Metal triflates combined with caffeine based imidazolium salts: A new family of highly efficient and reusable catalysts

The direct alkylation of caffeine (1), by ethyl triflate gave 1,3,7-trimethyl-9-ethylxanthinium triflate (2) which led to 1,3,7-trimethyl-9-ethylxanthinium bis(trifluoromethanesulfonyl)amide (3) after methathesis with LiNTf2; 3 proved to be an ionic solid which can be used for the recovery of metal triflates (M(OTf)n with M = Sc, La, Yb, Cu, Hf, Bi). These reusable catalysts proved to be efficient Lewis acids for Diels–Alder reactions leading to very little or no polymerisation of the diene. In the case of bismuth (III) triflate, the catalyst can be recovered and reused at least 10 times without loss of activity.