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

Abstract

Listen

Translate article

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(PPh3)2(C2H4) leads to rapid room-temperature insertion of Pt(PPh3)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 CC 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 η2-(CC) intermediate is blocked, the reaction rate slows, and insertion into the C(aryl)−S bond becomes possible or even dominant. An η1-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-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 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(PPh3)2(C2H4) 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 insertion product is reported. It is concluded that the 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 structures are reported for (η5-selenophene)Mn(CO)3+ and its Pt(PPh3)2 insertion product.