Reactions and mechanisms of rhenium catalyzed oxygen atom transfer.

摘要

The rhenium(V) monooxo complexes Re(O)(hoz)₂Cl, 1, and [Re(O)(hoz)₂(H₂O)][OTf], 2, have been synthesized and fully characterized (hoz = 2-(2′-hydroxyphenyl)-2-oxazoline). The reactive intermediate Re(O)₂(hoz)₂+, 3, has been made by reacting 2 with a variety of oxidants: R_lS(O)R₂, 4-picoline N-oxide, H ₂O₂, and ClO₄−. The analogous molybdenum compound Mo(O)₂(hoz)₂, 4, has also been prepared as well as derivatives of 1 and 2, Re(O)(hthoz)₂Cl, 5, and [Re(O)(hthoz)₂][OTf], 6, (hthoz = 2-(2′-hydroxyphenyl)-2-thiozoline).; 1 and 2 catalyze oxygen atom transfer from aryl sulfoxides to alkyl sulfides and oxygen scrambling between sulfoxides to yield sulfone and sulfide. The transition state is very sensitive to electronic influences. A Hammett correlation plot with para-substituted thioanisole derivatives gave a reaction constant ρ of −4.6 ± 0.4, which is in agreement with an electrophilic oxygen transfer from rhenium. Changing the ligand from hoz to hthoz enhances the rate of oxygen atom transfer 18-fold. In comparison, the reduction of Re(O)₂(hthoz)₂+, 9, by tbutyl sulfide yields a second-order rate constant k₃′ = 26,724 3079 L mol−1s−1, while a k₃′ = 1,482 79 L mol−1 s−1 was obtained for 3. The catalytic reaction features inhibition by sulfides at high concentrations. The high activity and oxygen electrophilicity of complex 3 are discussed.; Rhenium complexes 2 and 3 combine to form a dimer, [(hoz)₂(O)Re-O-Re(O)(hoz)₂]+2. The dimerization reaction competes with oxygen atom transfer to sulfides. The competing dimerization reaction is decreased when the oxidant is switched from diphenylsulfoxide to picoline N-oxide.; Catalyst 2 is an efficient catalyst for the selective oxidation of thiols to disulfides when sulfoxides are used as the oxidant. No overoxidation to either the thiosulfinates or thiosulfonates is observed. The selectivity is caused by the differences in reactivity between the sulfide and the disulfide products. The second-order rate constant for the oxidation of tBu₂S by 3 is 1.48 × 10 3 L mol−1s−1 compared to 4.15 × 10−2 L mol−1s −1 for the oxidation of PhSSPh. Another interesting application of catalyst 2 is the reduction of perchlorate to chloride with sulfides. The stoichiometry of the reaction is 4:1 sulfide to ClO₄ −.