Models for deep hydrodesulfurization (HDS). Remote activation of C-S bondsin alkylated benzothiophenes and dibenzothiophenes by metal coordination to a carbocyclic ring

A series of alkylated benzothiophene and dibenzothiophene complexes contain ing a manganese tricarbonyl moiety coordinated to a carbocyclic ring have b een synthesized. Reaction of these with the mild nucleophile Pt(PPh3)(2)(C2 H4) leads to rapid room-temperature insertion of Pt(PPh3)(2) into a C-S bon d to afford metallathiacyclic complexes. With benzothiophene complexes bear ing no substitutent at the 2- or at the 3-position, it is shown that initia l rapid coordination of the platinum to the C=C bond in the heterocyclic ri ng takes place prior to insertion into the C(vinyl)-S bond. When a substitu ent is present at the benzothiophene 2- and/or 3-position, formation of the eta (2)-(C=C) intermediate is blocked, the reaction rate slows, and insert ion into the C(aryl)-S bond becomes possible or even dominant. An eta (1)-S intermediate is suggested in these cases. Insertion into the C-S bond near er the coordinated ring in dibenzothiophene complexes, even ones alkylated at the 4- and/or 6-positions, occurs rapidly at rates similar to those foun d for alkylated benzothiphene complexes. Even the normally intractable 4,6- Me2DBT is "remotely activated" to rapid C-S bond cleavage by Pt(PPh3)2 when precoordinated to the Mn(CO)(3)(+) moiety. On the basis of observed regios electivities, low-temperature infrared studies, and room-temperature stoppe d-flow kinetics, a mechanism is proposed for the insertion of platinum into precoordinated benzothiophenes and dibenzothiophenes. The palladium comple x Pd(PPh3)(2)(C2H4) is capable of inserting into C-C, C-S, and C-Se bonds i n coordinated biphenylene, thiophenes, and selenophenes. The X-ray structur e of the biphenylene insertion product is reported. It is concluded that th e metallacycles formed from Pd(PPh3)(2)(C2H4) are in general not as rapidly formed or as stable as those obtained with Pt(PPh3)(2)(C2H4). X-ray struct ures are reported for (eta (5)-selenophene)Mn(CO)(3)(+) and its Pt(PPh3)(2) insertion product.