基于苯并硫氮杂卓的亚胺、烯胺在质子溶剂中的稳定性及其互变异构

用核磁共振法研究了亚胺型杂卓1和烯胺型杂卓2在不同质子溶剂—甲醇、乙醇、叔丁醇、乙酸及CF3CO2D的DMSO-d6溶液中的稳定性及互变异构.结果表明,烯胺型杂卓2在上述溶剂中稳定,亚胺型杂卓1在酸性较弱的质子溶剂中(乙醇、叔丁醇)及较低的温度下(≤30℃)基本稳定,在酸性较强的质子溶剂中(甲醇、乙酸、CF3CO2D的DMSO-d6溶液)向烯胺转化,而且随着溶剂酸性的增强、温度的升高以及反应物与溶剂接触时间的延长,亚胺向烯胺的转化进一步加强.本文还以1a和1b为例,研究了在乙酸溶液中亚胺型杂卓向烯胺型杂卓转化的热力学和动力学特征.研究表明,在283~333 K范围内,该转化的焓变(△H)和熵变(△S)均大于零,吉布斯自由能变(△G)小于零,因此其是吸热、自发及熵增加的过程,该过程可用二级动力学方程来描述,质子对亚胺向烯胺的转化起催化作用.     

3-乙酰基-2,3/2,5-二氢-1,5-苯并硫氮杂卓的选择性合成及其互变异构

研究了温度、时间等因素对合成反应的影响, 发现亚胺型杂卓4和烯胺型杂卓5分别为速度控制产物和平衡控制产物, 并且实现了两种互变异构体的选择性合成. 用核磁共振法研究了溶剂、酸碱度对4, 5互变异构的影响, 发现两种异构体在CD₃OD, DMSO-d₆, C₆D₆以及精制的CDCl₃中比较稳定, 在未精制的CDCl₃中容易发生互变及开环反应, 形成3, 4, 5的平衡混合物, 并以4为主要组分. 4, 5在酸中不稳定, 但在弱碱(吡啶)中能稳定存在. 采用密度泛函理论方法在B3LYP/6-31G基组水平上对四组(八种)不同取代基的上述异构体进行了几何优化和计算. 结果表明, 烯胺型杂卓5比亚胺型杂卓4稳定, 理论计算结果与实验结果基本一致.     

三羟甲基氨基甲烷水杨醛类席夫碱质子转移异构化的理论研究

采用密度泛函(DFT)中的B3LYP方法,在6-311+G(d,p)基组水平上对三羟甲基氨基甲烷水杨醛席夫碱气相、水溶剂及甲醇溶剂中的分子内质子转移和衍生席夫碱的互变异构反应机理进行了计算研究,获得了反应焓、活化能、活化吉布斯自由能和质子转移反应的速率常数等参数.液相计算采用Onsager模型.结果表明,不论在气相、水溶剂还是甲醇溶剂中,三羟甲基氨基甲烷水杨醛席夫碱(L3=H,L5=H)烯醇亚胺式异构体R1和醌型的酮烯胺异构体P1可以共存,但以苯环型的烯醇亚胺式R1为主要形式.当由苯环型的烯醇亚胺R1向醌型的酮烯胺P1分子内质子转移时活化能较低,室温常压下反应容易进行.水和甲醇溶剂对异构化反应影响较小.当—NO2,—OMe取代生成衍生席夫碱时(L3=H,L5=NO2;L3=OMe,L5=H),结果表明,苯环型的烯醇亚胺式和醌型的酮烯胺式异构体都能共存,质子转移异构化反应的活化能垒也较低.     

有机配体Schiff碱4-(2-羟基苯基)-亚氨基-戊-2-酮的从头算研究

本文通过从头算HF／6－31G（d）方法,对Schiff4－（2－羟基苯基）－亚氨基－戊－2－酮 进行了理论研究,提供了该化合物两种互变异构体的几何构型参数、电子结构、IR光谱性 、偶极矩数据,并借助热、动力学手段,分析了两种互变异构体的异构平衡过程。计算结果表明：（1）从几何构型、电子 结构和相对能量的角度考虑,由于较强的分子内氢键作用和较大的共轭体系,亚胺烯醇式更为稳定。（2）从分子极性的角度考虑,烯胺酮式具有较大的偶极矩,其较强的分子间作用力有利形成晶体,因而烯胺酮式以晶体的形式存在。（3）由烯胺酮式向亚胺烯醇式转化的互变异构反应是热力学自发反应,但受到较大活化能的控制,是一个动力学稳定体系。分析了极性溶剂存在的互变异构反应是热力学自发反应,但受到较大活化能的控制,是一个动力学稳定体系。分析了极性溶剂存在将有利于反应发生且标题化合物以亚胺烯醇式存在于极性溶剂中的作用。以上结论均与实验研究结果相符。     

