title: Radical cations from dicyclopropylidenemethane and its octamethyl derivative.
abstract: 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 gamma-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 pi-system. The intact cyclopropylidenemethylidene moiety assumes a "bisected" conformation, thus favoring an optimal interaction with the positively charged center on the pi-system.
authors.affiliation: Department of Chemistry, University of Fribourg, Switzerland. Thomas.Bally@unifr.ch
authors.forename: Thomas
authors.initials: T
authors.lastname: Bally
authors.forename: Beat
authors.initials: B
authors.lastname: Müller
authors.forename: Fabian
authors.initials: F
authors.lastname: Gerson
authors.forename: Xue-Zhi
authors.initials: XZ
authors.lastname: Qin
authors.forename: Malte
authors.initials: M
authors.lastname: von Seebach
authors.forename: Sergei I
authors.initials: SI
authors.lastname: Kozhushkov
authors.forename: Armin
authors.initials: A
authors.lastname: de Meijere
authors.forename: Vsevolod I
authors.initials: VI
authors.lastname: Borovkov
authors.forename: Pavel A
authors.initials: PA
authors.lastname: Potashov