Unraveling the Role of Water in the Stereoselective Step of Aqueous Proline‐Catalyzed Aldol Reactions

A multiscale computational study was performed with the aim of tracing the source of stereoselectivity and disclosing the role of water in the stereoselective step of propionaldehyde aldol self‐condensation catalyzed by proline amide in water, a reaction that serves as a model for aqueous organocatalytic aldol condensations. Solvent mixing and hydration behavior were assessed by classical molecular dynamics simulations, which show that the reaction between propanal and the corresponding enamine takes place in a fully hydrated environment. First‐principles molecular dynamics simulations were used to study the free‐energy profile of four possible reaction paths, each of which yields a different stereoisomer, and high‐level static first‐principles calculations were employed to characterize the transition states for microsolvated species. The first solvation shell of the oxygen atom of the electrophilic aldehyde at the transition states contains two water molecules, each of which donates one hydrogen bond to the nascent alkoxide and thereby largely stabilizes its excess electron density. The stereoselectivity originates in an extra hydrogen bond donated by the amido group of proline amide in two reaction paths.     

Alternative low-energy mechanisms for isotopic exchange in gas-phase D2O-H+(H2O)n reactions.

Molecular-dynamics (MD) trajectories and high-level ab initio methods have been used to study the low-energy mechanism for D(2)O-H(+)(H(2)O)(n) reactions. At low collisional energies, MD simulations show that the collisional complexes are long-lived and undergo fast monomolecular isomerization, converting between different isomers within 50-500 ps. Such processes, primarily involving water-molecule shifts along a water chain, require the surmounting of very-low-energy barriers and present sizable non- Rice-Ramsperger-Kassel-Marcus (RRKM) effects, which are interpreted as a lack of randomization of the internal kinetic energy. Interestingly, the rate of water shifts was found to increase upon increasing the size of the cluster. Based on these findings, we propose to incorporate the following steps into the mechanism for low-energy isotopic scrambling these D(2)O-H(+)(H(2)O)(n) reactions: a) formation of the collisional complex [H(+)(H(2)O)(n)D(2)O]* in a vibro-rotational excited state; b) incorporation of the heavy-water molecule in the cluster core as HD(2)O(+) by means of isomerization involving molecular shifts; c) displacement of solvation molecules from the first shell of HD(2)O(+) inducing de-deuteration (shift of a D(+) to a neighbor water molecule); d) reorganization of the clusters and/or expulsion of one of the isotopic variants of water (H(2)O, HDO or D(2)O) from the periphery of the complex.     

分子形貌所指示的羟基卡宾及其衍生物的质子转移反应

分子形貌(Molecular face, MF)定义分子的内禀电子转折边界面,同时在其上计算并描绘出前沿电子密度(MFED). MF不仅能显示分子的形状和大小,还能够指示分子的化学反应性.应用M06-2X/6-311++G(d,p)理论方法,对单线态和三线态羟基卡宾分子及其衍生物的质子化反应进行研究并计算了这些反应的活化能.结果表明,吸电性和供电性较强的取代基,均使单线态反应活化能增大,只有吸电性较强的─CN才能使三线态反应活化能增大.应用分子形貌理论研究了上述反应,不仅展示出分子的形貌变化、与反应位点的关联,以及有关物理量的变化倾向,而且还定量地显示出,单线态羟基卡宾及其衍生物分子边界面上前沿电子密度极大值与极小值的差值,与其质子转移反应的活化能之间存在线性相关.     

Kinetic Isotope Effects in Asymmetric Reactions

Kinetic isotope effects are exquisitely sensitive probes of transition structure. As such, kinetic isotope effects offer a uniquely useful probe for the symmetry‐breaking process that is inherent to stereoselective reactions. In this Concept article, we explore the role of steric and electronic effects in stereocontrol, and we relate these concepts to recent studies carried out in our laboratory. We also explore the way in which kinetic isotope effects serve as useful points of contact with computational models of transition structures. Finally, we discuss future opportunities for kinetic isotope effects to play a role in asymmetric catalyst development.     

Chirality Transfer and Memory of Chirality in Gold‐Catalyzed Reactions

Over the last few years, gold‐catalyzed reactions that involved chirality transfer and memory of chirality (MOC) have emerged as a powerful tool in enantioselective synthesis. This technique has allowed for the single‐step synthesis of enantioenriched compounds from readily available starting materials. This Focus Review discusses this emerging field with an emphasis on mechanistic aspects and their applications in synthetic organic chemistry.     

某些电荷转移反应中线性反应坐标确定的反应途径的非唯一性

在电子转移半经典模型的基础上,对氟代苯并菲组成的盘状液晶体系的电荷转移反应进行了研究,其反应动力学参数在量子化学B3LYP/6-31G(d,p)水平进行计算.研究中发现在反应物和产物的构型确定后,用线性反应坐标构造的反应途径和过渡态的构型不是唯一的.其原因是反应物和产物确定后,其独立内坐标的选择并不是唯一的.因此在计算电荷转移速率常数时必须考虑到多个过渡态构型才能得到定性正确的结果.     

