Nucleophilicity parameter reactions

E° is the standard potential of the reaction 2Y = Y, -I- 2e . The additive constants set Hu and n equal to zero for H2O. The quantities a and b are parameters measuring susceptibility to the corresponding nucleophilic parameters. Evidently /7n describes... 

On the basis of the nucleophilicity parameters B, NBs, and fi (see Table 8-2) one expects less of the homolytic product in water than in methanol. This is, however, not the case. It has been known for many decades that a very complex mixture of products is formed in the decomposition of diazonium ions, including polymeric products, the so-called diazo tars. In alcohols this is quite different. The number of products exceeds three or four only in exceptional cases, diazo tars are hardly formed. For dediazoniation in weakly alkaline aqueous solutions, there has, to the best of our knowledge, been only one detailed study (Besse et al., 1981) on the products of decomposition of 4-chlorobenzenediazonium fluoroborate in aqueous HCOf/ CO]- buffers at pH 9.00-10.30. Depending on reaction conditions, up to ten compounds of low molecular mass were identified besides the diazo tar. 

In Eq. (10-17), parameters a and b measure the sensitivity of the reaction to these nucleophilic parameters. Since H measures proton basicity and En the electron-donation ability, this treatment models nucleophilicity as a combination of electron loss and electron pair donation. The Edwards equation is thus an oxibase scale of nucleophilic reactivity. Table 10-5 summarizes the nucleophilic parameters. 

Nucleophilic reactivity toward Pt(II) complexes may be conveniently systematized via linear free energy relationships established between reactions of trans Ptpy2Cl2 (py = pyridine) with various nucleophiles and reactions of other Pt(II) complexes with the same nucleophiles. First, each nucleophile is characterized by a nucleophilicity parameter, derived from its reactivity toward the common substrate, trans Ptpy2Cl2. Reactivity toward other Pt(II) substrates is then quite satisfactorily represented by an equation of the form (21), wherein ky is the value of in the reaction with nucleophile Y... 

Correlation analysis of solvent effects on the heterolysis of p-methoxyneophyl tosyl-ate has been performed by using the Koppel-Palm and Kamlet-Taft equations. The reaction rate is satisfactorily described by the electrophilicity and polarity parameters of solvents, but a possible role for polarizability or nucleophilicity parameters was also examined. 

It is accepted that the acmal nucleophile in the reactions of oximes with OPs is the oximate anion, Pyr+-CH=N-0 , and the availability of the unshared electrons on the a-N neighboring atom enhances reactions that involve nucleophilic displacements at tetravalent OP compounds (known also as the a-effect). In view of the fact that the concentration of the oximate ion depends on the oxime s pATa and on the reaction pH, and since the pKs also reflects the affinity of the oximate ion for the electrophile, such as tetra valent OP, the theoretical relationship between the pATa and the nucleophilicity parameter was analyzed by Wilson and Froede . They proposed that for each type of OP, at a given pH, there is an optimum pK value of an oxime nucleophile that will provide a maximal reaction rate. The dissociation constants of potent reactivators, such as 38-43 (with pA a values of 7.0-8.5), are close to this optimum pK, and can be calculated, at pH = 7.4, from pKg = — log[l//3 — 1] -h 7.4, where is the OP electrophile susceptibility factor, known as the Brpnsted coefficient. If the above relationship holds also for the reactivation kinetics of the tetravalent OP-AChE conjugate (see equation 20), it would be important to estimate the magnitude of the effect of changes in oxime pX a on the rate of reactivation, and to address two questions (a) How do changes in the dissociation constants of oximes affect the rate of reactivation (b) What is the impact of the /3 value, that ranges from 0.1 to 0.9 for the various OPs, on the relationship between the pKg, and the rate of reactivation To this end, Table 3 summarizes some theoretical calculations for the pK. ... 

The diastereoselectivities in the nucleophilic addition reactions of l,3-dioxane-5-ones 37 and l,3-dithiane-5-ones 38 were studied by employing two newly available theoretical tools, the exterior frontier orbital electron (EFOE) density of the 7tc=o -orbitals and the 7t-plane-divided accessible space (PDAS) as quantitative measures of the 7t-facial steric effects <1999CRV1243, 1999CC621, 1999CL1161, 2000H(52)1435, 2001HAC358>. The two parameters predict correctly the experimentally observed stereochemical reversal of 37 and 38 (R = Ph see Table 1) in particular, the PDAS values for both substrates clearly show the opposite steric environment about the carbonyl carbon atom of these heterocyclic ketones and prove sizeable ground-state conformational differences to be responsible for the observed reversed facial stereoselection. 

The development of these various solvent parameters and scales has been accompanied by the realization that there are uncertainties in the physical property of the solvent that is correlated by a particular parameter in cases where systematic changes in solvent structure affect several solvent properties. Consider a reaction that shows no rate dependence on the basicity of hydroxylic solvents, and a second reaction that proceeds through a transition state in which there is a small transition state stabilization from a nucleophilic interaction with the hydroxyl group. The rate constants for the latter reaction will increase more sharply with changing solvent nucleophilicity than those for the former, and they should show a correlation with some solvent nucleophilicity parameter. This trend was observed in a comparison of the effects of solvent on the rate constants for solvolysis of 1-adamantyl and ferf-butyl halides, and is consistent with a greater stabilization of the transition state for reaction of the latter by interaction with nucleophilic solvents. ... 

