Homolytic Reactions of Cubanes. Generation and Characterization of Cubyl and Cubylcarbinyl Radicals

A series of 4-substituted cubyl radicals was generated by bromine atom abstraction from 1 -bromo-4-substituted cubanes. EPR observations showed that the cubyl radicals abstracted secondary hydrogen atoms from the ethyl groups of triethylsilane and decayed mainly by second-order combination reactions. tert-Butoxyl radicals abstracted hydrogen atoms from cubane at least 26 times more rapidly than from cyclopropane at -90 °C. Electron-withdrawing substituents on cubane greatly reduced this rate. tert-Butoxyl radicals selectively abstracted the cage hydrogens rather than primary methyl hydrogens from methylcubane. Reaction of bromine atoms with cubane occurred at the carbon atoms directly by homolytic substitution, thus initiating a rearrangement sequence which ended when the 7-bromotricyclo[4.2.0.0^2,5]oct-3-en-8-yl radical (28) rapidly transferred a bromine atom from molecular bromine. By contrast, reaction of cubane with chlorine atoms occurred predominantly via hydrogen abstraction. Cubylcarbinyl radicals were shown by EPR spectroscopy to rearrange with a rate constant ≥5 × 10⁹ s^-1 at 25 °C, by a cascade of three β-scissions. The intermediate tricyclo allyl radical 6 was characterized by spectroscopic methods. The first β-scission of the -hydroxycubylcarbinyl radical occurred at ca. 10^-3 times the rate for the parent cubylcarbinyl radical, enabling the former radical, and its rearrangement product, to be observed by EPR spectroscopy.