Regiospecific aromatic fluorodemetallation of group IVb metalloarenes using elemental fluorine or acetyl hypofluorite

Abstract

The reactions of fluorine and acetyl hypofluorite with trimethylaryltin, -germanium, and -silicon compounds were studied using fluorine-18 (T 1 2 = 110 min) as a radiochemical tracer. The fluorodemetallation of phenyl derivatives by sub-stoichiometric F 2 to yield fluorobenzene was almost independent of temperature (−78° to 22°C), reaction time (2 to 200 min) and substrate concentration (2 to 150 mM). The reactions of both fluorination agents were not affected by perhalogenated methanes as solvents, but completely suppressed in solvents with active hydrogens. In a series of p-substituted phenyl derivatives (CH 3O-, CH 3-, H-, F-, Br-, CF 3-,O 2N-), a decrease in the fluorodemetallation yield was observed in the order Sn > Ge > Si, and with rings containing electron-withdrawing groups. The reactivity of CH 3CO 2F with stannylated arenes was equal to that of F 2, but much lower than that of F 2 when arylgermanium or -silicon derivatives were used. These findings are in agreement with a substitution mechanism involving a σ-complex. The only side-products which formed from the corresponding metallated anisyl- and toluyl- compounds were 2-fluoroanisole (< 16%) and benzylfluoride (< 5.5%). No direct hydrogen substitution was observed in the other substrates investigated. The high regiospecific fluorination yields obtained make aromatic fluorination of group IVb metallated arenes (especially of tin) a very attractive preparative fluorination and radio-labelling method.