Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Nucleophilicity parameters review

Mayr has reviewed the nucleophilicity parameters of allylstannanes and related compounds.276 The allyl-tin bond is easily cleaved, and this has been exploited in the preparation of pentafluorophenyltin chlorides and bromides (Equation (98)).277... [Pg.837]

The mechanisms of action of a range of organocatalysts have been reviewed for a range of reaction types, focussing on the use of electro- and nucleophilicity parameters to tease out viable routes. ... [Pg.24]

In this section three main aspects will be considered. Firstly, the basic strengths of the principal heterocyclic systems under review and the effects of structural modification on this parameter will be discussed. For reference some pK values are collected in Table 3. Secondly, the position of protonation in these carbon-protonating systems will be considered. Thirdly, the reactivity aspects of protonation are mentioned. Protonation yields in most cases highly reactive electrophilic species. Under conditions in which both protonated and non-protonated base co-exist, polymerization frequently occurs. Further ipso protonation of substituted derivatives may induce rearrangement, and also the protonated heterocycles are found to be subject to ring-opening attack by nucleophilic reagents. [Pg.46]

A comprehensive review on the chemistry of mercapto- and thione-substituted 1,2,4-triazoles and their utility in heterocyclic synthesis has been published <06ARK59>. The thermodynamic parameters, AAH and AAS, were determined for the interception of an intermediate, with the structural characteristics of an aziridinium imide, by nucleophilic solvents during the reaction of 2-methy 1-2-butene with A-phenyltriazolinedione <06TL2961>. [Pg.231]

There has been a review of relationships between activation parameters and mechanisms for biomolecular reactions in solution, including both nucleophilic substitutions and additions.5 Several studies have been reported involving substitutions by amine nucleophiles where both electronic and steric effects may be important. Kinetic studies of the reactions of 2,4-dinitrophenyl 2,4,6-trinitrophenyl ether (1) with ring-substituted... [Pg.175]

However, even for the simple methyl transfer reactions, there is considerable confusion and some disagreement about the details of the mechanism. Some authors (Sneen, 1973) have suggested that ionization of RX always precedes attack by the nucleophile, while others have maintained that the nucleophile attacks the covalent substrate. Extensive references to both points of view are given by McLennan (1976). In the present review the application of the Marcus theory of atom transfer (Marcus, 1968a) allows us to deduce values of the parameter a which describes the symmetry of the transition state. We shall compare this information about the transition state with that from changing the solvent, from isotope effects, and from Hammett relations. We shall then attempt to deduce a model for the transition state which is consistent for all the different types of data. [Pg.89]

Picosecond absorption spectroscopy was employed to study the dynamics of contact ion pairs produced upon the photolysis of substituted diphenylmethyl acetates in the solvents acetonitrile, dimethyl sulfoxide, and 2,2,2-trifluoroethanol.66 A review appeared of the equation developed by Mayr and co-workers log k = s(N + E), where k is the rate constant at 20 °C, s and N are nucleophile-dependent parameters, and is an electrophilicity parameter 67 This equation, originally developed for benzhydrylium ions and n-nucleophiles, has now been employed for a large number of different types of electrophiles and nucleophiles. The E, N, and s parameters now available can be used to predict the rates of a large number of polar organic reactions. Rate constants for the reactions of benzhydrylium ions with halide ions were obtained... [Pg.186]

Meisenheimer complex formation as separate reactions. Meisenheimer complexes can be considered as the anionic pseudobases derived from neutral aromatic molecules, and in this light it is clear that heterocyclic Meisenheimer complexes are appropriately considered in the current Review. By so doing, it is hoped that attention can be drawn to potentially mutual benefits that may be derived from comparative studies of neutral and anionic pseudobases. Certainly, the spectroscopic techniques applicable to the study of pseudobase and Meisenheimer complex formation are identical. Quantitative studies of substituent effects and structural effects on rates and equilibria for nucleophilic addition should be relevant both to neutral and to anionic e-complex formation. The general rules enunciated by Strauss23,318 and Fendler319 for the prediction of the relative stabilities of Meisenheimer complexes should be directly applicable to analogous pseudobases. Terrier et al.2n have made an important contribution in this area with a detailed comparison of kinetic and thermodynamic parameters for formation of a benzofuroxan Meisenheimer complex and an isoquinoline pseudobase. [Pg.68]

