内消旋和外消旋-2,3-二异丁酰氧-2,3-二苯基丁二腈的晶体结构和分子结构: 受-授取代基效应对分子构象的影响

1979年Viehe等人提出了受-授取代基效应(capto-dative substituent effect)的概念,认为在六取代乙烷(1)中,除吸电子取代基CN外,有给电子取代基R,则1比较容易均裂成取代甲基自由基2。近年来,受-授取代基效应引起了许多研究者的兴趣,但对具有受-授取代基的多取代乙烷的分子结构和晶体结构,尚鲜见报道。     

内消旋2,3-二苯基-2,3-二乙氧基丁二腈的meso/dl热异构化

对内消旋2,3-二苯基2,3-二乙氧基丁二腈的meso/dl热异构化反应作了动力学NMR和ESR研究,证明该异构化是通过α-乙氧基苄腈自由基中间体进行的.在120～150℃测出反应的ΔH≠=31.9±0.9kcal/mol,ΔS^≠=6.4±0.4e.u.,表现出一定的受-授取代基效应.     

内消旋和外消旋2,3-二氰基-2,3-二苯基丁二酸二乙酯对苯乙烯聚合反应的影响

测定了题目化合物立体异构体在80℃和90℃引发苯乙烯的自由基聚合反应。在相同条件下, 较活泼的meso-体为引发剂时, 聚合速率大, 产物分子量小; 而较稳定的dl-体则聚合速率小, 产物分子量大。且在所有反应条件下分子量均随反应时间增长。研究结果认为它们不同的聚合性能主要与异构体热分解速率对聚合动力学的影响有关。     

内消旋和外消旋2,3-二氰基-2,3-二苯基丁二酸二乙酯对苯乙烯聚合反应的影响

测定了题目化合物立体异构体在80℃和90℃引发苯乙烯的自由基聚合反应.在相同条件下,较活泼的meso-体为引发剂时,聚合速率大,产物分子量小;而较稳定的dl-体则聚合速率小,产物分子量大.且在所有反应条件下分子量均随反应时间增长.研究结果认为它们不同的聚合性能主要与异构体热分解速率对聚合动力学的影响有关.     

内消旋-和外消旋-2,3-二氰基-2,3-二苯基丁二酸二乙酯的热异构化反应

用高效液相色谱法测定了2,3-二氰基-2,3-二苯基丁二酸二乙酯的内消旋体(1)和外消旋体(2)在1,1,2,2-四氯乙烷中的热异构化反应动力学和活化参数。结果表明,在热力学上2比1更稳定,并从它们的构型与形成空间键的能力间的关系给予解释。     

内消旋和外消旋2,3-二氰基-2,3-二苯基丁二酸二乙酯在苯乙烯中的分解动力学

用膨胀计方法测定了2,3-二氰基-2,3-二苯基丁二酸二乙酯的内消旋体(1)和外消旋体(2),于不同反应温度,分别在泰乙烯中的分解动力学和活化参数。结果表明,2比1更为稳定。用动力学链长综合常数k′来解释了1和2在较低温度时的单体极限转化率X_∞大于较高温度时的X_∞实验结果。     

2,3-二氰基-2,3-二苯基丁二酸二乙酯的两种异构体的晶体结构和分子结构

2,3-二氰基-2,3-二苯基丁二酸二乙酯由α-氰基苯乙酸乙酯经Cu~(++)-TMEDA-O_2系统氧化偶联制得。产物经柱层析得到两个结晶化合物1和2.1和2的单晶经X射线晶体结构分析鉴定,分别为内消旋和外消旋2,3-二氰基-2,3-二苯基丁二酸二乙酯。值得注意的是,在化合物1和2的分子中,中心碳—碳单键的键长均有明显的增长效应,前者为1.585(?),后者为1.62(?)。同时,与中心碳—碳键两端碳原子上相连接的取代基之间有明显的空间位阻效应。这些事实和分子中取代基的电子效应结合在一起,为标题化合物很容易分解为自由基,从而可有效地作为聚合引发剂提供了结构上的解释。用CNDO/2方法计算化合物1分子中各原子上的电荷分布和Wiberg键级,结果与晶体结构分析的结果吻合得很好。     

