Coordination-Induced Stereocontrol over Carbocations: Asymmetric Reductive Deoxygenation of Racemic Tertiary Alcohols

Pd-Catalyzed Enantioselective Creation of All-Carbon Quaternary Center with 2,3-Allenylic Carbonates

Enantioselective construction of all-carbon quaternary centers has been achieved via the palladium-catalyzed highly enantioselective allenylation of oxindoles with 2,3-allenylic carbonates to afford a variety of optically active allene products, which contain oxindole units with different functional groups, in high ee. The corresponding synthetic applications have also been demonstrated.

Ph3P/ICH2CH2I-promoted reductive deoxygenation of alcohols

Owing to the ubiquity of the hydroxyl group, reductive deoxygenation of alcohols has become an active research area. The classic Barton-McCombie reaction suffers from a tedious two-step procedure. New efficient methods have been developed, but they have some limitations, such as a narrow substrate scope and the use of moisture-sensitive Lewis acids. In this work, we describe the Ph3P/ICH2CH2I-promoted reductive deoxygenation of alcohols with NaBH4. The process is applicable to benzyl, allyl and propargyl alcohols, and also to primary and secondary alcohols, demonstrating a wide substrate scope and a good level of functional group tolerance. This protocol features convenient operation and low cost of all reagents.

Enantioselective synthesis of α,α-diarylketones by sequential visible light photoactivation and phosphoric acid catalysis

Chiral ketones and their derivatives are useful synthetic intermediates for the synthesis of biologically active natural products and medicinally relevant molecules. Nevertheless, general and broadly applicable methods for enantioenriched acyclic α,α-disubstituted ketones, especially α,α-diarylketones, remain largely underdeveloped, owing to the easy racemization. Here, we report a visible light photoactivation and phosphoric acid-catalyzed alkyne-carbonyl metathesis/transfer hydrogenation one-pot reaction using arylalkyne, benzoquinone, and Hantzsch ester for the expeditious synthesis of α,α-diarylketones with excellent yields and enantioselectivities. In the reaction, three chemical bonds, including C═O, C─C, and C─H, are formed, providing a de novo synthesis reaction for chiral α,α-diarylketones. Moreover, this protocol provides a convenient and practical method to synthesize or modify complex bioactive molecules, including efficient routes to florylpicoxamid and BRL-15572 analogs. Computational mechanistic studies revealed that C-H/π interactions, π-π interaction, and the substituents of Hantzsch ester all play crucial roles in the stereocontrol of the reaction.

Stereodivergent Synthesis of Allenes with α,β-Adjacent Central Chiralities Empowered by Synergistic Pd/Cu Catalysis

The stereodivergent synthesis of allene compounds bearing α,β-adjacent central chiralities has been realized via the Pd/Cu-catalyzed dynamic kinetic asymmetric alkylation of racemic allenylic esters. The matched reactivity of bimetallic catalytic system enables the challenging reaction of racemic aryl-substituted allenylic acetates with sterically crowded aldimine esters smoothly under mild reaction conditions. Various chiral non-natural amino acids bearing a terminal allenyl group are easily synthesized in high yields and with excellent diastereo- and enantioselectivities (up to >20 : 1 dr, >99 % ee). Importantly, all four stereoisomers of the product can be readily accessed by switching the configurations of the two chiral metal catalysts. Furthermore, the easy interconversion between the uncommon η³ -butadienyl palladium intermediate featuring a weak C=C/Pd coordination bond and a stable Csp² -Pd bond is beneficial for the dynamic kinetic asymmetric transformation process (DyKAT).

Role of Noncovalent Interactions in Inducing High Enantioselectivity in an Alcohol Reductive Deoxygenation Reaction Involving a Planar Carbocationic Intermediate

