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
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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
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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 Ph3 P is
used instead of Me3 P.
