铂催化2-烯炔苯甲醛水合环化反应的机理及化学选择性的理论研究（英文）

采用密度泛函理论(DFT)的计算方法,研究了铂催化2-烯炔基苯甲醛水合环化反应的微观机理及化学选择性的根源.计算结果表明,首先炔基被催化活化而发生亲核环化生成吡喃铂中间体;接着吡喃铂中间体与烯烃双键发生[3+2]环加成生成铂-碳卡宾复合物;之后,反应将沿2条路径进行,得到产物3a或4a,其中4a的生成需经两步水分子辅助的质子转移过程.生成产物3a需要克服的活化能垒为146.5 k J/mol;对4a的生成,烯醇式和酮式互变异构是决速步聚,当一个水分子参与反应时,对应的能垒为185.8 k J/mol,当2个和3个水分子参与反应时,能垒分别降低到128.1和64.9 k J/mol.因此,水分子参与催化得到产物4a的路径是有利的.另外,反应的选择性与在异构化过程中水的共催化作用有关.以上结果很好地解释了实验现象,并为铂催化水环化反应提供新的见解.     

(H_2O)_n(n=0～2)催化HS和HOCl反应机理的理论研究

在CCSD(T)/6-311+G(3df,2p)//M06-2X/6-311+G(3df,2p)水平上研究了(H_2O)n(n=0~2)催化HS和HOCl的反应机理.结果表明,HS与HOCl反应中HS夺取HOCl上的H原子形成产物H_2S和ClO.在无水催化时,该反应存在2种不同的路径(分别经过过渡态TS1和TS2,二者互为顺反结构),对应的能垒分别为100.28和100.91kJ/mol,到达产物(H_2S+ClO)需吸收18.99kJ/mol能量,反应不易发生;在单个水分子参与时,水分子可通过形成弱相互作用或者作为H原子转移桥梁影响反应机理,获得了4种水催化路径,能垒(间于53.97~92.39kJ/mol之间)均低于无水催化过程.同时发现,在反应到达产物前,水分子可以与产物形成中间体IM,IM相对能仅为0.46kJ/mol,有利于产物形成;有2个水分子参与反应时,找到了3条催化路径,最优反应路径过渡态TS7的能垒为45.05kJ/mol,低于无水催化过程,相比单个水分子最优路径能垒(53.97kJ/mol)并无显著降低.     

O-乙酰基-吡喃木糖热解反应机理的理论研究

为了从微观上理解半纤维素热解过程及其主要产物的形成演变机理,采用密度泛函理论方法B3LYP/6-31G++(d,p),对O-乙酰基-吡喃木糖的热解反应机理进行了量子化学理论研究。在热解过程中,O-乙酰基-吡喃木糖中的O-乙酰基首先脱出,形成乙酸和中间体IM₁,该步反应能垒为269.4 kJ/mol。IM₁进一步发生开环反应形成IM₂,开环反应能垒较低,为181.8 kJ/mol。对中间体IM₂设计了四种可能的热解反应途径,对各种反应的反应物、产物、中间体和过渡态的结构进行了能量梯度全优化,计算了各热解反应途径的热力学和动力学参数。计算结果表明,反应路径(4)和反应路径(2)是O-乙酰基-吡喃木糖热解的主要反应通道,乙酸、乙醛、乙醇醛、丙酮、CO、CO₂、CH₄等小分子产物是热解的主要产物。这与相关实验结果分析是一致的。     

