Cyclizations of 5-Hexenyl, 6-Heptenyl, 7-Octenyl, and 8-Nonenyl Radicals. The Kinetic and Regiochemical Impact of Fluorine and Oxygen Substituents

Using competition kinetic methodology, rate constants for cyclizations of a series of hydrofluorocarbon (HFC) and ether 5-hexenyl, 6-heptenyl, and 7-octenyl radicals have been determined. Remarkably large rate constants (>107 s-1) have been observed for 6-exo-cyclizations of 1,1,2,2-tetrafluoro- and 1,1,2,2,3,3,4,4,-octafluoro-6-heptenyl radicals (>103 those of analogous hydrocarbon radicals), whereas HFC hexenyl and heptenyl ethers exhibit lower cyclization reactivity, as do HFC 7-octenyl radical systems, which cyclize in an endo manner. HFC 8-nonenyl radicals were not observed to cyclize. The results can be rationalized in terms of transition state polar influences, though other factors may also play significant roles.