Structures 1,2-diphenylcyclopropane

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.

Typical aromatic donors and acceptors undergo only minor geometry changes upon oxidation or reduction or upon population of the triplet state for these compounds, the reaction sequence ET followed by BET has no effect on the structure. If the triplet state or biradical belongs to a different stmcture type than radical ion and ground-state precursor, as is the case for cis- or fraui-1,2-diphenylcyclopropane (65) or norbornadiene (16) BET may occur with cleavage or for-mation of one or more C—C bonds. In such cases, the sequence ET-BET may... 

Photostationary cis/trans ratios vary with sensitizer structure in a manner not yet perfectly understood, and both the time required for attainment of a photostationary state and actual flash spectroscopic quenching rates indicate that energy transfer to diphenylcyclopropane is quite inefficient. Such should be the case if nonvertical energy transfer with the production of a biradical is occurring. When care is taken so that only the sensitizer absorbs light, only cis-trans isomerization is observed. Direct excitation of the cyclopropane produces 1,3-diphenyl-propene and 1-phenylindane as well.298... 

Two simple phenyl derivatives have been studied by low-temperature XD phenylcyclo-propane (41) at -100°C and m-l,2-diphenylcyclopropane (42) at -40°C (Table 4). Structural data of 43 are also listed in the Table. 

Kinetic work on the isomeric 1,2-diphenylcyclopropanes (Scheme 2) made evident a substantial reduction in Ed and thus implied a stabilization of trimethylene diradical transition structure(s) by phenyl substituents142. In further work with 0.2 M (-)-l,2-diphenylcyclopropane in 1 -butanol, Crawford and Lynch143 uncovered a direct route from one trans antipode to the other at 220.7 °C the measured ratio of rate constants /trac(for loss of optical activity) to kK (for trans to cis geometrical isomerization) was found to be 1.49 0.05 and since krdC is (2k12 + 2/c,). and klc is 2/c,(Scheme 2), the implication is that one-center epimerizations (2kt) are favored over the two-center epimerization process (ka) by... 

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). 

Disubstitution of a gem-dibromocyclopropane synthesis of 1-butyl 1,2-diphenylcyclopropane (Structure S))81... 

Figure 7 RHF/3-21G optimized structures of Li+ bound 2(3,3(3-diphenylcyclopropane-la-carboxamides of 1-phenylethylamine (a) and 1-cyclohexylethylamine (b). Binding affinities are included at the bottom of each structure.

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. 

Reactions like the conversion of compound 7c to 7d are relatively uncommon. Most 4,5-dihydro-3//-pyrazoles are chemically too sensitive to permit extensive structural modification before deazetization. Those that can be modified are normally persubstituted at C3 and C5 or these positions are bridgehead carbons so that in either case tautomerism is avoided. Hence, in making optically active 1-hydroxymethyl-1-methyl-2,2-diphenylcyclopropane (10), the ester group of the diphenyldiazomethane, menthyl 2-methylprop-2-enoate adduct 9 was reduced... 

The competing side reaction is a cyclopropyl anionic ring opening (for a review see ref 13). In the case of isomeric l-benzoyl-2,3-diphenylcyclopropanes, no base-induced isomerization (MeOH/ NaOMe, 65 °C, 4 h) was detected. In many cases these rearrangements require rather drastic conditions but the interconversion can be facilitated by the participation of an adjacent substituent. For example, with an acyl-substituted cyclopropane, enolization permits cisitmns isomerization to proceed without ring opening and therefore without competing anionic structural isomerization. 

In contrast, substituents that accept electron density by resonance can stabilize carbanionic centers, thus lessening the interaction with metal ions and facilitating racemization. The 1-lithio derivative of (-)-(R)-l-cyano-2,2-diphe-nylcyclopropanecan be alkylated withmethyl iodide to yield racemic-l-methyl-l-cyano-2,2-diphenylcyclopropane, indicating racemization at the carbanionic center. This result could be ascribed either to a planar carbanion with appreciable C=C=N character or to a rapidly inverting tetrahedral carbanion. The X-ray crystal structure of l-cyano-2,2-dimethylcyclopropyIlithium... 

Photochemical Reactions.—The structural and geometrical isomerizations of 1,2-diphenylcyclopropane have been subjected to detailed analysis and the photochemical (and thermal) decomposition of cyclopropanone has been examined in the vapour phase. ... 

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. 

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