Asymmetric Transfer Hydrogenation of Prochiral α,β-Unsaturated Acids and Their Esters by Achiral or Chiral Alcohols with Ruthenium Chiral Diphosphine Complexes
- 1 June 1983
- journal article
- research article
- Published by Oxford University Press (OUP) in Bulletin of the Chemical Society of Japan
- Vol. 56 (6) , 1786-1790
- https://doi.org/10.1246/bcsj.56.1786
Abstract
Asymmetric transfer hydrogenations of α,β-unsaturated acids (MeCH=C(Me)CO2H, PhCH=C(Me)CO2H, CH2=C(CH2CO2H)CO2H, and HO2CCH=C(Me)CO2H) and esters (MeCH=C(Me)CO2R; R=achiral and chiral group) by alcohols (PhCH2OH, (RS)-PhCH(Me)OH, and α-D-glucofuranose derivatives) were carried out with RuCl2(PPh3)3 or Ru2Cl4((+) or (−)-diop)3 (diop=2,3-O-isopropylidene-1,4-bis(diphenylphosphino)-2,3-butanediol) at 160–190 °C. The optical purity (3.4–16.4%) of hydrogenated acids obtained with PhCH2OH and Ru2Cl4((−)-diop)3 is in the order MeCH=C(Me)CO2H>PhCH=C(Me)CO2H>CH2=C(CH2CO2H)CO2H>HO2CCH=C(Me)CO2H, suggesting that the substrates possessing two carboxyl groups would provide an asymmetrically unfavorable coordination to the Ru(II) complex for this asymmetric reaction. In the transfer hydrogenation of MeCH=C(Me)CO2R by PhCH2OH or (RS)-PhCH(Me)OH at 190 °C, the extent of asymmetric induction of Ru2Cl4((−)-diop)3 (1.7–11.4% e.e. with PhCH2OH and 7.4–18.2% e.e. with (RS)-PhCH(Me)OH) decreases with increase in bulkiness of groups R (R=H, Me, Et, n-Bu, and PhCH2) and is not enhanced by the introduction of any groups R with chiral carbon atoms into esters (maximum 15.4% e.e.). The structural change of hydrogen donors from PhCH2OH to (RS)-PhCH(Me)OH appreciably increases the optical purity of the saturated acids and esters, and the chiral α-D-glucofuranose derivatives afford optically active saturated products, even with achiral RuCl2(PPh3)3. Features of the present reaction are discussed in relation to the reaction mechanism.Keywords
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