Iridium-Catalyzed Asymmetric Allylic Substitutions

Iridium-Catalyzed Stereoselective Allylic Alkylation Reactions with Crotyl Chloride

The development of the first enantio-, diastereo-, and regioselective iridium-catalyzed allylic alkylation reaction of prochiral enolates to form an all-carbon quaternary stereogenic center with an aliphatic-substituted allylic electrophile is disclosed. The reaction proceeds with good to excellent selectivity with a range of substituted tetralone-derived nucleophiles furnishing products bearing a newly formed vicinal tertiary and all-carbon quaternary stereodyad. The utility of this protocol is further demonstrated via a number of synthetically diverse product transformations.

An Ir/Zn Dual Catalysis for Enantio- and Diastereodivergent α-Allylation of α-Hydroxyketones

An Ir/Zn dual catalysis has been developed for the enantio- and diastereodivergent α-allylation of unprotected α-hydroxyketones under mild conditions, in the absence of any additional base. The cooperative action of a chiral iridium complex derived from phosphoramidites and a chiral Zn-ProPhenol complex is most likely responsible for its high reactivity, excellent enantioselectivity (up to >99% ee), and good diastereoselectivity (up to >20:1 dr). All four product stereoisomers could be prepared from the same set of starting materials and under identical conditions by simple selection of appropriate catalyst combinations.

Iridium-Catalyzed Regio- and Enantioselective Allylic Substitution of Trisubstituted Allylic Electrophiles

The first Ir-catalyzed enantioselective allylation of trisubstituted allylic electrophiles has been developed. Through modification of the leaving group of allylic electrophiles, we found that trisubstituted allylic phosphates are suitable electrophiles for asymmetric allylation. The reaction of allylic phosphates with enol silanes derived from dioxinones gave allylated products in good yields with high enantioselectivities.

Highly efficient synthesis and stereoselective migration reactions of chiral five-membered aza-spiroindolenines: scope and mechanistic understanding

An Ir-catalyzed asymmetric synthesis of five-membered aza-spiroindolenines is achieved. Based on the detailed investigation of the reaction patterns of the aryl iminium migration, a one-pot asymmetric allylic dearomatization/migration sequence from racemic indole derivatives is realized, affording enantioenriched Pictet-Spengler-type products bearing an additional allylic stereogenic center adjacent to the C3 position of the indole core.

Enantioselective γ-Alkylation of α,β-Unsaturated Malonates and Ketoesters by a Sequential Ir-Catalyzed Asymmetric Allylic Alkylation/Cope Rearrangement

A catalytic, enantioselective γ-alkylation of α,β-unsaturated malonates and ketoesters is reported. This strategy entails a highly regio- and enantioselective iridium-catalyzed α-alkylation of an extended enolate, and a subsequent translocation of chirality to the γ-position via a Cope rearrangement.

Iridium-Catalyzed Diastereoselective and Enantioselective Allylic Substitutions with Acyclic α-Alkoxy Ketones

The asymmetric alkylation of acyclic ketones is a longstanding challenge in organic synthesis. Reported herein are diastereoselective and enantioselective allylic substitutions with acyclic α-alkoxy ketones catalyzed by a metallacyclic iridium complex to form products with contiguous stereogenic centers derived from the nucleophile and electrophile. These reactions occur between allyl methyl carbonates and unstabilized copper(I) enolates generated in situ from acyclic α-alkoxy ketones. The resulting products can be readily converted into enantioenriched tertiary alcohols and tetrahydrofuran derivatives without erosion of enantiomeric purity.

