Ring conversions, forming thiophenes

Thiol Decomposition. As explained previously, the elimination of sulfur from benzothiophene occurs stepwise after the aromatic thiol (o-thiocresol) has formed and not in a concerted fashion from the thiophenic ring system. Extrapolation of this implies that thio-phenic sulfur in coal is eliminated by conversion to an aromatic thiol that subsequently undergoes desulfurization. Since the aromatic thiol is the apparent organosulfur species that undergoes desulfurization, it is of interest to understand the chemistry involving the elimination of sulfur from aromatic thiols. Thio-phenol was used as a model compound to examine reactions, primarily the thermal decomposition reactions that might lead to sulfur elimination. In the experiments with caustic and benzothiophene, the intermediate (o-thiocresol) most likely exists in the salt... 

A three-level switching device has been demonstrated in which photochromic properties are used to control electrical properties, and vice versa. Such a system has been realized in the form of thiophene bisphenol [90, 91]. Conversion of the open (8a) to the closed (8b) form of the thiophene was achieved by absorption of 312 nm light, and revered by absorption of 600 nm light. The bisphenol oxidation occurs at +0.735 V (vs. SCE), forming the closed-ring bisquinone, compound 8c. This species has large absorptions at 400 and 534 nm. The optical properties of the quinone phenol couple have previously been used in a bianthrone-based system [87]. The bisquinone (8c) cannot be converted to the open thiophene, and locks the system in the closed form. The thiophene has also been incorporated as a component in two-level molecular switches [99, 128] and switchable molecular wires [30]. 

PSS closed forms the /u/ o value for the open form is very small, in agreement with the absence of Tt-conjugation between the two thiophene rings of the DTE fragment. Upon conversion to the closed form in the photosta-tionary state, the NLO activity dramatically increased, from 160 to... 

A novel method for preparing l-phenylthio-2,3-disubstituted butadienes (313) involves reaction of 3,4-diaryl-2,5-dihydrothiophenes with benzyne. The mechanism probably involves initial betaine formation, followed by conversion to the ylide. Disrotatory ring opening would then give the product. The major product has the Z configuration (Scheme 59). A minor product is the thiophene (314) this may be formed by a symmetry allowed hydrogen transfer <89TL847>. c -2,5-Dimethyl-... 

Very recently, the efficient photoswitching of the second-order NLO responses of some dipolar photochromic Zn° complexes (9) was reported [71]. The NLO activity for the open forms is very small, as expected for the absence of 7i-conjuga-tion between the two thiophene rings. Upon conversion to the closed form in the photo-stationary state, the NLO activity increases dramatically (/i)So(EFISH) from... 

The conversion of a 1,4-diketone into a thiophene ring is a very convenient method, but not when extremely pure products are needed, for example when biological or physical properties are to be measured. Furans are formed competitively with thiophenes the extent of their formation apparently depends on the substitution pattern and probably on reaction and work-up conditions as well. Occasionally, the separation of these two products is extremely difficult 260). 

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