Models for Deep Hydrodesulfurization (HDS).Remote Activation of C-S Bonds in Alkylated Benzothiophenes and Dibenzothiophenes by Metal Coordination to a Carbocyclic Ring

摘要

A series of alkylated benzothiophene and dibenzothiophene complexes containing a manganese tricarbonyl moiety coordinated to a carbocyclic ring have been synthesized. Reaction of these with the mild nucleophile Pt(PPh_3)_2(C)_2H_4) leads to rapid room-temperature insertion of Pt(Ph_3)_2 into a C-S bond to afford metallathiacyclic complexes. With benzothiophene complexes bearing no substitutent at the 2-or at the 3-position, it is shown that initial rapid coordination of the platinum to the C=C bond in the heterocyclic ring takes place prior to insertion into the C(vinyl)-S bond. When a substituent is present at the benzothiophene 2-and/or 3-position, formation of the #ea#~2-(C=C) intermediate is blocked, the reaction rate slows, and insertion 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 nearer the coordinated ring in dibenzothiophene complexes, even ones alkylated at the 4-and/or 6-positions, occurs rapidly at rates similar to those found for alkylated benzothiphene complexes. Even the normally intractable 4,6-Me_2DBT is "remotely activated" to rapid C-S bond cleavage by Pt(PPh_3)_2 when precoordinated to the Mn(CO)_3~+ moiety. On the basis of observed regioselectivities, low-temperature infrared studies, and room-temperature, stopped-flow kinetics, a mechanism is proposed for the insertion of platinum into precoordinated benzothiophenes and dibenzothiophenes. The palladium complex pd(PPh_3)_2(C_2h_4) is capable of inserting into C-C, C-S, and C-Se bonds in coordinated biphenylene, thiophenes, and selenophenes. The X-ray structure of the biphenylene insetion product is reported. It is concluded that the metallacycles formed from Pd(PPh_3)_2(C_2H_4) are in general not as rapidly formed or as stable as those obtained with Pt(PPh_3)_2(C_2H_4). X-ray structures are reported for (#eta#~5-selenophene)Mn(CO)_3~+ and its Pt(PPh_3)_2 insertion product.