The involvement of planar carbocation intermediates is generally considered undesirable in asymmetric catalysis due to the difficulty in gaining facial control and their intrinsic stability issues. Recently, suitably designed chiral catalyst(s) have enabled a guided approach of nucleophiles to one of the prochiral faces of carbocations affording high enantiocontrol. Herein, we present the vital mechanistic insights from our comprehensive density functional theory (B3LYP-D3) study on a chiral Ir-phosphoramidite-catalyzed asymmetric reductive deoxygenation of racemic tertiary α-substituted allenylic alcohols. The catalytic transformation relies on the synergistic action of a phosphoramidite-modified Ir catalyst and Bi(OTf)₃, first leading to the formation of an Ir-π-allenyl carbocation intermediate through a turn-over-determining S_N1 ionization, followed by a face-selective hydride transfer from a Hantzsch ester analogue to yield an enantioenriched product. Bi(OTf)₃ was found to promote a significant number of ionic interactions as well as noncovalent interactions (NCIs) with the catalyst and the substrates (allenylic alcohol and Hantzsch ester), thus providing access to a lower energy route as compared to the pathways devoid of Bi(OTf)₃. In the nucleophilic addition, the chiral induction was found to depend on the number and efficacy of such key NCIs. The curious case of reversal of enantioselectivity, when the α-substituent of the allenyl alcohol is changed from methyl to cyclopropyl, was identified to originate from a change in mechanism from an enantioconvergent pathway (α-methyl) to a dynamic kinetic asymmetric transformation (α-cyclopropyl). These molecular insights could lead to newer strategies to tame tertiary carbocations in enantioselective reactions using suitable combinations of catalysts and additives.

Construction of Vicinal All-Carbon Stereogenic Centers via Copper-Catalyzed Asymmetric Decarboxylative Propargylation: Enantio- and Diastereoselective Synthesis of Substituted Spirolactones

The formation of vicinal all-carbon quaternary stereocenters remains a formidable challenge. We report herein the synthesis of such highly congested structural dyads by copper-catalyzed decarboxylative propargylation between propargyl carbonates and indanone-based nucleophiles. The implementation of diphenylethylenediamine (DPEN)-based ligands is the key to success. A wide range of functional groups was tolerated, delivering the indanone-based spirolactones in good yields with high diastereo- and enantioselectivity. The mechanistic observations suggest the capability of the new copper complex to enable stereocontrolled addition to copper-allenylidene species.

Catalytic asymmetric C-H insertion reactions of vinyl carbocations

From the preparation of pharmaceuticals to enzymatic construction of natural products, carbocations are central to molecular synthesis. Although these reactive intermediates are engaged in stereoselective processes in nature, exerting enantiocontrol over carbocations with synthetic catalysts remains challenging. Many resonance-stabilized tricoordinated carbocations, such as iminium and oxocarbenium ions, have been applied in catalytic enantioselective reactions. However, their dicoordinated counterparts (aryl and vinyl carbocations) have not, despite their emerging utility in chemical synthesis. We report the discovery of a highly enantioselective vinyl carbocation carbon-hydrogen (C-H) insertion reaction enabled by imidodiphosphorimidate organocatalysts. Active site confinement featured in this catalyst class not only enables effective enantiocontrol but also expands the scope of vinyl cation C-H insertion chemistry, which broadens the utility of this transition metal-free C(sp³)-H functionalization platform.

Asymmetric Synthesis of Structurally Sophisticated Spirocyclic Pyrano[2,3-c]pyrazole Derivatives Bearing a Chiral Quaternary Carbon Center

A carbene-catalyzed enantio- and diastereoselective [2 + 4] cycloaddition reaction is developed for quick and efficient access to structurally complex multicyclic pyrano[2,3-c]pyrazole molecules. The reaction tolerates a broad scope of substrates bearing various substitution patterns, with the multicyclic pyrano[2,3-c]pyrazole products afforded in generally good to excellent yields and optical purities. The chiral molecules obtained from this approach has found promising applications in the development of novel bacteriacides for plant protection.

Stereoinvertive Nucleophilic Substitution at Quaternary Carbon Stereocenters of Cyclopropyl Ketones and Ethers

A highly regio‐ and diastereoselective nucleophilic substitution at the quaternary carbon stereocenter of cyclopropyl ketones and cyclopropyl carbinol derivatives using TMSBr, DMPSCl and TMSN₃ as nucleophiles has been developed. A variety of acyclic tertiary alkyl bromides, chlorides and azides were therefore prepared with excellent diastereopurity. The substitution occurs at the most substituted quaternary carbon center in a stereoinvertive manner, which may be attributed to the existence of a bicyclobutonium species.

