Intermediate in reaction mechanism

A. Polarization and Electrophilic Svhstitidion From the various accounts that have been given of the role of 77-complexes and <7-complexes as possible intermediates in reaction mechanisms, that described by Olah e al.(1961)is selected for special attention, since it... 

Carbanions play critical roles in a wide variety of reaction pathways. As stated in the Introduction, this chapter will not focus on the synthetic utility of carbanions, but will instead focus on their mechanistic significance. In this section, a sample of important reaction mechanisms that involve transient or relatively short-lived car-banion intermediates will be introduced. As you will see, the key element in these mechanisms is the ability to form a carbanion that is reasonably stable, and often the kinetics of the reactions are dominated by carbanion stability. The role of carbanion intermediates in elimination reactions will be presented in some detail as a way to illustrate some of the methods that have been developed to probe for carbanion intermediates in reaction mechanisms. Other processes including additions and rearrangement reactions will be presented in less detail, but the role of carbanion stability in these reactions will be outlined. 

Very often intermediates in reaction mechanisms are present in very small eoncentrations. In sueh a situation, the rate of change of the concentration of the intermediate with time is much smaller than the corresponding quantities for the reactants and products. The intermediate is then said to be in a steady state, and its time derivative can be set equal to zero (to a good approximation). The result is usually a vast simplification of the rate equation. For example, if (B) is in a steady state,... 

Likewise, there is also a minimum OS corresponding to maximrrm d" of d [Pt(PPh3)3] obeys the 18e rule so it might be thought possible however, it would be d Pt(—II) and forbidden. These limitations need to be borne in mind when proposing intermediates in reaction mechanisms. 

Open shell atoms and other molecular species that act as free radicals play a special role as intermediates in reaction mechanisms. Because they have incomplete electron shells they are usually highly reactive, even with stable molecules at ordinary temperatures. The concentrations of such free radicals in these reaction systems are usually low and the application of the steady-state hypothesis to them is valid. 

Entry 4 shows that reaction of a secondary 2-octyl system with the moderately good nucleophile acetate ion occurs wifii complete inversion. The results cited in entry 5 serve to illustrate the importance of solvation of ion-pair intermediates in reactions of secondary substrates. The data show fiiat partial racemization occurs in aqueous dioxane but that an added nucleophile (azide ion) results in complete inversion, both in the product resulting from reaction with azide ion and in the alcohol resulting from reaction with water. The alcohol of retained configuration is attributed to an intermediate oxonium ion resulting from reaction of the ion pair with the dioxane solvent. This would react until water to give product of retained configuratioiL When azide ion is present, dioxane does not efiTectively conqiete for tiie ion-p intermediate, and all of the alcohol arises from tiie inversion mechanism. ... 

An alternative view of these addition reactions is that the rate-determining step is halide-assisted proton transfer, followed by capture of the carbocation, with or without rearrangement Bromide ion accelerates addition of HBr to 1-, 2-, and 4-octene in 20% trifluoroacetic acid in CH2CI2. In the same system, 3,3-dimethyl-1-butene shows substantial rearrangement Even 1- and 2-octene show some evidence of rearrangement, as detected by hydride shifts. These results can all be accoimted for by a halide-assisted protonation. The key intermediate in this mechanism is an ion sandwich. An estimation of the fate of the 2-octyl cation under these conditions has been made ... 

In a word, POM and OCM reactions proceed at different active sites with different reaction intermediates and reaction mechanisms over the nickel-based catalysts proposed as follows ... 

The powerful nucleophilicity of enamines allows the addition of nitroalkenes to take place without the presence of Lewis acids. The isolation of secondary products, which can be explained by an initial Michael addition, suggests the participation of zwitterionic intermediates in the mechanism of the reaction (Eq. 8.97).154... 

Regarding mechanism 2 which involves H as an intermediate reducing agent, the author is unaware of reports of H as a possible intermediate in the mechanisms proposed for reactions for the hydrogen electrode. As in the case of mechanism 1, the supposed active intermediate is consumed in two reactions (equations (8) and (9)), an unlikely event for an unlikely intermediate. In his review of the H2 electrode reaction, Enyo [46] concluded that involvement of H intermediates in H2 electrode reactions had not gained sufficient experimental evidence to warrant serious consideration. Nevertheless, the mechanism has been occasionally invoked, as for example, recently by Gemmler et al. for electroless Ni-P deposition [47]. 

One of the more efficient CL substances, lucigenin (10,10 -dimethyl-9,9 -biscridinium nitrate), was discovered by Gleu and Petsch in 1935 (Fig. 5). They observed an intense green emission when lucigenin was oxidized in an alkaline medium [72], Other acridinium derivatives were shown to produce CL emission upon hydrogen peroxide oxidation of aqueous alkaline solutions. The main reaction product was /V-mcthylacridone, acting as an active intermediate in the mechanism proposed by Rauhut et al. [73, 74] (Fig. 6). 

Nitroxyl radicals are formed as intermediates in reactions of polymer stabilization by steri-cally hindered amines as light stabilizers (HALS) [30,34,39,59]. The very important peculiarity of nitroxyl radicals as antioxidants of polymer degradation is their ability to participate in cyclic mechanisms of chain termination. This mechanism involves alternation of reactions involving alkyl and peroxyl radicals with regeneration of nitroxyl radical [60 64],... 

H.-U. Siehl, unpublished results presented in a part at the 6th WATOC, Lugano, 2002, August 4-9 and the International Conference on Reaction Intermediates and Reaction Mechanism, Ascona, 2002, July 7-12... 

