THE REGIOSELECTIVITY IN THE FLUORINATION OF DIBENZOFURAN, DIPHENYLETHER AND BIPHENYL WITH N-F TYPE OF REAGENTS

Dibenzofuran, diphenylether and biphenyl were used as a target molecul es in an investigation of the effect of N-F type of reagent structure and reaction conditions on the fluorination process. The yields of flu orinated products formed and the regioselectivity depend on the reagen t used, and in the reactions with dibenzofuran three products were for med. The ratios of 2-fluorodibenzofuran (4) and 3-fluorodibenzofuran ( 5) formed were: 1.3 for -fluoro-4-hydroxy-1,4-diazoniabicyclo[2.2.2]oc tane bis(tetrafluoroborate) (NFTh, 1b), 1.4 for romethyl-4-fluoro-1,4- diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (F-TEDA, 1a), and 1.8 for N-fluoro-2,6-dichloropyridinium tetrafluoroborate (FPD-B, 1c). Ortho regioselectivity predominated for all reagents in the fluorinat ion of biphenyl (ortho : para ratio varied from 4.8 to 1.2), while par a attack prevailed for diphenylether (ortho : para = 0.9 - 0.7); howev er substitution of a phenyl ring by a methyl group enhanced the amount of 2-fluoroanisole. Second order rate constants for the fluorination of aromatic molecules with F-TEDA in acetonitrile at 65 degrees C were established: anisole 4.8x10(-3) Lmol(-1)s(-1), diphenylether 6.0x10(- 4) Lmol(-1)s(-1), dibenzofuran 2.5x10(-4) Lmol(-1)s(-1) and biphenyl 1 .0x10(-4) Lmol(-1)s(-1). Copyright (C) 1996 Published by Elsevier Scie nce Ltd