Base strength condensation

Because a base-catalyzed reaction involves the abstraction of a proton by the catalyst, one approach to measurement of the total number of basic sites and also the base strength distribution is to use the reactions of molecules with various values (96-100). For instance, the basic site distribution in calcined MgAl hy-drotalcites was determined by Corma et al. (99), who used the Knoevenagel condensation (Scheme 7) between benzaldehyde and methylene active compounds with various pKa values, i.e., ethyl cyanoacetate (pKa = 9), diethyl malonate (pKa = 13.3), and ethyl bromoacetate (pKa = 16.5). The authors found that this material has basic sites with pKa values up to 16.5, although most of the basic sites... 

Thieno[2,3-. ]pyridines, 115, form via a condensation reaction with halogenated carbonyl compounds, as shown in Scheme 35. Depending on the base strength of the reagent and reaction conditions, intermediate 114 may be isolated <2005CCA63>. 

The base One equivalent, at least, of a base relative to the aryl halide must be present to achieve the alkene substitution catalytically. Most often a tertiary amine is employed. Secondary amines also appear to be suitable but primary amines usually are not. The base strength of the amine is important since only quite basic amines such as triethylamine work well. Acetate salts, carbonates and bicarbonates also are suitable bases but solubility may cause difficulties in some instances. The addition of a phase transfer agent such as a quaternary ammonium salt has often solved this problem. The inorganic bases, of course, may cause other problems such as ester hydrolysis, aldol condensations and other undesired side reactions. 

The acid and base sensitivity of condensation polymers whether or not under stress, e.g. polycarbonate, polyesters, polyamides and polysilanes under influence of acid or base the condensation bonds are hydrolysed under the cooperative action of mechanical stresses and the environment. A striking example is shown in Fig. 26.11, where the strength retention of PpPTA fibres is plotted versus pH after an exposure of 3 months at room temperature (Van den Heuvel and Klop). The hydrolysis of the polyamide is acid or alkali catalysed, in particular below pH = 3 and above pH = 9. 

Note that this Principle is simply a restatement of the experimental evidence which led to Table 1.2. It is a condensed statement of a very large amount of chemical information. As such it might be called a law. But this label seems pretentious in view of the lack of a quantitative definition of hardness. HSAB is not a theory, since it does not explain variations in the strength of chemical bonds. The word prefer in the HSAB Principle implies a rather modest effect. Softness is not the only factor which determines the values of A/Z° in Equation (1.1). There are many examples of very strong bonds between mismatched pairs, such as H2, formed from hard H+ and soft H. H2O, OH and 0 are all classified as hard bases, but there are great differences in their base strength, by any criterion. 

Quaternary ammonium hydroxides immobilized on MCM-41 (MCM-410H1) [23] act a stronger base catalyst tban amine analogs. The condensation of salicyl-aldehyde with diethyl-2-pentenedicarboxylate in the presence of this catalyst led to a mixture of coumarin-3-acrylates and chromene derivatives [24], The ratio of the formation of coumarin-3-acrylates and chromene derivatives depended on the base strength of the catalyst (Scheme 5.8). 

A suitable solid base must have the appropriate base strength for the reaction under investigation. If the initial reaction step is the removal of a proton from a reactant of the form R1-CH2-R2 then the acidity of the proton to be removed depends on the identity of the R) and R2 groups (Table 1). The solid base selected should have sufficient base strength to carry out the reaction but should not have excessive base strength as this may lead to rapid catalyst deactivation or to side-product formation. For aldehyde and ketone condensation reactions therefore with a p.Ka of 19.7 - 20 a strong base is required but not a superbase material. Caustic can be used to carry out reactions with reactants with the removable proton having a pA a of up to around 20. 

The impetus for low temperature aldol processes in the absence or minimization of organic solvent has resulted in several new approaches. For example, Dewa et al. describe an aldehyde-ketone condensation catalyst that can be suspended in water, prepared by reacting lanthanum tris(isopropoxide) with anthracenebisresorcinol (ABR). A polymeric aquo complex with several 1,2-linked ABRs and two (LaOH) + groups is formed, coordinating 30 water molecules and relatively stable to decomposition. The base strength of this complex approached that of aqueous NaaCOs. 

The base strength of alkali ion-exchanged zeolites can be estimated by using the Knoevenagel condensation reactions of benzaldehyde with ethyl cyanoac-etate (pFTa = 8.6) [reaction (12)], ethyl acetoacetate (pKa = 10.7), and ethyl mal-onate(pKa = 13.3) over alkali ion-exchanged X and Y zeolites (55). 

Diacetone alcohol decomposition to acetone (reverse reaction of acetone aldol condensation) proceeds over alkidine earth catalysts. The active sites are poisoned by COz- The slope of the activity decrease with increasing amount of adsorbed CO2 represents the activity per unit base site. The activities per unit site are in order BaO>SrO>CaO>MgO. The order coincides with the base strength order the stronger the base strength the more effective the active sites. 

In contrast with the above findings in water, condensations in chloroform exhibit a considerable specificity of the base used as a catalyst, as shown by two model reactions ethyl nitroacetate with norbornene and benzoylnitromethane with styrene [65]. A screening of several organic bases (tertiary amines and heteroaromatic A-bases with one or two basic sites) applied to these reactions indicates that the results are not related to the base strength. In fact, the most effective bases are those with the highest H-bonding basicity [74], particularly those having two basic sites, like DABCO, NMI, etc. [75-77]. 

More substituted dipolarophiles like methacrylic derivatives, as well as nitroacetamides, with the same dipolarophiles, selectively give the condensation products in excellent yields. The results depend in some cases on the base employed in general, additions are favored by base strength and concentration and lower temperature, compared to condensations. 

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