Cross Cage Interactions in Substituted Bicyclo[1.1.1]pent-1-yl Radicals. Dissociation to [1.1.1]Propellane

A series of 3-substituted bicyclo[1.1.1]pent-1-yl radicals, including the 3-fluoro derivative, was generated by bromine atom abstraction from 1-bromo-3-substituted-bicyclo[1.1.1]pentanes and examined by EPR spectroscopy. The exceptionally large hyperfine splittings obtained from magnetic nuclei of the 3-substituents indicated that cross cage electronic interactions were substantial in these species. Bromine atom abstraction by triethylsilyl radicals from 1-bromo-3-fluorobicyclo[1.1.1]pentane was found to take place more rapidly than bromine abstraction from the unsubstituted parent, i. e., the fluorine substituent mediated a significant polar effect. Evidence was found of a novel disproportionation process in which the y-fluorine atom was transferred from the 3-fluoro radical to a triethylsilyl or to a second bicyclo[1.1.1 ]pent-1-yl radical; an analogous chlorine atom transfer process was found for the 3-chloro radical. Ab initio MO calculations (6-31G* basis with electron correlation up to MP4) on the 3-fluoro- and 3-methyl-substituted radicals indicated that loss of the substituent to give [1.1.1]propellane would be comparatively easy for both species.