Nucleophilic reactivity parameters

TABLE 12.12 Rate Constants and Nucleophilic Reactivity Parameters for Entering Groups... 

A more unusual fact observed in thiazole chemistiy is that also the other positions (4 and 5) are activated toward the nucleophilic substitution, as found independently by Metzger and coworkers (46) and by Todesco and coworkers (30, 47). Some kinetic data are reported in Table V-2. As the data in Table V-2 indicate, no simple relationship between nucleophilic reactivity and charge density, or other parameters available from more or less sophisticated calculation methods, can be applied. As a... 

The parameter 5 is a measure of the susceptibility of the substrate to nucleophilic attack, and n a measure of the nucleophilic reactivity as defined by a reference reaction. The rate constants for attack at saturated carbon are used to define the values of n.14 Table 10-4 lists the values of n for certain nucleophiles. This particular compilation lists the... 

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... 

Calculation of the appropriate local reactivity parameters (values for nucleophilic attack for the electrophilic reactant and vice versa)... 

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 nucleophilicity parameters for carbanions of nitronates and malonic acid derivatives have been investigated.143 The nucleophilic reactivities do not correlate with the acidity constants of the conjugate CH acids, and from the poor correlation of the reactivities of the substituted a-nitrobenzyl anions with Hammett s ex-constants it can be inferred that the nucleophilic reactivities are strongly controlled by solvation. 

For the reactivity parameters Y, n, a+ (but not a) andN+ the lack of curvature is not unexpected. This is because these parameters are defined with respect to the rate of some standard reaction (solvolysis of t-butyl chloride, substitution of methyl iodide, solvolysis of cumyl chlorides, combination reaction of nucleophiles with a standard electrophile). Therefore the resultant plot is of the type log k vs. log k, while the curvature shown in a typical Br nsted plot (Figure 5) results from a plot of log k vs. log K. This curvature is due to a gradual change from a reactant-like transition state, which is insensitive to a perturbation in the reactivity parameter, to a product-like transition state in which equilibrium perturbations are largely reflected in the transition state (and hence the rate). A log k — log k plot is not expected to show this effect and hence is not expected to show curvature. 

Another key feature of the metal complex cycle in which the iodide acts most effectively is the nature of the active catalyst itself. The oxidative addition step is considered to be nucleophilic in nature, based on activation parameters and relative rate data (23, 24a) (Section II,C), and the presence of a negative charge on the metal center appears to significantly enhance the nucleophilicity (and hence reactivity toward methyl iodide) of the metal relative to neutral rhodium(I) species (20). Extrapolations of available data (24-26) indicate that, at 25°C, the diiododicarbonylrhodium(I) species has a Pearson nucleophilicity parameter (25) toward methyl iodide of 5.5. In relation to other common nucleophiles, this value corresponds to nucleophilic reactivity toward methyl iodide comparable to that of pyridine (n = 5.2), an order of magnitude greater than chloride (n = 4.4), and two orders of magnitude slower than iodide (n = 7.4). 

Nucleophilic reactivity is in the same order DMF < DMSO < HMPT. In both cases, the activation parameters are characterized by highly negative activation entropy factors (AS = -40 to -65 e.u.). Only the enthalpy factors are markedly different for Si and P (AH — 0 and AH 7 kcal/mol). As shown for silicon compounds, identical rate laws (eqs. [101] and [102]) and energy data are con-... 

This paper focuses on the relationship between reactivity of a nucleophile with an electrophile and the distance separating the two species. The relationship, examined by means of rigid molecular frameworks bearing two functionalities at well-defined distances and angles, is shown to be extremely sensitive. Theoretical considerations also support the contention that distance is a key parameter in nucleophilic reactivity. Reactions in solution occur with enzyme-like rates when critical distances are achieved. 

Rates of reaction of transition metal nucleophiles correlate both with oxidation potentials for MLn and with the pKa values of the corresponding acids, HMLn. Therefore, the two parameters, E° and H, in the Edwards equation are not independent parameters. The same result is found for other nucleophiles, if the donor atom is C,N, O, or F. However, for bases with heavier donor atoms, E° and H are not as correlated with each other. For transition metal complexes, soft ligands, L, increase acidity and decrease nucleophilic reactivity. 

As explained earlier, Figures 1 and 2 will not be greatly changed if data in solution are used. Individual bases will move up or down, parallel to the lines shown. Therefore, under the normal conditions for nucleophilic substitution, ease of oxidation (E0 ) and Brpnsted basicity are not independent parameters for bases where the donor atom is a second-row element. Either parameter may be used as a measure of nucleophilic reactivity. 

The process represented by V — IV occurs, however, in the solvolysis of a preionized material, such as triethyloxonium fluorophosphate, which was employed by Kevill and Lin (21) to establish a scale of nucleophilicity parameters, N. Because the effect of variable anion stabilization by solvent was not subtracted, whether their N values measure true and only nucleophilic reactivities is uncertain. This doubt would be dispelled, however, by an experiment in which the reagent would be treated with small amounts of nucleophile in a better anion-stabilizing solvent, such as TFA, or even sulfuric acid. 

The concept of philicity (m (r)) introduced by Chattaraj et al. [310,311] signifies the aptitude towards reactivity (electrophihcity or nucleophilicity) of a local atomic site in a molecule. In other words, an increase (or decrease) in the reactivity of any local site of a molecule does not affect the electrophihcity (cn) of the entire species as a whole—the global reactivity parameter, co remains conserved. The transformation of the local electrophihcity ([Pg.158]

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... 

Complexes 2, 4-6, 8-10, and 13 promoted the t ROP of l-LA (Scheme 28.6, Table 28.3). Under the chosen conditions, the activity of complexes 4-6 supported by the LO amino-ether phenolate ligand decreased in the order Ca < Sr < Ba. This was linked to the increase of the nucleophilic reactivity of the L X Ae-OPLLA(Nu) species on descending from the least (Ca) to the most (Ba) electropositive metal. The catalysts based on 5 and 6 afforded moderate control over the parameters = 1.36-1.46) for the t ROP of 1000 equiv of l-LA, with turnover frequency (TOP) values of 2080 and... 

These benzhydrilium carbocations were also used to develop a comprehensive scale of hydride donor strength. Both Si-H and C-H donors were evaluated by their second-order rate constants of hydride transfers to the carbocation electrophiles. The respective nucleophilicity parameter, N and %, were then determined. The benzhydryl cation series was also used to evaluate the nucleophilic reactivities of azolium enolates (1). ... 

Kinetics of the reactions of pyridinium ylides (210) and their isoquinolinium and quinolinium congeners with arylidene malonates (211) and related electrophiles, such as diarylcarbenium ions and quinone methides, have been studied in DMSO by UV-vis spectroscopy. The second-order rate constants thus obtained were used to derive the nucleophile-specific parameters Nand % for these ylides. Pyridinium substitution turned out to have a similar effect as that of alkoxycarbonyl substituents on the reactivity of carbanionic reaction centres. Agreement between the experimental rate constants and those calculated from E, N, and % shows that this correlation can also be employed for predicting the absolute rate constants of step-wise or highly unsymmetrical concerted cycloadditions. On the other hand, deviation by a factor of 10 would indicate a change of reaction mechanism. ... 

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