Barton decarboxylation
| Barton decarboxylation | |
|---|---|
| Named after | Derek Barton |
| Reaction type | Substitution reaction |
| Identifiers | |
| Organic Chemistry Portal | barton-decarboxylation |
| RSC ontology ID | RXNO:0000135 |
The Barton decarboxylation is a radical reaction in which a carboxylic acid is first converted to a thiohydroxamate ester (commonly referred to as a Barton ester). The product is then heated in the presence of a radical initiator and a suitable hydrogen donor to complete the reductive decarboxylation of the initial carboxylic acid.[1][2] Using this reaction it is possible to remove a carboxylic acid moiety from an alkyl group and replace it with other functional groups.[3][4] (See Scheme 1) This reaction is named after its developer, the British chemist and Nobel laureate Sir Derek Barton (1918–1998).
-
File:Bartondecarbtotal.svgScheme 1
Mechanism
The reaction is initiated by homolytic cleavage of a radical initiator, in this case 2,2'-azobisisobutyronitrile (AIBN), upon heating. A hydrogen is then abstracted from tributylstannane to leave a tributylstannyl radical that attacks the sulfur atom of the thiohydroxamate ester. The N-O bond of the thiohydroxamate ester undergoes homolysis to form a carboxyl radical which then undergoes decarboxylation and carbon dioxide (CO2) is lost. The remaining alkyl radical (R·) then abstracts a hydrogen atom from remaining tributylstannane to form the reduced alkane (RH). (See Scheme 2) The tributyltin radical enters into another cycle of the reaction until all thiohydroxamate ester is consumed.
N-O bond cleavage of the Barton ester can also occur spontaneously upon heating or by irradiation with light to initiate the reaction. In this case a radical initiator is not required but a hydrogen-atom (H-atom) donor is still necessary to form the reduced alkane (RH). Alternative H-atom donors to tributylstannane include tertiary thiols and organosilanes.[5] The relative expense, smell, and toxicity associated with tin, thiol or silane reagents can be avoided by carrying the reaction out using chloroform as both solvent and H-atom donor.[6]
It is also possible to functionalize the alkyl radical by use of other radical trapping species (X-Y + R· -> R-X + Y·).[7] The reaction proceeds due to the formation of the stable S-Sn bond and increasing aromaticity of the thiohydroxamate ester. There is also an overall increase in entropy due to the formation of gas which drives the reaction forward.
See also
- Barton–McCombie deoxygenation
- Hunsdiecker reaction
- Kochi reaction
- Krapcho decarboxylation
- Kolbe electrolysis
References
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
