Cyclopropanation cation radical

FIGURE 6.Ex.l Two trajectories for the nucleophilic cleavage of the one-electron a-bond in a substituted cyclopropane cation radical, leading to retention or inversion of configuration. 

An excess (5 1 mole ratio) of ethyl diazoacetate is used in these reactions to suppress cyclobutadimerization or Diels-Alder cyclodimerization. In difunctional molecules which have non-equivalent ionizable functionalities, cyclopropanation is highly selective for the more easily oxidized functionality. The latter selectivity is perhaps the most attractive aspect of the reaction. In contrast to transition metal (e.g. rhodium) catalyzed cyclopropanations, cation radical additions to electron deficient alkene moieties do not occur at all. The reaction is relatively sensitive to... 

A. Ring-opening of Substituted Cyclopropane Cation Radicals... 

The chemistry of polysubstituted cyclopropane cation radicals seems also to be governed by processes comparable to the ones outlined in Schemes 1 and 2. For example, electron impact ionization of either 1,2-disubstituted (26) or 1,1-disubstituted cyclopropane (27) leads to the elimination of the Br radicaP. From the CA mass spectra of the resulting [M-Br] ions and those of C4H7 ions generated from other suitable precursors it must be concluded that both 26 and 27 give only the 1-methylallyl cation 28, and not the isomeric 2-methylallyl cation 29 (Scheme 3). 

The A,-type cation radical intermediates are fiiequently proposed for the nucleophilic substitution reactions of aryl-substituted cyclopropane cation radicals. Rao and Hixson were the first to report the unusual regioselective nucleophilic substitution of 4. The more highly substituted C-2 of 4 is regioselectively attacked by methanol and reduction of the resulting radical 5 followed by protonation gives... 

The anion radicals of the SET sensitizers, N-methylphthalimide, chloranil (CA), naphthoquinone, 2,3-dichloronaphthoqui-none, - and 9, lO-dicyanoanthracene (DCA) undergo similar nucleophilic additions to intermediate cyclopropane cation radicals in sensitized photoreactions of arylcyclopropanes. For instance, the CA-sensitized photoreaction of 9 in the presence of methanol gives the CA/methanol adduct 12. - Presumably, nucleophilic substitution of 9 by methanol initially takes place to form 11, to which CAH adds to form 12. The steady-state photolysis of a solution of 9, DCA, and Cu(BF4)2 in acetonitrile containing methanol or t-butanol gives 13 and 14, respectively. Similar photoreaction of 9 in acetonitrile containing methanol and t-butanol gives 15 and 16. Apparently, Cu oxidizes not only DCA but also radical 11. Under these sensitized conditions, the rate constants for the nucleophilic substitution,... 

No chemical reactions have been observed which require the intermediacy of the B2-type cation radical. However, CIDNP experiments with 33 provide evidence for the existence of the unique structure of 33 in which two cyclopropane bonds are simultaneously weakened. It is noteworthy that spiroannelation with the fluorene ring causes a remarkable change in the distribution of the charge and odd electron densities in the ring of cyclopropane cation radicals. 

Generally, at least in theory, an important aspect of cation-radical polymerization, from a commercial viewpoint, is that either catalysts or monomer cation-radicals can be generated electrochem-ically. Such an approach deserves a special treatment. The scope of cation-radical polymerization appears to be very substantial. A variety of cation-radical pericyclic reaction types can potentially be applied, including cyclobutanation, Diels-Alder addition, and cyclopropanation. The monomers that are most effectively employed in the cation-radical context are diverse and distinct from those that are used in standard polymerization methods (i.e., vinyl monomers). Consequently, the obtained polymers are structurally distinct from those available by conventional methods although the molecular masses observed so far are still modest. Further development in this area would be promising. 

ELECTRON TRANSFER PHOTOCHEMISTRY OF CYCLOPROPANE SYSTEMS RADICAL CATION REACTIVITIES... 

Exercise 6.10 The ground and promoted states of the reactants, R and R are shown in Fig. 6.Ans.8 below using the FO—VB representation, which is the simplest one for making predictions on stereochemistry. The electronic structure of R displays an electron transfer from the nucleophile to the C C bond. A transfer to the a-orbital is not relevant, as this would generate a closed-shell cyclopropane that cannot form a bond with the oxidized nucleophile. It is therefore the a orbital that accepts the transferred electron, thus generating the triplet oo- configuration of the C-C bond. Now, since R and R differ by one-electron shift from Nu to the a orbital of the cation radical, the corresponding... 

The details of bonding and spectroscopic properties of cyclopropane derivatives have recently been reviewed.In addition, the properties and energetics of cyclopropyl cations, anions, radicals, anion radicals and cation radicals have been amply reviewed, and comparisons have been made with their corresponding alkenic counterparts. 

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