Dehydration Condensation

Amin omethyl-3,5,5-trimethyl cyclohexyl amine (21), commonly called isophoronediamine (IPD) (51), is made by hydrocyanation of (17) (52), (53) followed by transformation of the ketone (19) to an imine (20) by dehydrative condensation of ammonia (54), then concomitant hydrogenation of the imine and nitrile functions at 15—16 MPa (- 2200 psi) system pressure and 120 °C using methanol diluent in addition to YL NH. Integrated imine formation and nitrile reduction by reductive amination of the ketone leads to alcohol by-product. There are two geometric isomers of IPD the major product is ds-(22) [71954-30-5] and the minor, tram-(25) [71954-29-5] (55). 

Although the selectivity is high, minor amounts of by-products can form by dehydration, condensation, and oxidation, eg, propylene [115-07-17, diisopropyl ether, mesityl oxide [141-79-7] acetaldehyde [75-07-0], and propionaldehyde [123-38-6]. Hydrotalcites having different Al/(A1 + Mg) ratios have been used to describe a complete reaction network for dehydrogenation (17). This reaction can also be carried out in the Hquid phase. 

By now the dehydration condensation of urea [57-13-6] has displaced the dicyandiamide process (see Urea). Although the latter is stiU used occasionally, the urea process predominates in North America. A flow sheet is shown in Figure 2 (43). 

Condensed Phosphates. The term condensed phosphate refers to any dehydrated, condensed orthophosphate for which the M20 P20 ratio is less than 3 1 (17). (1) is disodium orthophosphate [7558-79-4] and (2) tetrasodium pyrophosphate [7722-88-5]. 

The DBSA-system is also applicable for the dithioacetalization of aldehdyes and ketones with 1,2-ethanedithiol to give the corresponding dithioacetals (Scheme 5.4, d). Increasing the reaction temperature decreases the yield of the products. Interestingly, increases in the concentration of the surfactant also decrease the yield of products formed, while shortening the alkyl chain of the surfactant abolishes its catalytic activity. Optical microscopy shows the formation of micelles, which are proposed to form hydrophobic environments and decrease the effective concentration of water and facilitate the dehydrative condensation reactions. 

Dehydrative condensation of transition metal hydroxo complexes is a versatile synthetic method, when the conjugated acid of the incoming ligand (H-A) is a protic compound. There are reports of the synthesis of two types of square-planar hydroxopalladium complexes, mononuclear [TplPr2 x(L)Pd-OH] (X = H, Br) and dinuclear complexes [TplPr2Pd(H20)( -0H)2 (H20)PdTplPr2] (TplPr2 = hydrotris(3,5-diisopropylpyrazolyl)borate), and some dehydrative condensations of them.278... 

Hasegawa and co-workers illustrated the syntheses of substituted phenazine-5,10-dioxides 236 by a dehydrative condensation between benzofuroxan 234 and dihydroxybenzene derivatives 235 catalyzed by molecular sieves at room temperature <00H2151>. 

Ruthenium catalysts have also been used in this context.200,201 In particular, the cationic ruthenium complex, CpRu(CH3CN)3PF6, in conjunction with carboxylic acid ligand 3, has been used to achieve the remarkably chemoselective allylation of a variety of alcohols via dehydrative condensation with allyl alcohol (Equation (50)).202 It is worth noting that this transformation proceeds with 0.05 mol% catalyst loading and does not require the use of excess allyl alcohol. 

Tar formation is thought to occur under the following conditions As the biomass feedstock is heated, it dehydrates and then volatilizes as it thermally decomposes. The volatilized material either can undergo further decomposition to form permanent gases, or it can undergo dehydration, condensation, and polymerization reactions that result in tar formation. 

Carbodiimides are, in general, useful compounds for effecting certain dehydrative condensations, e.g., in the formation of amides, esters, and anhydrides. These two crystalline water-soluble carbodiimides are especially useful in the synthesis of peptides and in the modification of proteins. The excess of reagent and the co-product (the corresponding urea) are easily separated from products with limited solubility in water. The hydrochloride is best employed in nonaqueous solvents (methylene chloride, acetonitrile, dimethylformamide). The methiodide is relatively stable in neutral aqueous systems, and thus is recommended for those media. 

Dehydrative condensation of pyrrole-2-carboxaldehyde 77 and ethyl carbazate afforded carbethoxyhydrazone 78 in quantitative yield. Cyclization of 78 in the presence of a catalytic amount of sodium hydride (10mol%) in dimethyl-formamide (DMF) at 100°C led to the formation of pyrrolo[l,2-tf][l,2,4]triazin-4-one 27 in 75% yield (Scheme 8) <1999T13703>. 

Akita and coworkers established a dehydrative condensation procedure, starting from hydroxo metal precursors containing the hydrotris(3,5-diisopropylpyrazolyl) borato ligand, and they were able to obtain dinuclear (/r-peroxo)Pd complexes, as indicated in Scheme 4. With the same procedure peroxo and hydroperoxo species (for Pd and Rh) and alkyl peroxides (for Mn, Co, Ni and Pd) complexes may be obtained. 

