EPIMERIZATION OF SECONDARY ALCOHOLS BY NEW HOMOGENEOUS, LOW OXIDATION-STATE METAL-CATALYSTS - CARBON HYDROGEN-BOND ACTIVATION IN RHENIUM ALKOXIDE COMPLEXES (ETA-5-C5R5)RE(NO)(PPH3)(OCHRR')
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Diastereomerically pure secondary alcohols epimerize to mixtures of diastereomers in C6H5R at 65-90 C in the presence of 10 mol % (5-C5R5)Re(NO)(PPh3)(OCH3) (1; R = H, Me). The methoxide ligand of 1 first exchanges with the alcohol substrate to give alkoxide complexes (5-C5R5)Re(NO)(PPh3)(OCHRR) (2). Authentic samples of diastereomerically and enantiomerically pure 2 are prepared, where OCHRR is derived from (+)- and (-)-, exo- and endo-borneol. NMR data show that epimerization occurs first at rhenium (ca. 35 C) and then at carbon (ca. 65 C). Substitution reactions and rate experiments show that PPh3 initially dissociates from 2 with anchimeric assistance by alkoxide oxygen lone pairs. An intermediate with a trigonal-planar rhenium, which can either return to 2 (with epimerization at rhenium) or undergo -hydride elimination to a ketone hydride complex (leading to epimerization at carbon), is proposed. Accordingly, rates of epimerization at carbon (but not rhenium) are strongly inhibited by added PPh3, and show a significant kH/kD. 1992, American Chemical Society. All rights reserved.