Reaction Mechanisms Reagents

Antithetical connections (the reversal of synthetic cleavages) and rearrangements are indicated by a con or rcarr on the double-lined arrow. Here it is always practical to draw right away the reagents instead of synthons. A plausible reaction mechanism may, of course, always be indicated. 

In many cases, the a-haloketone does not appear to be an intermediate in this reaction, since reagents such as sulfur trioxide, sulfuric, or 60% nitric add lead to 2-aminothiazole but with lower yields (11 to 43%). Formamidine disulfide [-S-C(=NH)NH2]2, a product of the oxidation of thiourea, seems to be the intermediate in this reaction, since upon treatment with ketones, it gives 2-aminothiazole (604). However, the true mechanism of this reaction has not yet been completely elucidated. 

Hydrogen peroxide may react directiy or after it has first ionized or dissociated into free radicals. Often, the reaction mechanism is extremely complex and may involve catalysis or be dependent on the environment. Enhancement of the relatively mild oxidizing action of hydrogen peroxide is accompHshed in the presence of certain metal catalysts (4). The redox system Fe(II)—Fe(III) is the most widely used catalyst, which, in combination with hydrogen peroxide, is known as Fenton s reagent (5). 

Acetaldehyde. Acetaldehyde [75-07-0], has a slightly different reaction mechanism with hemoglobin (58). Although this reagent reacts... 

In contrast to electrophilic reagents, the highly -tt-deficient character of the pteridine nucleus is responsible for its vulnerability towards nucleophilic attack by a wide variety of reagents. The direct nucleophilic substitution of pteridine itself in a Chichibabin-type reaction with sodamide in diethylaniline, however, was unsuccessful (51JCS474). Pteridin-6-one, on the other hand, yielded pteridine-6,7-dione under the same conditions, via a still unknown reaction mechanism. 

The reaction mechanism has not been elucidated. It is possible that formaldehyde reacts by oxidation as in the Marquis reaction (see formaldehyde — sulfuric acid reagent), whereby colored salts are formed with naphthoquinone sulfonic acid. 

In the Meth-Cohn quinoline synthesis, the acetanilide becomes a nucleophile and provides the framework of the quinoline (nitrogen and the 2,3-carbons) and the 4-carbon is derived from the Vilsmeier reagent. The reaction mechanism involves the initial conversion of an acylanilide 1 into an a-iminochloride 11 by the action of POCI3. The a-chloroenamine tautomer 12 is subsequently C-formylated by the Vilsmeier reagent 13 derived from POCI3 and DMF. In examples where acetanilides 1 (r = H) are employed, a second C-formylation of 14 occurs to afford 15 subsequent cyclisation and... 

A generally applicable reaction scheme naturally cannot be given. The reaction mechanism of one particular carbinolamine with a particular reagent can depend on the reaction conditions in nonpolar solvents, the nondissociated carbinolamine obviously reacts (Sj 2 mechanism). In polar solvents, on the other hand, the mesomeric cation reacts (S l mechanism). Formally all these reactions belong to the general class of aminomethylation. The reaction products can be considered to be Mannich bases. ... 

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. 

In general, the reaction mechanism of elastomeric polymers with vulcanisation reagents is slow. Therefore, it is natural to add special accelerators to rubber compounds to speed the reaction. Accelerators are usually organic compounds such as amines, aldehyde-amines, thiazoles, thiurams or dithio-carbamates, either on their own or in various combinations. 

The procedure described here 1 is a convenient two-step reaction that relics on the base-induced elimination-isomerization reactions of gem-dichlorooyclopropanes.8-15 The reaction mechanism has been studied.16 The principal advantage of this method is the ready availability of necessary reagents. 

These basic reaction mechanisms occur with many of the reagents treated in Volumes la and lb. The following examples can be listed ... 

In addition to the reagent sequences with clearly detectable reaction mechanism, which have already been described, many sequences of reagents not covered by any of the reaction types described have also found application in thin-layer chromatograpl. The reaction sequences that remain to be described were all designed to provide as speciflc a detection of the separated substances as possible. 

Classical examples of this type of reaction are the various dimethylaminobenz-aldehyde reagents (q.v.) and vanillin-acid reagents, of which one, the vanillin-phosphoric acid reagent, is already included in Volume 1 a. The aldol condensation of estrogens is an example for the reaction mechanism (cf. Chapter 2, Table 6). According to Maiowan indole derivatives react in a similar manner [1]. Longo has postulated that catechins yield intensely colored triphenylmethane dyes [2]. 

Thermal solid-state reactions were carried out by keeping a mixture of powdered reactant and reagent at room temperature or elevated temperature, or by mixing with pestle and mortar. In some cases, the solid-state reactions proceed much more efficiently in a water suspension medium or in the presence of a small amount of solvent. Sometimes, a mixture of solid reactant and reagent turns to liquid as the reaction proceeds. All these reactions are called solid-state reactions in this chapter. Solid-state reactions were found to be useful in the study of reaction mechanisms, since it is easy to monitor the reaction by continuous measurement of IR spectra. 

A possible reaction mechanism involves the initial generation of the carbene 86 via the deprotonation of SIEt HCl 85 by the Grignard reagent. The carbene 86 binds with CoCl to generate the complex 87 which is also reduced by the Grignard... 

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