Alkyl radical fluorinated derivative

Fluorine substituents have a dramatic impact upon the structure of alkyl radicals. The methyl radical itself is planar UV, IR, PES and ESR spectroscopy, as well as the highest level of theoretical analysis, all indicate that its conformational properties are best defined as deriving from a single minimum [2], Fluoromethyl radicals, on the other hand, are increasingly pyramidal [3], with the trifluoromethyl radical being essentially tetrahedral [3 - 7], with a significant barrier to inversion [8,9]. 

The influence of fluorine substituents on the stability of alkyl radicals derives from the same complex interplay of inductive and resonance effects that affects their structure. Simple orbital interaction theory predicts that substituents of the -X type (that is, electronegative substituents bearing lone pairs) should destabilize inductively by virtue of their group electronegativities, and stabilize by resonance to the extent of their ability to delocalize the odd electron. 

Fluorinated radicals, in contrast, would be expected to derive their electro-philicities virtually entirely from fluorine s inductive effect. One would expect the reactivity of perfluoro-n-alkyl radicals to differ significantly from that of their hydrocarbon counterparts, since the latter are electron-rich, planar n-radicals, whereas the former are electron poor, nonplanar cr-radicals. 

The termination mode of poly(alkyl methacrylate) radicals has also been the subject of much research.[9] Model compound studies of the bimolecular reactions of l-methoxycarbonyH-methylethyl radicals and the higher esters ethyl and butyl have resulted in for MMA, 0.72 for EMA, and 1.17 for nBMA.[9,10]. A k /k =4.37 was obtained by means of Matrix-assisted laser desorption/ionisation-time of flight mass spectrometry (MALDI-TOF MS) to the end-group analysis of low MW PMMA.[11] The use of fluorinated derivatives of BPO in combination with F NMR analysis of PMMA also indicated that the termination occurred mainly due to the disproportionation in this system.[12]... 

Radical cyclization continues to be one of the most popular radical reaction classes used in synthesis. By far the majority of published cyclization reactions are 5-exo in nature and this year is no exception. The rates of 5-exo cyclization reactions continue to be measured. For example, the rate constants for the 5-exo cyclizations of the 6,6-diphenylhex-5-enyl, 1-methyl-6,6-diphenylhex-5-enyl and the 1,1-dimethyl-6,6-diphenylhex-5-enyl radicals have been measured by laser flash photolysis studies and Arrhenius parameters determined. The relative rate constants for cyclization, and the reaction with PhSeH, were determined at 20 °C. At 20 °C the rates of the three primary, secondary and tertiary radicals with PhSeH were approximately (1.2 0.1) X 10 lmol s The rate constants for alkyl radicals calibrated by competition reactions with PhSeH and PhSH were found to be 30-40% smaller than previously reported and thus it was concluded that derived rate constants for some fast radical reactions may have to be adjusted accordingly. The rate constants for the 5-exo cyclization of secondary radicals on to hydrazones and oxime ethers have been determined. The fastest rate constants were observed for the IV-benzoylhydrazone acceptor. The rate constants were found to be approximately 800 times faster than for the corresponding 5-exo cyclization on to alkenes." Other work has measured the rate constants of cyclization of a range of fluorinated hex-5-enyl, hept-6-enyl, oct-7-enyl and non-8-enyl radicals. Not only were the rate constants measured but the regioselectivity of cyclization was compared with that for the aU-carbon homologues." The 6-exo cyclization of 1,1,2,2-tetrafluoro- and l,l,2,2,3,3,4,4-octafluorohept-6-enyl radicals were found to be approximately 10 times faster than those for the parent... 

The trivalent fluoro-radicals which we shall discuss are SF3 and its derivatives. The ESR spectra of CF3 and other perfluoro-alkyl radicals have been adequately treated elsewhere (34, 35). The radical SF3 was first detected in electron-irradiated SF ( ). Its spectrum indicates the presence of two, rather than three equivalent F nuclei a iq(2) = 54. 3 G, jq(l) = 40.4 G. This observation is consistent with the structure of SF3 shown in Fig. 3, in which the semi-occupied orbital is described as anantibonding combination of F p(2p) orbitals and the S(3s, Sp ) orbitals. The bonds to the apical fluorines are weaker, longer, and more polarizable, resulting in the somewhat larger hyperfine interaction of the apical nuclei. 

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