Rhodium complexes with planar-chiral cyclopentadienyl ligands: synthesis from tert-butylacetylene and catalytic performance in C–H activation of arylhydroxamates

Abstract

The rhodium complex [(C₅H₂^tBu₂CH₂^tBu)RhCl₂]₂ with an asymmetric cyclopentadienyl ligand was prepared in 95% yield by the reaction of [(cod)RhCl]₂ with tert-butylacetylene in the presence of AlCl₃. A similar reaction in the presence of InBr₃ gave the cationic fulvene complex [(C₅H₂^tBu₂ = CH^tBu)Rh(cod)]InBr₄ (70%), which can add alcohols ROH and produce more bulky catalysts [(C₅H₂^tBu₂CH(OR)^tBu)RhCl₂]₂. The enantiomers of these planar-chiral complexes were separated by thin-layer chromatography in the presence of L-phenylglycinol. The complexes catalyze the reactions of arylhydroxamates with alkenes giving dihydroisoquinolones in excellent yields (80–90%), but with moderate enantioselectivity (typically 20–50% ee).