Cristol-Firth modification, Hunsdiecker reaction

Hunsdiecker reaction. The Cristol-Firth modification (1, 657 6,428) of this reaction generally proceeds in improved yield if irradiated with a 100-W bulb. ... 

Recently, Paquette and coworkers reported on the stereochemical consequences of having a trimethylsilyl substituent at the radical site. The Hunsdiecker reaction, as well as the Cristol-Firth modification thereof, on ( —)-(R)-l-trimethylsilyl-2,2-diphenyl-cyclopropanecarboxylic acid (71) resulted in racemic ( )-l-bromo-l-trimethylsilyl-2,2-diphenylcyclopropane (72). The trimethylsilyl group, bulky as it is, could not slow down the inversion frequency of the cyclopropyl a radical sufficiently to prevent complete racemization. More to the point, recentESR studies have demonstrated that the radical intermediate is planar, or nearly so. 

Australian chemists4 report an instance where chlorodecarboxylation was effective where the Hunsdiecker reaction (Cristol-Firth modification) failed. Thus treatment of the acid (5) with lead tetraacetate in benzene in the presence of lithium chloride gave a... 

Bromocyclopropane is also obtained from cyclopropanecarboxylic acid (3) in 41-46% yield by a Cristol-Firth modification of the Hunsdiecker reaction. ... 

The Cristol-Firth modification of the Hunsdiecker reaction applied to cis- and /ra 5-2-phenyl-cyclopropanecarboxylic acid (6) in tribromomethane leads to a mixture of cis- and trans-1-bromo-2-phenylcyclopropane (7) with predominant retention of configuration. "... 

Oxidative decarboxylation of optically active l-methyl-2,2-diphenylcyclopropanecarboxylic acid (10) with lead tetraacetate in the presence of iodine leads to racemized 1-iodo-l-methy 1-2,2-diphenylcyclopropane (11) in 45% yield. Subjecting 10 to the Cristol-Firth modification of the Hunsdiecker reaction (bromine and mercuric oxide in carbon tetrachloride) leads to racemic 1-bromo-l-methyl-2,2-diphenylcyclopropane (12) however, the yield is poor (5 /o). ... 

In addition to Ag, many other metals (Hg, Pb, Tl, and Mn) can be used in the Hunsdiecker reaction.13-20 The Cristol-Firth modification is a one-pot reaction using excess red HgO and one equivalent of halogen.13 The advantage of the Cristol-Firth modification is that the mercury carboxylate does not need to be isolated and purified, unlike the Hunsdiecker reaction using the silver carboxylate. Therefore, as depicted by transformations 6— 714 and 8— 9,15 the Cristol-Firth modification is operationally more straightforward and more widely used than the classical Hunsdiecker reaction using silver carboxylate 1. 

Decarboxylation. Della and Patney recommend a two-step method for decarboxylation of bridgehead carboxylic acids. The first step is the Cristol-Firth modification of the Hunsdiecker reaction (I, 657, improved by use of methylene bromide as solvent). The second is photochemical reduction of the bromides by tri- -butyltin hydride (I, 1192-1193) with azobisisobutyronitrile as initiator. Yields are 80-95% in the first step and 80-90% in the second step. 

In the Hunsdiecker reaction, treatment of silver salts of carboxylic acids with bromine furnishes the alkyl(aryl) bromides with one less carbon atom. Improvements that do not require the preparation of the dry silver salts include the use of mer-cury(II) salts (Cristol-Firth modification) (eq 28), thallium(I) salts, and photostimulation. ... 

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... 

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