Petasis反应研究进展

Petasis反应是构建氨基酸、氨基醇及其相应的衍生物等结构片段的有效方法之一,其中手性Petasis反应已应用于天然产物和药物的片段合成.就文献中机理、组分、反应条件等方面的研究进行了综述,并对Petasis反应的应用实例进行了讨论.     

不对称Petasis反应在手性胺类化合物合成中的应用

手性胺类化合物广泛存在于天然产物、药物分子和多功能材料中,而且作为重要中间体、催化剂和手性辅剂在有机合成中也有广泛的应用,因此,发展高效的方法合成各种手性胺化合物及相应的骨架结构具有重要的科学意义和应用价值.有机硼试剂、胺和羰基化合物参与的不对称Petasis三组分反应是构建手性胺及其衍生物最简洁、高效的方法之一.近年来,利用该策略来构建手性胺类化合物引起了广泛的关注.文章综述了不对称Petasis反应合成手性胺类化合物的近期研究进展,主要包括手性胺源、手性羰基化合物和手性硼试剂参与的底物诱导的不对称Petasis反应,以及手性催化剂促进的不对称Petasis反应,并对该领域的挑战和未来发展方向进行简要讨论.     

萤火虫生物发光中加氧反应机理的理论研究

萤火虫生物发光是最常见的生物发光,在生物和医药等领域已得到重要应用.发光过程涉及到其活体内的一系列复杂的化学反应.引起发光的起始反应是单重态荧光素分子与三重态氧气的加成反应.这是一个自旋禁阻的酶催化反应,通常效率很低,但萤火虫是目前将化学能转化为光能最高效的系统.这个自旋禁阻的反应为什么能高效率发生?实验研究认为该反应由单电子转移(SET)诱发而发生,但对反应的详细过程和机理并没有完整的描述.本工作通过理论计算,描述了该反应的完整过程,解释了这个自旋禁阻反应高效发生的原因.     

The Origin of Stereoselectivity in the Hydrogenation of Oximes Catalyzed by Iridium Complexes: A DFT Mechanistic Study

Herein the reaction mechanism and the origin of stereoselectivity of asymmetric hydrogenation of oximes to hydroxylamines catalyzed by the cyclometalated iridium (III) complexes with chiral substituted single cyclopentadienyl ligands (Ir catalysts A1 and B1) under acidic condition were unveiled using DFT calculations. The catalytic cycle for this reaction consists of the dihydrogen activation step and the hydride transfer step. The calculated results indicate that the hydride transfer step is the chirality-determining step and the involvement of methanesulfonate anion (MsO⁻) in this reaction is of importance in the asymmetric hydrogenation of oximes catalyzed by A1 and B1. The calculated energy barriers for the hydride transfer steps without an MsO⁻ anion are higher than those with an MsO⁻ anion. The differences in Gibbs free energies between TSA5−1fR/TSA5−1fS and TSB5−1fR/TSB5−1fS are 13.8/13.2 (ΔΔG^‡ = 0.6 kcal/mol) and 7.5/5.6 (ΔΔG^‡ = 1.9 kcal/mol) kcal/mol for the hydride transfer step of substrate protonated oximes with E configuration (E−2a−H⁺) with MsO⁻ anion to chiral hydroxylamines product R−3a/S−3a catalyzed by A1 and B1, respectively. According to the Curtin–Hammet principle, the major products are hydroxylamines S−3a for the reaction catalyzed by A1 and B1, which agrees well with the experimental results. This is due to the non-covalent interactions among the protonated substrate, MsO⁻ anion and catalytic species. The hydrogen bond could not only stabilize the catalytic species, but also change the preference of stereoselectivity of this reaction.     

不饱和硼烯HClN=B:重排反应的DFT研究

采用量子化学中的密度泛函方法, 在B3LYP/6-311G**水平上研究了不饱和硼烯HClN=B:的重排反应机理。结果表明, 无论是氢原子H迁移还是氯原子Cl迁移都经过1个三元环过渡态,生成直线型产物。但氢原子H迁移是在面内进行, 而氯原子Cl迁移是在面外进行。根据计算结果,详细研究了不饱和硼烯HClN=B:重排反应的热力学及动力学函数, 在此基础上讨论了不饱和硼烯HClN=B:的存在寿命问题。     