镍配合物催化N-烯丙基酰胺异构化反应机理

采用密度泛函理论(DFT)方法,对镍配合物Ni(PPh3)2催化N-烯丙基酰胺异构化生成N-丙烯基酰胺的微观反应机理进行了计算.反应涉及了C—H键活化、异构化及还原消除生成新的C—H键等步骤.对C—H键活化和异构化步骤,分别考虑了Ni(PPh_3)_2和Ni(PPh_3)的催化活性,发现均为前者对应的能垒更低;对异构化步骤,分别考虑了π-烯丙基和σ-烯丙基机理,发现前者能垒更低.在整个反应路径中,生成产物E异构体的决速能垒为141.8 kJ/mol,与生成Z异构体的决速能垒(141.1 kJ/mol)仅差0.7 kJ/mol,与实验上E/Z选择性不高(56/44)一致.Pd(PPh_3)_2催化的决速中间体和过渡态的计算表明,生成E和Z异构体的决速能垒较高,均超过175 kJ/mol,与实验上Pd(PPh_3)_4没有催化活性一致.Ni(PPh_3)_2和Pd(PPh_3)_2催化活性不同,可由Ni的d电子对烯丙基阴离子π*反键的反馈作用较Pd更强来解释.此外,通过反应物中不同取代基对产物E/Z选择性影响的分析,发现E/Z选择性不同是由各取代基在生成E和Z异构体的决速过渡态中所受空间位阻不同所导致.     

吡喃木糖和O-乙酰基吡喃木糖热解形成羟基乙醛的机理研究

以半纤维素主要成分木聚糖的两种单体--吡喃木糖和O-乙酰基吡喃木糖为模型化合物,运用密度泛函理论(DFT),采用B3LYP方法和6-31+G(d,p)基组进行计算,研究了吡喃木糖热解形成HAA的6条可能的反应路径和O-乙酰基吡喃木糖热解形成HAA的3条可能的反应路径。由此确定了吡喃木糖热解形成HAA的最优路径为:吡喃木糖首先开环得到链式木糖,然后C3羟基和C2氢脱水,随后经重排和逆醇醛缩合反应生成包含C4/C5的HAA;该路径的决速步骤为脱水反应,能垒为253.3 kJ/mol。O-乙酰基吡喃木糖热解形成HAA的最优路径为:O-乙酰基吡喃木糖首先支链断裂脱出乙酸(AA),开环后的链式中间体经氢转移反应得到包含C4/C5的HAA;该路径的决速步骤为最后的氢转移反应,能垒为317.6 kJ/mol。     

高温水中多产物反应模型的构建及机理研究

建立了高温水中的反应模型并采用密度泛函理论(DFT)对香叶醇在高温水中的反应路径进行了研究. 通过前线轨道分析确定了反应的可能性. 高温水中香叶醇的反应路径与常温水中不同, 从常态下的酸催化反应路径转变为高温水状态下的酸碱共催化路径, 计算所得反应能垒为205.8 kJ/mol. 分别计算分析了高温水环境和周围水分子对反应能垒的影响, 结果表明, 高温水对反应物零点能的影响最大为0.966 kJ/mol, 水分子个数的影响最大可达94.7 kJ/mol. 官能团周围水分子个数变化对反应所产生的的影响大于高温水环境的影响.     

改性羰基钴催化氢甲酰化反应系列基元反应的理论研究

在HF/LANL2DZ水平下,采用有效核势能近似(ECP)从头算方法,研究了有机膦配体改性羰基钴催化的氢甲酰化反应循环中部分基元反应步骤的微观反应机理.优化了基态势能面上诸反应中间体、过渡态和产物的几何构型.计算了反应活化位垒.结果表明,羰基插入、加氢氧化和脱氢还原的基元反应步骤的活化位垒分别为54.02,134.02和43.44kJ/mol.     

HZSM-5 分子筛上乙烯芳构化过程中 C4 至 C6 中间体的反应机理

 应用量子力学和分子力学联合的 ONIOM2 (B3LYP/6-31G(d,p):UFF) 方法, 采用包含分子筛孔道结构的 78T 簇模型, 对 HZSM-5 分子筛上乙烯芳构化过程中 C4 至 C6 中间体的反应历程进行了研究, 探讨了分子筛的酸催化机理和择形催化作用. 结果表明, 作为乙烯二聚产物的表面正丁基烷氧络合物 (C4) 直接与乙烯作用得到正己基烷氧络合物 (C6), 在分子筛孔穴尺寸的限制下, 很难实现碳链的折叠环化. 按照间歇反应历程, 丁基烷氧络合物先发生 C–O 键断裂, 脱质子生成 1-丁烯, 然后在酸性位上再与乙烯加成, 在分子筛表面生成 3-甲基戊基烷氧络合物. 该烷氧络合物脱除质子给分子筛, 同时环化生成甲基环戊烷, 后者再与分子筛酸性质子共同脱除氢分子, 生成不稳定的碳正离子中间体, 然后重构成环己烷正离子. 丁基烷氧络合物脱质子的活化能为 158.42 kJ/mol; 1-丁烯与乙烯加成反应的活化能为 130.71 kJ/mol; 3-甲基戊基烷氧络合物脱氢环化生成甲基环戊烷的活化能为 122.06 kJ/mol. 由于孔穴的限域作用, 五员环的甲基环戊烷是重要的中间体.     

