108. “[CpRu((R)-Binop-F)(H₂O)][SbF₆], a New Fluxional Chiral Lewis Acid Catalyst: Synthesis, Dynamic NMR, Asymmetric Catalysis, and Theoretical Studies”
     V. Alezra, G. Bernardinelli, C. Corminboeuf, U. Frey, E.P. Kündig, A.E. Merbach, C.M. Saudan, F. Viton, J. Weber,
     J. Am. Chem. Soc. 2004, 126, 4843-4853.

The C₂-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)₂H] complex followed by reaction with iodoform. Complex 6 was reacted with [Ag(SbF₆)] in acetone to yield [CpRu((R)-BINOP-F)(acetone)][SbF₆] ((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)][SbF₆] 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_α-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 ¹H, ³¹P, and ¹⁷O NMR spectroscopy, variable-pressure ³¹P and¹⁷O 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.

DOI : 10.1021/ja0374123 

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