Copper-Catalyzed Chemoselective Asymmetric Hydrogenation of C=O Bonds of Exocyclic α,β-Unsaturated Pentanones

Ru-Catalyzed Asymmetric Hydrogenation of α,β-Unsaturated γ-Lactams

A highly efficient Ru-catalyzed asymmetric hydrogenation of α,β-unsaturated γ-lactams has been developed by using a C2-symmetric ruthenocenyl phosphine-oxazoline as the chiral ligand. This method achieves the enantioselective synthesis of chiral β-substituted γ-lactams in high yields and with excellent enantioselectivities (up to 99% yield with 99% ee). Mechanistic studies based on detailed control experiments and computational investigation revealed that the cationic Ru-complex acts as the active catalytic species; the protonation process of the oxa-π-allyl-Ru complex, which is formed by the migratory insertion of the C=C double bond to the Ru-H bond (the stereocontrolling step) followed by an isomerization process, is the rate-determining step, and the existence of PPh3 is crucial for the highly efficient catalytic behavior. The protocol provides a straightforward and practical pathway for the synthesis of key intermediates for several chiral drugs and bioactive compounds, particularly for the 150 kg-scale industrial production of Brivaracetam, an antiepileptic drug that shows 13-fold more potent binding to the synaptic vesicle protein 2A compared with the well-known Levetiracetam.

Cobalt-Catalyzed Enantioselective Alkenylation of Aldehydes

Catalytic enantioselective alkenylation of aldehydes with easily accessible alkenyl halides promoted by a chiral cobalt complex derived from a newly developed tridentate bisoxazolinephosphine is presented. Such processes represent an unprecedented reaction pathway for cobalt catalysis and a general approach that enable rapid construction of highly diversified enantioenriched allylic alcohols containing a 1,1-, 1,2-disubstituted and trisubstituted alkene as well as axial stereogenicity in up to 99 % yield and 99 : 1 er without the need of preformation of alkenyl-metal reagents. DFT calculations revealed the origin of enantioselectivity.

Mild ketyl radical generation and coupling with alkynes enabled by Cr catalysis: stereoselective access to E-exocyclic allyl alcohols

The mild catalytic generation of ketyl radicals for organic transformations remains an unsolved issue, although it facilitates the discovery of metal-catalyzed reactions with the features of high functional group tolerance. Here, we report the generation of the ketyl radicals and coupling with alkynes that was enabled by cost-effective chromium catalysis, allowing for the formation of valuable E-exocyclic allyl alcohols with high stereo- and chemoselectivity. A broad range of synthetically useful functional groups that are sensitive to strong reductants are compatible with the catalytic system, providing access to diverse substituted E-exocyclic allyl alcohols under mild conditions. Appended hydroxyl groups in products are facilely late-stage functionalized in accessing numerous derivatives, as well as the enantio-enrichment of exocyclic allyl alcohol using chiral ligands. Mechanistic studies suggest that bipyridine-ligated Cr(ii) complex serves as a reactive catalyst enabling the generation of the ketyl radical for coupling, giving vinyl radical, followed by the combination of Cr and transmetalation with Cp2ZrCl moiety in affording oxazirconiumacycle. This reaction provides a new opportunity for the mild formation of transient ketyl radicals from widely accessible aliphatic aldehydes for coupling with Earth-abundant metal catalysis.

Iridium-Catalyzed Asymmetric Hydrogenation of Unfunctionalized Cycloalkenes to Access Chiral 2-Aryl Tetralins

The transition-metal catalyzed asymmetric hydrogenation of unfunctionalized alkenes is challenging. Herein, we report an efficient iridium-catalyzed asymmetric hydrogenation of unfunctionalized cycloalkenes, delivering chiral 2-aryl tetralins in excellent yields and with moderate to excellent enantioselectivities. The reaction can be performed on a gram-scale with a low catalyst loading (S/C = 1000), and the reduced product was obtained without erosion of the enantioselectivity. Deuterium experiments indicated that the C═C bond in the substrate is hydrogenated directly without isomerization.