Chemo-, Diastereo-, and Enantioselective Iridium-Catalyzed Allylic Intramolecular Dearomatization Reaction of Naphthol Derivatives

An iridium-catalyzed intramolecular asymmetric allylic dearomatization reaction of naphthol derivatives is described. Challenges confronted in this reaction include chemoselectivity between carbon and oxygen atoms as nucleophilic centers, diastereoselectivity when contiguous chiral centers are generated, and enantioselective control for constructing an all-carbon quaternary stereocenter. In the presence of an iridium catalyst generated from [{Ir(dbcot)Cl}2] (dbcot=dibenzocyclooctatetraene) and a new THQphos (tetrahydroquinolinedinaphthophosphoramidite) ligand, various spironaphthalenones were obtained with up to greater than 95:5 C/O selectivity, greater than 95:5 d.r., and 99 % ee, thus providing a general method for the dearomatization of naphthols.

Enantioselective synthesis of fluorinated branched allylic compounds via Ir-catalyzed allylations of functionalized fluorinated methylene derivatives

Introduction of the functionalized monofluorinated methylenes into the allylic fragment under Ir catalysis gave the fluorinated branched allyl products in good to high yields with excellent regio- and enantioselectivities.

Iridium-Catalyzed Intramolecular Asymmetric Allylic Dearomatization Reaction of Pyridines, Pyrazines, Quinolines, and Isoquinolines

The first Ir-catalyzed intramolecular asymmetric allylic dearomatization reaction of pyridines, pyrazines, quinolines, and isoquinolines has been developed. Enabled by in situ formed chiral Ir-catalyst, the dearomatized products were isolated in high levels of yield (up to 99% yield) and enantioselectivity (up to 99% ee). It is worth noting that the Me-THQphos ligand is much more efficient than other tested ligands for the dearomatization of pyrazines and certain quinolines. Mechanistic studies of the dearomatization reaction were carried out, and the results suggest the feasibility of an alternative process which features the formation of a quinolinium as the key intermediate. The mechanistic findings render this reaction a yet unknown type in the chemistry of Reissert-type reactions. In addition, the utility of this method was showcased by a large-scale reaction and formal synthesis of (+)-gephyrotoxin.

Origins of Regioselectivity in Iridium Catalyzed Allylic Substitution

Detailed studies on the origin of the regioselectivity for formation of branched products over linear products have been conducted with complexes containing the achiral triphenylphosphite ligand. The combination of iridium and P(OPh)3 was the first catalytic system shown to give high regioselectivity for the branched product with iridium and among the most selective for forming branched products among any combination of metal and ligand. We have shown the active catalyst to be generated from [Ir(COD)Cl]2 and P(OPh)3 by cyclometalation of the phenyl group on the ligand and have shown such species to be the resting state of the catalyst. A series of allyliridium complexes ligated by the resulting P,C ligand have been generated and shown to be competent intermediates in the catalytic system. We have assessed the potential impact of charge, metal-iridium bond length, and stability of terminal vs internal alkenes generated by attack at the branched and terminal positions of the allyl ligand, respectively. These factors do not distinguish the regioselectivity for attack on allyliridium complexes from that for attack on allylpalladium complexes. Instead, detailed computational studies suggest that a series of weak, attractive, noncovalent interactions, including interactions of H-bond acceptors with a vinyl C-H bond of the alkene ligand, favor formation of the branched product with the iridium catalyst. This conclusion underscores the importance of considering attractive interactions, as well as repulsive steric interactions, when seeking to rationalize selectivities.

Iridium-Catalyzed Enantioselective Allylic Substitution of Enol Silanes from Vinylogous Esters and Amides

The enol silanes of vinylogous esters and amides are classic dienes for Diels-Alder reactions. Here, we report their reactivity as nucleophiles in Ir-catalyzed, enantioselective allylic substitution reactions. A variety of allylic carbonates react with these nucleophiles to give allylated products in good yields with high enantioselectivities and excellent branched-to-linear ratios. These reactions occur with KF or alkoxide as the additive, but mechanistic studies suggest that these additives do not activate the enol silanes. Instead, they serve as bases to promote the cyclometalation to generate the active Ir catalyst. The carbonate anion, which was generated from the oxidative addition of the allylic carbonate, likely activates the enol silanes to trigger their activity as nucleophiles for reactions with the allyliridium electrophile. The synthetic utility of this method was illustrated by the synthesis of the anti-muscarinic drug, fesoterodine.

