Activation of the carbon-sulfur bonds in benzothiophenes by precoordination of transition metals to the carbocyclic ring

Coordination of an electrophilic transition-metal fragment, MLn, to the carbocyclic ring of benzothiophene (BT) to form (eta(6)-BT)MLnm+ activates a C-S bond to cleavage by the weak nucleophile Pt(PPh3)(3), with concomitant insertion of Pt(PPh3)(2). The rate of formation of the resulting metallathiacyclic insertion products, ((eta(6)-BT . Pt(PPh3)(2))MLnm+, depends on the metal fragment in the order MLn = Ru(C6Me6)(2+), Mn(CO)(3)(+) > FeCp+, RuCp+ >> Cr(CO)(3), with no reaction occurring in the absence of a MLn activating group. All of the unsubstituted benzothiophene complexes undergo regiospecific cleavage of the olefinic C-S bond rather than the aryl C-S bond, which is likely a consequence of steric congestion that would exist if insertion had occurred at the latter site. The X-ray structures of the metallathiacycles ((eta(6)-BT . Pt(PPh3)(2))Mn(CO)(3)(+) and ((eta(6)-BT . Pt(PPh3)(2))FeCp+ are reported. The complexes (eta(5)-2,5-dimethylthiophene)Mn(CO)(3)(+) and (eta 6-dibenzothiophene)Mn(CO)(3)(+) also undergo rapid C-S bond cleavage with metal insertion in the presence of Pt(PPh3)(3). The results suggest that pi-adsorption of a thiophenic substrate on a catalyst surface in hydrodesulfurization reactions is a viable way to facilitate C-S bond cleavage, as well as subsequent desulfurization and hydrogenolysis.