Reactive intermediates reactions with

Because activated 4-0-trimethylsilylated-2, 3, 5 -0-acyluridines such as 3 are also obtained as reactive intermediates in the Friedel-Crafts-catalyzed silyl-Hilbert-Johnson reaction [59, 59 a] of persilylated uracils or 6-azauracils such as 227 with sugars such as l-0-acetyl-2,3,5-tri-0-benzoyl-/9-D-ribofuranose 228 in the presence of SnCl4, treatment of the reactive intermediate 229 with a large excess of pyrrolidine neutralizes the SnCLi. used and aminates 229 to afford the protected 6-aza-cytidine 230, although in 57% yield only [49, 59] (Scheme 4.20). 

Bertani, R., Michelin, R.A., Mozzon, M., Traldi, P., Seraglia, R., Busetto, L., Cassani, M.C., Tagliatesta, P. and D Arcangelo, G. (1997) Mass Spectrometric Detection of Reactive Intermediates. Reaction Mechanism of Diazoalkanes with Platinum(O) and Gold (I) Complexes. Organometallics, 16(14), 3229-3233. 

Hydrogen Abstraction Photoexcited ketone intermolecular hydrogen atom abstraction reactions are an interesting area of research becanse of their importance in organic chemistry and dne to the complex reaction mechanisms that may be possible for these kinds of reactions. Time resolved absorption spectroscopy has typically been nsed to follow the kinetics of these reactions but these experiments do not reveal mnch abont the strnctnre of the reactive intermediates. " Time resolved resonance Raman spectroscopy can be used to examine the structure and properties of the reactive intermediates associated with these reactions. Here, we will briefly describe TR experiments reported by Balakrishnan and Umapathy to study hydrogen atom abstraction reactions in the fluoranil/isopropanol system as an example. 

Figure 25.4 An SC derivative of mPEG was first prepared through the use of phosgene to form a chloroformate intermediate. Reaction with NHS gives the amine-reactive SC-mPEG.

It is assumed137 that the reaction proceeds by path a so long as the carboxonium ions 284 are sufficiently reactive for reaction with weak nucleophilic nitriles (e.g. as is the case when the ions 284 are acyloxycarbocations 225, E = acyl). Otherwise (284, E = H, Aik), path b is followed because in this case the highly reactive nitrilium ions 287 can be generated from weak nucleophilic nitriles. Both paths lead to the same A-acyliminium intermediate 28949 (equation 77). 

Padwa and coworkers found that a-cyanoaminosilane 12a is a convenient synthon for azomethine ylide 15 which is extensively used in heterocyclic synthesis [7]. AgP has been adopted to generate the ylide 15 from 12a for the preparation of pyrrolidine derivative 14 (Sch. 4). Various dipolarophiles including A-phenylmaleimide (13) can be used for the cycloaddition. When iV-[(trimethylsilyl)methyl]-substituted indole 16 is reacted with AgP in the presence of maleimide 13, pyrrolo[l,2-a]indole 17 is formed in good yield, retaining the CN group [8]. A silver-bonded carbonium ion is assumed to be a reactive intermediate. Reaction of a cyano-substituted azomethine ylide, derived from (silylmethylamino)malononitrile 12b and AgP, with methyl propiolate (18) provides 3-carbomethoxy-A-benzylpyrrole (19) [9]. Epibatidine, a novel alkaloid, was successfully synthesized by employing the [3 + 2] cycloaddition of azomethine ylide with electron-deficient alkenes as a key step [10]. 

With the steady state hypothesis for the reactive intermediates, Reactions 1-15 lead to the rate law... 

The ease of halogenation depends on whether the hydrogen atom is bonded to a primary, secondary or tertiary carbon atom. A tertiary hydrogen atom is more reactive because reaction with a halogen atom (X") produces an intermediate tertiary radical, which is more stable (and therefore more readily formed) than a secondary or primary radical (see Section 4.3). 