Similar pyrone complexes were isolated by Semmelhack97a as the products of the reaction between tetracarbonyl[ethoxy(alkyl)carbene]iron(0) complexes and various acetylenes. Vinylketene complexes are proposed as key intermediates in the mechanism of this conversion, which closely matches analogous reactions with cobalt carbenes51 (see Section V,B), while showing crucial differences with the analogous reaction of a chromium carbene (see Section II,B). 

E = Faraday constant). The equilibrium potential E is dependent on the temperature and on the concentrations (activities) of the oxidized and reduced species of the reactants according to the Nemst equation (see Chapter 1). In practice, electroorganic conversions mostly are not simple reversible reactions. Often, they will include, for example, energy-rich intermediates, complicated reaction mechanisms, and irreversible steps. In this case, it is difficult to define E and it has only poor practical relevance. Then, a suitable value of the redox potential is used as a base for the design of an electroorganic synthesis. It can be estimated from measurements of the peak potential in cyclovoltammetry or of the half-wave potential in polarography (see Chapter 1). Usually, a common RE such as the calomel electrode is applied (see Sect. 2.5.1.6.1). Numerous literature data are available, for example, in [5b, 8, 9]. 

As discussed above, proteases are peptide bond hydrolases and act as catalysts in this reaction. Consequently, as catalysts they also have the potential to catalyze the reverse reaction, the formation of a peptide bond. Peptide synthesis with proteases can occur via one of two routes either in an equilibrium controlled or a kinetically controlled manner 60). In the kinetically controlled process, the enzyme acts as a transferase. The protease catalyzes the transfer of an acyl group to a nucleophile. This requires an activated substrate preferably in the form of an ester and a protected P carboxyl group. This process occurs through an acyl covalent intermediate. Hence, for kineticmly controlled reactions the eii me must go through an acyl intermediate in its mechanism and thus only serine and cysteine proteases are of use. In equilibrium controlled synthesis, the enzyme serves omy to expedite the rate at which the equilibrium is reached, however, the position of the equilibrium is unaffected by the protease. 

Similar qualitative relationships between reaction mechanism and the stability of the putative reactive intermediates have been observed for a variety of organic reactions, including alkene-forming elimination reactions, and nucleophilic substitution at vinylic" and at carbonyl carbon. The nomenclature for reaction mechanisms has evolved through the years and we will adopt the International Union of Pure and Applied Chemistry (lUPAC) nomenclature and refer to stepwise substitution (SnI) as Dn + An (Scheme 2.1 A) and concerted bimolecular substitution (Sn2) as AnDn (Scheme 2.IB), except when we want to emphasize that the distinction in reaction mechanism is based solely upon the experimentally determined kinetic order of the reaction with respect to the nucleophile. 

This notion that reaction mechanisms are strictly enforced by the intermediate lifetime implies the existence of a narrow borderline region and a sharp change in reaction mechanism with changing lifetime of the carbocation intermediate. However, a narrow borderline region is not observed in all cases. The problem is... 

The benzylic substrates X-l-Y and X-2-Y have provided a useful platform for examining the changes in reaction mechanism for nucleophilic substitution that occur as the lifetime of the carbocation intermediate is decreased systematically by varying the meta- and para- aromatic ring substituents. When X is strongly resonance electron-donating, X-l-Y and X-2-Y react by a stepwise mechan-... 

This chapter does not intend to provide a complete collection of newly synthesized organometallic or coordination complexes for alkene polymerization, but rather aims to review a cross-section of transition metal catalysts from the viewpoint of polymers and polymerization reactions. We focus particularly on polymers that are difficult or virtually impossible to prepare using conventional catalysts. In this light, we narrow our attention to well-defined molecular catalysts, including a study of progress in the understanding of active species, reactive intermediates, and reaction mechanisms that are indispensable for the synthesis of such polymers. 

A second mechanism by which impurities present in the solution interfere with measurements in electrode kinetics is associated less with their providing alternative paths to reactions and more with their adsorption on the electrode surface. The presence of adsorbed impurity material on the electrode surface may affect the rate of an intended reaction, particularly where there is an intermediate in the mechanism of the intended reaction.25 Thus, the adsorbed impurity may block reactive sites on the electrode. Since the intended reaction (say, the reduction of oxygen) cannot now use the blocked active sites on the electrode surface, its kinetics are slowed down, particularly when the impurity adsorbs on the most active catalytic sites on the electrode. Again,... 

At room temperature, the results are similar to, but somewhat more complex, than for C4F8-2. C3FeO is formed in a manner which required a complex mechanism to explain the results.86 The P02 intermediate in reaction (4-51) could have led to product formation by any of the following steps... 

The experienced practitioner in reaction mechanisms accepts a concerted mechanism for a reaction involving the breaking and making of more than two bonds as a last resort. He first will try to analyze the overall transformation in terms of discrete steps that are individually simple enough surely to be concerted and that also involve energetically reasonable intermediates. 

Hydro acylation of alkenes was achieved in the presence of Wilkinson s catalyst and microwave irradiation under solvent-free conditions. As an example, benzaldehyde was reacted with dec- 1-ene to give 1-phenylundecan- 1-one in 83%yield within 30 min. Both domestic microwave ovens and single-mode reactors have been used for this reaction. The presence of an amine such as 2-amino-3-picoline or aniline and a carboxylic acid is crucial for the success of the reaction, showing that the formation of an imine plays an important role as an intermediate in the mechanism of this reaction29. 

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