Fe(OTf)(3)-catalyzed addition of sp C-H bonds to olefins

We have developed a novel acid-catalyzed addition of acetylenes to olefins in the presence of catalytic triflic acid or its metal salts. Among the various triflates, the catalytic activities depend on the cation and decrease in the order Fe(3+) > Al(3+) >> H(+), In(3+), Sc(3+) >> Cu(2+), Ag(+). In general, "hard" acids gave higher yields than "soft" acids such as copper and silver triflates. Among relatively hard acids, Fe(OTf)(3) was the best catalyst, which is also the case for ester formation from carboxylic acids and olefins. Our procedure is unique and attractive for the following reasons: (i) The reaction proceeds even for isolated C=C double bonds, as in norbornene. (ii) The reaction is promoted by acid catalysts and does not include an oxidation-reduction cycle for transition metals. (iii) Moreover, these catalysts are inexpensive, abundant, and less toxic than precious-metal-based catalysts. The reaction proceeds even under air and does not require precious metals.