Metallocenium incorporated charge-enhanced thiourea catalysts.

Several metallocenium derivatives were prepared including -ferrocenium-'-phenylthiourea tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BAr, 4a), -methylferrocenium-'-phenylthiourea BAr (4b) and -cobaltocenium-'-phenylthiourea BAr (5). These compounds are competent catalysts for the Friedel-Crafts alkylation of indoles with -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 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.