金属异头碳效应

异头碳效应是有机化学中经典立体电子效应的重要组成部分。异头碳效应可以解释吡喃糖中异头碳上电负性较大的取代基倾向于占据直立键,而非立体位阻更具优势的平伏键的现象。本文简要回顾了异头碳效应的发展历史及其重要应用,重点介绍了有机糖化学前沿的金属异头碳效应。当吡喃糖中异头碳上连接金属元素时,可以通过改变金属的种类、价态、配体以及底物结构有效调控金属异头碳效应。通过科研反哺本科教学,有助于化学相关专业学生进一步加深对金属异头碳效应这一新兴研究领域的理解和认识。     

H2XP…SHY复合物中磷键与硫键的理论研究

用从头算量子化学方法MP2 与CCSD(T)研究了H₂XP和SHY (X, Y=H, F, Cl, Br)分子的P与S之间形成的磷键X―P…S与硫键Y―S…P的本质与规律以及取代基X与Y对成键的影响. 计算结果表明, 硫键比磷键强, 连接在Lewis 酸上的取代基的电负性增大导致形成的磷键或硫键增强, 键能增大, 对单体的结构和性质的影响也增大; 而连接在Lewis 碱上的取代基效应则相反. 硫键键能为8.37-23.45 kJ·mol^-1, 最强的硫键结构是Y 电负性最大而X 电负性最小的HFS…PH₃, CCSD(T)计算的键能是16.04 kJ·mol^-1; 磷键键能为7.54-14.65 kJ·mol^-1, 最强的磷键结构是X 电负性最大而Y 电负性最小的H₂FP…SH₂, CCSD(T)计算的键能是12.52 kJ·mol^-1. 对磷键与硫键能量贡献较大的是交换与静电作用. 分子间超共轭lp(S)-σ^*(PX)与lp(P)-σ^*(SY)对磷键与硫键的形成起着重要作用, 它导致单体的极化, 其中硫键的极化效应较大, 从而有一定的共价特征.     

立体电子效应

立体电子效应是近年来有机化学研究的一个热点，有关的研究论文逐年增多。对于立体电子效应的认识始于构象分析和异头效应。     

异头效应与“糖”教学中的几个问题

从异头效应这一视角,阐释了构成淀粉和纤维素的葡萄糖结构单元之间α苷键和β苷键的不同及其如何导致结构的悬殊差异。籍此分析了α异头物和β异头物的相对稳定性、淀粉和纤维素酸性水解的难易,指出大自然对食物链的划分是精妙的。     

有机化学中的异头效应

与共轭效应、诱导效应和螺共轭效应一样,异头效应(anomeric effect)是有机化学中特殊的立体电子效应。在新型有机染料、医药、农药等高科技领域具有重要的理论意义。对理解有机反应的机理、预测反应产物的结构以及生理活性与结构的关系具有重要的意义,并能达到相当高的准确性。     

α,β——异头物的稳定性问题讨论

一、异头效应概念及偶极——偶极作用在环已烷多元取代物的椅式构象中,一般e键(下称横位)上取代基较多的构象较稳定;但若分子内有氢键或偶极作用力时,优势构象取决于各种相反作用力的强弱。如图1示出一些有机物的优势构象。其中a组构象因两极性取代基间的偶极——偶极作用,稳定的优势构象为两极性基团相距远、静电斥力小的aa键(竖位)取代构象。b组构象因这种排列有利于形     

共轭效应的本质及其在有机反应中的特殊性

共轭效应是有机化学中特别重要的电子效应之一,存在于共轭体系之中;大学有机化学教学目标要求学生掌握并能熟练应用共轭效应理论解决有机化学中的实际问题。由于共轭效应涉及微观认知,学生普遍感觉不易理解。但作为教学重点与难点,又必须掌握;同时也是学生大学毕业后继续深造学习所必须的基础理论知识,理解、掌握并能熟练应用该理论知识解决有机化学中的学习问题显得尤为重要。本文采用密度泛函理论方法计算并给出了3个二烯烃分子轨道中最高占据分子轨道(HOMO)、最低空分子轨道(LUMO)结构图,揭示了共轭体系内部因存在电荷转移现象而产生电子云的平均化本质,表现在键参数中的键长平均化变化。     