Double‐Oxygen‐Atom Transfer in Reactions of CemO2m+ (m=2–6) with C2H2

Cerium oxide cluster cations (Ce_mO_n⁺, m=2–16; n=2m, 2m±1 and 2m±2) are prepared by laser ablation and reacted with acetylene (C₂H₂) in a fast‐flow reactor. A time‐of‐flight mass spectrometer is used to detect the cluster distribution before and after the reactions. Reactions of stoichiometric Ce_mO_2m⁺ (m=2–6) with C₂H₂ produce Ce_mO_2m?2⁺ clusters, which indicates a “double‐oxygen‐atom transfer” reaction Ce_mO_2m⁺+C₂H₂→Ce_mO_2m?2⁺+(CHO)₂ (ethanedial). A single‐oxygen‐atom transfer reaction channel is also identified as Ce_mO_2m⁺+C₂H₂→Ce_mO_2m?1⁺+C₂H₂O (at least for m=2 and 3). Density functional theory calculations are performed to study reaction mechanisms of Ce₂O₄⁺+C₂H₂, and the calculated results confirm that both the single‐ and double‐oxygen‐atom transfer channels are thermodynamically and kinetically favourable.     

质子转移反应质谱的建立与性能研究

报道了自行研制的质子转移反应质谱的基本结构和性能。利用水蒸气辉光放电产生了反应离子H3O ,以合成空气为反应气体,测量了H3O 与合成空气中的水反应产生的团簇离子H3O (H2O)n的质谱。实验发现,当漂移管电场与分子密度比值为144Td时,增加的离子能量可以阻止团簇离子H3O (H2O)n的形成,质谱观察到的离子主要是H3O ,其纯度可达99%以上,这时H3O 与有机物分子如甲苯的质子转移反应的产物离子也呈单一形式,团簇离子得到很好地抑制。根据离子强度和离子反应时间等参数,获得了PTR-MS目前的检出限为10-8(V/V)。利用PTR-MS对标准浓度甲苯及其稀释气体进行检测,表明PTR-MS在线定量检测准确性良好,线性动态范围跨越3个数量级,能够应用于大气中痕量挥发性有机物的实时在线测量。     

Electron Transfer and Charge Transfer: Twin Themes in Unifying the Mechanisms of Organic and Organometallic Reactions

The broad varieties of organic and organometallic reactions merge into a common unifying mechanism by considering all nucleophiles and electrophiles as electron donors (D) and electron acceptors (A), respectively. Comparison of outer-sphere and inner-sphere electron transfers with the aid of Marcus theory provides the thermochemical basis for the generalized free energy relationship for electron transfer (FERET) in Equation (37) and its corollaries in Equations (43) and (44) that have wide predictive applicability to electrophilic aromatic substitutions, olefin additions, organometallic cleavages, etc. The FERET is based on the conversion of the weak nucleophile–electrophile interactions extant in the ubiquitous electron donor—acceptor (EDA) precursor complex [D, A] to the radical ion pair [D^⊕, A^?], for which the free energy change can be evaluated from the charge-transfer absorption spectra according to Mulliken theory. FERET analysis thus indicates that the charge-transfer ion pairs [D^⊕, A^?] are energetically equivalent to the transition states for nucleophile/electrophile transformations. The behavior of such ion pairs can be directly observed immediately following the irradiation of the charge-transfer bands of various EDA complexes with a 25-ps laser pulse. Such studies confirm the radical ion pair [Arene^⊕, NO₂] as a viable intermediate in electrophilic aromatic nitration, as presented in the electron-transfer mechanism between arenes and the nitryl cation (NO) electrophile.     

Solvent‐Slaved Protein Motions Accompany Proton but Not Hydride Tunneling in Light‐Activated Protochlorophyllide Oxidoreductase

H ⁺ but not H ^? : The reduction reaction of protochlorophyllide catalyzed by protochlorophyllide oxidoreductase features solvent‐slaved motions that control the proton‐ but not the hydride‐tunneling mechanism. These motions imply a long‐range dynamic network from the solvent to the enzyme active site that facilitate proton transfer (see picture, left). Motions for hydride transfer are more localized and are not slaved by the solvent (see picture, right).

     

Distal charge transport in peptides

Biological systems often transport charges and reactive processes over substantial distances. Traditional models of chemical kinetics generally do not describe such extreme distal processes. In this Review, an atomistic model for a distal transport of information, which was specifically developed for peptides, is considered. Chemical reactivity is taken as the result of distal effects based on two-step bifunctional kinetics involving unique, very rapid motional properties of peptides in the subpicosecond regime. The bifunctional model suggests highly efficient transport of charge and reactivity in an isolated peptide over a substantial distance; conversely, a very low efficiency in a water environment was found. The model suggests ultrafast transport of charge and reactivity over substantial molecular distances in a peptide environment. Many such domains can be active in a protein.     

微带电极上同时受扩散、化学反应、电极反应速率控制的伏安方程

运用积分变换的方法推导了超微带电极上同时受扩散、化学反应、电化学反应动力学控制的伏安关系,得到了前行化学反应、平行化学反应和后行化学反应的准可逆伏安方程,并列出了计算所得的典型伏安曲线。     

Electron Transfer around a Molecular Corner

The distance dependence of electron transfer (ET) is commonly investigated in linear rigid rod‐like compounds, but studies of molecular wires with integrated corners imposing 90° angles are very rare. By using spirobifluorene as a key bridging element and by substituting it at different positions, two isomeric series of donor‐bridge‐acceptor compounds with either nearly linear or angled geometries were obtained. Photoinduced ET in both series is dominated by rapid through‐bond hole hopping across oligofluorene bridges over distances of up to 70 Å. Despite considerable conformational flexibility, direct through‐space and through‐solvent ET is negligible even in the angled series. The independence of the ET rate constant on the total number of fluorene units in the angled series is attributed to a rate‐limiting tunneling step through the spirobifluorene corner. This finding is relevant for multidimensional ET systems and grids in which individual molecular wires are interlinked at 90° angles.