Barton decarboxylation

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The Barton decarboxylation is a name reaction in organic chemistry . With their help, organic residues such. B. be split off from carboxylic acid chlorides or carboxylic acids . The reaction is named after the chemist and Nobel Prize winner Derek HR Barton .

Overview

The reaction allows the synthesis of an alkane from

a carboxylic acid chloride or a carboxylic acid.
Overview of Barton decarboxylation
Scheme of the Barton decarboxylation of a carboxylic acid.

mechanism

The mechanism is two-step. First you couple a carboxylic acid chloride with a thiopyridine. In the second step, the resulting product is mixed with a free- radical initiator - such as azobis (isobutyronitrile) (AIBN) or a peroxide .

Coupling of a carboxylic acid chloride with a thiopyridine, R 1 denotes an organic radical.

In the first step, 2-thioxopyridine-1 (2 H ) enolate 2 is added to a carboxylic acid chloride 1 . The carboxylic acid chloride is attacked by the 2-thioxopyridine-1 (2 H ) -olate 2 through the rearrangement of a pair of electrons from nitrogen , whereby molecule 3 is formed. The newly formed alcoholate group rearranges a pair of electrons. As a result, on the one hand, the chloride is split off from the molecule and, on the other hand, a carbonyl group is formed from 4 . This carbonyl group is now attacked intramolecularly by the alcoholate on the pyridine ring , as a result of which a five-membered ring is formed in the molecule 5 . A further electron pair rearrangement finally forms the desired intermediate molecule 6 , which contains an ester group .

Radical reaction mechanism for the formation of the deoxygenation product 11 from the sulfur compound 6 , R and R 1 each denote an organic radical.

In the second step, a radical initiator is added to tributyltin hydride 7 . The mechanism is described here using an isobutyronitrile radical, which is formed when AIBN decays . This forms a tributylstannane radical ( 8 ) and the protonated radical of the radical initiator. The radical 8 is now mixed with the pyridine derivative 6 previously formed . On the one hand, 2 - [(tributylstannyl) thio] pyridine and a carboxylic acid radical 9 are formed . By shifting electrons in the carboxylic acid radical, it decays. On the one hand, this creates carbon dioxide and a radical-safe residue 10 . The remaining radical 10 then reacts in turn with a tributyltin hydride 7 in such a way that the remainder is obtained in protonated form 11 and again a tributylstannane radical. The product 11 is thus a molecule in which a carboxylic acid chloride group 1 has been replaced by a hydrogen atom.

Sulfur-tin bonds are very stable. The driving forces for the entire reaction are, on the one hand, the formation of such a bond and, on the other hand, the increase in entropy due to the formation of CO 2 .

Barton decarboxylation is not to be confused with the Barton reaction or the closely related Barton-McCombie deoxygenation .

Web links

Commons : Barton decarboxylation  - collection of images, videos, and audio files

Individual evidence

  1. ^ A b c B. P. Mundy, MG Ellerd, FG Favaloro: Name Reactions and Reagents in organic Synthesis , 2nd edition, Wiley-Interscience, Hoboken, NJ 2005 , ISBN 978-0-471-22854-7 , p. 64.