学习化学元素的电负性的体会

在学习化学元素电负性的概念、化合物的键型和元素的化学活泼性等问题中,我有一些学习心得,现归纳如下: 1.电负性的定义和计算方法L.Pauling在1932年首先提出电负性的概念.元素的电负性就是“元素的原子在化合物分子中把电子吸引向自己的能力”.计算电负性的方法有下列四种: (1)Pauling的热化学方法Pauling     

锂键

所谓氢键,就是一种三中心键,X—H…Y,其中X和Y较氢的电负性大,且Y含有非成键电子对。由于氢键的广泛存在,所以它在化学领域甚被重视。但这未必意味着氢是唯一有能力参与三中心互相作用的原子。几年前就有人提出,氢的同族元素——锂也可能有类似的相互吸引作用。     

电负性均衡

电负性是分子中一个原子把电子拉向它自身的能力,是化学理论的基本概念之一。继Pauling建立第一个电负性标度后,提出了众多的电负性标度。只是在密度泛函理论的基础上,电负性概念和电负性均衡原理,才被精密地论证。近二十多年来,电负性理论的重要发展是:应用电负性均衡模型或方法,可以快速地计算分子体系的电荷分布,从而确定分子的其他性质,甚至包括分子的结构和反应性指标。通常的电负性均衡方法只把分子划分到原子区域,虽然简单直观,但其精度和应用范围受到限制。原子与键电负性均衡方法,把分子划分到包括原子区域、化学键区域和孤对电子区域,能够较快速精密地计算分子的电荷分布和其他性质,并被应用到构建新一代可极化或浮动电荷力场的探索中,有广阔的应用前景。     

How the Generalized Anomeric Effect Influences the Conformational Preference

The generalized anomeric effect refers to the conformational preference of a gauche structure over an anti structure for molecules with a R‐X‐C‐Y moiety. Whereas there are conflicting reports regarding the origin of this ubiquitous effect, a general consensus is that both the steric (more specifically electrostatic) and hyperconjugative interactions contribute. Here we employed the block‐localized wavefunction (BLW) method, which is the simplest variant of ab initio valence bond (VB) theory and can define reference electron‐localized states self‐consistently, to evaluate the magnitude of the hyperconjugation effect in a number of acyclic molecules exhibiting the generalized anomeric effect. The BLW‐based energy decomposition analysis revealed that both the steric and hyperconjugation effects contribute to the conformational preferences of methoxymethyl fluoride and methoxymethyl chlorides. But for the other systems under investigation, including methanediol, methanediamine, aminomethanol and dimethoxymethane, the hyperconjugative interactions play a negative role in the conformational preferences and the steric effect is solely responsible for the generalized anomeric effect.     

Alcohols,ethers, carbohydrates,and related compounds. II. The anomeric effect

The anomeric effect has been studied for a variety of compounds using the MM4 force field, and also using MP2/6-311++G(2d,2p) ab initio calculations and experimental data for reference purposes. Geometries and energies, including conformational, rotational barriers, and heats of formation were examined. Overall, the agreement of MM4 with the experimental and ab initio data is good, and significantly better than the agreement obtained with the MM3 force field. The anomeric effect is represented in MM4 by various explicit terms in the force constant matrix. The bond length changes are accounted for with torsion-stretch elements. The angle changes are accounted for with torsion-bend elements. The energies are taken into account with a number of torsional terms in the usual way. A torsion-torsion interaction is also of some importance. With all of these elements included in the calculation, the MM4 results now appear to be adequately accurate. The heats of formation were examined for a total of 12 anomeric compounds, and the experimental values were fit by MM4 with an RMS error of 0.42 kcal/mol.     

