Carboxylic acids, from alkyl halides

Strategy We ve seen two methods for preparing carboxylic acids from alkyl halides ... 

Many other examples of synthetic equivalent groups have been developed. For example, in Chapter 6 we discussed the use of diene and dienophiles with masked functionality in the Diels-Alder reaction. It should be recognized that there is no absolute difference between what is termed a reagent and a synthetic equivalent group. For example, we think of potassium cyanide as a reagent, but the cyanide ion is a nucleophilic equivalent of a carboxy group. This reactivity is evident in the classical preparation of carboxylic acids from alkyl halides via nitrile intermediates. 

Formation of carboxylic acid from alkyl halide using the hydrolysis of a nitrile. 

It is in many ways unfortunate that the study of cationic polymerization has, from its very start, been so intimately linked with the very complicated and ill-understood chemistry of the metal halides. This connection is largely fortuitous and there is the promise of much progress in this field when these two problems can be attacked independently. On the one hand, we need to know much more about the complex acids and esters which are formed when water, alcohols, carboxylic acids, and alkyl halides react with metal halides on the other hand, a study of olefin polymerizations catalysed by simple acids such as HBr [14], HC104 [25], and H2S04 [26] should be rewarding, because they would presumably be unobscured by the complications and uncertainties accompanying the formation of the initiating species when this involves a metal halide. 

Polymeric amines can be proton acceptors, acyl transfer agents, or ligands for metal ions. The 2- and 4-isomers of poly(vinylpyridine) (11) and (12) and the weakly basic ion exchange resins, p-dimethylaminomethylated PS (2) and poly(2-dimethylaminoethyl acrylate), are commercial. The ion exchange resins are catalysts for aldol condensations, Knoevenagel condensations, Perkin reactions, cyanohydrin formation and redistributions of chlorosilanes. " The poly(vinylpyridine)s have been used in stoichiometric amounts for preparation of esters from acid chlorides and alcohols, and for preparation of trimethylsilyl ethers and trimethylsilylamines from chlorotrimethylsilane and alcohols or amines. Polymer-suppored DBU (l,8-diazabicyclo[5.4.0]undec-7-ene) (52) in stoichiometric amounts promotes dehydrohalogenation of alkyl bromides and esterification of carboxylic acids with alkyl halides. The protonated tertiary amine resins are converted to free base form by treatment with aqueous sodium hydroxide. 

Carboxylic acids can be prepared from nitriles by reaction with hot aqueous acid or base by a mechanism that we ll see in Section 20.9. Since nitriles themselves are usually made by Sts 2 reaction of a primary or secondary7 alkyl halide with CN , the two-step sequence of cyanide displacement followed by nitiile hydrolysis is a good way to make a carboxylic acid from an alkyl halide (RBr —> RC=N RC02H). 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

Polymeric phosphonium salt-bound carboxylate, benzenesulphinate and phenoxide anions have been used in nucleophilic substitution reactions for the synthesis of carboxylic acid esters, sulphones and C/O alkylation of phenols from alkyl halides. The polymeric reagent seems to increase the nucleophilicity of the anions376 and the yields are higher than those for corresponding polymer phase-transfer catalysis (reaction 273). 

As these compounds all possess a nitrogen-containing functional feature, it is worth pointing out that they arise from alkyl halides (or methanesulphonates) in the case of azides and cyanides, or from carboxylic acid chlorides (in the case of amides). 

The reaction under consideration is typified by the formation of saturated carboxylic acids from olefins, carbon monoxide, and water. Other compounds have been used in place of olefins (alkyl halides, alcohols), and besides water, a variety of compounds containing active hydrogen may be employed. Thus, alcohols, thiols, amines, and acids give rise to esters, thio-esters, amides, and acid anhydrides, respectively (15). If the olefin and the active hydrogen are part of the same molecule, three or four atoms apart, cyclizations may occur to produce lactones, lactams, imides, etc. The cyclizations are formally equivalent to carbonylations, however, and will be considered later. 

Hydrides from carboxylic acids Carboxylic acids from hydrides Carboxylic acids from hydrides Esters from hydrides Hydrides from aldehydes Hydrides from aldehydes Alkyls from aldehydes Ketones from methylenes Ketones from ketones Alkyls from olefins Acetylenes from halides also acetylenes from acetylenes Esters from alcohols also esters from carboxylic acids Alkyls from olefins Alkyls from olefins... 

The Potassium complex of I8-Crown-6 or Potassium Naph-thalenide effects ring-opening to give acetates or their alkylated derivatives in good yield (eq 3). Treatment of the reaction mixture obtained from p-methyl-p-propiolactone and potassium-18-crown-6 with hydrochloric acid or alkyl halides gives the acetate (S) or its alkylated derivative (6), respectively. The a,3-unsaturated carboxylic acid (7) or its ester (8) is formed by the action of the potassium naphthalenide-18-crown-6 complex. ... 

---