2-酰基-1,3-环二酮类化合物互变异构的理论计算

采用MP2/6-31+G**//B3LYP/6-31+G**方法,系统地研究了2-酰基-1,3-环二酮类化合物的互变异构性质,探讨了环的饱和性、环的大小、环上杂原子以及溶剂效应对各互变异构体相对稳定性的影响。结果表明:(1)未烯醇化的异构体由于无法形成分子内氢键而具有较高的能量;(2)环上杂原子的推电子效应提高了邻位羰基氧上的负电荷,从而有利于相应环外烯醇式异构体的稳定;(3)对化合物3-酰基-1H-吡啶-2,4-二酮,3-酰基吡喃-2,4-二酮,2-酰基环戊-4-烯-1,3-二酮和3-酰基吡咯-2,4-二酮,由互变异构形成的环的芳香性在各异构体的相对稳定性中扮演着决定性的作用。     

C(3)-酯基取代的1,5-苯并硫氮杂卓的合成、晶体结构及抑菌和构效关系的研究

设计、合成了三类C(3)酯基取代的1,5-苯并硫氮杂卓衍生物: 2,3/2,5-二氢和2,3,4,5-四氢-1,5-苯并硫氮杂卓-3-甲酸乙酯, 采用元素分析、IR、MS、1H NMR及X射线衍射法确定了标题化合物的分子结构．结构分析表明, 2,5-二氢-1,5-苯并硫氮杂卓-3-甲酸乙酯属单斜晶系, C2/c空间群, 晶胞参数为: a＝2.0319(4) nm, b＝1.4985(3) nm, c＝1.3659(3) nm, α＝90°, β＝120.49(3)°, γ＝90°, V＝3.5840(12) nm3, Z＝8, Dc＝1.397 g/cm3, μ＝0.351 mm－1, F(000)＝1560, R＝0.0478, Rw＝0.1304; 研究了2,3/2,5-二氢-1,5-苯并硫氮杂卓的合成反应条件, 发现该两种互变异构体分别是速度控制产物和平衡控制产物; 抑菌活性及抑真菌构效关系研究表明, 亚胺型的2,3-二氢-1,5-苯并硫氮杂卓具有明显的抑菌活性, 亚胺官能团是其抑真菌的药效团．     

2-甲基-1,5-苯并硫氮杂的基态和激发态反应的研究III:2-甲基-1,5-苯并硫氮杂4-(5H)-酮与-4-甲基-1,5-苯并硫氮杂-2(3H)-酮间的互变异构

用化学反应和光谱分析证实了2-甲基(1,5)苯并硫氮杂环庚三烯-4(5H)-酮和4-甲基(1,5)苯并硫氮杂环庚三烯-2(3H)-酮是作为互变异构混合物存在的.     

4-甲基-3-乙酰基-2-杂环-2H,3H-苯并[f]-1,5-硫氮杂衍生物的合成、抑菌活性及副反应的研究

以含不同取代基的杂环醛和乙酰丙酮为原料,经三步反应合成了一系列新型的4-甲基-3-乙酰基-2-杂环-2H,3H-苯并[f]-1,5-硫氮杂衍生物3a～3l,产物的结构经1H NMR,IR,MS和元素分析确证,且通过1H NMR测试,发现目标化合物存在着烯胺型和亚胺型结构的互变.测定了目标化合物的抑菌活性,抑菌结果为杂类化合物的结构与抑菌的关系提供了启示.同时,确定了目标化合物3h发生分解反应时主要副产物的结构,提出了其可能的生成机理.     

2-甲基-1,5-苯并硫氮杂的基态和激发态反应的研究 II:2-甲基-1,5-苯并硫氮杂的光异构化和Fries重排

2-甲基-1,5-苯并硫氮杂(2a-2d)在苯溶液中光照生成相应的2,4-异构化产物3a-3d,其中2a和2d同时还生成双键位移产物4a和4d.4a经碱性醇解生成一对2, 4-异构体2b和3b.2-甲基-4-芳酰氧基-1,5-苯并硫氮杂(2e-2g)在苯中光照主要经Fries重排生成2-甲基-3-芳酰基-1,5-苯并硫氮杂-4(5H)-酮(5e-5g).同时还生成少量2,4-异构化产物2-芳酰氧基-4-甲基-1,5-苯并硫氮杂(3e-3g).但2-甲基-4-烷基酰氧基-1,5-苯并硫氮杂(2i,2j)在相同条件下光照只生成2,4-异构化产物-2-烷基酰氧基-4-甲基-1,5-苯并硫氮杂(3i,3j).2-甲基-4-对硝基苯甲酰氧基-1,5-苯并硫氮杂(2h)可发生热Fries重排,生成2-甲基-3-对硝基苯甲酰基-1,5-苯并硫氮杂-4(5H)-酮(5h).化学和光谱数据证明5e-5h和6e-6h为互变异构体.     

