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.

β-ketophosphonates - organomagnesium compounds - organocobalt compounds

References

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<A NAME="RP01302SS-21">21</A>

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).

<A NAME="RP01302SS-22">22</A>

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.

<A NAME="RP01302SS-23">23</A>

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

<A NAME="RP01302SS-24">24</A>

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).

<A NAME="RP01302SS-25">25</A>

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.