Cis-1,2-Diphenylcyclopropane

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). 

Fig. 18. 1H CIDNP spectra (cyclopropane resonances) observed during the electron transfer photoreaction of chloranil with cis-1,2-diphenylcyclopropane (top) and benzonorcaradiene (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...

Inter- and intramolecularly sensitized enantiodifferentiating photodecom-j positions of pyrazoline derivatives 19t and 23 were also examined [12]. Th triplet sensitized photodecomposition of frvms-3,5-diphenylpyrazoline 19t witty (— )-rotenone 21 and (4- )-testosterone 22 afforded chiral trans- and achiral cis 1,2-diphenylcyclopropanes 20t and 20c (Scheme 3, top). The enantioselectivity ... 

Achiral cis-1,2-diphenylcyclopropane photoisomerizes to the chiral trans isomer upon singlet- or triplet-photosensitized irradiation [64-67], It is expected that the reactant and chiral inductor immobilized in a zeolite supercage interact intimately with each other to afford more efficient photochirogenesis. Ramamurthy and coworkers reported that the enantio- and diastereodifferentiating photoisomeriza-tions of ris-2p,3p-diphenyl-la-cyclopropanecarboxylates 20 (Scheme 7) in chirally modified zeolite supercages lead to the corresponding chiral trans isomer 21 [68]. 

Photochemical Reactions.—Three-membered Rings. The importance of charge transfer to naphthalene from 1,2-diarylcyclopropanes in their naphthalene-sensitized isomerization has been discussed and an attempt has been made to influence the decay modes of 1,2-diphenylcyclopropane excited states using optically active solvents. Naphthalene-sensitized photolysis of cis-1,2-diphenylcyclopropane in optically active methyl ethers led in all cases to the racemic trans-isomer, but with acetone sensitization slight differences in the photostationary state in different solvents were observed, leading to different cisftrans ratios. ... 

The reaction of sulfur dioxide with cyclopropanes in trifluoroacetic acid at 20 °C affords mixtures of isomeric 1,2-oxathiolane 2-oxides (y-sultines). From cis-1,2-diphenylcyclopropane a mixture of the (2,3-cw)-2-oxide 19 (yield, 52 %) and the (2,3-trans)-2-oxide 20 (yield, 18 %) is obtained... 

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. ... 

The same has been observed by Dodson and Klose in previous work where cis- and trans-1,2-diphenylcyclopropane were formed by pyrolysis of 2,3-diphenylthietane 1,1-dioxide at 230°C. Analogous results have been achieved by both photolysis and thermolysis of l-phenyl-2-benzoylthiethane 1,1-dioxide to a cis-trans mixture of l-phenyl-2-benzoylcyclopropane. ... 

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... 

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... 

Purify the crude product by flash chromatography on silica gel using hexane as an eluent to obtain pure 1-butyl-1,2-diphenylcyclopropane (11 cis trans = 2.7 1) (0.23 g, 82%) as a clear oil. Characterize the product by 1H NMR, IR, MS spectroscopy, and HRMS spectroscopy. 

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... 

After early unsuccessful attempts to direct the photoreduction of ketones with chiral secondary alcohols [8-10]. Weiss et al. examined the sensitized cis-trans photoisomerization of 1,2-diphenylcyclopropane in chiral solvents but obtained the product without detectable optical rotation [11]. Seebach and coworkers were the first to achieve asymmetric induction for a photochemical reaction by a chiral solvent [12-15]. They examined the photopinacolization of aldehydes and ketones in the chiral solvent (S,S)-( + )-l,4-bis(dimethylamino)-2,3-dimethoxybutane (DDB, 4). Irradiation of acetophenone in the presence of 7.5 equiv. of DDB yielded a mixture of chiral d,/-pinacols 3/ent-3 and achiral meso-pinacol 2. At 25°C pinacol 3 was obtained with 8% ee, with the (R, / )-( + )-enantiomer prevailing. At lower temperatures the asymmetric induction was more effective, up to 23% ee at — 78°C in a 1 5 mixture of DDB and pentane (Scheme... 

E. Enantio- and Diastereodifferentiating cis-trans Photoisomerization of 1,2-Diphenylcyclopropane in Chirally Modified Zeolites... 