Nickel-catalyzed asymmetric hydrogenation for the preparation of α-substituted propionic acids

Transition metal-catalyzed asymmetric hydrogenation is one of the most efficient methods for the preparation of chiral α-substituted propionic acids. However, research on this method, employing cleaner earth-abundant metal catalysts, is still insufficient in both academic and industrial contexts. Herein, we report an efficient nickel-catalyzed asymmetric hydrogenation of α-substituted acrylic acids affording the corresponding chiral α-substituted propionic acids with up to 99.4% ee (enantiomeric excess) and 10,000 S/C (substrate/catalyst). In particular, this method can be used to obtain (R)-dihydroartemisinic acid with 99.8:0.2 dr (diastereomeric ratio) and 5000 S/C, which is an essential intermediate for the preparation of the antimalarial drug Artemisinin. The reaction mechanism has been investigated via experiments and DFT (Density Functional Theory) calculations, which indicate that the protonolysis of the C-Ni bond of the key intermediate via an intramolecular proton transfer from the carboxylic acid group of the substrate, is the rate-determining step.

Toward a Chemical Constructor: A Lego-Like Approach for Formal α-Alkylation of Cyclic Ketones

A conceptual strategy for a formal α-alkylation of α-methylene ketones was developed. Diverse 1° and 2° alkyl substituents were generated in the α-position of various ketones via synthesis of enaminone (step 1) and treatment with organomagnesium (step 2) with subsequent catalytic hydrogenation (step 3, 1° alkyl) or organocopper reagents (step 4, 2° alkyl). Tolerance toward ester, Boc-protected amine, and α-fluoro-substituted ketone moieties was demonstrated. The suitability of the method for late-stage natural product modification was shown.

Iridium-Catalyzed Enantioselective Transfer Hydrogenation of 1,1-Dialkylethenes with Ethanol: Scope and Mechanism

Despite half a century's advance in the field of transition-metal-catalyzed asymmetric alkene hydrogenation, the enantioselective hydrogenation of purely alkyl-substituted 1,1-dialkylethenes has remained an unmet challenge. Herein, we describe a chiral PCNOx-pincer iridium complex for asymmetric transfer hydrogenation of this alkene class with ethanol, furnishing all-alkyl-substituted tertiary stereocenters. High levels of enantioselectivity can be achieved in the reactions of substrates with secondary/primary and primary/primary alkyl combinations. The catalyst is further applied to the redox isomerization of disubstituted alkenols, producing a tertiary stereocenter remote to the resulting carbonyl group. Mechanistic studies reveal a dihydride species, (PCNOx)Ir(H)2, as the catalytically active intermediate, which can decay to a dimeric species (κ3-PCNOx)IrH(μ-H)2IrH(κ2-PCNOx) via a ligand-remetalation pathway. The catalyst deactivation under the hydrogenation conditions with H2 is much faster than that under the transfer hydrogenation conditions with EtOH, which explains why the (PCNOx)Ir catalyst is effective for the transfer hydrogenation but ineffective for the hydrogenation. The suppression of di-to-trisubstituted alkene isomerization by regioselective 1,2-insertion is partly responsible for the success of this system, underscoring the critical role played by the pincer ligand in enantioselective transfer hydrogenation of 1,1-dialkylethenes. Moreover, computational studies elucidate the significant influence of the London dispersion interaction between the ligand and the substrate on enantioselectivity control, as illustrated by the complete reversal of stereochemistry through cyclohexyl-to-cyclopropyl group substitution in the alkene substrates.

Cobalt-Catalyzed Efficient Asymmetric Hydrogenation of α-Primary Amino Ketones

Based on an amino-group-assisted coordination strategy and a proton-shuttle-activated outer-sphere mode, the cobalt-catalyzed asymmetric hydrogenation of α-primary amino ketones has been developed, resulting in the efficient synthesis of chiral vicinal amino alcohols bearing functionalized aryl rings in high yields and enantioselectivities (up to 99% enantiomeric excess (ee)) within 0.5 h.