Cyclopropane cation radical, ring

The cyclopropane radical cation can be prepared by y-radiolysis in rigid matrices. At temperatures as low as 4.2 K, its ESR spectrum shows evidence for static Jahn-Teller distortion, resulting in a structure of the (ring-closed) trimethylene ( Ai) type, r " ... 

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... 

Ring-opened cyclopropane radical cations have also been postulated to account for the stereochemistry of the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes [225]. These systems, of course, contain substituents that may veil the true nature of the cyclopropane radical cations by delocalizing spin and charge. In addition, we note that the experimental findings allow some latitude in their interpretations. 

Nucleophiles also add to radical cations of arylcyclopropanes to give anti-Markownikoff-type adducts [40-43]. There are two possible structures for the radical cations of arylcyclopropanes i.e., ring opened 1,3-radical cation and ring closed cyclopropane radical cation. Dinnocenzo reported that the photoinduced electron transfer reaction of chiral trans-l-methyl-2-phenylcyclopropane in methanol gives chiral 3-methoxy-l-phenylbutane via the radical cation [44], This result indicates that the radical cation in the ring closed form is attacked by methanol. The fact that this cyclopropane does not isomerize under the photochemical reaction conditions supports this conclusion (Scheme 11). 

The vinylcyclopropane radical cation, is another radical cation of structure type B, which is stabilized by conjugation. Its proposed structure was based exclusively on ab initio calculations (B3LYP/6-31G ) because the electron-transfer photochemistry of this species failed to provide clear-cut CIDNP effects [128], In this context it is worth noting that product studies cannot, in principle, establish the cyclopropane radical cation structure type. Irrespective of the structure, nucleophilic capture is expected to result in the cleavage of the strained ring. 

The potential ring-opening of cyclopropane radical cations, breaking the weakened bond of type A radical cations (21 +), has been a subject of both interest and controversy. The ESR spectra of cyclopropane radical cation and its methyl-substituted derivatives decayed at temperatures near 100 K. They were replaced by secondary spectra, in which the protons at one cyclopropane center do not interact with the electron spin. This coupling pattern was interpreted as evidence for a ring-opened trimethylene species (22 +) in which one terminal carbon has rotated into an orthogonal orientation [105, 106, 140]. 

Additional insight into the ring-closed or ring-opened structure of cyclopropane radical cations can be derived unambiguously from the stereochemical comse of their reactions. These reactions are discussed in detail in Section 2.4 here we briefly mention several illuminating examples of cyclopropane radical cation reactivity, chosen solely to illuminate the structure of cyclopropane radical cations. 

More recently, Dinnocenzo and colleagues showed that a 2,3-dimethyl derivative (30, R = CH3) and several 1-phenyl- and l,l-diphenyl-2-alkyl-substituted cyclopropanes are captured with complete inversion of configuration [154, 155]. The observed stereochemistry requires an intermediate radical cation, 30, with the unperturbed stereochemistry of the parent molecule. This result unambiguously rules out a ring-opened cyclopropane radical cation in solution. 

The unsubstituted cyclopropane radical cation has received considerable experimental [72] and eoretical [73] attention and is thought to exist as three equivalent 2A partially-ring-opened structures. These three equivalent structures are able to interconvert relatively easily, via three equivalent 23 2 structures. Although the symmetry is reduced in the methyl-substituted system, we are able to observe the appropriate 1,2-shift operating by a mechanism analogous to that... 

There are very few examples of cleavage initiated by loss of an electron from a C=C bond however, after attack of a nucleophile on the initially formed radical cation, further loss of electrons may result in a cleavage analogous to that described in Eq (15). The special bonds in a cyclopropane ring may lose an electron with formation of a ring-opened radical cation ... 

Cation radicals of arylcyclopropanes, produced by photoinduced one-electron transfer to an electron acceptor, undergo ring-opening reactions accompanied by regioselective polar additions. 1,4-Dicyanobenzene (5) sensitized photolysis of l,l-bis(4-tolyl)cyclopropane (4) in methanol/acetonitrile resulted exclusively in anti-Markovnikov addition of methanol to the cyclopropane. It is assumed that excitation of 5 followed by electron transfer from 4 gives a dicyanobenzene radical anion 5(- ) - cyclopropane radical cation 4(4-) pair. Attack by methanol on the radical cation 4(4-) followed by proton loss affords benzylic radical 7 that is reduced by 5( —) to give benzylic anion 8 and quenched by methanol to afford product... 

Irradiation of electron deficient arenes in the presence of cis-l,2-diphenylcyclopropane leads to formation of the trans isomer by an electron transfer mechanism. The reaction occurs by way of the radical cation of the cyclopropane which isomerises prior to back electron transfer. It has now been examined using menthyl and bornyl esters of benzene tetracarboxylic acid as chiral electron transfer sensitisers. °° Slight excesses of one of the enantiomers of the trans-1,2-diphenylcyclopropane were observed. The dicyanoanthracene sensitised reactions of 1,1,2,3-tetra-arylcyclopropanes have been studied.Depending on the substituents present on the arene rings these compounds rearrange to 1,1,3,3-tetra-arylpropenes. The rearrangement occurs in a ring opened radical cation intermediate. 

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