General Reaction Schemes

Scheme 43 The Barton-Zard reaction (general scheme)...

The first main idea of this work is to refuse the assumption of possible one-step transfer of several (more than one) electrons in one elementary electrochemical act and to consider any real many-electron process as a sequence of one-electron steps. Although this idea is not new (it follows immediately from quantum theories of electron transfer [4]), it is not followed consistently in research practice. The reason is that a number of significant problems ought to be overcome in such an approach description of the accompanying intervalence chemical reactions, general scheme of the mechanism, estimation of stability of low-valence intermediate species and... 

A further example is given below illustrating the use of a dibasic anhydride (succinic anhydride) the succinoylation reaction is a valuable one since it leads to aroyl carboxylic acids and ultimately to polynuclear hydrocarbons. This general scheme of synthesis of substituted hydrocarbons through the use of succinic anhydride is sometimes called the Haworth reaction. Thus a-tetralone (see below) may be reduced by the Clemmensen method to tetralin (tetrahydronaphthalene) and the latter converted into naphthalene either catal3d.ically or by means of sulphur or selenium (compare Section, VI,33). 

Several 4-amino-2,5-disubstituted thiazoles (257) have been obtained recently (702, 756, 776, 814, 820) by a ring cyclization reaction of halogeno compounds with cyanamide derivatives (263) according to the general Scheme 135. 

TFEO is by fai the most reactive epoxide of the series. However, ail the reported perfluoroepoxides undergo similar ring-opening reactions. The most important reactions of these epoxides ate those with the fluoride ion or perfluoroalkoxides. The reaction of PIBO and the fluoride ion is an example (27). It also illustrates the general scheme of oligomerization of perfluoroepoxides (eq 5). 

Other methods of generating a-aminoketones in situ are common, if somewhat less general than the methods already described. 2-Nitrovinylpyrrolidine, which is readily available, yields 2,3-bis(3-aminopropyl)pyrazine on reduction and this almost certainly involves ring opening of the intermediate enamine to an a-aminoketone which then dimerizes under the reaction conditions (Scheme 59) (78TL2217). Nitroethylene derivatives have also served as a-aminoketone precursors via ammonolysis of the derived epoxides at elevated temperatures (Scheme 60) (76S53). Condensation of 1,1-disubstituted hydrazine derivatives with a-nitro-/3-ethoxyethylene derivatives has been used in the synthesis of l,4-dialkylamino-l,4-dihydropyrazines (Scheme 61) (77S136). 

FIG. 16-9 General scheme of adsorbent particles in a packed bed showing the locations of mass transfer and dispersive mechanisms. Numerals correspond to mimhered paragraphs in the text 1, pore diffusion 2, solid diffusion 3, reaction kinetics at phase boundary 4, external mass transfer 5, fluid mixing. 

Scales for bases that are too weak to study in aqueous solution employ other solvents but are related to the equilibrium in aqueous solution. These equilibrium constants provide a measure of thermodynamic basicity, but we also need to have some concept of kinetic basicity. For the reactions in Scheme 5.4, for example, it is important to be able to make generalizations about the rates of competing reactions. 

The reaction is generally carried out in an alcoholic solvent, resulting in formation of the acetal the general scheme is called the Mattox rearrangement, and the yields are usually high. If a nonalcoholic solvent and acid are used, the aldehyde is obtained directly, but in lower yield. However, the use of zinc acetate as catalyst instead of an acid permits isolation of the aldehyde in ca. 50 % yield. Tire use of refluxing acetic acid alone... 

The general scheme for this type of tautomeric reaction is 310a 310b. 

The Diels-Alder reaction catalyzed by this chiral titanium catalyst 31 has wide generality (Scheme 1.53, 1.54, Table 1.22, 1.23). Acryloyl- and fumaroyl-oxazolidinones react with isoprene giving cycloadducts in high optical purity. 2-Ethylthio-l,3-buta-diene can also be successfully employed as the diene [42]. 

