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-o ctenyl radicals have been determined. Remarkably large rate constants (>10( 7) 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 (>10(3) those of analo gous hydrocarbon radicals), whereas HFC hexenyl and heptenyl ethers exhibit lower cyclization reactivity, as do HFC 7-octenyl radical systems, which c yclize in an endo manner. HFC 8-nonenyl radicals were not observed to cycli ze. The results can be rationalized in terms of transition state polar infl uences, though other factors may also play significant roles.