Carboxylic acids Hunsdiecker reaction

Halodecarboxylation of aromatic Q ,/3-unsaturated carboxylic acids (Hunsdiecker reaction) to make /3-halostyrenes has been accomplished with Oxone and sodium halide (eq 82). 7 Bromodecarbonylation and bromodecarboxylation of electron-rich benzaldehydes and benzoic acids has also been observed.7 ... 

The Hunsdiecker Reaction. The classical Hunsdiecker reaction is somewhat restricted due to the relatively harsh conditions required. In the Barton version, alkyl radicals generated from O-acyl thiohydroxamates, under either thermal or photolytic conditions, are efficiently trapped either by CI3C-X (X=C1 or Br CbC is the chain carrier) or by IodoformThe method is applicable to sensitive substrates for which the classical methods are unsuitable. thus allowing the preparation of a wide range of alkyl chlorides, bromides, and iodides by the one-carbon degradation of a carboxylic acid. Similar reactions of aromatic acid derivatives tend to require an additional radical initiator (e.g. Azo-bisisobutyronitrile), if high yields (55-85%) are to be obtained. ... 

Suitable substrates for the Hunsdiecker reaction are first of all aliphatic carboxylates. Aromatic carboxylates do not react uniformly. Silver benzoates with electron-withdrawing substituents react to the corresponding bromobenzenes, while electron-donating substituents can give rise to formation of products where an aromatic hydrogen is replaced by bromine. For example the silver /)-methoxybenzoate 6 is converted to 3-bromo-4-methoxybenzoic acid 7 in good yield ... 

In a modified procedure the free carboxylic acid is treated with a mixture of mercuric oxide and bromine in carbon tetrachloride the otherwise necessary purification of the silver salt is thereby avoided. This procedure has been used in the first synthesis of [1.1.1 ]propellane 10. Bicyclo[l.l.l]pentane-l,3-dicarboxylic acid 8 has been converted to the dibromide 9 by the modified Hunsdiecker reaction. Treatment of 9 with t-butyllithium then resulted in a debromination and formation of the central carbon-carbon bond thus generating the propellane 10." ... 

The Hunsdiecker reaction is the treatment of the dry silver salt of a carboxylic acid with bromine in carbon tetrachloride. Decarboxylation occurs, and the product isolated is the corresponding organic bromide 16). Since dry silver salts are tedious to prepare, a modification of the reaction discovered by Cristol and Firth (77) is now... 

Bromotrithiatriazepine 8 can also be obtained from the carboxylic acid 6 in 52 % yield by a modified Hunsdiecker reaction involving irradiation of a mixture of the acid, bromine, mer-cury(II) oxide and carbon tetrachloride. The iodo derivative is formed when iodine is employed418... 

The Hunsdiecker reaction has been used to convert indazole carboxylic acids into the bromo heterocycles [67HC(22)1],... 

Reaction of a silver salt of a carboxylic acid with bromine is called the Hunsdiecker... 

When iodine is the reagent, the ratio between the reactants is very important and determines the products. A 1 1 ratio of salt to iodine gives the alkyl halide, as above. A 2 1 ratio, however, gives the ester RCOOR. This is called the Simonini reaction and is sometimes used to prepare carboxylic esters. The Simonini reaction can also be carried out with lead salts of acids." A more convenient way to perform the Hunsdiecker reaction is by use of a mixture of the acid and mercuric oxide instead of the salt, since the silver salt must be very pure and dry and such pure silver salts are often not easy to prepare. 

A related method for conversion of carboxylic acids to bromides with decarboxylation is the Hunsdiecker reaction.276 The usual method for carrying out this transformation involves heating the carboxylic acid with mercuric oxide and bromine. 

The decarboxylation of carboxylic acid in the presence of a nucleophile is a classical reaction known as the Hunsdiecker reaction. Such reactions can be carried out sometimes in aqueous conditions. Man-ganese(II) acetate catalyzed the reaction of a, 3-unsaturated aromatic carboxylic acids with NBS (1 and 2 equiv) in MeCN/water to afford haloalkenes and a-(dibromomethyl)benzenemethanols, respectively (Eq. 9.15).32 Decarboxylation of free carboxylic acids catalyzed by Pd/C under hydrothermal water (250° C/4 MPa) gave the corresponding hydrocarbons (Eq. 9.16).33 Under the hydrothermal conditions of deuterium oxide, decarbonylative deuteration was observed to give fully deuterated hydrocarbons from carboxylic acids or aldehydes. 

The procedure described here allows for a convenient and efficient preparation in very high yields of large quantities of bromides from carboxylic acids containing an olefinic functionality. The Hunsdiecker reaction is traditionally accomplished by treating anhydrous silver carboxylates with bromine or iodine.2 Heavy metal salts such as mercury,3 lead,4 and thallium5 have also been used successfully as well as tert-butyl hypoiodite.6 The major disadvantages associated with the above methods, such as use of heavy metal salts and non-tolerance towards olefins, has led to the development of a more versatile method using O-acyl thiohydroxamates.7 8 The O-... 