四氯化硅催化氢化合成三氯氢硅机理研究

针对四氯化硅催化氢化过程采用第一性原理机理对其进行模拟研究,结果表明:没有催化剂时,SiCl4与H2反应能垒为464.45 kJ/mol,反应能量为74.94 kJ/mol,与热力学计算结果 71.85 kJ/mol一致.负载在HZSM-5分子筛上的氯化钡可催化四氯化硅氢化反应,其最具催化活性表面为(111)面;H2在BaCl2(111)面上表现排斥性;SiCl4表现为吸附性,可在BaCl2(111)表面稳定吸附并生成.SiCl3自由基,过程吸附能为448.33 kJ/mol;在催化剂BaCl2存在条件下,SiCl4与H2反应为自由基反应,反应步骤能垒为400.23 kJ/mol;氢化过程能垒降为184.97kJ/mol;催化氢化反应过程所需能量为64.20 kJ/mol.催化氢化过程反应条件相对无催化剂过程更为温和.     

CuCl2催化（2-甲基辛烷-2,3-二烯-4-基）磷酸乙酯氯代环化反应机理理论研究

采用密度泛函理论(DFT)中的B3LYP方法对CuCl2催化的(2-甲基辛烷-2,3-二烯-4-基)磷酸乙酯氯代环化反应机理进行了理论研究.在6-31+G(d)基组水平上对反应机理中所有反应物、过渡态、中间体和产物进行了优化,通过能量和振动频率分析以及IRC计算证实了中间体和过渡态的合理性.在相同基组水平上应用自然键轨道(NBO)理论和分子中的原子(AIM)理论分析了复合物的成键特征和轨道间相互作用.反应物R和催化剂CuCl2可通过IA和IB两条可行反应通道生成中间体IM9,控制步骤活化能分别是129.61和142.10kJ/mol.中间体IM9到产物P也有两条反应路径PA和PB,控制步骤活化能分别是179.55和9.83kJ/mol.整个反应机理中IA→PB和IB→PB反应通道可能同时发生,反应控制步骤活化能最低反应通道为IA→PB.     

Theoretical Study on the Tautomerization Reaction of Formamide to Formamidic Acid Encapsulated in a Nano-sized Molecular Container

The hybrid calculations with ONIOM(B3LYP/6-31G*:AM1) method were carried out on the tautomerization reaction of formamide to formamidic acid in the microcontainer-encapsulated state. The free-state tautomerization process was also investigated with B3LYP/6-31G**//B3LYP/6-31G* method for the purpose of comparison. Bare tautomerization, H2O-assisted(single-H2O or multiple-H2O) and self-assisted mechanisms were all taken into consideration for the encapsulated state. The results show that only bare tautomerization and single-H2O catalysis mechanisms are possible to the encapsulated for-mamide tautomerization owing to the container's size confinement effect. Geometrical changes in the complexed container and guest molecules are discussed to deeply understand the complex's structural properties. The bare tautomerization barrier in the encapsulated state increases by 23.826 kJ/mol, ac-counting for 12% of the corresponding total energy barrier in the free state, and the increased values for the single-H2O catalysis process are 12.958 kJ/mol, accounting for 16% of the corresponding total energy barrier, respectively. This finding suggests that the encapsulation can make the tautomerization process slightly difficult.     

