Mechanistic investigations in the

Annis, D. A. Jaeobsen, E. N. (1999) Polymer supported ehiral Co(salen) complexes synthetie applieations and mechanistic investigations in the hydrolytic kinetic resolution of terminal epoxides., Y. Am. Chem. Soc., 121 4147-4154. 

DMSO has been used in a multitude of mechanistic investigations. In the present section we have chosen to highlight certain studies which illustrate the principles and methods discussed in earlier sections of this article. This will be done by reference to several contrasting situations. The systems chosen illustrate rate phenomena, both retardation and acceleration, resulting from use of DMSO. Various techniques for analysing these effects are presented, including the use of acidity functions and thermodynamic transfer functions, and their value as a guide to mechanisms demonstrated. 

Summary Vinylsilanes are known to react with lithium metal either to 1,2-dilithioethanes by reduction or to 1,4-dilithiobutanes by reductive dimerization. The reaction of substituted vinylsilanes with lithium metal is employed in the approach to vicinal and geminal dilithiated vinylsilanes by two consecutive additions of lithium metal and subsequent eliminations of lithium hydride. A mechanistic investigation in the reactivity of a- and (3-substituted vinylsilanes towards lithium metal discloses several new reaction pathways, whereby the choice of solvent plays an important role in apolar solvents like toluene vinyllithium compounds are obtained. Compound 14, R = Ph, which is not stable under the reaction conditions, finally affords the 1,4-dilithium compound 27. Compound 18, R = SiMes, on the other hand either adds to the starting vinylsilane (forming the monolithium compound 39) or shows an unusual dimerization to 47, which is studied in detail. 

Buscemi, S., Pace, A., Vivona, N., Caronna, T., and GaHa, A., Photoinduced single electron transfer on 5-aryl-1,2,4-oxadiazoles some mechanistic investigations in the synthesis of quanazolin-4-ones, /. Org. Chem., 64, 7028, 1999. 

In one of the earliest mechanistic investigations in organic chemistry, Arthur Lapworth discovered in 1904 that the rates of chlorination and bromination of acetone were the... 

A widely significant and crucial investigation of the mechanism of diazotization was made in 1899 by Hantzsch and Schumann widely significant because it was one of the first mechanistic investigations using appropriate methods of organic chemistry - extremely crucial because it influenced all subsequent investigations in the field of diazotization mechanisms until 1958. 

Huffman in the X-ray Molecular Structure Center. From 1992 to 1994 he joined the research group of Professor John D. Corbett at Iowa State University where he pursued synthetic solid-state chemistry research exploring structure/property relationships. In 1994 he joined the faculty of the Department of Chemistry at North Carolina State University where he is a full professor of Inorganic Chemistry pursuing synthetic, structural and mechanistic investigations in inorganic condensed matter. He has published more than 70 research papers and has several patented discoveries. He received an NSF CAREER award in 1995, was named a Cottrell Scholar of the Research Corporation in 1997, and received a Sigma Xi Research Award in 1999. 

Mechanistic investigation in this case has shown that there is an equilibrium between an alkenyl silane and an allylic silane by a rapid 3,3-sigmatropic process. The cyclization occurs through the more reactive allylic silane. 

Such results seem to be rather indicative of the opportunity to exploit TD-DFT and the PBEO functional in predicting spectroscopic properties of mixtures containing the three tautomers 1-Q, 1-QM, and 1-QI in aqueous solution. This approach should be very useful for future experimental mechanistic investigations clarifying the complex mechanisms of dihydroxyindole oxidation. 

The solvent dependence of the reaction rate is also consistent with this mechanistic scheme. Comparison of the rate constants for isomerizations of PCMT in chloroform and in nitrobenzene shows a small (ca. 40%) rate enhancement in the latter solvent. Simple electrostatic theory predicts that nucleophilic substitutions in which neutral reactants are converted to ionic products should be accelerated in polar solvents (23), so that a rate increase in nitrobenzene is to be expected. In fact, this effect is often very small (24). For example, Parker and co-workers (25) report that the S 2 reaction of methyl bromide and dimethyl sulfide is accelerated by only 50% on changing the solvent from 88% (w/w) methanol-water to N,N-dimethylacetamide (DMAc) at low ionic strength this is a far greater change in solvent properties than that investigated in the present work. Thus a small, positive dependence of reaction rate on solvent polarity is implicit in the sulfonium ion mechanism. 