取代基效应对二取代二苯基硝酮还原电位的影响

合成了36种二取代二苯基硝酮XArCH＝N（O）ArY （简称XPNY）化合物,研究了取代基效应对其还原电位（E_red）的影响,并系统对比了XPNY与XArCH＝NArY （简称XBAY）和XArC（Me）＝NArY （简称XPEAY）还原电位的差异。研究结果表明：XPNY的E_red与C＝N键的¹³C NMR化学位移δ_C（C＝N）没有线性关系;XPNY、XBAY和XPEAY三类化合物的E_red之间没有线性关系,表现出不同的变化规律;X基团的激发态取代基效应和间位基团位置指示变量对化合物XPEAY和XBAY的E_red都有贡献,而对化合物XPNY的E_red贡献都很小,可忽略;Y基团的激发态取代基效应对化合物XPNY的E_red有一定贡献,而对化合物XPEAY和XBAY的E_red贡献很小,可忽略;XBAY和XPNY的母体有近似的还原电位,而XPEAY的母体其还原电位更低,一般而言,X-Y基团对相同时,XPEAY化合物更难被还原。     

2,3-二氰基-2,3-二(p-X苯基)丁二酸二乙酯的分子构型和取代基电子效应对13C NMR谱的影响

测定了meso-和dl-2,3-二氰基-2,3-二(p-X苯基)丁二酸二乙酯(X=OCH₃,CH₃,H,Cl,NO₂)的¹³CNMR谱。结果表明,中心碳-碳键两端相连基团的各碳原子的化学位移值相同,与dl-异构体相比,所有相应meso-异构体的乙氧羰基¹³CNMR吸收峰均处于高场。苯环对位取代基的Hammett基团常数σ与氰基碳原子和乙氧羰基中的羰基碳及次甲基碳的化学位移间线性相关,而且meso-异构体比dl-异构体有更好的线性关系。     

2,3-二氰基-2,3-二(对-硝基苯基)丁二酸二乙酯的分子结构和晶体结构

我们曾报导了2,3-二氰基-2,3-二苯基丁二酸二乙酯的分子结构,并对该化合物对烯烃单体进行自由基聚合的引发作用,热分解动力学和固态变温ESR谱等进行了研究。为了进一步探讨这类新型引发剂的结构与性能间的关系,我们继续用X射线衍射方法研究了2,3-二氰基-2,3-二(对-硝基苯基)丁二酸二乙酯的两种异构体1和2的分子结构和晶体结构。并讨论了分子中空间立体效应和电子效应对该化合物稳定性的影响。实验异构体1从乙酸乙酯和四氯化碳(9∶1,V/V)混合溶剂中重结晶,在空气中基本稳定。     

Crystal and Molecular Structure of 2,3-Tetramethylen-3,4-dihydroquinazoline

The crystal and molecular structure of the alkaolid 2,3-tetramethylen-3,4-dihydroquinazoline hydrochloride was solved by X-ray structure analysis. It was demonstrated that the crystal contains an aqua system similar to those in the isostructural 2,3-trimethylen-3,4-dihydroquinazoline (deoxypeganine), and 2,3-pentamethylen-3,4-dihydroquinazoline hydrochlorides.     