Enantioselective Construction of Spiroindolines with Three Contiguous Stereogenic Centers and Chiral Tryptamine Derivatives via Reactive Spiroindolenine Intermediates

The highly efficient synthesis of the enantioenriched spiroindolines by iridium-catalyzed asymmetric allylic dearomatization and reduction is presented. Spiroindolines containing three contiguous stereogenic centers were obtained with excellent diastereo- and enantioselectivity. In addition, a chiral tryptamine derivative could be easily accessed in good yield with excellent ee value through an unprecedented dearomatization/retro-Mannich/hydrolysis cascade reaction of an indole derivative.

Design of Modified Amine Transfer Reagents Allows the Synthesis of α-Chiral Secondary Amines via CuH-Catalyzed Hydroamination

The CuH-catalyzed hydroamination of alkenes and alkynes using a silane and an amine transfer reagent represents a simple strategy to access chiral amine products. We have recently reported methods to prepare chiral amines with high efficiency and stereoselectivity using this approach. However, the current technology is limited to the synthesis of trialkylamines from dialkylamine transfer reagents (R2NOBz). When monoalkylamine transfer reagents [RN(H)OBz] were used for the synthesis of chiral secondary amines, competitive, nonproductive consumption of these reagents by the CuH species resulted in poor yields. In this paper, we report the design of a modified type of amine transfer reagent that addresses this limitation. This effort has enabled us to develop a CuH-catalyzed synthesis of chiral secondary amines using a variety of amine coupling partners, including those derived from amino acid esters, carbohydrates, and steroids. Mechanistic investigations indicated that the modified amine transfer reagents are less susceptible to direct reaction with CuH.

Ligand-enabled Ir-catalyzed intermolecular diastereoselective and enantioselective allylic alkylation of 3-substituted indoles

An Ir-catalyzed asymmetric allylic alkylation of 3-substituted indoles is reported. The reaction provides indoline products containing multiple contiguous stereocenters with high site-, regio-, diastereo- and enantioselectivities in one step from a wide range of readily available starting materials. The key to this method is the high level of diastereocontrol enabled by an iridium catalyst derived from a N-aryl phosphoramidite ligand (Me-THQphos, 1c).

Rhodium-Catalyzed Enantioselective Hydroamination of Alkynes with Indolines

The hydroamination of internal alkynes via tandem rhodium catalysis gives branched N-allylic indolines with high regio- and enantioselectivity. An acid switch provides access to the linear isomer in preference to the branched isomer by an isomerization mechanism. Mechanistic studies suggest formation of an allene intermediate, which undergoes hydroamination to generate allylic amines instead of the enamine or imine products typically observed in alkyne hydroaminations.

Enantioselective Synthesis of Pyrrole-Based Spiro- and Polycyclic Derivatives by Iridium-Catalyzed Asymmetric Allylic Dearomatization and Controllable Migration Reactions

The first highly diastereo- and enantioselective synthesis of five-membered spiro-2H-pyrroles was achieved using an Ir-catalyzed asymmetric allylic dearomatization reaction. The spiro-2H-pyrrole derivatives readily undergo a controllable and stereospecific allylic migration under acid catalysis, providing polycyclic pyrrole derivatives in excellent yields and ee values. Additionally, the novel Ir-complex K1, derived from [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene) and N-benzhydryl-N-phenyldinaphthophosphoramidite (BHPphos), showed excellent control of both diastereo- and enantioselectivities.

