Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

The radical cations of dicyclopropylidenemethane (2) and its octamethyl derivative (2-Me8) are prone to rearrangements into those of (2-methylallylidene)cyclopropane (2a) and its octamethyl derivative (2a-Me8), respectively, by opening one three-membered ring. In contrast to the radical cations of bicyclopropylidene (1) and its octamethyl derivative (1-Me8), 2¥+ and 2-Me8¥+ are stable to opening of the second ring, because in this case the resulting species would be a non-Kekule ́ hydrocarbon with a quartet ground state. Similarly to 1, octamethyl substitution in 2 promotes the tendency to rearrangement. Thus, ESR and ENDOR studies indicate that the primary radical cation 2¥+, which is formed upon ç-irradiation of 2 in a CFCl3 matrix at 77 K, does not rearrange up to 150 K. On the other hand, when 2-Me8 is treated in the same way, only the re-arranged radical cation 2a-Me8¥+ can be observed and characterized by its ESR and ENDOR spectra. Nev-ertheless, the existence of the two “missing ” species, 2a¥+ and 2-Me8¥+, is revealed by other methods. According to UV and IR studies, X irradiation of 2 in an Ar matrix leads directly to the ring-opened radical cation 2a¥+. Moreover, magnetic field effects on the decay of fluorescence, which appears upon recombination of the radical anion of p-terphenyl with a radical cation generated from 2-Me8 in liquid octane, strongly suggest that 2-Me8¥+ (and not 2a-Me8¥+) is formed initially. From the temperature dependence of the decay, the activation energy of the ring-opening process 2-Me8¥+ f 2a-Me8¥+ is estimated. The radical cations 2a¥+ an