The aqua complexes (S M ,R C )-[(η 5 -C 5 Me 5 )M(PROPHOS)-(H 2 O)][SbF 6 ] 2 [PROPHOS = (R)-propane-1,2-diylbis(diphenylphosphane); M = Rh (1), Ir (2)] are active catalysts for the asymmetric Diels−Alder reaction between ketones and dienes. At low temperatures, enantioselectivities of up to 89% ee are achieved. The intermediate Lewis acid−dienophile complexes (S M ,R C )-[(η 5 -C 5 Me 5 )M(PROPHOS)(MVK)][SbF 6 ] 2 (MVK = methyl vinyl ketone; M = Rh (3), Ir (4)) and (S Ir ,R C )-[(η 5 -C 5 Me 5 )Ir(PROPHOS)(EVK)]-[SbF 6 ] 2 (EVK = ethyl vinyl ketone (5)) have been isolated and characterized by analytical and spectroscopic means, including the determination of the crystal structure of the iridium complexes 4 and 5 by X-ray diffractometric methods. Structural parameters indicate that the dispositions of the coordinated dienophiles are controlled by the CH/π attractive interactions established between a phenyl group of the PROPHOS ligand and the α-vinyl proton of the ketones. Proton NMR parameters indicate that these interactions are maintained in solution. From these data, the stereoselectivity of the catalytic reaction is discussed.
The aqua complexes [(η5-C5Me5)M(PP*)(H2O)][SbF6]2 (M = Rh, Ir;
PP* = chiral diphosphane) (1–10)
are prepared and characterized. These complexes efficiently catalyze
the Diels–Alder reaction between methacrolein and HCp with
enantioselectivities of up to 96% e.e. The norphos complexes 9 and 10, which are obtained as diastereomeric
mixtures of the two epimers at the metal, afford the highest e.e.
values. The intermediate complexes [(η5-C5Me5)M(PP*)(methacrolein)][SbF6]2 (11–20) are also isolated and characterized,
including the molecular structure determination of the [(η5-C5Me5)M(benphos)(methacrolein)][SbF6]2 (M = Rh (13), Ir (14)) derivatives. An NMR study of the reactivity of the metallic intermediates
under catalytic conditions explains the high enantioselectivities
achieved with the norphos systems.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.