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

Abstract

The s-BuLi–sparteine base combination deprotonated the C-2′ position of 1,2,3,4,5-pentamethylazaferrocene1 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 Ph₂CO, MeI and Ph₂S₂. Transmetallation of the 2-lithiopentamethylazaferrocene (∼80% ee) with ZnCl₂ 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 C₂-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–Et₂O solvent mixture proceeded in 97% ee.