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.