Catalysis condensation reactions

Absence of catalysis Condensation reaction between basic amino acids and sugars in the absence of catalysts results in N-rich polymers similar to humic substances in marine environments Hedges (1978)... 

Side chain reactivity is also enhanced and is typified by the difference in reactivity of 2-methylpyrazine and 2-methylpyrazine 1,4-dioxide towards anion formation and subsequent condensation reactions. 2-Methylpyrazine undergoes condensation with benzal-dehyde at 180 °C, with zinc chloride catalysis, to yield the styrylpyrazine (58), whereas the corresponding reaction of 2-methylpyrazine 1,4-dioxide proceeds at 25 °C under base catalysis (67KGS419). 

Stereoselectivity in the condensation reaction of 2-arylethylamines with carbonyl compounds to give 1,2,3,4-tetrahydroisoquinoline derivatives was somewhat dependent on whether acid catalysis or superacid catalysis was invoked. Particularly in the cases of 2-alkyl-N-benzylidene-2-phenethylamines, an enhanced stereoselectivity was observed with trifluorosulfonic acid (TFSA) as compared with the weaker acid, trifluoroacetic acid (TFA). Compound 43 was cyclized in the presence of TFA to give modest to good transicis product ratios. The analogous compound 44 was cyclized in the presence of TFSA to give slightly improved transicis product ratios. 

Tetrahydrocarbazoles have been prepared in one-flask syntheses from indoles, ketones and maleic anhydride, with acid catalysis. The reactions involve a condensation of the indole 121 with the ketone or aldehyde, followed by in situ trapping of the vinylindole 122 with maleic anhydride to afford tetrahydrocarbazoles 123 after double bond isomerization <96T4555>. 

Polycondensation pol5mers, like polyesters or polyamides, are obtained by condensation reactions of monomers, which entail elimination of small molecules (e.g. water or a hydrogen halide), usually under acid/ base catalysis conditions. Polyolefins and polyacrylates are typical polyaddition products, which can be obtained by radical, ionic and transition metal catalyzed polymerization. The process usually requires an initiator (a radical precursor, a salt, electromagnetic radiation) or a catalyst (a transition metal). Cross-linked polyaddition pol5mers have been almost exclusively used so far as catalytic supports, in academic research, with few exceptions (for examples of metal catalysts on polyamides see Ref. [95-98]). 

The addition of glyceraldehyde dimer had a significant impact on the catalysis of glucose. CrCf is the preferred catalyst and resulted in a 94% conversion of glucose with a 70% yield of HMF. In the presence of glyceraldehyde the conversion decreased to about 60% and the yield of HMF fell to 20%. This means that HMF selectivity also decreased (selectivity = yield/conversion). The loss of selectivity was primarily due to formation of heavies via intermolecular condensation reactions. The heavies were not characterized. 

Exactly the same distinction can be made over catalysis by bases as was made above for acids. Thus in specific base catalysis the reaction rate is again found to be oc pH, this time rising as the pH rises, i.e. is oc [eOH]. Thus in the reversal of an aldol condensation (cf. p. 224) it is found that,... 

In 2002 Mehnert and co-workers were the first to apply SILP-catalysis to Rh-catalysed hydroformylation [74], They described in detail the preparation of a surface modified silica gel with a covalently anchored ionic liquid fragment (Scheme 7.7). The complex N-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazole was reacted with 1-chlorobutane to give the complex l-butyl-3-(3-triethoxysilylpropyl)- 4,5-dihydroimidazolium chloride. The latter was further treated with either sodium tetrafluoroborate or sodium hexafluorophosphate in acetonitrile to introduce the desired anion. In the immobilisation step, pre-treated silica gel was refluxed with a chloroform solution of the functionalised ionic liquid to undergo a condensation reaction giving the modified support material. Treatment of the obtained monolayer of ionic liquid with additional ionic liquid resulted in a multiple layer of free ionic liquid on the support. 

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 ... 

In 1899 R. C. Guerbet discovered the self-condensation reaction of alcohols, which, via the aldehyde as an intermediate, lead to branched structures (2-alkyl alcohols) (Fig. 4.21) - the Guerbet alcohols. Starting with fatty alcohols from vegetable sources, such as octanol and decanol, the corresponding C1(, and C2o alcohols are produced (2-hexyldecanol and 2-octyldecanol, respectively). The reaction is carried out under alkali catalysis and high temperatures (>200 °C). Over the years, both products have proven to be efficient emollients, but are also used for other applications, such as plasticizers or components for lubricants (Fig. 4.21). 

