title: [CpRu((R)-Binop-F)(H2O)][SbF6], a new fluxional chiral Lewis acid catalyst: synthesis, dynamic nmr, asymmetric catalysis, and theoretical studies.
abstract: The C(2)-symmetric electron-poor ligand (R)-BINOP-F (4) was prepared by reaction of (R)-BINOL with bis(pentafluorophenyl)-phosphorus bromide in the presence of triethylamine. The iodo complex [CpRu((R)-BINOP-F)(I)] ((R)-6) was obtained by substitution of two carbonyl ligands by (R)-4 in the in situ-prepared [CpRu(CO)(2)H] complex followed by reaction with iodoform. Complex 6 was reacted with [Ag(SbF(6))] in acetone to yield [CpRu((R)-BINOP-F)(acetone)][SbF(6)] ((R)-7). X-ray structures were obtained for both (R)-6 and (R)-7. The chiral one-point binding Lewis acid [CpRu((R)-BINOP-F)][SbF(6)] derived from either (R)-7 or the corresponding aquo complex (R)-8 activates methacrolein and catalyzes the Diels-Alder reaction with cyclopentadiene to give the [4 + 2] cycloadduct with an exo/endo ratio of 99:1 and an ee of 92% of the exo product. Addition occurs predominantly to the methacrolein C(alpha)-Re face. In solution, water in (R)-8 exchanges readily. Moreover, a second exchange process renders the diastereotopic BINOP-F phosphorus atoms equivalent. These processes were studied by the application of variable-temperature (1)H, (31)P, and (17)O NMR spectroscopy, variable-pressure (31)P and(17)O NMR spectroscopy, and, using a simpler model complex, density functional theory (DFT) calculations. The results point to a dissociative mechanism of the aquo ligand and a pendular motion of the BINOP-F ligand. NMR experiments show an energy barrier of 50.7 kJ mol(-1) (12.2 kcal mol(-1)) for the inversion of the pseudo-chirality at the ruthenium center.
authors.affiliation: Department of Organic Chemistry, University of Geneva, CH-1211 Geneva 4, Switzerland.
authors.forename: Valérie
authors.initials: V
authors.lastname: Alezra
authors.forename: Gérald
authors.initials: G
authors.lastname: Bernardinelli
authors.forename: Clémence
authors.initials: C
authors.lastname: Corminboeuf
authors.forename: Urban
authors.initials: U
authors.lastname: Frey
authors.forename: E Peter
authors.initials: EP
authors.lastname: Kündig
authors.forename: André E
authors.initials: AE
authors.lastname: Merbach
authors.forename: Christophe M
authors.initials: CM
authors.lastname: Saudan
authors.forename: Florian
authors.initials: F
authors.lastname: Viton
authors.forename: Jacques
authors.initials: J
authors.lastname: Weber