Stereoselective Synthesis of Atropisomeric Acridinium Salts by the Catalyst‐Controlled Cyclization of ortho ‐Quinone Methide Iminiums

Quinone methides are fundamental intermediates for a wide range of reactions in which catalyst stereocontrol is often achieved by hydrogen bonding. Herein, we describe the feasibility of an intramolecular Friedel–Crafts 6π electrocyclization through ortho‐quinone methide iminiums stereocontrolled by a contact ion pair. A disulfonimide catalyst activates racemic trichloroacetimidate substrates and imparts stereocontrol in the cyclization step, providing a new avenue for selective ortho‐quinone methide iminium functionalization. A highly stereospecific oxidation readily transforms the enantioenriched acridanes into rotationally restricted acridiniums. Upon ion exchange, the method selectively affords atropisomeric acridinium tetrafluoroborate salts in high yields and an enantioenrichment of up to 93 : 7 e.r. We envision that ion‐pairing catalysis over ortho‐quinone methide iminiums enables the selective synthesis of a diversity of heterocycles and aniline derivatives with distinct stereogenic units.

Iridium-Catalyzed Enantioselective and Chemodivergent Allenylic Alkylation of Vinyl Azides for the Synthesis of α-Allenylic Amides and Ketones

The first enantioselective synthesis of α-allenylic amides and ketones through allenylic alkylation of vinyl azides is reported. In these chemodivergent reactions, cooperatively catalyzed by a Ir^I /(phosphoramidite,olefin) complex and Sc(OTf)₃ , vinyl azides act as the surrogate for both amide enolates and ketone enolates. The desiccant (molecular sieves) plays a crucial role in controlling the chemodivergency of this enantioconvergent and regioselective reaction: Under otherwise identical reaction conditions, the presence of the desiccant led to α-allenylic amides, while its absence resulted in α-allenylic ketones. Utilizing racemic allenylic alcohols as the alkylating agent, the overall process represents a dynamic kinetic asymmetric transformation (DyKAT), where both the products are formed with the same absolute configuration. To the best of our knowledge, this is the first example of the use of vinyl azide as the ketone enolate surrogate in an enantioselective transformation.

Recent advances in iridium-catalyzed enantioselective allylic substitution using phosphoramidite-alkene ligands

This minireview describes the recent progress of iridium-catalyzed enantioselective allylic substitution using phosphoramidite-alkene ligands realizing highly enantioselective carbon–carbon and carbon–heteroatom bond formation.

Catalytic Asymmetric Hydroalkoxylation of C-C Multiple Bonds

Asymmetric hydroalkoxylation of alkenes constitutes a redox-neutral and 100% atom-economical strategy toward enantioenriched oxygenated building blocks from readily available starting materials. Despite their great potential, catalytic enantioselective additions of alcohols across a C-C multiple bond are particularly underdeveloped, especially compared to other hydrofunctionalization methods such as hydroamination. However, driven by some recent innovations, e.g., asymmetric MHAT methods, asymmetric photocatalytic methods, and the development of extremely strong chiral Brønsted acids, there has been a gratifying surge of reports in this burgeoning field. The goal of this review is to survey the growing landscape of asymmetric hydroalkoxylation by highlighting exciting new advances, deconstructing mechanistic underpinnings, and drawing insight from related asymmetric hydroacyloxylation and hydration. A deep appreciation of the underlying principles informs an understanding of the various selectivity parameters and activation modes in the realm of asymmetric alkene hydrofunctionalization while simultaneously evoking the outstanding challenges to the field moving forward. Overall, we aim to lay a foundation for cross-fertilization among various catalytic fields and spur further innovation in asymmetric hydroalkoxylations of C-C multiple bonds.

Organocatalytic enantioselective reactions involving prochiral carbocationic intermediates

Since the discovery of carbocations in 1901, the past 120 years have witnessed many marvelous advances in the chemistry of carbocations. The state-of-the-art research in this field is to overcome the intrinsic instability and high reactivity of the prochiral carbocationic intermediates to develop catalytic asymmetric reactions. Such transformations enable the facile synthesis of structurally diverse value-added products from readily available starting materials such as alkenes, alcohols, and carbonyl derivatives, and enjoy high and even perfect atom-economy in most cases. Notably, such allows catalytic stereoconvergent synthesis from racemic substrates and can realize regioselectivity in olefin functionalization reactions complementary to radical processes. With the rapid developments in modern asymmetric organocatalysis, a variety of highly enantioselective protocols evolving prochiral carbocationic intermediates have been achieved by employing three strategies, namely chiral ion-pairing, chiral nucleophile, or chiral carbenium ion strategy. This feature article aims to summarize the exciting advances in this emerging area and briefly showcase the possible mechanisms. The advantages and limitations of each strategy are presented as well as their synthetic applications in the synthesis of natural products or bioactive compounds.

