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Facile Ruthenium(IV)-Catalyzed Single and Double Allylation of Indole Compounds using Alcohols as Substrates: Aspects of Ruthenium(IV) Allyl Chemistry

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posted on 11.08.2008, 00:00 by Stefan Gruber, Alexey B. Zaitsev, Michael Wörle, Paul S. Pregosin, Luis F. Veiros
The dicationic Ru(IV) salt [Ru(Cp*)(η3-C3H5)(CH3CN)2](PF6)2 is shown to be an excellent and rapid catalyst for the C-allylation of indole compounds using allyl alcohols as substrates. Selective, one-pot N,C-double allylation is also possible. Preliminary experiments suggest that vinyl epoxides may be used as an allyl source. Stoichiometric reactions of CH2CHCH(OH)CHCH2, CH2CHCH(OAc)CHCH2, and CH2CHCHCHCH2Br with [Ru(Cp*)(CH3CN)3](PF6) afford new Ru(IV) η3-vinyl-allyl salts, two of which, [Ru(Cp*)(κ2-OAc)(η3-vinyl-allyl)](PF6) (13) and [Ru(Cp*)Br(η3-vinyl-allyl)(CH3CN)](PF6) (14), have been studied by X-ray diffraction methods. In addition to the Ru(IV) η3-vinyl-allyl bonding mode, we also find an isomer of the bromo salt 14 possessing a dynamic Ru(IV) S (rather than U)-shaped η5-pentadienyl moiety, 20. DFT computational results for 20 indicate that the S-shaped rather than the U-shaped form is the most stable Ru(IV) species and that the stability difference between these two forms is mainly due to interligand repulsion between the Br and pentadienyl ligands.