Cyclopropane cation radical, ring opening

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

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates. ... 

A more detailed evaluation of the diverse structures proposed for the secondary species goes beyond the scope of this review. We mwely emphasize that the ESR results provide detailed evidence for the nature of the radical center, but fail to elucidate the cationic site. The identity of this center is left to secondary considerations or speculation. We also note that any alternative structure has the virtue of not contradicting the ab irutio calculations the potential c ture of chloride ion has precedent in the nucleophilic substitution at a cyclopropane carbon (see Section 7). Another type of ring-opened structure has been postulated as an intermediate in the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes (see Section 5). 

As mentioned above, the conversion of cyclopropane to propene radical cation has been investigated by ab initio calculations. The general course of this reaction was confirmed, or anticipated, by product studies in the electron transfer-sensitized conversion of 1,1,2,2-tetraphenylcyclopropane (37) to 1,1,3,3-tetra-phenylpropene (38). The sequence of the key steps, migration versus ring opening cannot be derived from the results. In the case of 37, the four phenyl substituents may actually favor a ring-opened bifunctional radical cation. 

Substituted cyclopropane systems also undergo nucleophilic addition of suitable solvents (MeOH). For example, the photoinduced ET reaction of 1,2-dimethyl-3-phenylcyclopropane (112, R = Me) with p-dicyanobenzene formed a ring-opened ether by anti-Markovnikov addition. The reaction occurs with essentially complete inversion of configuration at carbon, suggesting a nucleophilic cleavage of a one-electron cyclopropane bond, generating 113. The retention of chirality confirms that the stereochemistry of the parent molecule is unperturbed in the radical cation 112 " ". 

Cyclopropane Fragmentations. Cyciopropanes themselves can also be cleaved. A variety of tri- and tetra-substituted rings lead to radical cations in the presence of irradiated acceptors like chloranil (119,120). With the bicyclic radical cation 11, radical cationic ring opening is followed by dehydrogenation to generate oxidized product, eq. 42 (120) ... 

The electron transfer reaction of gem-diarylmethylenecyclopropanes (60) with singlet sensitizers results in the exchange of the exo-methylene and the secondary cyclopropane carbons [241], The chloranil photo-sensitized reaction generates two unusual cycloaddition products (61, 62) [242], whereas the tetracyanoethylene sensitized oxygenation produces the respective dioxolanes [238]. These reactions are compatible with a ring-opened radical cation, and CIDNP experiments have... 

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