An experimental and theoretical study of the enantioselective deprotonation of cyclohexene oxide with isopinocampheyl-based chiral lithium amides

The mechanism of the enantioselective deprotonation of cyclohexene oxide with isopinocampheyl-based chiral lithium amide was studied by quantum chemical calculations. The transition states of eight molecules were fully optimized at the ab initio HF/3-21G and density functional B3LYP/3-21G levels with Gaussian 98. The activation energies were calculated at the B3LYP/6-31+G(3df,2p)//B3LYP/3-21G level. We found the theoretical evaluation to be consistent with the experimental data. At the best case, an enantiomeric excess of up to 95% for (R)-2-scyclohexen-1-ol was achieved with (−)-N, N-diisopinocampheyl lithium amide

Platinum group metals as catalysts in enantioselective 1-phenyl-1,2-propandione hydrogenation

Different γ-Al2O3 supported Ir, Pd, Ru, Rh and Pt catalysts were tested in enantioselective 1-phenylpropane-1,2-dione hydrogenation using cinchona alkaloid modifiers. Activity and enantioselectivity over Ir and Ru catalysts were low. Pd catalyst was active in the hydrogenation of 1-phenylpropane-1,2-dione, however, the enantioselectivity over this catalyst was almost negligible. Over Pd hydrogenation proceeded mainly via hydrogenation of the C1O1 carbonyl group, which is attached to the phenyl ring. Hydrogenation over Pd did not proceed in the second hydrogenation step via an enol form as found for ethyl pyruvate hydrogenation over Pd. The structure-selectivity relationship and solvent effects are similar over Pt and Rh in the first hydrogenation step. However, in the second hydrogenation step of hydroxyketones to diols large mechanistical differences between Pt and Rh were observed. Although the activity over Rh catalysts was lower than over Pt after optimization the best result obtained with Rh/γ-Al2O 3 (5754 Lancaster) was 60% ee in toluene at maximum yield of 28%, which makes Rh a promising metal for enantioselective hydrogenation

Enantioselective hydrogenation of α-ketoesters:An in Situ Surface-Enhanced Raman Spectroscopy (SERS) study

Adsorption of ethyl pyruvate (EP) and methyl pyruvate (MP) has been studied on a SERS-active platinum electrode. Gas-phase experiments under a hydrogen atmosphere show that decarbonylation to form adsorbed CO together with formation of a half-hydrogenated state of MP/EP are the dominant surface reaction pathways occurring under these conditions. Similar findings were obtained using a hydrogen-evolving platinum electrode under potentiostatic control in 0.1 M aqueous sulfuric acid containing EP or MP. DFT calculations have been used to support the assignment of vibrational bands in the SERS obtained from the half-hydrogenated state intermediate. At hydrogen evolution potentials and using concentrations of MP and EP < 0.02M, adsorbed hydrogen adatoms were detected. At higher EP concentrations, hydrogen-atom formation was inhibited by the greater surface coverages of the half-hydrogenated state of EP/MP and to a lesser extent adsorbed CO derived from decarbonylation side reactions. Addition of the chiral modifier cinchonidine (CD) to the acidic electrolyte under conditions of hydrogen evolution resulted in significantly less CO production and a reduction in the intensity of all SERS peaks associated with the half-hydrogenated state. Most importantly, however, the presence of surface hydrogen, previously absent on the unmodified surface, was always observed irrespective of the EP concentration. The significance of this result in relation to the question of the origin of rate enhancement during heterogeneously catalyzed enantioselective hydrogenation of α-ketoesters is discussed