Abstract
The main product in the direct elemental fluorination of 4-fluorophenyltrimethylsilane was 1,4-difluorobenzene, produced by the ipso electrophilic substitution of the trimethylsilyl group. An unexpected product, 2,5-difluorophenyltrimethylsilane, was also formed as the result of a novel 1,2-migration of the trimethylsilyl group. The same trimethylsilyl 1,2-shifts were observed in the elemental fluorinations of 2-fiuoroand 2,4-difluoro-phenyltrimethylsilanes. We propose that the carbocations formed in the rearrangements were stabilised by the mesomeric effect of the α-fluorine and the greater stability of these intermediates provided the driving force for the trimethylsilyl group migrations. Although the main products in all of the fluorinations were the fiuorodesilylated product, the reactions were not totally regioselective and some competing ftuorodeprotonation also occurred in the reactions of 2-fluoro-and 2,4-difluoro-phenyltrimethylsilanes. The role of protic acids and Lewis acids in particular triflic acid, trifluoroacetic acid and boron trifluoride in a variety of co-solvents, respectively, was crucial in all the fluorinations we studied, experiments done in their absence gave very poor yields. Optimal conditions for fluorination of silylated substrates are described.
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