Scales of Lewis basicities toward C-centered Lewis acids (carbocations).

Equilibria for the reactions of benzhydryl cations (Ar2CH(+)) with phosphines, tert-amines, pyridines, and related Lewis bases were determined photometrically in CH2Cl2 and CH3CN solution at 20 °C. The measured equilibrium constants can be expressed by the sum of two parameters, defined as the Lewis Acidity (LA) of the benzhydrylium ions and the Lewis basicity (LB) of the phosphines, pyridines, etc. Least-squares minimization of log K = LA + LB with the definition LA = 0 for (4-MeOC6H4)2CH(+) gave a Lewis acidity scale for 18 benzhydrylium ions covering 18 orders of magnitude in CH2Cl2 as well as Lewis basicities (with respect to C-centered Lewis acids) for 56 bases. The Lewis acidities correlated linearly with the quantum chemically calculated (B3LYP/6-311++G(3df,2pd)//B3LYP/6-31G(d,p) level) methyl anion affinities of the corresponding benzhydrylium ions, which can be used as reference compounds for characterizing a wide variety of Lewis bases. The equilibrium measurements were complemented by isothermal titration calorimetry studies. Rates of SN1 solvolyses of benzhydryl chlorides, bromides, and tosylates derived from E(13-33)(+), i.e., from highly reactive carbocations, correlate excellently with the corresponding Lewis acidities and the quantum chemically calculated methyl anion affinities. This correlation does not hold for solvolyses of derivatives of the better stabilized amino-substituted benzhydrylium ions E(1-12)(+). In contrast, the correlation between electrophilic reactivities and Lewis acidities (or methyl anion affinities) is linear for all donor-substituted benzhydrylium ions E(1-21)(+), while the acceptor-substituted benzhydrylium ions E(26-33)(+) react more slowly than expected from their thermodynamic stabilities. The boundaries of linear rate-equilibrium relationships were thus defined.

Annelated pyridines as highly nucleophilic and Lewis basic catalysts for acylation reactions.

New heterocyclic derivatives of 9-azajulolidine have been synthesized and characterized with respect to their nucleophilicity and Lewis basicity. The Lewis basicity of these bases as quantified through their theoretically calculated methyl-cation affinities correlate well with the experimentally measured reaction rates for addition to benzhydryl cations. All newly synthesized pyridines show exceptional catalytic activities in benchmark acylation reactions, which correlate only poorly with Lewis basicity or nucleophilicity parameters. A combination of Lewis basicity with charge and geometric parameters in the framework of a three-component quantitative structure-activity relationship (QSAR) model is, however, highly predictive.

Lewis acid-triggered selective zincation of chromones, quinolones, and thiochromones: application to the preparation of natural flavones and isoflavones.

A Lewis acid-triggered zincation allows the regioselective metalation of various chromones and quinolones. In the absence of MgCl(2), a C(3) zincation is observed, whereas in the presence of MgCl(2) or a related Lewis acid, C(2) zincation occurs. Applications to a natural flavone, isoflavone, and quinolone are shown.

Isothiourea-mediated asymmetric O- to C-carboxyl transfer of oxazolyl carbonates: structure-selectivity profiles and mechanistic studies.

The structural motif within a series of tetrahydropyrimidine-based isothioureas necessary for generating high asymmetric induction in the asymmetric Steglich rearrangement of oxazolyl carbonates is fully explored, with crossover and dynamic (19)F NMR experiments used to develop a mechanistic understanding of this transformation.

Nucleophilic reactivities of hydrazines and amines: the futile search for the α-effect in hydrazine reactivities.

The kinetics of the reactions of amines, hydrazines, hydrazides, and hydroxylamines with benzhydrylium ions and quinone methides were studied in acetonitrile and water by UV-vis spectroscopy, using conventional spectrometers and stopped-flow and laser-flash techniques. From the second-order rate constants k(2) of these reactions, the nucleophilicity parameters N and s(N) were determined according to the linear free energy relationship log k(2) = s(N)(N + E). While methyl groups increase the reactivities of the α-position of hydrazines, they decrease the reactivities of the ß-position. Despite the 10(2) times lower reactivities of amines and hydrazines in water than in acetonitrile, the relative reactivities of differently substituted amines and hydrazines are almost identical in the two solvents. In both solvents hydrazine has a reactivity similar to that of methylamine. This observation implies that replacement of one hydrogen in ammonia by Me increases the nucleophilicity more than introduction of an amino group, if one takes into account that hydrazine has two reactive centers. Plots of log k(2) versus the corresponding equilibrium constants (log K) or Brønsted basicities (pK(aH)) do not show enhanced nucleophilicities (α-effect) for either hydrazines or hydroxylamine relative to alkylamines.

Photogeneration of benzhydryl cations by near-UV laser flash photolysis of pyridinium salts.

Laser flash irradiation of substituted N-benzhydryl pyridinium salts yields benzhydryl cations (diarylcarbenium ions) and/or benzhydryl radicals (diarylmethyl radicals). The use of 3,4,5-triamino-substituted pyridines as photoleaving groups allowed us to employ the third harmonic of a Nd/YAG laser (355 nm) for the photogeneration of benzhydryl cations. In this way, benzhydryl cations can also be photogenerated in the presence of aromatic compounds and in solvents which are opaque at the wavelength of the quadrupled Nd/YAG laser (266 nm). To demonstrate the scope and limitations of this method, the rate constants for the bimolecular reactions of benzhydryl cations with several substituted pyridines were determined in acetonitrile and with water in acetone. The obtained data agree with results obtained by stopped-flow UV-vis spectroscopic measurements. The rate constants for the reaction of the 4,4'-bis[methyl(2,2,2-trifluoroethyl)amino]benzhydrylium ion with 4-(dimethylamino)pyridine were also determined in dimethyl sulfoxide, N,N-dimethylformamide, and acetone. From the second-order rate constants, we derived the nucleophilicity parameters N and s(N) for the substituted pyridines, as defined by the linear free energy relationship, log k(2) = s(N)(N + E).

Nucleophilicities and Lewis basicities of isothiourea derivatives.

Rate and equilibrium constants for the reactions of a series of isothioureas with benzhydrylium ions have been measured photometrically. The data were employed to determine the nucleophilicities and nucleofugalities of isothioureas and compare them with those of other organocatalysts.