The changes in the geometric parameters of the CISiC/jO coordination centre are typical of a bimolecular nucleophilic substitution reaction at a tetrahedral atom and consistent with the results obtained using the Burgi-Dunitz structure correlation method with the experimental results of Macharashvili and coworkers26. 

Far from confirming a dependence of nucleophilic selectivity on the reactivity of the carbocations, Ritchie observed that selectivities were unchanged over a 106-fold change in reactivity.15 He enshrined this result in an equation (29) analogous to that of Swain and Scott, but with the nucleophilic parameter n modified to N+ to indicate its reference (initially) to reactions of cations, and with the selectivity parameter s taken as 1.0, that is, with no dependence of the selectivity of the cation on its reactivity (as measured by the rate constant for the reference nucleophile, kn2o for water). 

Dinitrobenzofuroxan (DNBF) is known as a superelectrophile due to its high reactivity both as an electrophile and in its pericyclic addition reactions. NMR studies show that reaction with 2-aminothiazole and its 4-methyl derivative yield anionic carbon-bonded adducts such as (11) by reaction at the 5-position, whereas the 4,5-dimethyl derivative reacts via the exocyclic amino group. Kinetic studies of the first two compounds, both in acetonitrile and in 70 30 (v/v) water-DMSO, have been used to assess their carbon nucleophilicities and place them on the Mayr nucleophilicity scale.55 In a related study, the nucleophilic reactivity, in acetonitrile, of a series of indoles with both DNBF and with benzhydryl cations have been compared and used to determine nucleophilicity parameters for the indoles.56... 

The kinetics of reaction of four indoles with a number of benzhydryl cations has allowed the determination of the nucleophilicity parameters N and S for these reactions.49 Kinetic data from the reaction of a number of indoles with 4,6-dinitroben-zofuroxan then allow determination of their N values. Correlation of these data with those for protonation at C(3) of 5-substituted indoles and 5-substituted 2-methylindoles... 

Figure 1. Estimated reaction rates, Scaled, using nucleophilicity parameter N and elec-trophilicity parameter E.

The simplest linear free energy relation that has been used to describe the attack of a series of nucleophiles in an SN2 reaction is the Swain-Scott relation (Swain and Scott, 1953). In our notation the relation becomes (45), where the nucleophilicity parameter for X, x, is defined by (46) or its equivalent (47)... 

Finally, we would like to emphasize that, besides obtaining values of ccfrom the Marcus analysis, another great advantage is that one can separate kinetic and thermodynamic contributions to the parameters in linear free-energy relations. In this article we have done this for the Swain-Scott nucleophilicity parameter n, for m describing the change of reaction rate with solvent, and for the Hammett p-values. 

Equation (33) suggests that the failure to observe a reactivity-selectivity relationship for electrophile-nucleophile combination reactions is due to the counter influence of the solvent. The parameter 0 represents the inherent reactivity of the electrophile and is large for unreactive electrophiles, while for reactive electrophiles P is small. Now, it is the reactive electrophiles which are... 

Mayr and Patz have recently evaluated 56 reaction series, mostly for reactions as described in this article, and derived Eq. (23), in which carbo-cations are characterized by the electrophilicity parameter E, whereas nucleophiles are characterized by the nucleophilicity parameter N and the slope parameter s [182]. The latter quantity, s, which basically describes the slopes of plots as shown in Figs. 10 and 11, ranges from 0.8 to 1.2 for 91 % of the 7r-nucIeophiles investigated. The mathematical form of Eq. (23) implies that the exact value of s will usually only be of importance when rate constants, which strongly deviate from 1 (e.g., (log > 5), are considered. Some of the characterized nucleophiles and electrophiles are listed in Scheme 53, where the two scales are arranged in such a way that electrophiles and nucleophiles which are located at the same level are predicted to combine with rate constants of lg k = -5 s. With s 1 one expects slow combinations for electrophile-nucleophile pairs at the same level, whereas reactions of nucleophiles with electrophiles located below them are expected to be very slow or not to occur at all at 20° C. 

Table 5-16. Nucleophilic parameters N+ for various (nucleophile + solvent) systems, based on reactions of malachite green or the tris-4-anisylmethyl cation [597],...

Fig. 4 log kiK vs the Pearson nucleophilicity parameter n for reactions of N-methyl-N-nitrosoaniline and N-nitrosodiphenylamine with nucleophiles... 

The vertical ionization potential for a solvated chemical species can be the measure of its reactivity in the solution phase, especially for a single electron transfer reaction. It has been reported that the ionization potentials of anions in solution are conelated with the kinetic parameter for nucleophilic substitution reaction. This implies that an important aspect of the activation process of the reaction is a single electron transfer from anion to substrate. The ionization potential for solvated species has been available as the threshold energy E by photoeiectron emission spectroscopy for solution (PEES). This spectroscopic technique is able to provide the , values of almost any solvated species, such as organic, inorganic, cations, anions and neutral molecules in aqueous and nonaqueous solutions. 

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. ... 

Evaluation of 7i-nucleophilicity parameters of heterocyclic compounds in carbon-carbon bond-forming reactions 03ACR66. 

The results indicate that in this case the parameter is a good model and the dissociation of the conjugate acid is not a predominant factor in the nucleophilic substitution reaction. A correlation between logA and... 

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