Hydrogenolysis of Sulfur Heterocycles

The reaction which transforms a thiophenic substrate into the corresponding unsaturated thiol is referred to as hydrogenolysis (eq. (3)). Because of the facile hydrogenation of the unsaturated thiols derived from T or BT, the actual hydrogenolysis products obtained with molecular catalysts are generally the saturated thiols. 

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]. 

The importance of the metal oxidation state in controlling the chemoselectivity of hydrogenation of thiophenes is highlighted by the Ru-TRIPHOS case (cf. [9 a, b]). 

A modified version of TRIPHOS has recently been anchored to a crosslinked styrene/divinylbenzene polymer yielding a polymeric material, named POLYTRI-PHOS, containing pendant tripodal triphosphine moieties -C(CH2PPh2)3 [31 a]. The simple reaction of POLYTRIPHOS with a CH2CI2 solution of [RhCl(COD)]2 in the presence of AgPF gives the polystyrene-supported complex [Rh(COD)-(P0LYTRIPH0S)]PF6 (Rh 0.94 wt. %) (eq. (4) and Structure 5). 

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