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

[1,3]-Claisen Rearrangement via Removable Functional Group Mediated Radical Stabilization

A thermal O-to-C [1,3]-rearrangement of α-hydroxy acid derived enol ethers was achieved under mild conditions. The 2-aminothiophenol protection of carboxylic acids facilitates formation of the [1,3] precursor and its thermal rearrangement via stabilization of a radical intermediate. Experimental and theoretical evidence for dissociative radical pair formation, its captodative stability via aminothiophenol, and a unique solvent effect are presented. The aminothiophenol was deprotected from rearrangement products as well as after derivatization to useful synthons.

Strong and Confined Acids Catalyze Asymmetric Intramolecular Hydroarylations of Unactivated Olefins with Indoles

In recent years, several organocatalytic asymmetric hydroarylations of activated, electron-poor olefins with activated, electron-rich arenes have been described. In contrast, only a few approaches that can handle unactivated, electronically neutral olefins have been reported and invariably require transition metal catalysts. Here we show how an efficient and highly enantioselective catalytic asymmetric intramolecular hydroarylation of aliphatic and aromatic olefins with indoles can be realized using strong and confined IDPi Brønsted acid catalysts. This unprecedented transformation is enabled by tertiary carbocation formation and establishes quaternary stereogenic centers in excellent enantioselectivity and with a broad substrate scope that includes an aliphatic iodide, an azide, and an alkyl boronate, which can be further elaborated into bioactive molecules.

Bi(OAc)3/chiral phosphoric acid catalyzed enantioselective allylation of seven-membered cyclic imines, dibenzo[b,f][1,4]oxazepines

An efficient asymmetric allylation reaction of allylboronates with seven-membered cyclic imines, dibenzo[b,f][1,4]oxazepines, is described. The reaction, which is catalyzed by a Bi(OAc)₃/CPA system, gives a range of chiral nitrogen-containing heterocycle structures in high yields and with good enantioselectivities. The conversion of these products to nitrogen-containing heterocycles is also demonstrated.

Catalytic enantiocontrol over a non-classical carbocation

Carbocations can be categorized into classical carbenium ions and non-classical carbonium ions. These intermediates are ubiquitous in reactions of both fundamental and practical relevance, finding application in the petroleum industry as well as the discovery of new drugs and materials. Conveying stereochemical information to carbocations is therefore of interest to a range of chemical fields. While previous studies targeted systems proceeding through classical ions, enantiocontrol over their non-classical counterparts has remained unprecedented. Here we show that strong and confined chiral acids catalyse enantioselective reactions via the non-classical 2-norbornyl cation. This reactive intermediate is generated from structurally different precursors by leveraging the reactivity of various functional groups to ultimately deliver the same enantioenriched product. Our work demonstrates that tailored catalysts can act as suitable hosts for simple, non-functionalized carbocations via a network of non-covalent interactions. We anticipate that the methods described herein will provide catalytic accessibility to valuable carbocation systems.

Chemoselective Homologation-Deoxygenation Strategy Enabling the Direct Conversion of Carbonyls into (n+1)-Halomethyl-Alkanes

The sequential installation of a carbenoid and a hydride into a carbonyl, furnishing halomethyl alkyl derivatives, is reported. Despite the employment of carbenoids as nucleophiles in reactions with carbon-centered electrophiles, sp³-type alkyl halides remain elusive materials for selective one-carbon homologations. Our tactic levers on using carbonyls as starting materials and enables uniformly high yields and chemocontrol. The tactic is flexible and is not limited to carbenoids. Also, diverse carbanion-like species can act as nucleophiles, thus making it of general applicability.

Ir-Catalyzed Enantioconvergent Synthesis of Diversely Protected Allenylic Amines Employing Ammonia Surrogates

The first iridium catalyzed, enantioconvergent amination of allenylic carbonates is reported. This process utilizes various commercially available carbamates and sulfonamides to generate allenylic amines including commonly employed protected groups (Boc, Fmoc, Cbz, Ts, Ns) in 62-82 % yield and 87-98 % ee. The products generated through this scalable procedure serve as effective linchpins for the rapid, enantiospecific synthesis of a wide range of complex structures.

Stereocontrolled Addition of Scrambling ortho-Sulfinyl Carbanions: Easy Access to Homopropargylamines and α-Allenylamines

An unprecedented behavior of ortho-sulfinylpropargyl carbanions in the presence of optically active sulfinylimines affords two different families of compounds: this peculiar chemodivergency is importantly affected by the nature of the employed base, and assisted by the configuration of the electrophile, displaying no alteration in the stereocontrol of both reactions. α-Allenylamines are formed exclusively, using R-sulfinyl aldimines as electrophiles, while homopropargylamines result when S-sulfinyl aldimines are employed.

