Asymmetric Allylic Substitution Reactions Research Articles

Chiral P,Olefin Ligands with Rotamers for Palladium-Catalyzed Asymmetric Allylic Substitution Reactions

We synthesized a series of phosphine–olefin-type chiral aminophosphines, and we confirmed that these each exists as two rotamers at the C(aryl)–N(amine) bond. We also investigated the ability of these aminophosphines to act as chiral ligands for Pd-catalyzed asymmetric allylic substitution reactions, such as the alkylation of allylic acetates with malonates or indoles, and we found they gave high enantio­selectivities (up to 98% ee).

Exploring the Chemistry of Spiroindolenines by Mechanistically-Driven Reaction Development: Asymmetric Pictet-Spengler-type Reactions and Beyond.

The Pictet-Spengler reaction is a fundamental named reaction in organic chemistry, and it is the most straightforward method for the synthesis of tetrahydro-β-carbolines, a core structure embedded in numerous alkaloids. Spiroindolenines are often proposed as possible intermediates in Pictet-Spengler reactions. However, whether the spiroindolenine species is an intermediate in the mechanism of the asymmetric Pictet-Spengler reaction remains unclear. Questions about the role of the spiroindolenine species regarding the mechanism include the following: Can the spiroindolenine species be formed effectively under Pictet-Spengler conditions? If so, what is its fate? Is the delivery of the enantioenriched tetrahydro-β-carboline product related to the spiroindolenine intermediate? Previous studies regarding these questions have not reached a consensus. Therefore, elucidating these questions will advance the field of synthetic organic chemistry.The first highly enantioselective synthesis of spiroindolenines that have the same molecular scaffold as the proposed key intermediate of the Pictet-Spengler reaction was accomplished by an Ir-catalyzed intramolecular asymmetric allylic substitution reaction of an indol-3-yl allylic carbonate. In this reaction, a piperidine, pyrrolidine, or cyclopentane ring can be introduced in conjunction with the indolenine structure.Spiroindolenines were found to undergo ring-expansive migration reactions when treated with a catalytic amount of an acid, leading to tetrahydro-β-carbolines or related tetrahydrocarbazoles. Comprehensive DFT calculations and Born-Oppenheimer molecular dynamics simulations have provided insight into the mechanism of the migration process. It has been found that the stereochemistry is strongly correlated with the electronic properties of the migratory group along with the acidity of the catalyst. Close interactions between the positively charged migratory group and the electron-rich indole ring favor the stereospecificity of the migration. Furthermore, a continuous mechanistic spectrum of the Pictet-Spengler reactions can be obtained on the basis of two readily accessible energetic parameters that are derived from computed energies for competing transition states relative to a key intermediate species. This theoretical model provides a unified mechanistic understanding of the asymmetric Pictet-Spengler reaction, which has been further supported by rationally designed prototype reactions. Chemically and stereochemically controllable migration can be achieved when multiple potential migratory groups are available.The reactivity of spiroindolenines has also been explored beyond Pictet-Spengler reactions. A one-pot Ir-catalyzed asymmetric allylic dearomatization/stereoconvergent migration allows the facile synthesis of enantioenriched tetrahydro-β-carbolines from racemic starting materials. An unprecedented six- to seven-membered ring-expansive migration can be achieved when a vinyliminium moiety is involved as a highly reactive migratory group. This reaction facilitates the stereoselective synthesis of thermodynamically challenging indole-annulated seven-membered rings. It has also been found that the migration process can be interrupted. The electrophilic migratory group released from the retro-Mannich reaction of a spiroindolenine can be captured by an inter- or intramolecular nucleophile, thus providing new entries into structurally diverse polycyclic indole derivatives.Therefore, the mechanism of the Pictet-Spengler reaction can be probed by manipulating the reactivity of the spiroindolenine species. In turn, the mechanistic insights gained herein will aid in chemical transformations toward various target molecules. This study serves as a vivid example of the positive interplay between experimental and theoretical investigations in synthetic organic chemistry.

