The mechanism of acid-base catalysis

New concepts of the mechanism of acid-base catalysis are developed, incorporating electron transfer in low-lying electronic excited states of the reagent-catalysts complex. In base catalysis, the base-catalyst lowers the ionization potential of one of the reagents, the proton and electron dnor, and thereby lowers the energy of the excited level. Acid catalysis consists in intensifying the electron-accepting properties of the reagent acting as an electron acceptor.Published to initiate discussion.Institute of High-Molecular-Weight Compounds, Russian Academy of Sciences, Leningrad. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 6, pp. 710–715, November–December, 1991. Original article submitted July 13, 1987.     

Electron and proton transfer in acid-base catalysis

The role of electron and proton transfer in acid-base catalysis is discussed, with two reactions as examples, in one of which (polymerization of cyclobutenes) an acid, and in another (nitramide decomposition), a base acts as the catalyst.     

Protonation of reagents and acid-base catalysis in acylation

The kinetic characteristics of the model reaction of electron transfer and the reaction of acylation of aromatic amines by aromatic acid anhydrides were investigated as a function of the concentration of acid catalyst and a correlation was established between the type of this function and the characteristics of protonation of the amines. The rate constants of the catalytic and noncatalytic flows of the forward and reverse reactions in the phthalic anhydride—p-toluidine system were determined as a function of the molarity and proton-acceptor properties of the solvent. The mechanism of acid-base catalysis was examined as a sequence of proton and electron transfer processes.Institute of Macromolecular Compounds, Russian Academy of Sciences, 199004 St. Petersburg. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 5, pp. 1048–1056, May, 1992.     

Surface Lewis acid-base properties of polymers measured by inverse gas chromatography

Surface Lewis acid-base properties are significant for polymers materials. The acid constant, K(a) and base constant, K(b) of many polymers were characterized by some researchers with inverse gas chromatography (IGC) in recent years. In this paper, the surface acid-base constants, K(a) and K(b) of 20 kinds of polymers measured by IGC in recent years are summarized and discussed, including seven polymers characterized in this work. After plotting K(b) versus K(a), it is found that the polymers can be encircled by a triangle. They scatter in two regions of the triangle. Four polymers exist in region I. K(b)/K(a) of the polymers in region I are 1.4-2.1. The other polymers exist in region II. Most of the polymers are relative basic materials.     

Functionalization of polyoxometalates: towards advanced applications in catalysis and materials science

Functionalization via covalent grafting of organic functions allows to tune the redox and acid-base properties, and the solubility of polyoxometalates, to enhance their stability and biological activity and to reduce their toxicity, to facilitate their implementation in extended structures and functional devices. We discuss herein the electronic and binding connections, and the various synthesis methodologies. We emphasize on organonitrogen, organosilyl and organophosphonyl derivatives with special attention to synthesis, characterization and potential applications in catalysis and materials science. We also consider the giant molybdenum oxide-based clusters especially the porous capsule-type clusters (Keplerates) which have high relevance to this context.     

Lewis base catalysis by thiourea: N-bromosuccinimide-mediated oxidation of alcohols

In recent times, (thio)urea derivatives have become synonymous with hydrogen bonding owing to their extensive applicability as small molecule organocatalysts. In this paper, another activation mode by thiourea derivatives, namely via Lewis base catalysis, is disclosed for the NBS-mediated oxidation of alcohols. The mild reaction conditions employed here is suitable for chemoselective oxidation of secondary alcohol in the presence of primary alcohol.     

Regioselective alkenylation of imidazoles by nickel/Lewis acid catalysis

Nickel/Lewis acid binary catalysis is found effective to direct regioselective alkenylation of imidazoles through C-H bond activation and stereoselective insertion of alkynes. Use of P(t-Bu)₃ as a ligand allows exclusive regioselective C(2)-alkenylation, while PCyp₃ is found effective for C(5)-alkenylation of C(2)-substituted imidazoles. The reaction demonstrates a broad scope of imidazoles and internal alkynes to give trisubstituted ethenes highly regio- and stereoselectively in modest to good yields.     

Molecular rearrangement of epoxide derived from sesquiterpenes by Lewis acid catalysis

The molecular rearrangement under mild conditions of two epoxides derived from the sesquiterpenic himachalenes has been studied using BF₃·Et₂O as the Lewis acid catalyst. Both reactions are efficient and selective affording in each case one new enantiomerically pure ketone isolated in good yield. Their structures have been fully characterized by spectroscopic methods. Mechanisms for the BF₃-catalyzed reactions are proposed.     

Dual catalysis: combining photoredox and Lewis base catalysis for direct Mannich reactions

A dual catalytic system combining photoredox and Lewis base catalysis has been developed. By the appropriate choice of light source and catalyst, the photoredox cycle can be optimally modulated to match the base catalyzed reaction cycle to provide the corresponding products under mild reaction conditions.     