Matrix-induced Linear Stark Effect of Single Dibenzoterrylene Molecules in 2,3-Dibromonaphthalene Crystal

Absorption and fluorescence from single molecules can be tuned by applying an external electric field – a phenomenon known as the Stark effect. A linear Stark effect is associated to a lack of centrosymmetry of the guest in the host matrix. Centrosymmetric guests can display a linear Stark effect in disordered matrices, but the response of individual guest molecules is often relatively weak and non-uniform, with a broad distribution of the Stark coefficients. Here we introduce a novel single-molecule host-guest system, dibenzoterrylene (DBT) in 2,3-dibromonaphthalene (DBN) crystal. Fluorescent DBT molecules show excellent spectral stability with a large linear Stark effect, of the order of 1.5 GHz/kVcm⁻¹, corresponding to an electric dipole moment change of around 2 D. Remarkably, when the electric field is aligned with the a crystal axis, nearly all DBT molecules show either positive or negative Stark shifts with similar absolute values. These results are consistent with quantum chemistry calculations. Those indicate that DBT substitutes three DBN molecules along the a-axis, giving rise to eight equivalent embedding sites, related by the three glide planes of the orthorhombic crystal. The static dipole moment of DBT molecules is created by host-induced breaking of the inversion symmetry. This new host–guest system is promising for applications that require a high sensitivity of fluorescent emitters to electric fields, for example to probe weak electric fields.     

Crystal Structure and Conformation of a Dipyrrylmethane. The gem-Dimethyl Effect

Summary.  A crystal structure determination of the new dipyrrylmethane diethyl-2,3,5,5,7,8-hexamethyl-5,10-dihydrodipyrrin-1,9-dicarboxylate (1) is only the third reported for a dipyrrylmethane and the first with a gem-dimethyl group at the bridging carbon atom. Conformation determining torsion angles are compared to those from molecular mechanics calculations and to the corresponding data for an analogous dipyrrylmethane (2) with no gem-dimethyl moiety. The crystal structures of 1 and 2 differ significantly: 1 adopts the +ac,+ac or −ac,−ac conformation, whereas 2 exists in the −ac,+sc conformation in an intermolecularly hydrogen bonded dimer. There is no evidence for hydrogen bonding in crystals of 1, and its ac conformation is unlike that found about the central core of bilirubin (sc,sc). Taken collectively, the data indicate that the presence of a sterically demanding and potentially conformation distorting gem-dimethyl group located at the bridging carbon of a dipyrrylmethane (i) stabilizes a conformation that brings the pyrrole NH groups syn to the gem-dimethyls and (ii) would destabilize the ridge-title conformation of 10,10-dimethylbilirubin. Received December 3, 1999. Accepted December 15, 1999     

Crystal Structure and Conformation of an Imino Sugar

The title compound 3 was synthesized and characterized by IR,NMR and H-RMS,and its crystal structure was determined by X-ray diffraction analysis. The crystal is of monoclinic system (C16H23NO7S,Mr=373.41),space group P21,with a=12.000(2),b=5.5970(11),c=13.689(3),β=93.25(3)°,V=917.9(3)3,Z=2,Dc=1.351 g/cm3,F(000)=396,μ=0.213 mm-1,the final R=0.0489 and wR=0.0777 for observed reflections (I > 2σ(I)). Three intramolecular and three intermolecular hydrogen bonds were found. The absolute configuration of this molecule was confirmed by comparing with the original material.     

Crystal Structure and Conformation of a 10-Thiabilirubin

 A crystal structure determination of a bilirubin analog with a sulfur instead of a C(10)–CH₂ linking the two dipyrrinones is reported. Conformation-determining torsion angles and key hydrogen bond distances and angles are compared to those obtained from molecular dynamics calculations as well as to the corresponding data from X-ray determinations and molecular dynamics calculations of bilirubin. Like other bilirubins, the component dipyrrinones of the analog are present in the bis-lactam form with (Z)-configurated double bonds at C(4) and C(15). Despite the large differences in bond lengths and angles at –S–vs.–CH₂–, the crystal structure shows considerable similarity to bilirubin: both pigments adopt a folded, intramolecularly hydrogen-bonded ridge-tile conformation stabilized by six hydrogen bonds – although the interplanar angle of the ridge-tile conformation of the title compound is smaller (∼ 86°) than that of bilirubin (∼ 98°). The collective data indicate that even with long C–S bond lengths and a smaller C–S–C bond angle at the pivot point on the ridge-tile seam, intramolecular hydrogen bonding persists.     