As cyanide ions operate as ambident nucleophiles, alkylation reactions may generate isonitriles as well as nitriles (equation 2). A whole range of parameters is responsible for the outcome of reactions of this type and their particular role together with special counter influences is not easily evaluated. There is a large and growing number of papers on this topic, but one can concentrate here on a few selected review articles.Suffice it to say that Komblum s seminal article s from 1955 is still of special importance in this field. Pearson s principle of soft and hard acids and bases (HSAB) proved to be particularly helpful in the interpretation of experimental results. ... [Pg.226]

Of course, the study of medium effects is one of the major concerns of chemistry, and an extensive review (35) of the subject would be inappropriate here. However, two recent contributions are particularly pertinent to the foregoing discussion of solvent nucleophilicity these contributions are the work of Swain et al. (36) and the work of Taft et al. (37). In these studies, the goal is to correlate all available medium-dependent phenomena, not just nucleophilic reactions, with a single LFER parameters that may be related to solvent nucleophilicities are produced. [Pg.21]

A review paper examines the nucleophilic properties of solvents. It is based on accumulated data derived from calorimetric measurements, equilibrium constants, Gibbs free energy, nuclear magnetic resonance, and vibrational and electronic spectra. Parameters characterizing Lewis-donor properties are critically evaluated and tabulated for a large number of solvents. The explanation of the physical meaning of polarity and discussion of solvatochromic dyes as the empirical indicators of solvent polarity are discussed (see more on this subject in Chapter 10). ... [Pg.705]

The 5[tertiary alkyl halides have been reviewed in the light of the three parameters, Y (the ionizing power of the solvent), N (the nucleophilicity of the solvent), and 1 (an aromatic ring or jr-bond parameter) of the Grunwald-Winstein equation. ... [Pg.325]

The nucleophilic reactivities towards cations of several nucleophiles has been reviewed . A parameter which is characteristic of the nucleophile system and independent of the cation has been defined as... [Pg.374]

The efficiency of such a DKR-based synthesis depends on the kinetic parameters of the parallel reaction [kg and kj) and racemization (kj ). This is determined not only by the structural parameters of the catalyst and substrate, but also by the conditions that facilitate or enforce in situ racemization of the chiral substrate. In this review, examples of DKR have been classified into five categories according to the method of racemization (i) tautomerization, (ii) internal motion (pyramidal inversion, and deformation and rotation), (iii) elimination-addition and addition-elimination, (iv) nucleophilic substitution and (v) others. Representative equilibrium for each type of racemization is shown in Figure 5.2. [Pg.163]

As in the previous volume, this chapter reviews kinetic and mechanistic studies of the stoichiometric reactions of coordinated hydrocarbons with nucleophiles and electrophiles, together with some related processes such as cycloadditions. Nucleophilic addition and substitution, in particular, continue to attract considerable interest, especially with dienyl- and arene-metal substrates. Even here, however, quantitative studies still lag far behind the extensive synthetic literature. However, fundamental reactivity patterns and parameters are beginning to emerge from the mechanistic studies which should assist in the design of rational syntheses. [Pg.301]


See other pages where Nucleophilicity parameters review is mentioned: [Pg.208]    [Pg.209]    [Pg.224]    [Pg.158]    [Pg.237]    [Pg.644]    [Pg.2527]    [Pg.1108]    [Pg.204]    [Pg.145]    [Pg.290]    [Pg.251]    [Pg.217]    [Pg.41]    [Pg.102]    [Pg.361]    [Pg.192]    [Pg.196]    [Pg.147]    [Pg.60]    [Pg.151]    [Pg.7]    [Pg.738]    [Pg.644]   
See also in sourсe #XX -- [ Pg.24 ]




SEARCH



Nucleophilic parameter

Nucleophilicity parameter

© 2024 chempedia.info