trifluoromethanesulfonyl amine or corresponding metal salts of the amine with the general formula me n so 3 cf 3 2 n wherein me represents a metal and n has the same value as the valency of the corresponding metal also suitable as proton acids are in particular mixtures of boric acid on the one hand and oxalic acid on the other hand in a molar ratio of 1 1 to 1 5 in particular 1 2 the use of proton acids takes place in a concentration range 10 u2212 2 mol to 100 mol with respect to the tmhq da used whereas lewis acids may be used in a concentration range 10 100 mol with respect to the tmhq da used larger amounts of catalyst are also suitable for performing the reaction but an increase in the amount of catalyst does not provide any further economic advantage although there are no specific restrictions with regard to the amount of solvent used the amount of solvent used is preferably 0 05 to 100 g g of tmhq da used wherein 0 1 10 g g of tmhq da are particularly preferred suitable organic solvents for the reaction are the carbonates known from ep a 0 24 208 including carbonate esters including the following ethyl isovalerate n propyl acetate i propyl acetate n butyl acetate i butyl acetate n butyl propionate ethyl butyrate i propyl butyrate methyl isobutyrate ethyl isobutyrate and methyl valerate are particularly preferred.