Fluorocyclopropyl radicals

Methyl radicals are essentially planar but ESR measurements [116], supported by theoretical calculations [114], show that fluoromethyl radicals deviate from planarity to increasingly pyramidal structures upon further fluorination, with CF3 measured to be 49.1° from planarity. The barriers to inversion of fluoromethyl radicals increase in the order CFH2 < CF2H < CF, while fluorocyclopropyl radicals (Figure 4.46) adopt a fixed pyramidal conformation at the radical centre [117], as determined by ESR at — 108°C. The tendency for fluorine to induce pyrimidalisation of radical centres has also been used to account for the stereochemistry of products [118]. 

As the most electronegative element, fluorine would be expected to have the greatest effect on the stereochemical stability of the cyclopropyl radical and it does. When comparing ESR spectra of cyclopropyl radicals with 1-fluoro cyclopropyl radicals Kawamura and coworkers found the inversion frequencies of 1, 2 and 3 at — 99°C comparable to that found for the parent cyclopropyl radical, 10 s at — 175°C. In contrast, the inversion frequency of the a-fluorocyclopropyl radicals 4,5 and 6 is estimated to be lowered to 10 s at — 108°C. 

Generation and stereospecific reaction of fluorocyclopropyl radicals provided experimental evidence for the nonplanarity of the fluoroalkyl radical [23]. Hunsdiecker reaction... 

Cyclopropyl radicals are nonplanar but rapidly inverting species1 2. The inversion rate, however, depends strongly on the substituent at the radical center3. In the 1-fluorocyclopropyl radical 1, the rate of inversion is sufficiently slow for intermolecular reactions to be able to compete and occur with retention of configuration. This is seen in reactions of optically active precursors, in which no racemization occurs4. 

CFj-CFBr-CHPh CHj yields the same cis-trans mixture of l,l,2-trifluoro-3-phenylcyclobutanes, indicating that if the a-fluorocyclobutyl radical is non-planar its rate of inversion is faster than that of the a-fluorocyclopropyl radical energy barriers for radical inversions, calculated using the CNDO/2 approximation, can be correlated with this and other observations on radical inversions (see the Table). 

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