Direct enantioselective allylic substitution of 4-hydroxycoumarin derivatives with branched allylic alcohols via iridium catalysis.

A highly efficient direct asymmetric allylic substitution (AAS) reaction of 4-hydroxycoumarin derivatives with branched allylic alcohols was realized by combining a chiral iridium complex catalyst with a Lewis acid under mild reaction conditions, delivering various chiral allylation products in remarkably high yields and excellent enantioselectivities. The salient features of this transformation include mild reaction conditions, general substrate scope, good functional group tolerance, high yields, excellent selectivities and easy scale-up. Furthermore, the obtained products can be readily transformed into several kinds of bioactive compounds.

Progress in Iridium-Catalyzed Asymmetric Allylic Substitution Reactions via Synergetic Catalysis

Progress in Iridium-Catalyzed Asymmetric Allylic Substitution Reactions via Synergetic Catalysis

Synthetic approach for optically active polymers through the combination of asymmetric chirogenic polymerization and postpolymerization modification

A new type of asymmetric chirogenic polymerization by asymmetric allylic substitution catalyzed by planar–chiral ruthenium complexes was designed. The polymerization systems function in a highly stereoselective manner to afford optically active polymers with high selectivity. The asymmetric carbon in the main chain is precisely controlled. Each monomer unit of the polymer has a potentially reactive terminal olefin, which can be used for further transformations. Optically active polymers bearing chiral cyclic architecture were prepared by a combination of asymmetric allylic substitution and ring-closing metathesis reaction employing the terminal olefin of the side chains. Additionally, the efficient introduction of a wide range of substituents into the side chain of the optically active polymer without any racemization has been made possible by using the thiol–ene reaction. Poly-N-alkoxyamides obtained by our asymmetric polymerization can be transformed into nonnatural polypeptides containing an aromatic ring on the peptide backbone, called a poly “arylopeptide”, through reductive cleavage of the N–O bond in N-alkoxyamide. The resulting polymer adopts a one-handed stable helical conformation in solution. A new type of asymmetric chirogenic polymerization by asymmetric allylic substitution catalyzed by planar–chiral ruthenium complexes was designed. The polymerization systems function in a highly stereoselective manner. The asymmetric carbon in the main chain is precisely controlled. Each monomer unit of the polymer has a potentially reactive terminal olefin, which can be used for further transformations, ring-closing metathesis and thiol–ene reaction. Additionally, the resulting polymer can be transformed poly “arylopeptide”, through reductive cleavage of the N–O bond in N-alkoxyamide, which adopts a one-handed stable helical conformation in solution.

New Rigid Polycyclic Bis(phosphane) for Asymmetric Catalysis.

A simple, highly efficient synthesis of a series of novel chiral non-racemic rigid tetracyclic phosphorus ligands, applicable in important chemical asymmetric transformations, was performed. In a tandem cross-coupling/C-H bond activation reaction, a well-recognised and readily available ligand (R,R)-NORPHOS was used as the starting material. The palladium complexes of new ligands were obtained and characterised on the example of a crystalline dichloropalladium complex of [(1R,2R,9S,10S,11R,12R)-4-phenyltetracyclo[8.2.1.02,9.03,8]trideca-3,5,7-triene-11,12-diyl]bis(diphenylphosphane). A notably high activity and stereoselectivity of the palladium catalysts based on the new ligands were confirmed in a model asymmetric allylic substitution reaction. Herein, we discuss the geometry of the palladium complexes formed and its impact on the efficiency of the catalysts. A comparison of their geometric features with other bis(phosphane) ligand complexes found in the Cambridge Structural Database and built density functional theory (DFT) commutated models is also presented and rationalised.

Ketones and Aldehydes as O-Nucleophiles in Iridium-Catalyzed Intramolecular Asymmetric Allylic Substitution Reaction.

