Benzo thiophene desulfurization

Matsui, T. Onaka, T. Maruhashi, K., and Kurane, R., Benzo(b)thiophene desulfurization by Gordonia rubropertinctus strain T08. Applied Microbiology and Biotechnology, 2001. 57 pp. 212-215. 

Thienothiophenes, like thiophenes and benzo[h]thiophenes, easily undergo reductive desulfurization with Raney nickel. The method was fvst applied in this series by Challenger et to determine the structures of 2-acetylthieno[3,2-h]thiophene and 2-acetylthieno[2,3-hl-thiophene the former gave 2-octanone (229), and the latter 5-methyl-heptan-2-ol (230) [Eqs. (75) and (76)]. 

Nickel boride (prepared by adding NaBH4 to nickel chloride in methanolTHF) is an efficient, nonpyrophoric reagent for the desulfurization of benzo[A]thiophene and dibenzothiophene. The reaction proceeds under very mild conditions and is probably mediated by nickel hydride. 

Desulfurization of polynuclear thiophenes was performed by continuous stirring with a Raney nickel catalyst in ethanol at 0-78.8 °C. The major products were biphenyl in the desulfurization of dibenzothiophene, a-phenylnaphthalene in the desulfurization of benzo[b]naphtho[l,2-d]thiophene, and -phenylnaphthalene in the desulfurization of benzo[b]naphtho[2,3-d]thiophene and benzo[b]naphtho[2,l-d]thiophene. Observation of these products shows that the main reaction pathway is the extrusion of a sulfur atom to give the corresponding hydrocarbon. The tt-electron densities of the sulfur atoms, which were calculated by using simple Huckel molecular orbital theory, are considered to be related to the adsorption of the sulfur compounds to the surface and consequently to the C-S bond-breaking rate. 

Hydrodesulfurization [HDS, Eq. (1)] is the process by which sulfur is removed from fossil materials upon treatment with a high pressure of H2 (3.5-17 MPa) at high temperature (300-425 °C) in the presence of heterogenexius catalysts, generally transition metal sulfides (Mo, W, Co, Ni) supported on alumina [1]. About 90% of the sulfur in fossil materials is contained in thiophenic molecules, which comprise an enormous variety of substituted thiophenes, and benzo[b]thiophenes, di-benzo[b,d]thiophenes as well as other fused-ring thiophenes, most of which are generally less easily desulfurized over heterogeneous catalysts than any other sulfur compound in petroleum feedstocks (e.g., thiols, sulfide, and disulfides). 

HDS catalysts generally consist of (heterogeneous) Mo or W sulfides on alumina supports. However, Bianchini et al. described a two-step procedure for HDS of thiophenes by the hydrogenolysis of thiols, followed by the desulfurization of the thiols by applying their zwitterionic rhodium(I) complex, [Rh(sulphos((cod)] (see previous section) [17]. This complex is soluble in polar solvents, such as methanol and methanol-water mixtures, but not in hydrocarbons. Benzo[b]thiophene was chosen as substrate since it is one of the most difficult thiophene derivatives to degrade. Under the mild reaction conditions of the two-step process, the benzene rings of the (di)benzothiophenes were not affected. In the absence of a base, the double bond of benzo[b]thiophene was hydrogenated, while in the presence of a base (NaOH) 2-ethylthiophenolate was the major product (Scheme 1). 

Hydrogenolysis of benzo[fc]thiophene was performed in 20 mL of solvent mixture (methanol/n-heptane or methanol-water/n-heptane) with 180 mg of NaOH present. The catalyst/substrate ratio was 1 100, at an H2 pressure of 3 MPa, and 160 °C for 5 h. After the reaction, hydrochloric acid was added to obtain 2-ethylthiophenol. Both biphasic systems gave good yields of 2-ethylthiophenol (95% for methanol/ n-heptane and 89% for mefhanol-water/n-heptane). These results were similar to those obtained in the monophasic systems of methanol (93%) and methanol-water (84%). In both biphasic systems, aU the 2-efhylthiophenol is found in the polar phase as sodium 2-ethylthiophenolate, leaving the hydrocarbon phase almost completely desulfurized. 

The widely accepted mechanism of the regioselective hydrogenation of benzo[h] thiophenes over solid catalysts is shown in Scheme 140 [156]. Path A begins with the regioselective hydrogenation to 2,3-dihydrobenzo[h]thiophene prior to the C-S cleavage and subsequent desulfurization, while path B firstly breaks the C-S bond... 

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