Perhaloalkyl Radicals

Several reactions of halogen-substituted carbon-centered radicals with silanes have been studied, but limited kinetic information is available for reactions of halogen-substituted radicals with tin hydrides. A rate constant for reaction of the perfluorooctyl radical with Bu3SnH was determined by competition against addition of this radical to styrenes, reactions that were calibrated directly by LFP methods.93 At ambient temperature, the n-C8F17 radical reacts with tin hydride two orders of magnitude faster than does an alkyl radical, consistent with the electron-deficient nature of the perflu-oroalkyl radical and the electron-rich character of the tin hydride. Similar behavior was noted previously for reactions of silanes with perhaloalkyl radicals. 

Shoute LCT, Alfassi ZB, Neta P, Eluie RE (1994) Rate constants for reactions of (perhaloalkyl)peroxyl radicals with alkenes in methanol. J Phys Chem 98 5701-5704 Sies H (1986) Biochemie des oxidativen Stress. Angew Chem 98 1061-1075 Sies H (1991) Oxidative stress - oxidants and antioxidants. Academic Press, London Simic MG, Jovanovic SV, Niki E (1992) Mechanisms of lipid oxidative processes and their inhibition. ACS Symp Ser 500 14-32... 

Evidence based on product mixtures now suggests, at least in the cases of a-halocarbonyl and perhaloalkyl starting marterials, that these reactions are in fact atom transfer radical cyclizations (equation 166)324,325. In them, the palladium catalyst is proposed to have roles both as the radical initiator and as a trap for iodine, similar to the more commonly used hexabutylditin. Intramolecular allyl halide-alkyne cyclizations proceed with trans-addition to the triple bond this is evidence that a still different mechanism may be operating in these cases (equation 167)1,326. 

In the perhaloalkyl halide addition to C = C bonds, the radical attack is compatible with the presence of functional groups in the structure of the acetylenic substrate (see Table 6). For example, formation of alkcne 2 and amine 3. ... 

BrCF2X (X = Br, Cl, Cp2Br) without irradiation or use of radical initiators, affording, after hydrolysis, the corresponding ot-perhaloalkyl aldehydes and ketones in moderate yields (equation 3). The yields are much higher if an additional tertiary amine is used to trap HI. 

The observations that these reactions are inhibited by nitrobenzene (a free radical inhibitor), no hydrogenated by-products are formed and that CF BrCl gives only a-CFjCl carbonyl compounds, led the authors to propose a radical chain mechanism for these reactions (Scheme 1). The chain initiation step is the formation of XFiC radical and enamine radical cation by electron transfer from the enamine to BrCFjX. The addition of this perhaloalkyl radical to the enamine generates a RjNC R R" type radical which is known to have an unusually low oxidation potential with 1/2 in the range of — 1 V (sce). An electron transfer from this radical to another molecule of perhaloalkane then takes place to form the iminium salt and another perhaloalkyl radical which continues the chain. A similar mechanism operates in the case of Rp. ... 

The electrophilic character of perhaloalkyl radicals (and in particular perfluoroalkyl radicals) makes them particularly attractive for hydrogen atom abstraction [100], Indeed, their reactivity resembles that of alkoxyl radicals, and rates of hydrogen atom abstraction from C-H bonds are much higher than those of the corresponding hydrocarbon radicals [lOlj. Their electrophilic character facilitates abstraction at... 

The perhalomethanes react in the chain transfer process (as shown in eq. 38) to give a perhaloalkyl radical, which in turn is capable of initiating the polymerization. 

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