Eschenmoser coupling
The Eschenmoser coupling is a name reaction in organic chemistry, which was first introduced in 1967 by Albert Fischli and Albert Eschenmoser and named after Eschenmoser. This reaction was discovered in the synthesis of vitamin B 12 .
Overview reaction
This reaction is the formation of vinylogous amides and urethanes by the alkylation of secondary or tertiary thioamides with an electrophilic reagent followed by desulfurization. This method is not suitable for the introduction of meso substituents . It is unclear whether the majority of the results are in the cis or trans configuration. The atom marked with X must be a halogen .
Reaction mechanism
The mechanism of the reaction is not yet fully understood. At the beginning, an electrophilic reagent 1 is added and the carbon is attacked because it is partially positively charged due to its direct bond to a halogen. At the same time, the rearrangement of the lone pair of electrons in nitrogen creates a double bond between nitrogen and carbon. The nitrogen is now positively charged. The coupled reagent can then be deprotonated by a hydroxide ion 2 and the now free electron pair of the negatively charged carbon attacks the central carbon atom, 3 as a result of which the electron pair returns to nitrogen and the nitrogen is again neutrally charged. Triphenylphosphine 4 is added for desulfurization . This forms a bond with the sulfur atom, whereby it detaches from the carbons and is split off as triphenylphosphine sulfide 5 .
application
This reaction is used in particular in the production of vinylogous amides for vitamin B 12 .
Individual evidence
- ↑ Eschenmoser Coupling . In: Comprehensive Organic Name Reactions and Reagents . Wiley, 2010, ISBN 978-0-470-63885-9 , pp. 1001-1004 , doi : 10.1002 / 9780470638859.conrr218 .