Xiao X. Rong

Kinetic effects of fluorine substituents in free-radical chemistry. Hydrogen-atom abstraction reactions of perfluoro-n-alkyl radicals

Abstract Using competitive methods, the rates of hydrogen-atom abstraction by a typical perfluoro-n-alkyl radical, n-C 7 F 15 ·, have been determined for a series of silane and stannane reductants including n Bu 3 SnH (2.03×10 8 M −1 s −1 ), Et 3 SiH (7.5×10 5 ), (TMS) 2 SiMeH (1.63×10 7 ) and (TMS) 3 SiH (5.1×10 7 ), all of which exhibited substantial rate enhancements relative to their analogous reductions of hydrocarbon radicals. The reduction by PhSH is, in contrast, ∼400-times slower than for hydrocarbon radicals. Transition state polar effects are invoked to rationalize the relative reactivity of perfluoro- versus hydrocarbon radicals in these hydrogen-transfer reactions. A Hammett study for H-atom transfer from arene thiols ( p + = −0.56) provided further substantiation for this conclusion. A discussion of the relative reactivity of t-butoxyl and perfluoro-n-alkyl radicals is presented.

Cyclization reactivities of fluorinated hex-5-enyl radicals

A kinetic study of the effect of fluorine substitution on the rates and regiochemistry of hex-5-enyl radical cyclization is reported. One or more fluorines on or proximate to the double bond of the radical have relatively little electronic effect on either rate or regiochemistry, whereas fluorines substituted at the radical end can have a dramatic impact on both. The relative reactivities of such partially-fluorinated hex-5-enyl radicals can be understood largely in terms of polar effects on the transition state, but radical pyramidalization and, to a lesser extent, addition thermodynamics play a role. The relationship between these fluorine substituent effects and the cyclopolymerizations of fluorinated α,ω-dienes are discussed.

Reactivity of perfluoro-n-alkyl radicals : a Hammett study of hydrogen transfer from arene thiols

Abstract A Hammett study of hydrogen atom abstraction by the perfluoro-n-heptyl radical is presented. Excellent correlation of the rates with σ+, with a ϱ+ value of −0.56, provides important corroboration of the substantial influence of transition state polar effects on this unexpectedly slow process.

The kinetic impact of vinylic fluorine substituents on 5-hexenyl radical cyclizations

Abstract A study of the effect of vinylic fluorine substituents on the 5-hexenyl radical cyclization indicates that substituents at the 6-position, or vicinal 5,6-difluoro substitution have little effect on the rate, whereas a 5-fluoro substituent slows down the rate by a factor of 10.8, and 5,6,6-trifluoro substitution enhances the rate by a factor of 2.3. The results are explained in terms of a combination of steric and enthalpic effects, with possible intervention of polar influences in the trifluorovinyl case.