Stereodivergent Coupling of Aldehydes and Alkynes via Synergistic Catalysis Using Rh and Jacobsen's Amine

Stereodivergent synthesis of enantioenriched azepino[3,4,5-cd]-indoles via cooperative Cu/Ir-catalyzed asymmetric allylic alkylation and intramolecular Friedel-Crafts reaction

The development of enantioselective annulation reactions using readily available substrates for the construction of structurally and stereochemically diverse heterocycles is a compelling topic in diversity-oriented synthesis. Herein, we report efficient catalytic asymmetric formal 1,3-dipolar (3 + 4) cycloadditions of azomethine ylides with 4-indolyl allylic carbonates for the construction of azepino[3,4,5-cd]-indoles fused with a challenging seven-membered N-heterocycle, a frequently occurring tricyclic indole scaffold in bioactive compounds and pharmaceuticals. Through cooperative Cu/Ir-catalyzed asymmetric allylic alkylation followed by intramolecular Friedel-Crafts reaction, an array of azepino[3,4,5-cd]-indoles were obtained in good yields with excellent diastereo-/enantioselective control. More importantly, the full stereodivergence of this transformation was established via synergistic catalysis followed by acid-promoted epimerization, and up to eight stereoisomers of the cycloadducts bearing three stereogenic centers could be predictably achieved from the same set of starting materials for the first time. Quantum mechanical computations established a plausible mechanism for the synergistic Cu/Ir catalysis to stereodivergently introduce two vicinal stereocenters whose stereochemical information is remotely delivered across the fused azepine ring to control the third chiral center. Epimerization of the last center involves protonation-enabled reversal of the thermodynamically controlled relative configuration.

Water enables diastereodivergency in bispidine-based chiral amine-catalyzed asymmetric Mannich reaction of cyclic N-sulfonyl ketimines with ketones

Tuning diastereoselectivity is a great challenge in asymmetric catalysis for the inherent stereochemical bias of the substrates. Here, we report a diastereodivergent asymmetric Mannich reaction of cyclic N-sulfonyl ketimines with ketones catalyzed by a bispidine-based chiral amine catalyst, in which additional water switches the diastereoselectivity efficiently. Both chiral anti- and syn-benzosultams with potential anti-HIV-1 activity are obtained in excellent yields and good to excellent ee values. Control experiments and density functional theory (DFT) calculations were applied to study the diastereodivergent mechanism, which reveal that the diastereodivergent catalysis should be state-determined, and the water reverses the energies of states to realize the diastereodivergency. The findings are quite new and might inspire more diastereodivergent asymmetric synthesis.

A diastereodivergent asymmetric Mannich reaction of cyclic N-sulfonyl ketimines with ketones is realized by employing bispidine-based chiral amine as catalyst and additional water switching the diastereoselectivity.

Desymmetric Partial Reduction of Malonic Esters

Desymmetrization of easily available disubstituted malonic esters is a rewarding strategy to access structurally diverse quaternary stereocenters. Particularly, asymmetric reduction of malonic esters would generate a functional group with a lower oxidation state than the remaining ester, thus allowing for more chemoselective derivatization. Here, we report a new set of conditions for the zinc-catalyzed desymmetric hydrosilylation of malonic esters that afford aldehydes as the major product. Compared with alcohol-selective desymmetrization, the partial reduction uses a higher concentration of silanes and new pipecolinol-derived tetradentate ligands, proposedly to switch the pathway of zinc hemiacetal intermediates from elimination to silylation. As a result, high aldehyde-to-alcohol ratios and enantioselectivity of aldehydes are obtained from malonic esters with a large collection of substituents. Together with the abundant reactivity of aldehydes, the partial reduction has enabled an expeditious synthesis of bioactive compounds and natural metabolites containing a quaternary stereocenter.

Stereodivergent propargylic alkylation of enals via cooperative NHC and copper catalysis

Despite that asymmetric stereodivergent synthesis has experienced great success to provide unusual processes for the creation of chirality complexity, concepts appliable to asymmetric stereodivergent catalysis are still limited. The dependence on the unusual capacity of each catalyst to precisely control the reactive site planar in the region poses unparalleled constraints on this field. Here, we first demonstrate that the chiral Cu-allenylidene species can participate in the stereodivergent propargylic alkylation of enals, in concert with chiral N-heterocyclic carbenes (NHCs). Thus, all four stereoisomers were obtained with excellent enantioselectivity and diastereoselectivity (up to >99% e.e. and >95:5 d.r.) from the same starting materials by simply altering chiral Cu-Pybox complex and NHC combinations. The rich chemistry workable in the products enables the structurally diverse synthesis of chiral functional molecules and holds great potential in alkaloid synthesis, as showcased by the preparation of the key building block to access (-)-perophoramidine.

