Silver-Nanoparticle-Catalyzed Dearomatization of Indoles toward 3-Spiroindolenines via a 5-exo-dig Spirocyclization

We present a supported silver-nanoparticle-catalyzed dearomatization of 3-substituted indoles toward 3-spiroindolenines. Two scaffolds were investigated for this transformation. The yields range from moderate to high. In the case of chiral reactants (Ugi four-component reaction adducts), the process is diastereoselective with a diastereomeric excess between 75% and 92%. The catalyst was characterized by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Metal leaching was investigated using hot-filtration and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) experiments. An apparent turnover frequency (TOF) was determined.