The ultra high pressure conjugate addition of indoles to electron-deficient olefins

The addition of indole and methyl indole at both high and ambient pressures to a series of Michael accepters under the influence of ytterbium triflate was investigated. Under ambient pressure the more reactive and less sterically hindered electrophiles gave the expected 3-alkylated indoles in good to excellent yields. The more problematic Michael accepters were subjected to pressures of 13 kbar. In all cases a dramatic reduction in reaction time and a significant improvement in yields was observed. In the cases involving 3-methylcyclohex-2-en-1-one, a by-product was formed and was characterized by single crystal X-ray diffraction. alpha,beta-Unsaturated ketones gave the best yields. Enals tended to polymerize while enoates proved to be much too unreactive. A particularly reactive malonate derived ester and beta-nitrostyrene gave good yields at ambient pressures.