Asymmetric Hydrogenation of β-Aryl Alkylidene Malonate Esters: Installing an Ester Group Significantly Increases the Efficiency

Unlocking the Asymmetric Hydrogenation of Tetrasubstituted Acyclic Enones

Asymmetric hydrogenation (AH) of tetrasubstituted olefins generating two stereocenters is still an open topic. There are only a few reports on the AH of tetrasubstituted olefins with conjugated functional groups, while this process can create useful intermediates for the subsequent elaboration of relevant end products. Most of the tetrasubstituted olefins successfully submitted to AH belong to a small number of functional classes; remarkably, the AH of tetrasubstituted acyclic enones still represents an unsolved challenge. Herein, we disclose a class of air-stable Ir-P,N catalysts, prepared in three steps from commercially available amino alcohols, that can hydrogenate, in minutes, a wide range of electronically and sterically diverse acyclic tetrasubstituted enones (including exocyclic ones) with high yields and high enantioselectivities. The factors responsible for the excellent selectivities were elucidated by combining deuterogenation experiments and theoretical calculations. The calculations indicated that the reduction follows an IrI /IrIII mechanism, in which enantioselectivity is controlled in the first migratory insertion of the hydride to the most electrophilic olefinic Cβ and the formation of the hydrogenated product via reductive elimination takes place prior to the coordination of dihydrogen and the subsequent oxidative addition. The potential of the new catalytic systems is demonstrated by the derivatization of hydrogenation products.

P-Stereogenic Ir-MaxPHOX: A Step toward Privileged Catalysts for Asymmetric Hydrogenation of Nonchelating Olefins

The Ir-MaxPHOX-type catalysts demonstrated high catalytic performance in the hydrogenation of a wide range of nonchelating olefins with different geometries, substitution patterns, and degrees of functionalization. These air-stable and readily available catalysts have been successfully applied in the asymmetric hydrogenation of di-, tri-, and tetrasubstituted olefins (ee's up to 99%). The combination of theoretical calculations and deuterium labeling experiments led to the uncovering of the factors responsible for the enantioselectivity observed in the reaction, allowing the rationalization of the most suitable substrates for these Ir-catalysts.

Highly Efficient Asymmetric Hydrogenation Catalyzed by Iridium Complexes with Tridentate Chiral Spiro Aminophosphine Ligands

ConspectusCatalytic asymmetric hydrogenation is one of the most reliable, powerful, and environmentally benign methods for the synthesis of chiral molecules with high atom economy and has been successfully applied in the industrial production of pharmaceuticals, agrochemicals, and fragrances. The key to achieving highly efficient and highly enantioselective hydrogenation reactions is the design and synthesis of chiral catalysts.Our recent studies involving iridium complexes of bidentate chiral spiro aminophosphine ligands (Ir-SpiroAP) have revealed that adding another coordinating group on the nitrogen atom to form a tridentate ligand can provide catalysts with markedly higher stability, enantioselectivity, and efficiency. Specifically, chiral Ir-SpiroAP catalysts bearing an added pyridine group (designated Ir-SpiroPAP) exhibit high activity and excellent enantioselectivity in the asymmetric hydrogenation of a wide range of carbonyl compounds, including aryl ketones, β- and δ-ketoesters, α,β-unsaturated ketones and esters, and racemic α-substituted lactones, as well as highly electron-deficient alkenes such as α,β-unsaturated malonates and analogues. The efficiency of the Ir-SpiroPAP catalysts is extremely high: in the hydrogenation of aryl ketones, turnover numbers reach 4.5 million, which is the highest value reported to date for a molecular catalyst. Moreover, when a thioether or a bulky triarylphosphine group is added to afford tridentate ligands designated SpiroSAP and SpiroPNP, respectively, the resulting iridium catalysts show high efficiency and enantioselectivity for asymmetric hydrogenation of β-alkyl-β-ketoesters and dialkyl ketones, which are challenging substrates. Furthermore, chiral spiro catalysts containing an added oxazoline moiety (Ir-SpiroOAP) show high enantioselectivity for asymmetric hydrogenation of α-keto amides and racemic α-aryloxy lactones. The above-described catalysts have been used for enantioselective synthesis of chiral pharmaceuticals and other bioactive compounds.We have shown that chiral spiro ligands that combine a rigid skeleton with tridentate coordination stabilize iridium catalysts. The careful tailoring of the substituents on the ligand creates a chiral environment around the active metal center of the catalyst that can precisely discriminate between the two faces of a substrate carbonyl group. These factors are key for controlling the activity, enantioselectivity, and turnover numbers of asymmetric hydrogenation catalysts. We expect that catalysts based on iridium, and other transition metals, coordinated by tridentate chiral ligands with a rigid skeleton will find more applications in asymmetric hydrogenation and other asymmetric transformations.