Acylation Reactions of Dibenzo-7-phosphanorbornadiene: DFT Mechanistic Insights

Extensive DFT calculations provide deep mechanistic insights into the acylation reactions of tert-butyl dibenzo-7-phosphanobornadiene with PhCOX (X=Cl, Br, I, OTf) in CH₂Cl₂ solution. Such reactions are initialized by the nucleophilic P⋅⋅⋅C attack to the carbonyl group to form the acylphosphonium intermediate A⁺ together with X⁻ anion, followed either by nucleophilic X⁻⋅⋅⋅P attack (X=Cl, Br, and I) toward A⁺ to eliminate anthracene or by slow rearrangement or decomposition of A⁺ (X=OTf). In contrast to the first case (X=Cl) that is rate-limited by the initial P⋅⋅⋅C attack, other reactions are rate-limited by the second X⁻⋅⋅⋅P attack for X=Br and I and even thermodynamically prevented for X=OTf, leading to isolable phosphonium salts. The rearrangement of phosphonium A⁺ is initiated by a P-C bond cleavage, followed either by sequential proton-shifts to form anthracenyl acylphosphonium or by deprotonation with additional base Et₃N to form neutral anthracenyl acylphosphine. Our DFT results strongly support the separated acylphosphonium A⁺ as the key reaction intermediate that may be useful for the transfer of acylphosphenium in general.     

Mechanistic Study of Ozone-Water Reaction and Their Reaction Products with Gas Phase Elemental Mercury:A Computational Study

The interaction of oxygen of water and central oxygen of ozone produces stable H₂O‐O₃ complex with no barrier. With decomposition of this complex through H‐abstraction by O₃ and O‐abstraction by H₂O, four possible product channels have been found. The reaction of mercury and the products of water‐ozone reaction have been studied. All geometrical and AIM parameters of intermediate, transition states, and the products of reactions are calculated and thermodynamic parameters are obtained. The negative value of free energy show that channels Hg+H₂OO, Hg+H₂O₂ and Hg+H₂O₄ in hydrogen tetroxide form (HTO) may be the main reaction channels.     

甲醇溶剂中Baylis-Hillman反应机理的DFT研究

在B3LYP/6-311＋＋G**水平下计算研究了三甲胺催化的丙烯醛和甲醛在甲醇溶剂中的Baylis-Hillman反应, 甲醇溶剂效应采用超分子模型研究, 获得了反应中各物种的优化构型及其能量, 探讨了甲醇溶剂中Baylis-Hillman反应的微观过程.     

2，5－二叔丁基对二甲氧基苯的去叔丁基反应的研究

２，５－二叔丁基对二甲氧基苯（１）在ＣＦ_３ＣＯ_２Ｈ，ｎ－Ｃ_３Ｆ_７ＣＯ_２Ｈ或ＡｌＣｌ_３－ＣＨ_２Ｃｌ_２体系中易发生去叔丁基化反应，而在强质子酸ＨＩ（ａｑ）．ＨＢＦ_４或ＣＣｌ_３ＣＯ_２Ｈ中不发生此反应。在前一类体系中，都可在反应过程中观察到较强的阳离子基１￣＋的ＥＰＲ信号，而在后一类体系中则无此信号。动力学测定表明，受物１与三氟乙酸的反应表现为三级动力学，其中１为二级，ＣＦ_３ＣＯ_２Ｈ为一级。这些结果揭示了去叔丁基化反应与离子自由基的形成有着内在的联系。     

New Mechanistic Aspects of the Fenton Reaction

The kinetics of the Fenton reaction was studied in detail. A second reaction step in the presence of excess H₂O₂ is attributed to formation of the complex Fe^III(^?O₂H)_aq. Therefore, the reaction of Fe(H₂O)₆²⁺ with Fe^III(^?O₂H)_aq in the presence of Fe^II to form Fe^III_aq (k=(7.7±1.5)×10⁵ M ^?1 s^?1) may contribute to the overall Fenton reaction, and could account for some of the debate in the literature concerning its detailed mechanism. If this is correct for LFe^III(^?O₂H)_aq also, then it might be of significant biological importance. The activation parameters ΔH^≠, ΔS^≠, and ΔV^≠ for the Fenton reaction were measured under various experimental conditions, and are used in the mechanistic interpretation.     

Solvent-free Synthesis of Alkylaminophenols via Petasis Boronic Mannich Reaction in One Pot without Catalysts

By using solvent‐free and heat conditions, the Petasis boronic Mannich reaction of salicylaldehydes with various boronic acids and secondary amines without catalyst is described. The alkylaminophenols were obtained in moderate to good yields in a shorter reaction time than the traditional methods.     

Au(111)表面上乙醇选择性氧化反应机理的密度泛函理论研究(英文)

采用密度泛函理论研究了吸附有O原子的Au(111)表面上乙醇选择性氧化的反应机理.反应结果表明,除O原子和中间产物二齿醋酸根(CH3CHOO)外,其他中间产物在Au(111)表面扩散能垒均较低,不会对反应速控步骤的确定造成影响.乙醇羟基氧化脱氢为反应的第一步骤,当氧化剂为吸附态的O原子或者为OH基时,反应活化能分别为0.20和0.17eV.氧化产物乙氧基(CH3CH2O)进一步氧化脱氢生成乙醛则需要表面吸附的O原子或另一表面吸附的OH基的参与,所需活化能为0.29或0.27eV.同时,乙醛易与表面吸附的乙氧基反应生成乙氧基半缩醛(CH3CHOOC2H5),其可进一步与O原子作用,脱氢形成乙酸乙酯.此外,在乙醛深度氧化成酸的过程中需要克服较高的反应能垒,因而在表面反应温度较低时无法进行,这与实验结果相符.     

