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

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.