The ability to construct multiple stereocenters in a modular fashion is an important goal of synthetic organic chemistry. Here the authors present a method to construct oxindoles in four stereoisomers with high enantioselectivity and diastereoselectivity from the same starting materials by using cooperative copper- and organocatalysis.

Acyclic Quaternary Carbon Stereocenters through Transition-Metal-Catalyzed Enantioselective Functionalization of Unsaturated Hydrocarbons

Acyclic quaternary carbon stereocenters occur frequently in natural products, bioactive molecules, and pharmaceutical compounds. Construction of a carbon stereogenic center attached to four different carbons with defined spatial arrangement is a daunting challenge in asymmetric catalysis. Significant efforts have been directed towards the stereoselective construction of such acyclic quaternary carbon stereocenters. In particular, catalytic generation of acyclic quaternary carbon stereocenters through functionalization of unsaturated hydrocarbons is an extremely attractive approach because unsaturated hydrocarbons are easily accessible both in industry and in organic synthesis. In this short review, we summarize the recent advances achieved in this research area, with the aim to inspire future development.

1 Introduction

2 Acyclic Quaternary Carbon Stereocenters through Functionalization of Allenes

3 Acyclic Quaternary Carbon Stereocenters through Functionalization of Dienes

4 Acyclic Quaternary Carbon Stereocenters through Functionalization of Mono-alkenes

5 Acyclic Quaternary Carbon Stereocenters through Functionalization of Alkynes

6 Summary and Outlook

A Convenient Procedure for Sonogashira Reactions Using Propyne

A modified Sonogashira coupling of aryl iodides and propyne­ was achieved using only two equivalents of propyne in THF from –78 °C to room temperature.

Hydrogen-Bond-Donor-Directed Switching of Enantioselectivity in the Vinylogous Aldol-Cyclization Cascade Reaction of Prostereogenic 3-Alkylidene Oxindoles with Isatins and o-Quinones

In this study, we reported a hydrogen-bond-donor-directed enantiodivergent vinylogous aldol-cyclization cascade reaction of 3-alkylidene oxindoles with isatins and o-quinones. Both enantiomers can be prepared by thiourea or squaramide cinchona alkaloid bifunctional organocatalysts with the same quinine scaffold. Kinetic study data provided the possible reaction mechanism for the vinylogous aldol-cyclization cascade reaction. The DFT calculation data showed the geometry of the generated dienolates from pronucleophiles dominated the observed switch of enantioselectivity.

Construction of quaternary carbon centers by KOtBu-catalyzed α-homoallylic alkylation of lactams with 1,3-dienes

We report a KOtBu-catalyzed α-homoallylic alkylation of lactams with 1,3-dienes.

Diastereodivergent synthesis of fully disubstituted spiro[indoline-3,2′-pyrrolidin]-2-ones via tuneable Lewis base/Brønsted base-promoted (3 + 2) cycloadditions

We have developed unique base-promoted diastereoselective (3 + 2) cycloadditions. A diastereodivergent synthesis of fully disubstituted spiro[indoline-3,2′-pyrrolidin]-2-ones was realized by tuning a Lewis base/Brønsted base.

Combining Palladium and Chiral Organocatalysis for the Enantioselective Deconjugative Allylation of Enals via Dienamine Intermediates

A catalytic enantioselective deconjugative allylation of enals is reported. A variety of enals underwent this transformation in high yield and ee, and products can be readily transformed into γ-allyl enals via a Cope rearrangement without erosion of ee. This transformation was used to install the quaternary stereocenter in (S)-bakuchiol, enabling completion of a concise formal synthesis.