Novel metal-free Lewis acid catalysis by phosphonium salts through hypervalent interaction

Phosphonium salts as a novel metal-free Lewis acid catalyst can be considered as organocatalysts. The introduction of a five-membered dioxaphosphacycle to the phosphonium salt is critical to its function as a Lewis acid catalyst for the Diels–Alder reaction. The key to the successful catalysis by the phosphonium salt is the utilization of hypervalent bonding as a strategic interaction for the generation of an active species.     

Highly enantioselective Michael-cyclization cascade promoted by synergistic asymmetric aminocatalysis and Lewis acid catalysis

The novel dual cooperative asymmetric aminocatalysis and Lewis acid catalysis has been successfully developed for promoting cascade Michael-cyclization reaction with high enantio-, regio- and chemo-selectivity. The simple and practical process affords a one-pot approach to synthetically useful cyclopentenes.     

Enantioselective fluoride ring opening of aziridines enabled by cooperative Lewis acid catalysis

The enantioselective ring opening of aziridines using a latent source of HF is described. A combination of two Lewis acids, (salen)Co and an achiral Ti(IV) cocatalyst, provided optimal reactivity and enantioselectivity for the trans β-fluoroamine product. The use of a chelating aziridine protecting group was crucial. Acyclic and cyclic meso N-picolinamide aziridines underwent fluoride ring opening in up to 84% ee, and the kinetic resolution of a piperidine-derived aziridine was performed with k_rel=6.6. The picolinamide group may be readily removed without epimerization of the fluoroamine. Preliminary studies revealed a bimetallic mechanism wherein the chiral (salen)Co catalyst delivers the nucleophile and the Ti(IV) cocatalyst activates the aziridine.     

Mechanisms of acid-base catalysis of beta-elimination reactions in systems activated by a pyridine ring

Beta-elimination reactions from 1 (in quinuclidine/quinuclidinium chloride, imidazole/imidazolium, and acetate/acetic acid buffers) and from 2 (in imidazole/imidazolium and acetate/acetic acid buffers) with formation of 4-vinylpyridine and 2-vinylpyridine, respectively, were studied. The results of a kinetic study of acid-base catalysis and H/D exchange are consistent with NH(+), the protonated substrate, as the species that undergoes carbon deprotonation with an E1cb mechanism. The comparison with previously studied reactions in acetohydroxamate/acetohydroxamic acid buffer confirms this assignment. The high proton activating factor, PAF, value observed (PAF = 1.2 x 10 (6) with isomer 1 in quinuclidine/quinuclidinium buffer) can be explained with the high stability by the resonance of the intermediate carbanion.     

Structural characterization of a completely alkyl-substituted Al-Sb Lewis acid-base adduct

The Lewis acid-base adduct t-Bu₃Al-SbMe₃ (1), which was synthesized by reaction of equimolar amounts of t-Bu₃Al and trimethylstibine SbMe₃, was characterized by multinuclear NMR (¹H, ¹³C) spectroscopy, elemental analyses as well as by single crystal X-ray diffraction.     

Inherent stability limits of intramolecular boron nitrogen Lewis acid-base pairs

The reaction of (C(6)F(5))(2)BH (1) with N,N-dimethylallylamine (2), N,N-diethylallylamine (3) and 1-allylpiperidine (4) afforded the five-membered ring systems (C(6)F(5))(2)B(CH(2))(3)NR(2) (R = Me (5), Et (6)) and (C(6)F(5))(2)B(CH(2))(3)N(CH(2))(5) (7) with an intramolecular dative B-N bond. A different product was obtained from the reaction of (C(6)F(5))(2)BH (1) with N,N-diisopropylallylamine (8), which afforded the seven-membered ring system (C(6)F(5))(2)B(CH(2))(3)N(iPr)CH(Me)CH(2) (9) under extrusion of dihydrogen. All compounds were characterised by elemental analysis, NMR spectroscopy and single-crystal X-ray diffraction experiments. Density functional theory (DFT) studies were performed to rationalise the different reaction mechanism for the formation of products 6 and 9. The bonding situation of compound 9 was analysed in terms of its electron density topology to describe the delocalised nature of a borane-enamine adduct.     

Testing the veracity of claims of Lewis acid catalysis

Are reactions employing Lewis acids really catalysed by those Lewis acids, or by “hidden Brønsted acids”, i.e. Brønsted acids generated in situ by hydrolysis? Testing of a series of reactions using Sc(III), Fe(III), In(III) and Y(III) by addition of 2,6-di-t-butyl-4-methylpyridine reveal that all are likely to follow the latter pathway. A reaction claimed to be catalysed by CBr₄ through halogen bonding is also likely to be Brønsted acid catalysed.     

Transition metal-catalyzed dehydrocoupling of group 13-group 15 Lewis acid-base adducts

Transition metal-catalyzed dehydrocoupling is a relatively new method for main group element-element bond formation, providing a facile alternative to salt metathesis processes. Lewis acid-base adducts of Groups 13 and 15 can undergo dehydrocoupling with both early and late transition metals and complexes thereof to yield rings, chains and macromolecules based on a framework of alternating B-N or B-P atoms. Evidence is also provided for Group 13 hydrides acting as poisons towards heterogeneous dehydrocoupling catalysts.