A DFT Study of the Reaction of Acrylamide with L-Cysteine and L-Glutathione

Thermal processing of certain foods implies the formation of acrylamide, which has been proven to provoke adverse effects on human health. Thus, several strategies to mitigate it have been developed. One of them could be the application of organosulfur compounds obtained from natural sources to react with the acrylamide, forming non-toxic adducts. A DFT study of the acrylamide reaction with the organosulfur model compounds L-cysteine and L-glutathione by Michael addition and a free radical pathway complemented by a kinetic study of these model molecules has been applied. The kinetic evaluation results demonstrate that the L-glutathione reaction exhibited a higher rate constant than the other studied compound.     

Intramolecular Oxonium-Ene Reaction: Scope and Mechanistic Studies

Substituted tetrahydrofurans are widely used as synthetic intermediates and they are also featured widely in natural products. Among the many methods available for the synthesis of this class of compounds, intramolecular-ene reaction is one of the most attractive methods. Recently, Lob group has established a convergent approach to various cyclic ethers via an intramolecular (3,5)-oxonium-ene type cyclizations.[1,2] We employed E-homoallylic alcohols with aldehydes using indium triflate as a catalyst to study intramolecular (3,5)-oxonium-ene type cyclizations, with the yields ranging from 37% to 95% and the selectivities de from 60 : 40 to 99; 1.     

丙烯醛和甲醛的Baylis-Hillman反应机理的DFT研究

采用密度泛函DFT-B3LYP方法在6-311++G**基组下计算研究了三甲胺催化的丙烯醛和甲醛的Baylis-Hillman反应的微观过程,获得了两种反应通道(分别对应于顺式-丙烯醛和反式-丙烯醛)的势能面.结合CH3OH溶剂效应,探讨了两种反应通道的微观机理.     

Mechanistic Study of Pd/NHC-Catalyzed Sonogashira Reaction: Discovery of NHC-Ethynyl Coupling Process

The product of a revealed transformation—NHC-ethynyl coupling—was observed as a catalyst transformation pathway in the Sonogashira cross-coupling, catalyzed by Pd/NHC complexes. The 2-ethynylated azolium salt was isolated in individual form and fully characterized, including X-ray analysis. A number of possible intermediates of this transformation with common formulae (NHC)_nPd(C₂Ph) (n=1,2) were observed and subjected to collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) experiments to elucidate their structure. Measured bond dissociation energies (BDEs) and IRMPD spectra were in an excellent agreement with quantum calculations for coupling product π-complexes with Pd⁰. Molecular dynamics simulations confirmed the observed multiple CID fragmentation pathways. An unconventional methodology to study catalyst evolution suggests the reported transformation to be considered in the development of new catalytic systems for alkyne functionalization reactions.     

Mechanistic Aspects of the Palladium-Catalyzed Suzuki-Miyaura Cross-Coupling Reaction

The story of C−C bond formation includes several reactions, and surely Suzuki-Miyaura is among the most outstanding ones. Herein, a brief historical overview of insights regarding the reaction mechanism is provided. In particular, the formation of the catalytically active species is probably the main concern, thus the preactivation is in competition with, or even assumes the role of the rate determining step (rds) of the overall reaction. Computational chemistry is key in identifying the rds and thus leading to milder conditions on an experimental level by means of predictive catalysis.     

Acid-Catalyzed Rearrangements of 3-Aryloxirane-2-Carboxamides: Novel DFT Mechanistic Insights

Efficient synthesis of 3-arylquinolin-2(1H)-ones and N-(2-carboxyaryl)-oxalamides from protic acid-catalyzed rearrangements of 3-aryloxirane-2-carboxamides was achieved recently but not well understood. In contrast to the classical Meinwald rearrangement, extensive DFT calculations reveal that the proximal aryl and amide groups have strong synergetic effects to control the amide-aided and aryl-directed oxirane-opening and further rearrangement sequences. The ortho-nitro substituent of the proximal aryl is directly involved in a nucleophilic oxirane ring-opening, the amide C=O is an important proton shuttle for facile H-shifts, while the N-aryl may act as a potential ring-closing site via Friedel-Crafts alkylation. The mechanistic insights are useful for rational design of novel synthesis by changing the aryl and amide functional groups proximal to the oxirane ring.