Epimerization of secondary alcohols by new homogeneous, low oxidation state metal catalysts: Carbon-hydrogen bond activation in rhenium alkoxide complexes ({eta}{sup 5}-C{sub 5}R{sub 5})Re(NO)(PPh{sub 3})(OCHRR{prime})
- Univ. of Utah, Salt Lake City, UT (United States)
Diastereomerically pure secondary alcohols epimerize to mixtures of diasteromers in C{sub 6}H{sub 5}R at 65-90 {degrees}C in the presence of 10 mol% ({eta}{sup 5}C{sub 5}R{sub 5})Re(NO)(PPh{sub 3})(OCH{sub 3}) (1; R = H, Me). The methoxide ligand of 1 first exchanges with the alcohol substrate to give alkoxide complexes {eta}{sup 5}-C{sub 5}R{sub 5})Re(NO)(PPh{sub 3})(OCHR{prime}R{double_prime} is derived from (+)- and (-)-, exo- and endo-borneol. NMR data show that epimerization occurs first at rhenium (ca. 35 {degrees}C) and then at carbon (ca. 65 {degrees}C). Substitution reactions and rate experiments show that PPh{sub 3} 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 {beta}-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 PPh{sub 3}, and show a significant k{sub H}/k{sub D}. 38 refs., 7 figs., 3 tabs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 232291
- Journal Information:
- Journal of the American Chemical Society, Vol. 114, Issue 6; Other Information: PBD: 11 Mar 1992
- Country of Publication:
- United States
- Language:
- English
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