Benzothiophene mechanisms

Thiophene is the typical model compound, which has been extensively studied for typifying gasoline HDS. Although, some results are not completely understood, a reaction network has been proposed by Van Parijs and Froment, to explain their own results, which were obtained in a comprehensive set of conditions. In this network, thiophene is hydrodesulfurized to give a mixture of -butenes, followed by further hydrogenation to butane. On the considered reaction conditions, tetrahydrothiophene and butadiene were not observed [43], The consistency between the functional forms of the rate equations for the HDS of benzothiophene and thiophene, based on the dissociative adsorption of hydrogen, were identical [43,44], suggesting equivalent mechanisms. 

The sulfur-specific pathway for desulfurization of benzothiophene (BT) has been reported in Gordonia sp. Strain 213E. The metabolites of BT conversion were determined by ethylacetate extraction of the culture medium followed by GC-MS analysis [33,34], The reaction mechanism was proposed to be very similar to that of DBT for the first two steps (Fig. 4) however, the third step was quite different. 

Two patents were awarded on microbial desulfurization of sulfur-containing heterocyclic compound [155,156], the first targeting DBT and alkylated DBTs and the other benzothiophenes and alkylated benzothiophenes. In both cases, the selective cleavage of the C—S bonds is reported as the main mechanism. The claimed bacteria strains are Mycobacterium G3 strain (PERM P-16105) and R. erythropolis KA2-5-1 strain (PERM P-16277), respectively. Special emphasis was made to the desulfurization of the recalcitrant 4,6-dimethyl-dibenzothiophene. The main product from DBT... 

Keywords thiophene, methyl-thiophene, benzothiophene, H202, Ti-Beta, mechanism, XANES. 

The oxidation of thiophene and its derivatives with H202 was studied using a Ti-Beta molecular sieve. The oxidation product is very dependent from the aromaticity of model compounds. The thiophene oxidation product was mostly sulfates and the benzothiophene oxidation product was benzothiophene sulfone. Oxidation of mono and di-alkyl thiophenes also produced sulfates and sulfones. The diffusivity and aromaticity of the relevant sulfur compounds, intermediates and stable product, as well as the proposed new mechanism of oxidation will be discussed. This proposed new reaction pathway is different from current literature, which reports the formation of sulfones as a stable oxidation product. 

A versatile and regioselective synthesis of benzo[b]furans, naphthalenes, indoles and benzothiophenes was achieved by reaction of o-alkynylarene and heteroarene carboxaldehyde derivatives in the presence of iodonium ions. The reaction mechanism was also discussed <06CEJ5790>. 

Several heteroaromatic compounds can be hydrogenated by [Rh(COD) (PPh3)2]+ species. Thus, this cationic complex has been reported to be a catalyst precursor for the homogeneous hydrogenation of heteroaromatic compounds such as quinoline [32] or benzothiophene [33]. Detailed mechanistic cycles have been proposed by Sanchez-Delgado and coworkers. The mechanism of hydrogenation of benzothiophene by the cationic rhodium(III) complex, [Rh(C5Me5) (MeCN)3]2+, has been elucidated by Fish and coworkers [34]. 

As discussed in Section III, when the sulfur content is lowered from 0.20 to 0.05%, the chemistry of HDS of gas oils is essentially the chemistry of alkyl-substituted dibenzothiophenes. Though gas oils initially contain mostly alkyl-substituted benzothiophenes, these are completely removed by the time 0.20% S is achieved. Thus, this review will deal predominantly with the reaction pathways involved in the HDS of alkyl-substituted dibenzothiophenes. There are many excellent reviews on reaction pathways of the more reactive sulfur species such as thiophenes and benzothiophenes (2, 5, 8, 23, 24), and the reader is referred to those reviews for information on the reaction pathways and mechanisms of HDS for the more reactive... 

Whitehurst, Isoda, and Mochida write about catalytic hydrodesulfurization of fossil fuels, one of the important applications of catalysis for environmental protection. They focus on the relatively unreactive substituted di-benzothiophenes, the most difficult to convert organosulfur compounds, which now must be removed if fuels are to meet the stringent emerging standards for sulfur content. On the basis of an in-depth examination of the reaction networks, kinetics, and mechanisms of hydrodesulfurization of these compounds, the authors draw conclusions that are important for catalyst and process design. 

A number of reactions have been investigated which could form a thiophene ring by forming a bond y to the sulfur atom, but which have not been generalized. Photocyclization of phenylthioethylenes has led to benzothiophenes, usually in low yields, and sometimes accompanied by rearrangement (68JOC2218). Irradiation of (115 R = H or Me, and R1 = Ph) in the presence of iodine gave a mixture of (116) and (117) in about 11% maximum yield. If R1 = H or Me, no product (117) was found, and if R was Ph, only traces of cyclized products were detected. A complex mechanism was proposed to account for these results. 

The well-known Elbs reaction has been applied to thiophene syntheses, but the reaction may involve some isomerization. Thus pyrolysis of 3-o-toluoylbenzo[6]thiophene (347) for 3 hours at 340-360 °C gave naphtho[2,1 -b]benzothiophene (348) in 35% crude yield, instead of the expected naphtho[2,3-Z>]benzothiophene. A radical mechanism (Scheme 26) was proposed to explain this result (56JCS3435). 

