Strong Solid Bases for Organic Reactions

BY YOSHIO ONO AND TOSHIHIDE BABA

1 Introduction

Carbanions are important intermediates in many organic reactions such as isomerizations, additions, alkylations, and cyclizations. They are formed by abstraction of a proton from a C-H bond of an organic molecule by a base.

These organic reactions often require a stoichiometric amount of liquid base to generate carbanions and produce a stoichiometric amount of metal salts as a by-product. For example, the methylation of phenylacetonitrile with methyl iodide proceeds in the presence of base under a phase-transfer condition.

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In this case, more than a stoichiometric amount of sodium hydroxide is required to neutralize the hydrogen iodide produced and to keep the system basic. Furthermore, a stoichiometric amount of sodium iodide is inevitably formed and has to be disposed of in an appropriate manner. Organometallic compounds such as Grignard reagents and alkyl lithium serve as donors of carbanion-like species. Here, again, a stoichiometric use of these reagents is required. Therefore, there is a need to develop solid bases to avoid these problems.

Solid base catalysts have many advantages over liquid bases. They are non-corrosive and environmentally benign, presenting fewer disposal problems, while allowing easier separation and recovery of the products, catalysts and the solvent. Thus, solid base catalysis is one of the economically and ecologically important fields in catalysis and the replacement of liquid bases with heterogeneous catalysts is becoming more and more important in the chemical industry. Furthermore, high activities and selectivities are often obtained only by solid base catalysts for various kinds of reaction.

Since the ability of bases to abstract a proton from a C-H bond is directly connected to the base strength, stronger bases are in general more effective in forming carbanions. Alkaline earth oxides such as magnesium oxide are strongly basic when properly pretreated. Extensive works by Tanabe, Hattori and their co-workers have been carried out using these materials.

Recently, other strong base catalysts have been reported. Potassium amide supported on alumina (KNH2/Al2O3) is effective for a number of base-catalysed reactions. Even toluene is activated to react with silanes at 329 K,8 and the isomerization of 2,3-dimethylbut- l-ene proceeds even at 201 K. Potassium fluoride supported on alumina (KF/Al2O3) has been used by organic chemists for a long time, but Tsuji and Hattori revealed that this catalyst becomes much more active when pretreated at 573-673 K under vacuum. Yamaguchi et al. reported that catalysts prepared by loading alkali- metal salts such as KNO3, followed by heating at 773-873 K, were very strongly basic and active for the isomerization of cis-but-2-ene at 273 K. Fu et al. used alkali-metal compounds supported on alumina for the reaction of catechol with dimethyl carbonate and found that the rate and selectivity depended strongly on the catalyst used. Furthermore, modified zeolites and calcined hydrotalcites are often reported as strong bases.

In this review, we will describe the preparation and characterization of these strong bases. Then, application of these catalysts to a variety of catalytic reactions is described. The reactions include the isomerization of alkenes and alkynes, the dimerization of alkynes, aldol reactions, and the formation of Si-C, Si-N and Si-O bonds.

2 Role of Solid Base and Basic Sites as a Catalyst

2.1 Abstraction of Protons - On the surface of solid bases, there are specific sites or centers, which function as a base. Basic sites (centers) abstract protons from the reactant molecules (AH) to form carbanions (A-).

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Here, the basic site B- on the solid surface acts as a Bronsted base. Stronger bases can abstract a proton with molecules with higher pKa values.

2.2 Activation of Reactants without Proton Abstraction - Reactants such as ketones and aldehydes are often activated by bases without proton transfer, as expressed by the following equation.

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Here, the basic sites B- act as a Lewis base.

It should be noted that a same surface site can serve as a Bronsted base as well as a Lewis base, depending on the nature of the adsorbate.

2.3 Cooperative Action of Acidic and Basic Sites - Magnesium oxide is active for the hydrogenation of 1,3-butadiene. It is assumed that hydrogen heterolytically dissociates in the presence of a pair of a coordinatively unsaturated Mg2+ and an oxide ion. Hydrogen adsorption is schematically expressed as:

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3 Base Strength of Basic Sites

3.1 H_ Acidity Function - The H_ acidity function is defined as a measure of the ability of the basic solution to abstract a proton from an acidic neutral solute.

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To determine the H_ value of a solution, the concentrations of AH and A- have to be measured accurately. When half of a solute AH is deprotonated in the solution, i.e.,]A-]= [AH], the H_ value of the solution is equal to the pKa value of AH. The basic strength of a solution is stronger when a neutral molecule of larger pKa value is deprotonated.

Tanabe proposed transferring this concept to solid bases as a measure of their strength. The base strength of solid bases is expressed by means of the H_ value, equated to the highest among the pKa values of the adsorbates from which the basic site is able to abstract a proton.

Tanabe defined solid superbases as materials with H_ values higher than 26. This value, like that of superacids (H_ ≤ -12), is 19 units from a neutral solution of 7.

In the use of this concept for solid bases, two important points should be noted:

(a) In the discussion of solid bases, the H_ value is treated as a parameter to describe the nature of individual basic sites. It is often assumed that there are a certain number of basic sites on solid surfaces and that each of the sites has its own basic strength. In the original definition, H_ scale is used to describe basic property of the solution, not that of individual basic molecules (or ions) in the solution.

(b) In principle, the idea of the H_ scale is only applicable to the Bronsted base. It is not, at least directly, related to the ability of the sites to function as Lewis bases, as shown in eqn. ( 1.5).