Acid Base Cooperativity Research Articles

Decoration of chitosan microspheres with inorganic oxide clusters: Rational design of hierarchically porous, stable and cooperative acid–base nanoreactors

One of the fundamental enzymatic catalyst assets, which is the most difficult to engineer in synthetic systems, is the coexistence of multifunctional sites and their synergetic cooperation. In this work, an efficient approach toward cooperative acid–base materials using natural matrices is proposed. Taking advantages from chitosan polysaccharide as nano-assembling system and on the supercritical drying technique to preserve their porosity, the mutual interactions between different glucosamine units and the Lewis acidic precursors (Ti, Zr, Al, Sn) allowed the preparation of hierarchically porous microspheres in which well-separated amino groups from chitosan are replicated with highly dispersed acidic inorganic oxides. This decoration at the nano-scale entails a notable improvement on the hydrothermal stability of the resulting organic–inorganic hybrid materials. The resulting acid–base hybrid materials are assessed for three carbon–carbon forming reactions (Henry condensation, Michael addition and jasminaldehyde synthesis) and systematically compared to the pure acidic inorganic oxide and basic chitosan microspheres. The bifunctional materials displayed interesting catalytic activity and selectivity, with respect to monofunctional ones, witnessing thus on the cooperative effect attainable in chitosan@inorganic oxide microspheres.

Cooperative Acid–Base Effects with Functionalized Mesoporous Silica Nanoparticles: Applications in Carbon–Carbon Bond‐Formation Reactions

Acid-base bifunctional mesoporous silica nanoparticles (MSN) were prepared by a one-step synthesis by co-condensation of tetraethoxysilane (TEOS) and silanes possessing amino and/or sulfonic acid groups. Both the functionality and morphology of the particles can be controlled. The grafted functional groups were characterized by using solid-state (29)Si and (13)C cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopy, thermal analysis, and elemental analysis, whereas the structural and the morphological features of the materials were evaluated by using XRD and N(2) adsorption-desorption analyses, and SEM imaging. The catalytic activities of the mono- and bifunctional mesoporous hybrid materials were evaluated in carbon-carbon coupling reactions like the nitroaldol reaction and the one-pot deacetalization-nitroaldol and deacetalization-aldol reactions. Among all the catalysts evaluated, the bifunctional sample containing amine and sulfonic acid groups (MSN-NNH(2)-SO(3)H) showed excellent catalytic activity, whereas the homogeneous catalysts were unable to initiate the reaction due to their mutual neutralization in solution. Therefore a cooperative acid-base activation is envisaged for the carbon-carbon coupling reactions.

Rationalizing the Reactivity of Frustrated Lewis Pairs: Thermodynamics of H2 Activation and the Role of Acid−Base Properties

The acid-base strengths of recently reported frustrated Lewis pairs and their relation with the thermodynamic feasibility of heterolytic hydrogen splitting reactions are analyzed in terms of quantum chemical calculations. Reaction free energies of hydrogenation processes are computed, and an energy partitioning scheme is introduced, which involves quantitative measures of the acidity and basicity of the reacting Lewis centers. Additional terms are also included that account for possible dative bond formation between the active sites and for stabilizing electrostatic interactions occurring in the product species. For intermolecular combinations of donor-acceptor components, the calculated reaction free energies are found to correlate well with the cumulative acid-base strengths. Product stabilization for these systems represents a notable contribution to the overall energetics; however, it generally shows only a slight variation for the investigated series. The reactivity of linked donor-acceptor pairs is primarily governed by acid-base properties as well, but the magnitude of stabilizing effects arising from acid-base cooperativity of active sites is also of significant importance in determining the thermodynamic feasibility of the reactions.

Acid–base cooperativity of heterogeneous catalyst containing acidic framework and sterically hindered base for aldol condensation

A bifunctional heterogeneous catalyst containing two mutually incompatible acidic and basic sites, which exhibits cooperative catalytic behavior in the aldol condensation of acetone and various aldehydes, was synthesized by postgrafting of 1,5,7-triazabicyclo[4.4.0] dec-5-ene (TBD, a sterically hindered organic base) onto Al-MCM-41 molecular sieve.