2,6-硫代黄嘌呤质子互变异构反应机理的理论研究

在密度泛函B3LYP/6-311G~(**)理论水平上,对气相和水相中2,6-硫代黄嘌呤各烯醇式与酮式水助质子互变异构体及其过渡态进行几何构型全自由度优化,获得它们在气相和水相中的几何结构和电子结构,PCM反应场溶剂模型用于水相计算.结果显示在气相和水相中,水参与反应降低了互变异构质子迁移的反应活化能,对互变异构质子迁移的反应起到催化作用,但是没有改变各异构体的稳定性顺序,其顺序为W1＞W3＞W2.进一步研究了2,6-硫代黄嘌呤各烯醇式与酮式水助质子互变异构的反应机理,提出了2,6-硫代黄嘌呤各烯醇式与酮式互变异构质子迁移的反应为平面六元环的过渡态结构.探讨了溶剂化效应对互变异构体的几何结构、能量、电荷分布以及互变异构反应活化能的影响等.     

Relative Stability of the Key Enamine,Oxazolidinone, and Imine Intermediates in Some Proline-catalyzed Asymmetric Reactions

Many proline-catalyzed asymmetric addition reactions with ketones as substrates were assumed to involve a key intermediate, an enamine, produced by the condensation of proline and ketone. In this paper, the key intermediate enamines derived from L-proline and cyclohexanone （or acetone） as well as the corresponding oxazolidinone and imine tautomers have been investigated by means of density functional calculations at the B3LYP/6-311＋G^＊＊ level. The predicted order of stability for these tautomers is oxazolidinones 〉 enamines 〉 imines in gas phase and oxazolidinones 〉 imines 〉 enamines in aprotic THF solvent. This prediction explains why enamine intermediate can not be observed experimentally. The predicted energy/enthalpy difference between the formal oxazolidinone structure and the zwitterionic imine structures is very small in THF solvent, suggesting the oxazolidinone-to-imine tautomerization can be readily induced in solvent. ^13C NMR chemical shifts of the oxazolidinone and imine structures have been computed and used to explain the experimental NMR spectra observed in oxazolidinone-to-imine tautomerization induced by protic solvent.     

How to drive imine–enamine tautomerism of pyronic derivatives of biological interest – A theoretical and experimental study of substituent and solvent effects

An investigation of the tautomerism of five series of aminated pyronic compounds of pharmacological interest was carried out using NMR experiments and standard quantum mechanical B3LYP/6-311+G** calculations. The obtained results indicate that among four possible tautomers, imine and enamine forms are the two predominating ones in the gas phase as well as in solution. Depending on the nature of the substituting group, the enamine or the imine form is the most stable tautomer, the calculations being in agreement with experiment. The calculated equilibrium constants in the gas phase and in solution show that the enamine form is stabilized by polar solvents, in all cases. NBO analysis explains well the predominance of a form over another one when changing a substituting group. We give indications on how to favour the imine form which is preferred for synthesis purposes.     

Synthesis and tautomerism of spiro-pyrazolines

An experimental study on the synthesis, tautomerism, and acid promoted structural changes of spiro-pyrazolines is described. The target was achieved through a [3+2]-dipolar cycloaddition of an alkene with nitrile imines generated in situ and was isolated in high yield. The synthesized cycloadduct displayed a tendency to exhibit an imine–enamine type of tautomerism as evidenced by X-ray crystal and NMR studies. Furthermore, addition of an acid resulted in the transformation of an imine tautomer to an enamine. The current report constitutes a first formal observation of this kind of tautomerism observed in spiro-indoline pyrazolines.     

Imines et enamines stanniques : IV. Protolyse: synthese d'enamines secondaires; structure,isomerisation, reactivite vis a vis de l'acrylonitrile

The partial protolysis of organotin imines—enamines leads to organic secondary enamines, which we found to be stable in completely aprotic media.Their structure was determined by IR and NMR methods, and their reactivity towards acrylonitrile studied. The results show a much greater reactivity for enamines, as compared to that of the corresponding imine tautomers.     

