Trans- 1,2-Diphenylcyclopropane

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. ... 

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). 

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. 

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 ... 

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. 

Goez and Frisch have used photo-CIDNP to measure the activation energy of a biradical rearrangement reaction. Roth ° has studied the electron transfer back reactions of photoexcited cis- and trans-1,2-diphenylcyclopropane in the presence of singlet acceptors. He finds that the pair energy relative to the reactant ground states and an accessible triplet state, respectively, determines whether the back reaction is competitive. 

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... 

TABLE 4. Bond lengths (A) and bond angles (deg) of phenylcyclopropane (41), m-l,2-diphenylcyclopropane (42) and trans-1 -cyano-2-phenylcyclopropanc (43) from XD ... 

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... 

Thermolysis of l,l-difluoro-2,3-diphenylcyclopropane in supercritical CO2 has allowed the rate of geometrical isomerization [i.e. cis-( 109) to trans-( 109)] and racemization [i.e. (/J)-(109) to (5j-(109)] to be determined from O2 dependence of the trapping rate of the postulated intermediate 1,3-biradical.246 Above 150 °C, the formation of 2,2-difluoroindane and its decomposition products is reported. A similar thermally induced equilibrating series of stereomutations has been observed with the analogous non-fluorinated cyclopropane in which rate constants and deuterium exchange isotope effects are reported.247 Theoretical studies of this isomerization have focused on classical248 and quasi-classical trajectories.249... 

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... 

A solution of cis, trans-2,3-diphenylcyclopropylmagnesium bromide (70 ml, 6.1 mmol) is prepared from -brovno-cis,trans-2,3-diphenylcyclopropane (2.46 g, 9.0 mmol) and magnesium (2.46 g, 10.0 mmol) in diethyl ether (80 ml) and added at -10° to a suspension of A.A-dimethyl-O-mesitylsulfonylhydroxylamine (1.50 g, 6.1 mmol) in THF (20 ml). The mixture is allowed to warm to 25° and stirred for 15 h. Hydrochloric acid (2 m, 20 ml) is added, and the aqueous layer is separated, extracted twice with ether, and basified (2 m NaOH, 20 ml). The resulting alkaline solution is extracted three times with ether. The extract is dried (MgS04), the solvent is evaporated, and the residue (880 mg, 52%) is recrystallized from ether/pentane to give 1-dimethylamino-cw,frarts-2,3-diphenylcyclopropane (790 mg, 47%), m.p. 

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 ... 

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... 

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]. 

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. 

Photoisomerization of 1,2-diphenylcyclopropane has played a central role in photochemical asymmetric induction processes. A number of such systems have been examined within zeolites. Ethyl ester 40 undergoes photoisomerization as shown in Scheme 41. The reaction in an isotropic medium gave a racemic mixture of the corresponding trans isomer. Enantiomeric excess of 17% was achieved by the chiral inductor approach with cyclohexylethylamine [298]. Compound 41 is similar to 40, except that the ester group is changed to a keto group. Upon excitation, 41 is converted to the chiral trans isomers (Scheme 41). Within NaY, 20% enantiomeric excess was achieved with norephedrine as the chiral inductor [299]. 

In this context, we mention two zeolite-induced conversions of cyclopropane derivatives. Incorporation of tra -l,2-diphenylcyclopropane trans-Vi) and its 3,3-D2-isotopomer into the channels of a redox-active pentasil zeolite (Na-ZSM-5) generated exo,exo-l,3-diphenylallyl radical (24 ) and its 2-Di-isotopomer. This conversion is a zeolite-specific reaction it requires a series of reactions, including oxidation, ring opening, and deprotonation [70]. 

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. 

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... 

Other reversible ET-catalyzed stereoisomerizations of cyclopropanes have been observed with cjs-l-methyl-2-phenyl-, r-1-phenyl-l-methyl-c-2-methyl- and optically active l-methyl-2,2-diphenylcyclopropane (190, 191 and ( + )-(/ )-49, respectively) . Experimental evidence for the existence of intermediate cyclopropane radical anions like CIS- or trans-llH" (Scheme 18) has not been found in the course of these investigations. 

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... 

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