Esters of Substituted Aliphatic Acids

Cellulose esters of halogenated acids are exceptionally difficult to prepare. This is particularly true if the halogen is in the alpha position to the carboxyl group. Chloroacetic anhydride in the presence of acid catalysts esterifies cellulose only after severe degradation. The use of pyridine is prohibited because of side reactions with the reagent. Mixed 

Alkoxy fatty acid anhydrides show a behavior similar to that of the halogenated anhydrides and do not esterify cellulose in the presence of acid catalysts. In the presence of pyridine, however, methoxyacetic and ethoxyacetic anhydrides or their acid chlorides give soluble cellulose derivatives.  

Investigations of the cellulose esters of benzoic acid have so far not led to products of commercial interest. Benzoyl chloride reacts in the presence of pyridine at elevated temperatures, most suitably by the use of a higher boiling neutral solvent as diluent. Benzoyl derivatives have also been prepared by the reaction of the acid chloride on alkali cellulose. A combination of pyridine and alkali has been reported to be advantageous.  

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The factor 2.48 puts a on the same scale as Hammett s er, and the k0 values are rate constants for acid and base hydrolysis of acetic acid esters (i.e., R is a methyl group in the reference compound). Usually R is an ethyl or methyl group, but in many cases the rate constants do not depend on the nature of R. Equation 8 is based on the fact that acid hydrolysis rates of substituted benzoic acid esters are only slightly affected by the nature of the substituent, but acid hydrolysis rates of aliphatic esters are strongly affected by substituents. These effects were taken to be caused by steric factors thus log(/c//c0)acid defines s. It is reasonable to assume that steric factors affect base-catalyzed rates in the same way. Substituent effects on base hydrolysis of aliphatic compounds are composed of both polar and steric effects, and subtraction of the latter yields a measure of the former. The parameter a is important because it allows one to evaluate substituent effects on aliphatic reaction rates by a formula analogous to the Hammett equation, or by a bivariate relationship, the Taft-Pavelich equation (Pavelich and Taft, 1957) ... 

As the proton release is often too slow under the acidic conditions used for the diazotization of aromatic amines, syntheses of aliphatic diazo compounds by this method are carried out without an excess of mineral acid. Usually, equimolar amounts of amine, HCl and NaN02, or amine and NOCl, are used. A better alternative is nitrosation with pentyl nitrite in the presence of up to 30% acetic acid, as found by Takamura et al. (1975). Yields higher than 60% were obtained with a-amino-substituted esters of some aliphatic carboxylic acids. 

The acylation of cellulose with acid chlorides in DMA/LiCl is most suitable for the homogeneous synthesis of readily soluble partially functionalized long-chain aliphatic esters and substituted acetic acid esters (Table 16.3). In contrast to the anhydrides, the fatty acid chlorides are soluble in the reaction mixture and soluble polysaccharide esters may be formed with a very high efficiency. Even in the case of stearoyl chloride, 79 per cent of the reagent is consumed for the esterification of cellulose. 

The occurrence of steric as well as polar substituent effects in aliphatic systems and orf/io-substituted aromatic systems complicates the devising of correlation equations. A typical situation is shown in Figure 2. There is clearly no simple relationship between the rates of alkaline hydrolysis of the ethyl esters of the alkanoic acids and the strengths of the acids themselves. Little progress was made until the early 1950s, when Taft made an excellent start in developing linear free energy relationships in this area. This topic is sometimes referred to as the separation of polar, steric, and resonance effects and is the subject of Section 3. 

The Pictet-Spengler reaction has mainly been investigated as a potential source of polycyclic heterocycles for combinatorial apphcations or in natural product synthesis [149]. Tryptophan or differently substituted tryptamines are the preferred substrates in a cyclocondensation that involves also aldehydes or activated ketones in the presence of an acid catalyst. Several versions of microwave-assisted Pictet-Spengler reactions have been reported in the hter-ature. Microwave irradiation allowed the use of mild Lewis acid catalysts such as Sc(OTf)3 in the reaction of tryptophan methyl esters 234 with different substituted aldehydes (aliphatic or aromatic) [150]. Under these conditions the reaction was carried out in a one-pot process without initial formation of the imine (Scheme 86). 

Polar-substituted alkenes where the functionality is not attached to a strained ring are considerably more discriminating in their compatibility with metathesis catalysts and as a rule require relatively high catalyst charges. In the aliphatic series, unsaturated esters have received the most attention. Boelhouwer reported in 1972 the metathesis of the ester methyl oleate and its trans isomer, methyl elaidate, with a homogeneous catalyst based on a 1/1.4 molar combination of WCl6/(CH3)4Sn (23). At 70°C and an ester/W molar ratio of 33, near-thermodynamic equilibrium was attained, and 49 and 52% of the respective esters were converted to equal amounts of 9-octadecene and the dimethyl ester of 9-octadecene-1,18-dioic acid. 