Rydon and co-workers (73) have shown that the reaction of simple alcohols with triphenylphosphite methiodide and triphenylphosphite dihalides gives alkyl halides according to the general scheme. 

Bacterial bioluminescence, 30-46 factors required, 31 general scheme, 32 in vivo luminescence, 41 luminescence reaction, 37, 38 Bacterial luciferase, 33-35, 343 assay, 39 cloning, 34 crystal structure, 34 extraction and purification, 34 inactivation, 34, 35 molecular weight, 34 properties, 34 storage, 35 subunits, 34... 

Destructive hydrazinolysis is of importance for the establishment of the structure of the PCS main chain, since the reaction products can easily be identified. The importance of this reaction also results from the fact that C=C bonds do not undergo hydrazinolysis. A general scheme of the process of destructive hydrazinolysis may be given as follows ... 

Many reactions, nitration, nitrosation, oxidation, etc are then possible. Titov s generalized scheme is... 

A generalized scheme, which summarizes certain of the most frequently observed kinetic characteristics for the reactions of a solid alone or with a gas, a liquid (solute) or another solid, is given in Table 2. The following processes may control the rate of product formation. 

In this section indirect methods will be explored. One was given in Section 3.6. There, data were presented for two members of a large family of reactions of the phenyl radical with organic halides. Pairs of halides were used, and the ratio of two competing products was determined. Expanding the general scheme of Eq. (5-1) further, to use a pair of competing traps, we have... 

The reaction is generally believed to proceed via the formation of ionic acylam-monium intermediate compounds (Reaction 1, Scheme 2.27). The equilibrium constant of the acylammonium formation depends mostly on steric and resonance factors, while the basicity of the tertiary amine seems to play a secondary role.297 In die case of the less basic compounds, such as acidic phenols, and of strong tertiary amines, such as Uialkylamines, the reaction has been reported to proceed through a general base mechanism via the formation of hydroxy-amine H-bonded complexes (Reaction 2, Scheme 2.27).297... 

The reaction of the monomeric metallo-ester with the monomer in THF allows the study of the initial stages of polymerization 50). The course of the reaction is followed by gas-chromatographic and GPC analyses of the terminated products. Oligomerization of the monomeric metallo-ester proceeds very fast. In a fraction of a second all the available monomer is consumed. In contrast, the disproportionation of the independently studied 67) dimeric metallo-esters, proceeds very slowly in a time scale of 10s s. Both reactions are described by the general scheme 67). 

Figure 30 General scheme of hydrosilylation reactions using Ts[OSiMe2H]8.

The carbon-centered radical R, resulting from the initial atom (or group) removal by a silyl radical or by addition of a silyl radical to an unsaturated bond, can be designed to undergo a number of consecutive reactions prior to H-atom transfer. The key step in these consecutive reactions generally involves the intra-or inter-molecular addition of R to a multiple-bonded carbon acceptor. As an example, the propagation steps for the reductive alkylation of alkenes by (TMSfsSiH are shown in Scheme 6. 

Fundamental advances are offered by the knowledge of energy states and their electronic distributions in organic compounds and the relationship of these to reaction mechanisms. The development, for example, of even an empirical and approximate general scheme for the estimation of activation energies would indeed be most notable. 

FIGURE 29.3 General scheme of the reaction of a polysulfidic silane with an unsaturated polymer. 

The hydrogen transfer reaction (HTR), a chemical redox process in which a substrate is reduced by an hydrogen donor, is generally catalysed by an organometallic complex [72]. Isopropanol is often used for this purpose since it can also act as the reaction solvent. Moreover the oxidation product, acetone, is easily removed from the reaction media (Scheme 14). The use of chiral ligands in the catalyst complex affords enantioselective ketone reductions [73, 74]. 

Figure 4.6. General scheme for the synthesis of linear alkyl benzenes, precursors to surfactants. Control over pore size of the catalyst can suppress the second alkylation almost completely. Given the ease with which the pore size can be chosen, one can design an effective catalyst for any particular reaction, and allow the selective and clean production of the desired mono-alkyl product, thus eliminating much of the waste associated with the process.

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