Acid–base cooperativity in condensation reactions with functionalized mesoporous silica catalysts

The Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate was used to test the acid–base cooperativity of SBA-15 mesoporous silica co-condensed with aminopropyl groups as well as silica co-condensed with dihydroimidazole groups. Surface silanols were capped with trimethylsilyl groups using hexamethyldisilazane to determine the effects of the silanol groups on catalytic activity. The activities of the mesoporous silicas with or without capped silanols were compared to the activity of propylamine free in solution. Silylation resulted in a significant loss of activity, where turnover frequencies dropped nearly to those of the homogeneous base. The same behavior was also observed with dihydroimidazole-functionalized SBA-15, which catalyzes the Knoevenagel condensation by a different mechanism. Not only did these results indicate cooperative effects in heterogeneous systems, but cooperativity between a homogeneous base and silanols on unfunctionalized SBA-15 was also observed to a lesser extent. These results demonstrated that acid–base cooperativity seen in well-defined single sites can also be demonstrated on an extended catalytic domain on the surface of mesoporous silica where the active sites are not rigorously isolated.

Acid−Base Bifunctional and Dielectric Outer-Sphere Effects in Heterogeneous Catalysis: A Comparative Investigation of Model Primary Amine Catalysts

Dielectric and acid-base bifunctional effects are elucidated in heterogeneous aminocatalysis using a synthetic strategy based on bulk silica imprinting. Acid-base cooperativity between silanols and amines yields a bifunctional catalyst for the Henry reaction that forms alpha,beta-unsaturated product via quasi-equilibrated iminium intermediate. Solid-state UV/vis spectroscopy of catalyst materials treated with salicylaldehyde demonstrates zwitterionic iminium ion to be the thermodynamically preferred product in the bifunctional catalyst. This product is observed to a much lesser extent relative to its neutral imine tautomer in primary amine catalysts having outer-sphere silanols partially replaced by aprotic functional groups. One of these primary amine catalysts, consisting of a polar outer-sphere environment derived from cyano-terminated capping groups, has activity comparable to that of the bifunctional catalyst in the Henry reaction, but instead forms the beta-nitro alcohol product in high selectivity (approximately 99%). This appears to be the first observation of selective alcohol formation in primary amine catalysis of the Henry reaction. A primary amine catalyst with a methyl-terminated outer-sphere also produces alcohol, albeit at a rate that is 50-fold slower than the cyano-terminated catalyst, demonstrating that outer-sphere dielectric constant affects catalyst activity. We further investigate the importance of organizational effects in enabling acid-base cooperativity within the context of bifunctional catalysis, and the unique role of the solid surface as a macroscopic ligand to impose this cooperativity. Our results unequivocally demonstrate that reaction mechanism and product selectivity in heterogeneous aminocatalysis are critically dependent on the outer-sphere environment.

Kinetic Analysis of Diamine-Catalyzed RNA Hydrolysis.

The catalysis of various amines for the hydrolysis of RNA has been kinetically investigated, and the catalytic rate constants for each of the ionic states of these amines are determined. Ethylenediamine and 1,3-propanediamine are highly active under the physiological conditions, mainly because they preferentially take the catalytically active monocationic forms. The catalysis of these diamines is further promoted by the intramolecular acid-base cooperation of the neutral amine and the ammonium ion. In contrast, monoamines overwhelmingly exist at pH 7 as the inactive cations. Potential application of the catalysis by the diamines and the related oligoamines is discussed.

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Highly active catalysts for the hydrolysis of phosphodiester linkage in RNAs and DNAs, which are applicable to artificial ribonucleases and nucleases, have been reported. Oligoamines such as ethylenediamine and triethylenetetramine (trien), as well as cobalt (III) complexes such as [Co (trien) (H2O) 2] 3+ and [Co (tme) 2 (H2O) 2] 3+ (tme : 1, 1, 2, 2-tetramethylethylenediamine), are so active (105 to 106 fold acceleration) that RNAs are promptly hydrolyzed near neutrality. The large activities are ascribed to intramolecular acid-base cooperation in the catalysts. In addition, pheophorbide, a derivative of chlorophyll, efficiently cleaves DNAs under irradiation of visible light, even in the absence of oxygen. The photocleavage proceeds via a novel mechanism, which involves direct attack of the dye to DNAs (and not attack by singlet oxygen). Simple structures of these catalysts as well as their large activities strongly indicate high potentialities as the catalytic sites in artificial ribonucleases and nucleases.

Transition-state stabilization via dynamic molecular recognition : a concerted acid-base bifunctional catalysis in ester hydrolysis

An imidazole derivative 1a having a bisresorcinol moiety catalyzes the hydrolysis of p-nitrophenyl acetate in H 2O-CH 3CN (9:1) by the mechanism involving a concerted acid-base cooperation; nucleophilic attack of the imidazolyl group on substrate is assisted by the hydroxyl group of the bisresorcinol moiety as a general acid.