Hydrogenation asymetrique a l'aide du complexe DIOXOP-Rh(I) voie dihydro ou voie insaturee.

Entropy-Controlled and Enantiodivergent Lewis Acid Catalysis in Water

“…D-alanine, for example, afforded (R)-3 with an enantiomeric excess of only 9% at 10 °C, but, under otherwise identical reaction conditions, at 90 °C a 67% ee was obtained. This difference in enantiomeric excess of +58% equals the largest reversed temperature effect on enantioselectivity that we have seen reported to date, 42 and is likely the single largest that cannot be explained by a significant change in the reaction mechanism. The use of L-proline gave a nearly as large reversed temperature effect (Dee = +54%) and also afforded the highest enantiomeric excess (79%).…”

“…We then proceeded to study several of the a-amino acids in more detail in order to get a clearer picture of the observed effects and their origin. Reversed temperature effects in enantioselective reactions are indeed unusual, but not without precedence 33,34,42,43 and have commonly been explained by analyzing the differential activation enthalpy and entropy factors of the reaction. An Eyring plot allows the activation parameters to be determined from the temperature dependence of the rate constants.…”

A series of new C2‐symmetric amino alcohols with multicoordination groups: Promising catalysts for synthesis of both enantiomers in the borane‐mediated reduction of prochiral ketones

“…25,28- perature are rare. [32][33][34][35][36][37][38][39] In a recent work on enantioselective borane-mediated reduction of prochiral ketones, Basavaiah et al 31,40 reported that their catalysts containing NÀ À(C¼ ¼NH)À ÀN and NÀ À(C¼ ¼NH)À ÀO frameworks exhibited temperature dependant reversal of stereoselectivity. To further understand the structural effect of new C 2symmetric amino alcohol ligands on the temperaturedependent reversal of stereochemistry, the mono prolinol analogue 8 (Fig.…”

Asymmetric Lewis Acid Catalysis in Water: α‐Amino Acids as Effective Ligands in Aqueous Biphasic Catalytic Michael Additions

“…3‐(3‐Oxocyclohexyl)pentane‐2,4‐dione (6): A) Without Acetic Acid: Compound 3 36 (50.2 mg, 0.24 mmol) was stirred in H 2 O (1 mL) and then acetylacetone (78 μL, 0.76 mmol) and 2‐cyclohexene‐1‐one (291 μL, 3.0 mmol) were added. The reaction mixture was stirred vigorously for 24 h at 60 °C.…”