1.2- diphenylcyclopropane radical cations

In the context of the potential Cope rearrangement of hexa-1,5-diene radical cations (Section 2.4.1), we mentioned the triplet recombination of radical ion pairs generating a biradical [202, 203]. Because of continuing interest in this type of reaction we briefly mention two additional examples involving radical cationic systems discussed in this review, viz., the isomeric 1,2-diphenylcyclopropane radical cations, cis- and trans- 3 , and norbornadiene radical cation, 91 +. 

Exposure of several methyl-substituted derivatives to y-radiolysis at 77 K in cryogenic matrices gave rise to a family of radical cations of the same structure type, some of which had been previously identified on the basis of CIDNP results. We begin with a discussion of the CIDNP investigations, since they preceded the ESR studies of all species but the prototype. The first CIDNP results attributed to a cyclopropane radical cation were observed during the photoreaction between 1,4-dicyanonaphthalene and cis-l,2-diphenylcyclopropane. However, the nature of the cyclopropane radical cation was characterized by CIDNP effects observed during the reaction of chloranil with cis- and /rans-l,2-diphenylcyclo-propane. ... 

Radical cations of the same general structure type as those derived from cis- and /ro/tf-diphenylcyclopropane have been established for numerous cyclopropane... 

Figures, h CIDNP spectra (cyclopropane resonances) observed during the electron transfer photoreaction of chloranil with c/s-1,2-diphenylcyclopropane (fop) and ben-zonorcaradiene (.bottom). The opposite signal directions observed for analogous protons in the two compounds constitute evidence that the two radical cations belong to two different structure types.

Radical cations of the same general structure type as those derived from cis- and tratw-diphenylcyclopropane have been established for numerous cyclopropane derivatives, including the parent, 1,2-di-, 1,1,2-tri- and 1,1,2,2-tetramethylcyclo-propane (Table 3). Two of these systems provide a direct comparison between the results of CIDNP and ESR experiments. In both cases, the ESR spectra observed by Williams and coworkers following pulse radiolysis in frozen solutions [293, 296, 297] show splitting patterns supporting the presence of spin density on two carbon centers, thus confirming the structure type (102) assigned on the basis of CIDNP results. 

The assignment of an antisymmetrical cyclopropane SOMO to the radical cation of 105 is based on a comparison of CIDNP effects (Fig. 18) with those for cis-1,2-diphenylcyclopropane. While the nuclei of the aromatic segments show identical or very similar polarization, the cyclopropane protons show characteristic differences. This suggests significantly different spin density distributions for the cyclopropane moieties of the two species and, thus, different structures [229]. The benzonorcaradiene radical cation should owe its structure to the symmetry of the fragment FMOs at the points of union. The styrene HOMO is antisymmetric at the positions of attachment, suggesting preferred interaction with the antisymmetric cyclopropane HOMO (as shown below). 

The nature of the cyclopropane radical cation was first characterized unambiguously by CIDNP effects of a 1,2-disubstituted derivative. The pattern of benzylic and geminal polarization observed during the reaction of chloranil with cis- and tru 5-l,2-diphenylcyclopropane (Figure 16) supported radical ions with spin-density on the benzylic carbons [107]. 

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. 

Streith and Nastasi have reviewed the photoreactions of three-membered rings. A study of the photo-ring-opening reactions of the azirines (125) has been reported. A CIDNP study of photoelectron transfer from cis- and trans-, 2-diphenylcyclopropane to chloranil has been carried out. The evidence collected from this study indicates that the intermediate involved is the radical cation (126), since no polarized rearrangement product was observed. The failure to observe reaction is in marked contrast to the behaviour when the cyclopropane is irradiated in the presence of 1,4-dicyanonaphthalene. ... 

Asymmetric induction within 1,2-diphenylcyclopropane on photolysis in P-cyclodextrin has been studied.Recent research has shown that cis-2,3-diphenylcyclopropane-1-carboxylic acid does not undergo ISC on direct irradiation. The reaction encountered is isomerization to the corresponding trans isomer via a 1,3-biradical intermediate. Exothermic bond cleavage is the dominant reaction within radical cations of cyclopropylamines formed by SET to DCA. The photoheterolysis of 9-cyclopropyl-9-fluorenol has been studied in non-acidic zeolites. The rate of formation of the resultant cation is dependent upon the alkali metal counterion. 

For example, the structure of the radical cation of 1,2-diphenylcyclopropane has been assigned on the basis of the analysis of CIDNP data formed in the act of photoinduced reversible electron transfer from cyclopropane to chloranil. The choice has been made between closed and open structures of the radical cation of 1,2-diphenylcyclopropane (Figure 4). The observed CIDNP effects of 1,2-diphenylcyclopropane (absorption of aromatic ortho-, para-, and Hd, and the emission of Hp) comply only with the open structure. 

Roth, Schilling and coworkers [47-54] have investigated the nuclear spin polarization behavior of cation-radicals of numerous strained hydrocarbon systems produced by PET to strong electron acceptors such as chloranil, anthraquinone and cyanoaromaties. These systems include cyclopentadiene dimers, methyl-enebicyclo[2.2.0]hexenes [48], bicyclo[1.1.0]butanes [49], hexamethyl (Dewar benzene) [50], norbomadiene [53], quadricyclene [53], and 1,2-diphenylcyclopropanes [54]. 

The second type of structural adjustment which is pertinent to the present chemistry is the long bond . An especially good example of this phenomenon is available in the case of the 1,2-diphenylcyclopropane cation radical [10], Any of the three possible structures of this cation radical illustrated in Scheme 6 might be considered to be plausible, a priori. 

---