Alkyltrifluoromethanesulphonates as alkylating reagents for aromatic compounds

Abstract

Methyl and ethyl trifluoromethanesulphonates (‘triflates’), prepared by conventional routes involving either trifluoromethanesulphonic acid (‘triflic acid’) or its anhydride, contain traces of triflic acid as an impurity, which catalyse their alkylation reactions with aromatic compounds. Pure methyl triflate, obtained from reaction between CH₃I and CF₃SO₃Ag, does not alkylate p-cymene after several hours at 100 °C. Pure ethyl triflate, prepared by a similar method, is thermally less stable under these conditions, and alkylation takes place only after long induction periods during which some breakdown to triflic acid occurs. With aromatic substrates such as p-cymene or mesitylene the onset of alkylation is followed rapidly by the formation of isomerisation and disproportionation products. Benzyl triflate, prepared from PhCH₂Br and CF₃SO₃Ag, alkylates p-cymene even at room temperature. The strong Lewis acids SbF₅ and AlCl₃ similarly catalyse alkylation reactions of methyl and ethyl triflates, but BF₃, FeCl₃, and SnCl₄ are much less effective.