Ketones and aldehydes are employed as enol O-nucleophiles in an iridium-catalyzed asymmetric allylic substitution reaction. The reaction proceeds well in the presence of a well-defined chiral iridium complex under mild conditions. A series of chiral 2 H-1,4-oxazine skeletons can be obtained in up to 94% yield with 99% ee. The utility of this novel method has been demonstrated by its implementation in the first enantioselective synthesis of (+)-chelonin A.

Iridium-Catalyzed Asymmetric Allylic Substitution Reactions

In this review, we summarize the origin and advancements of iridium-catalyzed asymmetric allylic substitution reactions during the past two decades. Since the first report in 1997, Ir-catalyzed asymmetric allylic substitution reactions have attracted intense attention due to their exceptionally high regio- and enantioselectivities. Ir-catalyzed asymmetric allylic substitution reactions have been significantly developed in recent years in many respects, including ligand development, mechanistic understanding, substrate scope, and application in the synthesis of complex functional molecules. In this review, an explicit outline of ligands, mechanism, scope of nucleophiles, and applications is presented.

Asymmetric Rhodium-Catalyzed Allylic Substitution Reactions: Discovery, Development and Applications to Target-Directed Synthesis.

The transition metal-catalyzed allylic substitution reaction is a particularly versatile method for the construction of carbon-carbon and carbon-heteroatom bonds. In this regard, the rhodium-catalyzed variant has emerged as a powerful method for the regioselective and stereospecific allylic substitution of chiral nonracemic secondary and tertiary allylic carbonates with a variety of carbon- and heteroatom-based nucleophiles. In addition, recent developments have made the analogous enantioselective process possible using prochiral nucleophiles with achiral allylic electrophiles, which represents a significant advance in this area. In this Perspective, the discovery, development and applications of these conceptually orthogonal strategies to target-directed synthesis are discussed, with a particular emphasis given to those methods developed in our laboratory.

THQphos in Ir-catalyzed Asymmetric Allylic Substitution Reactions.

The 1,2,3,4-tetrahydroquinoline phosphoramidites (THQphos) have been found to be efficient ligands for iridium-catalyzed asymmetric allylic substitution reactions. In this short review article, an overall introduction on the development of THQphos, involving their original design, preparation, mechanistic studies, applications and features will be described.

Generation and Application of (Diborylmethyl)zinc(II) Species: Access to Enantioenriched gem-Diborylalkanes by an Asymmetric Allylic Substitution.

We report the successful generation of (diborylmethyl)zinc(II) species by transmetallation beteween isolable (diborylmethyl)lithium and zinc(II) halide (X=Br, Cl) and their application in the synthesis of enantioenriched gem-diborylalkanes bearing a stereogenic center at the β-position of the diboryl groups by an asymmetric allylic substitution reaction. The reaction has a broad substrate scope, and various enantioenriched gem-diborylalkanes can be obtained in good yields with excellent enantioselectivity. Further elaboration of the enantioenriched gem-diborylalkanes provides access to a diverse set of valuable chiral building blocks.

Generation and Application of (Diborylmethyl)zinc(II) Species: Access to Enantioenriched gem‐Diborylalkanes by an Asymmetric Allylic Substitution

AbstractWe report the successful generation of (diborylmethyl)zinc(II) species by transmetallation beteween isolable (diborylmethyl)lithium and zinc(II) halide (X=Br, Cl) and their application in the synthesis of enantioenriched gem‐diborylalkanes bearing a stereogenic center at the β‐position of the diboryl groups by an asymmetric allylic substitution reaction. The reaction has a broad substrate scope, and various enantioenriched gem‐diborylalkanes can be obtained in good yields with excellent enantioselectivity. Further elaboration of the enantioenriched gem‐diborylalkanes provides access to a diverse set of valuable chiral building blocks.

