Cationic Planar Chiral (η 6 -Arene)Mn(CO) 3 + Complexes: Resolution, NMR Study in Chiral-Oriented Solvents, and Applications to the Enantioselective Synthesis of 4-Substituted Cyclohexenones and (η6 -Phosphinoarene)Mn(CO)3+ Complexes
Résumé
An easy round-trip of (D)-(þ)-camphor enolate to a racemic mixture of cationic (η6-arene)Mn-(CO)3
complexes is the base of the strategy adopted for the first resolution of such complexes. X-ray
structures of one of the (η5-cyclohexadienyl)Mn(CO)3 diastereoisomers obtained after addition of
the chiral auxiliary as well as of the corresponding enantiopure η6 cationic complex after rearomatization
have been established. Proton-decoupled deuterium 2D NMR in chiral polypeptide
liquid crystals proved to be an efficient tool for the determination of the enantiomeric purity of such
planar chiral cationic η6 complexes. The potential of this unprecedented resolution is exemplified by
enantioselective syntheses in organic and organometallic fields. Thus, starting from the enantiopure
(η6-meta-halogenoanisole)Mn(CO)3 complexes, enantiopure 2,4-disubstituted cyclohexenones
were easily generated through a Mn-assisted dearomatization process, and the first examples of
enantiopure η5- and η6-phosphino-substituted Mn complexes were obtained through lithiation/
electrophilic quench sequence.