NAD(P)H辅酶模型在Mg2+离子催化条件下还原N-芳基芴亚胺反应的研究

对Hantzsch酯在Mg²⁺存在和不存在的情况下还原N-芳基芴亚胺的反应进行了研究, 并与BNAH的类似还原做了系统的比较.研究结果表明:Mg²⁺在该还原反应中起亲电催化剂的作用;还原能力较BNAH弱的Hantzsch酯在反应中所呈现的强的反应性是由于其3, 5-位两个极性谈基氧通过静电作用降低过渡态的能量的缘故;本文反应届H-一步转移机理.     

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.     

Insights into the Decomposition of Olefin Metathesis Precatalysts

The decomposition of a series of benzylidene, methylidene, and 3‐phenylindenylidene complexes has been probed in alcohol solution in the presence of base. Tricyclohexylphosphane‐containing precatalysts are shown to yield [RuCl(H)(H₂)(PCy₃)₂] in isopropyl alcohol solutions, while 3‐phenylindenylidene complexes lead to η⁵‐(3‐phenyl)indenyl products. The potential‐energy surfaces for the formation of the latter species have been probed using density functional theory studies.     

Complementary routes to both enantiomers of pipecolic acid and 4,5-dihydroxypipecolic acid derivatives

Complementary new routes to both enantiomers of N-protected pipecolic acid and the corresponding 4,5-dihydroxylated derivatives are developed, which involve stereo-divergent allylation of a chiral N-allylimine and ring-closing metathesis as key steps.     

Oxygen Transfer from Inorganic and Organic Peroxides to Organic Substrates: A Common Mechanism?

In this progress report an attempt is made to rationalize, from a mechanistic point of view, the different ways in which oxygen is transferred from inorganic and organic peroxides to nucleophilic substrates, particularly olefins. Oxygen transfer from transition-metal peroxides, which is relevant to catalytic oxidations using O₂, H₂O₂ or ROOH, occurs via a cyclic or “pseudocyclic” peroxymetalation in which a dioxametallacycle is formed. Owing to the wide discrepancy between peroxymetalation and the conventional oxidation mechanism, i.e. nucleophilic attack of the substrate at the electrophilic “active oxygen”, we propose an alternative mechanism involving dioxiranes as the reactive species. The generation of dioxiranes appears to be a common denominator in the reactions of most organic peroxides e.g. peroxy acids, the reaction of electrophilic ketones with H₂O₂, or ozonizations. Oxygen transfer from dioxirane reagents probably involves the formation of a charge-transfer π-complex between the substrate and the carbon atom of the dioxirane, and the subsequent formation of a cyclic peroxidic intermediate.     

Electron Transfer Reactions in Chemistry: Theory and Experiment (Nobel Lecture)

Since the late 1940s, the field of electron transfer processes has grown enormously, both in chemistry and biology. The development of the field, experimentally and theoretically, as well as its relation to the study of other kinds of chemical reactions, presents to us an intriguing history, one in which many threads have been brought together. In this lecture, some history, recent trends, and my own involvement in this research are described.     

Efficient stereodivergent synthesis of 1,4-dideoxy-1,4-iminohexitols from an (S)-glyceraldimine

A stereodivergent synthesis of 1,4-dideoxy-1,4-imino-d-mannitol I and d-allitol III from an (S)-glyceraldimine, which is easily prepared from d-mannitol, has been achieved with overall yields of 62% and 49%, respectively. The synthesis is based on the addition of vinylmagnesium bromide to N-benzylimine 1, derived from readily available (R)-2,3-O-isopropylideneglyceraldehyde, followed by N-allylation or N-acryloylation, ring-closing metathesis and asymmetric dihydroxylation.     

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).

     

Proposed reaction mechanisms for selenium UV photolysis vapor generation by computational methods

The production of volatile analyte species by UV photolysis in the presence of low-molecular-weight organic acids as an alternative to chemical vapor generation has been of recent interest. The mechanism of this process is not well understood. Proposed mechanisms often involve photolytic cleavage of the organic acid as the initial step. Evidence suggests that this may not be the dominant route for UV photolysis vapor generation. In this work computational methods were applied to determine a possible alternative mechanism in the absence of free-radical production. The proposed mechanism specifically focused on selenium vapor generation. An energetically favored mechanism was found for UV photolysis of inorganic selenium in the presence of formic and acetic acids which is consistent with previously reported experimental results.     