Aminoalkyl-phosphine (P,N) ligands with pentane-2,4-diyl backbone in asymmetric allylic substitution reactions

The asymmetric allylic substitution reaction of rac-1,3-diaryl-2-propenyl acetates with several C- and N-nucleophiles catalyzed by the palladium-complexes of eleven structurally analogous aminoalkyl-phosphines (P,N) with pentane-2,4-diyl backbone is reported. The role of the N-substituents and the influence of the ligand/palladium molar ratio on the activity and enantioselectivity of the catalytic system are studied. The solvent and the temperature dependence of the catalytic reaction were also assessed yielding enantioselectivities up to 95% in alkylation and 90% in amination processes under optimized reaction conditions.

Anilines as C-Nucleophiles in Ir-Catalyzed Intramolecular Asymmetric Allylic Substitution Reactions.

Anilines generally act as N-nucleophiles in transition-metal-catalyzed allylic substitution reactions. In this paper, a highly enantioselective intramolecular Friedel-Crafts-type allylic alkylation of aniline derivatives was realized by using an iridium catalyst derived from [Ir(cod)Cl]2 and (Ra )-BHPphos. Various tetrahydroisoquinilin-5-amines were obtained in moderate to good yields, excellent enantioselectivity and regioselectivity under mild reaction conditions. BHPphos=N-benzhydryl-N-phenyldinaphthophosphoramidite.

Applications of Iridium-Catalyzed Asymmetric Allylic Substitution Reactions in Target-Oriented Synthesis.

Metal catalyzed allylic substitution is a cornerstone of organometallic and synthetic chemistry. Enantioselective versions have been developed with catalysts derived from transition metals, most notably molybdenum, nickel, ruthenium, rhodium, iridium, palladium, and copper. The palladium- and the iridium-catalyzed versions have turned out to be particularly versatile in organic synthesis because of the very broad scope of the nucleophile and great functional group compatibility. Assets of the iridium-catalyzed reaction are the formation of branched, chiral products from simple monosubstituted allylic substrates, high degrees of regio- and enantioselectivity, and use of modular, readily available chiral ligands. The possibility to use carbon, nitrogen, oxygen, and sulfur compounds as well as fluoride as nucleophiles allows a wide range of chiral building blocks to be prepared. Our Account begins with the presentation of fundamental reaction schemes and chiral ligands. We will focus our discussion on reactions promoted by phosphoramidite ligands, though numerous chiral ligands have been employed. The subsequent section presents a brief overview of reaction mechanism and experimental conditions. Two versions of the iridium-catalyzed allylic substitution have emerged. In type 1 reactions (introduced in 1997), linear allylic esters are commonly used as substrates under basic reaction conditions. In type 2 reactions (introduced in 2007), environmentally friendly branched allylic alcohols can be reacted under acidic conditions; occasionally, derivatives of allylic alcohols have also been applied. A unique feature of the type 2 reactions is that highly electrophilic allylic intermediates can be brought to reaction with weakly activated alkenes. The subsequent text is ordered according to the strategies followed to transform allylic substitution products to desired targets, most of which are natural products or drugs. Syntheses starting with an intermolecular allylic substitution are discussed first. Some fairly complex targets, for example, the potent nitric oxide inhibitor (-)-nyasol and the drug (-)-protrifenbute, have been synthesized via less than five steps from simple starting materials. Most targets discussed are cyclic compounds. Intermolecular allylic substitution with subsequent ring closing metathesis is a powerful strategy for their synthesis. Highlights are stereodivergent syntheses of Δ9-tetrahydrocannabinols (THC), wherein iridium- and organocatalysis are combined (dual catalysis). The combination of allylic alkylation with a Diels-Alder reaction was utilized to synthesize the ketide apiosporic acid and the drug fesoterodine (Toviaz). Sequential allylic amination, hydroboration and Suzuki-Miyaura coupling generates enones suitable for conjugate addition reactions; this strategy was employed in syntheses of a variety of alkaloids, for example, the poison frog alkaloid (+)-cis-195A (pumiliotoxin C). Intramolecular substitutions offer interesting possibilities to build up stereochemical complexity via short synthetic routes. For example, in diastereoselective cyclizations of chiral compounds, substrate control can be overruled by catalyst control in order to generate cis- and trans-isomers selectively from a given precursor. This approach was used to prepare a variety of piperidine and pyrrolidine alkaloids. Finally, complex polycyclic structures, including the structurally unusual indolosesquiterpenoid mycoleptodiscin A, have been generated diastereo- and enantioselectively from olefins by polyene cyclizations and from electron-rich arenes, such as indoles, in dearomatization reactions.

