Journal article

[CpRu((R)-Binop-F)(H2O)][SbF6], a New Fluxional Chiral Lewis Acid Catalyst: Synthesis, Dynamic NMR, Asymmetric Catalysis, and Theoretical Studies

The C2-sym. electron-poor ligand (R)-BINOP-F (4; (R)-2,2'-[bis(pentafluorophenyl)phosphinoxy]-1,1'-binaphthalene) was prepd. by reaction of (R)-BINOL ((R)-1,1'-binaphthalene-2,2'-diol) with bromobis(pentafluorophenyl)phosphine in the presence of NEt3. 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-prepd. [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 1-point binding Lewis acid [CpRu((R)-BINOP-F)][SbF6] derived from either (R)-7 or the corresponding aqua 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. Addn. occurs predominantly to the methacrolein Calpha-Re face. In soln., H2O in (R)-8 exchanges readily. Moreover, a 2nd exchange process renders the diastereotopic BINOP-F P atoms equiv. These processes were studied by the application of variable-temp. 1H, 31P, and 17O NMR spectroscopy, variable-pressure 31P and17O NMR spectroscopy, and, using a simpler model complex, d. functional theory (DFT) calcns. The results point to a dissociative mechanism of the aqua ligand and a pendular motion of the BINOP-F ligand. NMR expts. show an energy barrier of 50.7 kJ mol-1 (12.2 kcal mol-1) for the inversion of the pseudo-chirality at the Ru center. [on SciFinder (R)]


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