Highly enantioselective hydrogenation of (E)-beta-(acylamino)acrylates catalyzed by Rh(i)-complexes of electron-rich P-chirogenic diphosphines

A New Class of Versatile Chiral-Bridged Atropisomeric Diphosphine Ligands: Remarkably Efficient Ligand Syntheses and Their Applications in Highly Enantioselective Hydrogenation Reactions

A series of chiral diphosphine ligands denoted as PQ-Phos was prepared by atropdiastereoselective Ullmann coupling and ring-closure reactions. The Ullmann coupling reaction of the biaryl diphosphine dioxides is featured by highly efficient central-to-axial chirality transfer with diastereomeric excess >99%. This substrate-directed diastereomeric biaryl coupling reaction is unprecedented for the preparation of chiral diphosphine dioxides, and our method precludes the tedious resolution procedures usually required for preparing enantiomerically pure diphosphine ligands. The effect of chiral recognition was also revealed in a relevant asymmetric ring-closure reaction. The chiral tether bridging the two aryl units creates a conformationally rigid scaffold essential for enantiofacial differentiation; fine-tuning of the ligand scaffold (e.g., dihedral angles) can be achieved by varying the chain length of the chiral tether. The enantiomerically pure Ru- and Ir-PQ-Phos complexes have been prepared and applied to the catalytic enantioselective hydrogenations of alpha- and beta-ketoesters (C=O bond reduction), 2-(6'-methoxy-2'-naphthyl)propenoic acid, alkyl-substituted beta-dehydroamino acids (C=C bond reduction), and N-heteroaromatic compounds (C=N bond reduction). An excellent level of enantioselection (up to 99.9% ee) has been attained for the catalytic reactions. In addition, the significant ligand dihedral angle effects on the Ir-catalyzed asymmetric hydrogenation of N-heteroaromatic compounds were also revealed.

Efficient Synthesis of β-Aminoacrylates and β-Enaminones Catalyzed by Zn(OAc)2·2H2O

The direct condensation of amines with β-ketoesters and β-diketones to produce functional enamine derivatives has been investigated with zinc Lewis acid catalysts. Zn(OAc)2·2H2O shows good catalytic activity and leads to a chemo- and stereoselective formation of (Z)-enamine derivatives from aliphatic primary amines and ring-substituted anilines under mild conditions.

On the mechanism of stereoselection in Rh-catalyzed asymmetric hydrogenation: a general approach for predicting the sense of enantioselectivity

This account brings together the recent experimental and computational data on the mechanism of Rh-catalyzed asymmetric hydrogenation of activated double bonds. Two alternative reaction pathways (unsaturated and dihydride) are compared. It is suggested that the differences in these mechanisms are not primarily important for stereoselection, since they join in a single pathway before stereoselection occurs. This approach was used to rationalize the present discrepancies in the prediction of the sense of enantioselection for the P-stereogenic ligands and the ligands with backbone chirality.

Rhodium-Catalyzed Asymmetric Hydrogenation of Functionalized Olefins Using Monodentate Spiro Phosphoramidite Ligands

Novel chiral monodentate phosphorus ligands, SIPHOS, were conveniently synthesized from 1,1'-spirobiindane-7,7'-diol. The Rh complexes of SIPHOS can catalyze the hydrogenation of alpha-dehydroamino esters in mild conditions, providing alpha-amino acid derivatives in up to 99% ee. Enamides and beta-dehydroamino esters can also be hydrogenated in good to excellent enantioselectivities (up to 99% and 94% ee, respectively). The SIPHOS ligand with smaller alkyl groups on the N-atom afforded higher enantioselectivity. The X-ray analysis of single crystal showed that the structure of Rh/SIPHOS catalyst is [Rh(COD)((S)-SIPHOS-Me)(2)](+), which clarified the configuration of the catalyst with the monodentate chiral phosphorus ligand in Rh-catalyzed asymmetric hydrogenation. A positive nonlinear effect in the relationship of the optical purities of ligand and product was observed in the hydrogenation of dehydroamino acid derivatives. The kinetic study of hydrogenation showed that the reaction is zero order in the concentration of substrate and first order in the concentration of Rh catalyst and the hydrogenation pressure. The rate of hydrogenation decreased when the Rh/L ratio changed from 1:1 to 1:4.

Asymmetric hydrogenation of alpha,beta-unsaturated phosphonates with Rh-BisP* and Rh-MiniPHOS catalysts: scope and mechanism of the reaction

Optically active 1,2-bis(alkylmethylphosphino)ethanes and bis(alkylmethylphosphino)methanes are unique diphosphine ligands combining the simple molecular structure and P-stereogenic asymmetric environment. This work shows that these ligands exhibit excellent enantioselectivity in rhodium-catalyzed asymmetric hydrogenation of alpha,beta-unsaturated phosphonic acid derivatives. The enantioselective hydrogenation mechanism elucidated by NMR study is also described.

