Mechanism Mechanistic features

However, mechanistic features not involved in the simplified mechanism of Scheme 2 can also play a role. In particular, interaction of the bromonium-bromide ion pair with its close solvent environment, which cannot be readily estimated from kinetics or product formation in bromination. An example of this control is shown in the following paragraph. 

The general mechanistic features of the aldol addition and condensation reactions of aldehydes and ketones were discussed in Section 7.7 of Part A, where these general mechanisms can be reviewed. That mechanistic discussion pertains to reactions occurring in hydroxylic solvents and under thermodynamic control. These conditions are useful for the preparation of aldehyde dimers (aldols) and certain a,(3-unsaturated aldehydes and ketones. For example, the mixed condensation of aromatic aldehydes with aliphatic aldehydes and ketones is often done under these conditions. The conjugation in the (3-aryl enones provides a driving force for the elimination step. 

Another important family of elimination reactions has as its common mechanistic feature cyclic TSs in which an intramolecular hydrogen transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.20 depicts examples of these reaction types. These are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic TS dictates that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are often referred to as thermal syn eliminations. 

The model proposed by Brandt et al. is consistent with the experimental observations, reproduces the peculiar shape of the kinetic curves in the absence and presence of dioxygen reasonably well, and predicts the same trends in the concentration dependencies of t, p that were observed experimentally (80). It was concluded that there is no need to assume the participation of oxo-complexes in the mechanism as it has been proposed in the literature (88-90). However, the model provides only a semi-quantitative description of the reaction because it was developed at constant pH by neglecting the acid-base equilibria of the sulfite ion and the reactive intermediates, as well as the possible complex-formation equilibria between various iron(III) species. In spite of the obvious constraints introduced by the simplifications, the results shed light on the general mechanistic features of the reaction and could be used to identify the main tasks for further model development. 

One pervasive mechanistic feature of many of the hydrogenations described in other chapters of this handbook concerns the bonding of the unsaturated substrate to a metal center. As illustrated in generalized form in Eq. (1) for the hydrogenation of a ketone, a key step in the traditional mechanism of hydrogenation is migratory insertion of the bound substrate into a metal hydride bond (M-H). 

Various approaches to epoxide also show promise for the preparation of chiral aziridines. Identification of the Cu(I) complex as the most effective catalyst for this process has raised the possibility that aziridination might share fundamental mechanistic features with olefin cyclopropanation.115 Similar to cyclo-propanation, in which the generally accepted mechanism involves a discrete Cu-carbenoid intermediate, copper-catalyzed aziridation might proceed via a discrete Cu-nitrenoid intermediate as well. 

Horseradish peroxidase has been intensively studied for the elucidation of the mechanism of peroxidase catalysis [24-28]. Some important mechanistic features of peroxidase-catalyzed reaction are briefly described here. 

Protonation to the conjugate acid (iminium cation) increases the potential of the itnine to act as an electrophile (compare carbonyl see Section 7.1), and this is followed by nucleophilic attack of water. The protonated product is in equilibrium with the other mono-protonated species in which the nitrogen carries the charge. We shall meet this mechanistic feature from time to time, and it is usually represented in a mechanism simply by putting H+, +H+ over the equilibrium arrows. Do not interpret this as an internal transfer of a proton such transfer would not be possible, and it is necessary to have solvent to supply and remove protons. 

Various mechanisms have been proposed to explain the stereoselectivity of Ziegler-Natta initiators [Boor, 1979 Carrick, 1973 Corradini et al., 1989 Cossee, 1967 Ketley, 1967a,b Tait and Watkins, 1989 Zambelli and Tosi, 1974]. Most mechanisms contain considerable details that distinguish them from each other but usually cannot be verified. In this section the mechanistic features of Ziegler-Natta polymerizations are considered with emphasis on those features that hold for most initiator systems. The major interest will be on the titanium-aluminum systems for isoselective polymerization, more specifically, TiCl3 with A1(C2H5)2C1 and TiCLt with A1(C2H5)3—probably the most widely studied systems, and certainly the most important systems for industrial polymerizations. 

