Hammett a parameter

A few examples have been reported in which no steric parameter is involved in the correlation analysis of cyclodextrin catalysis. Straub and Bender 108) showed that the maximal catalytic rate constant, k2, for the (5-cyclodextrin-catalyzed decarboxylation of substituted phenylcyanoacetic acid anions (J) is correlated simply by the Hammett a parameter. 

Lindberg et al.17 have shown that the carbon Is shifts of some substituted benzenes are linearly correlated with the Hammett a parameters of the substituents. However, Hammett parameters are of rather limited applicability a given set of a values can be used to correlate data only for similar chemical systems. It has recently been shown18 that a wider variety of core binding energy shifts can be correlated by the four-parameter relation... 

Rates of oxidation of para-substituted arylphosphines with singlet oxygen show good correlation with the Hammett a parameter (p = —1.53) and with the Tolman electronic parameter. The only products are the corresponding phosphine oxides. However, for ortho-substituted phosphines with electron-donating substituents, there are two products, namely a phosphinate formed by intramolecular insertion and phosphine oxide. Kinetic analyses demonstrated that both products are formed from the same intermediate, a phosphadioxirane. VT NMR experiments showed that perox-idic intermediates can only be detected for highly hindered and very electron-rich arylphosphines 243... 

The cathodic process (reduction) has been studied in different experimental conditions being the potential of the one-electron reaction correlated with other structural parameters, i.e., with the reduced-product lowest vacant orbital energy [86], with the substituent Hammett a parameter [87], or with the biological activity (see below). Table 4 shows reduction potentials for some QDO and PDO derivatives in different experimental conditions [88,89]. 

Evaluation of steric effects can also be made by separating electronic from steric effects with the help of linear free energy relationship and appropriate parameters. Applications of the Hammett equation to heterocycles have been reviewed (64AHC(3)209 76AHC(20)1) and the influence of substituent effects on the basicity and N-alkylation of pyridines, which have been by far the most widely studied, shows the difficulties in this approach. Jaffe and Jones (64AHC(3)209) reported a good correlation between pKA of 3- and 4-substituted pyridines and Hammett a parameters ([Pg.179]

These results are used to calculate the following Hammett a parameters of the 2-, 4-, and 5-thiazolyl groups (taking a value of -5.12 for p for the reaction of solvolysis) 5-thiazolyl =—0.18, 4-thiazolyl =-0.01, 2-thiazolyl =-1-0.26. 

Scheme 33. Kinetic studies of the cation radical Diels-Alder reactions of aryl vinyl sulfides with cyclopentadiene and of aryl propenyl ethers with 2,3-dimethyl 1,3-butadiene. In both solvents, both extended reaction series have rates which correlate excellently with the Hammett a parameters and poorly with the Hammett-Brown o-+ values. The preference is statistically significant at or above the 95 % confidence level.

The values obey the Jaffe equation employing Hammett a parameters for the 4-nitro substituents (Figure 7) ... 

In 1962, Hansen [14] derived a first Hammett-type relationship between the toxicities of substituted benzoic acids and the electronic a constants of their substituents. However, later, it turned out that this was a chance correlation that only resulted from a close interrelationship between the Hammett a parameter and the lipophilicity constant % (Sec. 4 Eqs. (42) and (43)). In the same year, for the very first time, a nonlinear multiparameter equation (Eq. 6) [15] was used to describe biological activity values ... 

Many of the neutral complexes [FeX(CO)2 L Cp] are reversibly oxidized at potentials sufficiently negative to allow the chemical isolation of radical cations (325, 342-346). Representative examples are given in Table VI. The E° values for [FeBr(CNR)2Cp] (R = aryl), which correlate linearly with the Hammett a parameters for R, are relatively low. However, chemical or electrolytic oxidation yields only [Fe(CNR)3Cp] +, which are, in turn, oxidized at a potential 0.9 V more positive than the neutral bromides (342). The analog [Fe(PMe3)3Cp]+ (E° = 0.71 V in CH2C12) and [NO][PF6] afford the isolable dication (344). 

Molar volume itself is not strictly additive, but the corrected volume parameters MR and PA are additive constitutive molecular properties, like log P and the Hammett a parameter. While molar refractivity has attracted much attention [50, 286 — 288], molar volume [55, 289] and parachor [50, 290] have only rarely been used in QSAR studies. 

Finally, we mention the correlation between the Hammett a-parameters and the recently introduced activation hardnesses [46], both also being well suited for describing site preferences in electrophilic aromatic substitution. A fundamental study of the link between such longstanding concepts and the present ones is highly recommended. 

The rates summarized in Tables 3.51 and 3.52 were correlated with substituent effects using Hammett a parameters [225]. Hammett plots for the reactions of Zn with aryl bromides (THE, 70°C), aryl iodides (THE, 0°C), Mg with aryl bromides (Et20, THE, 0°C), and aryl iodides (Et20, 0°C) [226] are represented in Figure 3.3. The wide range of reactivity exhibited and o values covered, as well as the good fitting of the lines for the Zn reaction, permits an estimation... 

---