Barton-Kellogg reaction

The Barton-Kellogg reaction is a coupling reaction between a ketone and a thioketone through a diazo intermediate forming an alkene.



This reaction has been pioneered by Hermann Staudinger and therefore the reaction also goes by the name Staudinger type diazo-thioketone coupling.

Reaction mechanism
In the reaction mechanism for this reaction the diazo compound is a 1,3-dipole which reacts with the thioketone in a 1,3-dipolar cycloaddition to a thiadiazoline. This intermediate is unstable and through nitrogen gas expulsion and formation of an intermediate thiocarbonyl ylide it forms a stable episulfide. Triphenylphosphine opens the three-membered ring and then forms a sulfaphosphatane in a manner similar to the Wittig reaction. In the final step triphenylphosphinesulfoxide is expulsed liberating the alkene.



Scope
The diazo compound can be obtained from a ketone by reaction with hydrazine to a hydrazone followed by oxidation. Many reagents exist for this conversion for example silver(I) oxide and (bis(trifluoroacetoxy)iodo)benzene. The thioketone required for this reaction can be obtained from a ketone and phosphorus pentasulfide. Desulfurization of the episulfide can be accomplished by many phosphines and also by copper powder.

The main advantage of this reaction over the McMurry reaction is the notion that the reaction can take place with two different ketones. In this regard the diazo-thioketone coupling is a cross-coupling rather than a homocoupling.