of NMR spectral parameters of aldoximes and ketoximes has been studied for several decades . In particular it is necessary to select investigations of 1 H, 13 C, and 15 N spectra of oximes having a heterocyclic substituent. The 1 H NMR spectra of furfural oximes were investigated in the earlier studies [4,5]. The most interesting data were obtained for oximes enriched with 15 N isotope, in which a dependence was discovered of the geminal 15 N-1 H coupling constant on the steric orientation of the unshared pair of the nitrogen atom [5]. Ideas on the stereospecificity of parameters of 15 N NMR spectra in 15 N-labeled furfural oximes were developed in [9, 12], in which the stereospecificity of the 15 N nuclear shielding constants, the geminal 15 N-13 C coupling constants, and the vicinal 15 N-1 H coupling constants was shown. The aim of the present work was to study the stereospecificity of the shielding constants of the carbon-13 nuclei in 13 C NMR spectra of the series of oximes 1-6 having various heterocyclic substituents.The chemical shifts of 13 C atoms of oximes 1-6, and also of model oximes 7a,b, are given in Table 1. Two-dimensional 1 H-13 C correlation procedures HSQC and HMBC were used to assign the signals in the 13 C NMR spectra. Configurational assignment of aldoximes 1 and 2 was made on the basis of the differences of the chemical shift values of the "oxime" proton for the E-and Z-isomers, taking into account that in the E-isomer _______ * Dedicated to Academician of the Russian Academy of Sciences B. A. Trofimov on his 70th jubilee. __________________________________________________________________________________________

Comparative analysis of 13C shielding constants stereospecificity in the silicon and germanium derivatives of acetylenic aldehyde and ketone oximes based on the 13C NMR spectroscopy and GIAO calculations

“…Unambiguous assignment of the 13 C-2 and 13 C-3 quaternary carbon signals of the triple bond for the studied aldoximes and ketoximes 1-4 is very important as these signals reveal an inversion of the ordering on the 13 C NMR scale on going from one studied compound to another studied compounds (vide infra). The three-bond connectivity of the 13 C-3 signal with the (CH 3 ) 3 Si group singlet at 0.2 ppm or CH 2 group quartet at 0.9 ppm in the HMBC spectra of 1, 3 and 2, 4, respectively, gives its correct distinction from the 13 C-2 signal.…”

“…It is worth noticing that the two-bond 2 J(C-2,H) coupling constant in the aldoximes 1 and 2 is larger than the three-bond 3 J(C-3,H) one (6.8 vs 4.4 Hz) in both E and Z isomers. Since these coupling constants are invariable in the different configurational isomers of 1, 2 they may be also useful for the assignment of the 13 C-2 and 13 C-3 resonances in the further investigation of the acetylenic aldehyde compounds.…”

“…[1 -12] Much attention was paid to the stereospecificity of the one-bond 13 C-1 H and 13 C-13 C coupling constants [2,7] as well as two-bond 15 N-1 H and 15 N-13 C coupling constants. [4 -6] These coupling constants have been shown [2,4 -7] to be always larger in the absolute size at a syn-orientation of the relevant C-H or C-C bond to the nitrogen lone pair.…”

“…[12] Also, an unusually large orientational effect of the nitrogen lone pair on the 1 J(C,C) coupling constant (13-16 Hz) was found in these compounds. [11] Further investigation of the 13 C NMR spectra for series of oximes with a heteroaryl substituent discovered that the shielding constants of the double bond carbons (C-2 and C-3, Scheme 1) reveal the same stereospecificity in the configurational isomers as those of the triple bond carbons in the propynal oxime (C-2 and C-3, Scheme 1) [13,14] indicating the common character of the stereospecific effect on the carbon shielding constants in the configurational isomers of the conjugated oximes.…”

“…[11,12] In order to develop an idea about the stereospecificity of the 13 C shielding constants in the oximes having a substituent with triple bond, in this work, the silicon and germanium containing acetylenic ketoximes were synthesized and their 13 C NMR spectra were studied. A comparative analysis of 13 C shielding constants stereospecificity in the acetylenic aldoximes and ketoximes was carried out. Besides, the observed trends in the change of the measured chemical shifts in the acetylenic aldoximes and ketoximes were examined by the gauge-including atomic orbital (GIAO) calculations of shielding constants.…”