Reactions acid-catalyzed condensation

Certain catalytic modes have been well exploited in flame retardant systems, namely the dehydrating action of compounds which yield strong acids under flaming or smoldering conditions. Friedel-Crafts and other acid catalyzed condensation reactions have been exploited to increase char. These mechanisms don t work very well for polymers of mainly hydrocarbon character. Are there other modes of catalysis which might work better ... 

Because of their ring-chain tautomeric character, tetrahydro-l,3-oxazines can be used as aldehyde sources in acid-catalyzed condensation reactions involving the carbon transfer via the open forms. This approach is especially advantageous in those cases where the aldehydes required for the condensations are unstable or difficult to access <2003EJ03025>. 

Position 5 is also the most reactive in the nitration of guaiacyl propane units containing a free phenolic hydroxyl group and in analogous model compounds (7). By contrast, acid-catalyzed condensation reactions appear to proceed more rapidly at position 6 (15) and in this respect resemble the protodedeuteration reactions. 

Cucurbituril (cucrbit[6]uril or CB[6]), a hexameric macrocycle 141 (R = H, = 6) with a 1,3,5,6-tetraazocane core unit, is self-assembled from an acid-catalyzed condensation reaction of glycouril and formaldehyde (see Section 14.09.9.1.1 and 1996CHEC-II(9)705). Although its synthesis first appeared in 1905, its chemical nature and structure remained unknown until 1981, when full characterization was reported by Mock and co-workers <1981JA7367>. 

A recent patent granted to Kurihara and co-workers of Toray Industries describes membranes formed by acid-catalyzed condensation reactions of a variety of reactive monomers on polysulfone support films under conditions similar to those used for NS-200 membranes (J ). The data (Example 2 in the patent) show that 1,3,5-tris (hydroxyethyl)isocyanuric acid (THEIC), when polymerized at 150°C on a polysulfone support film, produced a membrane exhibiting 96.7 percent salt rejection with a flux of 0.49 gfd (0.82 L/sq m/hr) when tested toward 0.25 percent NaCl at 568 psi (3916 kPascals)and 25°C. 

Based on these biological data, two commercial forms of a-tocopherol (or their more stable acetate derivatives) are currently being produced by independent approaches [7, 8], Totally synthetic vitamin E, which is an equimolar mixture of all eight stereoisomers of a-tocopherol, is produced at a rate of over 25000 tons per year for the application in feed, food, and the pharma industry. The large-scale industrial synthesis of (all-rac)-a-tocopherol uses 2,3,5-trimethylhydroquinone (11) as the aromatic building block and the C2o compound isophytol (12). The acid-catalyzed condensation reaction in the last step delivers (all-rac)-3 (Fig. 2) [21-25],... 

The PEC-1000 membrane, developed by Toray Industries, has many similarities to the NS-200 membrane. This membrane has been described by Kurihara and coworkers as a thin film composite membrane in which the barrier layer was formed by an acid-catalyzed condensation reaction on the surface of micro-porous polysulfone.7S The cross section of this membrane, under high magnification in a scanning electron microscope is quite similar in structure to the NS-200 membrane. The barrier layer is 300 angstroms thick. 

Scheme 6.19 Acid-catalyzed condensation reaction of novoiac resins by melamine cross-linker.

Another belt-shaped molecule is cucurbituril (53) which was presumably synthesized as early as 1905 by Behrend et al. [42]. The structural formula of 53 was resolved in 1981 by Freeman. Mock, and Shih [43] only when modern spectroscopic methods were available. Cucurbituril (53) is easily obtained from urea, glyoxal, and formaldehyde in an acid-catalyzed condensation reaction with glycoluril (52) as an intermediate. The initially formed polycondensation product, insoluble in all common solvents, is treated with hot concentrated sulfuric acid. After dilution with water and subsequent boiling a solid is obtained in a yield of 40-70%. The H-NMR spectrum shows only three signals of equal intensity and the infrared spectrum suggests retention of the glycoluril moieties. The compound is not suitable for usual mass spectrometry due to its low volatility. However, an X-ray crystal structure of the calcium complex obtained from sulfuric acid solution was undertaken. 

The synthesis of trimetrexate, reported by Davoll and Johnson, began with an acid-catalyzed condensation reaction between l-nitro-4-chloro-o-toluonitrile and guanidine to produce 2,4-diamino-5-methyl-6-nitro-quinazoline as an intermediate for further functionalization. Hydrogenation of 2,4-diamino-5-methyl-6-nitro-quinazoline produced the corresponding 2,4,6-triamino-quinazoline, which was readily converted the corresponding nitrile upon treatment with hydrochloric acid and cuprocyanide. Condensation of the nitrile with 2,3,4-trimethoxyaniline produced trimetrexate in 26% overall yield. 

Cucurbituril (cucurbit[6]uril, or CB[6]) is a hexameric macrocyclic compoxmd self-assembled from an acid-catalyzed condensation reaction of glycoluril and formaldehyde (Scheme 1). Although its synthesis was first reported by Behrend and coworkers in 1905j its chemical nature and stmcture were unknown until they were revealed by Mock and coworkers in 198iP CB[6] has a cavity of 5.5 A diameter, accessible from the exterior by two carbonyl-laced portals of 4 A diameter. Although the size of the cavity is similar to that of a-cyclodextrin ( < highly symmetrical structure... 

Fig. 64.12 Putative pathway from monomeric C-glycosidic ellagitaimins to oligomeric and glycosilated C-glycosidic ellagitannins as well as to llavano-ellagitannins. Acid-catalyzed condensation reactions of C-glycosidic ellagitannins via benzylic-type carbocation intermediates...

Fig. 4.22 Acid-catalyzed condensation reactions of C-glucosidic ellagitannins via benzylic-type carbocation intermediates.

Because the acid-catalyzed condensation reaction in which amides are made is reversible, the amides can be decomposed in the presence of an add catalyst and water. Acid rain gives precisely that combination. 

Another mention of the versatile and recyclable catalyst Mont K-10 (10 mol%) has been found for one-pot quinoxa-line S5mthesis in water at room temperature [79] (Scheme 55). Mont K-10 has long been used as acidic and the condensation reaction of 1,2-diamine with 1,2-dicarbonyl compounds follows the regular mechanism of acid-catalyzed condensation reactions. The catalyst can be directly reused four times without any treatment. 

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