The chemistry of nitrilium salts. Part 3. The importance of triazinium salts in Houben–Hoesch reactions catalyzed by trifluoromethanesulphonic acid

Abstract

In the presence of trifluoromethanesulphonic (triflic) acid, isobutyronitrile reacts with anisole, 1,3-dimethoxybenzene, resorcinol, 1,3,5-trimethoxybenzene, and phloroglucinol at room temperature to give acylation products. A study of the reactions of acetonitrile and isobutyronitrile with triflic acid has shown that these modified Houben–Hoesch reactions occur by initial cyclotrimerization of the nitriles to 1,3,5-triazinium triflate salts followed by a slow reaction of the salts with the aromatic substrate. Support for this comes from the isolation of 2-(4-methoxyphenyl)-2,4,6-tri-isopropyl-1,2-dihydro-1,3,5-triazine from the reaction between anisole, triflic acid, and isobutyronitrile. Similar 1,2-dihydro-s-triazines and their salts have been prepared by reaction of trimethyl-s-triazinium triflate with 1,3-(MeO)₂C₆H₄, and tri-isopropyl-s-triazinium triflate with 1,3-(RO)₂C₆H₄(R = H or Me) and 1,3,5-(MeO)₃C₆H₃; these salts are easily hydrolyzed to the corresponding aromatic ketones. The salt [1,3,5-Me₃C₃N₃H₂]⁺ 2Ō₃SCF₃ also reacts with o-phenylenediamine to give 2-methylbenzimidazolinium triflate in 83% yield. Nitrilium salts [RCNMe]⁺Ō₃SCF₃(R = Me, Ph, or PhCH₂) undergo rapid reactions with 1,3-(MeO)₂C₆H₄ to form acylation products after hydrolysis, and N,N-dimethylaniline similarly affords 4-Me₂NC₆H₄COMe on reaction with [MeCNMe]⁺Ō₃SCF₃ at room temperature.