RHENIUM(V) OXO-ALKOXIDE COMPLEXES - SYNTHESES AND OXIDATION TO ALDEHYDES

New rhenium(V) oxo-alkoxide compounds (HBpz(3))ReO(OR)(2) (R = Me (1). Et (2), Pr-i (3), (n)Bu (4), CH(2)Ph (5)) have been prepared in good yields from reaction of (HBpz(3))ReOCl2 with excess alcohol in the presence of NEt(3) [HBpz(3) = hydrotris(1-pyrazolyl)borate]. Treatment of these compounds with Me(3)SiOTf (OTf = OSO2CF3) forms the reactive triflate compounds (HBpz(3))ReO(OR)(OTf) (6-8). The triflate complexes are rapidly oxidized by the oxygen atom donors pyridine N-oxide and Me(2)SO to give aldehydes or ketones, alcohols, and (HBpz(3))ReO3 (9). Addition of DMSO to 2 initially gives the sulfoxide adduct [(HBpz(3))ReO(OEt)(OSMe(2))]OTf (10). Mechanistic studies indicate that 10 rapidly and reversibly loses SMe(2) (k(298) = 8.2 (6) s(-1)) to give a cationic rhenium(VII) dioxo complex, [(HBpz(3))ReO2(OEt)]OTf (11), in which oxidation of the ethoxide ligand occurs. An isotope effect of 4.6 is determined for C-H bond cleavage. Oxidation of the ethoxide ligand in 11 is proposed to occur by hydride transfer to an oxo group. The oxo ligands in 11 are shown to be electrophilic. based on the relative rates of Me(2)S and Me(2)SO oxidation, which explains their ability to act as hydride acceptors.