Selective oxidation of the alkyl ligand in rhenium(V) oxo complexes

Rhenium(V) oxo alkyl triflate compounds (HBpz(3))ReO(R)OTf [R = Me (4), Et (5), n-Bu (6); HBpz(3) = hydrotris(1-pyrazolyl)borate; OTf = OSO2CF3, triflate] are formed on sequential reaction of (HBpz(3))ReOCl2 with dialkyl zinc reagents and AgOTf. These triflate compounds are rapidly oxidized at ambient temperatures by oxygen atom donors pyridine N-oxide (pyO) and dimethyl sulfoxide (DMSO) to give (HBpz(3))ReO3 (7) and the corresponding aldehyde. In the eases of 5 and 6 this transformation is quantitative. The addition of 2,6-lutidine to a low-temperature oxidation of 5 by Dh?ISO redirects the reaction to form cis-2-butene instead of acetaldehyde. These oxidation reactions do not proceed through alkoxide intermediates, as shown by independent studies of alkoxide oxidations. Reaction of 5 with pyO or DMSO at -47 degrees C results in the formation of intermediates which are assigned as ylide or ''trapped-carbene'' complexes [(HBpz(3))ReO(OH){CH(CH3}}3OTf (L = py (8) or SMe(2) (9), respectively). Mechanistic studies and analogies with related systems suggest that oxygen atom transfer to 4-6 forms [(HBpz(3))ReO(2)R](+). Transfer of an alpha-hydrogen from the alkyl. group to an oxo ligand then forms an alkylidene complex which is trapped by SMe(2) or py to give the observed intermediates. Further oxidation of the ylide complex gives the aldehyde.