Spectacular
solvent effects in the asymmetric hydrogenation of methyl 3,5-dioxohexanoate (3) and 2,4-pentanedione
(5) have been observed using Ru[(S)-Ph,Ph-oxoProNOP]X2
complexes (X=η3-C4H7, IIa; CF3CO2, IIb; (R)-MTPA, IIc) as catalyst
precursors. β-Diketones 3 and 5 are respectively reduced to the corresponding β-diols 4 and 6. In both cases, an almost
complete reversal in the diastereoselectivity of the reaction is observed when changing the solvent from CH2Cl2
(syn-4 in up to 92% de; meso-6 in up to 84% de) to a 1:1 CH2Cl2âCH3OH mixture (anti-4
and anti-6 in up to 84% de).
The extent of this
solvent effect
is much less
marked with Ru-atropisomeric
diphosphine catalysts.
使用Ru[(S)-Ph,Ph-oxoProNOP]X2(X=η3-
C4H7,IIa;CF3CO2,IIb;(R)-M
TPA,IIc)作为催化剂前体,在甲基3,5-二氧代
己酸(3)和
2,4-戊二酮(5)的不对称氢化反应中观察到了壮观的光学溶剂效应。
β-二酮3和5分别被还原为相应的β
-二醇4和6。在这两种情况下,当将溶剂从
CH2Cl2(顺式-4的脱除率高达92%;中式-6的脱除率高达84%)变为1:1的 -CH3OH混合物(反式-4和反式-6的脱除率高达84%)时,反应的立体选择性几乎完全相反。Ru-对映异构体
二膦催化剂的溶剂效应程度要小得多。