Synthesis of Phosphaferrocenes

All the derivatives prepared from aldehyde 2 can be obtained as pure enantiomers if desired, e.g., for use as ligands in asymmetric catalysis. Investigations of diastereoselective processes can of course be carried out using racemic phospha-ferrocene compounds. 

All the reactions described above rely on the electrophilic reactivity of the carbonyl C atom of the aldehyde 2. Much effort was devoted to the development of phosphaferrocenes with nucleophilic reactivity at this position. For example, transformation of the formyl group into a halomethyl function would pave the way for the preparation of Grignard or lithium derivatives by halogen-metal exchange. However, all attempts to do this were unsuccessful. 

Quite a number of different metal complexes were prepared with the bidentate ligands described above. The common characteristics of those complexes will be described for illustrative examples. 

This is also true for the donor/acceptor properties, because the phosphaferrocene is a reasonable Jt-acceptor due to the large p orbital contribution of the P to the LUMO of the molecule [15]. 

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Phosphole radicals are known to be more readily made and more stable than analogous phosphine radicals . This is probably due to the delocalization of the unpaired electron over the ring. On the other hand, complexes such as XLVIII are isolated from the reaction mixture and their mass spectra contain a medium-intensity peak corresponding to the final phosphaferrocene however, their thermolysis in the pure state does not produce phosphaferrocenes in detectable amounts. Phenyl-substituted phosphaferrocenes are always among the by-products of the synthesis of phosphaferrocenes. No satisfactory explanation of this fact (the formation of phenyl substituted phosphaferrocene by radical arylation as initially proposed is not very likely since ferrocene cannot be arylated in this way could be found until the very recent discovery that, at around... 

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