Development of a synthetic equivalent of α,α-dicationic acetic acid leading to unnatural amino acid derivatives via tetrafunctionalized methanes

Diethyl mesoxalate (DEMO) exhibits high electrophilicity and accepts the nucleophilic addition of a less nucleophilic acid amide to afford N,O-hemiacetal. However, our research showed that elimination of the amide moiety proceeded more easily than dehydration upon treatment with a base. This problem was overcome by reacting DEMO with an acid amide in the presence of acetic anhydride to efficiently obtain N,O-acetal. Acetic acid was eliminated leading to the formation of N-acylimine in situ upon treatment with the base. N-Acylimine is also electrophilic, accepting the second nucleophilic addition by pyrrole or indole to form α,α-disubstituted malonates. Subsequent hydrolysis followed by decarboxylation resulted in (α-indolyl-α-acylamino)acetic acid formation; homologs of tryptophan. Through this process, DEMO serves as a synthetic equivalent of α,α-dicationic acetic acid to facilitate nucleophilic introduction of the two substituents.

Catalytic Asymmetric Hydrogenation of Tetrasubstituted Unsaturated Lactams: An Efficient Approach to Enantioenriched 3,4-Disubstituted Piperidines

Asymmetric hydrogenation of tetrasubstituted alkenes remains a formidable challenge in asymmetric catalysis. We report herein an unprecedented Rh-catalyzed enantioselective and diastereoselective hydrogenation of easily accessed α,β-disubstituted unsaturated lactams to afford synthetically valuable chiral lactams with 1,2-consecutive stereocenters. The reaction could be performed on the gram scale, and the products could be concisely transformed to enantiomerically pure trans-3,4-disubstituted piperidines, which are prevalent structural units in medicinal agents.

Biomimetic Asymmetric Reduction of Tetrasubstituted Olefin 2,3-Disubstituted Inden-1-ones with Chiral and Regenerable NAD(P)H Model CYNAM

Because of the formidable development of the asymmetric reduction of tetrasubstituted olefins, an effective method is in urgent demand. Herein, through the biomimetic protocol of the coenzyme NAD(P)H, the reduction of tetrasubstituted olefin 2,3-substituted 1H-inden-1-ones has been successfully realized with the catalytic chiral NAD(P)H model CYNAM, which is hard to bring about via the common rhodium or iridium-based catalytic system, producing the corresponding products in good yield (up to 98%) with good enantioselectivity (up to 99% ee). Furthermore, the chiral bioactive molecule can be concisely synthesized from the reduced product.

Catalytic Asymmetric Hydrogenation of 3-Ethoxycarbonyl Quinolin-2-ones and Coumarins

A protocol of iridium catalyzed asymmetric hydrogenation of 4-alkyl substituted 3-ethoxycarbonyl quinolin-2-ones and coumarins has been reported, providing a wide range of chiral dihydroquinolin-2-ones and dihydrocoumarins in high yields with excellent enantioselectivities (up to 99% ee) and high turnover numbers (up to 28 000). This efficient protocol was successfully applied for the synthesis of MPR3160 and the key chiral intermediate of R-106578.