Isomer open-ring

Ester functions are not saponified under these ring opening conditions. However, a trans-a-acetoxy function hinders the epoxide opening reaction and a noticeable decrease in yield is observed in comparison to the cw-a-acetoxy isomer. The ring opening reaction is also dependent on the concentration of sulfuric acid. Polymer formation results when the acid concentration is too low and the reaction is markedly slower with excessive concentrations of acid. A 0.5% (vol./vol.) concentration of acid in DMSO is satisfactory. Ring opening does not occur when ethanol, acetone, or dioxane are used as solvent. 

Irradiation of l,3-benzodioxan-4-ones appropriately substituted at position 2 with an aryloxy or thioaryloxy group gives rise to the formation of the same photoproducts as in the PFR of the open ring isomers. Scheme 60 depicts the structures of para-methoxyphenyl ortho- acetoxybenzoate 229 and its cyclic isomer, 2-(para-methoxyphenoxy)-2-methyl-l,3-benzodioxan-4-one 230. Irradiation of 229 or... 

Scheme 11.5 Reaction of 16 with singlet oxygen, and structure motif of the open ring within the cluster framework of 22 and 25. Curly arrows within the formulas 20 and 23 indicate the cluster opening reaction leading to the valence isomers 22 and 25.

The stability of related isomers towards ring opening in both ground and excited state can be compared using electronic overlap populations of the C4a-C4b bonds. Thus in the ground state for 44 n = 0.7274 while for n = 0.7201 reflecting the greater stability of 44 relative to 45. 

When dihydro-1,3-benzothiazine 203 was allowed to react with KF in DMF (or NaOMe in MeOH), an open ring tautomeric isomer 204 was formed. When 2-bromoacetophenone 169 (R = H) is present in the reaction medium, a mixture of two diastereoisomers of 206 was obtained (75%) after enolate/imine addition of intermediate 205 with a slight excess of the m-isomer 206a over the trans-isomer 206b (Scheme 35). The same result was obtained by a synthesis from [6+1] fragments, reported in the next section <1995TL753>. 

Hereafter, a and b indicate the closed- and open-ring form isomers, respectively. The photocyclization and cycloreversion quantum yields were determined to be 0.46 and 0.015, respectively.1121 In the absence of oxygen, the coloration/decoloration cycle could be repeated more than 2000 times.[13] The basic performance of diaryl-ethenes is described below. 

Some diarylethene derivatives that possess strongly electron-withdrawing substituents deviate from the general ruleJ5,23 The closed-ring isomers of 12b and 13b, possessing dicyanoethylene substituents, reverted to the open-ring isomers in 3.3 min and 186 min, respectively, at 60 °C. The dithienylethenes 14b, with pyridinium ion substituents, and 15b, with formyl residues, also underwent thermally reversible photochromic reactions. 

The open-ring isomer was excited with a 355 nm laser pulse (fwhm 22 ps) and the formation of the closed-ring isomer was followed at 560 nm in hexane. A rapid spectral evolution in a few tens of picoseconds was observed, and attributed to the photocycliza-tion reaction. The rise curve was reproduced by taking into account the pulse duration and the time constant of formation (t = 8 ps). Taking the rather long pulse duration into account, it was concluded that the switching time is shorter than 10 ps. 

In the open-ring isomer, free rotation is possible between the ethene moiety and the aryl group. Therefore, the open-ring isomer is non-planar and the it-electrons are localized in the two aryl groups. On the other hand, the closed-ring isomer has a bond-alternative polyene structure and the jt-electrons are delocalized throughout the molecule. These geometric and electronic structural differences result in some differences in their physical properties. For example, the closed-... 

Fig. 9.2 The open-ring isomer 8a and the closed-ring isomer 8b of the radical-substituted diarylethene. Two SOMOs of 8a and HOMO of 8b are also shown.

The electronic structural change of radical-substituted diarylethenes accompanying the photoisomerization is the transformation between a disjoint non-Kekule structure and a closed-shell Kekule structure. One may infer from the above consideration that the interaction between spins in the open-ring isomer of diarylethene is weak, whereas significant antiferromagnetic interaction takes... 

In solution, 10a showed typical photochromic behavior on irradiation with UV and visible light (Fig. 9.4). Although the radical moiety has a weak absorption in the region from 550 to 700 nm, this did not prevent the photochromic reaction. Almost 100 % photochemical conversions were observed in both the cyclization from the open-ring isomer 10a to the closed-ring isomer 10b and the cycloreversion from 10b to 10a. For the practical use of photochromic devices, high conversion is one of the most important characteristics. 

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