Catalytic condensation reaction

Edeleanu Dimethyl ether Methanol Catalytic condensation reaction with 99.9% purity 2 NA... 

Dextrose and sorbitol undergo a catalytic condensation reaction with an acid. Further purification may be performed to... 

Aniline 77 was converted into its diazonium salt with nitrous acid and this was followed by reduction with stannous chloride to afford the corresponding arylhydrazine 78. Condensation of 78 with 3-cyanopropanal dimethylacetal 79 gave the arylhydrazone 80. Treatment of 80 with PPE resulted in cyclization to indole 81. The nitrile group was then reduced to the primary amine by catalytic hydrogenation. Reaction of the amine with excess formalin and sodium borohydride resulted in Imitrex (82). 

On the other hand, carbonyl condensation reactions require only a catalytic amount of a relatively weak base rather than a full equivalent so that a small amount of enolate ion is generated in the presence of unreacted carbonyl compound. Once a condensation has occurred, the basic catalyst is regenerated. To carry out an aldol reaction on propanal, for instance, we might dissolve the aldehyde in methanol, add 0.05 equivalent of sodium methoxide, and then warm the mixture to give the aldol product. 

The first step in sol-gel processing is the catalytic hydrolysis of TEOS and the second step is the polycondensation of SiOH moieties framing into silica (Scheme 3.1). In the first step of the reaction, water is present as a reactant while it is the by-product in the second step. It is likely that the molar ratio of TEOS/H2O would influence the sol-gel chemistry and hence the end properties of the resultant hybrids. The most interesting part of the sol-gel chemistry is that the catalytic hydrolysis of TEOS is an ion-controlled reaction, while polymerization of silica is not. Usually, the ionic reactions are much faster than the condensation reactions. The stoichiometric equation showing the silica formation from TEOS is presented in Scheme 3.3. 

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 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. 

The condensation reactions described above are unique in yet another sense. The conversion of an amine, a basic residue, to a neutral imide occurs with the simultaneous creation of a carboxylic acid nearby. In one synthetic event, an amine acts as the template and is converted into a structure that is the complement of an amine in size, shape and functionality. In this manner the triacid 15 shows high selectivity toward the parent triamine in binding experiments. Complementarity in binding is self-evident. Cyclodextrins for example, provide a hydrophobic inner surface complementary to structures such as benzenes, adamantanes and ferrocenes having appropriate shapes and sizes 12) (cf. 1). Complementary functionality has been harder to arrange in macrocycles the lone pairs of the oxygens of crown ethers and the 7t-surfaces of the cyclo-phanes are relatively inert13). Catalytically useful functionality such as carboxylic acids and their derivatives are available for the first time within these new molecular clefts. 

Freeder, B. G. et al., J. Loss Prev. Process Ind., 1988, 1, 164-168 Accidental contamination of a 90 kg cylinder of ethylene oxide with a little sodium hydroxide solution led to explosive failure of the cylinder over 8 hours later [1], Based on later studies of the kinetics and heat release of the poly condensation reaction, it was estimated that after 8 hours and 1 min, some 12.7% of the oxide had condensed with an increase in temperature from 20 to 100°C. At this point the heat release rate was calculated to be 2.1 MJ/min, and 100 s later the temperature and heat release rate would be 160° and 1.67 MJ/s respectively, with 28% condensation. Complete reaction would have been attained some 16 s later at a temperature of 700°C [2], Precautions designed to prevent explosive polymerisation of ethylene oxide are discussed, including rigid exclusion of acids covalent halides, such as aluminium chloride, iron(III) chloride, tin(IV) chloride basic materials like alkali hydroxides, ammonia, amines, metallic potassium and catalytically active solids such as aluminium oxide, iron oxide, or rust [1] A comparative study of the runaway exothermic polymerisation of ethylene oxide and of propylene oxide by 10 wt% of solutions of sodium hydroxide of various concentrations has been done using ARC. Results below show onset temperatures/corrected adiabatic exotherm/maximum pressure attained and heat of polymerisation for the least (0.125 M) and most (1 M) concentrated alkali solutions used as catalysts. 

Scheme 10.30. Domino Knoevenagel condensation/catalytic hydrogenation reaction in ionic liquid.

Capacity to utilise catalytic Ni2+, Fe2+, W(Mo) and Mg2+ in cell metabolism (see Table 5.2). All primitive cells had to reduce CO (C02) before engaging in condensation reactions. We suppose nitrogen was available as NH3 for protein synthesis and HCN for the synthesis of nucleotide bases. 

The condensation reaction of neat carbonyl compounds with nitroalkanes to afford nitroalkenes, Henry reaction, also proceeds rapidly via this MW approach in the presence of only catalytic amounts of ammonium acetate, thus avoiding the use of a large excess of polluting nitrohydrocarbons normally employed (Scheme 6.22) [72],... 

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 ... 

Each CHS monomer consists of two structural domains (Fig. 12.5, left). The upper domain exhibits the a-p-a-p-a pseudo-symmetric motif observed in fatty acid P-ketoacyl synthases (KASs) (Fig. 12.5, right).20 Both CHS and KAS use a cysteine as a nucleophile in the condensation reaction, and shuttle reaction intermediates via CoA thioester-linked molecules or ACPs, respectively. The conserved architecture of the upper domain maintains the three-dimensional position of the catalytic residues of each enzyme Cysl64, His303, and Asn336 in CHS correspond to a Cys, His, and His in KAS I and II. 

Although condensation reactions nsnally resnlt in achiral products they represent important additional reactivity of the active imininm ion which must be considered. Design of condensation reactions into cascade processes will provide further intriguing catalytic seqnences. 

In another example, a catalytic amount of a Bronsted acid such as H3PO4 was dissolved in [NRR 3][NTf2] (R = hexyl, R = butyl) 119). The catalyst was applied for the condensation reaction of alcohols, which usually requires strongly acidic media and dehydrating conditions. The condensation of veratryl alcohol was facilitated because the water that formed was continuously removed as vapor, which assisted in driving the reaction to high yields. The product (cyclotriveratrylene) separation, however, required the addition of a co-solvent. 

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