Asymmetric synthesis of chiral Roche ester and its derivatives via Rh-catalyzed enantioselective hydrogenation with chiral phosphine-phosphoramidite ligands

Chiral phosphine-phosphoramidite ligands for highly enantioselective hydrogenation of N-arylimines

The asymmetric hydrogenation of N-arylimines with the chiral phosphine-phosphoramidite ligand, (Sc,Sa)-PEAPhos 2b, has been developed, in which high turnover numbers and excellent enantioselectivity were achieved.

Highly enantioselective hydrogenation of 2-substituted-2-alkenols catalysed by a ChenPhos-Rh complex

Highly enantioselective hydrogenation of a variety of 2-substituted-2-alkenols has been achieved using a ChenPhos-Rh complex as catalyst, giving ≥99% ee for most substrates. Optically active antifungal agent amorolfine was first synthesised using hydrogenation as the key step.

Homo-Roche ester derivatives by asymmetric hydrogenation and organocatalysis

Asymmetric hydrogenation routes to homologues of The Roche ester tend to be restricted to hydrogenations of itaconic acid derivatives, that is, substrates that contain a relatively unhindered, 1,1-disubstituted alkene. This is because in hydrogenations mediated by RhP2 complexes, the typical catalysts, it is difficult to obtain high conversions using the alternative substrate for the same product, the isomeric trisubstituted alkenes (D in the text). However, chemoselective modification of the identical functional groups in itaconic acid derivatives are difficult; hence, it would be favorable to use the trisubstituted alkene. Trisubstituted alkene substrates can be hydrogenated with high conversions using chiral analogs of Crabtree's catalyst of the type IrN(carbene). This paper demonstrates that such reactions are scalable (tens of grams) and can be manipulated to give optically pure homo-Roche ester chirons. Organocatalytic fluorination, chlorination, and amination of the homo-Roche building blocks was performed to demonstrate that they could easily be transformed into functionalized materials with two chiral centers and α,ω-groups that provide extensive scope for modifications. A synthesis of (S,S)- and (R,S)-γ-hydroxyvaline was performed to illustrate one application of the amination product.

Asymmetric bioreduction of activated alkenes to industrially relevant optically active compounds

Ene-reductases from the ‘Old Yellow Enzyme’ family of flavoproteins catalyze the asymmetric reduction of various α,β-unsaturated compounds at the expense of a nicotinamide cofactor. They have been applied to the synthesis of valuable enantiopure products, including chiral building blocks with broad industrial applications, terpenoids, amino acid derivatives and fragrances. The combination of these highly stereoselective biocatalysts with a cofactor recycling system has allowed the development of cost-effective methods for the generation of optically active molecules, which is strengthened by the availability of stereo-complementary enzyme homologues.

Asymmetric Rh-catalyzed hydrogenation using a furanoside phosphite-phosphoroamidite and diphosphoroamidite ligand library

A furanoside phosphite-phosphoroamidite and diphosphoroamidite ligand library L1-L5a-f was tested in the asymmetric Rh-catalyzed hydrogenation of α,β-unsaturated carboxylic acid derivatives and enamides. Enantioselectivity depended strongly on the ligand parameters. High enantioselectivities were obtained in the reduction of dimethyl itaconate (up to >99% ee), α-dehydroamino acid esters (up to 99% ee) and several enamides (up to 92% ee). Kinetic and NMR studies on the intermediates of the catalytic cycle of the reaction indicate that the [Rh(P(1)-P(2))(substrate)](+) species is the resting state of the reaction and that the rate dependence is first order in rhodium and hydrogen pressure and zeroth order in the substrate.

Competition of Ester, Amide, Ether, Carbonate, Alcohol and Epoxide Ligands in the Dirhodium Experiment (Chiral Discrimination by NMR Spectroscopy) [1]

Thirty-eight derivatives of 3-hydroxy-2-methylpropanoic acid, each with two different oxygen functionalities, were synthesized and subjected to the standard dirhodium experiment (1H NMR in the presence of an equimolar amount of the chiral dirhodium tetracarboxylate complex Rh*). Their structures represent ester, amide, carbonate, ether, alcohol and/or epoxy groups. Significant selectivity in the binding of those oxygen groups to the complex were determined. From these results, a priority list in binding to a rhodium atom of Rh* was established:

epoxides > primary alcohols > ethers ≥ esters ≥ amides > carbonates > tertiary alcohols

This sequence allows the prediction of the preferred binding site of oxygen-containing groups in polyfunctional compounds, which frequently occur among natural products, and, particularly, in asymmetric synthesis of such compounds. Differentiation of the enantiomers by the dirhodium experiment is easily accomplished due to numerous signal dispersions in nearly all cases.

Hybrid bidentate phosphorus ligands in asymmetric catalysis: privileged ligand approach vs. combinatorial strategies

In this perspective the development of chiral phosphorus ligands for asymmetric catalysis is discussed, with a special focus on hybrid bidentate phosphorus ligands, in particular phosphine-phosphoramidites. An attempt is made to compare privileged ligand and combinatorial approaches to ligand development--for which the class of phosphine-phosphoramidite ligands is well suited--highlighting differences, similarities and their complementary use.