Reaction mechanism recent studies

In spite of a great number of studies devoted to curing with cyclic anhydrides, there existed a number of contraversial views concerning the reaction mechanism. Recent studies have revealedthat in the presence of tertiary amines the reaction can proceed also in the absence of proton donors. The tertiary amine reacts first with epoxide most probably via a zwitterion The simplified mechanism is as follows... 

The universality of intermolecular electron transfer (ET) makes the mechanism one of the central questions to be solved in chemistry and biology. ET is strongly influenced by the nature of solvent and its dynamics in solution. Dynamical solvent effects on the course of reaction have recently studied both theoretically [1-5] and experimentally [6-15]. For ET with a rate comparable to the solvent flucmation rate, the motion and structure of the solvent can determine the rate of ET and it becomes the "solvent-controlled" reaction [2,3,6-9]. 

Most of the research reported in the literature was oriented towards analyses of product distribution, influence of operating conditions and reaction mechanisms. Recently, spectroscopic characterization of coke deposits formed on Y-zeolite catalysts was reported (13,14), and Stocker et al presented regeneration studies of the EMT catalyst, by heating in air up to 600°C (15). 

Transmetalation forms new metal-carbon bonds under mild conditions. It serves to alter the reactivity of the organometallic intermediates in the metal complex-promoted synthetic organic reactions. The recent studies of the transmetalation have replaced previously postulated and obscure mechanisms of several metal complex-promoted synthetic organic reactions with the more convincing mechanism based on the precise description of the transmetalation reactions. This chapter intended to concentrate on the activation and formation of metal-carbon... 

Another issue is the formation of oxazolidinones, which has been the subject of study by several research groups and is considered to be part of a parasitic equilibrium for proline-catalysed aldol reactions. More recent studies have indicated that this parasitic equilibrium may not be true, and that reversible oxazolidinone formation may help keep proline in solu-tion. Figure 5.3 illustrates a generalised mechanism for proline catalysis involving enamine intermediates. As aforementioned, the formation of oxazolidinones may or may not be part of a parasitic equilibrium. 

The characterization of reaction mechanisms and intermediates of complex chemical reactions forms a central topic in electrochemistry [113-118]. The electrochemical reductive cleavage of carbon-halogen bonds is an important process in electro-organic synthesis, waste stream treatment and electron-transfer mechanisms. Benzyl chloride (PhCH2Cl) reduction in organic solvents has been widely investigated following debate as to whether the carbon-chlorine bond would be reductively cleaved via a concerted or a nonconcerted reaction pathway. Recent studies have shown that Pd, Cu - and especially Ag - cathodes... 

The photochemical isomerization of various spiropyran molecules in different media (gas phase, solution, mono-layers, polymers) has been extensively experimentally [1-3, 20-30] and theoretically [31, 32] studied in order to understand the reaction mechanism. Less studied, however, are the effect of protonation, charge, and external fields on the isomerization pathway of this electrocyclic reaction. It was recently reported that the rate of isomerization of the SP form is considerably increased by protonation of the... 

The key step in the proposed oxetane repair mechanism is the rapid fragmentation of the oxetane radical anion. This ring-opening reaction was recently studied with model compounds. Falvey and coworkers " prepared stable oxetane adducts from dimethylthymine and various benzaldehyde derivatives. Laser flash photolysis investigations in the presence of external electron donors hke dimethyl aniline showed efficient oxetane sphtting, which allowed investigation of the radical intermediates. The radical cations of the various sensitizers and the radical anions of the carbonyl products formed after... 

In the first chapter, devoted to thiazole itself, specific emphasis has been given to the structure and mechanistic aspects of the reactivity of the molecule most of the theoretical methods and physical techniques available to date have been applied in the study of thiazole and its derivatives, and the results are discussed in detail The chapter devoted to methods of synthesis is especially detailed and traces the way for the preparation of any monocyclic thiazole derivative. Three chapters concern the non-tautomeric functional derivatives, and two are devoted to amino-, hydroxy- and mercaptothiazoles these chapters constitute the core of the book. All discussion of chemical properties is complemented by tables in which all the known derivatives are inventoried and characterized by their usual physical properties. This information should be of particular value to organic chemists in identifying natural or Synthetic thiazoles. Two brief chapters concern mesoionic thiazoles and selenazoles. Finally, an important chapter is devoted to cyanine dyes derived from thiazolium salts, completing some classical reviews on the subject and discussing recent developments in the studies of the reaction mechanisms involved in their synthesis. 

Novolaks. Novolak resins are typically cured with 5—15% hexa as the cross-linking agent. The reaction mechanism and reactive intermediates have been studied by classical chemical techniques (3,4) and the results showed that as much as 75% of nitrogen is chemically bound. More recent studies of resin cure (42—45) have made use of tga, dta, gc, k, and nmr (15). They confirm that the cure begins with the formation of benzoxazine (12), progresses through a benzyl amine intermediate, and finally forms (hydroxy)diphenyknethanes (DPM). 

The mechanism of the reaction between a hydrocarbon organohthium compound and a carbonyl compound is well-documented [69] and has been suggested earlier for fluonne-contaming compounds [32, 68, 70] More recent studies on the mechanism have resulted in a better understanding of this reaction [71] (equauon 37)... 

The cleavage proceeds under extremely mild conditions in aqueous solution at room temperature and can therefore be used for complicated syntheses. The mechanism of this reaction proved to be very complicated and has been elucidated only recently. The ring opening of quaternary salts of trisubstituted isoxazoles by nucleophiles proceeds under similar conditions, and the mechanism of this reaction was extensively studied by Kohler et The authors... 

The first systematic theoretical study on dihydro-1,2,4-triazines was recently carried out (98JOC5824) the stabilities of all the possible unsubstituted dihydro-1,2,4-triazines were calculated using various theoretical methods, all reliable calculation methods consistently show that the 2,5-dihydro isomer 98 is the most stable. This is in perfect agreement with the experimental observations all the synthetic methods used for the preparation of dihydro-1,2,4-triazines result in 2,5-dihydro isomer 98, provided the structures of the reactants and the reaction mechanism allow its formation. Thus, although Metze and Scherowsky (59CB2481) claimed the formation of 1,2-dihydro-1,2,4-triazine 92 (R = = Ph) in the reduction... 

Because of the complexity of the pathway, the sensitivity of the reagents involved, the heterogeneous nature of the reaction, and the limitations of modern experimental techniques and instrumentation, it is not surprising that a compelling picture of the mechanism of the Simmons-Smith reaction has yet to emerge. In recent years, the application of computational techniques to the study of the mechanism has become important. Enabling theoretical advances, namely the implementation of density functional theory, have finally made this complex system amenable to calculation. These studies not only provide support for earlier conclusions regarding the reaction mechanism, but they have also opened new mechanistic possibilities to view. 

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