Theoretical studies of proton-transfer reactions of 2-hydroxypyridine--(H2O)n (n = 0-2) in the ground and excited states

The potential energy profiles for proton-transfer reactions of 2-hydroxypyridine and its complexes with water were determined by MP2, CASSCF and MR-CI calculations with the 6-31G** basis set. The tautomerization reaction between 2-hydroxypyridine (2HP) and 2-pyridone (2PY) does not take place at room temperature because of a barrier of approximately 35 kcal/mol for the ground-state pathway. The water-catalyzed enol-keto tautomerization reactions in the ground state proceed easily through the concerted proton transfer, especially for the two-water complex. The S1 tautomerization between the 2HP and 2PY monomers has a barrier of 18.4 kcal/mol, which is reduced to 5.6 kcal/mol for the one-water complex and 6.4 kcal/mol for the two-water complex. The results reported here predict that the photoinduced tautomerization reaction between the enol and keto forms involves a cyclic transition state having one or two water molecules as a bridge.     

Theoretical studies of mechanisms of cycloaddition reaction between difluoromethylene carbene and acetone

Mechanisms of the cycloaddition reaction between singlet difluoromethylene carbene and acetone have been investigated with the second‐order Møller–Plesset (MP2)/6‐31G* method, including geometry optimization and vibrational analysis. Energies for the involved stationary points on the potential energy surface (PES) are corrected by zero‐point energy (ZPE) and CCSD(T)/6‐31G* single‐point calculations. From the PES obtained with the CCSD(T)//MP2/6‐31G* method for the cycloaddition reaction between singlet difluoromethylene carbene and acetone, it can be predicted that path B of reactions 2 and 3 should be two competitive leading channels of the cycloaddition reaction between difluoromethylene carbene and acetone. The former consists of two steps: (i) the two reactants first form a four‐membered ring intermediate, INT2, which is a barrier‐free exothermic reaction of 97.8 kJ/mol; (ii) the intermediate INT2 isomerizes to a four‐membered product P_2b via a transition state TS2b with an energy barrier of 24.9 kJ/mol, which results from the methyl group transfer. The latter proceeds in three steps: (i) the two reactants first form an intermediate, INT1c, through a barrier‐free exothermic reaction of 199.4 kJ/mol; (ii) the intermediate INT1c further reacts with acetone to form a polycyclic intermediate, INT3, which is also a barrier‐free exothermic reaction of 27.4 kJ/mol; and (iii) INT3 isomerizes to a polycyclic product P₃ via a transition state TS3 with an energy barrier of 25.8 kJ/mol. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

BF3活化高锰酸氧化丙烯酸反应机理的 密度泛函研究

用量子化学密度泛函理论(DFT)的B3LYP方法对高锰酸根离子与丙烯酸的环加成反应机理进行了系统研究, 全参数优化了反应势能面上各驻点的几何构型、振动频率和能量. 计算结果表明: 反应有两个竞争通道, 即[2+3]反应通道和[2+2]反应通道, 其中[2+3]通道比[2+2]通道的反应势垒降低了183.89 kJ/mol, 并通过在高锰酸根的氧原子上配位一个或两个BF3分子来研究BF3分子对反应体系的活化效应, 结合两个BF3分子使得[2+3]通道的反应势垒降低为23.97 kJ/mol, 则有利于反应按该通道进行, 然而[2+2]通道的反应势垒仍较高(>195 kJ/mol).这进一步表明该反应体系中加入一定量BF3能提高高锰酸氧化烯烃双键的化学活性.     

BF_3活化高锰酸氧化丙烯酸反应机理的密度泛函研究

用量子化学密度泛函理论(DFT)的B3LYP方法对高锰酸根离子与丙烯酸的环加成反应机理进行了系统研究,全参数优化了反应势能面上各驻点的几何构型、振动频率和能量.计算结果表明:反应有两个竞争通道,即[2+3]反应通道和[2+2]反应通道,其中[2+3]通道比[2+2]通道的反应势垒降低了183.89kJ/mol,并通过在高锰酸根的氧原子上配位一个或两个BF3分子来研究BF3分子对反应体系的活化效应,结合两个BF3分子使得[2+3]通道的反应势垒降低为23.97kJ/mol,则有利于反应按该通道进行,然而[2+2]通道的反应势垒仍较高(195kJ/mol).这进一步表明该反应体系中加入一定量BF3能提高高锰酸氧化烯烃双键的化学活性.     