The preferred reagent for Hunsdiecker-type reactions is lead(IV) acetate in the presence of an inorganic halide.18,19 The yields are usually good to excellent. Reaction of cyclobutanecar-boxylic acid with lead(IV) acetate in the presence of lithium chloride gave chlorocyclobutane (1) in good yield.18 Other carboxylic acids reacted under similar conditions.18 However, care should always be taken when lead(IV) acetate is used, as the use of this reagent in the absence of the halide results in decarboxylative elimination to give an alkene, which is found as a byproduct.19... 

Mercury(II) oxide together with a halogen is an early development of the classic Hunsdiecker reaction (bromodecarboxylation of a carboxylic acid silver salt, see below) which is still in use.20 22 A double Hunsdiecker reaction of cyclobutane-1,1-dicarboxylic acid with red mer-cury(ll) oxide in the presence of bromine gave 1,1-dibromocyclobutane (2) in 46% yield.21 However, a similar reaction performed on spiro[3.3]heptane-2-carboxylic acid afforded 2-bro-mospiro[3.3]heptane (3) in only 16% yield.22... 

Carboxylic acids may be converted to alkyl bromides with the loss of one carbon atom by the Hunsdiecker reaction ... 

Reaction of a silver salt of a carboxylic acid with bromine is called the Hunsdiecker reaction438 and is a way of decreasing the length of a carbon chain by one unit.439 The reaction is of... 

Thiophene- and benzo[6]thiophene-carboxylic acids undergo all the normal reactions of an aromatic carboxylic acid (63AHC(1)1, 70AHC(11)177). They can be converted to acid chlorides, amides and esters the esters can be used to make hydrazides. Benzo[6]thiophene-2-carboxylic acid chloride has been converted to the methyl ketone with dimethylcadmium and to the diazoketone with diazomethane. Bromodecarboxylation of the silver salts (Hunsdiecker reaction) has been used to prepare the dibromo compounds (340) and (341). 

Silver(I) carboxylates have been obtained by the addition of equivalent amounts of freshly prepared silver oxide to aqueous solutions of the appropriate acid.247 Their degradation by halogens provides a convenient method for the preparation of alkyl halides (Hunsdiecker reaction) or esters (Simonini reaction).248 Equations (14)-(18) have been proposed to account for the products obtained and were the result of extensive studies. 

Decarboxylation of the silver salts of carboxylic acids in the presence of bromine or chlorine, the Hunsdiecker reaction, often is useful for the synthesis of alkyl halides ... 

The Hunsdiecker reaction has been used only twice as a potential source of bromobenzo[6]thiophenes with 3-bromo-5-nitrobenzo[6]-thiophene-2-carboxylic acid it proceeds normally to give 2,3-dibromo-5-nitrobenzo[6]thiophene,152 but with 5-nitrobenzo[6]thiophene-2-carboxylic acid the main product is 3-bromo-5-nitrobenzo[6]thio-phene-2-carboxylic acid, accompanied by smaller amounts of 2,3-dibromo-5-nitrobenzo[6]thiophene.152 497... 

Modified Hunsdiecker reaction. Primary carboxylic acids are converted into thallium(I) carboxylates by reaction with T12C03. The salts are not isolated, but treated with Br2 (1.5 equiv.) to give primary alkyl bromides (equation I).1... 

In a Hunsdiecker reaction, the silver salt of an aromatic carboxylic acid is converted by bromine treatment to an acyl halide. 

The Kochi Reaction is a one-carbon oxidative degradation of carboxylic acids, and is a valuable alternative to the Hunsdiecker Reaction. A Pb(IV) reagent is the oxidant, and this reaction is suitable for synthesis of secondary and tertiary chlorides. 

Hunsdiecker reaction does not work so well in aromatic carboxylic acids [75, 76]. The rate constants for decarbonylation of acyl radicals are lowered as shown in Table 1.18. 

Perhaloalkanes serve as bromination or iodination agents in the radical decarbox-ylative halogenation of carboxylic acids. In an interesting modification of the Hunsdiecker-Bodin reaction Barton and coworkers have applied iV-hydroxypyridine-2-thione esters as nonelectrophilic intermediates for the decarboxylative bromination and iodination of primary, secondary and tertiary aliphatic and alicyclic592, as well as aroma-... 

Of lesser relevance to this discussion are halogenation methods involving the modification of the carbon skeleton (synthesis and degradation). The Hunsdiecker reaction, as applied to certain heterocyclic acids, has had limited application for the synthesis of halogen derivatives. The preparation of 3-bromo-4,6-dimethyl-2-pyridone from the silver salt of the respective 3-carboxylic acid by treatment with bromine in carbon tetrachloride is a rare example of success.13 The interaction of carbenes with heterocycles also has been employed infrequently, but recent advances in carbene generation may reactivate this approach.14 The Ciamician-Dennstedt ring expansion of pyrrole to / -halopyridines is a case in point18 [Eq. (4)] ... 

Silver-mediated C—X bond formation was known for many years as the Hunsdiecker reaction, in which substrates bearing carboxylic acids are oxidatively decarboxylated to give alkyl halides in the presence of halogens (142). If olefins are used with silver benzoate and I2, the Prevost reaction occurs to yield diols (143). 

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