Types of reaction scheme

More complicated reactions that combine competition between first- and second-order reactions with ECE-DISP processes are treated in detail in Section 6.2.8. The results of these theoretical treatments are used to analyze the mechanism of carbon dioxide reduction (Section 2.5.4) and the question of Fl-atom transfer vs. electron + proton transfer (Section 2.5.5). A treatment very similar to the latter case has also been used to treat the preparative-scale results in electrochemically triggered SrnI substitution reactions (Section 2.5.6). From this large range of treated reaction schemes and experimental illustrations, one may address with little adaptation any type of reaction scheme that associates electrode electron transfers and homogeneous reactions. 

Type (ii) cyclizations are more common. Thus, acid derivatives of pyrrolo[l,2-f>]-[l,2,5]benzothiadiazepine 5,5-dioxide 346 and 347 have been prepared from amino sulfonyl pyrrole 345 with acetal (2006JMC5840) or the semiacetal (1994JHC867) of ethyl glyoxalate or ethyl 2,2-diethoxy propionate (1996FES425) in the presence of PTSA catalyst in boiling absolute ethanol by a Pictet-Spengler type of reaction (Scheme 73). 

In this section, a description of the state of the art is attempted by (i) a review of the most fundamental types of reaction schemes, illustrated by some examples (ii) formulation of corresponding sets of differential equations and boundary conditions and derivation of their solutions in Laplace form (iii) description of rigorous and approximate expressions for the response in the current and/or potential step methods and (iv) discussion of the faradaic impedance or admittance. Not all the underlying conditions and fundamentals will be treated in depth. The... 

An organometallic radical may undergo several different types of reactions Scheme 3 illustrates some different reactions of CpM(CO)3 radicals (21) including (i) dimerization (ii) halide abstraction from an alkyl hahde or metal halide (iii) hydrogen atom abstraction from metal hydrides (iv) electron-transfer reduction (v) electron-transfer oxidation and ligand addition (vi) electron-transfer induced disproportionation (see Electron Transfer in Coordination Compounds). 

General support for this type of reaction scheme is afforded by investigations on the effect of reactant concentrations. An excess of phosphorus pentachloride gives a high yield of the linear end-stopped bodies the proportion of these petrol-insoluble compounds can be reduced almost to zero by extracting the phosphorus pentachloride slowly into a refluxing suspension of ammonium chloride in tetrachloroethane (/). 

Another interesting alkene synthesis starts from a cyclopropyl-substituted alcohol, which, on reaction with magnesium halides, is converted to a haloalkene MesSiX has also been used in this type of reaction (Scheme 22). ... 

In the former general type of reaction (Scheme 1 (1)), the nucleophile (Nuc) can be (i) aprotic, to give (Scheme 2 (1)) an azavinylidene (or methyleneamide) species, [M]—N=C(Nuc)R (ii) protic (HNuc), to afford (Scheme 2 (2)) the corresponding imino derivative, [M]-NH=C(Nuc)R or (iii) aprotic but bearing an electrophilic center that can remain attached to the nucleophilic site... 

Nuc E), forming a dipole for cycloaddition, to produce (Scheme 2 (3)) a cyclic imino species. The latter general type of reaction (Scheme 1 (2)), involving the addition of an electrophile (E) to the ligated nitrile, yields (Scheme 2 (4)) azavinylidene products, [M]-N=C(E)R. Sequential nucleophilic-electrophilic additions have also been performed (Scheme 3), leading to a stepwise and alternative mode of ligand activation. 

This type of reaction scheme is fairly common in organic synthesis. If the second reaction is faster than the first reaction ( i< 2)j h is difficult to stop the reaction at the first stage. P2 is formed as a major product. If the first reaction is faster than the second reaction ki > 2) there is a chance to obtain PI selectively. When one equivalent of A relative to B is used, PI should be obtained selectively. In other words, when the stoichiometric ratio of two reaction components is approximately unity (A/B = 1), the selectivity for the primary product PI should be high. Experiments,... 

The mode of transport of olefin intermediates between metal and acidic sites must be considered in this type of reaction scheme. In the isomerization of alkanes or dehydroisomerization of alkylcyclopentanes, a reaction sequence involving transport of olefin intermediates from metal to acidic... 

Further fluorination of perfluoroaromatic compounds328,329 occurs quite smoothly in many cases, using cobalt trifluoride and related reagents (Eq. 94). These reactions are of course much easier to control than fluorinations of the corresponding parent compounds containing hydrogen and the dimeric compound (103), obtained from perfluoropyrimidine, presents some of the best evidence for a radical cation process for this type of reaction (Scheme 29). Even direct fluorination of pentafluoropyridine, with fluorine of low... 