Hammond and Cole reported the first asymmetric photosensitized geometri-r cal isomerization with 1,2-diphenylcyclopropane (Scheme 2) [29]. The irradiation of racemic trans-1,2-diphenylcylcopropane 2 in the presence of the chiral sensitizer (R)-N-acetyl-1 -naphthylethylamine 4 led to the induction of optical activity in the irradiated solution, along with the simultaneous formation of the cis isomer 3. The enantiomeric excess of the trans-cyclopropane was about 1% in this reaction. Since then, several reports have appeared on this enantiodifferentiating photosensitization using several optically active aromatic ketones as shown in Scheme 2 [30-36]. The enantiomeric excesses obtained in all these reactions have been low. Another example of a photosensitized geometrical isomerization is the Z-E photoisomerization of cyclooctene 5, sensitized by optically active (poly)alkyl-benzene(poly)carboxylates (Scheme 3) [37-52]. Further examples and more detailed discussion are to be found in Chap. 4. 

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 +. 

The very low activation energy and frequency factor obtained for the cis-trans isomerization of 1,2-diphenylcyclopropane (Table 4) is a matter of some interest. The values were obtained for isomerization in the liquid phase, but it is unlikely that the difference can be ascribed to a medium effect. The low activation energy can be interpreted in terms of stabilization by the phenyl groups of a biradical intermediate or of an activated complex having some biradical character. 

SCHEME 18. Cis/trans isomerization of the cis- and trans-1,2-diphenylcyclopropanes (cis- and trans-178a-c) and formation of the 1,3-diphenylpropanes (181a-c) and 1,3-diphenylpropenes (179a,b) with Na/K in THF (gegenions omitted). 

The cis-trans isomerization in the case of the 1,2-diphenylcyclopropanes 178a in the presence of Na/K is not a base-catalyzed reaction with the cyclopropyl anions 183 as... 

The comparison of the ionization potentials of identically substituted cyclopropenones, cyclopropenes and cyclopropanes is interesting, if not yet particularly informative to date. The ionization potentials of cyclopropane, cyclopropene and cyclopropenone are much closer, 9.86, 9.67 and 9.47 eV, than for their diphenyl derivatives. Diphenylcyclopropene has an adiabatic ionization potential of 7.45 eV while those of the cis and trans isomers of 1,2-diphenylcyclopropane (18) are 8.20 and 8.05 eV respectively. These latter values for the saturated species correspond to ring-opening to l,3-diphenylprop-l-yl-3-ium (19) (equation 24) a result corroborated by both experiment via solution phase chemi-ionization and ab initio calculations on the analogous divinylcyclopropane. (The... 

Arnold and Wong [555] have reported that the oxidation potentials of cis and trans-1,2-diphenylcyclopropanes have simple linear correlations with both their ionization energies and charge transfer adsorption energies with tetracyanoethylene as an acceptor. Similar correlations have been observed in cis- and ra/z5-2,3-diphenyloxiranes. 

The styrene (10) that was formed also underwent a secondary reaction giving cis-and trans-l-methyl- 1,2-diphenylcyclopropanes (9).81a These three-atom rings are not... 

A typical example is given by the reaction of cz s-1,2-diphenylcyclopropane cis-78a with Na/K alloy at 0 °C to yield on protonation trans-1,2-diphenylcyclopropane trans-79a, /rans-l,3-diphenylpropene 84a (together with some m-l,3-diphenyl-propene), and 1,3-diphenylpropane 82 a. Of special importance is the time dependence of the yields of cis-78a, trans-79a, 84a and 82a. The results are given in Table 4. Table 4 can be summarized as follows ... 

The fate of diazirines on decomposition can be influenced by the addition of y -cyclodextrin (j6-CD). When 3-methyl-3-phenyl-3jf/-diazirine (3) was thermolyzed under argon, 1 -methyl-1,2-diphenylcyclopropane (4) was obtained as an isomeric mixture in close to 20% yield the main product (43% yield) was acetophenone azine (5). Cyclodextrin complexation prior to pyrolysis, however, increased the yield of 4 tremendously. The carbohydrate, therefore, facilitates both styrene (6) and cyclopropane formation.It is also interesting to note that the trans-41 cis-4 ratio increased concomitantly and that styrene appears as an isolable product after photochemical, but not after thermal degradation of the 3 /(-CD complex. 

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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