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 th e carbocyclic ring of benzothiophene (BT) to form (eta(6)-BT)MLnm+ act ivates a C-S bond to cleavage by the weak nucleophile Pt(PPh3)(3), wit h concomitant insertion of Pt(PPh3)(2). The rate of formation of the r esulting 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 occurri ng in the absence of a MLn activating group. All of the unsubstituted benzothiophene complexes undergo regiospecific cleavage of the olefini c C-S bond rather than the aryl C-S bond, which is likely a consequenc e of steric congestion that would exist if insertion had occurred at t he 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)(+) an d (eta 6-dibenzothiophene)Mn(CO)(3)(+) also undergo rapid C-S bond cle avage with metal insertion in the presence of Pt(PPh3)(3). The results suggest that pi-adsorption of a thiophenic substrate on a catalyst su rface in hydrodesulfurization reactions is a viable way to facilitate C-S bond cleavage, as well as subsequent desulfurization and hydrogeno lysis.