Secondary Coordination Sphere Alkylation Promotes Cyclometalation

Diphosphines have taken on a dominant role as supporting ligands in transition metal chemistry. Here, we describe complexes of the type [Cp*Fe(diphosphine)(X)] (X = Cl, H) where for diphosphine = tetraallylphosphinoethane (tape), a Lewis-acidic secondary coordination sphere (SCS) was installed via allyl group hydroboration using dicyclohexylborane (HBCy2). The resulting chloride complex, [Cp*Fe(P2BCy4)(Cl)] (P2BCy4 = 1,2-bis(di(3-cyclohexylboraneyl)propylphosphino)ethane), was treated with n-butyllithium (1-10 equivs.), resulting in SCS butylation, followed by cyclometalation at iron. This reactivity is contrasted with [Cp*Fe(dnppe)(Cl)] (dnppe = 1,2-bis(di-n-propylphosphino)ethane), whereby addition of n-butyllithium provides a mixture of products. Overall, transmetalation is a common elementary transformation in organometallic chemistry, and here we describe how its outcome is altered due to Lewis acid SCS incorporation.