Highly Functionalized Ferrocenes

Ferrocene is unique among organometallic compounds, and serves notably as a versatile platform towards the production of ligands useful to promote transition metals chemistry. A general limiting aspect of the synthesis of ferrocene derivatives is the efficient access to sophisticated highly functionalized polysubstituted ferrocenes, i.e. bearing four or more substituents replacing hydrogen atoms on the cyclopentadienyl rings. These ferrocene derivatives can bear various functional or/and structuring spectator substituents. Their preparation involves synthetic difficulties resulting from the need of multiple functionalizations coexisting altogether, and satisfying functional group compatibility and high selectivity issues. In the last decades, our group initially designed highly functionalized polyphosphines and hybrid ligands (1,1',3,3'-tetrafunctionalized Fc) using dialkylated 1,1'-tert-butylferrocene as a scaffold, which opened the way to various new classes of hybrid compounds. Some of these original ferrocenes were used as ligands, promoting metal catalysed C-C and C-X bond formation (X = O, S, N, etc.). Thus, highly functionalized ferrocenes, which include notably (P,P,P,P)-, (P,P,N,N)-, (P,P,P')-, (P,P,B)-, (P,B)- and (N,B)-compounds were developed in which the heteroatoms coexist in a close proximity on a common ferrocene platform with a controlled conformation. The present review details the concepts attached to the synthesis of these highly functionalized ferrocene species, and illustrates their main features and applications related to organometallic chemistry directed towards organic synthesis by metal catalysis.