Silica-supported ceric ammonium nitrate catalyzed chemoselective formylation of indoles

Selective formylation of free (N–H) indoles at C3 can be achieved by using formylating species generated from hexamethylenetetramine (HMTA) and silica-supported ceric ammonium nitrate (CAN–SiO2). The use of a catalytic amount of this solid-supported reagent was found to be compatible with a range of substituents on the indoles and generated the corresponding products with good yields. A plausible mechanism for the formylation involving an electron transfer process is discussed.