1.2- diphenylcyclopropane cations

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. 

Cathodic reduction of bicyclic gem-dibromocyclopropane in the presence of chlorotrimethylsilane provides the exo-silylated isomer selectively. With a sacrificial Mg anode the current efficiency can be increased by sonication as the anode acts additionally as a chemical reducing agent [358]. The 2e reduction of (5 )-(+)-l-bromo-l-carboxy-2,2-diphenylcyclopropane showed that the stereoselectivity at a Hg cathode was strongly determined by the supporting electrolyte cation. With NH4+, a preferential retention of configuration was observed, which increased with a more negative reduction potential. By contrast, a R4N+ cation gives rise to a major inversion, which increases with the bulkiness... 

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). 

Differential interactions between cations in zeolites and the products of a photoreaction may result in selectivity. One such example is the selective photoisomerization of nms-l,2-diphenylcyclopropane to the cis isomer [136]. Triplet sensitization of 1,2-diphenylcyclopropane in solution results in a photostationary state mixture consisting of 55% cis and 45% trans isomers. When the same process is carried out within NaY zeolite the cis isomer is formed in excess of 95%. The preference for the cis isomer within NaY is attributed to the preferential binding of the cation to the cis... 

We analyze one system, the photoisomerization of 2,3-diphenylcyclopropane-l-carboxylic acid derivatives, in detail to illustrate the role of alkali ions during the asymmetric induction process within zeolites [282,296]. All other systems investigated here could be analyzed on the basis of the same model. Computational results have been used to gain an insight into the interaction among the reaction site, the chiral auxiliary, and the cation. While the computations provide information about interactions in the gas phase, the reactions that we are concerned with occur in much more complex environments, namely zeolites. In spite of this severe limitation, the computational results serve to build a preliminary model that can be used to plan further experiments. 

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 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]. 

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 +. 

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. ... 

One electron transfer reactions of pyrylium cations and some related systems have been reviewed (94H116S). A single electron transfer is thought to be involved in the photochemical conversion of the diphenylcyclopropane (19) into the xanthylium salt (20) in the presence of an arene donor such as naphthalene (94CC1681). 

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. 

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