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 % (η5-C5R5)Re(NO)(PPh3)(OCH3) (1; R = H, Me). The methoxide ligand of 1 first exchanges with the alcohol substrate to give alkoxide complexes (η5-C5R5)Re(NO)(PPh3)(OCHR′R″) (2). Authentic samples of diastereomerically and enantiomerically pure 2 are prepared, where OCHR′R″ 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 β-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.
author list (cited authors)
Saura-Llamas, I., & Gladysz, J. A.