Fluorinated nitroxide radicals

Kinetic data for the reaction of the persistent nitroxides such as TEMPO (21) and the (CF3)2NO- radical with Ph3GeH and Bu3GeH, respectively, were determined by ESR spectroscopy.61,72 The fluorinated nitroxide was more than four orders of magnitude more reactive than TEMPO. On the other hand, the similar reactivities of the (CF3)2NO- and PhCMe200-radical toward Bu3GeH are expected because both the thermochemistries and spin densities for these two radicals are rather similar.72,73... 

The interpretation of the experimental a constants for the fluoroalkyl and nitroxide radicals is complicated because the stereochemistry at the radical center is not well defined. This complication does not exist for radicals derived from aromatic compounds. Consequently, several research groups have investigated aromatic radicals to characterize the factors governing spin delocalization to /3-fluorine atoms. One aspect of this work concerns the epr spectra of nitrobenzene anion radicals. Another concerns the contact chemical shifts of paramagnetic transition metal complexes. The latter approach was initiated by Eaton, Josey, and Sheppard who examined stable bis(phenylaminotroponiminato)-nickel(II) complexes (55a). More recently, we have examined the contact chemical shifts in the nmr spectra of nickel acetylacetonate complexes of aniline derivatives (556). 

The observations made in the work on the fluoroalkyl and nitroxide radicals suggests that the deloealization of spin density to /3-hydrogen and /3-fluorine atoms depends importantly on the electronegativity of the other atoms in the alkyl group. This aspect of epr spectroscopy has also been investigated by the study of the constants for the 4-substituted nitrobenzene anion radicals (Table 21). 

Becker, M. L., Liu, J., and Wooley, K. L. (2001). Preparation of tritrpticin block copolymer bio-conjugates by fluorinated nitroxide mediated radical polymerization on solid support. Abstracts of Papers, 222nd ACS National Meeting, Chicago, IL, United States, August 26-30, 2001, p. OLY-017. 

Few j8-fluorine splittings have been reported for radicals in solution. Smith et al. (1966) used a flow technique to examine CFs.CHOH and found to be 31-8 G. Scheidler and Bolton (1966) found that % for the nitroxide (CF8)2NO is temperature-dependent and suggested that there is a direct 1,3-interaction between fluorine and nitrogen involving overlap of the fluorine p orbitals and the nitrogen orbital (see also Strom and Bluhm, 1966). 

LAC Lacroix-Desmazes, P., Andre, P., Desimone, J.M., Ruzette, A.-V., and Boutevin, B., Macromolecular surfactants for supercritical carbon dioxide applications Synthesis and characterization of fluorinated block copolymers prepared by nitroxide-mediated radical polymerization (experimental data by P. Lacroix-Desmazes), J. Polym. Sci. Part A Polym. Chem., 42, 3537, 2004. 

Numerous nitroxide free radicals have been found to be sufficiently stable for the spin probe technique. In the ESR studies of fluorinated surfactants, N-oxyl derivatives of piperidine (I) or oxazolidine (II) have been used as the spin probes. 

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