Synthesis of diarylethenes

The synthesis of diarylethenes from hetaryl bromides and octafluoro-cyclopentene using a microflow system allows the preparation of various symmetrical and unsymmetrical diarylperfluorocyclopentenes in 47-87% yields at 0°C (07CC2947). 

With conventional protocols requiring low reaction temperatures, typically —78 °C, to prevent side reactions from occurring, scaling the reaction for industrial production of such compounds has proved difficult. As such, the authors evaluated the process under continuous flow, proposing that the effective temperature control and accurate residence times attainable within miniaturized flow reactors would enable the synthesis of diarylethenes at temperatures above — 78 °C and thus facilitate the large-scale synthesis of such compounds. 

Scheme 5.8 Programmable and diversity oriented synthesis of diarylethenes 43 and triarylethenes 44 (a) [Pd2(dba3)], P(2-furyl)3, NEt3, THF, 60°C (b) PdCI2(PhCN)2, Bu4NF, THF, 60°C.

Fig. 5.15 Aryl iodides and diiodides applied in the combinatorial synthesis of diarylethenes 43 and triarylethenes 44.

As a typical example of diarylethene derivatives, the procedure for the synthesis of diarylethene biradical 10a [37] is given below. 

The optimized yield is 81 % at 0 °C and the total residence time is ca. 6 s. Therefore, the synthesis of diarylethenes can be accomplished at 0°C, which is easily attained in industry, by virtue of an effective temperature... 

Figure 10.3 Effect of temperature on the synthesis of diarylethene in a microflow...

Diarylethenes, 1,1-diarylallylalcohols and aryl vinyl ethers were succesfully hydroformylated in water/toluene or water/cyclohexane biphasic mixtures with a catalyst prepared in situ from[ RhCl(COD) 2] and TPPTS (Scheme 4.15). Yields of the desired linear aldehyde product were around 80%. This method was applied for the synthesis of the neuroleptics Fluspirilen and Penfluridol (Scheme 4.16) and for other pharmaceutically active compounds containing the 4,4-bis(p-fluorophenyl)butyl group [153]. 

Following similar trials with the formation of diarylbenzenes [63-65], the same research group has reported a multistep synthesis of photochromic diarylethenes using a microflow system that contained two linked micromixers and microreactors (MRi 2) [66]. Similarly to the previously reported linked microreactors, the reactors used in this setup were made of stainless steel tubes and T-shaped micromixers. Initial experiments were conducted in two steps in a continuous sequence to afford symmetrical octasubstitued diaryUiexafluoro cyclopentene (Scheme 29). 

Scheme 30 Synthesis of an unsymmetrical diarylethene using a multistep microflow system [66]...

Ushiogi Y, Hase T, linuma Y et al (2007) Synthesis of photochromic diarylethenes using a microflow system. Chem Commun 28 2947-2949... 

Recently, the synthesis of photochromic compounds containing two or more diarylethene fragments bound either directly or through linkers has been performed. Scheme 32 illustrates one approach to 96 containing two hexafluorocyclopentene-bridged dihetarylethene fragments (05JACS8922). 

Scheme 63 Schematic detailing the general protocol employed for the continuous flow synthesis of (a) symmetrical and (b) unsymmetrical diarylethenes.

The photochemical reactions of naphthoquinones and quinolinediones with 1,1-diarylethenes have led to the synthesis of many new polycyclic aromatic and heteroaromatic compounds. A review has appeared16 in which a number of these results are tabulated. 

The mono- and disubstituted perfluorocyclopentenes were selectively prepared by controlling the ratio of perfluorocyclopentene and aryllithium. The monosub-stituted compound can be used for the synthesis of nonsymmetric diarylethenes. 

Topics which have formed the subjects of reviews this year include theoretical studies of the photochemistry of thiiranes, photoaddition of amines to aryl olefins and arenes, the synthesis of heterocyclic compounds, photoamination directed towards the synthesis of heterocycles, selective addition of organic dichalcogenides to carbon-carbon unsaturated bonds, photocyclisation mechanisms of c/5-stilbene analogues, synthetic utility of the photocyclisation of aryl-and heteroarylpropenoic acids, photochromic diarylethenes, spiropyrans, cy-clophanes, and polycondensed aromatics," photochromic organic media, photophysics and photochemistry of P-carbolines, and the photochemical synthesis of macrocycles.Chirality switching by light has also been described. ... 

Topics of relevance to the content of this chapter which have been reviewed during the year include photoactive [2]rotaxanes and [2]catenanes, photochemical synthesis of macrocycles, phototransformations of phthalimido amino acids, photoaddition reactions of amines with aryl alkenes and arenes, photoreactions between arenenitriles and benzylic donors, photostability of drugs, polycyclic heterocycles from aryl- and heteroaryl-2-propenoic acids, photoreactions of pyrroles, photoamination reactions in heterocyclic synthesis, switching of chirality by light, photochromic diarylethenes for molecular photoionics and solid state bimolecular photoreactions. 

Figure 10.4 Synthesis of unsymmetrical diarylethenes in a microflow system...

The successful synthesis of unsymmetrical diarylethenes using two different aryl bromides in a single flow speaks well for the potential of microfiow systems in making new functional materials, which would be otherwise difficult to synthesize in a conventional manner. Acceptable productivity from the viewpoint of industrial production of functional materials can be attained using microtube reactors with a relatively large inner diameter (800/rm) without numbering-up. 

Kmi, M.-S., Kawai, T, and Irie, M. Synthesis of fluore.scent amorphous diarylethenes. Chem. Lett. 1188, 2000. 

Several reviews have been published within the year which are of general relevance to the photoreactions of aromatic compounds. The subjects of these reviews include photochemistry in ionic liquids and in isotropic and anisotropic media, organic synthesis utilizing photoinduced electron-transfer reactions," heteroatom-directed photoarylation processes, photochromism, and photochemical molecular devices. Reviews more directly pertinent to the sections in the present chapter include those of the photoisomerization of five-membered heteroaromatic azoles, the photocycloaddition of benzene derivatives to alkenes, Diels-Alder additions of anthracenes, advances in the synthesis of polycyclic aromatic compounds, diarylethene-based photochromic switches, the photo-Fries rearrangement, and the application of Diels-Alder trapping of photogenerated o-xylenols to the synthesis of novel compounds. " A number of chapters in the two recently published handbooks of photochemistry and photobiology and in the revised edition of the text on photochromism are also pertinent to the current subject matter. 

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