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New Routes to Phosphametallocene Derivatives

Abstract

While the syntheses of phosphaferrocene and diphosphaferrocene have been known for several decades, derivatization of these molecules has been limited. Prior to our work, functionalization of the phosphametallocenes has been through two routes: electrophilic aromatic substitution (EAS) and functionalization of phospholide ions that are later coordinated to an iron source. The most common method, EAS, is limited to alpha carbon substitution and only with acetyl or benzoyl groups.

A small number of 2,5-silyl substituted phosphametallocenes have been reported in literature, via coordination of substituted phospholide ions with Cp*FeCl or FeCl2. However, the 2,5-silyl substituted phosphaferrocene and diphosphaferrocene have not been found to be useful precursors of phosphametallocene macrocyles.

Our group has explored other routes for making substituted phosphaferrocene molecules and this work is presented hereafter. Using phospholide ions that are carbonyl substituted at the alpha position to react with iron (II) chloride in the presence of zinc chloride, we were able to prepare the corresponding 2,2'-difunctional-1,1' diphosphaferrocenes as mixtures of meso and rac diastereomers. When the 2,5-bis(ethoxycarbonyl)phospholide was reacted under the same conditions however the product was found to be the tetrafunctional 1,1'-biphospholyl which lead us to believe that the reaction is occurring through a bis(eta1-phospholyl) iron intermediate.

We were successful at synthesizing 2,5-bis(ethoxycarbonyl)phosphaferrocene, 2,5-dibenzoylphosphaferrocene, and the 2,2',5,5'-tetrapivaloyl-1,1'diphosphaferrocene by reacting iron (II) chloride with a 1:1 mixture of Cp*Li and the corresponding 2,5 substituted phospholide ion in the presence of zinc chloride. We attributed the stability of the 2,5-substituted phosphametallocenes to a push-pull type stabilization, where the Cp* unit acts as an electron donor to the metal center and the phospholide plays the role of an electron acceptor.

Our new protocol for making phosphaferrocene derivatives was used in the synthesis of a new 2-(azacymantrenylmethyl)phosphaferrocene ligand, and in the preparation of building blocks for phosphametallocene macrocycles 2,5-bis(hydroxylmethyl)phosphaferrocene and 2,5-bis(pyrrolylmethyl)phosphaferrocene.

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