Photosensitizer optically active

Irradiation of [2.2]paracyclophane, under different conditions (various solvents, light sources of different wavelength, addition of photosensitizers) always leads only to open-chain cleavage products of 2. The counterpart of 775, the polycyclic equinene (77(5), could not be detected 22>. Cram and Delton 96> even ruled out the intermediate occurrence of 116 analogs in the photo-racemization of a number of optically active nuclear- and bridge-substituted [2.2]paracyclophanes. 

More recently, investigations by Schenck et al. on the photosensitized oxygenation164 and on the autooxidation176 of optically active olefins such as (+)-limonene 16) and (+)-carvomenthene [(+)-A1-p-... 

Hydrogen abstraction from the two optically active compounds 16 and 19 gives rise to the formation of symmetrical radicals from which only the racemic cis- and frans-alcohols in a ratio of 1 1 were formed. On the other hand, photosensitized oxygenation of 16 and 19 resulted... 

The method of photosensitized oxygenation was successfully applied in the preparation of alcohols 265-270 from sylvestrene (264),207 and seems to be the most simple and successful method for the preparation of optically active rose oxides (272,273) from (+)- or (—)-citronellol C271).177 It may also be used for the preparation of certain organo-metallic hydroperoxides. Thus, the triphenyl-tin derivative of tri-methylethylene (274) undergoes a photosensitized oxygenation reaction with a rate similar to that of tetramethylethylene, giving rise to the hydroperoxides 275 and 276 219... 

Besides photoconductivity, photo-electro-motive (e.m.f.) forces were observed in copper polyacetylenides. The common results characterize the hypso-chromic shift of the photo-emf spectra compared to the photoconductivity ones. The maximum of the photoconductivity spectra coincide with the minima of the photo-emf spectra. The optical activation energies of the photosensitivity defined from the photoconductivity and photo-emf spectra are in close agreement and equal to the energy of the absorption edge. 

Perhaps more important, Foote has shown that chemically generated singlet oxygen gives the same product distribution from (+)-limonene as obtained by the rose bengal-photosensitized oxidation.467 Moreover, the (—)-tra 5,-carveol formed in both types of oxidation has the same large optical activity. 

As a consequence of the restricted jump-rope rotation around the trans double bond, (i j-cyclo-octene 38E is chiral. Optically active (E)-cyclo-octene has long been known, but the conventional multistep synthesis is rather tedious [138-140]. In contrast, direct-preparation of optically active (Ej-cyclo-octene through asymmetric photosensitization is an attractive alternative. The first enantiodifferentiating Z-E photoisomerization of cyclo-octene 38Z sensitized by simple chiral alkyl benzenecarboxylates was reported in 1978 to give low enantiomeric excesses (ee s) of <6% [141] a variety of systems and conditions have been examined since then to raise the product ee. For an efficient transfer of chiral information... 

The smaller-sized cyclohexene and cycloheptene have also been subjected to enantiodifferentiating photoisomerization, although the corresponding ( )-isomers are short-lived transient species. Photosensitization of (Z)-cyclohexene 31Z with chiral benzene(poly)carboxylates affords trans-anti-trans-, cis-trans-, and cis-anti-cis-cyclodimers 34 (Sch. 32). Interestingly, of the former two chiral products, only the trans-anti-trans isomer is optically active and its ee reaches up to 68%, whilst the cis-trans isomer is totally racemic under a variety of irradiation conditions, for which two competing, concerted, and stepwise cyclodimerization mechanisms are responsible. Thus, the enantiodifferentiating photoisomerization of 31Z to the optically... 

There are several excellent photosensitizers one of them is [Ru(bpy)3]2+ [6]. There are two optical isomers in this complex one is A [Ru(bpy)3]2+ and the other is A-[Ru(bpy)3]2 +, as shown in Scheme 1. Thus one can expect to perform the stereoselective electron transfer reaction with A- and A-[Ru(bpy)3]2 +. Unfortunately, however, the racemization of [Ru(bpy)3]2+ is induced photochemically [7]. The reasonable way to suppress the photoracemiza-tion of this complex is to introduce the optically active organic functional group into the transition metal complexes, as will be discussed in Sec. II.B. The other photosensitizer that is useful for the photoinduced electron transfer reaction is the copper(I) complexes with 1,10-phenanthroline and their derivatives [8,9]. Zinc(II) porphyrin is also an excellent photosensitizer for photoinduced electron transfer reaction [10]. In these complexes, molecular chirality does not exist, unlike in [Ru(bpy)3]2 +. Thus one must introduce some chiral functional group into these compounds, to use these complexes as chiral photosensitizers. 

An intriguing logical extension of such work is the construction of a supramolecular host modified with a chiral sensitizer for executing enantiodifferentiating photosensitization. Inoue and coworkers reported a novel supramolecular photochirogenic system, in which an optically active sensitizer immobilized in zeolite supercages sensitizes the enantiodifferentiating photoisomerization of an excess amount of substrate dissolved in bulk solution [90]. 

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

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. 

Irradiation of aqueous solutions of (5)-tryptophan at pH 6-9 containing a photosensitizer (e.g., eosin) caused destruction of the amino acid and formation of N -formylkynurenine as the major product. Similar irradiation in dilute ammonia at pH 8-9 gave an optically active compound in 13% yield, which was shown to be 4-(2 -amino-2 -carboxyethyl)quinazoline (12). 

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