New Methods of Free-Radical Perfluoroalkylation of Aromatics and Alkenes. Absolute Rate Constants and Partial Rate Factors for the Homolytic Aromatic Substitution by n-Perfluorobutyl Radical

Abstract

New methods of free-radical perfluoroalkylation of aromatics and alkenes are reported. n-C(4)F(9)I has been utilized as source of C(4)F(9)(*) radical through iodine abstraction by phenyl or methyl radical. The reaction with alkenes, carried out in the presence of catalytic amount of Cu(OAc)(2), leads to substitution by a mechanism substantially identical to the aromatic substitution and not to the usual chain addition of perfluoroalkyl group and iodine atom to the double bond. This has allowed to measure for the first time the absolute rate constants and the partial rate factors for the homolytic aromatic perfluoroalkylation by competition kinetics. The C(4)F(9)(*) radical shows a clear-cut electrophilic character in the aromatic substitution, as already reported for the addition to alkenes, but the low regio- and chemoselectivities suggest that the polar effect is not the main factor in determining the high reactivity of perfluoroalkyl radicals toward aromatics (10(5)-10(6) M(-1) s(-1), 2-3 orders of magnitude more reactive than alkyl radicals). The enthalpic factor, related to the involved bond energies, appears to be the major cause of the increased reactivity. The polar effect is considered as related more to the polarizability than to the polarity of a radical (the sigma-perfluoroalkyl radicals are considered less polarizable and hence less sensitive to polar effects than pi-radicals).

Photocatalytic fluoroalkylation reactions of organic compounds

“…A variety of reagents, such as CF 3 I, 50,[55][56][57]59,62,66,67 silver trifluoroacetate, triflyl chloride (CF 3 SO 2 Cl), 49,64 the Langlois reagent, the Togni reagent, 51,53,54,60,65 the Umemoto reagent, 48,52,58,63,65 and the Shibata reagent, 61 can act as trifluoromethyl radical (•CF 3 The excited catalyst can reduce the R f X source (oxidative quenching pathway), regeneration of the catalyst follows by means of an electron donor or else, the photoexcited catalyst could be reduced by an electron donor to a one-electron reduced species that transfers an extra electron to the R f X source (reductive quenching pathway).…”

New, Simple and Selective Synthesis of Perfluoroalkylquinones by Perfluoroalkyl Radicals – Enthalpic and Polar Effects

“…Also, the relative rates of the addition of R f · to the aromatic rings [7] strongly confirm that electrophilic character is a significant but not the major factor, for the increased aromatic reactivity of R f · compared to alkyl radicals. The fact that n-C 4 F 9 · is more reactive (ca.…”

“…However, more recently we have shown [7] that both polar and enthalpic effects are very important in determining the reactivity and selectivity of perfluoroalkyl radicals, but with the prevalence of the latter in the reaction with alkenes, aromatic rings and alkanes. This is due to the fact that C-C and C-H bonds formed by R f · are stronger than those formed by the corresponding alkyl radicals (i.e.…”

Perfluoroalkylation of alkenes and alkynes in water

“…26,27 In the first step, the azo bond of the radical initiator (ACCN) is thermally broken forming two radicals, that react with the C-I bond of the perfluoroalkyl iodide (R F I), forming a perfluoroalkyl radical, R F˙. This radical then adds to the double bound of the alkene, at the terminal carbon, forming the more stable secondary radical, I.…”