Asymmetric synthesis of planar chiral 2-mono- and 2,2′-disubstituted 1,1′-bisbenzoylferrocenes

An efficient and flexible asymmetric synthesis of planar chiral 2-mono- and 2,2′-disubstituted 1,1′-bisbenzoylferrocenes 4 and 6 is reported. Key step is a highly diastereoselective ortho-metalation of 1,1′-bisbenzoylferrocene 1 via the corresponding bis-SAMP-hydrazone 2 (de≥96%), followed by trapping with various carbon, silicon, phosphorus and sulfur electrophiles. Cleavage of the monosubstituted hydrazones 3 led to monosubstituted ketones 4 (ee≥98%). Further ortho-substitution of the hydrazones 3 afforded 2,2′-disubstituted hydrazones 5, which could be cleaved to disubstituted ferrocenyl diketones 6 (ee≥99%). The new methodology allows a broad and flexible fine-tuning of ferrocenyl ligands desired in asymmetric catalysis. Ozonolysis or reductive hydrazone cleavage using TiCl3 or SnCl2 were the methods of choice to remove the auxiliary.