Solvents can be classified into three categories according to their polarity namely, polar protic, dipolar aprotic and non-polar. Most of the common solvents fall under one of following chemical classes Aliphatic hydrocarbons, aromatic hydrocarbons, alcohols, phenols, ethers, aldehydes, ketones, carboxylic acids, esters, halogen-substituted hydrocarbons, amines, nitriles, nitro-derivatives, amides and sulfur-containing solvents (Marcus, 1998). In certain cases a mixture of two or more solvents would perform better than a single solvent. 

Esters of aromatic (instead of aliphatic) acids react rapidly at room temperature (701). but the addition of a condensing agent is still desirable when 1-substituted biguanides are used. 

Compound 211 and several related compounds are readily accessible by stereospecific deprotonation of the appropriate optically active carbamic esters with 5-BuLi/TMEDA ° . Much of the knowledge about the stereochemical course of substitution in benzyUithium derivatives was obtained from experiments with these compounds. Only the reaction with proton acids, aliphatic aldehydes, ketones or esters as electrophiles proceed with retention for alkyl, silyl and stannyl halides, acid chlorides. 

In effect this equation defines an acidity function, Hs = mH0, for the ionization of protonated acetate esters. It is an experimental observation that Hs is the same linear function of H0 for several of the esters concerned, so that the approximations involved are greatly reduced from those implicit in the use of // itself, while the treatment remains more general than the experimentally unattainable ideal of measuring the acidity function directly for every substrate. In practice the excellent results obtained with aliphatic esters are in contrast to the slightly less clear-cut picture for aryl esters (see below) and suggest that H describes the protonation behaviour of substituted phenyl acetates less than perfectly. However, there is little doubt that Hs will be a linear function of H0 for any given ester, since this appears to be the case for the acidity functions defined for a wide variety of neutral substrates. Yates and McClelland37 show that all available acidity functions measured in sulphuric... 

Carbamates are substituted esters of carbamic acid (NH2COOH) with aliphatic or aromatic substituents on the oxygen and nitrogen atoms. Carbamate insecticides have an aryl or oxime N-methylcarbamate structure, and their mode of action is based on the inhibition of the enzyme acethylcholine esterase (1). However, this inhibition is reversible, and recovery from sublethal doses occurs rapidly. Some carbamate fungicides have a dithio, bisdithio, or benzimidazole carbamate basic structure, and dithiocarbamate fungicides inhibit the enzyme aldehyde deshydro-genase (2). The herbicides have an /V-alkylthiocarbamate or A-phenylcarbamate structure and interfere with photosynthetic activity or affect meristematic activity or lipid metabolism (3). Representative structures of carbamate pesticides are shown in Fig. 1. 

Fries rearrangement—that is, the transformation of phenolic esters to isomeric hydroxyphenyl ketones—is related to Friedel-Crafts acylations.392,393 Olah et al.394 have found a convenient way to perform the Fries rearrangement of a variety of substituted phenolic esters in the presence of Nafion-H in nitrobenzene as solvent [Eq. (5.153)]. A catalytic amount of Nafion-H is satisfactory, and the catalyst can be recycled. In contrast, Nafion-silica nanocomposites, in general, exhibit low activities in the Fries rearrangement of phenyl acetate to yield isomeric hydroxyacetophe-nones.239,395 In a recent study, BF3-H20 was found to be highly efficient under mild conditions (80°C, 1 h) to transform phenolic esters of aliphatic and aromatic carboxylic acids to ketones (71-99% yields).396 In most cases the para-hydroxyphenyl isomers are formed with high (up to 94%) selectivity. 

Gallotannins are hydrolysable tannins with a polyol core (referring to a compound with multiple hydroxyl groups) substituted with 10-12 gallic acid residues. Gallotannins contain the characteristic raeto-depside bonds (1.91) between gallic acid residues. This bond is more labile than an aliphatic ester bond, and can be methanolyzed with a weak acid in methanol. In contrast, methanolysis of an aliphatic ester bond requires methanol with a strong mineral acid and heat. 

Because the electronic nature of substituents has little effect on the rate of acid-catalyzed hydrolysis of meta- or para-substituted benzoates (e.g., p for the acid hydrolysis of XC6H4COOR esters is close to zero), Taft suggested that the electronic nature of substituents will also have little effect on acid-catalyzed hydrolysis of aliphatic esters (Lowry and Richardson, 1987). Nevertheless, a strong electronic effect occurs in basic hydrolysis, as can be examined from the large p values for base-catalyzed hydrolysis of meta- or para-substituented benzoates. Hence, the effect of X on acid hydrolysis is purely steric but is a combination of steric and electronic effects in basic hydrolysis. Taft defined Es, a steric substituent constant, by Equation (5.11) ... 

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