ETC: A Mechanistic Concept for Inorganic and Organic Chemistry

The concept of electron transfer catalysis (ETC), or more specifically “Double Activation Induced by Single Electron Transfer” (DAISET) gives an opportunity to connect experimental facts never previously correlated. The first activation results from the transfer of an electron to (or from) a molecular species; the second activation results from the build-up of a reaction chain able to reproduce the species formed in the first step. The starting point of this review is the S_RN 1 mechanism where principle and experimental diagnostic criteria are critically discussed. The thermal and photochemical exchange and substitution reactions of Pt^IV complexes are then reviewed together with the exchange reaction [AuCl₄]^?/Cl^?, reactions with Grignard reagents and other organometallic reagents, as well as the redox behavior of electronically excited organic compounds. Photochemical applications, including solar energy conversion are discussed. New aspects are also presented for the mechanistic problem “S_N 2 reaction or SET process?” Moreover, the concept has significance for S_H2 reactions at metal centers, molecule-induced homolyses, reactions of complexes, as well as electrochemical processes.–Unless otherwise specified, only double activation (DAISET) processes will be discussed in this article.     

Catalytic hydroboration of aldehydes and ketones with sodium hydride: Application to chemoselective reduction of aldehydes over ketones

Sodium hydride-promoted catalytic hydroboration of aldehydes and ketones with pinacolborane (HBpin) was examined, and 10?mol% of NaH was found to cause the HBpin to participate in hydroboration in a convenient and efficient manner at mild reaction conditions. Further chemoselective hydroboration of aldehyde over ketone functionality was also analyzed. In addition, no hydroboration was observed form ester, acyl chloride, amide, nitrile, alkene, alkyne, alkyl halide and epoxide functional groups indicate that present system (HBpin, NaH) is highly selective for aldehydes and ketones.     

Alkane Activation Initiated by Hydride Transfer: Co‐conversion of Propane and Methanol over H‐ZSM‐5 Zeolite

Co‐conversion of alkane with another reactant over zeolite catalysts has emerged as a new approach to the long‐standing challenge of alkane transformation. With the aid of solid‐state NMR spectroscopy and GC‐MS analysis, it was found that the co‐conversion of propane and methanol can be readily initiated by hydride transfer at temperatures of ≥449 K over the acidic zeolite H‐ZSM‐5. The formation of ¹³C‐labeled methane and singly ¹³C‐labeled n‐butanes in selective labeling experiments provided the first evidence for the initial hydride transfer from propane to surface methoxy intermediates. The results not only provide new insight into carbocation chemistry of solid acids, but also shed light on the low‐temperature transformation of alkanes for industrial applications.     

Reaction of a 6-dimethylaminopentafulvene with the Mes2PCH2CH2B(C6F5)2 frustrated Lewis pair

The frustrated Lewis pair Mes(2)PCH(2)CH(2)B(C(6)F(5))(2) reacts readily with 6-dimethylamino-6-methylfulvene at room temperature to yield the trans-1-[B(C(6)F(5))(2)]-2-[CH(2)CH(2)PMes(2)] disubstituted fulvene derivative 9 that features an internal N-B contact. Thermolysis (80 °C in toluene) results in a complete isomerization to the respective 1-[B(C(6)F(5))(2)]-3-[CH(2)CH(2)PMes(2)] isomer 10. Both compounds were characterized by using X-ray diffraction. A reaction scheme is formulated to rationalize the specific formation of these compounds, involving a retro-hydroboration/hydroboration sequence. The reaction of the 6-dimethylaminofulvene with HB(C(6)F(5))(2) yielded the corresponding parent compound 13 that was also characterized by X-ray diffraction.     

含负氢配体的单核钌催化的降冰片烯开环易位聚合催化研究

设计的含负氢配体的单核钌配合物Ru(p-cymene)HClPCy₃高活性地催化了降冰片烯开环易位聚合反应(ROMP), 并通过设计的Ru(p-cymene)H₂PCy₃对降冰片烯的催化活性研究证明了单核催化剂需同时含有氯原子配体, 提出了反应机理.     

Ab initio and density function theory computational studies of the CH4 + H → CH3 + H2 reaction

The activation barrier for the CH₄ + H → CH₃ + H₂ reaction was evaluated with traditional ab initio and Density Functional Theory (DFT) methods. None of the applied ab initio and DFT methods was able to reproduce the experimental activation barrier of 11.0-12.0 kcal/mol. All ab initio methods (HF, MP2, MP3, MP4, QCISD, QCISD(T), G1, G2, and G2MP2) overestimated the activation energy. The best results were obtained with the G2 and G2MP2 ab initio computational approaches. The zero-point corrected energy was 14.4 kcal mol⁻¹. Some of the exchange DFT methods (HFB) computed energies which were similar to the highly accurate ab initio methods, while the B3LYP hybrid DFT methods underestimated the activation barrier by 3 kcal mol⁻¹. Gradient-corrected DFT methods underestimated the barrier even more. The gradient-corrected DFT method that incorporated the PW91 correlational functional even generated a negative reaction barrier. The suitability of some computational methods for accurately predicting the potential energy surface for this hydrogen radical abstraction reaction was discussed.