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Carbosilane Dendrimers Peripherally Functionalized with P-Stereogenic Monophosphines. Catalytic Behavior of Their Allylpalladium Complexes in the Asymmetric Hydrovinylation of Styrene

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journal contribution
posted on 13.03.2006, 00:00 by Lara-Isabel Rodríguez, Oriol Rossell, Miquel Seco, Arnald Grabulosa, Guillermo Muller, Mercè Rocamora
The first dendrimers containing P-stereogenic monophosphines as peripheral groups were obtained by reacting the protected BH3-LiCH2PPhR (R = 2-biphenylyl, 9-phenanthryl) with zeroth- and first-generation chlorocarbosilane dendrimers, followed by treatment with morpholine and subsequent chromatographic purification. The first-generation dendrimer with the 9-phenanthryl-substituted phosphine could not be obtained, due to the incomplete functionalization of all the arms of the dendrimer. The reaction of the chiral dendrimers with the dinuclear allyl complex [Pd(μ-Cl)(η3-2-MeC3H4)]2 allowed us to graft PdCl(η3-2-MeC3H4) units on their surface. Complete characterization of these species was achieved by multinuclear NMR spectra (including 2D experiments), mass spectrometry, and optical rotation determinations. The catalytic properties of the new palladodendrimers were tested in the asymmetric hydrovinylation of styrene. To evaluate possible dendritic effects, two model chiral compounds, (CH3)3SiCH2PPhR, were prepared. The results in terms of activity, selectivity, and enantiomeric excess depend strongly on the nature of the phosphine and the halide abstractor. The best results were achieved with the employment of the first-generation dendrimer containing the 2-biphenylyl-substituted phosphine and NaBARF (79% ee toward the S enantiomer). In all experiments, the major enantiomer of 3-phenyl-1-butene had the absolute configuration S, with the exception of that catalyzed by the 9-phenanthryl-substituted phosphine dendrimer, which afforded (R)-3-phenyl-1-butene as the predominant enantiomer.