Enantioselective and regioselective aza-Friedel–Crafts reaction of electron-rich phenols with isatin-derived ketimines

An efficient asymmetric aza-Friedel–Crafts reaction of phenols with isatin-derived ketimines is described.

Iridium-Catalyzed Asymmetric Synthesis of Functionally Rich Molecules Enabled by (Phosphoramidite,Olefin) Ligands

The catalytic, asymmetric synthesis of complex molecules has been a core focus of our research program for some time because developments in the area can have an immediate impact on the identification of novel strategies for the synthesis of value-added molecules. In concert with this central interest, we have emphasized the design of ligand scaffolds as a tactic to discover and develop novel chemistry and overcome well-recognized synthetic challenges. Based on our group's work on chiral pool-derived diolefin ligands, we designed and implemented a class of hybrid (phosphoramidite,olefin) ligands, which combines the properties of both phosphoramidite and olefin motifs to impact, fine-tune, and even override the inherent reactivity of the metal center. Specifically, we have utilized these unique modifying ligands to address several recognized limitations in the field of iridium-catalyzed, asymmetric allylic substitution. The methods we have documented typically employ branched, unprotected allylic alcohols as substrates and obviate the need for rigorous exclusion of air and moisture. Following Takeuchi's seminal report demonstrating the high aptitude of Ir(I)-phosphite catalysts for  branch-selective allylic substitution, concerted efforts from numerous research laboratories have led to a broadening of the synthetic utility of this reaction class. The first section of this Account outlines the process leading to our discovery of an unprecedented (phosphoramidite,olefin) ligand and its validation in the first iridium-catalyzed amination of branched, unprotected allylic alcohols. This section continues with our work involving heteroatom-based nucleophiles within inter- and intramolecular etherification, thioetherification and spiroketalization processes. The second section highlights the use of readily available carbon nucleophiles possessing sp, sp², and sp³ hybridization in a series of enantioselective carbon-carbon bond-forming reactions. We describe how alkylzinc, allylsilane, and several classes of organotrifluoroborate nucleophiles can be coupled enantioselectively to enable construction of several key motifs including 1,5-dienes, 1,4-dienes, and 1,4-enynes. Since the unique electronic and steric properties of this class of ligands renders the (η³-allyl)-Ir(III) intermediate highly electrophilic, even weak nucleophiles such as alkyl olefins can be used. We also show that more nucleophilic alkene motifs such as enamines and in situ generated ketene acetals smoothly participate in substitution reactions with allylic alcohols to yield valuable piperidines and γ,δ-unsaturated esters, respectively. The concept of stereodivergent dual catalysis, which synergistically combines chiral amine catalysis with iridium catalysis to furnish α-allylated aldehydes containing two independently controllable stereocenters is then discussed. This process has enabled the independent, stereoselective synthesis of all four possible product stereoisomers from a single set of starting materials, and was highlighted in the stereodivergent synthesis of Δ⁹-tetrahydrocannabinol. This Account concludes with an overview of our organometallic mechanistic studies regarding relevant intermediates within the catalytic cycle of this class of allylic substitution. These studies have allowed us to better understand the origin of the unique characteristics exhibited by this catalyst in comparison to related systems.

Bi(III)-Catalyzed Enantioselective Allylation Reactions of Ketimines

Chiral homoallylic amines not only are found in pharmaceutically relevant compounds but also serve as versatile building blocks for chemical synthesis. However, catalytic allylation of ketimines with allylboronates, an attractive approach to synthesize chiral homoallylic amine scaffolds remain scarce. Herein, we develop a highly enantioselective allylation of isatin-derived ketimines with boron allylation reagents catalyzed by a Bi(OAc)₃-chiral phosphoric acid catalyst system. The reactions are remarkably efficient and mild, most of which were completed in less than an hour at room temperature with only 1/2 mol% (Bi(OAc)₃/CPA) catalyst loading. A wide range of chiral 3-allyl 3-aminooxindoles were obtained in excellent yields and enantioselectivities. The synthetic utility was demonstrated by efficient formal synthesis of (+)-AG-041R and (−)-psychotriasine. Preliminary mechanism was studied by control experiments and theoretical calculations.

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Asymmetric allylation of ketimines

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Bi(OAc)₃-chiral phosphoric acid catalyst

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Downstream synthetic transformations