Kinetics and Mechanism of Rhenium-Catalyzed O Atom Transfer from Epoxides

O atom transfer from epoxides cis-stilbene oxide and styrene oxide to triphenylphosphine catalyzed by Tp‘ReO₃ (Tp‘ = hydridotris(3,5-dimethylpyrazolyl)borate) is shown to proceed via an unexpectedly complex combination of mechanisms. Reduction of Tp‘ReO₃ with PPh₃ in THF is rapid above room temperature to form a highly reactive species suggested to be Tp‘ReO₂. Spectroscopic examination and attempts to isolate this by chromatography lead only to Tp‘Re(O)(OH)₂ (1); exposure of the crude reduction mixture to ethanol results in formation of Tp‘Re(O)(OEt)(OH) (3). Both 1 and 3 are as efficient catalysts for O atom transfer as the unpurified mixture resulting from reaction of PPh₃ with Tp‘ReO₃; all three rhenium reactants give the same turnover frequency to within 10% at identical [Re]_total and [epoxide]. The kinetic behavior of the catalytic system (epoxide:Re = 20) is complex; an initial “burst” of alkene production is seen, which quickly tapers off and falls into a pseudo-zero-order reaction. The majority of rhenium is observed to exist as the syn-Tp‘Re(O)(diolate) complex, formed by ring expansion of the epoxide. However, cycloreversion of this diolate is incapable of accounting for the observed catalytic turnover frequency. An additional intermediate, a coordinated epoxide, is proposed to form and partition between ring expansion and direct fragmentation to alkene; eventually a steady-state concentration of diolate forms. Competition between direct atom transfer and ring expansion followed by diolate cycloreversion is demonstrated by reaction of 3 with excess cis-stilbene oxide and styrene oxide in the absence of reductant to give a 4:1 mixture of alkene and syn-diolate from cis-stilbene oxide or a 5.5:1 mixture of alkene and syn-diolate from styrene oxide under conditions where diolate cycloreversion is a negligible contributor.