Role of the anomeric effect in methanediamines in the gas phase and aqueous solutions

An ab initio study of methylenediamine and several methylated derivatives in the gas phase and aqueous solution was performed. The conformational preferences can be considered adequately described at the HF/6‐31G**//HF/6‐31G** level, because these results agree with those obtained using larger basis sets and including ZPE and electron correlation. The energy ordering is clearly dependent on the number and position of the methyl groups present in a molecule. For a first set of the compounds, the energies obtained were interpreted in terms of the anomeric effect because the favored conformers show two or one anti orientation between the nitrogen lone pair and the C N polar bond. Reverse anomeric effects were found for a second set of compounds. The NBO analysis was used to interpret these energetic tendencies and the rotational barrier around the N C bonds. Thus, the preference for the anti‐Lp N C N orientations is mainly due to charge delocalization, always stronger than the electrostatic and steric contributions included in the Lewis term. However, the origin for the reverse anomeric effect may be related to the steric hindrance associated with the methylation. The influence of water on the conformational preferences was evaluated by means of the PCM method. Contrary to expectation, the anomeric effect is not reduced in water, and the most stable conformers in the gas phase are maintained in solution. The electrostatic term of the free solvation energy is the main responsible of the energetic changes, and depends strongly on local solute–solvent interactions. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 135–150, 2001     

Atoms in molecules interpretation of the anomeric effect in the O--C--O unit

The conformational preferences of two model compounds for the O--CH2--O anomeric unit: methanediol and dimethoxymethane analyzed within the framework of the QTAIM theory provide a new interpretation of the anomeric effect. The characteristic stabilization of the gauche conformers of these compounds is accompanied by a progressive reduction of the electron population of the hydrogens of the central methylene as the number of their gauche interactions to lone pairs rises. The electron population removed from these atoms during the conformational change is gained in the gauche conformers by atoms of larger atomic number, which results in a more negative molecular energy. Also, the variations displayed by atomic populations and the QTAIM delocalization indexes are not keeping in line with the hyperconjugative model of the anomeric effect.     

Comparative performance of MM3(92) and two TINKER MM3 versions for the modeling of carbohydrates

The 1992 version of MM3 was largely used for modeling mono-, di-, and trisaccharides. In later versions of MM3 improvements were made in some parameters that may be important for carbohydrates. This corrected MM3 force field is part of the Tinker package, freely available (as its 4.1 version), and included in the Chem 3D Ultra 8.0 package (as the 3.7 version). The latter version lacks the corrections to the standard bond lengths produced by electronegativity and anomeric effects, whereas the Tinker 4.1 version only lacks the latter correction. The present work compares the performance of the three MM3 versions (and in some cases, DFT and/or HF/ab initio procedures) on several carbohydrate model problems as the chair and rotamer equilibria in 2-hydroxy- and 2-methoxytetrahydropyran, hydrogen bonding in cis-2,3-dihydroxytetrahydropyran, and the potential energy surfaces around the glycosidic bonds of two sulfated disaccharides and two trisaccharides. Tinker MM3 can be used accurately to estimate carbohydrate energies and geometries, and-with the help of some programming-to pursue studies on the potential energy surfaces of di- and trisaccharides. In most cases results obtained using the three MM3 versions are similar, although large energy differences are obtained when comparing a rotameric distribution around a O-C-O-H dihedral, which is almost forced to the exo-anomeric position by the Tinker versions. In other systems smaller energy differences are found, but they can nevertheless lead to a different global minimum when comparing conformers of similar energy. MM3(92) establishes better the differences between the bond lengths in both anomers, as an expected expression of the anomeric correction.     

