Electrophilic Substituent Constant σ⁺ of Electron Donor Substituents in Nonpolar Media

Abstract

The electrophilic substituent constant sigma(+) was derived from the relative rate constants of the solvolysis of substituted alpha-cumyl chlorides in an acetone/water solvent mixture in the original work by Brown and Okamoto (J. Am. Chem. Soc. 1958, 80, 4979). As an extension of this procedure, we were looking for methods to determine sigma(+) values in nonpolar media. We have calculated the exocyclic charges q(R) for alpha-cumyl cations (R-C(6)H(4)CMe(2)(+)) as a measure of the electron-donating capacity of meta and para substituents. The UV-vis absorption nu(max) of the analogously substituted nitrobenzene derivatives has been used for the experimental measurement of the electron donating capacity of substituents R in nonpolar media. A linear relationship between nu(max) and Deltaq(R) (= q(R-C(6)H(4)CMe(2)(+)) - q(C(6)H(5)CMe(2)(+))) was obtained. A description of the electron-donating capacity in nonpolar media can be achieved both for substituents investigated already and for those for which the experimental determination of sigma(+) has previously been difficult. Special interest was directed to boron-based substituents in which the electron-donating ability is dependent on the coordinational environment of the boron atom.

Role of substituents on the reactivity and product selectivity in reactions of naphthalene derivatives catalyzed by the orphan thermostable cytochrome P450, CYP175A1

“…The rates of hydroxylation of the substituted naphthalene substrates by H 2 O 2 catalyzed by the enzyme were analyzed based on the electron releasing property of the substituents [41] given by the Hammett-Brown substituent constant (r + ). The plot of the logarithm of k cat against the Hammett-Brown substituent constant (r + ) [41][42][43][44] is shown in Fig. 5.…”

“…5), though the magnitude of r + [42] for the AOH substituent (À0.92) is larger than that of the later (À0.78). The Hammett-Brown electrophilic substituent constant (r + ) of substituents are prone to solvation effects and pH dependencies particularly in case of hydrogen bonding substituents [43]. The electron-donating strength of AOH substituent have hence been assumed to be the same r + (= À0.78) for the AOH and the AOMe in some reports [43].…”

“…The Hammett-Brown electrophilic substituent constant (r + ) of substituents are prone to solvation effects and pH dependencies particularly in case of hydrogen bonding substituents [43]. The electron-donating strength of AOH substituent have hence been assumed to be the same r + (= À0.78) for the AOH and the AOMe in some reports [43]. In view of this, the lower rate constant for the AOH substituted naphthalene compared to that for the AOMe substituted substrate may be ascribed to the partial ionization or hydrogen bonding effects of AOH in the solution.…”

“…A single peak Here the k cat is based on the peroxygenase activity of the enzyme. The behavior of AOH group in determining the r + value is known to be less certain and controversial because of partial ionization of AOH, which modifies its electron donating ability [42][43][44].…”

Benzhydrylium and tritylium ions: complementary probes for examining ambident nucleophiles

“…The short lifetimes of the benzylium ions in these solvents hamper the kinetic investigation of a large set of nucleophiles of widely differing reactivities [1][2][3]. Furthermore, at present little is known about the reactivity of benzylium ions towards nucleophiles in organic solvents [1][2][3][4][5][6].…”