Zhang, X. and Man Lai, E.S. and Martin-Aranda, R. and Yeimg, K.L. (2004). An investigation of Knoevenagel condensation reaction in microreactors using zeolite catalyst. Applied Catalysis A General, 261,109-118. 

In general, the product ratio of a mixed aldol condensation will depend upon the individual reaction rates. Most ketones show a pattern similar to butanone in reactions with aromatic aldehydes. Base catalysis favors reaction at a methyl position over a methylene group, whereas acid catalysis gives the opposite preference. 

Aldol condensation reaction may be either acid or base catalysed. However, base catalysis is more common. The product of this reaction is called an aldol, i.e. aid from aldehyde and ol from alcohol. The product is either a P-hydroxyaldehyde or P-hydroxyketone, depending on the starting material. For example, two acetaldehyde (ethanal) molecules condense together in the presence of an aqueous base (NaOH), to produce 3-hydroxybutanal (a P-hydroxyaldehyde). 

Catalysis in reaction systems with undissolved substrates and products is not restricted to biocatalysis. High yields in sobd-state synthesis, sohd-to-sohd reactions, and solvent-free systems have also been reported for aldol condensation, Baeyer-Villiger oxidation, oxidative coupling of naphthols, and condensation of amines and aldehydes [1, 2]. 

The effect of the basicity of aldol condensation catalysts on their activity was thoroughly investigated by Malinowski et al. [372—379]. The observed linear dependence of the rate coefficients of several condensation reactions on the amount of sodium hydroxide contained in silica gel (Figs. 12 and 13) supported the view that the basic properties of this type of catalyst were actually the cause of its catalytic activity, though the alkali-free catalyst was not completely inactive. The amphoteric nature of the catalysis by silica gel, which can act also as an acid catalyst, was demonstrated [380]. By a stepwise addition of sodium acetate to a HN03-pretreated silica gel catalyst the original activity for acetaldehyde self-condensation was decreased to a minimum (when an equivalent amount of the base was added) by further addition of sodium acetate, the activity increased again because of the transition to a base... 

A further demonstration of the Lewis acidity of Os(III) is the aldol condensation reaction of [Os(NH3)5( j1-acetone)]3+, to form the diacetone alcohol complex (67,90). The catalysis of this reaction can occur in one of two ways. Either Os(III) catalyzes the deprotonation of a methyl group of the bound acetone ligand to produce a nucleophile for attack at a second acetone ligand, or the Os(III) center polarizes the C=0 bond of... 

The rapid spontaneous mutarotation of glucose-6-phosphate has been shown to result from an intramolecular catalysis of the reaction by the phosphate group at carbon 6 (81). The cleavage of glucose into three carbon fragments, which is essentially a reversal of the aldol condensation reaction, requires the ketohexose as substrate. The necessary isomerization reaction to form the ketohexose then uses the open-chain form intermediate of the mutarotation reaction. Salas et al. (80) have speculated that the enhanced mutarotation of glucose-6-phosphate may thus have been the key requirement which led to the evolution of the phosphorolytic pathway for glucose metabolism. 

The condensation reactions of silanols are catalyzed by acids [19, 25-27,63—68, 72], Grubb measured the hydrogen chloride catalyzed silanoi condensation reaction of trimethylsilanol in methanol [19]. Lasocki and Michalska studied the effect of acid type on the condensation of dialkylsilanediols in dioxane [68]. Under anhydrous conditions, the rate of acid catalysis by strong acids (such as hydrogen bromide and perchloric acid) was directly related to the acid concentrations. The catalytic effects of weaker acids, such as hydrogen chloride, were not linearly related to the concentration. They postulated that in anhydrous dioxane, the strong acids were completely ionized while the weaker acids were not [68]. When small amounts of water were added to the solvent, all the acids behaved in a similar manner. Lasocki [64-67] extended the studies to examine the effects of alkyl or aryl substitution of silanediols on the condensation rate in aqueous dioxane [64-67]. The rate constants for acid catalyzed condensation of... 

Keywords cyanoacetamide, malodinitril, methyl cyanoacetate, aromatic aldehyde, Knoevenagel condensation, solid-solid reaction, base catalysis, melt reaction, uncatalyzed, cinnamamide, cinnamonitril, methyl cinnamate... 

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