Synergistic Peptide and Gold Catalysis: Enantioselective Addition of Branched Aldehydes to Allenamides

The combination of a peptide catalyst and a gold catalyst is presented for enantioselective addition reactions between branched aldehydes and allenamides. The two catalysts act in concert to provide γ,δ-enamide aldehydes bearing a fully substituted, benzylic stereogenic center - a structural motif common in many natural products and therapeutically active compounds - with good yields and enantioselectivities. The reaction tolerates a variety of alkyl and alkoxy substituted aldehydes and the products can be elaborated into several chiral building blocks bearing either 1,4- or 1,5- functional group relationships. Mechanistic studies showed that the conformational features of the peptide are important for both the catalytic efficiency and stereochemistry, while a balance of acid/base additives is key for ensuring formation of the desired product over undesired side reactions.

Stereodivergent synthesis via iridium-catalyzed asymmetric double allylic alkylation of cyanoacetate

Methods that enable the rapid construction of multiple C–C bonds using a single catalyst with high diastereo- and enantio-control are particularly valuable in organic synthesis. Here, we report an Ir-catalyzed double allylic alkylation reaction in which bisnucleophilic cyanoacetate reacted successionally with electrophilic π-allyl-Ir species, producing various pseudo-C₂-symmetrical cyanoacetate derivatives in high yield with excellent stereocontrol. More challenging sequential allylic alkylation/allylic alkylation with two distinct allylic carbonates that can deliver the corresponding products bearing three contiguous tertiary–quaternary–tertiary stereocenters was also developed by using a modified catalytic system, which is revealed to be associated with the quasi-dynamic kinetic resolution of the initially formed diastereomeric monoallylation intermediates. Notably, stereodivergence for this sequential process depending on a single iridium catalyst was successfully realized, and up to six stereoisomers could be predictably prepared by combining the appropriate enantiomer of the chiral ligand for the iridium catalyst and adjusting the adding sequence of two distinct allylic precursors.

Ir-catalyzed asymmetric double AAA reaction of cyanoacetate was developed, affording cyanoacetate derivatives in high yield with excellent stereocontrol. Notably, quasi-DKR is involved in the sequential protocol with two distinct allylic carbonates.

Synergistic Copper/Enamine Catalysis for the Regio-, Stereo-, and Enantioselective Intermolecular α-Addition of Aldehydes to Allenamides

We herein describe an intermolecular enantioselective α-addition of aldehydes to allenamides using a dual copper/enamine catalytic system. Highly enantioselective addition of aldehydes was obtained thanks to secondary amine catalysts. The process was found to be highly regio-, stereo-, and enantioselective under mild conditions.

Enantio- and Diastereodivergent Synthesis of Spirocycles through Dual-Metal-Catalyzed [3+2] Annulation of 2-Vinyloxiranes with Nucleophilic Dipoles

The development of efficient and straightforward methods for obtaining all optically active isomers of structurally rigid spirocycles from readily available starting materials is of great value in drug discovery and chiral ligand development. However, the stereodivergent synthesis of spirocycles bearing multiple stereocenters remains an unsolved challenge owing to steric hindrance and ring strain. Herein, we report an enantio- and diastereodivergent synthesis of rigid spirocycles through dual-metal-catalyzed [3+2] annulation of oxy π-allyl metallic dipoles with less commonly employed nucleophilic dipoles (imino esters). A series of spiro compounds bearing a pyrroline and an olefin were easily synthesized in an enantio- and diastereodivergent manner (up to 19:1 dr, >99 % ee), which showed great promise as a new type of N-olefin ligand. Preliminary mechanistic studies were also carried out to understand the process of this bimetallic catalysis.

Stereodivergent Synthesis of Enantioenriched γ-Butyrolactones Bearing Two Vicinal Stereocenters Enabled by Synergistic Copper and Iridium Catalysis

By virtue of a fundamentally new reaction model of azomethine ylide serving as a two-atom synthon, we present the first example of stereodivergent preparation of γ-butyrolactones via synergistic Cu/Ir-catalyzed asymmetric cascade allylation/lactonization, and all four stereoisomers of γ-butyrolactones bearing two vicinal stereocenters are accessible with excellent diastereoselective and enantioselective control. The chiral Ir^III -π-allyl intermediate was separated and characterized to understand the origin of the regio- and stereoselectivity of the initial C-C bond formation process. Control experiments shed some light on the catalyst/substrate and catalyst/catalyst interactions in this dual catalytic system to rationalize the related kinetic/dynamic kinetic resolution process with different catalyst combinations. The enantioenriched γ-butyrolactone products were converted into an array of structurally complex chiral molecules and organocatalysts that were otherwise inaccessible.

