Metallocenium Lewis Acid Catalysts for Use in Friedel–Crafts Alkylation and Diels–Alder Reactions

A series of ferrocenium (Fc, FcCO2H, FcCO2Me, and Fc­(Me)10) and cobaltocenium (Cc, CcCO2H, CcCO2Me) salts were prepared and explored as air- and water-tolerant homogeneous catalysts. They were found to be active catalysts at room temperature for the Friedel–Crafts alkylation of trans-β-nitrostyrene and N-methylindole and the Diels–Alder cycloaddition of 1,3-cyclohexadiene with methyl vinyl ketone. These catalysts are valuable additions to more traditional Lewis acids in that they are soluble in nonpolar media, did not decompose the starting materials, and dual activation exploiting both the Lewis (metal) and Brønsted acid (CO2H) centers was observed.