Aromatization of tetrahydrothiophenes

Dichlorothiophene has become easily available through chlorination and dehydrochlorination of tetrahydrothiophened Another example of the aromatization of tetrahydrothiophene derivatives is the preparation of 3-substituted thiophenes by the reaction of 3-ketotetrahydrothiophene with Grignard reagents followed by the aromatization of the intermediate dihydrothiophene. Recent gas chromatographic analysis showed, however, that 2,3-dichlorothio-phene is the main product from the dehydrochlorination of tetra-chlorotetrahydrothiophene. 

The potential of MIL-47 and MIL-53(A1) for adsorption of other types of aromatic adsorbates has also been explored, for instance, of dichlorobenzene, cresol, or alkylnaphthalene isomers [17, 98]. The removal of sulfur-containing aromatics from fuels via physisorption on MOFs has been investigated on several instances in literature, for instance, via the selective removal of thiophene from a stream of methane gas by MIL-47 [99], the removal of tetrahydrothiophene from methane by... 

Thiophenes and Tetrahydrothiophenes. Thiophenes (thioles) are subject to aromatic hydroxylation tetrahydrothiophenes (thiolanes) undergo oxidation of the sulfur to give sulfoxides or sulfones. 

Does the fact that thiophene reacts similarly to benzene mean that it is aromatic One way to tell is to calculate first and second hydrogenation energies of thiophene, leading to dihydrothiophene and tetrahydrothiophene, respectively. (The energy of hydrogen is provided at right.)... 

The thienothienoimidazolium salts 29 were prepared by the reaction of thiophanes 362 with HX (X = halogen) and crystallization from solvents selected from ketones, aromatic hydrocarbons, and halohydrocarbons. l-(—)-3,4-(l, 3 -dibenzyl-2 -ketoimidazolido)-2-(u -ethoxypropyl)tetrahydrothiophene 362 was reacted with HBr at 99-103 °G for 2h and crystallized from methyl-Tro-butyl ketone to give l-(—)-3,4-(T,3 -dibenzyl-2 -ketoimidazolido)-l,2-trimethyle-nethiophanium bromide 29 (95%, 98.7% purity) (Scheme 75) <2001JAK100477>. 

This technique has been applied to the determination of aromatic hydrocarbons, alcohols, aldehydes, ketones, chloroaliphatic compounds, haloaromatic compounds, acrylonitrile, acetonitrile, mixtures of organic compounds and tetrahydrothiophene in soils, chloroaliphatic and haloaromatic compounds and organotin compounds in non-saline sediments, and organotin compounds in saline sediments. 

The resulting tetrahydropyrroles and tetrahydrothiophenes could be easily aromatized under basic conditions (Scheme 36), thus allowing a convenient access to a new class of pyrrole and thiophene derivatives, which have found application in the preparation of new organic materials [304-308]. 

The fact that the lone pair on sulfur contributes to the aromaticity is seen in the lower dipole moment of thiophene as compared to its saturated analogue tetrahydrothiophene (0.52 D vs. 1.90 D) <1972JA8854>. In thiophene, the dipole is directed from the ring toward the heteroatom. 

Hence the currently most favoured mechanism of thiophen hds is that thiophen adsorbs parallel to the catalyst surface through the rr-system of the ring, and then undergoes aromatic-type hydrogenation to tetrahydrothiophen prior to S-elimination. 

Removal of Aromatic Compounds. Because of the demand for high-purity aromatic compounds for petrochemical feedstocks, several processes have been developed for BTX (benzene, toluene, and xylenes) recovery from distillate streams. In these processes, aromatic compounds are separated from nonaromatic compounds by liquid—liquid extraction using polar solvents. The three major processes in use are the UOP—Dow UDEX process (di- or triethylene glycol solvent), the UOP sulfolane process (tetrahydrothiophene 1,1-dioxide), and the Union Carbide TETRA process (tetraethylene glycol). 

The role of heteroatoms in ground- and excited-state electronic distribution in saturated and aromatic heterocyclic compounds is easily demonstrated by a comparison of a number of heteroaromatic systems with their perhydro counterparts. In Jt-excessive heteroaromatic systems, because of their resonance structures, their dipole moments are less in the direction of the heteroatom than in the corresponding saturated heterocycles furan (1, 0.71 D) vs. tetrahydrofliran (2, 1.68 D), thiophene (3, 0.52 D) vs. tetrahydrothiophene (4, 1.87 D), and selenophene (5, 0.40 D) vs. tetrahydroselenophene (6, 1.97 D). In the case of pyrrole (7, 1.80 D), the dipole moment is reversed and is actually higher than that of pyrrolidine (8, 1.57 D) due to the acidic nature of the pyrrole ring (the N-H bond) In contrast, the dipole moment of n-deficient pyridine (9, 2.22 D) is higher than that of piperidine (10, 1.17 D). In all these compounds, with the exception of pyrrole (7), the direction of the dipole moment is from the ring towards the heteroatom [32-34]. 

Aromatic thiophenes play no part in animal metabolism biotin, one of the vitamins, is a tetrahydrothiophene, however aromatic thiophenes do occur in some plants, in association with polyacetylenes with which they are biogenetically linked. Banminth (Pyrantel), a valuable anthelminth used in animal husbandry, is one of the few thiophene compounds in chemotherapy. 

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