Radical cations from dicyclopropylidenemethane and its octamethyl derivative

Abstract

The radical cations of dicyclopropylidenemethane (2) and its octamethyl derivative (2-Me-8) are prone to rearrangements into those of (2-methylallylidene)cyclopropane (2a) and its octamethyl derivative (2a-Me-8), respectively, by opening one three-membered ring. In contrast to the radical cations of bicyclopropylidene (1) and its octamethyl derivative (1-Me-8), 2(.+) and 2-Me-8(.+) 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 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-Me-8(.+) can be observed and characterized by its ESR and ENDOR spectra. Nevertheless, the existence of the two '' missing '' species, 2a(.+) and 2-Me-8(.+), 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-Me-8 in liquid octane, strongly suggest that 2-Me-8(.+) (and not 2a-Me-8(.+)) is formed initially. From the temperature dependence of the decay, the activation energy of the ring-opening process 2-Me-8(.1) -> 2a-Me-8(.+) is estimated. The radical cations 2a(.+) and 2a-Me-8(.+) 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 pi-system.