Stereospecificity of 13C shielding constants in acetone azine as a tool for configurational assignment of ketone azines

Abstract

According to the 13 C NMR data, the chemical shift of the methyl carbon atom in acetone azine in the trans position with respect to the lone electron pair on the neighboring nitrogen atom is lower by 7 ppm than that of the methyl carbon atom in the cis position. The corresponding direct 13 C-13 C coupling constant for the trans-methyl groups is lower by 10 Hz as compared to the cis-methyl groups. The experimental spectral data are reproduced well by nonempirical quantum-chemical calculations. The observed stereospecificity of 13 C chemical shifts and 13 C-13 C coupling constants may be used as an effective tool in configurational analysis of various ketone azines.We previously synthesized a representative series of ketazines and aldazines and studied their physicochemical properties [1,2]. Due to the presence of two azomethine moieties in their molecules, ketazines can exist as three configurational isomers, E,E, Z,Z, and E,Z (structure A).

Stereospecificity of1H,13C and15N shielding constants in the isomers of methylglyoxal bisdimethylhydrazone: problem with configurational assignment based on1H chemical shifts

“…In the E (or EE ) isomer of the aldoximes and aldazines, the 1 H proton of the –CH═N– fragment resonates at higher frequency by 0.6–0.9 ppm than that of the Z (or ZZ ) isomer. Furthermore, when the methyl, α‐methylene or α‐methine group in the alkyl ketoximes and ketazines has the trans orientation relative to the nitrogen lone pair, the relevant 13 C resonance appears at lower frequencies by 4–7 ppm in comparison with the cis oriented groups . For the ketoximes and ketazines with the phenyl substituent, the 13 C carbon in the ipso position is shielded by 2–3 ppm if the phenyl ring also adopts the trans orientation with respect to the nitrogen lone pair as compared with that of the cis oriented phenyl group .…”