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-Kekulé 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 γ-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 rearranged radical cation 2a-Me8•+ can be observed and characterized by its ESR and ENDOR spectra. Nevertheless, 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•+ → 2a-Me8•+ is estimated. The radical cations 2a•+ and
2a-Me8•+ are formally distonic with the spin residing in the allylic moiety and the charge accommodated on
the central carbon atom of the allene π-system. The intact cyclopropylidenemethylidene moiety assumes a
“bisected” conformation, thus favoring an optimal interaction with the positively charged center on the π-system.