Direct access to spirocycles by Pd/WingPhos-catalyzed enantioselective cycloaddition of 1,3-enynes

Spirocycles play an important role in drug discovery and development. The direct, catalytic, and enantioselective synthesis of spirocycles from readily available starting materials and in an atom economic manner remains a highly sought-after task in organic synthesis. Herein, an enantioselective Pd-hydride-catalyzed cycloaddition method for the synthesis of spirocyclic compounds directly from two classes of commonly available starting materials, 1,3-enynes and cyclic carbon−hydrogen (C−H) bonds, is reported. The reactions employ a chiral Pd/WingPhos catalyst to both suppress the formation of bis-allenyl by-products and control the stereoselectivity. 1,3-Enynes are used as dielectrophilic four-carbon units in the cycloaddition reactions, which also enables an enyne substrate-directed enantioselectivity switch with good levels of stereocontrol. The present spirocycle synthesis tolerates a broad range of functional groups of 1,3-enyne substrates, including alcohols, esters, nitriles, halides, and olefins. A variety of diverse cyclic nucleophiles, including pharmaceutically important heterocycles and carbocycles, can be flexibly incorporated with spiro scaffolds.

Spirocycles are traditionally difficult structures to synthesize due to the congested nature of the central atom. Here the authors show a method to synthesize quaternary carbon spirocycles in one step from 1,3-enynes and pyrazolidine-type heterocycles, both relatively unactivated structures, proceeding via palladium catalysis.

Enantioselective Addition of Pyrazoles to Dienes*

We report the first enantioselective addition of pyrazoles to 1,3-dienes. Secondary and tertiary allylic pyrazoles can be generated with excellent regioselectivity. Mechanistic studies support a pathway distinct from previous hydroaminations: a Pd0 -catalyzed ligand-to-ligand hydrogen transfer (LLHT). This hydroamination tolerates a range of functional groups and advances the field of diene hydrofunctionalization.

A Diastereodivergent and Enantioselective Approach to syn- and anti-Diamines: Development of 2-Azatrienes for Cu-Catalyzed Reductive Couplings with Imines That Furnish Allylic Amines

We introduce a new reagent class, 2-azatrienes, as a platform for catalytic enantioselective synthesis of allylic amines. Herein, we demonstrate their promise by a diastereodivergent synthesis of syn- and anti-1,2-diamines through their Cu-bis(phosphine)-catalyzed reductive couplings with imines. With Ph-BPE as the supporting ligand, anti-diamines are obtained (up to 91% yield, >20:1 dr, and >99:1 er), and with the rarely utilized t-Bu-BDPP, syn-diamines are generated (up to 76% yield, 1:>20 dr, and 97:3 er).

Enantio- and Diastereodivergent Construction of 1,3-Nonadjacent Stereocenters Bearing Axial and Central Chirality through Synergistic Pd/Cu Catalysis

In contrast to the widely explored methods for the asymmetric synthesis of molecules bearing a single stereocenter or adjacent stereocenters, the concurrent construction of 1,3-stereogenic centers in an enantio- and diastereoselective manner remains a challenge, especially in acyclic systems. Herein, we report an enantio- and diastereodivergent construction of 1,3-nonadjacent stereocenters bearing allenyl axial and central chirality through synergistic Pd/Cu-catalyzed dynamic kinetic asymmetric allenylation with racemic allenylic esters. The protocol is suitable for a wide range of substrates including the challenging allenylic esters with less sterically bulky substituents and provided chiral allenylic products bearing 1,3-nonadjacent stereocenters with high levels of enantio- and diastereoselectivities (up to >20:1 dr and >99% ee). Furthermore, several representative transformations involving axial-to-central chirality transfer were conducted, affording useful structural motifs containing nonadjacent stereocenters in a diastereodivergent manner.

Synergistic Pd/Amine-Catalyzed Stereodivergent Hydroalkylation of 1,3-Dienes with Aldehydes: Reaction Development, Mechanism, and Stereochemical Origins

