1.2- Diphenylcyclopropanes, isomers

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... 

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... 

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]. 

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... 

Although pyrolysis of several phenyl-substituted cyclopropanes results in extrusion of phenyl-carbene, which undergoes addition to C-C double bonds and forms phenylcyclopropanes under the experimental conditions employed, the overall efficiency of the process is generally too low to make it an attractive method for synthesis on a preparative scale. Thus, phenylcarbene, generated by photolysis of trans-l,2-diphenylcyclopropane or 1,2,3-triphenylcyclo-propane, forms 2-ethyl-2-methyl-1-phenylcyclopropane in only 6% yield as a mixture of isomers when the reaetions are carried out in 2-methylbut-l-ene. ... 

The stereochemistry of the cyclopropanation reactions is difficult to predict. (Dimethoxy-phosphoryl)carbene reacts stereospecifically with ( )-l,2-diphenylethene to give 3-dimethoxy-phosphoryl-/ra .v-l,2-diphenylcyclopropane. In contrast, photolysis of (dimethoxyphosphor-yl)(phenyl)diazomethane in the presence of (Z)-but-2-ene gave a mixture of diastereomeric m-isomers, (la,2, 3 )-2 and (la,2a,3a)-2, along with the /ranj-isomer, (la,2ot,3/ )-2, in 15, 3 and 5% yield, respectively. Reaction with ( )-but-2-ene afforded only two of the possible cyclopropanes, namely (la,2, 3 )-2, and (Ia,2a,3/S)-2. ... 

In the palladium(0)-catalyzed reaction of (l-methylethylidene)cyclopropane (1, R = Me) with cycloalk-2-enones, mixtures of isomers with respect to the stereochemistry of the ring fusion are formed, the preferences being dependent on the ring size of the cycloalkenone. Analogous reactions with (diphenylmethylene)cyclopropane (1, R = Ph) lead to improved product yields but decreased stereoselectivity. In the case of cyclohex-2-enone (n = 3), palladium(0)-catalyzed disproportionation of the enone to yield cyclohexanone and phenol may also occur. With (diphenylmethylene)cyclopropane, this side reaction is of minor importance because of the high tendency of this substrate to undergo cycloaddition. However, with 2-methylene-l,l-diphenylcyclopropane as substrate, the disproportionation reaction predominates. " " ... 

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. 

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. 

The photoinduced cis-trans interconversions of 1,2-diarylethenes are reviewed in Chapter 3 of this Volume. It is, however, pertinent to note here the study into the geometric isomerization of cis 2,3-diphenylcyclopropane-l-earboxylic acid derivatives. Such compounds, contrary to what is known about 1,2-di-phenylcyclopropanes, are now reported to have triplet energies of approximately 311 kJ mol , to undergo the less common adiabatic isomerization to the Irons isomers, to display emission from the electronically excited 1,3-diradical intermediates, and not to undergo intersystem crossing on direct excitation. 

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. ... 

C)S-Diphenylcyclopropane is achiral while the trans-isomer is chiraL Energy- or electron-transfer sensitized irradiation of cis-diphenylcydopropane in isotropic solvents provides the trans-isomer as a... 

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