Catalytic enantioselective construction of quaternary stereocenters: assembly of key building blocks for the synthesis of biologically active molecules

The ever-present demand for drugs with better efficacy and fewer side effects continually motivates scientists to explore the vast chemical space. Traditionally, medicinal chemists have focused much attention on achiral or so-called "flat" molecules. More recently, attention has shifted toward molecules with stereogenic centers since their three-dimensional structures represent a much larger fraction of the chemical space and have a number of superior properties compared with flat aromatic compounds. Quaternary stereocenters, in particular, add greatly to the three-dimensionality and novelty of the molecule. Nevertheless, synthetic challenges in building quaternary stereocenters have largely prevented their implementation in drug discovery. The lack of effective and broadly general methods for enantioselective formation of quaternary stereocenters in simple molecular scaffolds has prompted us to investigate new chemistry and develop innovative tools and solutions. In this Account, we describe three approaches to constructing quaternary stereocenters: nucleophilic substitution of 3-halooxindoles, conjugate addition of boronic acids to cyclic enones, and allylic alkylation of enolates. In the first approach, malonic ester nucleophiles attack electrophilic 3-halooxindoles, mediated by a copper(II)-bisoxazoline catalyst. A variety of oxindoles containing a benzylic quaternary stereocenter can be accessed through this method. However, it is only applicable to the specialized 3,3-disubstituted oxindole system. To access benzylic quaternary stereocenters in a more general context, we turned our attention to the enantioselective conjugate addition of carbon nucleophiles to α,β-unsaturated carbonyl acceptors. We discovered that in the presence of catalytic palladium-pyridinooxazoline complex, arylboronic acids add smoothly to β-substituted cyclic enones to furnish ketones with a β-benzylic quaternary stereocenter in high yields and enantioselectivities. The reaction is compatible with a wide range of arylboronic acids, β-substituents, and ring sizes. Aside from benzylic quaternary stereocenters, a more challenging motif is a quaternary stereocenter not adjacent to an aromatic group. Such centers represent more general structures in chemical space but are more difficult to form by asymmetric catalysis. To address this greater challenge, and motivated by the greater reward, we entered the field of palladium-catalyzed asymmetric allylic alkylation of prochiral enolate nucleophiles about a decade ago. On the basis of Tsuji's work, which solved the issue of positional selectivity for unsymmetrical ketones, we discovered that the phosphinooxazoline ligand effectively rendered this reaction enantioselective. Extensive investigations since then have revealed that the reaction exhibits broad scope and accepts a range of substrate classes, each with its unique advantage in synthetic applications. A diverse array of carbonyl compounds bearing α-quaternary stereocenters are obtained in excellent yields and enantioselectivities, and more possibilities have yet to be explored. As an alternative to palladium catalysis, we also studied iridium-catalyzed asymmetric allylic alkylations that generate vicinal quaternary and tertiary stereocenters in a single transformation. Overall, these methods provide access to small molecule building blocks with a single quaternary stereocenter, can be applied to various molecular scaffolds, and tolerate a wide range of functional groups. We envision that the chemistry reported in this Account will be increasingly useful in drug discovery and design.

Iridium-catalyzed enantioselective allylation of silyl enol ethers derived from ketones and α,β-unsaturated ketones

The unified Ir-catalyzed enantioselective allylic substitution reactions of silyl enol ethers derived from ketones and α,β-unsaturated ketones with branched, racemic allylic alcohols are described.

A unified lead-oriented synthesis of over fifty molecular scaffolds

Sourcing large numbers of lead-like compounds is a major challenge; a unified synthetic approach enabled the efficient synthesis of 52 diverse lead-like molecular scaffolds from just 13 precursors.

Enantioselective construction of C-chiral allylic sulfilimines via the iridium-catalyzed allylic amination with S,S-diphenylsulfilimine: asymmetric synthesis of primary allylic amines

We have devised a highly regio- and enantioselective iridium-catalyzed allylic amination reaction with the sulfur-stabilized aza-ylide, S,S-diphenylsulfilimine. This process provides a robust and scalable method for the construction of aryl-, alkyl- and alkenyl-substituted C-chiral allylic sulfilimines, which are important functional groups for organic synthesis. Additionally, the combination of the allylic amination with an in situ deprotection of the sulfilimine constitutes a convenient one-pot protocol for the construction of chiral nonracemic primary allylic amines.

