Cyclooctene photoisomerization

Optically active benzene(poly)carboxamides and benzene(poly)carboxy-lates were used by Inoue and co-workers as sensitizers for the geometrical photoisomerization of (Z)-cyclooctene and (Z,Z)-cyclooctadienes in various solvents at different temperatures. Under energy-transfer conditions, enantiomeric excesses up to 64% ee in unpolar solvents like pentane were reported. The use of polar solvents diminished the product ee s due to the intervention of a free or solvent-separated radical ion pair generated through the electron transfer from the substrate to the excited chiral sensitizer (Scheme 58) [105-109]. 

Figure 2 Pressure switching of product chirality in enantiodifferentiating photoisomerization of cyclooctene 47 sensitized by (— )-menthyl pyromellitate 45a in pentane at 25°C.

Sensitization with Chiral Aromatic Amides, Phosphoryl Esters. Besides the above-mentioned studies employing aromatic carboxylic esters as sensitizers and naturally occurring alcohols as chiral auxiliaries, some attempts have been made to use other types of sensitizers, such as aromatic amides [43] and phosphoryl esters [44] with (— )-menthyl, as well as synthetic C2-symmetric chiral auxiliaries, shown in Scheme 8. These chiral compounds can efficiently sensitize the Z-E photoisomerization of cyclooctene 47 to give moderate E Z ratios of up to 0.17 and 028 and low-to-moderate ees of up to 5% and 14% for the aromatic amides and phosphoryl esters, respectively [43,44]. 

The photoisomerization of (Z)-cyclooctene (30) (Scheme 12) to the (E)-isomer (31) was sensitized by enantiopure alkyl benzenecarboxylates immobilized in zeolite to give modest ees. The use of an antipodal sensitizer pair of (R)-and (S)-1 -methyIheptyl benzoates, 32d and 32e, yielded enantiomeric 31 in — 5% and +5% ee, respectively, while the same sensitizers gave practically racemic 31 upon irradiation in homogeneous solutions. This small, but apparent, enhancement of the product ee observed upon irradiation in modified zeolite supercages is likely to arise from the decreased conformational freedom of the adsorbed sensitizer, the hindered approach of 30 to the sensitizer, and/or the different exciplex structure in confined media. In this context, it is interesting to examine the effect of temperature on the supramolecular photochirogenesis in modified zeolites and to compare the results with those obtained in the homogeneous phase. Such an examination will reveal the distinctly different role of entropy in confined media, which should be clarified in a future study. 

Prior to such a sophisticated attempt, a more straightforward strategy was examined in order to evaluate the chiral discrimination ability of the CDx cavity. Thus the direct photoisomerization at 185 nm of a 1 1 complex of (Z)-cyclooctene 30 with P-CDx was carried out in the solid state to give an E-Z mixture of E Z = 0.47 [122]. The ee of the obtained (E)-isomer 31 was low (0.24 %) [123], but this work paved the way for the supramolecular photosensitization of 30 with chromophore-modified 3-CDx derivatives 53-55 in solution [123,124]. 

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. 

Further studies on the photoisomerization of ci5-cyclohexene and cycloocta-1,3-diene have been reported. Again the work has focused on enantiodiffer-entiation. In this case a series of optically active chiral sensitisers (3) have been used under conditions where solvent and temperature have been varied. Some of the o-disubstituted and tetra substituted amide sensitisers afford mixtures with enantioisomeric excesses of 14%. The influence of pressure and temperature on the asymmetric photochemistry of cyclooctene has been reported. A variety of chiral sensitisers were used. Some of these are shown in (4). Other work has shown that aromatic phosphates, phosphinates and phosphines (e.g. 5-8) can also sensitise the isomerism of cyclooctene. Moderate stationary-state ratios were obtained. 

Upon sensitized irradiation, the triplet excited acyclic alkenes and large-ring cyc-loalkenes undergo E Z isomerization in both aprotic and protic media. Mediumring cycloalkanes cyclohexenes, cycloheptenes or cyclooctenes can, however, be protonated via the corresponding thermodynamically unstable (strained) /i-isomers formed initially by a Z —> E photoisomerization step (Section 6.1.1).662-664 For example, acid-catalysed irradiation of (Z)-l-methylcyclohexene (99) in the presence of /)-xylene as a triplet sensitizer in methanol affords the Markovnikov adduct 1-methoxy-l-methylcyclohexane (100) and the elimination product methylenecyclohexane (101), both in approximately 40% chemical yield (Scheme 6.41).671 The Zs-isomer intermediate exhibits extensive incorporation of deuterium in the presence of CH3OD. 

Inoue, Y., Yamasaki, N., Yokoyama, T., Tai, A., Highly Enantiodifferentiating Photoisomerization of Cyclooctene by Congested and or Triplex forming Chiral Sensitizers, J. Org. Chem. 1993, 58, 1011 1018. 

Fig. 2.1 Some model compounds used for studying photoisomerization processes Z-l, 2-bis-a-naphthylethylene 1, retinal 2, -carotene 3, cyclooctene 4.

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