Dehydration reactions anhydrides

On the other hand, when the pure anhydride (XXXIV) was heated on a steam bath, without hydrochloric acid, analyses with periodic acid at room temperature showed that the reaction is reversible and that an equilibrium is reached when the solution contains approximately 92 % of compound XXXIV. The investigators deduce from their experimental data that the dehydration reaction is reversible and pseudo-monomolecular. The data fit the following equation. 

It is well documented that the isoimide is the kinetically favoured product and that isomerization yields the thermodynamically stable imide when sodium acetate is used as the catalyst. High catalyst concentrations provide maleimides with low isoimide impurity. The mechanism by which the chemical imidization is thought to occur is shown in Fig. 3. The first step in the dehydration reaction may be formation of the acetic acid-maleamic acid mixed anhydride. This species could lose acetic acid in one of the two ways. Path A involves participation by the neighboring amide carbonyl oxygen to eject acetate ion with simultaneous or subsequent loss of proton on nitrogen to form the isoimide. Path B involves loss of acetate ion assisted by the attack of nitrogen with simultaneous or subsequent loss of the proton on nitrogen to form the imide. If the cyclodehydration is run in acetic anhydride in the absence of the base catalyst, isoimide is the main reaction product. 

An unusual dehydration reaction was observed in the case of sulfoxide 290, which yielded 99% of 2//-thiopyran 291 on heating with acetic anhydride.302... 

Acid anhydrides, as their name implies, are formed from the dehydration reaction of two carboxylic acid groups (Fig. 72). Anhydrides are highly reactive toward nucleophiles and are able to acylate a number of the important functional groups of proteins and other macromolecules. Upon nucleophilic attack, the anhydride yields one carboxylic acid for every acylated product. If the anhydride was formed from monocar-boxylic acids, such as acetic anhydride, then the acylation occurs with release of one carboxylate group. However for dicarboxylic acid anhydrides, such as succinic anhydride, upon reaction with a nucleophile the ring structure of the anhydride opens, forming the acylated product modified to contain a newly formed carboxylate group. 

The set of catalysts selected for the dehydration of 2-butanol was also tested for the Friedel-Crafts acylation of anisole [69, 70]. The catalytic test was performed in the liquid phase due to the high boiling points of the reactants and products of this reaction. Anisole was reacted with acetic anhydride at 120 °C in the absence of solvent. In principle, acylation can occur on both the ortho and para positions of anisole. The main product (>99%) over all catalysts in this study was para-methoxyacetophenone, indicating that the reaction predominantly takes place inside the zeolite micropores. The same trend in catalytic activity as in the 2-buta-nol dehydration reaction is observed the conversion of anisole into para-nicihoxy-acetophenone increases upon increasing Ge content of the catalyst (Fig. 9.17) [67]. The main cause of deactivation for this reaction is accumulation of the reaction products inside the micropores of the zeolite. The different behavior of Ge-ZSM-5, compared with ZSM-5, may therefore be due to improved diffusional properties of the former, as the presence of additional meso- and macropores allows for... 

The meehanism of the conjugate addition is the same as that in the previous example and the meehanism for ester hydrolysis was covered in Chapter 12, The key step in the dehydration reaction is the formation and eyclization of the mixed anhydride formed from the diacid and acetic anhydride, Both steps have the same mechanism, attack of an acid on an anhydride, but the second step is intramolecular. Like most eyelizations the reaction is entropically favoured as two molecules react to give three—the cyclic anhydride and two molecules of acetic acid... 

Carboxylic acids cannot be readily converted to anhydrides, but dicarboxylic acids can be converted to cyclic anhydrides by heating to high temperatures. This is a dehydration reaction because a water molecule is lost from the diacid. 

Perkin reaction Anhydride II II 0 0 Aromatic aldehyde usually follows Dehydration... 

Then the hydroxyl at C-5 is protected by means of acetylation carried out with acetic anhydride in pyridine affording conq>ound 13g. The analysis of the H-NMR spectrum of acetyl derivative 13g allows to claim surely the regiospecificity of the dehydration reaction in fact in the acetylation reaction fi om 13f to 13g, it is showed the usual deshielding of about 1 ppm of proton whose chemical shift (6 5.15) is, of course, attributable to a not allyhc position. 

