Homogeneous Catalytic Hydrogenolysis and Hydrodesulfurization of Thiophenes

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. 

These unprecedented homogeneous transformations have been achieved for a variety of substrates, viz. T [72], BT [72-74], DBT [29, 72], and dinaphthothiophene (DNT) [75], by use of the electron-rich 16-electron neutral or anionic fragments [(triphos)MH] (M = Rh, Ir, n = 0 M = Ru, n = -1). The reactions have been typically performed at about 160 °C under 30 atm H, conditions that are well tolerated by the very stable triphos derivatives the presence of strong bases such as NaOH or KO Bu dramatically increases the overall rate of the catalytic hydrogenolysis. 

The homogeneity of the hydrogenolysis reaction -at temperatures up to 180° C- was demonstrated by the fact that metallic mercury did not affect the reaction rate. The hydrogen pressure did not seem to have a marked effect on the catalysis, within the range employed. Also, using THF as the solvent instead of acetone did not result in any important effect. 

In an similar manner, the anionic 16-electron d Ru(0) fragment [(triphos)RuH] that is isoelectronic with the Rh complex described above, is also an active and selective catalyst for the conversion of BT to 2-ethylthiophenol above 60° C and 30 atm Hj [77]. The catalytic cycle for this ruthenium system was shown experimentally to be in essence analogous to that discussed for Rh. Very interestingly, as noted in Section 

Hydrogenolysis and hydrogenation of beniolhiophene by [(triphos)RhH] (Adapted from ref. 73). 

---