Novel Cobalt(0)- or Magnesium-Mediated Approaches to β-Ketophosphonates

Abstract

Two novel approaches to β-ketophosphonates, based on cobalt(0)- or magnesium-mediated reactions of α-halophosphonates with esters are described.

Key words

β-ketophosphonates - organomagnesium compounds - organocobalt compounds

References

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21

Diethyl methanephosphonate may form both from acid-base exchange between the β-ketophosphonate and the α-metallated phosphonate (as observed for lithium phosphonate) or from hydrolysis, most likely during workup, from an unreacted organometallic reagent (as observed in Reformatsky-type reactions).

22

This behaviour would suggest a mechanism involving the oxidative addition of the α-halophosphonate to the cobalt(0)-complex to give a cobalt(II) species, as well as the substitution of a phosphine ligand by a carbonyl donor group. Reaction between the two organic moieties bound to the cobalt center would then give the product together with a Co(II) species from which the Co(0) precursor would be regenerated by reduction with Mg metal.

23

Mg metal is used in excess and can be recycled after use.

24

Mg was activated by adding a few drops of 1,2-dichloroethane to the metal turnings in THF so that a vigorous reaction ensued. After few minutes the mixture was cooled in an ice-bath, the solvent was removed and the Mg was washed with THF (3 × 2 mL).

25

The solution can be also warmed with an external bath at about 50 °C to speed the reduction of Co(II) to Co(0). Warming may be useful when Ph₃P is used instead of Me₃P.