Iridium-Catalyzed Asymmetric Umpolung Allylation of N-Fluor­enyl Imines to Prepare 1,4-Disubstituted Homoallylic Amines

The discovery and development of an Ir-catalyzed asymmetric umpolung allylation of imines is discussed here. This method produces 1,4-disubstituted homoallylic amines, a class of compounds that are difficult to access by conventional methods. This reaction proceeds through a sequence involving an allylation and a 2-aza-Cope rearrangement event. The unique mechanistic feature of this reaction could be the reason for its broad substrate scope. The products of this reaction are useful intermediates for various bioactive and natural products. Besides its immediate synthetic utility, we expect this transformation to inspire the development of other umpolung functionalizations of imines and Ir-catalyzed asymmetric allylic substitution (AAS) reactions.1 Introduction2 Reaction Discovery3 Substrate Scope4 Conclusion

Iridium-Catalyzed Intermolecular Asymmetric Dearomatization of β-Naphthols with Allyl Alcohols or Allyl Ethers.

An Ir-catalyzed intermolecular asymmetric dearomatization reaction of β-naphthols with allyl alcohols or allyl ethers was developed. When an iridium catalyst generated from [Ir(COD)Cl]2 (COD=cyclooctadiene) and a chiral P/olefin ligand is employed, highly functionalized β-naphthalenone compounds bearing an all-carbon-substituted quaternary chiral center were obtained in up to 92 % yield and 98 % ee. The direct utilization of allyl alcohols as electrophiles represents an improvement from the viewpoint of atom economy. Allyl ethers were found to undergo asymmetric allylic substitution reaction under Ir catalysis for the first time. The diverse transformations of the dearomatized product to various motifs render this method attractive.

Iridium‐Catalyzed Intermolecular Asymmetric Dearomatization of β‐Naphthols with Allyl Alcohols or Allyl Ethers

AbstractAn Ir‐catalyzed intermolecular asymmetric dearomatization reaction of β‐naphthols with allyl alcohols or allyl ethers was developed. When an iridium catalyst generated from [Ir(COD)Cl]2 (COD=cyclooctadiene) and a chiral P/olefin ligand is employed, highly functionalized β‐naphthalenone compounds bearing an all‐carbon‐substituted quaternary chiral center were obtained in up to 92 % yield and 98 % ee. The direct utilization of allyl alcohols as electrophiles represents an improvement from the viewpoint of atom economy. Allyl ethers were found to undergo asymmetric allylic substitution reaction under Ir catalysis for the first time. The diverse transformations of the dearomatized product to various motifs render this method attractive.

Progress in Iridium-Catalyzed Asymmetric Allylic Substitution Reactions with Allylic Esters

Progress in Iridium-Catalyzed Asymmetric Allylic Substitution Reactions with Allylic Esters

Cooperative bimetallic catalysis in asymmetric allylic substitution.

Synergistic catalysis is gaining increasing attention due to its advantages over traditional catalytic methodologies, such as improved catalytic activity, broader substrate scope, increased selectivity and lower cost. Methodologies involving the synergistic combination of metal catalysts and organocatalysts have been intensively studied. Given the clear benefits of bimetallic catalyst systems consisting of two distinct metal catalysts, cooperative bimetallic catalysis has proved to be successful for a number of difficult asymmetric transformations. This review highlights the recent advances in bimetallic systems for catalytic asymmetric allylic substitution reactions. Strategies using a chiral metal catalyst and the cooperative effect of a second achiral metal catalyst for asymmetric transformations are discussed. Additionally, several challenging asymmetric reactions realized by employing two different chiral metal catalysts in a synergistic manner are also covered.