A highly active catalyst for the reductive cyclization of ortho-nitrostyrenes under mild conditions

A mild and efficient method for the palladium-catalyzed reductive cyclization of ortho-nitrostyrenes to afford indoles is reported. Treatment of ortho-nitrostyrenes 3 with 0.1 mol% palladium (II) trifluoroacetate [Pd(TFA)2] and 0.7 mol% 3,4,7,8-tetramethyl-1,10-phenanthroline (tm-phen) in DMF at 15 psig CO and 80 °C gives indole 2 in 98% yield. The mechanism of the reaction was explored in detail.