Nitrogen bridgehead compounds. Part 35. Structures of α-formyl-2,3-polymethylene-3,4-dihydroquinazolin-4-ones

The structures of the title compounds bearing a five-, six- or seven-membered A ring have been investigated by uv and ¹H and ¹³C nmr spectroscopy. The imine-enol-enamine (I-II-III) tautomerism of these compounds depends greatly on the ring size. A significant solvent-dependence is observed only for the five-membered-ring compounds 1 and 2 , which in ethanolic solution exist predominantly in the imine form I, and in chloroform solution in the enol form II. The compounds with a six-membered A ring, 3 and 4 , are mainly in the enamine form III. On protonation, 3 and 4 change into the E and Z isomeric mixture of the enol tautomer II. The seven-membered-ring compound 5 is a mixture of the imine I and the enamine III tautomers.     

Relative tendency of carbonyl compounds to form enamines

Equilibria between carbonyl compounds and their enamines (from O-TBDPS-derived prolinol) have been examined by NMR spectroscopy in DMSO-d(6). By comparing the exchange reactions between pairs (enamine A + carbonyl B → carbonyl A + enamine B), a quite general scale of the tendency of carbonyl groups to form enamines has been established. Aldehydes quickly give enamines that are relatively more stable than those of ketones, but there are exceptions to this expected rule; for example, 1,3-dihydroxyacetone acetals or 3,5-dioxacyclohexanones (2-phenyl-1,3-dioxan-5-one and 2,2-dimethyl-1,3-dioxan-5-one) show a greater tendency to afford enamines than many α-substituted aldehydes.     

DFT study on the isomerization in vitamin B6

Isomerization and tautomerism reactions of the active form of vitamin B6, pyridoxal phosphate, are studied at B3LYP level of theory using 6-311++G(2df,p) basis set in gas and aqueous phases. Twenty-three transition state (TS) structures for vitamin B6 isomerization are optimized, including 13 TS structures for O–H and C–C rotations, 8 TS structures for imine–enamine tautomerism, and 2 TS structures for keto–enol tautomerism. Activation energy (E _a), imaginary frequency (υ), and Gibbs free energy of activation (ΔG ^#) for the isomerization reactions are calculated. The activation energies of the imine–enamine tautomerism are in the range of 190–280 kJ/mol and of O–H and C–C rotations are mainly less than 60 kJ/mol. Also, our calculation shows that the imine forms of B6 are mainly more stable than the enamine forms. Effect of microhydration on the TS structures and activation energies is also investigated. It is found that the presence of water molecules catalyzes only the imine–enamine tautomerism.     

Structure and tautomerism of 4-substituted 3(5)-aminopyrazoles in solution and in the solid state: NMR study and Ab initio calculations

Annular tautomerism of 3(5)-aminopyrazoles containing a cyano, thiocyanato, or aryl substituent in the 4-position has been studied by ¹H and ¹³C NMR in solution, cross-polarization and magic-angle spinning ¹³C NMR in the solid state, and ab initio quantum chemical calculations (B3LYP/6-31G**). The title compounds in the solid state exist as 3-amino tautomers. A rare case of slow (on the NMR time scale) annular prototropic tautomerism has been observed in DMSO-d ₆: signals of particular tautomers (3- and 5-aminopyrazoles) have been detected in the NMR spectra. 4-Cyano and 4-thiocyanato derivatives exist preferentially as 5-amino tautomers, whereas 4-methoxy analog is represented mainly by the 3-amino tautomers. Ab initio calculations (B3LYP/6-31G**) for the gas phase and DMSO solution (in terms of the polarizable continuum model) have shown increase of the relative stability of more polar 5-amino tautomer in going to DMSO.     

Theoretical and experimental 15N NMR study of enamine–imine tautomerism of 4‐trifluoromethyl[b]benzo‐1,4‐diazepine system

The tautomeric structure of 4‐trifluoromethyl[b]benzo‐1,4‐diazepine system in solution has been evaluated by means of the calculation of ¹⁵N NMR chemical shifts of individual tautomers in comparison with the averaged experimental shifts to show that the enamine–imine equilibrium is entirely shifted toward the imine form. The adequacy of the theoretical level used for the computation of ¹⁵N NMR chemical shifts in this case has been verified based on the benchmark calculations in the series of the push–pull and captodative enamines together with related azomethynes, which demonstrated a good to excellent agreement with experiment. Copyright © 2015 John Wiley & Sons, Ltd.