Since its discovery by Chandross and to this day, peroxy-oxalate chemiluminescence has been controversial because of its enormous complexity in view of the many alternative steps involved in this process. The principal mechanistic feature of the peroxy-oxalate chemiluminescence pertains to the base-catalyzed (commonly imidazole) reaction of an activated aryl oxalate with hydrogen peroxide in the presence of a chemiluminescent activator, usually a highly fluorescent aromatic hydrocarbon with a low oxidation potential . A variety of putative high-energy peroxide intermediates have been proposed for the generation of the excited states . In the context of the present chapter, it is of import to mention that recent work provides experimental evidence for the intervention of the 1,2-dioxetanedione 18 (Scheme 11) as the high-energy species responsible for the chemiexcitation. Furthermore, clear-cut experimental data favor the CIEEL mechanism as a rationalization of the peroxy-oxalate chemiluminescence . 

Any discussion of the mechanism of xanthine oxidase should attempt to incorporate the special features of xanthine oxidase (and xanthine dehydrogenase and aldehyde oxidase) which are not present, for example, in sulfite oxidase. There are two such features at least (a) the involvement of two protons rather than the one found for sulfite oxidase, and (b) the presence of the cyanolyzable sulfur atom. The mechanistic features discussed so far involve the abstraction of two electrons and a proton. This means that a carbonium ion is generated, which could undergo attack by a nucleophile. Thus, the presence of a nucleophile at the active site could lead to the formation of a covalent intermediate that will break down to give the products.1032 The nucleophile could either be the cyanolyzable sulfur atom or a group associated with the second proton. A possible scheme is shown in Figure 41. 

Besides the potential-dependent adsorption of a poison, i.e. a species that is different from the electroactive species or current carrier, other mechanisms are discussed that may hide a region of negative differential resistance and thus give rise to oscillations on a branch of the I-U curve with positive slope. According to specific mechanistic features, a subdivision of HN-NDR oscillators was proposed into originally three subclasses [12]. 

In this review we will present the distinctive features of the proposed mechanism as well as main experimental and theoretical evidence for it. The reaction of organic halides will be further discussed according to the following sequence mechanistic features of the S l reaction in Section II, alkyl halides with electron-withdrawing groups in Section III, alkyl halides without electron-withdrawing groups in Section IV, aromatic halides in Section V and vinyl halides in Section VI. 

The photodehalogenation of haloarenes has attracted the attention of scientists interested in reaction mechanisms, synthesis, and environmental processes [1]. Considerable effort has been devoted to developing our understanding of the mechanism of photodehalogenation of monohaloarenes. Since in the environment polyhaloarenes are of more importance than their monohalo parents, interest has been stimulated in applying current theory to achieving an understanding of the mechanistic features of the phototransformation of polyhaloarenes. 

The mechanism of the enaminonitrile - imidazole conversion has been the subject of extensive study, although only a few mechanistic features are known with certainty. The nitrile and the amine must have a cis relationship in the starting material. In the case of diaminomaleonitrile, the initial step would therefore be a photoisomerization to diaminofumaronitrile, via the triplet excited state of the enaminonitrile (equation 22)60. The rearrangement to imidazole, however, is presumed to occur via the singlet excited state, since triplet sensitizers do not promote the reaction and triplet quenchers do not inhibit it56b 59b. 

Our objective in this article will be to select critical examples dealing with the photooxygenation of organic substrates, in which experimental evidence strongly support a photochemically initiated electron-transfer mechanism, thus opening an entirely new perspective in peroxide chemistry. Our aim will be also to introduce the interested reader to mechanistic features which must be dealt with in investigating photoinduced transformations. On the other hand, because this area is inherently interdisciplinary, it is rather hard to provide a detailed compilation of all work relevant to the photoinduced reactions of concern. Thus, except for few cases, no attempt will be made to discuss the diversified chemistry of photosensitized electron-transfer processes, extensively reported in several excellent reviews [69-72]. 

These structural features help to define class A GPCRs and may be important components in the activation mechanism for individual receptors in this class. However, as the structural dynamics of class A receptors are beginning to be studied on a molecular level, important differences are emerging between the functional and structural roles of conserved sequence motifs in different class members. Thus, while the presence of conserved sequence motifs may in some cases help predict the mechanistic features of a novel receptor, careful experimental studies are stiU needed to test such predictions. 

A comment on deducing mechanistic details of Rice-Herzfeld-type reactions from apparent reaction orders is called for. Usually, a termination mechanism giving the desired result is postulated or, failing that, a collision partner in initiation, termination, or propagation steps is invoked. A formal scheme relating overall reaction orders to such mechanistic features, developed as early as 1948 by Goldfinger et al. [44], is quoted to this day in some textbooks. However, uncritical application... 

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