Nucleophilicity Swain-Scott parameters

Solvolysis of both a- and p-2-deoxyglucopyranosyl-isoquinolinium salts allowed generation of oxocarbenium ions with a poorly nucleophilic leaving group (Scheme 7). It was observed that solvolysis of the p-anomer in the presence of the series of nucleophilic monoanions AcO-, Cl-, Br-, and N+ gave a Swain-Scott parameter of 0.03 + 0.05. Similarly, solvolysis of the a-compound in the presence of the same series of salts afforded a Swain-Scott parameter of 0.03 + 0.10. The results indicate little or no sensitivity to anion nucleophilicity and therefore point to a mechanism that does not involve an AnDn transition state.27,28... 

Second-order rate constants for the reactions of phenacyl bromide with a number of anionic or neutral nucleophiles in 3 2 (v/v) acetone-water have been measured at several temperatures.141 Correlation analysis with the Bronsted equation or Swain-Scott equation is not satisfactory. Better results were obtained with the two-parameter Edwards equation. 

Correlation of nucleophilic rate data for phenyldimethylsulfonium ions with common nucleophiles, with pX e values shows that the slopes of the lines, jS[ e, correlate qualitatively with the Edwards hardness parameter for the nucleophile and not with the Swain-Scott n parameter.144 cw,cw-2,4,6-Trimethyl-l,3,5-triaminocyclohexane is weakly basic in aqueous solution, because of steric inhibition to solvation of the conjugate acid.145 The three NH2 groups are axial and the steric effect also results in reduced reactivity as a nucleophile in, S n2 reactions. Highly stereoselective syntheses of N-. and O-glycosides have been carried out by addition of anionic nucleophiles to glycosyl iodides.146 5 n2 reactions are involved, but some substrates are susceptible to E2 elimination when treated with highly basic anions. 

In principle, these measurements represent an application of the Swain-Scott relationship to two nucleophiles only. This is apparent from Equation (28), in which nAz corresponds to n for the azide ion and the electrophilic parameter s is seen to measure the selectivity of the carbocation between azide... 

Mayr initially defined a set of electrophilic parameters for the benzhydryl cations using a reference nucleophile, which was chosen as 2-methyl-1-pentene.268,269 Values of E were then defined as log k/k0, where k0 refers to a reference electrophile (E= 0), which was taken as the 4,4 -dimethoxybenzhydryl cation. Plots of log k against E for other alkenes are thus analogous to the plots of logk against p fR in Fig. 7 except that the correlation is referenced to kinetic rather than equilibrium measurements. However, they differ from plots based on the Swain-Scott or Ritchie relationships in which log k is normally plotted against a nucleophilic parameter, that is, n or N+, rather than E. 

Note again that 5 here differs from s in the Swain-Scott equation which refers to the slope of a plot of log k versus the nucleophilicity parameter n (or N) rather than electrophilicity 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 (31) has an electrophilic ( ) and a nucleophilic (N component and the value, s, is a nucleophile specific parameter. This equation has a close family relationship with the Ritchie and Swain-Scott equations (Chapter 2) and the Edwards equation (29). The equation successfully correlates rate constants for a wide range of disparate structures and... 

Table 5 Swain-Scott, Pearson and Edwards parameters for nucleophilic... 

A major contribution of Ritchie s has been his observation that a large number of nucleophiles show a constant selectivity toward a variety of electrophiles. In LFER terms, the reactivity of the nucleophile can be given by a single parameter with no selectivity coefficient (such as Brpnsted (5 or Swain-Scott s) (26b) ... 

The two major differences between these two equations are as follows (1) Unlike the Swain-Scott equation, the Ritchie equation lacks a selectivity parameter. Thus, a plot of log k versus N+ always gives a unity slope. (2) Nucleophilicity ranking by the two scales is different. This difference is especially noticeable for the three nucleophiles, CN-, CH30-, and N3-. According to the N+ scale, the nucleophilicity order is N3- > CH30- > CN-, whereas the reverse is true for the Swain-Scott n scale. 

Representative s values are collected in Table 24 and n values in Table 25. The order of nucleophhcity (n) in the Swain-Scott approach bears no relationship to the pK of the conjugate acid of the nucleophile. Since in a given constant series of nucleophiles basicity and nucleophiUcity are related there have been attempts to correlate nucleophiUcity with one or more parameters. If the relative importance of these parameters is the same as in the methyl bromide reaction then the Swain-Scott equation will hold. 

To create an LFER for nucleophilicity, Swain and Scott defined the reference reaction to be the Sn2 nucleophilic displacement of bromide from methyl bromide by the reference nucleophile water. A more widely accepted scale based upon methyl iodide is now used (Eq. 8.43). Eq. 8.44 shows the LFER, with rix as the parameter for nucleophile X, and s is the sensitivity parameter for any new reaction under study. Additionally, fcnew, nucx is the rate constant for the new reaction under study with each nucleophile X, while new, Hpis the rate constant for the same reaction with water as the nucleophile. Table 8.5 gives a series of x values for various nucleophiles (based upon CH3I), and the pX s of the conjugate acids. Note, as discussed above, that the pK s do not necessarily correlate with the nucleophilicities, and that the effects of resonance on nucleophilicity are well accounted for with the n values (compare the n values for acetate, phenoxide, and methoxide). 

A set of A+ nucleophilicity parameters were obtained for a wide array of nucleophiles, using the second-order reaction of bis-dimethylamino benzhydrylium cation with the nucleophiles. This approach also included a Swain-Scott response/selectivity parameter in the hnear Ifee energy equation. ... 

In contrast to organic chemistry, where extensive studies have attempted to correlate nucleophilicity with various parameters (e.g., the Swain-Scott, Edwards, and Ritchie equations), there is currently little quantitative information concerning nucleophilic reactivities toward coordinated ir-hydrocarbons. Recent kinetic studies of reactions shown in equations (10) and (11) have established... 

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). 

A single-parameter LFER for correlating nucleophilicity was presented by Swain and Scott in 1953 (17) ... 

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