FLUORINATION WITH XEF2 .40. THE IMPORTANT ROLE OF PI-BOND DISRUPTION IN FLUORINE ADDITION TO PHENYL-SUBSTITUTED ALKENES

Ionization potentials (IP) for 11 mono- and alpha,alpha-diphenyl-subst ituted acyclic and cyclic alkenes 1-11 were measured under the same co nditions, using a mass spectrometric technique and applying electron i mpact ionization. The values, ranging from 8.68 eV for styrene (1) to as low as 7.67 eV for 1,2-dihydro-4-phenylnaphthalene (10), were corre lated with the logarithms of the relative rates (k(rel), relative to 1 ,1-diphenylethene) of fluorine addition across the carbon-carbon doubl e bond in HF-catalyzed reaction of the alkenes with XeF2 in CH2Cl2 at room temperature. The linear relationship between log k(rel) and IP wi th a regression slope of -2.08 and a correlation constant of 0.942, pr esented for the first time for any fluorination reaction of organic co mpounds, demonstrates that ct-bond disruption is the rate-determining step in the fluorine addition process for the series of alkenes 1-11. The course of reaction of the series of three 1-phenyl-benzocyclenes ( 10-12) with XeF2 was investigated. The addition-elimination process, f orming 3-phenyl-2-fluoro-1H-indene (15a), was found exclusively in the case of 9-phenyl-1H-indene (12), while the formation of diastereoisom eric pairs of vicinal difluorides (13 and 14), with the trans isomers slightly predominant, was established in the case of 1,2-dihydro-4-phe nylnaphthalene (10) and 6,7-dihydro-9-phenyl-5H-benzocycloheptene (11) . On heating the vicinal difluorides 13 and 14 HF was released, thus y ielding 3-fluoro-4-phenyl-1,2-dihydronaphthalene (15b) and 6,7-dihydro -8-fluoro-9-phenyl-5H-benzocycloheptene (15c), respectively.