Competition between oxazolium and sulphonium salt formation in the acid-induced interaction of 2-diazoacetophenones with diaryl sulphides in acetonitrile

Abstract

Diphenyl sulphide and dibenzothiophen react with 2-diazoacetophenone (1a) and trifluoromethanesulphonic acid in dichloromethane to provide the corresponding phenacylsulphonium salts (3a) and (3c), respectively. Under similar conditions, 4-(diazoacetyl)dibenzothiophen (1c) gives the trifluoromethanesulphonate ester (4c) and not the cyclic sulphonium salt (3f). Interaction of diazoacetophenone (1a) and trifluoromethanesulphonic acid in acetonitrile yields 2-methyl-3-phenacyl-5-phenyloxazolium trifluoromethanesulphonate (7a) by phenacylation of the initially formed 2-methyl-5-phenyloxazole (6a); 4-(diazoacetyl)dibenzothiophen (1c) behaves analogously. 2-Diazoacetophenone (1a), trifluoromethanesulphonic acid, and benzonitrile afford 2,5-diphenyloxazole (6b) which does not undergo phenacylation. Both phenacyl-diphenylsulphonium (3a) and -dibenzothiophenium (3d) salts readily transfer their phenacyl groups to the nitrogen of 2-methyl-5-phenyloxazole (6a).