Models for Deep Hydrodesulfurization of Alkylated Benzothiophenes. Reductive Cleavage of C-S Bonds Mediated by Precoordination of Manganese Tricarbonyl to the Carbocyclic Ring

The reduction of (η6-benzothiophene)Mn(CO)3+ with cobaltocene under CO leads to insertion of the Mn(CO)4- fragment into the C(aryl)−S bond to afford a neutral bimetallic complex. When there are methyl substituents at the 2,3-, 2,7-, or 3,7-positions, the regioselectivity and product distribution in the reductive insertion reactions are significantly affected, and several new types of complexes are formed. Low-temperature chemical reduction of (η6-benzothiophene)Mn(CO)3+ complexes affords unstable η4-species, (η4-BT)Mn(CO)3-, which do not react with the η6-cation to give insertion products. Similarly, electrochemical reduction of (η6-benzothiophene)Mn(CO)3+ changes from a one-electron chemically irreversible process at room temperature to a two-electron chemically reversible but electrochemically irreversible process at −90 °C. It is concluded from these results that the room-temperature reductive cleavage of a C−S bond in (η6-benzothiophene)Mn(CO)3+ occurs by a radical mechanism. Crystallographic data and...