A mild and versatile synthesis of bis(indolyl)methanes and tris(indolyl)alkanes catalyzed by antimony trichloride

Antimony trichloride is found to be a mild and efficient catalyst for electrophilic substitution reaction at 3-position of 3-unsubstituted indole derivatives with a variety of carbonyl compounds in acetonitrile to afford the corresponding bis(indolyl)methanes in excellent yields. alpha, beta-Unsaturated carbonyl compounds give tris(indolyl)alkanes under the same reaction conditions. The versatility of this method has been proved with a wide range of aromatic aldehydes with various stereo-electronic factors. This method shows much better selectivity between aldehydes and ketones as well as greater reactivity towards the electron deficient over the electron rich aromatic aldehydes, which are in contrast to the existing methods.