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Kinetics and Mechanism of Oxygen Atom Transfer from Methyl Phenyl Sulfoxide to Triarylphosphines Catalyzed by an Oxorhenium(V) Dimer

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journal contribution
posted on 12.08.2003, 00:00 by Nobuyoshi Koshino, James H. Espenson
An oxorhenium(V) dimer, {MeReO(mtp)}2, D, where mtpH2 is 2-(mercaptomethyl)thiophenol, catalyzes oxygen atom transfer reaction from methyl phenyl sulfoxide to triarylphosphines. Kinetic studies in benzene-d6 at 23 °C indicate that the reaction takes place through the formation of an adduct between D and sulfoxide. The equilibrium constants, KDL, for adduct formation were determined by spectrophotometric titration, and the values of KDL for MeS(O)C6H4-4-R were obtained as 14.1(2), 5.7(1), and 2.1(1) for R = Me, H, and Br, respectively. Following sulfoxide binding, oxygen atom transfer occurs with either internal or external nucleophilic assistance. Because {MeReO(mtp)}2 is a much more reactive catalyst than its monomerized form, MeReO(mtp)PPh3, loss of the active catalyst during the time course of the reaction must be taken into account as a part of the kinetic analysis. As it happens, sulfoxide catalyzes monomerization. Monomerization by triarylphosphines was also studied in the presence of sulfoxide, and a mechanism for that reaction was also proposed. Both the phosphine-assisted monomerization and the phosphine-assisted pathway for oxygen atom transfer involve transition states with ternary components, D, sulfoxide, and phosphine, which we suggest are structural isomers of one another.