One-pot synthesis of indeno-1,2-thiazines, -[1,2]dithioles and thiophenes; new liquid crystalline materials

Abstract

In a search for further examples of a new type of discotic liquid crystal, we have examined the reactions of S₂Cl₂ in the presence of a chlorinating agent (NCS) and a base (Hünig’s base or DABCO), mostly in THF, with cyclopent-1-enylacetic acid 6, inden-3-ylacetic acid 8 and its nitrile 16 and 1-(dicyanomethylene)indane 22. The cyclopentene acid 6 gives purple crystals of the trichlorocyclopenta[1,2]dithiole ester 7, the product of heterocyclic ring formation, chlorination and dehydrochlorination and, unexpectedly, conversion of the acid in THF into its 4-chlorobutyl ester. Indenylacetic acid 8 gives methyleneindenes 10 and 11, the tricyclic 1,2-dithiolone 12 and the deep purple thiophene 13. Indenylacetonitrile 16 gives the chlorocyanomethylene indane 17 and the E- and Z- isomers of both the dichloroindene 18 and the monochloroindane 19, depending upon the precise reaction conditions. Dicyanomethylene indane 22 gives the dichloroindene 23 and the red, thermochromic indeno-1,2-thiazine 24; the analogous cyano-ester 25 gives the corresponding products. On melting, crystals of 13 and 23 are strongly birefringent. Mechanisms are proposed for the new transformations.

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