Condensation reactions, direction

Two other important sol-gel parameters are temperature and solvent. Both hot and cold plates are commercially available and can be used to increase and decrease the reaction rates, respectively. Varying the temperature is most effective when it can alter the relative rates of competing reactions. Solvent can change the nature of an alkoxide through solvent exchange or affect the condensation reaction directly. It is also possible to prepare a gel without a solvent as long as another means, such as ultrasound irradiation [10] (see Section A.8.6), is used to homogenize an otherwise immiscible alkoxide/water mixture. 

Another use of imines is in a directed aldol condensation reaction, directing the condensation in a way that the aldehyde, after reacting with the base, will react with the other carbonyl compound, with no self-condensation. Imines, in contrast to their precursors, aldehydes, are too weak electrophiles to undergo self-condensation. However, after lithiation with LDA, the lithium salts 19 are sufficiently good nucleophiles to react... 

Activation of sol-gel precursors is needed when they are chemically inert and cannot participate in condensation reactions directly. It most often involves hydrolysis of solvated metal ions in aqueous solutions or metal aUcoxides in... 

There are two great families of synthetic polymers, those made by addition methods (notably, polyethylene and other polyolefines), in which successive monomers simply become attached to a long chain, and those made by condensation reactions (polyesters, polyamides, etc.) in which a monomer becomes attached to the end of a chain with the generation of a small by-product molecule, such as water. The first sustained programme of research directed specifically to finding new synthetic macromolecules involved mostly condensation reactions and was master-... 

Model studies directed toward the synthesis of Ecteinascidin 743 employed an elegant Pictet-Spengler cyclization of phenethylamine 54 and the 1,2-dicarbonyl compound 55 to assemble the spiro tetrahydroisoquinoline 56 in a stereospecific fashion. " The silica-catalyzed condensation reaction provided 56 in excellent yield. 

Co-condensation reaction of the vapors of l,3-di-rcrt-butylimidazol-2-ylidene and nickel, palladium, or platinum gives the coordinatively unsaturated 14-electron sandwiches [L M] (M=Ni, Pd, Pt) of the carbene type (990M3228). Palladium(O) carbene complexes can also be prepared by the direct interaction of l,3-R2-imidazol-2-ylidenes (R=/-Pr, r-Bu, Cy, Mes) (L) with the palladium(O) compound [Pd(P(o-Tol)3)2] (OOJOM(595)186), and the product at the first stage is [(L)PdP(o-Tol)3l, and then in excess free carbene [PdL ]. 

Step 4 of Figure 29.3 Chain Cleavage Acetyl CoA is split off from the chain in the final step of /3-oxidation, leaving an acyl CoA that is two carbon atoms shorter than the original. The reaction is catalyzed by /3-ketoacyl-CoA thiolase and is mechanistically the reverse of a Claisen condensation reaction (Section 23.7). In the forward direction, a Claisen condensation joins two esters together to form a /3-keto ester product. In the reverse direction, a retro-Claisen reaction splits a /3-keto ester (or /3-keto thioester) apart to form two esters (or two thioesters). 

Note Several condensing agents for the direct poly condensation reaction such as diphenyl chlorophosphate-LiBr and tosyl chloride-DMF have also been developed by Higashi et al.311,312 (see Section 2.3.3.2). 

Either particulate sol or polymeric sol has been used for thin film coatings. The polymeric sol was fabricated by partial hydrolysis of corresponding metal alkoxide. If the rate of hydrolysis or condensation is very fast, then some kinds of organic acids, beta-dicarbonyls, and alkanolamines have been used as chelating agent in sol-gel processes to control the extent and direction of the hydrolysis-condensation reaction by forming a strong complex with alkoxide. [2]. 

Representative condensation polymers are listed in Table I. The list is by no means exhaustive, but it serves to indicate the variety of condensation reactions which may be employed in the synthesis of polymers. Cellulose and proteins, although their syntheses have not been accomplished by condensation polymerization in the laboratory, nevertheless are included within the definition of condensation polymers on the ground that they can be degraded, hydrolytically, to monomers differing from the structural units by the addition of the elements of a molecule of water. This is denoted by the direction of the arrows in the table, indicating depolymerization. 

Amidation is particularly well adapted to use as a polymer-forming condensation reaction. The reaction is rapid above 180° to 200°C, it is remarkably free from side reactions, no catalysts are required (indeed, none are known), and the process is of the second order so that the molecular weight increases directly as the time of reaction. Molecular weights of 20,000 to 30,000 are attainable with no great difficulty under favorable conditions. This is not true of particular polyamide reactants susceptible to side reactions, as, for example, in the reaction of a diamine with glutaric acid wherein the inherent instability of the glutaric amide unit leads to decomposition. 

Direct amide formation in aqueous solution between carboxylic acids and amines can occur and the rates are first order in the anion of the acid and the basic form of the amine (Eq. 9.12).23 The second-order rate constant is independent of the acidity of the medium. The condensation reaction of glycine to form di- and triglycine occurs in aqueous solution... 

We have intentionally selected example reactions (Figs. 29-33) that would not usually be immediately obvious to a chemist. The examples chosen have all been concerned with rearrangements of various types, since their courses are frequently difficult to predict. It remains to emphasize that the reactivity functions contained in EROS perform perfectly well with other types of reaction. This is true, for example, with reactions that a chemist could derive directly from an analysis of the functional groups in a molecule. Thus, EROS predicts addition reactions to carbonyl compounds, nucleophilic substitutions, and condensation reactions, to name just a few examples. In all these reaction types, the possibility of assigning a quantitative estimate to the reactivity at the various sites via the reactivity functions is of particular merit. It... 

Carbonyl alkylation and condensation reactions are always of great value in synthesis, and the formation of o-ANIS ALDEHYDE via 4,4-dimethyl-2-oxazoline, 2,2-DIMETHYL-3-PHENYLPROPION-ALDEHYDE via alkylation of the magnesio-enamine salt and threo-4-HYDROXY-3-PHENYL-2-HEPTANONE via a directed aldol... 

The observations suggest that the two condensations are direct and distinct. Had the base-catalysed reaction first given the aryl-dihydro-triazine (CXXXIV) which, under the prevailing basic conditions rearranged to the anilino-dihydro-triazine (CXXXV), an equilibrium mixture of the two would have been expected. Since, however, the pure anilino-dihydrotriazine (CXXXV) was isolated, its formation is considered to be direct and independent of that of its isomer (ffS). The formation of (CXXXIII) from 1-aryl-l-alkylbiguanides (see preceding Section), which cannot proceed by way of an aryl-dihydrotriazine, bears out this conclusion. 

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