Fluorination of Fluorene, Dibenzofuran and Their Open Analogues with Caesium Fluoroxysulfate and Related Fluorinating Reagents

Abstract

Fluorination of fluorene (1) with caesium fluoroxysulfate (CFS), 2,6-dichloro-1-fluoropyridinium tetrafluoroborate (FP-B800) and 1-fluoro-4-hydroxy-1,4-diazoniabicyclo[2.2.2]- octane bis(tetrafluoroborate) (Accufluor NFTh) occurred only on the aromatic ring in the position ortho and para to the biphenyl central bond with the ratio 2-fluoro- (2a) vs 4-fluorofluorene (2b) 1.7-2.4:1. Regioselectivity of fluorination of both open-chain analogues - diphenylmethane (3a) and biphenyl (3b) was different and more ortho-fluorinated product was formed. Furthermore, the reaction of diphenylmethane (3a) with CFS occurred also on central carbon forming benzophenone (6) and fluorodiphenylmethane (7), while fluorination with FP-B800 and Accufluor NFTh occurred only at the aromatic ring. Similar effect of the structure of fluorinating reagent on the regioselectivity was also observed with dibenzofuran (8) and its open-chain analogues diphenyl ether (10) and biphenyl (3b), where the regioselectivity of fluorination with CFS (1- (9a):2- (9b):3- (9c) = 27:46:27) was similar to fluorination with Selectfluor. Product distribution of fluorination of fluorene (1) and dibenzofuran (8) with CFS is similar to nitration and is in accordance with the calculated HOMO electron density, which indicates the presence of the electron transfer pathway.