Photochromism diastereoselective

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

The spiro carbon is a stereogenic center in spiropyrans, but because of the achiral structure of the open merocyanine form, the photochromic process will always lead to racemization unless additional chiral moieties are present. When a chiral substituent was introduced, remote from the spiro center, it was possible to isolate diastereo-isomers of the spiropyrans, but rapid epimerization at the spiro center occurred.1441 Diastereoselective switching was successful with 28, in which a stereogenic center was present close to the spiro carbon (Scheme 15).[45] Distinct changes in CD absorption at 250 nm were monitored upon irradiation with UV (250 nm) and with visible light (>530 nm) and a diastereomeric ratio of 1.6 1.0 was calculated for the closed form 28. Furthermore, a temperature-dependent CD effect was observed with this system it was attributed to an inversion of the diastereomeric composition at low temperatures. It might be possible to exploit such effects in dual-mode chiral response systems. A diastereoselective ring-closure was also recently observed in a photochromic N6-spirobenzopyran tricarbonyl chromium complex. 451 ... 

Scheme 17 Diastereoselective photochromism of a binaphthol-based indolylfuIgide 31.

In general, if a photochromic compound has an intrinsic chirality that is preserved during the photochromic reaction, it is just an extension of photochromism of achiral molecules unless the intrinsic chirality does not induce the generation of the second chirality in the biased ratio. If the intrinsic chirality induces another chirality selectively, it is called diastereoselective photochromism [2]. 

Similar diastereoselective photochromism was observed for F-5, in whi< a chiral xylenediol was condensed onto the acid anhydride moiety of an indolyl gide [28]. 

The diastereoselective photochromism was observed for bis(thienylethylidei ulgide F-6 by Yokoyama et al., although the resolution of the enantiomers oj colorless forms was impossible at room temperature [29]. 

The first example of diastereoselective photochromism of a diaryle was reported by Irie s group for the bisbenzothienylethene D-l possessing or (d)-menthyloxy substitutent on C-2 of one of the two benzothiophene [32]. The diastereoselectivity was dependent on solvent polarity and tempe... 

A diastereoselective photochromism of a bisbenzothienylethene was reported by Yokoyama et al. By employing A1,3-strain around the double bond of one of the benzothiophene of D-9, the conformation of the stereogenic carbon atom with regard to the hexafluorocyclopentene ring was fixed. As a result, the second benzothiophene lay closer to the medium-sized substituent so that it would generate one diastereomer predominantly by photoirradiation. Indeed, the diastereosel-ectivity of photocyclization of D-9 in toluene at room temperature was 88% de at 85% conversion [41]. 

Scheme 2 Diastereoselective photochromic cyclization of the chiral diarylethene (S)-11.

A remarkable photochromic system in which molecular and supramolecular chirality seem to communicate with each other has recently been described <2004SCI278>. The compound 16 (X = F1) shows exceptional stereoselectivity upon aggregation of the molecules during gel formation in toluene. This supramolecular chirality is translated into molecular chirality on photocyclization wherein a diastereoselectivity of 96% is obtained. 

In another work on photoreactions, Yokoyama et al. <03JA7194> have shown diastereoselective photochromism of bisbenzothienylethene. Preliminary studies on racemic mixtures of 97 showed that the R form shows a high preference towards ring closure as a result of steric and electronic Interactions. Furthermore, the pure enantiomer was synthesized and shown to undergo thermally stable photoreactions. An example of diastereocyclization in chiral diarylethylene crystals was reported by Irie and co-workers. A diarylethylene consisting of photoreactive thiophenes and a chiral side-chain 98 was shown to exhibit selectivity in the crystalline phase, while there was no selectivity observed in solution <03OL1769>. 

Yashushi Yokoyama, S. Uchida, Yayoi Yokoyama, Y. Sugawara, and Y. Kurita, Diastereoselective photochromism of an (7 )-binaphthol-condensedindolylfulgide, /. Am. Chem. Soc. 118, 3100-3107... 

A bisthienylethene possessing a chiral oxazoline group on C-5 of both thiophene rings (D-6), reported by Branda et al., showed diastereoselective photochromism when Cu(I) salt was present [37]. It recorded 98% de mol dm concen-... 

Yokoyama, Y, Okuyama, T., Yokoyama, Y, and Asami, M., Highly diastereoselective photochromic cyclization of an indolylfulgide derivative possessing C2-symmetric chiral diol as an auxiliary, Chem. Lett., 1112, 2001. 

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