Kinetics and mechanisms of catalytic oxygen atom transfer with oxorhenium(V) oxazoline complexes

The rhenium(V) monooxo complexes (hoz)(2)Re(O)Cl (1) and [(hoz)(2)Re(O)(OH2)] [OTf] (2) have been synthesized and fully characterized (hoz = 2-(2 ' -hydroxyphenyl)-2-oxazoline). A single-crystal X-ray structure of 2 h as been solved: space group = P (1) over bar, a = 13.61(2) Angstrom, b = 14.76(2) Angstrom, c = 11.571(14) Angstrom, alpha = 93.69(4)degrees, beta = 99.43(4)degrees, gamma = 108.44(4)degrees, Z = 4; the structure was refined to final residuals R = 0.0455 and R-w = 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. 4 Hammett correlation plot with para-substituted thioanisole derivatives gave a reaction constant p 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 3 (cause of inhibition), K-2 (L mol(-1)), were determined for a few sulfides: Me2S (22 +/- 3), Et2S (14 +/- 2). and (Bu2S)-Bu-t-S-t (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.