Halides and Acids

Considerably more attention has been paid to transition-metal halide intercalates than to those of halides of Main-Group elements, compounds of TiF4,  

Chun-Hsu, H. Selig, M. Rabinowitz, I. Agranat, and S. Sarig, Inorg. Nuclear Chem. Letters, 1975, 

and AuCla being studied. Interca- 

The use of graphite bisulphate, C24,HS04,2H2S04, as an esterification reagent in organic synthesis has been considered.It is very efficient for the production of formates and acetates, reaction times being less than one hour esterification of other acids is slower, but in most cases the yields are very high. 

---

Closely related to the carboxylic acids and nitriles discussed in the previous chapter are the carboxylic acid derivatives, compounds in which an acyl group is bonded to an electronegative atom or substituent that can net as a leaving group in a substitution reaction. Many kinds of acid derivatives are known, but we ll be concerned primarily with four of the more common ones acid halides, acid anhydrides, esters, and amides. Esters and amides are common in both laboratory and biological chemistry, while acid halides and acid anhydrides are used only in the laboratory. Thioesters and acyl phosphates are encountered primarily in biological chemistry. Note the structural similarity between acid anhydrides and acy) phosphates. 

In addition to acyl halides and acid anhydrides, there are a number of milder and more selective acylating agents that can be readily prepared from carboxylic acids. Imidazolides, the (V-acyl derivatives of imidazole, are examples.115 Imidazolides are isolable substances and can be prepared directly from the carboxylic acid by reaction with carbonyldiimidazole. 

Alternatively, esterification of carboxylic acid can be carried out in aqueous media by reacting carboxylic acid salts with alkyl halides through nucleophilic substitutions (Eq. 9.10).20 The reaction rate of alkyl halides with alkali metal salts of carboxylic acids to give esters increases with the increasing concentration of catalyst, halide, and solvent polarity and is reduced by water. Various thymyl ethers and esters can be synthesized by the reactions of thymol with alkyl halides and acid chlorides, respectively, in aqueous medium under microwave irradiation (Eq. 9.11).21 Such an esterification reaction of poly(methacrylic acid) can be performed readily with alkyl halides using DBU in aqueous solutions, although the rate of the reaction decreases with increasing water content.22... 

Catalytic amounts of I fCl4-AgC104 and Hf(OTf)4 are used for activation of acid halides and acid anhydrides for Friedel -Crafts acylation (Scheme 42) 178 the reactions of both reactive and unreactive aromatic substrates proceed smoothly in the presence of Hf(OTf)4. Furthermore, the Fries rearrangement179,180 and direct C-acylation of phenolic compounds181,182 take place using Hf(OTf)4. Formation of esters and Mannich-type reactions and allylation of imines have been also reported.152... 

Acyl-transfer reactions are some of the most important conversions in organic chemistry and biochemistry. Recent work has shown that adjacent cationic groups can also activate amides in acyl-transfer reactions. Friedel-Crafts acylations are known to proceed well with carboxylic acids, acid chlorides (and other halides), and acid anhydrides, but there are virtually no examples of acylations with simple amides.19 During studies related to unsaturated amides, we observed a cyclization reaction that is essentially an intramolecular acyl-transfer reaction involving an amide (eq 15). The indanone product is formed by a cyclization involving the dicationic species (40). To examine this further, the related amides 41 and 42 were studied in superacid promoted conversions (eqs 16-17). It was found that amide 42 leads to the indanone product while 41... 

Rearrangement of acyl and halo groups are the most common reactions of triazoles with functions on nitrogen. The chemistry of 4-hydroxytriazoles , in effect Af-oxides, has been less extensively studied than that of triazolinones. The N—O bond is cleaved by phosphorus halides and acidic anhydride but not by oxidation with permanganate or by alkaline bromination (70jpr610). 

Another consequence of the reactivity differences among carboxylic acid derivatives is that only esters and amides are commonly found in nature. Acid halides and acid anhydrides undergo nucleophilic attack by water so rapidly that they can t exist in living organisms. Esters and amides, however, are stable enough to occur widely. All protein molecules, for example, contain amide functional groups. 

Reactive Materials or Acid Halides and (Acid Anhydrides or Concentrated Acids)... 

Phosphanes are widely used as reagents in organic synthesis, triphenylphos-phane being the most commonly applied due to its stability towards oxidation. The polymer-supported analog has so far found use in the transformation of alcohols into alkyl halides and acids into acid halides, using either carbon tetrachloride or carbon tetrabromide as the halogen source (Scheme 6.4) [7, 11-14], This system can also successfully transform primary amides and oximes into nitriles, whereas secondary amides are transformed into imidoyl chlorides (Scheme 6.5) [15],... 

TABLE 15.6 Bond Dissociation Energies for Hydrogen Halides and Acid Strengths for Hydrohalic Acids... 

---