Enantioselective functionalisation of the C-2′ position of 1,2,3,4,5-pentamethylazaferrocene via sparteine mediated lithiation: potential new ligands for asymmetric catalysis

摘要

The s-BuLi-sparteine base combination deprotonated the C-2′ position of 1,2,3,4,5-pentamethylazaferrocene 1 and subsequent reaction with a range of electrophiles gave C-2 substituted products in 76-93% yield and ～80% ee. The products could be recrystallised to enrich ee's to > 90%. Resubjection of the initial addition products (～80% ee) to the deprotonation conditions led to a kinetic resolution to give products with > 90% ee and superior overall yields compared to recrystallisation for the cases where the electrophiles were Ph_2CO, MeI and Ph_2S_2. Transmetallation of the 2-lithiopentamethylazaferrocene (～80% ee) with ZnCl_2 allowed palladium catalysed cross coupling with a variety of C-2 haloaryl, heteroaryl and vinyl groups to give some novel C-2′ substituted pentamethylazaferrocene derivatives in 61-77% yield in 80% ee. Potential N,N-chelate ligands were recrystallised to > 95% ee. A novel C_2-symmetric bis-pentamethylazaferrocene 10 could be synthesised by an iron catalysed oxidative coupling of the enatioenriched C-2 lithio derivative and in the presence of a PhMe-Et_2O solvent mixture proceeded in 97% ee.