The number of different tetrazines which have been used in this type of reaction (Scheme S) is small and is essentially limited to the species already mentioned. However, a large number of olefins have been treated with these few tetrazines, yielding a large variety of dihydropyridazines. 

Another variety of this type of reaction scheme, which we will designate ECE, occurs when the reduction of O2 takes place at more negative potentials than Oi (i.e., e > E. In this case the reaction observed at the first reduction wave is an EC process however, the second reduction wave will be characteristic of an ECE reaction. 

Two types of reaction schemes are usually considered. Below we will use the notation of Cleland for denoting reactants and products. In sequential mechanism both substrates bind to the enzyme before any product is released. Such mechanism can be ordered with an obligatory order of addition of substrates and release of products. Substrate A binds E first, giving an EA complex. Then B binds to EA, giving a ternary complex (EAB). A and B are converted to products P and Q on (EAB), giving an (EPQ) complex. P dissociates first from (EPQ), then Q from EQ, regenerating E. 

For some types of reaction schemes this overall selectivity is a function of the degree of conversion, so often it is convenient to define in addition a point or differential selectivity referring to a particular conversion level ... 

The typical reaction mechanism for tri-liquid PTC in a batch reactor under agitation is illustrated in the schematic diagram of Fig. 9. Three types of reaction scheme considering the partition of the catalyst in the different phases and the place where the inherent reaction occurred have been proposed [226,227]. For the substitution reaction of alkyl halide (RX) and aqueous reactant metal salt (MY) using quaternary ammonium salt (QX) as the catalyst, the different types of reaction are addressed as follows [226]. 

Classic methods of pyrimidine synthesis enable the preparation of azulenopyrimidines, azuleno-4-pyrimidones, and azulenop5u-imidine-diones. One type of reactions (Scheme 27) consists in the cydization of 2-aminoazulenes (95, 110) by means of nitrogen-containing reagents... 

Kinetic data were obtained under pseudo-first-order conditions in the presence of a large excess of a-CD The inclusion reactions with S-CD were too fast to measure by the stopped-flow method. The values of the rate constants are summarized in Table 3. The two-step process was observed in the formation of LH 3- aCDx MLH aCDx Figure k shows the plots of the observed rate constants, kg and k, for the fast and the slow processes respectively against the total a-CD concentration, [a-CD ] From the dependence of kg and k- on [a-CD ], two types of reaction Schemes, 3 and can be considered. 

Figure 3. Illustration of difference types of reaction schemes. Explicit schemes are risgistered into the database implicit schemes are found by computer programs...

This type of reaction scheme is readily identified in steady-state or cyclic voltammetric [99] investigations based on the effect of the substrate concentration on the current [100]. The reaction layer thickness for the EC process is given by reaction = (f / h[C]buik)° where [C]buik denotes the concentration of C (Eq. II. 1.24) and fch is a second-order rate constant. The ratio of diffusion and reaction layer thickness yields an important kinetic parameter A = dpeak/Reaction = ([fch[C]buik RT]l[v nF]y- which describes the competition between diffusion and chemical reaction in the diffusion layer. With A < 0.1, diffusion is fast compared to the chemical reaction (e.g. fast scan rate) and the limiting case of a process... 

The photochemical Nazarov reaction is also known, but it has not been broadly used in synthesis. 4-Pyrones have commonly been used as substrates for the photochemical Nazarov. West and co-workers example illustrates this type of reaction (Scheme 19.35). Exposure of pyrone 133 to quartz-filtered ultraviolet (UV) irradiation in methanol solution leads to a 2 3 mixture of cyclopentenones 134 and 135. The two products presumably originate from zwitter-ionic epoxide intermediate 136 that forms through symmetry-allowed disrotatory ring closure from the excited triplet state. Nucleophilic attack by methanol at either of the two positively charged allylic carbon atoms leads to the observed products. This reaction is successful for pyrones that are fused to rings larger than cyclohexane, but it fails... 

After the first works on the copper-catalyzed aiylation of alkyl amines in the presence of ethylene glycol (2 equiv.) L15 [134] or diethylsalicylamide (5-20 mol%) L16 [135] as ligands, Buchwald reported in 2006 a room-temperature procedure for this type of reaction (Scheme 4). The condensation of primary or secondary alkyl amines [136, 137], and amino alcohols (selective N- versus O-arylation) with aryl iodides was performed at 25°C, thanks to the help of cheap 1,3-diket(Mie ligands L17 associated with Cul (5%). Harsher cmiditions (90°C) had to be applied from aryl bromides (Scheme 4). 

Two types of reaction schemes are usually considered. Below, we will use the notation of Cleland for denoting reactants and products. In sequential mechanism, both substrates bind to the enzyme before any product is released. Such mechanism can be ordered with an obligatory order of substrates addition and products release. 

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