Copper(II) triflate is the copper(II) salt of trifluoromethanesulfonic acid (known simply as triflic acid) which has a chemical formula of Cu(OSO<sub>2</sub>CF<sub>3</sub>)<sub>2</sub>, abbreviated Cu(OTf)<sub>2</sub>. This substance, first reported in 1972, is a powerful Lewis acid. It is used as a catalyst in several organic reactions, such as the DielsâÂÂAlder reaction and cyclopropanation reactions (much like rhodium(II) acetate).