吡啶酰胺类化合物中酰胺键异构化的变温核磁共振研究

合成了一系列六氢吡啶酰胺化合物以研究酰胺键的双键性质. 采用¹H 及¹³C NMR 技术确证了不同取代的酰胺化合物中酰胺键的双键性质. 进一步采用变温核磁共振技术对2-(3,5- dinitrophenyl)-1-(piperidin-1-yl)ethanone(化合物1)进行了¹H 及¹³C NMR研究. 结果表明，在20 ℃（293 K）时，a, b及c, d上的氢和碳原子具有不同的化学位移，且距酰胺键越近，化学位移的差异越大. 随着温度的升高，两种异构体的转换速率增加，酰胺双键的性质越来越不明显，导致化学位移的差异逐渐减小，直至融合为一个峰. 经计算，在融合温度下两种异构体的相互转换的速率常数是380 s-¹，能垒约为67 kJ• mol^-1.  

Isomerization of the Amide Bond in Piperidin Amides Studied by Variable-temperature NMR Spectroscopy

A series of amides of piperidin were synthesized and used for studies on the partial double-bond feature of the amide bonds. 1H and 13C NMR spectra of 2-(3, 5-dinitrophenyl)-1-(piperidin-1-yl) ethanone (compound 1) were acquired at various temperatures. The results showed that the C-N bond of the piperidin amides exhibited partially double-bond character at 20 ℃ (293 K) , suggesting that the chemical shifts of a and b, c and d in compound 1 were very different due to the two rotameric states. With increasing temperature, the two peaks of non-equivalent -CH2- groups collapsed into one peak due to the fast rotation of the C-N bond. The rate constant for the dynamic process detected for compound 1 was 380 s-1, and the free energy of activation for compound 1 at coalescence temperature (343 K) was 67 kJ·mol-1.