Thiophene benzothiophenes

Although desulfurization is a process, which has been in use in the oil industry for many years, renewed research has recently been started, aimed at improving the efficiency of the process. Envii onmental pressure and legislation to further reduce Sulfur levels in the various fuels has forced process development to place an increased emphasis on hydrodesulfurization (HDS). For a clear comprehension of the process kinetics involved in HDS, a detailed analyses of all the organosulfur compounds clarifying the desulfurization chemistry is a prerequisite. The reactivities of the Sulfur-containing structures present in middle distillates decrease sharply in the sequence thiols sulfides thiophenes benzothiophenes dibenzothio-phenes (32). However, in addition, within the various families the reactivities of the Substituted species are different. 

For commonly encountered heterocycles, the chemical shifts of trifluo-romethyl substituents will depend somewhat upon where in the heterocycle they are located. Examples of trifluoromethyl derivatives for a number of common heterocycles, including pyridines, quinolines, pyrroles, indoles, thiophenes, benzothiophenes, furans, benzofurans, imidazoles, and uracils are given below. 

Thiophene Benzothiophene (BT) Dibenzothiophene (DBT) Benzo(a)naphtho(2,3-d)thiophene Nitrogen compounds... 

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

Oxazoles and benzoxazoles are viable participants in the heteroaryl Heck reactions. In their monumental work published in 1992, Ohta and colleagues demonstrated that oxazoles and benzoxazoles, along with other rc-sufficient aromatic heterocycles such as furans, benzofurans, thiophenes, benzothiophenes, pyrroles, thiazole and imidazoles, are acceptable recipient partners for the heteroaryl Heck reactions of chloropyrazines [22b]. Therefore, treatment of 2-chloro-3,6-diethylpyrazine (27) with oxazole led to regioselective addition at C(5), giving rise to adduct 28. By contrast, a similar reaction between 2-chloro-3,6-diisobutylpyrazine (29) and benz[fc]oxazole took place at C(2) exclusively to afford pyrazinylbezoxazole 30. 

Reports detailing the chemistry and syntheses of thiophenes, benzothiophenes, and related ring systems that have appeared during the past year are the primary focus of this review. The synthesis of heterocycles including thiophenes has been reviewed <99JCS(P1)2849, 99JH1469>. As always, the author apologizes in advance for all errors and omissions. 

Figure 28 presents the calculated electron densities and bond orders of a variety of thiophenes, benzothiophenes, and dibenzothiophenes. It can be seen in the figure that the electron density on sulfur actually increases as the thiophene ring system becomes more condensed, e.g., with the parent molecules thiophene < benzothiophene < dibenzothiophene. This has also been noted by Rauchfuss (92). Thus, if electron density were the only controlling factor, one might expect dibenzothiophenes to be more reactive than thiophenes. However, as was discussed previously, the reverse is true. Thus other factors must also be important. 

As mentioned earlier, the MOPAC-PM3 calculations also helped to determine the importance of bond order in the hydrogenative route to desulfurization. Figure 28 shows the calculated bond orders of all bonds in a wide variety of thiophenes, benzothiophenes, and dibenzothiophenes (38). These values were correlated with the rates of desulfurization of sterically hindered alkyl-substituted benzothiophenes and alkyl-substituted 1,1 -diox-... 

This statement could well be expanded to include studies describing the kinetics of model compounds. In reviewing the literature, one finds that there are almost as many kinetic representations as there are researchers and/or model compounds. Even the same authors have found it necessary to use different equations to describe the different responses to inhibitors for closely related sulfur species such as thiophene, benzothiophene, and dibenzothiophene (104, 122, 123). The inhibiting effect of H2S for the hydrogenation of a simple molecule, such as toluene, has been found to require extremely complex equations to adequately describe mathemati-... 

Perhaps the largest discrepancies in reported results are the relative values for the adsorption constants of H2S and thiophene molecules (THs, including thiophene, benzothiophene, and dibenzothiophene). The reported preference for adsorption on the direct desulfurization site ranges from H2S THs (122,123,125) to about the same (104) to H2S < c THs (125). 

This chapter will review synthetic methods for the preparation of thiophenes, benzothiophenes and polycyclic thiophenes. In addition, a survey of the applications of thiophenes and derivatives will supplement the specific discussion included in Chapters 1.04 through 1.16 of this work. 

The hds of thiophen, benzothiophen, and dibenzothiophen and their methyl-substituted derivatives were compared in pulse experiments (623-723 K) over a sulphided CoO(5.6)-Mo03(11.2)/Al203 catalyst. Reactivities at 1 atm pressure were roughly the same but at hi er pressures, reactivity decreased with the number of rings. For benzothiophen, methyl substituents did not affect the reactivities, but for dibenzothiophen methyl substituents in the 4- or 4 - and 6- positions caused a decrease of desulphurization rate. Aromatic ring hydrogenation was not a prerequisite of C-S scission, which was the slow step. 

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. 

Upon hydrotreating under standard conditions, a good proportion of these species can be removed with relative ease, such as the thiols, sulfides, and disulfides, so they do not represent a major problem with today s technology, but others are more refractory, like the thiophenes, benzothiophenes, and specially the dibenzothiophenes, as a result of their aromatic character. 

Examples of S-bonded thiophene complexes are known for a wide variety of metals, viz. Mn, Re, Cr, Mo, W, Fe, Ru, Co, Rh and Ir, as shown in Table 2.1 the procedures used to synthesize such compounds are usually straightforward, involving addition of the thiophene to an unsaturated metal precursor or displacement of a labile ligand. This type of binding has generally turned out to be weak, resulting in rather unstable compounds the stability increases along the trend thiophenes < benzothiophenes < dibenzothiophenes. 

Photochromic reactions of cw-l,2-diarylethenes are the extension of photochemical electrocyclization of cw-stilbene, which yields dihydrophenanthrene. When the aromatic stabilization energy (aromaticity) of at least one aryl group is low (such as furan, thiophene, benzothiophene) and the nonhydrogen substituents are located on the ring-forming carbon atoms, the thermally irreversible photochromism is observed. When the aromaticity of both aryl groups are high (such as phenyl, indolyl, or pyrryl), the diarylethene is thermally reversible [31]. 

Sulfur is present in petroleum as sulfides, thiophenes, benzothiophenes, and dibenzothiophenes. In most cases, the presence of sulfur is detrimental to the processing because sulfur can act as catalytic poisons during processing. 

---