Confalone et al. (85) also made use of an intramolecular cycloaddition step in the construction of a range of tri- and tetracyclic products. Phenyl allyl ethers, of the type shown in Scheme 3.94, underwent dehydrative condensation with the requisite amine to furnish the intermediate ylides, which suffered cycloaddition resulting in 285 and 286 in essentially quantitative yield. The ratio of cis/trans fused products was in the range of 10 1. Such a process has been developed to construct the alkaloid (+ / ) sceletium A4 by reaction of the intermediate 287 with amine 288 via the cycloaddition protocol already developed, followed by further chemical manipulation, in an efficient five step synthesis (Scheme 3.94). 

Fragmentation and recombination reactions predominate in nearneutral, alkaline solutions. Fragments from the cleavage of the carbohydrate, and from its dehydration products, undergo further dehydration, condensation, and intermolecular, Cannizarro-type reactions. The benzilic acid rearrangement, an intramolecular Cannizarro reaction, seems to be inoperative. 

Zinc bromide, ZnBr2, used as a desiccant Zinc chloride, ZnCl2, used as a dehydrating condensing agent in org synthesis Commercial dehydration operations are best performed on solids by an apparatus that provides air to catty off water vapor on its release Some of the equipment is convertible from heat removal to heat supply by simply changing the temp level of the fluid or air (Ref 3)... 

The dehydrative condensation of 1,2-amino alcohols with C02 was found to be catalyzed also by n-Bu2SnO (10mol%) in N-methylpyrrolidone (NMP) as solvent [70b], After 16h at 453 K (C02 = 5 MPa), 2-oxazolidinones were obtained in 53-94% yields. In the solvent used (NMP), in the absence of any catalyst, even N-unsubstituted 1,2-amino alcohols gave tbe corresponding cyclic carbamate in fair yield, although the addition of -Bu2SnO further improved yields by more than 50%. 

In most of its applications the reagent acts as a dehydrating condensing agent and is often recovered as dicyclohexylurea. Although the reagent is not unduly expensive, it may be desirable to make use of the recovered urea the latter may... 

Dehydrative condensation of lf/-pyrrole-2-carbaldehyde and ethyl 1-hydrazinecarboxylate gives ethyl pyrrol-2-ylmethylidene)-l-hydrazinecarboxylate 1350 in quantitative yield (Scheme 257) <1999T13703>. A cycli-zation of the hydrazinecarboxylate 1350 by a stoichiometric amount of NaH was then examined, but a complicated mixture was obtained. It was found that pyrrolo[l,2-i7 [l,2,4]triazin-4(3/7)-one 1351 can be prepared by treatment of compound 1350 with a catalytic amount (0.1 equiv) of the base, in 75% yield. It is noteworthy that the liberated ethoxide ion which functioned as a base instead of NaH provided a catalytic system. 

This suggests that in the case that both intramolecular C- and 0-arylations are of geometrically equal possibility, the former preferentially takes place. The reaction of o-bromobenzaldehydes with dibenzylketones gives l,3-diaryl-2-naphthols that may be useful materials as bulky 0-ligands (Eq. 24) [62]. This cyclization involves palladium-catalyzed a-arylation and base-promoted dehy-drative condensation. Treatment of o-iodoanilines with ketones produces indole derivatives (Eq. 25) [63]. In this case, dehydrative condensation between the amino group and the carbonyl group occurs initially and the subsequent car-bopalladation (mechanism A) leads to the products. 

Diagenesis in oxidizing conditions results mainly in dehydration, condensation, compaction, and crystallization of amorphous iron hydroxide and silica-gel sediments. The stable mineral forms—goethite and quartz—are formed in several stages via intermediate metastable phases. 

The main reaction of PCS oxidation by heating in a nitrogen atmosphere involves the formation of Si-O-Si by a dehydration condensation reaction of Si-OH bonds (equation 3). This reaction can be deduced from the fact that the number of Si-OH bonds decreases notably compared with those of other groups. The decrease in Si-H Awlw < 6.2%) and Si-CH3 bonds at >300 °C may be explained by equations 6 and 7. At >400 C, a decrease in Si-H and Si-CH3 bonds was noted for every PCS sample. This finding suggests that Si-CH2-Si bond formation from Si-H and Si-CH3 proceeds by equation 8. 

The previous results indicate that the Si-OH groups introduced by the oxidation reaction proceed to form Si-O-Si bonds through dehydration condensation even with heating in a nitrogen atmosphere. 

Si-O-Si bond formation by dehydration condensation of Si-OH groups continues during gradual heating in a nitrogen atmosphere. PCS becomes infusible even with oxidation weight gains of 3-6%. 

The methyl derivative thus produced forms a multi-nucleus structure by the dehydration condensation. Generally, the numbers of benzene nuclei contained in resol for making phenolic foam are less than 10. The addition and condensation reactions used in synthesizing resol, and the reaction during foaming and curing are summarized as follows ... 

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