Versatile Friedel-Crafts-type alkylation of benzene derivatives using a molybdenum complex/ortho-chloranil catalytic system

A variety of molybdenum complexes catalyze Friedel-Crafts-type alkylation reactions of benzene derivatives with alkenes and alcohols in the presence of an organic oxidant, o-chloranil. The utilization of Mo(CO)(6)] and two equivalents of o-chloranil catalytically furnished the hydroarylation product of norbornene with p-xylene at 80 degrees C, whereas Cr(CO)(6)] and W(CO)(6)] failed to catalyze the same reaction, thus indicating the importance of the molybdenum source. The best results were obtained when a molybdenum(II) complex CpMoCl(CO)(3)] (Cp=cyclopentadienyl) was used as a precatalyst. The hydroarylation reactions also took place with styrenes, cyclohexenes, and 1-hexene as olefin substrates. The electrophilic-substitution mechanism was proposed on the basis of the ortho/para selectivities and the Markovnikov selectivities observed for the hydroarylation products. Our hypothesis was further corroborated by the fact that in the presence of the CpMoCl(CO)(3)]/o-chloranil catalytic system, secondary, benzylic, or allylic alcohols participated in the alkylation of benzenes with similar selectivities.