Recent Developments in the Catalytic Asymmetric Synthesis of α- and β-Amino Acids

The stereoselective synthesis of amino acids is of great importance for the construction of optically active natural products and pharmaceuticals. Apart from enzymes, a broad repertoire of chiral reagents, auxiliaries, and catalysts can be used for the formation of amino acids. Asymmetric reactions using catalytic amounts of chiral molecules provide efficient methods for the generation of optically active proteinogenic and nonproteinogenic amino acids. This minireview collects recent work on catalytic asymmetric synthesis of alpha- and beta-amino acids.

A catalytic asymmetric method for the synthesis of gamma-unsaturated beta-amino acid derivatives

Catalytic enantioselective synthesis of beta-amino acid derivatives is an area of intense interest, due to the importance of these compounds as components in pharmaceutical agents and peptidomimetics. In this report, we present the first catalytic enantioselective method for the synthesis of gamma-unsaturated beta-amino acids and their corresponding 1,3-amino alcohol derivatives. This methodology takes advantage of a highly enantioselective vinylzinc addition to an aldehyde to set chirality. The resulting allylic alcohols are then transformed into the corresponding allylic amines via Overman's [3,3]-sigmatropic imidate rearrangement, and subsequent one-pot deprotection-oxidation of a pendant oxygen leads to the gamma-unsaturated beta-amino acid derivatives of high enantiopurity.

Enantioselective hydrogenation of tetrasubstituted olefins of cyclic beta-(acylamino)acrylates

Hydrogenation of a series of cyclic beta-(acylamino)acrylates with tetrasubstituted olefins has been accomplished successfully with the use of Ru catalysts with chiral biaryl ligands such as C3-TunaPhos, and up to over 99% ee's have been achieved. This methodology provides an efficient catalytic method for the synthesis of both cis and trans chiral cyclic beta-amino acid derivatives.

Ru-catalyzed highly enantioselective hydrogenation of beta-alkyl-substituted beta-(acylamino)acrylates

Highly enantioselective hydrogenation of beta-alkyl-substituted (E)-beta-(acylamino)-acrylates catalyzed by Ru((R)-Xyl-P-Phos)(C(6)H(6))Cl(2) complex (cat. 1c) was achieved in up to 99.7% ee. Moderate to good enantioselectivities in the hydrogenation of corresponding (Z)-isomers in the presence of [Rh((R)-Xyl-P-Phos)(COD)]BF(4) (cat. 2c) were also obtained. The results demonstrated that the electronic and steric properties of the dipyridylphosphine ligands as well as the different transition metal ions have significant influences on the catalytic properties in the hydrogenation of beta-(acylamino)acrylates.

Highly enantioselective rhodium-catalyzed hydrogenation of beta-dehydroamino acid derivatives using monodentate phosphoramidites

New and very easily accessible monodentate phosphoramidite ligands have been developed that lead to excellent ee's and full conversions in the hydrogenation of (E)- and (Z)-beta-dehydroamino acid derivatives with both aliphatic and aromatic side chains. Particularly, two different catalytic systems were established for (E)-beta-(acylamino)acrylates (98-99% ee) and (Z)-beta-(acylamino)acrylates (92-95% ee) based on phosphoramidites 2 and 3, respectively.

Highly efficient synthesis of chiral beta-amino acid derivatives via asymmetric hydrogenation

The Rh-TangPhos complex is an efficient hydrogenation catalyst for making chiral beta-amino acid derivatives. With the Rh-TangPhos system, high enantioselectivities (up to 99.6%) and turnover numbers have been obtained in the hydrogenation of E/Z isomeric mixtures of both beta-alkyl and beta-aryl beta-(acylamino)acrylates. [reaction: see text]

Rh(I)-catalyzed enantioselective hydrogenation of (E)- and (Z)-beta-(acylamino)acrylates using 1,4-bisphosphine ligands under mild conditions

[reaction: see text] Rh-Me-BDPMI (1a) complex can be an effective catalyst for the hydrogenations of (E)- and (Z)-beta-(acylamino)acrylates, in which the Z-isomers hydrogenated with the same or even higher ee values than the corresponding E-isomers. The conversion yield and enantioselectivity of E- and Z-isomers were largely dependent on the solvent, and thus, the E-isomers were hydrogenated more effectively in CH(2)Cl(2), whereas the Z-isomers were hydrogenated more effectively in polar MeOH solvent.

Highly effective chiral ortho-substituted BINAPO ligands (o-BINAPO): applications in Ru-catalyzed asymmetric hydrogenations of beta-aryl-substituted beta-(acylamino)acrylates and beta-keto esters

A novel family of chiral ortho-substituted BINAPO ligands (o-BINAPO) were synthesized from BINOL, and their Ru complexes were highly efficient catalysts for asymmetric hydrogenation of beta-aryl-substituted beta-(acylamino)acrylates and beta-aryl-substituted beta-keto esters. The Ru-bisphosphinite catalysts can tolerate an E/Z mixture of beta-aryl-substituted beta-(acylamino)acrylates. These highly enantioselective hydrogenations provide a useful way to prepare beta-aryl-substituted beta-amino acids and beta-hydroxyl acids.