Asymmetric hydrogenation enantioselective organocatalytic

Kunz RK, MacMillan DWC (2005) Enantioselective organocatalytic cyclopro-panations. The identification of a new class of iminium catalyst based upon directed electrostatic activation. J Am Chem Soc 127 3240-3241 Lacour J, Hebbe-Viton V (2003) Recent developments in chiral anion mediated asymmetric chemistry. Chem Soc Rev 32 373-382 Li X, List B (2007) Catalytic asymmetric hydrogenation of aldehydes. Chem Commun 17 1739-1741... 

Wilson RM, Jen WS, MacMillan DWC (2005) Enantioselective organocatalytic intramolecular Diels-Alder reactions. The asymmetric synthesis of solana-pyrone D. J Am Chem Soc 127 11616-11617 Xie JH, Zhou ZT, Kong WL, Zhou QL (2007) Ru-catalyzed asymmetric hydrogenation of racemic aldehydes via dynamic kinetic resolution efficient synthesis of optically active primary alcohols. J Am Chem Soc 129 1868-1869... 

In 2007, AntiUa and coworkers disclosed the first asymmetric organocatalytic reduction of acyclic a-imino esters (Scheme 23) [39], Chiral VAPOL phosphate (5)-16 (5 mol%) served as a catalyst for the transfer hydrogenation of the latter (62) employing commercially available dihydropyridine 44a to give both aromatic and aliphatic a-amino esters 63 in very high yields (85-98%) and enantioselectivities (94-99% ee). 

It should be noted that not only the Lewis base but also typical Lewis acid roles can be emulated by organocatalytic systems. The proton is arguably the most common Lewis acid found in Nature, and these exist in two forms classified by the nature of the hydrogen bond polar covalent (RX-H) and polar ionic (RX+H-Y ). In the former case, in asymmetric transformations the chiral information is dictated by the chiral anion, whilst in the latter case the anion is non-chiral and the enantioselectivity is introduced by a chiral ligand (usually an amine base), which complexates the proton. This activation is discussed more extensively in Chapter 7. 

Kumaraswamy G, Padmaja M (2008) Enantioselective Total Synthesis of Eicosanoid and its Congener, Using Organocatalytic Cyclopropanation, and Catalytic Asymmetric Transfer Hydrogenation Reactions as Key Steps. J Org Chem 73 5198... 

Recently, considerable efforts have been made to discover new organocatalytic systems for asymmehic epoxidation. In 2003, A arwal and coworkers reported that the asymmetric epoxidation of olefins proceeded in good yields and with moderate enantioselectivities using Oxone (Wako Chemicals, Osaka, Japan) as an oxidant in the presence of a 48-type catalyst (Scheme 1.22) [261]. According to their proposal, the protonated ammonium salt species can act not only as a phase-transfer catalyst to carry the real oxidant species to the organic phase but also as a promoter to activate the chiral oxidant via hydrogen-bonding stabilization, as depicted in 63. 

The asymmetric organocatalytic enantioselective conjugate hydrogen transfer reaction is clearly inspired by the mode of action in biological processes, in which reductions are accomplished by enzymes using hydride reduction cofactors like NADH. 

Kumaraswamy G, Padmaja M. Enantioselective total synthesis of eicosanoid and its congener, using organocatalytic cyclopropanation, and catalytic asymmetric transfer hydrogenation reactions as key steps. J. Org. Chem. 2008 73 (13) 5198-5201. 

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