Base-catalyzed condensations table

TABLE 16.1 Base Catalyzed Condensations Showing the Active Components and the Carbonyl Compounds... 

TABLE 16.1 Base-catalyzed condensations showing the active-hydrogen components and the carbonyl components... 

Table 1. Distribution ratios a and rate constants K and Kof the acid-and base-catalyzed condensation.

These solvents may be categorized as polar protic (water, methanol, formamide), polar aprotic (acetonitrile, dimethylformamide) and nonpolar aptotic (dioxane). (See Table 7.) Artaki et al. [89] explained that under base-catalyzed condensation conditions (pH > 2.5), the aprotic solvent, dioxane, is unable to hydrogen bond to the SiO" nucleophile. In addition, because it is nonpolar, it does not tend to stabilize the reactants with respect to the activated complex. Therefore, dioxane should result in a significant enhancement of the condensation rate and cause an efficient condensation leading to the formation of large, compact spherical particles. The polar, aprotic solvents, dimethylformamide and acetonitrile, also do not hydrogen bond to the silicate nucleophile involved in the condensation reaction. However, due to their polarity the anionic reactants are stabilized with respect to the activated complex slowing down the reaction to some extent. 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

Representatives of this novel class of meso-ionic compounds in which the exocyclic substituent f (see Table I) is a stabilized carbanionoid residue [-C(CN)C02Me or -C(CN)2l have been recently synthesized. Base-catalyzed (potassium carbonate or triethylamine) condensation of AT-acylhydrazines (147) with 3,3-dichloroacrylonitriles (165) yield the greenish-yellow meso-ionic l,3,4-oxadiazol-2-enes (164) directly. 

The kraft lignin is constituted mainly of guaiacylpropanoid units (2) of which about half are condensed, i.e., substituted at the 5-position (16). The number of aromatic sites reactive towards base catalyzed electrophilic substitution reactions, therefore, is fairly low at about 0.3 per Cg unit (Table II). 

The reaction is further complicated by thermodynamic equilibrium limitations, as indicated in Table I. The condensation/dehydration of acetone to MO is limited to about 20% conversion at 120 C (16). However, there is no equilibrium limitation to the overall acetone-to-MIBK reaction. This, coupled with the possibility of numerous thermodynamically favorable side reactions that are also acid/base-catalyzed (Fig. 1), suggests the need to balance the acid/base and hydrogenation properties of the selected catalyst. 

Protease-Catalyzed Condensation Study. Based on the exceptional activity of trypsin and a-chymotrypsin from bovine pancreas (Table I), protease enzymes were identified as target catalysts. Consequently, a series of proteases (i.e. serine, cysteine, aspartic, and metallo) were selected in order to screen their ability to catalyze siloxane condensation with trimethylsilanol. The reactions... 

The results in Table 2 show that the pyridine is less active than any of the X zeolites and Ge faujasite except the lithium form which shows slightly lower activity, whereas all Y zeolites show lower activity than pyridine. Piperidine, however, is more active than any of the zeolite samples studied here. From this comparison, it appears that, most of the basic sites of the zeolites must have pK<10.3. However, the fact that zeolites are also active for catalyzing the condensation of benzaldehyde with ethyl malonate, indicate that these samples have some basic sites with pK< 13.3. On a quantitative bases, and comparing the activity of zeolites for condensation with ethyl cyanoacetate, ethyl acetoacetate and ethyl malonate (Fig. 2), we can conclude that most of the basic sites of the zeolite have pK<9.0 with a sensible amount with 9.0basic strength of different solid base catalysts. 

Acid catalyzed Sehiff base condensation of 2,6-diformyl-p-cresol and TETA (triethylenetetraamine) with Pr3+ as the template cation (in methanol) afforded a 1 1 product containing a tetraclinching alcohol (Eq. 9b, Table 9) [132]. One of the carbonyl groups was acetylated under mild acidic conditions. The 10-coord-ination around Pr3 + achieved by one aryloxide site, one imine and three amino nitrogen, two bidentate nitrate anions and one molecule of methanol is best described as a bicapped square antiprism. 

The enzymatic aldol reaction represents a useful method for the synthesis of various sugars and sugar-like structures. More than 20 different aldolases have been isolated (see Table 13.1 for examples) and several of these have been cloned and overexpressed. They catalyze the stereospecific aldol condensation of an aldehyde with a ketone donor. Two types of aldolases are known. Type I aldolases, found primarily in animals and higher plants, do not require any cofactor. The x-ray structure of rabbit muscle aldolase (RAMA) indicates that Lys-229 is responsible for Schiff-base formation with dihydroxyacetone phosphate (DHAP) (Scheme 13.7a). Type II aldolases, found primarily in micro-organisms, use Zn as a cofactor, which acts as a Lewis acid enhancing the electrophilicity of the ketone (Scheme 13.7b). In both cases, the aldolases accept a variety of natural (Table 13.1) and non-natural acceptor substrates (Scheme 13.8). 

There are, however, clear stereomechanistic differences between these two classes of enzyme-catalyzed reactions. The Claisen-type condensations uniformly involve inversion of configuration at the a-carbon of the esteratic substrate, involving C-C bond formation at either the re or the si face of the ketonic or aldehydic substrate (Table VII) (196-211). Moreover, neither Schiff bases nor metal ions have been directly implicated in the catalytic mechanisms of these enzymes. Unlike the aldolases, these enzymes do not catalyze rapid enolization of the nucleophilic substrate in the absence of the second substrate. Inversion of configuration suggests that at least two catalytic groups, perhaps operating in concert, facilitate C-C bond formation. Physicochemical measurements on citrate synthase are consistent with this interpretation of inversion of configuration. 

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