取代基效应对二芳基硝酮CH=N(O)桥基1H NMR化学位移的影响

摘要

A series of substituted diaryl nitrones XArCH=N(O)ArY were synthesized, and their 1H NMR spectra were measured. The 1H chemical shift of the CH=N(O) bridge groups, δH[CH=N(O)], in each compound was determined. The effects of substituents X and Y on δH[CH=N(O)] were investigated quantitatively. A four parameters correlation equation was constructed to model the changes of δH[CH=N(O)] with substituents, yielding a standard error of 0.020. The result indicated that δH[CH=N(O)] in substituted diaryl nitrones are mainly affected by four factors: field/inductive effect (S) of substituent X [σF(X)], conjugative effect of substituent Y [σR(Y)], substituent specific cross-interaction effect between X and Y (Δσ2) and substituent specific crossinteraction effect between X and O- [Δσ2(X-O-)]. Among these factors, the contribution of Δσ2(X-O-)to δH[CH=N(O)] was more than 70%. A comparison of δH[CH=N(O)] in substituted diaryl nitrones to δH(CH=N) of the CH=N bridge group in diaryl Schiff base XArCH=NArY revealed no good linear relationship between δH[CH=N(O)] and δ H(CH=N). Therefore it should be noted that one cannot simply use the change of δH(CH=N) to deduce the change of δH[CH=N(O)] when carrying out organic molecule structure elucidation with NMR spectra.%合成一系列取代二芳基硝酮XArCH=N(O)ArY化合物,测定其核磁共振氢谱(1H NMR),指认出桥基CH=N(O)上质子的化学位移δH[CH=N(O)],定量研究取代基效应对δH[CH=N(O)]的影响.得到一个4参数定量方程,标准偏差(S)为0.020,较好地表达了δH[CH=N(O)]的变化规律.结果表明,该类化合物的δH[CH=N(O)]主要受4个因素影响:X基团的场/诱导效应[σF(X)];Y基团的共轭效应[σR(Y)];基团X和Y之间的特殊交叉作用(Δσ2);以及基团X和O-之间的特殊交叉作用 [Δσ2(X-O-)].其中,Δσ2(X-O-)对δH[CH=N(O)]变化的贡献超过70%.通过δH[CH=N(O)]与二芳基希夫碱XArCH=NArY桥基CH=N上质子化学位移的δH(CH=N)比较发现,这两类化合物桥基上质子的化学位移之间没有良好的线性关系.因而,在应用NMR谱图解析有机化合物分子结构时,不能简单地用δH(CH=N)的变化去类比δH[CH=N(O)]的变化.