Hunsdiecker reaction

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Hunsdiecker reaction
Named after Heinz Hunsdiecker
Cläre Hunsdiecker
Alexander Borodin
Reaction type Substitution reaction
Identifiers
Organic Chemistry Portal hunsdiecker-reaction
RSC ontology ID RXNO:0000106

The Hunsdiecker reaction (also called the Borodin reaction or the Hunsdiecker–Borodin reaction) is a

organic halide.[1] It is an example of both a decarboxylation and a halogenation reaction as the product has one fewer carbon atoms than the starting material (lost as carbon dioxide) and a halogen atom is introduced its place.[2][3] A catalytic approach has been developed.[4]

The Hunsdiecker reaction

History

The reaction is named after Cläre Hunsdiecker and her husband Heinz Hunsdiecker, whose work in the 1930s[5][6] developed it into a general method.[1] The reaction was first demonstrated by

methyl bromide (CH3Br) from silver acetate (CH3CO2Ag).[7][8] Around the same time, Angelo Simonini, working as a student of Adolf Lieben at the University of Vienna, investigated the reactions of silver carboxylates with iodine.[2] They found that the products formed are determined by the stoichiometry within the reaction mixture. Using a carboxylate-to-iodine ratio of 1:1 leads to an alkyl iodide product, in line with Borodin's findings and the modern understanding of the Hunsdiecker reaction. However, a 2:1 ratio favours the formation of an ester product that arises from decarboxylation of one carboxylate and coupling the resulting alkyl chain with the other.[9][10]

The Simonini reaction

Using a 3:2 ratio of reactants leads to the formation of a 1:1 mixture of both products.[9][10] These processes are sometimes known as the Simonini reaction rather than as modifications of the Hunsdiecker reaction.[2][3]

3 RCOOAg + 2 I
2 → RI + RCOOR + 2 CO
2 + 3 AgI

Reaction mechanism

In terms of reaction mechanism, the Hunsdiecker reaction is believed to involve organic radical intermediates. The silver salt 1 reacts with bromine to form the acyl hypohalite intermediate 2. Formation of the diradical pair 3 allows for radical decarboxylation to form the diradical pair 4, which recombines to form the organic halide 5. The trend in the yield of the resulting halide is primary > secondary > tertiary.[2][3]

Radicalic mechanism of Hunsdiecker reaction

Variations

The reaction cannot be performed in

acetyl hypohalite.[citation needed
]

Other counterions than silver typically have slow reaction rates. The toxic[11] relativistic metals (mercury, thallium, and lead) are preferred: inert counterions, such as the alkali metals, have only rarely led to reported success.[12]: 464 

In the presence of

multiple bonds, the intermediate acetyl hypohalite prefers to add to the bond, producing an α-haloester. Steric considerations suppress this tendency in α,β-unsaturated carboxylic acids, which instead polymerize (see below).[12]
: 468 

Mercuric oxide and bromine convert 3-chlorocyclobutanecarboxylic acid to 1-bromo-3-chlorocyclobutane. This is known as Cristol-Firth modification.[13][14][15] The 1,3-dihalocyclobutanes were key precursors to propellanes.[16] The reaction has been applied to the preparation of ω-bromo esters with chain lengths between five and seventeen carbon atoms, with the preparation of methyl 5-bromovalerate published in Organic Syntheses as an exemplar.[17]

The Kochi reaction is a variation on the Hunsdiecker reaction developed by Jay Kochi that uses lead(IV) acetate and lithium chloride (lithium bromide can also be used) to effect the halogenation and decarboxylation.[18]

The Kochi reaction
The Kochi reaction

Reaction with α,β-unsaturated carboxylic acids

Synthesis of β-arylvinyl halide by microwave-induced Hunsdiecker reaction.

For unsaturated conmpounds, the radical conditions associated with the Hunsdiecker reaction can also induce polymerization instead of decarboxylation.[12]: 468  Consequently, reactions with α,β-unsaturated carboxylic acids typically give low yield.[11] Kuang et al have found that an alternate radical halogenating agent, N-halosuccinimide, combined with a lithium acetate catalyst, gives a higher yield of β-halostyrenes. The reaction also improves in the presence of microwave irradiation, which preferentially synthesizes (E)-β-arylvinyl halides.[19]

For a green metal-free reaction, tetrabutylammonium trifluoroacetate serves as an alternative catalyst.[20] However, it only exhibits comparable yields to the original lithium acetate when performed with micellular surfactants.[19][21][22]

See also

References

  1. ^
    ISBN 9783319039794
    .
  2. ^
    doi:10.1021/cr50008a002
    .
  3. ^
    ISBN 0471264180
    .
  4. PMID 22316183
    .
  5. ^ US patent 2176181, Hunsdiecker, C.; Vogt, E. & Hunsdiecker, H., "Method of manufacturing organic chlorine and bromine derivatives", published 1939-10-17, assigned to Hunsdiecker, C.; Vogt, E.; Hunsdiecker, H. 
  6. doi:10.1002/cber.19420750309
    .
  7. doi:10.1002/jlac.18611190113
    .
  8. ^ Borodin, A. (1861). "Ueber de Monobrombaldriansäure und Monobrombuttersäure" [About the monobromovaleric acid and monobromobutyric acid]. Zeitschrift für Chemie und Pharmacie (in German). 4: 5–7.
  9. ^
    S2CID 197766447
    .
  10. ^
    S2CID 104367588
    .
  11. ^
    PMID 11671382
    .
  12. ^
    LCCN 70-37114 – via the Internet Archive
    .
  13. doi:10.15227/orgsyn.051.0106
    ; Coll. Vol., vol. 6, p. 179.
  14. doi:10.15227/orgsyn.051.0055
    ; Coll. Vol., vol. 6, p. 133.
  15. doi:10.15227/orgsyn.043.0009
    ; Coll. Vol., vol. 5, p. 126.
  16. doi:10.1016/S0040-4020(01)98361-9
    .
  17. doi:10.15227/orgsyn.026.0052
    ; Coll. Vol., vol. 3, p. 578.
  18. doi:10.1021/ja01089a041
    .
  19. ^
    ISSN 0936-5214
    .
  20. ISSN 0040-4039
    .
  21. PMID 12398515
    .
  22. S2CID 96943205
    .

External links
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