Metal-hydride-catalyzed hydroalkylation of 1,3-dienes with enolizable carbonyl compounds is an atom- and step-economical method for preparing chiral molecules with allylic stereocenters. Although high diastereo- and enantioselectivities have been achieved for many coupling partners, aldehydes have not yet been used for this purpose because they are less stable than other carbonyl compounds under basic conditions and they have the potential to rapidly epimerize at the α-position. Moreover, stereodivergent hydroalkylation reactions of 1,3-dienes to access complementary diastereomers with vicinal stereocenters is challenging. Herein, we describe a synergistic palladium/amine catalyst system that allowed us to achieve the first stereodivergent hydroalkylation reactions of 1,3-dienes with aldehydes. By choosing an appropriate combination of chiral palladium and amine catalysts, we could obtain either syn or anti coupling products, and this method therefore provides highly diastereo- and enantioselective access to complementary diastereomers of chiral aldehydes with α,β-vicinal stereocenters. Density functional theory calculations revealed a mechanism involving PdH formation and migratory insertion into the alkene, followed by C-C bond formation. The origin of the stereoselectivities was investigated by means of distortion/interaction analysis.

Stereodivergent Synthesis of Enantioenriched 2,3-Disubstituted Dihydrobenzofurans via a One-Pot C-H Functionalization/Oxa-Michael Addition Cascade

A one-pot rhodium-catalyzed C-H functionalization/organocatalyzed oxa-Michael addition cascade reaction has been developed. This methodology enables the stereodivergent synthesis of diverse 2,3-disubstituted dihydrobenzofurans with broad functional group compatibility in good yields with high levels of stereoselectivity under exceptionally mild conditions. The full complement of stereoisomers of chiral 2,3-disubstituted dihydrobenzofurans and 3,4-disubstituted isochromans could be accessed at will by appropriate permutations of the two chiral catalysts. The current work provides a rare example of two chiral catalysts independently controlling two contiguous stereogenic centers subsequently via a two-step reaction in a single operation.

Palladium-Catalyzed Allenamide Carbopalladation/Allylation with Active Methine Compounds

A palladium-catalyzed allenamide carbopalladation/allylation with active methine compounds has been developed. Various indoles and isoquinolinones bearing a quaternary carbon center were achieved with good efficiency, a broad substrate scope and good functional group tolerance. This reaction underwent cascade oxidative addition, carbopalladation, and allylic alkylation, and two new C-C bonds were formed in one pot.

CF3SO3H-enabled cascade ring-opening/dearomatization of indole derivatives to polycyclic heterocycles

A novel dearomatization process to produce fused polycyclic indolines via a CF3SO3H-mediated cascade ring-opening of a β-lactam and hydroaminative cyclization is demonstrated. It provides a new strategy for the synthesis of important polycyclic indoline-2-amine derivatives in moderate to excellent yields, as well as with good functional group tolerance. Moreover, transformation of the product was performed to deliver the corresponding acid, alcohol and amide smoothly.

Highly Regio- and Enantioselective Reductive Coupling of Alkynes and Aldehydes via Photoredox Cobalt Dual Catalysis

A Co-catalyzed highly regio- and enantioselective reductive coupling of alkynes and aldehydes has been developed under visible light photoredox dual catalysis. A variety of enantioenriched allylic alcohols have been obtained by using unsymmetrical internal alkynes and commercially available catalyst, chiral ligand, and reagents. It is noteworthy that this approach has considerable advantages, such as excellent regio- (>95:5 for >40 examples), stereo- (up to >95:5 E/Z), and enantioselectivity (92-99% ee, >35 examples) control, mild reaction conditions, broad substrate scope, and good functional group compatibility, making it a great improvement to enantioselective alkyne-aldehyde reductive coupling reactions.

Reducing Challenges in Organic Synthesis with Stereoselective Hydrogenation and Tandem Catalysis

Tandem catalysis enables the rapid construction of complex architectures from simple building blocks. This Perspective shares our interest in combining stereoselective hydrogenation with transformations such as isomerization, oxidation, and epimerization to solve diverse challenges. We highlight the use of tandem hydrogenation for preparing complex natural products from simple prochiral building blocks and present tandem catalysis involving transfer hydrogenation and dynamic kinetic resolution. Finally, we underline recent breakthroughs and opportunities for asymmetric hydrogenation.

Enantioselective Addition of α-Nitroesters to Alkynes

By using Rh-H catalysis, we couple α-nitroesters and alkynes to prepare α-amino-acid precursors. This atom-economical strategy generates two contiguous stereocenters, with high enantio- and diastereocontrol. In this transformation, the alkyne undergoes isomerization to generate a Rh^III -π-allyl electrophile, which is trapped by an α-nitroester nucleophile. A subsequent reduction with In powder transforms the allylic α-nitroesters to the corresponding α,α-disubstituted α-amino esters.