Carboxylic esters, exchange with

Carboxylic acids exchange with amides and esters these methods are sometimes used to prepare anhydrides if the equilibrium can be shifted, for example. 

Poly(vinyl acetate), PVAc, of molecular weights (M ) 6.1x1 01, 1.9xl05, and 6.0xl05, was randomly labeled by ester exchange with 2,2,5,5-tetramethyl-3-pyrrolin-l-oxyl-3-carboxylic acid to give a spin labeled polymer containing typically 1-10 nitroxides per polymer molecule. Polystyrene was prepared by emulsion... 

The thiocyano group can be introduced into a phosphonic acid through the displacement of a tosyloxy group (generated from a hydroxyalkyl acid) with KCNS alternatively, and for substrates of relatively high acidity, for example, triesters of phosphonoacetic acid, a reaction between CISCN and a phosphonocarbanion is feasible, but the common use of Al(OPr )3 as a catalyst. sometimes suffers from the drawback of (carboxylic) ester exchange. ... 

These reactions are most important for the preparation of acyl fluorides. " Acyl chlorides and anhydrides can be converted to acyl fluorides by treatment with polyhydrogen fluoride-pyridine solution" or with liquid HF at — 10°C. Formyl fluoride, which is a stable compound, was prepared by the latter procedure from the mixed anhydride of formic and acetic acids. Acyl fluorides can also be obtained by reaction of acyl chlorides with KF in acetic acid or with DAST. Carboxylic esters and anhydrides can be converted to acyl halides other than fluorides by the inorganic acid halides mentioned in 10-77, as well as with PhsPXa (X = Cl or but this is seldom done. Halide exchange can be carried out in a... 

The occurrence of isotopic exchange of between water and carbonyl compounds has been observed to take place slowly with acetone (Cohn and Urey, 1938) and much more rapidly with acetaldehyde (Herbert and Lauder, 1938). This gives qualitative evidence for reversible hydration (since no other reasonable mechanism exists for isotopic exchange), but gives no quantitative information about the equilibrium position. Similarly, the fact that exchange occurs in the unhydrolysed ester during the hydrolysis of carboxylic esters (Bender, 1951) shows that the species RC(0H)20R is a stable intermediate rather than a transition state. 

Carboxylic acids can also be protected as ortho esters. Ortho esters derived from simple alcohols are very easily hydrolyzed, and a more useful ortho ester protecting group is the 4-methyl-2,6,7-trioxabicyclo[2.2.2]octane structure. These bicyclic orthoesters can be prepared by exchange with other ortho esters, by reaction with iminoethers, or by rearrangement of the ester derived from 3-hydroxymethyl-3-methyloxetane. 

Carboxylic acids generated in acylation reactions have been effectively sequestered by ion pair formation with Amberlite IRA-68 ion exchange resin.29 3 Similarly, 4-nitrophenoxide formed in acylations of 4-nitrophenyl esters has been captured by anion exchange with the quaternary ammonium hydroxide resin 12.31 An exception to ion pair-mediated sequestration of byproducts is the polyamine functionalized resin 3 that was shown to covalently sequester the (3-methoxy enone hydrolysis byproduct from Danishefsky s diene via imine formation.23... 

It is generally assumed that the ionized carboxyl group R COO- is not susceptible to nucleophilic attack, and essentially all the available evidence supports this assumption. A solitary exception is discussed below (p. 160). Thus ester formation with acyl-oxygen cleavage, and oxygen exchange with the solvent, are not expected under alkaline conditions. There is no reason, however, why they should not occur in neutral solution, where a significant proportion of the undissociated acid (R COOH) is present, and both reactions are well established under these conditions. 

It has been shown by ]H NMR that protonation of 2-amino-4,5-dimethylthiophene-3-carboxylic ester occurs at position 5 to give (385). The methyl group located at that carbon is seen as a doublet (68TL4605). Similarly, in 2,4-diaminothiophenes of the type (386), there is a rapid exchange of the 5-proton with D20 (70JHC285). 

As is to be expected, an alkynic ketone undergoes a Michael addition with a carbanion, leading eventually to a pyranone (50JA1022). Using malonic esters, a 3-alkoxycarbonyl derivative results, which is hydrolyzed to the 2-oxopyran-3-carboxylic acid under alkaline conditions, but to the pyranone by sulfuric acid. Rapid ester exchange is observed with the initial products, the alcohol used as solvent determining the nature of the alkyl group in the 3-carboxylic esters (Scheme 90). 

The acyl group (—COOR) in esters and amides is determined by hydrolysis in alcoholic sodium hydroxide solution, followed by ion exchange with an acidic resin. The carboxylic acid formed is then titrated with standard sodium hydroxide, as in (1). The reactions are... 

In the NMR experiments carried out by Wenthe and Cordes [187] with methyl orthobenzoate and methyl orthocarbonate in CD3OD—D20 solutions, the rate coefficients for the disappearance of orthoester and those for the formation of CH3OD and of carboxylic ester have been found identical within experimental error (Table 15). This indicates that there is no exchange of methoxy groups prior to hydrolysis. The same result has been obtained from product analysis studies of the carboxylic esters formed. Consequently, the rate-determining step must be carbonium ion formation or a previous step. The findings do not support an A2 mechanism, for the following reason. As the nucleophilic reactivities of water and methanol are similar, the A2 reaction with attack of water... 

The reactions of other important Lewis bases such as amines and silanes, have not been studied as extensively as that of esters. It is known that both the substituted piperidines and the silanes form much weaker complexes with the Al-alkyls than the carboxylic esters do 73,74). Other reactions may, however, take place. In fact, the piperidines are able to interact with Al-alkyls at moderately high temperatures (90— 120 °C) giving rise to Al-piperidides 7S), while the silanes can exchange their —OR, —NR2, or other substituents with the alkyl groups of Al-alkyls even at room temperature 76). Therefore, the cocatalyst must be considered also in these cases as a rather complex mixture of components, each of which is potentially endowed with its own catalytic effect. 

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