Crystal Structure and Conformation of a Bilirubin Ester

Summary. Crystal structures determined for three bilirubin analogs with gem-dimethyl groups at C(10) are reported, including the first X-ray structure of a bilirubin dimethyl ester. Conformation-determining torsion angles and key hydrogen bond distances and angles were compared to those from molecular dynamics calculations. Like other rubins, the component dipyrrinones of the three compounds were found to adopt the syn conformation, with Z-configuration double bonds at C(4) and C(15) and bis-lactam tautomeric structures of the end rings. No large differences in bond lengths and bond angles at C(10) were found, and the crystal structures of the two 10,10-dimethyl rubin acids showed considerable similarity to that of bilirubin: both pigments adopt a folded, intramolecularly hydrogen bonded ridge-tile conformation stabilized by six hydrogen bonds, with an interplanar angle in ridge-tile of 98° and 86°. In contrast, the dimethyl ester is intermolecularly hydrogen bonded in the crystal. Each molecule of the ester has its two syn-Z-dipyrrinones rotated into a conformation syn to the gem-dimethyl group, whereas in the acids they are anti.     

Crystal Structure and Conformation of a 10-Thiabilirubin

Summary.  A crystal structure determination of a bilirubin analog with a sulfur instead of a C(10)–CH₂ linking the two dipyrrinones is reported. Conformation-determining torsion angles and key hydrogen bond distances and angles are compared to those obtained from molecular dynamics calculations as well as to the corresponding data from X-ray determinations and molecular dynamics calculations of bilirubin. Like other bilirubins, the component dipyrrinones of the analog are present in the bis-lactam form with (Z)-configurated double bonds at C(4) and C(15). Despite the large differences in bond lengths and angles at –S–vs.–CH₂–, the crystal structure shows considerable similarity to bilirubin: both pigments adopt a folded, intramolecularly hydrogen-bonded ridge-tile conformation stabilized by six hydrogen bonds – although the interplanar angle of the ridge-tile conformation of the title compound is smaller (∼ 86°) than that of bilirubin (∼ 98°). The collective data indicate that even with long C–S bond lengths and a smaller C–S–C bond angle at the pivot point on the ridge-tile seam, intramolecular hydrogen bonding persists. Received August 16, 2001. Accepted September 12, 2001     

Synthesis and Crystal Structure of 2,3-Dihydroxymethyl-N-cyclohexylaziridine

1 INTRODUCTION It is well known that aziridine compounds are wi- dely present in many natural products[1～3]. They contain unique nitrogen skeletons of three-membe- red heterocycles. Aziridine derivatives are also hi- ghly versatile synthetic precursors and have been used as synthons for chiral amines, amino acids, amino alcohos, alkaloids, and β-lactam antibiotics[4, 5]. On the basis of previous work, we have performed the tandem asymmetric Michael addition/internal nucleophilic substit…     

Conformation and Crystal Structure of Dipyrrinones with Oxindole Components

Summary.  Pyrrole α-aldehydes (2-formyl-4,5-dimethyl-1H-pyrrole and 2-formyl-N-methylpyrrole) condense readily at C(3) of indolin-2-ones to give dipyrrinone analogs, such as (3Z)-[(4,5-dimethylpyrrol-2-yl)-methylidenyl]-indolin-2-one and (3E)-[(1-methylpyrrol-2-yl)-methylidenyl]-indolin-2-one. ¹H-NMR NOE analyses and X-ray crystallography confirm the syn-(Z) configuration for the former and the syn-(E) configuration for the latter. The former is stabilized by intramolecular hydrogen bonding. Molecular mechanics calculations of the latter indicate no energy difference between the syn and anti conformations. Corresponding author. E-mail: lightner@scs.unr.edu Received August 2, 2002; accepted August 30, 2002