Benzhydrylium Ions (benzhydrylium + ion)

Distribution by Scientific Domains

Selected Abstracts

Nucleophilicities and Nucleofugalities of Organic Carbonates,

Nicolas Streidl
Abstract The kinetics of the reactions of the methyl carbonate ion with benzhydrylium ions in acetonitrile have been studied by UV/Vis spectrophotometry. Substitution of the resulting second-order rate constants and the electrophilicity parameters E of the benzhydrylium ions into the linear free energy relationship log,k = s(N + E) yielded the nucleophilicity parameters N25 = 16.03 and s25 = 0.64 for methyl carbonate in acetonitrile. The kinetics of the reverse reactions, i.e., of the solvolyses of ring-substituted benzhydryl alkyl carbonates in different aqueous solvents were followed by conductimetry. The obtained first-order rate constants and the known electrofugality parameters Ef of benzhydrylium ions were used to determine the nucleofugality parameters Nf and sf of the ROCO2, groups by using the linear free energy relationship log,k = sf(Nf + Ef). The leaving group abilities of carbonates decrease by a factor of about 300 from PhOCO2, over MeOCO2, and iBuOCO2, to tBuOCO2, in various alcoholic and aqueous solvents. tert -Butyl carbonates (tBocO-R) are, thus, considerably more stable with respect to heterolytic cleavage of the O,R bond than other organic carbonates. [source]

Nucleophilic Reactivities of Azulene and Fulvenes,

Mariusz K, dziorek
Abstract The kinetics of the reactions of azulene (1), 6,6-dimethylfulvene (2), 6-[4-(dimethylamino)phenyl]fulvene (3) and 6-(julolidin-9-yl)fulvene (4) with a set of benzhydrylium ions (reference electrophiles) have been investigated in MeCN. The second-order rate constants for these reactions correlate linearly with the electrophilicity parameters (E) of the benzhydrylium ions. According to the linear free-enthalpy relationship log,k2(20 °C) = s(N + E), the nucleophilicity parameters N and s of the ,-nucleophiles 1,4 were determined and compared with those of other types of nucleophiles. Azulene (1, N = 6.66) is about 10 times more nucleophilic than N -methylpyrrole and comparable to 2-methylindole. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Nucleophilic Reactivities of Pyrroles,

Tobias A. Nigst
Abstract The second-order rate constants of the reactions of alkyl-substituted pyrroles with a series of benzhydrylium ions were determined in acetonitrile, and the reaction products were fully characterized by NMR spectroscopy and mass spectrometry. The formation of the , adducts is the rate-limiting step of these reactions. Because the second-order rate constants correlate linearly with the electrophilicity parameters of the benzhydrylium ions, the determination of the nucleophilicity parameters N and s according to the linear free energy relationship log k2 (20 °C) = s(N + E) was achieved. With these findings, a direct comparison of the nucleophilic reactivities of these ,-excessive heterocycles with other nucleophiles became possible, and the pyrroles were integrated into the comprehensive scale of nucleophilicity, covering a range of 8,9 orders of magnitude from N -(triisopropylsilyl)pyrrole (N = 3.12), the weakest nucleophile of this series, to kryptopyrrole (3-ethyl-2,4-dimethylpyrrole, N = 11.63). Thus, highly reactive pyrroles show similar nucleophilic reactivities as enamines, whereas those of less-reactive pyrroles are comparable to allylsilanes or indoles. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

Nucleophilicities and Carbon Basicities of Pyridines

Frank Brotzel Dipl.-Chem.
Abstract Rate and equilibrium constants for the reactions of pyridines with donor-substituted benzhydrylium ions have been determined spectrophotometrically. The correlation equation log,k(20,°C)=s(N+E), in which s and N are nucleophile-specific parameters and E is an electrophile-specific parameter, has been used to determine the nucleophilicity parameters of various pyridines in CH2Cl2 and aqueous solution and to compare them with N of other nucleophiles. It is found that the nucleophilic organocatalyst 4-(dimethylamino)pyridine (DMAP) and tertiary phosphanes have comparable nucleophilicities and carbon basicities despite widely differing Brønsted basicities. For that reason, these reactivity parameters are suggested as guidelines for the development of novel organocatalysts. The Marcus equation is employed for the determination of the intrinsic barriers of these reactions. Geschwindigkeits- und Gleichgewichtskonstanten der Reaktionen von Pyridinen mit Donor-substituierten Benzhydrylium-Ionen wurden spektralphotometrisch bestimmt. Die Korrelationsgleichung log,k(20,°C)=s(N+E), die mit s und N Nucleophil-spezifische und mit E einen Elektrophil-spezifischen Parameter enthält, wurde verwendet, um die Nucleophilie-Parameter von verschiedenen Pyridinen in CH2Cl2 und in wässrigen Lösungen zu bestimmen und um diese mit N anderer Nucleophile zu vergleichen. Es wurde gefunden, dass der nucleophile Organokatalysator 4-(Dimethylamino)pyridin (DMAP) und tertiäre Phosphane vergleichbare Nucleophilien und Kohlenstoffbasizitäten haben, obwohl ihre Brønsted-Basizitäten sehr unterschiedlich sind. Daher wird vorgeschlagen, die Reaktivitätsparameter als Richtgrößen zur Entwicklung neuer Organokatalysatoren heranzuziehen. Die Marcus-Gleichung wird eingesetzt, um die intrinsischen Barrieren dieser Reaktionen zu bestimmen. [source]