Two other dehydration reactions, reversion and anhydride formation, may possibly complicate the transformations in acidic solutions. However, they seem to be generally unimportant. 

Polymer-supported triphenylphosphine ditriflate (37) has been prepared by treatment of polymer bound (polystyrene-2% divinylbenzene copolymer resin) triphenylphosphine oxide (36) with triflic anhydride in dichloromethane, the structure being confirmed by gel-phase 31P NMR [54, 55] (Scheme 7.12). This reagent is effective in various dehydration reactions such as ester (from primary and secondary alcohols) and amide formation in the presence of diisopropylethylamine as base, the polymer-supported triphenylphosphine oxide being recovered after the coupling reaction and reused. Interestingly, with amide formation, the reactive acyloxyphosphonium salt was preformed by addition of the carboxylic acid to 37 prior to addition of the corresponding amine. This order of addition ensured that the amine did not react competitively with 37 to form the unreactive polymer-sup-ported aminophosphonium triflate. 

Dehydration reactions using the tertiary phosphine-carbon tetrachloride adduct have appeared quite regularly in the literature again this year. Among those reported have been the dehydrations of oximes to nitriles, carboxylic acids to anhydrides, and the amides (37) to the cumulenes (38). Further reaction of the dehydration product from treatment of the... 

Binary mbctures of acetic, propionic and butyric acid are converted in the vapor phase over zeolite H-T. From hydroxyl stretching vibration. spectra of zeolite H-T with adsorbed butyric acid, it is concluded that the carboxylic acids have access to the different proton locations in this zeolite. The acids undergo ketonization reactions inside the erionite cavities of the zeolite, and dehydration reactions into anhydrides on the outer surface of the zeolite crystal. The ketonization activity and selectivity is rationalized by transition-state shape-selectivity in erionite cages. Zeolite H-T is particularly suitable for converting an equimolar mixture of propionic and butyric acid into 3-hexanone. 

These compounds contain the characteristic ether linkage C-O-C, and can be thought of either as alcohols in which the hydrogen atom of the hydroxyl group is replaced by an alkyl group, or as alcohol anhydrides . Diethyl ether and some other ethers are made by a dehydration reaction ... 

In situ trapping of isocyanate. The mild conditions of the "dehydration reaction using o-sulfobenzoic acid anhydride provided an opportunity to look at generating isocyanates in situ followed by immediate conversion of the isocyanate into urethane materials. This method may prove to be valuable for the generation of materials based on toxic, volatile isocyanates (i.e. methyl isocyanate). An example is shown below in Scheme V(/5) which shows the formation of 1-naphthyl N-methylcaibamate (common insecticide). 

Some reviews on the preparation of cyanides from aldoximes [1045, 1046] and from carboxamides [1047,1048] are available. Often used dehydration reagents are acetic anhydride for aldoximes and phosphorus pentoxide for carboxamides. In the following sections, dehydration reactions affording cyanides are described with various dehydration reagents, classified into acidic and basic reagents. 

Acidic reagents seem to offer milder conditions. Dehydration reactions forming cyanides can be performed with phosgene [1049-1052], diphosgene [1053-1055], triphosgene [1056], phenyl chloroformate [1057], oxalyl chloride [1058, 1059], tri-chloroacetyl chloride [1060-1062], acetic anhydride [1063-1074], TFAA [1075-1082], phosphorus oxides [1083-1088], phosphorus oxychloride [1089-1098], phosphorus pentachloride [1099], triphenylphosphine/haloalkanes [1100-1103], thionyl chloride [1104-1118], p-tosyl chloride [1119-1124], triflic anhydride [1125-1127], chlorosulfonyl isocyanate [1128], the Burgess reagent [1129], phenyl chloro-thionoformate [1130], cyanuric chloride [1131-1134], carbodiimides [1135, 1136], CDC [1137], PyBOP [1138], AlCU/Nal [1139], and acetonitrile/aldehyde [1140], and by pyrolysis [1141]. 

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