MDA does not form in the absence of acid The aminals and the A/-methylolaniline decompose rapidly in acid solution to form an anilinium ion (intermediate B). This reactive intermediate combines with aniline to form A/-(p-anainoben2yl)aniline [17272-83-2] (C 2H 4N2) which reacts with intermediate B in the presence of acid to form oligomeric ben2ylamines (PBA) that exist in equiHbrium with the monomer. The formation of this equihbrium mixture completes the condensation phase of the synthesis. Almost aU of the side reactions take place during the condensation phase of the reaction. The typical side products formed are the N-methyl and quina2oline derivatives of aniline and MDA (13,15). Commercial processes have been successful in minimising these side reactions. 

Computational studies have compared substituent effects on the stability of ketenes, allenes, diazomethanes, diazirines, and cyclopropenes. Ketenes belong to the first generation of reactive intermediates along with carbocations, carbanions, radicals, and carbenes, and are intensively studied members of the cumulene family, with many useful synthetic applications. Ketenes were first recognized in 1905, when diphenylketene, a stable and isolable example, was obtained from the dehalogenation of the a-bromodiphenylacetyl bromide (Scheme 7.37). The most characteristic reaction of ketene is cycloaddition, as in the formation of p-laclams. 

Because of the enormous number of collisions occurring at any moment in a gas-phase reaction and the rapid formation and decomposition of reactive intermediates, reaction mechanisms can never be known with certainty. However, by observing reaction rates and comparing proposed mechanisms with experimental rates and with the overall stoichiometry, we can propose a mechanism that fits... 

The dication [CpWR(CO)2(NCMe)2]2 (R = Et) is a reactive compound. Reaction with CO in MeCN is by insertion and substitution to form [CpWMe(T 2-COR)(NCMe)3]2+. Photolysis of [CpWMe(CO)2(NCMe)2]2+ affords [CpWMe(NCMe)4]2+ via the intermediate... 

Equations 13-15 are a subset of the reactions shown in Figure 5 but they are sufficient to represent the mesophase fuel generation kinetics. In particular, polymer P thermally degrades to a reactive intermediate P with rate constant d-The intermediate produces gas G and/or char C, with rate constants and kc, respectively. Figure 7 shows data for a variety of pure, imfilled polymers plotted... 

Organoboranes are important in organic synthesis as reactive intermediates. Reactions have been developed by which the boron atom may be replaced by a wide variety of functional groups, such as —H, —OH, —NH2, —Br, —1, and —COOH.The present experiment demonstrates the conversion of an organoborane to an alcohol by oxidation with alkaline hydrogen peroxide. It is not necessary to isolate the organoborane prior to its oxidation. This simplification is particularly fortuitous in this case, since most alkylboranes, when not in solution, are pyrophoric (spontaneously flammable in air). 

Acylium ions are useful reactive intermediates in organic synthesis because their high reactivity allows reactions with relatively weak nucleophiles, in this case the carbonyl oxygen of an a,p-unsaturated enone.The product in this experiment, a dienol acetate of a cholesterol derivative, is itself a useful intermediate in the synthesis of sferoids such as cortisone. 

In the presence of O, NO shows sufficiently high reactivity in reactions with nitrogen-containing organic compounds. It has been found that primary aromatic amine reacted with NO + O in benzene to give triazene derivatives in 60-64% yield [32]. The proposed reaction mechanism includes the formation of intermediate N-nitroso compounds in reaction with dimers of NO ... 

Since the rates of the individual steps were measured at very different temperatures, measured activation parameters were used to estimate rate constants at the temperature of a given catalytic reaction [69]. The rate of dissociation of bridging hydride 1-tol at 60°C kj) was obtained by measuring the rate of reaction with PPh3 to give 2-tol and phosphine complex C3 the rate was first order in [1-tol] and independent of [PPha]. The rate of benzaldehyde reduction by 2-tol ( 3) had been measured below 0°C, and the rate of loss of H2 from 2-tol had been measured at 90°C. The rates of reaction of reactive intermediate A with H2 ( -2) and with 2-tol... 

Thioglycosides can be activated for gfycosylation reactions with sulfur electrophiles, e.g., with dimethyl(methylthio)sulfonium triflate or with methanesulfenyi bromide and silver(l +) to form reactive sulfonium intermediates (F. Dasgupta, 1988). 

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