Metallocenium incorporated charge-enhanced thiourea catalysts

Abstract

Several metallocenium derivatives were prepared including N-ferrocenium-N′-phenylthiourea tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BAr^F₄, 4a), N-methylferrocenium-N′-phenylthiourea BAr^F₄ (4b) and N-cobaltocenium-N′-phenylthiourea BAr^F₄ (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.