Diastereo- and enantioselective iridium-catalyzed allylation of cyclic ketone enolates: synergetic effect of ligands and barium enolates

We report asymmetric allylic alkylation of barium enolates of cyclic ketones catalyzed by a metallacyclic iridium complex containing a phosphoramidite ligand derived from (R)-1-(2-naphthyl)ethylamine. The reaction products contain adjacent quaternary and tertiary stereocenters. This process demonstrates that unstabilized cyclic ketone enolates can undergo diastereo- and enantioselective Ir-catalyzed allylic substitution reactions with the proper choice of enolate countercation. The products of these reactions can be conveniently transformed to various useful polycarbocyclic structures.

Iridium-catalyzed regio- and enantioselective allylic substitution of silyl dienolates derived from dioxinones

Reported herein is the iridium-catalyzed regio- and enantioselective allylic substitution reactions of unstabilized silyl dienolates derived from dioxinones. Asymmetric allylic substitution of a variety of allylic trichloroethyl carbonates with these silyl dienolates gave γ-allylated products selectively in 60-84% yield and 90-98% ee.

Transition-metal-catalyzed asymmetric allylic dearomatization reactions

Dearomatization reactions serve as powerful methods for the synthesis of highly functionalized, three-dimensional structures starting with simple planar aromatic compounds. Among processes of this type, catalytic asymmetric dearomatization (CADA) reactions are attractive owing to the large number of aromatic compounds that are readily available and the fact that they enable direct access to enantiopure polycycles and spirocycles, which frequently are key structural motifs in biologically active natural products and pharmaceuticals. However, as a consequence of their high stabilities, arenes only difficultly participate in dearomatization reactions that take place with high levels of enantioselectivity. Transition-metal-catalyzed asymmetric allylic substitution reactions have been demonstrated to be powerful methods for enantioselective formation of C-C and C-X (X = O, N, S, etc.) bonds. However, the scope of these processes has been explored mainly using soft carbon nucleophiles, some hard carbon nucleophiles such as enolates and preformed organometallic reagents, and heteroatom nucleophiles. Readily accessible aromatic compounds have been only rarely used directly as nucleophiles in these reactions. In this Account, we present the results of studies we have conducted aimed at the development of transition-metal-catalyzed asymmetric allylic dearomatization reactions. By utilizing this general process, we have devised methods for direct dearomatization of indoles, pyrroles, phenols, naphthols, pyridines, and pyrazines, which produce various highly functionalized structural motifs bearing all-carbon quaternary stereogenic centers in a straightforward manner. In mechanistic investigations of the dearomatization process, we found that the five-membered spiroindolenines serve as intermediates, which readily undergo stereospecific allylic migration to form corresponding tetrahydro-1H-carbazoles upon treatment with a catalytic amount of TsOH. It is worth noting that no notable loss of the enantiomeric excess of the spiroindolenine derivatives takes place during the rearrangement process as a consequence of the intervention of a "three-center-two-electron"-type transition state, a proposal that has gained support from the results of DFT calculations. Equally intriguing, upon tuning of the electronic nature of the tethers, pyrroles or indoles undergo unprecedented Ir or Ru catalyzed intramolecular allylic alkylation promoted dearomatization/migration reactions. The operation of this novel reaction pathway provides additional information leading to a greater mechanistic understanding of the transition-metal-catalyzed enantioselective intramolecular functionalizations of pyrroles and indoles. The combined results of this effort provide not only methods for the efficient synthesis of highly enantioenriched fused and spiro polycycles but also novel strategies in the field of asymmetric catalysis.

Iridium-catalyzed enantioselective allylic substitution of unstabilized enolates derived from α,β-unsaturated ketones

We report Ir-catalyzed, enantioselective allylic substitution reactions of unstabilized silyl enolates derived from α,β-unsaturated ketones. Asymmetric allylic substitution of a variety of allylic carbonates with silyl enolates gave allylated products in 62-94% yield with 90-98% ee and >20:1 branched-to-linear selectivity. The synthetic utility of this method was illustrated by the short synthesis of an anticancer agent, TEI-9826.