Theoretical study of 5-aminolevulinic acid tautomerization: a novel self-catalyzed mechanism

5-Aminolevulinic acid (5ALA) is the key synthetic building block in protoporphyrin IX (PpIX), the heme chromophore in mitochondria. In this study density functional theory calculations were performed on the tautomers of 5ALA and the tautomerization reaction mechanism from its enolic forms (5-amino-4-hydroxypent-3-enoic acid and 5-amino-4-hydroxypent-4-enoic acid) to the more stable 5ALA. The hydrated form 5-amino-4,4-dihydroxypentanoic acid was also studied. The lowest energy pathway of 5ALA tautomerization is by means of autocatalysis, in that an oxygen of the carboxylic group transfers the hydrogen atom as a "crane", with an activation energy of approximately 15 kcal/mol. This should be compared to the barriers of about 35 kcal/mol for water assisted tautomerization, and 60 kcal/mol for direct hydrogen transfer. For hydration of 5ALA, the water catalyzed activation barrier is found to be approximately 35 kcal/mol, approximately 5 kcal/mol lower than direct hydration.     

Tautomerization of adenine facilitated by water: computational study of microsolvation

We present calculations for the mechanism and the barrier heights of tautomerization of adenine. We find various pathways for the 9(H) <--> 7(H) and 9(H) <--> 3(H) tautomerization. One mechanism for the 9(H) --> 7(H) tautomerization involves an sp(3)- or carbene-type intermediate, whereas the other proceeds via imine intermediates. Tautomerization from the 9(H) tautomer to 7(H) or 3(H) is predicted to occur with a very large activation barrier (60-70 kcal/mol), indicating that the processes may not occur readily in the gas phase. Interactions with the water molecule(s) are found to lower the barrier tremendously. We suggest that dramatic lowering of the 9(H) --> 3(H) and 9(H) --> 7(H) barriers by microsolvating water molecules may facilitate the formation and observation of the 7(H) and 3(H) tautomers in the solution phase.     

DFT explorations of tautomerism of 2-mercaptoimidazole in aqueous solution

A systematic investigation of the proton transfer in the tautomerization of 2-mercaptoimidazole was undertaken. Calculations in aqueous solution were performed using the combined supramolecular/continuum and the direct continuum models, respectively. Complexes containing one and two water molecules around the hydrophilic site of 2-mercaptoimidazole were used for the combined supramolecular/continuum calculation. DFT results predict that the barrier height for non-water-assisted intramolecular proton transfer is very high (175.8 kJ/mol). Hydrogen bonding between 2-mercaptoimidazole and the water molecule(s) will dramatically lower the barrier by the concerted multiple proton transfer mechanism. The proton transfer process through a eight-member ring formed by 2-mercaptoimidazole and two water molecules is found to be more efficient one and the calculated barrier height is reduced to ca. 72 kJ/mol.     

Gold-catalyzed hydrative carbocyclization of 1,5- and 1,7-allenynes mediated by pi-allene complex: mechanistic evidence supported by the chirality transfer of allenyne substrates

We report PPh3AuCl/AgOTf-catalyzed hydrative carbocyclization of 1,5- and 1,7-allenynes to give cyclized ketones chemoselectively. In this transformation, hydration occurrs regioselectively at the C[triple bond]CPh carbon, accompanied by addition of the C[triple bond]CPh carbon to the two terminal allenyl carbons. This method is effective for the construction of a quaternary carbon center. On the basis of the chirality transfer of allenyne substrates, control experiments, and theoretic calculations, we propose that this hydrative carbocyclization proceeds through an initial pi-allene complex with a small energy barrier.     

Self-catalyzed keto-enol tautomerization of malonic acid

We demonstrate through quantum chemical calculations that the keto-enol tautomerization of malonic acid can be catalyzed by the two tautomers of malonic acid itself. This self-catalyzed process proceeds with a relatively low barrier (Gibbs energy ca. 13 kcal/mol in gas phase, 20 kcal/mol in aqueous phase), and involves the concerted transfer of two protons between the substrate and the carboxylic acid functionality of the malonic acid catalyst. This mechanism is expected to compete with the proton relay mechanism currently favored to explain the tautomerization of malonic acid in aqueous media. Malonic acid is an important constituent of secondary organic aerosol where the present chemistry may play a role in determining chemical composition.