Mechanistic features

An important mechanistic feature of such interactions is in the molecular detail of the interaction. Two general... 

Thus, time-resolved absorjDtion measurements are often useful to initially characterize tire kinetic characteristics of a reaction, but otlier spectroscopic metliods may also be useful in probing more subtle or stmcture-specific mechanistic features. In tire many cases in which one would like to obtain more infonnation about tire stmctural features of intennediates... 

Sections 2.1—2.3 give accounts of kinetic and mechanistic features of the three rate-limiting processes (i) diffusion at a surface or in a gas (including the nucleation step), (ii) reaction at an interface, and (iii) diffusion across a barrier phase, [(ii) and (iii) are growth processes.] In any particular reaction, the slowest of these processes will, at any particular instant, control the rate of product formation. (A kinetic analysis of rate measurements must also incorporate an allowance for the geometric factors.)... 

In summary, the evidence described above demonstrates three main mechanistic features of the rearrangement of allylic sulfenates to sulfoxides (1) spontaneous and wholly concerted [2,3]-sigmatropic shift of allyl or a-substituted allyl esters (7 a, b) at one extreme (2) complete stability of the y-aryl and y,y-dialkyl substituted allyl sulfenates as well as... 

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. 

A useful division of the contributing mechanistic features of any chemiluminescence process involving excitation of a "spectator" fluorophore is given in Equation 2. To optimize the chemical efficiency > we have also undertaken a systematic investigation of... 

Observations that have been made will be critically discussed in the light of their relevance to other approaches that attempt to elucidate mechanistic features of this complex phenomenon. No claim is made for a complete coverage of the literature, but an attempt has been made to collate all relevant information and to discuss the results that best illustrate the principles involved. 

This complex and structurally related molecules served as a functional homogeneous model system for commercially used heterogeneous catalysts based on chromium (e.g. Cp2Cr on silica - Union Carbide catalyst). The kinetics of the polymerization have been studied to elucidate mechanistic features of the catalysis and in order to characterize the potential energy surface of the catalytic reaction. 

The special kinetic and mechanistic features of this reaction began to be studied at platinum electrodes in the early 1960s. In recent years a number of groups in different countries published a large amount of research work concerning this reaction (see, e.g., Bagotsky et al., 1977 Parsons and Van der Noot, 1988 Iwasita and... 

Finally, the most significant mechanistic feature of the Ramberg-Backlund rearrangement is the stereoselective formation of ds-olefin products, as a result of the preferential cis-positioning of the pair of R groups in the episulfone-forming transition state, variously attributed to London forces , to diastereoselectivity in carbanion formation and to steric attraction . However, with the use of stronger bases such as potassium t-butoxide °, the trans-olefin predominates (equation 52), apparently due to prior epimerization of the kinetically favoured cts-episulfone, and subsequent loss of the sulfur dioxide. Similarly, when the episulfone intermediates possess unusually acidic... 

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. 

Rate studies show that chlorination is subject to acid catalysis, although the kinetics are frequently complex.13 The proton is believed to assist Cl-Cl bond breaking in a reactant-Cl2 complex. Chlorination is much more rapid in polar than in nonpolar solvents.14 Bromination exhibits similar mechanistic features. 

The chemistry of chlorocyclophosphazenes continues to be of great interest. Many of the mechanistic features involving the aminolysis reactions are now understood. However, rigorous kinetic studies on reactions of chlorocyclophosphazenes with other types of nucleophiles are quite rare. The most promising utility of the chlorocyclophosphazenes is their reactive periphery combined with a robust framework. It... 

Second is the application of a wide range of experimental designs and techniques. DNA CT is observed in a diverse array of systems over different distance and time regimes. Consequently, a versatile approach which draws upon complementary methods is required to explore different facets of this chemistry and develop a complete picture. We interrogate a variety of nucleic acid assemblies using spectroscopic, biochemical and electrochemical tools to define mechanistic features, exploit biological applications, and explore biological consequences of DNA CT. 

These results prompted us to investigate the reactions of tetrafluoro-benzyne with other alkoxybenzenes 114> not only in order to attempt to explore mechanistic features more closely, but also because we envisaged that the ketonic products would undergo interesting reactions. 

Laboratory studies have reported an excellent correlation between the toxicities of the dibenzo-p-dioxins and related compounds and their abilities to induce aryl hydrocarbon hydroxylase (AHH) activity (Poland and Knutson 1982), suggesting that some mechanistic features may be common to both the toxic and AHH-inducing activities of these compounds. Indeed, both the toxicities and the AHH-inducing activities of the dibenzo-p-diox-ins have found to correlate well with binding affinities to a cytosolic receptor (Poland, Greenlee, and Kende 1979 Poland and Knutson 1982). 

Vaughey J.T, Johnson C. S, Kropf A. J, Benedek R, Thackeray M. M, Tostmann H., Sarakonsri T., Hackney S., Fransson L., Edstrom K., Thomas J.O., Structural and mechanistic features of intermetallic materials for lithium batteries, J. Power Sources,... 

In conclusion, the reversibility of bromonium ion formation is at present inferred from particular experiments only nothing allows us to conclude that this mechanistic feature is general. However, when nucleophilic trapping... 

The photocycloaddition of an aldehyde or ketone with an olefin to yield an oxetane was reported by Paterno and Chieffi in 1909. 58> Contemporary studies on the synthetic utility and mechanistic features were initiated nearly 50 years later by Biichi et al. 59) Two review articles summarizing synthetic aspects of Paterno-Biichi reactions have been published 6.12)) and mechanistic studies have been reviewed several times. 6,38,60-62) The reaction involves the addition to olefin of a photo-excited carbonyl moiety. This circumstance makes it advantageous to review this reaction before a discussion of olefin-olefin additions, because the solution photochemistry of carbonyl compounds is probably better understood than any other aspect of organic photochemistry. Many of the reactions of carbonyl compounds have been elucidated during studies of the important phenomena of energy transfer and photosensitization. 63-65)... 

There are obviously no common mechanistic features for the photorearrangement of heterocycles in general. Yet three-membered rings349) and five-membered ring heterocycles 350) exhibit a quite particular behaviour. This topic has also been reviewed in a heterocyclic chemistry series 351). In this chapter only some selected reactions of preparative interest will be outlined. 

In the case of the bis phosphine derivatives of the group 7b metals, the lability of the oxygen atoms was so markedly retarded by substitution that it was necessary to enhance their reactivity by means of base catalysis (15, 14), a process having mechanistic features common wi Tx general base catalysis of the hydration of ketones (eq. 6) (15). 

Low molecular weight mimics for phosphoesterases are relevant from several perspectives. First, it is easier to study mechanistic features on a... 

Examples of autocatalytic reactions include the decomposition of C2H4I2 either in the gas phase or in solution in CC14 (Arnold and Kistiakowsky, 1933), hydrolysis of an ester, and some microbial fermentation reactions, The first of these may be used to illustrate some observed and mechanistic features. 

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. 

To confirm this mechanistic feature and to examine the interaction between the niobium ethoxide species and the postadsorbed molecule, various electron-donating compounds were postadsorbed on the surface that had been preadsorbed with the same amount of ethanol, Si0 2Nb(=0)(0H)(0C2H5)... 

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