Axially chiral cyclopentadienyl ligands: stereoselective synthesis of 1-substituted-9-(1′-naphthyl)fluorenes and retention of axial chirality in the fluorenyl carbanions 1

Abstract

1-(tert-Butyl- or 1-(p-tolyl-sulfinyl)naphthalene-2-carboxylate esters undergo coupling reactions with fluorenyllithiums substituted at the 1-position, providing 1-(1′-substituted-fluoren-9′-yl)naphthalene-2-carboxylate esters as single rotamers where the naphthalene ester substituent is syn to the fluorene 9-H. The stereoselectivity of the coupling reaction, with respect to asymmetric induction at the fluorene 9-C, varies from 21–95% (ee or de) dependant on the sulfoxide, ester and fluorene substituents, and the reaction temperature. The stereomutation of +ac(R)-1-methyl-9-(2′-methoxymethyl-1′-naphthyl)fluorene 33 into ent-33 was achieved through thermal atropisomerisation of 33 to the –sc(R)-rotamer 34, followed by lithiation of 34 and then reprotonation of the resultant fluorenyllithium 35, demonstrating the retention of axial chirality in the fluorenyl carbanion.