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

Abstract

6 ) 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 CH 2 dCHCH(OH)CHdCH 2 , CH 2 dCHCH(OAc)CHdCH 2 , and CH 2 dCHCHdCHCH 2 Br with [Ru(Cp*)(CH 3 CN) 3 ](PF 6 ) afford new Ru(IV) η 3 -vinyl-allyl salts, two of which, [Ru(Cp*)(κ 2 -OAc)(η 3 -vinyl-allyl)](PF 6 ) (13) and [Ru(Cp*)Br(η 3 -vinyl-allyl)(CH 3 CN)](PF 6 ) (14), have been studied by X-ray diffraction methods. In addition to the Ru(IV) η 3 -vinyl-allyl bonding mode, we also find an isomer of the bromo salt 14 possessing a dynamic Ru(IV) S (rather than U)-shaped η 5 -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.

Cationic Ruthenium‐Cyclopentadienyl‐Diphosphine Complexes as Catalysts for the Allylation of Phenols with Allyl Alcohol; Relation between Structure and Catalytic Performance in O‐ vs. C‐Allylation

“…[11] Often intermedate protonation of allyl alcohol is implicated to increase the reactivity of allyl alcohol for allylation reactions. [10,11,22,23] The strong increase in allylation reaction rate observed with addition of a catalytic quantity of a strong acid such as HOTs (Table 5), can thus be explained by protonation of the allyl alcohol, making H 2 O the better leaving group.…”

Scope of the allylation reaction with [RuCp(PP)]+ catalysts: changing the nucleophile or allylic alcohol

“…[36 -39] However, thus far for Ru-based catalysts this effect has not been reported; mostly a preference for the branched product is reported, starting from either a branched or a linear allylic substrate. [19,21] The 'memory effect' observed in our experiments indicates that the isomerization of an initially formed branched σ -allyl to a linear σ -allyl species (Scheme 6 from B to D) or vice versa, via the π -allyl species (C) is a relatively slow process relative to reductive elimination (from B to A or D to E). However, in the homoallylic coupling reaction with substrate 7 (Table 4; entry 2), a small amount of branched isomer is also formed, which indicates that a Ru(IV)(π -allyl) species is formed during the catalytic cycle.…”

“…The C3-position apparently is more nucleophilic than the nitrogen atom as has been reported previously. [19] We have shown that indole cannot be allylated with an acidic [CpRu(PPh 3 ) 2 ] + catalyst system, since the acid necessary for activity in allylation is neutralized by indole. [10] Thus, although the indole NH moiety does not seem to coordinate to a Ru(II) species, it is basic enough to be protonated by the strong acid HOTs.…”

“…For ruthenium catalysts often the branched product is favored over the linear product. [16,19,21,22] The reactivity of several substituted allylic alcohols in [RuCp(PP)] + -catalyzed allylation reactions is reported in this paper.…”

“…[18] Apart from allyl alcohol as allylating agent, substituted allylic alcohols have also been used in allylation reactions. [16,19,20] Whereas reactions with allyl alcohol yield only one product, substituted allyl alcohols may yield different products: they react with a certain regioselectivity depending on the catalyst and substrate structure and such reactions often lead to interesting clues for the mechanism. For ruthenium catalysts often the branched product is favored over the linear product.…”