Coin-cyclam322-324 and Nin-cyclam322 catalyze the electroreduction of nitrate in aqueous electrolytes with good current efficiencies and turnover numbers, giving mixtures of ammonia, nitrite, and hydroxylamine at a variety of electrode materials. Mechanistic investigations suggested the adsorption of electroreduced Co1- and Ni1 cyclam onto the electrode surface,322 and the formation of an oxo-metal bond via reduction of coordinated nitrate.323... 

In the last decade, numerous attempts have been made to understand the mechanism of the partial oxidation of methane (3,13-15,17,25,37,97,107, 128-137,142-148). Mechanistic investigations of the partial oxidation are still challenging, because this exothermic reaction is very fast and causes extremely high catalyst temperature rises, so that the usual methods of investigation are unsuitable. 

As mentioned earlier, it was originally assumed that this reaction is mechanistically related to the copper-catalyzed diazo-transfer cyclopropanation. As such, the intervention of a metal complexed nitrenoid intermediate has been theorized as the principal mode of action. Mechanistic investigations in this reaction have paralleled the development of the asymmetric version and hence, will be discussed in concert. 

No reports of mechanistic investigations in this area have appeared and the authors offer no speculation. Bolm et al (133) characterized a related catalyst by X-ray crystallography (Fig. 15). In the solid state, this catalyst forms a dimer with... 

The ex vivo methods lend themselves easily for the performance of mechanistic investigations. In order to optimize selection of drug candidates prior to further clinical development, it is important to decipher the contributive roles of permeation, metabolism, efflux, and toxicity. This will then make it possible to properly channel the optimization process, for instance, by permeation enhancement, mucoadhesion, modification of the physicochemical characteristics of the drug, or even change in the route of administration in case the drug and/or formulation turns out to be too toxic. Regarding permeability studies, it is possible not only to quantify passive diffusion but also to identify and characterize (compound)-specific carrier-mediated transport routes. These tools have been used to identify and characterize the relative contribution of... 

More attention has been devoted to aromatic and heteroaromatic substrates since first reported in 1983 [40]. The results are shown in Table 2 [25, 41-51]. All these reactions were run with nickel complexes associated with a phosphane or bpy ligand. Depending on the experimental conditions, the polymers were either precipitated during the electrolysis or deposited as films at the surface of the electrode. The method is also convenient to prepare copolymers from a mixture of two aryl dihalides. A mechanistic investigation on the nickel-bpy catalyzed polymerisation has been reported very recently [52]. 

Halo-lactonization of ketophosphoranes has been achieved via reaction with cyclic anhydrides and subsequent halogenation. " The products, halo enol lactones (75), are synthetically useful compounds, and an alternative synthesis via incorporation of the halogen at the ylid stage is also described. Mechanistic investigation of the Wittig reactions involved reveals subtle variations in pathway, allowing optimum experimental conditions to be selected to allow for the variation in reactivity of different anhydrides and halides. 

Extensive mechanistic investigation of the ring expansion 33 —> 34 was performed in order to differentiate between a ring-opening reaction to give a silyl radical 39 (path a), followed by the 6-endo cyclization, or a pentavalent silicon transition state 40 (path b). It was clearly demonstrated that the ring expansion proceeds via a pentavalent silicon transition state (Scheme 6.10) [16]. 

The syn addition of the adducts suggests a mechanism different from that observed in rhodium-catalyzed alcoholysis and aminolysis reactions. Mechanistic investigations from the Tautens laboratory have revealed that the most likely mechanism involves an enantioselective carbopalladation followed by a (3-alkoxide elimination to afford the ring-opened product." ... 

Therefore, the following mechanistic investigation of the epoxidation of olefins with hydrogen peroxide is constrained to reaction pathways which (1) involve HFIP in an sc or even sp conformation and (2) to hydrogen bonded HFIP aggregates comprising up to four alcohol monomers. 

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