gem‐Difluorocarbadisaccharides: Restoring the exo‐Anomeric Effect

Molecular mimicry is an essential part of the development of drugs and molecular probes. In the chemical glycobiology field, although many glycomimetics have been developed in the past years, it has been considered that many failures in their use are related to the lack of the anomeric effects in these analogues. Additionally, the origin of the anomeric effects is still the subject of virulent scientific debates. Herein, by combining chemical synthesis, NMR methods, and theoretical calculations, we show that it is possible to restore the anomeric effect for an acetal when replacing one of the oxygen atoms by a CF₂ group. This result provides key findings in chemical sciences. On the one hand, it strongly suggests the key relevance of the stereoelectronic component of the anomeric effect. On the other hand, the CF₂ analogue adopts the natural glycoside conformation, which might provide new avenues for sugar‐based drug design.     

二氯亚甲基环己烷的卤化反应和2-取代二氯亚甲基环己烷的构象

多卤代烃在有机反应机理和结构研究中有着重要意义,作为一个有用的合成中间体,其制备也受到人们重视。我们在此报道二氯亚甲基环已烷(1)与氯及溴的反应和其它有关反应的实验结果。 1 于氯作用,主要得到加成产物1—氯—1—     

Anomeric-Schleyer hyperconjugative interaction as a convenient avenue for aromaticity enhancement of phospholes

Due to the insufficient interaction of the phosphorus lone pair with the butadiene moiety, the aromaticity of the phosphole ring is lower than that of its counterpart's pyrrole, furan, and thiophene. Considering the high importance of phosphole core in organic chemistry, increasing its stability through reinforcement its aromaticity can be very valuable. In the present work, the aromaticity of the phosphole on the anomeric carbon in both the axial and equatorial conformers of the unsaturated six-membered heterocycles, using structural, electronic, energetic, and magnetic indices were investigated by the DFT-B3LYP/6-311++G(d,p) computational method. Electron pumping through anomeric and then Schleyer hyperconjugative interaction increase the aromaticity of the phosphole ring in axial conformer of compounds 1–11 . Based on various types of aromaticity indices, the results showed that the phosphole ring in the axial position has far higher aromaticity than the equatorial position. The phosphole ring containing cyano groups shows an efficient anomeric effect and, as a result, higher aromaticity. Excellent correlations were observed between aromaticity indices with different backgrounds.     

When anomeric effects collide

Rotational coordinates about the C(3)–O(4) bonds of 2,4‐dioxaheptane (DOH) and 2,4,6‐trioxaheptane (TOH) are compared at correlated levels of electronic structure theory for gauche and trans orientations of the O(2)–C(3) bonds. TOH has overlapping anomeric effects, while DOH does not. The overlapping stereoelectronic effect shows its largest impact on the length of the O(2)–C(3) bond, which is typically 0.02 Å longer in DOH than in TOH. However, the energetic consequences of the overlapping anomeric effect in TOH are very small, as judged by total conformational energies and analysis of delocalization energies within a natural bond orbital framework. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1194–1204, 2001     

Anomeric DNA Strand Displacement with α-D Oligonucleotides as Invaders and Ethidium Bromide as Fluorescence Sensor for Duplexes with α/β-, β/β- and α/α-D Configuration

DNA strand displacement is a technique to exchange one strand of a double stranded DNA by another strand (invader). It is an isothermal, enzyme free method driven by single stranded overhangs (toeholds) and is employed in DNA amplification, mismatch detection and nanotechnology. We discovered that anomeric (α/β) DNA can be used for heterochiral strand displacement. Homochiral DNA in β-D configuration was transformed to heterochiral DNA in α-D/β-D configuration and further to homochiral DNA with both strands in α-D configuration. Single stranded α-D DNA acts as invader. Herein, new anomeric displacement systems with and without toeholds were designed. Due to their resistance against enzymatic degradation, the systems are applicable to living cells. The light-up intercalator ethidium bromide is used as fluorescence sensor to follow the progress of displacement. Anomeric DNA displacement shows benefits over canonical DNA in view of toehold free displacement and simple detection by ethidium bromide.