Thiophenes catalytic hydrogenolysis

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>. 

The catalysts (50 mg) were heated up to 673 K in He at a rate of 6 K/min and pretreated at this temperature in situ either in He or in a mixture of 5% HgS in H2 for 1 h. Thiophene HDS test reactions were carried out at 673 K and atmospheric pressure in a flow reactor system (30 cm min flow of 3% thiophene in Hg). Thiophene and the products were detected by GC. The conversion is the fraction of thiophene converted to coke and gaseous products, the yield is the fraction of thiophene converted to gaseous products. The catalytic properties were characterized by activities in C-S hydrogenolysis without C-C bond breaking (yield - (Ci+Cj+Cg products)), cracking (yield - C4 products), and coking (conversion - yield). The catalytic conversions determined after 5 minutes time on stream are discussed here, because all samples deactivated fast due to coke formation. 

Irrespective of the metal catalyst, the hydrogenolysis rate of any thiophenic substrate is significantly accelerated when a strong Brpnsted base, generally KOBu in THF, is added to the catalytic mixture in the same concentration as the substrate [9, 10]. The main role of the base is that of speeding up the removal of the thiol product from M(H)(SR) intermediates, which constitutes the rds of all hydrogenolysis reactions reported so far. In some cases, strong bases have... 

The mechanisms for the model substrates BT (a) and DBT (b) involve the steps of C-S insertion, hydrogenation of the C-S inserted thiophene to the corresponding thiolate, base-assisted reductive elimination of the thiol (rds) to complete the cycle (in the catalytic reactions carried out in the absence of base, the displacement of the thiol by the substrate occurs thermally [10 b, c]). The addition of a strong base to the catalytic mixtures results in a remarkable rate enhancement for example, the TOP relative to the hydrogenolysis of BT to 2-ethylthiophenol catalyzed by [RhH(TRIPHOS)] increases from 12 to 40 by simply adding an excess of KOBu to the catalytic mixture [10 b, c]. 

Until fairly recent times, most of the reported homogeneous HDS-related reactions were limited to stoichiometric model transformations in which thiophenes and related molecules could be activated by coordination to the metal centers so as to undergo C-S bond scission and eventually hydrogenolysis to the corresponding free thiol, or in some cases even complete desulfurization to yield hydrocarbons. This has represented a remarkable contribution to the understanding of many of the elementary steps involved in complex heterogeneous HDS schemes, but it had obvious limitations, and it offered little promise in terms of developing catalytic systems of any mechanistic or practical importance. 

The catalytic conversion of thiophenic substrates to the corresponding thiols (hydrogenolysis) (Eq. 3) is a reaction of much relevance in the HDS process as the thiols can subsequently be desulfurized over conventional HDS catalysts with greater efficiency and under milder reaction conditions than those required to ac-... 

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]. 

Catalytic reductions of the thiophene ring, or of substituents attached to it, are complicated by two factors poisoning of the catalyst, and the possibility of competing hydrogenolysis - reductive removal of sulfur, particularly with Raney nickel - indeed the use of thiophenes as templates on which to elaborate a structure, followed finally by desulfurisation, is an important synthetic strategy... 

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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