of indoles with aldehydes to afford the corresponding bis(indolyl)methanes in high yields
under solvent free conditions. … According to the literature data,17 I think that TiO2
catalyzes the reaction as a mild Lewis acid even under solvent-free conditions. TiO2 has (i)
Ti4+ site, which is of Lewis-acid type, (ii) O2– site, which is of Lewis-base type, and (iii)
surface –OH groups.The activation of carbonyls through hydrogen bonding by Lewis acids …