Photoisomerization solid state

Abstract After a brief introduction and summary of various methods of asymmetric induction in organic photochemistry, the main part of the review covers the solid-state ionic chiral auxiliary approach to asymmetric photochemical synthesis. Application of this technique to the Norrish type II reaction, as well as to the di-n-methane and oxa-di-n-methane photorearrangements, and the cis,trans-photoisomerization of diarylcyclopropane derivatives is presented and discussed. 

Thus, the solid-state photoisomerization of 10 proceeds via a topotactic reaction mechanism, while some (Z,Z)-muconic esters (e.g., 13 and 15) can pho-toisomerize to the corresponding ( , )-muconic esters (14 and 16) without any change in the space group, i.e., proceed via topochemical EZ isomerization (Scheme 13, Fig. 15) [115]. 

Irradiation of 2,6-dimethylpyran-4-one (357) in ethanol or benzene or in the solid state yields the cage-like molecule (358) from which the pyran-4-one is regenerated by hydrochloric acid (63JA1208). When the photodimerization is conducted in water but in the absence of air, a small amount of 4,5-dimethylfuran-2-aldehyde is also formed (63JA2956). Pyran-4-one is photoisomerized to pyran-2-one (359) the intermediates postulated resemble those suggested in the irradiation of pyrylium salts (Section 2.23.2.1) (79JA7521). 

Most solid-state photoreactions that have been reported involve photoredox, although examples of photoisomerization and photoaquation involving lattice water have also appeared. For example,... 

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]. 

To what aspect of the solid-state environment can the high ees be attributed In general, one can imagine two scenarios a topochemical effect, in which the course of the reaction is governed by steric effects between the reactant and its lattice neighbors, and a conformational effect, in which reactivity is controlled by the conformation of the individual reacting molecules in the crystal. Zimmerman and Flechtner showed that the photoisomerization of cyclopropyl ketone 5 most likely involves rupture of the C1-C2 (or C1-C3) bond followed by C2-C3... 

The host-guest and ionic chiral auxiliary approaches have been most intensively applied for solid-state asymmetric induction. A number of achiral organic compounds could be converted into chiral compounds in high enantioselectivities. However, all the photoreactions in themselves are well-known intramolecular photoreactions photocyclization, [2 + 2] photocyclization, Norrish type II photo-cyclization, di-ir-methane photorearrangement and photoisomerization. New types of asymmetric photoreactions have never been reported. 

Salt crystals were prepared between cyclopropane mesoacid 76 and ten different optically pure amines. Solid-state irradiation of these crystals caused photoisomerization affording the optically active frans-cyclopropane 77 in moderate enantiomeric excesses (Scheme 17) [84]. The best results were obtained using (S)-proline, which gave 79% ee at low conversion. When salt crystals of the diphen-ylbenzoyl acid 78 with several optically pure amines were irradiated, then the compounds 79 or 80 of near quantitative enantiomeric excesses were obtained, with a slight diminution in ee as the conversion was increased [85]. 

Figure 2 Solid state 2-1 photoisomerization of the 2-cyanoethyl group. The chiral 1-cyanoethyl group is produced after photoirradiation.

Figure 10 presents the kinetic trans-cis photoisomerization process, under UV irradiation in the solid state, hi this case, significant differences appear between samples behaviour, as a function of the nucleobase chemical structures. It is interesting to note that, in the case of azo-polysiloxane substituted with adeiune (sample 2 -Table 1), the behaviours in the solid state and in solution are similar. That means that the polysiloxane chain flexibility, combined with the amorphous polymer ordering assure enough free volume for the trans-cis isomerization process. 

Sawada, K., Hashizume, D., Sekine, A., Uekusa, H., Kato, K., Ohashi, Y., Kakinuma, K. and Ohgo, Y. (1996). Four polymorphs of a cobaloxime complex with different solid-state photoisomerization rates. Acta Crystallogr. B, 52, 303-13. [237]... 

This actinometric procedure was utilized to determine the photochemical quantum yield for the solid-state photoisomerization of the Diels-Alder adduct (I) (formed from 2,5-dimethylbenzoquinone and cyclopentadiene) to the pentacyclic diketone (II), i.e.. 

Interestingly, identical photoisomerization quantum yields of 0=1.00 are obtained for substrate I in solution and adsorbed on silica gel. Thus, the photoisomerization reaction on silica gel is proposed as a new solid-state actinometer with well-known quantum yield. 

Luminescence properties of and phenomena in polymer systems continues to be widely researched in connection with mechanisms of polymer degradation and stabilization, molecular dynamics, solubility, blend miscibility, and solar energy harnessing. A number of interesting reviews have appeared. Molecular dynamics of polymers in solution and in the solid state have been covered, as has excimer formation,photoresponsive polymers,behaviour of polymer gels, and photochromic phenomena. Photoisomerization of enzymes and model compounds has also been discussed in depth, with particular emphasis on proteins and synthetic polymers containing azo-compounds or spirobenzopyrans. ... 

Several years ago, we found that the isomerization of n-butylammonium ( Z,Z)-muconate produces the corresponding EE-isomer in a high yield in the crystalline state under photoirradiation [41]. This solid-state photoisomerization was re e e< t0 he a one-way reaction and no EZ-isomer was formed during the reaction, unsaturated compounds such as olefins, polyenes, and azo compounds generally undergo reversible one-bonded photoisomerization to form a mixture of omers. Previously, we pointed out the possibility that the isomerization of the but thni(i er Va ves the s°hd state f°ll°ws the bicycle-pedal reaction mechanism, et al [4 eta s °ffhe molecular dynamics ofthe reaction had not been clarified. Saltiel react an< fl have independently discussed volume-conserving... 

Kinbara, K., Kai, A., Maekawa, Y., Hashimoto, Y., Naruse, S., Hasegawa, M. and Saigo, K. (1996) Photoisomerization of ammonium a,b-unsaturated carboxylates in the solid state effect of the hydrogen-bond network on the reactivity. J. Chem. Soc. Perkin. Trans. 2, 247-253. 

Furukawa, D., Kobatake, S. and Matsumoto, A. (2008) Direct observation of change in molecular structure of benzyl (Z,Z)-muconate during photoisomerization in the solid state. Chem. Commun., 55-57. 

Because the ratio of these processes is independent of a LMCT state is the origin of both reactions . In the solid state or in viscous media such as polyvinyl alcohol, photoredox reactions are suppressed and the principal photoproduct is the nitrito iso-mer. Similar linkage photoisomerizations include ... 

Diphenyl-1-benzoylcyclopropane and its derivatives (Scheme 2) provide ideal vehicles for testing the solid-state ionic chiral auxiliary approach to asymmetric synthesis. Early work by Zimmerman and Flechtner had shown that the parent unsubstituted compound, which is achiral, undergoes very efficient (d> = 0.94) photoisomerization in solution to afford the chiral cisjrans isomer [10]. Accordingly, we prepared the j9-carboxylic acid derivative 5a and treated this with a variety of optically pure amines to give salts 5b-5f. Irradiation of crystals of these salts followed by diazomethane workup yielded methyl ester 6, which was analyzed by chiral HPLC for enantiomeric excess. The results are summarized in Table 1 [11]. 

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