Unsymmetrical photochromes

The most widely used method for the preparation of perfluorocyclopen-tene-containing DTEs is based on the reactions of lithium derivatives of thiophene with octafluorocyclopentene (98KGS927, 99IZV979, 99JP183). This method is used for the assembly of photochromic products from components indifferent to butyllithium or for the formation of the core of the molecules for their subsequent functionalization. This approach is suitable for the synthesis of both symmetrical and unsymmetrical photochromes. 

Moreover, the monosubstitution products can be isolated and involved in subsequent transformations giving unsymmetrical photochromes (Scheme 9) (99EJ02359). 

The structures of unsymmetrical photochromic 42, which is also intended for the use in the synthesis of metal nanoparticles enclosed by dihetarylethenes (07CC1355), and fluorescent 43, which was prepared in 05JOC5545, are illustrated. 

A series of symmetrical and unsymmetrical photochromic 3,4-dihe-taryl-2,5-dihydropyrroles 147-150 containing the furan, thiophene, oxa-zole, or indole rings were synthesized as described above in 60-80% yields starting from p-anisidine (05JOC5001). 

The syntheses of photochromes in which the heterocycles are separated by the maleimide ring described in the literature were as time-consuming as the syntheses of compounds based on maleic anhydride. A general approach to these photochromes is exemplified in Scheme 55 using unsymmetrical photochrome 180 (98BCJ1101). Here, the treatment of 2-methoxybenzothiophene with oxalyl chloride and aminoacetonitrile... 

A typical system for an unsymmetrical photochromic compound is molecule 26. It is prepared by reaction of 10 with pyridosalicylideneanil (Scheme 10)33... 

The main method for the preparation of perfluorocyclopentene DHE used in the majority of studies is the reaction of the lithium derivatives of thiophene with octafluorocyclopentene [43 5, 41]. The photochromic products are thus assembled from the components indifferent to butyllithium or the molecule backbone is formed for further functionalization. This approach allows one to synthesize both symmetrical and unsymmetrical photochromes. 

In several cases, at an equimolar ratio of the reactants, the reaction can be directed to the predominant formation of the mono- or disnbstituted products. The nitration of photochrome 54 gave the monosubstitution product 62 in 75 % yield, which allowed the authors to synthesize unsymmetrical photochromes 64 and 65 [74] (Scheme 25). 

Product 117 is a convenient starting compound for the subsequent modification of photochromes. Publication (09TL1614) gives an efficient synthetic route to both symmetrical 118 and unsymmetrical 119 phenyl-substituted dihetarylethenes bearing amino, hydroxy, or carboxy groups based on a Suzuki reaction of dichloride 117 with commercially available substituted boronic acids (or their pinacol esters) in a dimethyl ether (DME)-H20 mixture (4 1). For the symmetrical products, the yields are 85-95% for the unsymmetrical products, they are 60%. 

The chlorine atoms in the key starting 121 can be easily replaced by metal in the presence of BuLi at 20°C, which offers considerable promise in preparing various symmetrical and unsymmetrical derivatives. In solution, this product exhibits typical photochromic properties. However, an X-ray diffraction study of 121 showed that the thiophene rings in the molecule are parallel to each other, which is responsible for the absence of the photochromic properties in the crystalline state (05AC (E)o951). 

The transformations of 121 into various symmetrical and unsymmetrical perhydrocyclopentene-bridge photochromes, for example, 122-127, were documented (98CC2313,03EJOC155, 04CSR85, 07CC1698). 

Symmetrical and unsymmetrical thenoines, diketones, and chloro ketones 213-225 containing substituted thienyl, benzothiophene, and other moieties are versatile precursors of structures containing various heterocycles as bridges. Chloro ketones were used, for example, in the synthesis of photochromic thiazoles 226 (01IZV113) and tetrathiafulva-lenes 227 (99CL1071) (Scheme 65). 

Over the past few years, in terms of atom economy and extension the conjugated system of dithienylethene, we fused photochromic dithienylethene directly to the position of the pyrrole units of porphyrazines (tetra-azaporphyrines) or phthalocyanines, and developed a class of unsymmetrical and symmetrical phthalocyanine and porphyrazine hybrids containing 2,4, 6, and 8 thiophenyl groups (BTE-TAPs), as illustrated in Scheme 9 [37-39],... 

Applying one of the four synthetic approaches (a-d) (see Sections 6.2.1.1 to 6.2.1.4) in combination with the same or a second approach, can lead to biphotochromic systems.30-33 In principle, these are symmetrical systems such as DHI-DHI and unsymmetrical systems such as DHI-a-DHI-b DHI, photochromic systems-c are possible. 

Unlike the symmetrically substituted analogue phenl, the unsymmetrically substituted DTE containing 1,10-phenanthroline ligand pheu2 and the corresponding Ru-pheu2 exhibit no photochromic properties [75]. 

M. M. Krayushkin, M. A. Kahk, D. V. Kozhinov, A. Y. Mart5mkin, Photochromic dihetarylethenes. 22. Synthesis and photochromic properties of unsymmetric alkylthio-substituted l,2-bis-(dithienyl)-perfluorocyclopentenes, Chem. Heterocycl. Compd., 41, 592-597 (2005). 

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