Nonstabilized alkyl complexes and alkyl-cyano-ate complexes of iron(II) and cobalt(II) as new reagents in organic synthesis

Alkyl transition metal reagents are being increasingly used for alkylations in organic synthesis. They have various advantages over alkyllithium and alkyl-magnesium reagents including higher selectivity, lower basicity, and- as long as the transition metal is not in its highest oxidation state- their willingness to undergo oxidative addition with elec-trophiles. Alkyl derivatives of Feii and Coii, which are not stabilized by special ligands but still can be easily handled, are in many cases superior to the well-known alkyl-CuI and -MnII reagents and can also undergo unexpected reactions. The introduction of alkyl-cyano-ate complexes of FeII and Coii, the cyanide ligands of which (in contrast to neutral z-acidic ligands) do not reduce the reactivity, has led to further advances. Reaction mechanisms will be discussed and comparisons will be made with alkylating reagents containing Cui, Mnii, Niii, or Tiiv as well as with Pd-cat-alyzed coupling reactions. Furthermore, it will be shown that super-ate Feii complexes are almost certainly the reactive species in highly selective catalytic alkylations.