Cp*Rh complexes with pyridyloxazolines: synthesis, fluxionality and applications as asymmetric catalysts for Diels–Alder reactions

Abstract

Half-sandwich complexes [RhCl(pymox)Cp*][SbF₆] (1–7) (pymox = pyridyloxazoline) have been synthesised as single diastereomers. Treatment of these with AgSbF₆ generates dications [Rh(OH₂)(pymox)Cp*]²⁺ which are fluxional at room temperature and which are enantioselective catalysts for the Diels–Alder reaction of methacrolein and cyclopentadiene. Treatment of the dication [Rh(OH₂)(ⁱPr-pymox)Cp*]²⁺ with [X]⁻ gives [RhX(ⁱPr-pymox)Cp*][SbF₆] (X = Br, I) as single diastereomers whilst reaction with 4-Mepy (4-methylpyridine) gives [Rh(4-Mepy)(ⁱPr-pymox)Cp*][SbF₆] as a mixture of diastereomers. Two complexes, [RhCl(ⁱPr-pymox)Cp*][SbF₆] (3) and [RhCl(Bz-pymox)Cp*][SbF₆] (6) have been characterised by X-ray crystallography.