Xenon difluoride fluorination. 3. Mechanism and selectivity of boron trifluoride etherate catalysis in the norbornene model

Abstract

Abstract : The boron trifluoride etherate initiated fluorination of norbornene with xenon difluoride in dichloromethane solvent selectively produces the novel 2-exo-5-exo-difluoronorbornane and its analogous 2-endo 5-exo-difluoronorbornane represent approximately half the directly fluorinated reaction products, and the well-known anti- and syn-2,7-difluoronorbornane isomers comprise the remainder. Subsequently the boron trifluoride etherate catalyzes selective isomerization of both 2,7-difluoronorbornanes to produce a nearly exclusive yield of the 2-exo-5-exo- and 2-endo-5-exo-difluoronorbornane isomers. A change to diethyl ether solvent yields the normal anti- and syn-2,7-difluoronorbornane isomers as the major difluoride products and quenches the boron trifluoride etherate isomerization. Substitution of lithim tetrafluoroborate for the boron trifluoride etherate in diethyl ether solvent again provides the unique 2,5-difluoronorbornanes as the major difluoride products. The 2,5-difluoronorbornane isomers were unambiguously identified by the direct fluorination of deuterated norbornene. The deuterated norbornene also permitted the initial fluorination mechanism and the selective isomerization pathway in dichloromethane solvent to be studied. (Author)