The literature on conductivity in poly(p-phenylene sulfide) is confused. According to Shacklette et al.74) heavy doping with AsF5 causes reduction of the polymer with the formation of fused benzothiophene structures which are responsible for conjugation. This would more properly place poly(p-phenylene sulfide) in the category of precursor polymers, discussed later. On the other hand, Friend and Giles 75) proposed an intrinsic conduction mechanism, based on optical measurements and Tsukamoto et al. 76) have presented XPS and 13C NMR measurements to support this view. 

The reaction of diphenylacetylene with benzothiophene allowed the isolation of low quantity of the first cycloadduct 90, but this reaction occurs with a mechanism different from that of propiolate considering that the excited states of diphenylacetylene is probably involved (69TL4791 71JOC3755). 

The flavonoid crysin and benzothiophenes have been shown to inhibit casein kinase II (CKII), a cellular protein that may regulate HIV-1 transcription by phosphorylating (other) cellular proteins involved in the HIV-1 transcription transactivation process. This mechanism of action is independent of the nuclear factor kB-driven transcription pathway. Thus, flavonoids interfere with HIV-1 transcription and hence prevent HIV expression in latently infected cells. Their specificity and usefulness as HIV transcription inhibitors remain to be assessed. 

The weight percents in Table XI demonstrate that the thiophenes were very reactive toward sulfur removal under all experimental conditions. The absence of partially hydrogenated thiophenes in the products is consistent with the mechanism of di-benzothiophene desulfurization (18,22). 

Two other minor products formed in the pyrolysis, not present in the starting benzothiophene, were phenylthiophene and bithienyl. According to our proposed scheme, these seem to demand the formation of thiophyne and/or thiophene from benzothiophene. Mass spectra of some aromatic dicarboxylic anhydrides give a clue to a possible mechanism. 

Solution-phase studies are more important preparatively. Two main mechanisms seem to operate in solution. The first is attack of the radical cation of a heteroaromatic donor on a tz nucleophile, as happens in the arylation reactions reported above. Other examples include photochemical reactions in which the heterocycle participates as a donor—for example the formation of 2- and 3-(l,2-diphenylethyl)-pyrroles (yield 44 and 10 %, respectively) from the irradiation of ( )-stilbene in the presence of pyrrole, a reaction which evidence implies is initiated by SET from pyrrole [88]. 2-(2, 2 -Diphenylethyl)furans are cleanly formed on irradiation of the corresponding furans in the presence of 1,1-diphenylethylene and an electron-accepting sensitizer [89]. Likewise, irradiation of naphthalene and benzothiophene in the presence of pyrrole results in electron transfer from the latter and leads eventually to pyrrolyldihydronaphthalene or benzothiophene, 44, respectively (Scheme 29) [90]. 

Anodic methoxylation of aromatic ethers via the EEQCp mechanism has found its widest application in the synthesis of quinone bisketals (LXX) from para-dimethox-ybenzenes (LXIX) [79]. Yields are generally excellent, and the reaction is conveniently carried out at constant current in a single cell [42,80]. The reaction has also been shown to work well for naphthalenes [81] and benzothiophenes [82]. As shown in Table 4, a variety of substituents may be present on the ring. When substituents sensitive to cathodic reduction such as -CHO and -CH CHCO Me are present, a divided cell apparatus may be required to obtain reasonable yields of the corresponding bisketals. 

The hds of benzothiophen (473-670 K, 85 atm, 8 wt.% S in decalin) proceeded by two separate mechanisms (Scheme 5) both pseudo-first order in reactant.Desulphurization to styrene was enhanced in a sulphided catalyst thiophenic ring hydrogenation was more favoured in a pre-reduced catalyst. At increased H2S levels, reaction between H2S and styrene gave 1- and 2-phenylethanethiol, the amounts of which were also increased by the presence of FeS2. 

The rates of hds of thiophen, benzothiophen, and polyaromatic thiophens were compared over a sulphided commercial C0O-M0O3/AI2O3 catalyst (573 K, 71 atm). Pseudo-first-order kinetics were obeyed. The mechanism of the reaction with thiophen (involving ring hydrogenation) was different from that of other compounds (S extmsion). The reactivity was not governed solely by the size of the ring compound interaction of the ir-electron system with the catalyst surface may be more important than the interaction of S except for thiophen. 

While the intramolecular Heck reaction has been widely used to synthesize indoles and benzofurans, not many applications have been found in the preparation of benzothiophenes because of the thiophilicity of the Pd(ll) species. Pleixats and coworkers treated iodophenylsulfide 276, obtained from o-iodoaniline and crotyl bromide in two steps, with Pd(Ph,P)4 and EtjN in refluxing acetonitrile to form the intramolecular Heck cychzation product 277 [175]. The mechanism is akin to that of the Mori-Ban indole synthesis (see page 27). In another case, the intramolecular Heck cyclization of enamidone 278 with a pendant thienylbromide moiety furnished the 6-endo-trig product, indolizine 279, in 63% yield, along with the debrominated enamidone 280 in 37% yield [179]. 

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