Cyclopropyl/cyclobutyl radical

Inversion of the radical center in cyclobutyl radicals is fast when compared to their cyclopropyl analogs. The steric outcome in hydrogen-abstraction reactions is therefore mainly determined by the substitution pattern at the / -carbon atoms in the radical7. 

Cyclopropyl and Cyclobutyl Radicals. Ab initio calculations on the transformation of tetrahedrane into the bicyclobiityl biradical (856) indicate that the former is actually an energy minimum on the surface and that there is a barrier of at least 18 kcal moP for this allowed transition. Semiempirical MO calculations on the cyclo-propenyl radical have also been reported. ... 

Efforts to produce the cyclopropylamino-radical for e.s.r. studies gave only ring-opened imine radicals. Ring-openings of cyclopropyl, cyclobutyl, and bicyclo[n,l,0]alkyl radicals have been reported and the disrotatory nature of the ring-opening of cyclopropyl radicals has been established by examination of ring-fused systems. ... 

Homolytic aromatic substitution with cyclopropyl and cyclobutyl radicals has been reported. ... 

Extensive studies have been devoted to these radicals in connection with their structure (classical or nonclassical) and their reactivity (homoallylic rearrangement and 1,2-vinyl migration) and the main results have been reviewed.For instance, the classical nature of the allylcarbinyl, cyclopropyl carbinyl, and cyclobutyl radicals now seems well established. 

Cyclopropyl and Cyclobutyl Radicals. The reaction of 1,1-dichloro-cyclopropane with photochemically produced chlorine atoms at 0°C in carbon tetrachloride gives 1,3,3-trichloropropane (which may react further with chlorine). The products of the reactions of chlorine atoms with the specifically labelled cyclopropanes (616a) and (616b) have been shown to be (617a) and (617b) respectively, by 250 MHz n.m.r. spectroscopy. This stereospecificity corresponds to initial radical substitution with inversion of configuration. 

Despite our earlier enunciated electronegativity and bond polarity logic, we must forego nearly all comparison with the free (uncomplexed) carbanions. Unlike the rather stable cyclopropyl anion, the cyclobutyl and cyclopentyl ions are unbound with regard to loss of their extra electron. That is, the gas phase ionization process to form the radical from the carbanion, Ru R" -E e, is energetically favorable. 

Schleyer and coworkers calculated the electron affinity of ethyl, 2-propyl, cyclobutyl and cyclopropyl radicals. Apart from the cyclopropyl radical, these radicals have negative electron affinities suggesting that the corresponding anions cannot be observed as long-lived species in the gas phase. For the cyclopropyl radical, an electron affinity of 5.1 kcal moT was predicted in reasonable agreement with the experimental value of 8 kcal moT Accordingly, it is probable that the cyclopropyl anion is the only saturated carbanion that can be observed experimentally in the gas phase. 

Bond cleavage reactions of cyclopropyl and cyclobutyl rings by adjacent ketyl radical anions generated via photoinduced electron transfer from amines have also been reported by Pandey and coworkers (Scheme 80) [336, 337]. 

In cases where two bonds are cleaved, it is often suggested that the cleavage occurs in one step. This assumption is based on the fact that a stepwise reaction would lead to the formation of either cyclopropyl or cyclobutyl derivatives which are usually inert under the conditions employed. However, it is well established that the pervasive hydrogen species chemisorbed on transition metal surfaces is the hydrogen atom . Thus the mechanism could still be stepwise since the cyclic intermediates are obtained as highly reactive radicals which can further add hydrogen in a series of successive steps. 

In morphine (agonist), the replacement of the A-methyl group by a more bulky radical such as A-allyl, A-cyclopropyl-methyl or A-cyclobutyl-methyl leads to powerful antagonists of the opiate receptors (see Section III in Chapter 20). 

HOMOs of this molecule are indeed the degenerate pair 3e. The distortion of the cyclobutane radical cation leads presumably to a Z)2h ground-state geometry The investigation of the PE spectra of cyclopropyl and cyclobutyl halidesshows that hyperconjugative interactions are smaller in the latter compounds than in the former, but still larger than in non-cyclic alkyl halides. 

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