Rapid, Selective Ru-Sulfonate-Catalyzed Allylation of Indoles Using Alcohols as Substrates

The Ru-catalyzed allylation of differing indole compounds using ArCH(OH)CHCH2 (Ar = Ph, 2-CH3C6H4, 2-CH3OC6H4 2-ClC6H4, and 1-naphthyl) is relatively fast (usually <30 min for 100% conversion) at ambient temperature in the presence of [Ru(Cp*)(CH3CN)3](PF6) + a sulfonic acid cocatalyst, RSO3H (R = p-tolyl, a camphor fragment or CH3). The new catalysts allow allyl alcohols rather than carbonates, halides, acetates, etc., to be used as substrates in this allylation chemistry, thereby avoiding the use of a (wasted) leaving group. The branched products are favored, and in a number of examples b/l ratios in excess of 20 are observed. The catalyst is selective in that there is little or no N-allylation. The use of acids such as CF3SO3H or HBF4 results in slower and/or less selective reactions. Although phosphoric acid additives show some promise, carboxylic acids are much less effective or do not catalyze the reaction at all. The preformed isolated complexes [Ru(Cp*)(η3-C3H5)(RSO3)2] are efficient catalyst precursors; however, NMR experiments suggest that one of the active species is the monosulfonate cationic complex [Ru(Cp*)(η3-C3H5)(CH3CN)(RSO3)]+. Stoichiometric oxidative addition reactions are shown to be relatively rapid. New Ru(Cp*) complexes have been generated and characterized by NMR methods. DFT calculations suggest that the large b/l ratios observed have a structural basis in that the Ru−C(allyl) bond length for the substituted terminal allyl carbon, Ru−C(1), is relatively long. X-ray structures for Ru(Cp*)(η3-C3H5)(SO4) and the dication [Ru(Cp*)(η3-C3H5)(H2O)2]2+ (as a camphor sulfonate salt) are reported.