The structural optimization of a family of modular, enantiopure β-amino alcohol ligands with a common 2-amino-2-aryl-1,1-diphenylethanol skeleton, whose stereogenicity was introduced through the ...Jacobsen epoxidation of 1,1-diphenyl-2-arylethylenes, has led to the identification of a small set of optimal catalysts with enhanced activity and enantioselectivity in the addition of alkylzinc and arylzinc reagents to aldehydes. Criteria for the discrimination between apparently analogous, highly enantioselective ligands are proposed.
Using l-proline as catalyst in the asymmetric aldol reaction and a series of benzamides and naphthamides, we have accomplished a dynamic kinetic resolution that simultaneously establishes the ...stereochemistry of the atropisomeric amide's chiral axis and a stereogenic center. The enantioselectivities ranged from 82% to 95% and the diastereoselectivities from 2.1:1 to 7.0:1.
The optimization of asymmetric catalysts for enantioselective synthesis has conventionally revolved around the synthesis and screening of enantiopure ligands. In contrast, we have optimized an ...asymmetric reaction by modification of a series of achiral ligands. Thus, employing (S)-3,3‘-diphenyl BINOL (S)-Ph2-BINOL and a series of achiral diimine and diamine activators in the asymmetric addition of alkyl groups to benzaldehyde, we have observed enantiomeric excesses between 96% (R) and 75% (S) of 1-phenyl-1-propanol. Some of the ligands examined have low-energy chiral conformations that can contribute to the chiral environment of the catalyst. These include achiral diimine ligands with meso backbones that adopt chiral conformations, achiral diimine ligands with backbones that become axially chiral on coordination to metal centers, achiral diamine ligands that form stereocenters on coordination to metal centers, and achiral diamine ligands with pendant groups that have axially chiral conformations. Additionally, we have structurally characterized (Ph2-BINOLate)Zn(diimine) and (Ph2-BINOLate)Zn(diamine) complexes and studied their solution behavior.
A series of enantiopure ligands based on the aminoindanol scaffold, but differing in regio‐ and stereochemistry has been synthesized. These ligands have been conveniently derivatized and their ...catalytic efficiency in different enantioselective reactions has been screened to determine privileged candidates with respect to regio‐ and stereochemistry for each considered process. The nature of the amino substituent has been optimized for specific applications and this has led to the development of an efficient method for the preparation of bulky bicyclic amines by reductive amination.
A new family of enantiomerically pure (1S,2R)-1-alkyl-2-(dialkylamino)-3-(R-oxy)-1-propanols containing a very bulky alkyl substituent (tert-butyl or 1-adamantyl) on their hydrocarbon chains has been ...synthesized from the corresponding enantiopure epoxy alcohols, arising from the catalytic Sharpless epoxidation, by protection of the primary hydroxy group and subsequent regioselective ring opening of the epoxide by a secondary cyclic amine (C-2 attack). The performance of these amino alcohols as ligands for the catalytic enantioselective addition of diethylzinc to benzaldehyde has been studied, with enantioselectivities of 92−96% being recorded. The best performing ligands, those with a bulky R-oxy group, also depict a convenient activity and selectivity profile in the addition of Et2Zn to a representative family of aldehydes. An anomalous structure/enantioselectivity relationship of some ligands in the tert-butyl series has been studied using PM3 calculations, and conclusions have been drawn on the possible effects of including in modular designs structural fragments giving rise to a variety of rotameric transition states.
4-Hhydroxyproline has been anchored to a polystyrene resin through click chemistry, and the resulting catalyst has been successfully applied to the direct aldol reaction in water. The high ...hydrophobicity of the resin and the presence of water are key to ensuring high stereoselectivity, whereas yield can be increased by using catalytic amounts of DiMePEG. This effect has been further demonstrated by the inefficiency of a homogeneous, more polar analogue.
Enhanced cooperativity leading to high catalytic activity and stereoselectivity has been achieved through a complex network of simple species interacting reversibly. This novel dynamic catalytic ...system relies on bipyridine-based organocatalytic ligands and zinc(
ii
) as the template. It demonstrates the effectiveness of dealing with mixtures rather than single species in asymmetric catalysis.
Enhanced cooperativity leading to high catalytic activity and stereoselectivity in the direct aldol reaction has been achieved through a complex network of simple species interacting reversibly.
A degenerate zinc-templated catalytic system containing two bipyridine ligands with redundant functional groups for either enamine or hydrogen bond formation was applied to the asymmetric aldol ...reaction. This concept led to both a higher probability of reaction and rate acceleration. Thus, the catalyst loading could be decreased to a remarkable 2 mol % in what we think is a general approach.
Long chain carboxylic acids functionalized at the omega position with chiral amino alcohol fragments have been used to stabilize cobalt nanoparticles prepared by thermal decomposition of Co2(CO)8. ...The first examples of chirally modified cobalt nanoparticles have been prepared in this manner and used as magnetically decantable ligands in the Ru-catalyzed asymmetric transfer hydrogenation of ketones.
A new functional polymer where proline is bonded to polystyrene through a 1,2,3-triazole linker depicts characteristics targeted for an artificial aldolase. In spite of the hydrophobicity of the ...polymer backbone, the resin swells in water with building of an aqueous microenvironment. This property, arising from the formation of a hydrogen-bond network connecting the proline and 1,2,3-triazole fragments, is translated into a very high catalytic activity and enantioselectivity toward direct aldol reactions in water.
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