Metallocenium incorporated charge-enhanced thiourea catalysts.

Several metallocenium derivatives were prepared including N -ferrocenium- N '-phenylthiourea tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BArF 4 , 4a), N -methylferrocenium- N '-phenylthiourea BArF 4 (4b) and N -cobaltocenium- N '-phenylthiourea BArF 4 (5). These compounds are competent catalysts for the Friedel-Crafts alkylation of indoles with trans -β-nitrostyrenes, and are much more active than their reduced non-charged forms. The iron derivatives are less stable than the cobalt analog and were generated and used in situ whereas the cobalt-containing thiourea was isolated and characterized by X-ray crystallography. All three of these metallocenium salts have Lewis and Brønsted acidic sites which were exploited in tandem to afford charge-activated catalysts.