Kinetics and Mechanisms of Catalytic Oxygen Atom Transfer with Oxorhenium(V) Oxazoline Complexes

The rhenium(V) monooxo complexes (hoz)2Re(O)Cl (1) and [(hoz)2Re(O)(OH2)][OTf] (2) have been synthesized
and fully characterized (hoz = 2-(2‘-hydroxyphenyl)-2-oxazoline). A single-crystal X-ray structure of 2 has been
solved:  space group = P1̄, a = 13.61(2) Å, b = 14.76(2) Å, c = 11.871(14) Å, α = 93.69(4)°, β = 99.43(4)°,
γ = 108.44(4)°, Z = 4; the structure was refined to final residuals R = 0.0455 and Rw = 0.1055. 1 and 2 catalyze
oxygen atom transfer from aryl sulfoxides to alkyl sulfides and oxygen-scrambling between sulfoxides to yield
sulfone and sulfide. Superior catalytic activity has been observed for 2 due to the availability of a coordination
site on the rhenium. The active form of the catalyst is a dioxo rhenium(VII) intermediate, [Re(O)2(hoz)2]+ (3). In
the presence of sulfide, 3 is rapidly reduced to [Re(O)(hoz)2]+ with sulfoxide as the sole organic product. 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, in agreement with an electrophilic oxygen transfer from
rhenium. The catalytic reaction features inhibition by sulfides at high concentrations. The equilibrium constants
for sulfide binding to complex 2 (cause of inhibition), K2 (L mol-1), were determined for a few sulfides:  Me2S
(22 ± 3), Et2S (14 ± 2), and tBu2S (8 ± 2). Thermodynamic data, obtained from equilibrium measurements in
solution, show that the SO bond in alkyl sulfoxides is stronger than in aryl sulfoxides. The ReO bond strength
in 3 was estimated to be about 20 kcal mol-1. The high activity and oxygen electrophilicity of complex 3 are
discussed and related to analogous molybdenum systems.