Hydrogenolysis of thiophene

The hydrogenolysis of thiophenes to thiols is a relevant reaction in the HDS process (Equation (16)). 

The hydrogenolysis of thiophene and its homologs has been investigated in the presence of Ni-Al alloy and aqueous alkali (65) and over vanadium oxide (47). It has been shown that / -(2-thenoyl)-propionic acid (II) on treatment with nickel aluminum alloy in aqueous alkali solution yielded... 

Mechanism of hydrogenolysis of thiophene on bimetallic sulfide catalysts 92UK332. 

Catalytic hydrogenolysis of thiophenes, benzo[, ]thiophene, and dibenzothiophene can be brought about with the help of the 16-electron species [(triphos)MH] (M = Rh or Ir triphos = MeC (CH2PPh2)3). These are generated in situ by thermolysis of the appropriate precursors. The rhodium complex 246 effectively catalyzes such hydrogenolyses. The required experimental conditions are rather drastic (Scheme 71) <1998ACR109>. 

T-b. Poisoning by adsorption and hydrogenolysis of thiophene, the hydrogenolysis of the adsorbed thiophene being the rate-determining step (ref, 17) ... 

Thiophene metal poisoning as well as hydrogenation of ethylbenzene on metal catalysts require, as a first step, the chemisorption of both organic molecules on the metal active sites. Afterwards, catalyst deactivation can simply take place by the blocking of these sites or by further hydrogenolysis of thiophene and subsequent formation of an inactive surface metal sulfide. We believe that, in our conditions, this last mechanism is probably operating. This hypothesis is supported by the fact that butane was detected in our experiments and, furthermore, XPS analysis showed the formation of metal sulfides (S ) on the deactivated catalysts. 

In general, Ru-based catalysts are more efficient than Rh-based catalysts for the selective hydrogenation of BT to DHBT in water/hydrocarbon mixtures [19]. Rhodium forms much better catalysts for the hydrogenolysis of thiophenes to thiols (vide infra). 

In conclusion, an extensive literature is available on the reaction networks that are thought to operate in HD,S of various types of thiophenic molecules besides the great advances that have been made in direct studies on molybdenum sulfides and related catalysts, this is another area in which organometallic chemistry has made an impressive contribution to HD,S catalysis, as a number of reaction pathways and mechanisms for the hydrogenation and hydrogenolysis of thiophenes on metal complexes in solution has been well established with the aid of a variety of physical techniques. 

Fig. 4.17. Some catalyst precursors used for the homogeneous and liquid-bi phasic hydrogenolysis of thiophenes...

Hydrogenolysis of 2,3-dihydrobenzothiophene by (triphos)Ir. Hydrogenolysis of thiophene by ReH7(PPh3)2. 

At variance with the homogeneous reactions with [(TRIPHOS)RhH] in which the base simply accelerates the catalytic process, in the biphase reactions the presence of NaOH is mandatory for catalysis to occur. This is most probably because the concentration of BT in the polar phase is too low to promote effectively the elimination of the thiol product. A similar dependence of the reaction rate on Bronsted bases has been observed for the homogeneous hydrogenolysis of thiophene by TRIPHOS/Rh catalysis, and was attributed to the weak nucleophilic properties of thiophene [7]. 

Subsequent experiments with thiophene and butylmercaptan (8) showed that a complete hydrogenolysis of these substances takes place with the production of hydrogen sulfide. The hydrogenolysis of thiophene proceeds probably by the following steps ... 

All the above-named substances were identified in the products of hydrogenolysis of thiophene. Further support of this mechanism was given by the experiments of hydrogenolysis of n-butylmercaptan and hydrogenation of butadiene. 

Several more complex reactions such as the catalytic reforming of heptanes on Pt/Re/alumina were dealt with in terms of sets of rate equations of the Hougen-Watson type by Van Trimpont et al. [1986]. The hydrogenolysis of thiophene and benzothiophene on Co/Mo/alumina was studied along the same lines by Van Parijs et al. [1986a, b] and is also discussed in Examples 2.6.4.A and 2.7.2.2.A. 

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