Allylic Electrophiles Research Articles

Difluoroenoxysilanes in Catalytic Asymmetric Allylic Alkylation.

An allylic substitution with difluoroenoxysilanes as the nucleophile is accomplished for the enantioselective synthesis of α-allylic α,α-difluoroketones. With racemic branched allylic alcohols as the easily accessible allylic electrophile, this branched-selective and enantioconvergent allylic alkylation reaction is catalyzed by an Ir(I)/(P,olefin) complex and overcomes the low nucleophilicity of difluoroenoxysilanes to furnish β-chiral α,α-difluoroketones in moderate to good yields with high enantioselectivity (up to >99.9:0.1 er).

Molybdenum Complex-Catalyzed N-Alkylation of Bulky Primary and Secondary Amines.

Aliphatic allylic amines are present in a large number of complex and pharmaceutically relevant molecules. The direct amination of allylic electrophiles serves as the most common method toward the preparation of these motifs. However, the use of feedstock reaction components (allyl alcohol and aliphatic amine) in these transformations remains a great challenge. Such a challenge primarily stems from the high Lewis basicity and large steric hindrance of aliphatic amines, in addition to the low reactivity of allylic alcohols. Herein, we report a general solution to these challenges. The developed protocol allows an efficient allylic amination of allyl alcohols with sterically bulky aliphatic amines in the presence of an inexpensive earth-abundant molybdenum complex. This simple and economic protocol also enables regioselective branched amination; the practicality of the reaction was shown in an efficient, scaled-up synthesis of several drugs.

B/Pd Synergistic Catalysis for the Decarboxylative Allylation of 2-(2-Azaaryl)acetic Acids.

We describe an allylation reaction between 2-(2-azaaryl)acetic acids and allylic electrophiles catalyzed synergistically by a dual system consisting of borinic acid and a Pd complex under mild conditions. The decarboxylative allylation proceeds via a boron-bound enamine intermediate, which then interacts with a π-allylpalladium intermediate from the allylic electrophile. High yields of diallylation products highlight the method's efficiency. Intriguingly, when using 2-(2-pyridyl)acetic acid with a C3 substituent on the pyridyl ring, the reaction exclusively yields monoallylation products.

Palladium-Catalyzed Ligand-Enabled Cyclization of Substituted Aliphatic Carboxylic Acids with Allylic Electrophiles.

A palladium-catalyzed cyclization of the β-C(sp3)-H bond of aliphatic carboxylic acids with allylic electrophiles providing five-membered γ-lactones in good to excellent yields is demonstrated. An acetyl-protected aminoethyl phenyl thioether ligand is used to promote the C-H activation reaction. A diverse range of allylic electrophiles such as allyl alcohols, allyl acetates, allyl sulfones, allyl phosphonate, allyl amine, and allyl ester have been utilized for this reaction. A feasible reaction mechanism has been proposed to account for the present cyclization reaction.

Desymmetrization of Cyclic Sulfonimidamides by Asymmetric Allylation

AbstractHerein we report the first transition metal‐catalyzed approach to the enantioenriched synthesis of cyclic sulfonimidamides relying on commercially available palladium catalysts and ligands. High‐throughput experimentation (HTE) was employed to identify the optimal catalyst system and solvent. The method is applied to a variety of saturated and unsaturated rings and exhibits the highest selectivity for 2‐substituted allyl electrophiles. The products are further elaborated to complex, tricyclic scaffolds. DFT experiments presented herein highlight the key ligand substrate interactions leading to the high levels of enantioselectivity.

Desymmetrization of Cyclic Sulfonimidamides by Asymmetric Allylation.

Herein we report the first transition metal-catalyzed approach to the enantioenriched synthesis of cyclic sulfonimidamides relying on commercially available palladium catalysts and ligands. High-throughput experimentation (HTE) was employed to identify the optimal catalyst system and solvent. The method is applied to a variety of saturated and unsaturated rings and exhibits the highest selectivity for 2-substituted allyl electrophiles. The products are further elaborated to complex, tricyclic scaffolds. DFT experiments presented herein highlight the key ligand substrate interactions leading to the high levels of enantioselectivity.

Dual-Catalysis-Enabled Construction of Vicinal Stereogenic Centers through Diastereo- and Enantioselective Allylic Substitution

AbstractVicinal stereogenic centers represent prevalent structural motifs in organic synthetic chemistry, and their construction poses a longstanding challenge. Transition-metal-catalyzed asymmetric allylic substitution has become a well-established enantioselective C–C bond-forming reaction. When these reactions involve a prochiral nucleophile and an allylic electrophile with a terminal substituent, the creation of vicinal stereogenic centers becomes feasible. However, despite remarkable achievements having been accomplished, realizing this transformation with precise control over both the enantio- and diastereoselectivity remains a significant challenge. To address the stereoselective challenges, the introduction of a second catalyst to the transition-metal-catalyzed asymmetric allylic alkylation to control the diastereoselectivity during C–C bond formation has proven particularly fruitful. In this short review, we aim to highlight recent advances in dual catalysis that enable diastereo- and enantioselective allylic substitutions.1 Introduction2 Construction of Vicinal Stereogenic Centers by Organo and Metal Dual Catalysis2.1 Chiral Phase-Transfer Catalysis and Transition-Metal Dual Catalysis2.2 Chiral Amine and Transition-Metal Dual Catalysis2.3 NHC and Transition-Metal Dual Catalysis2.4 Chiral Aldehyde and Transition-Metal Dual Catalysis2.5 Chiral Lewis Base and Transition-Metal Dual Catalysis3 Construction of Vicinal Stereogenic Centers by Metal and Metal Dual Catalysis3.1 Lewis Acidic Metal and Iridium Dual Catalysis3.2 Lewis Acidic Metal and Palladium Dual Catalysis3.3 Palladium and Ruthenium Dual Catalysis3.4 Other Advancements in the Construction of Vicinal Stereogenic Centers through Synergistic Bimetallic Catalysis Enabling Asymmetric Allylic Alkylation4 Conclusions and Future Outlook

Interception of Transient Allyl Radicals with Low-Valent Allylpalladium Chemistry: Tandem Pd(0/II/I)-Pd(0/II/I/II) Cycles in Photoredox/Pd Dual-Catalytic Enantioselective C(sp3)-C(sp3) Homocoupling.

We present comprehensive computational and experimental studies on the mechanism of an asymmetric photoredox/Pd dual-catalytic reductive C(sp3)-C(sp3) homocoupling of allylic electrophiles. In stark contrast to the canonical assumption that photoredox promotes bond formation via facile reductive elimination from high-valent metal-organic species, our computational analysis revealed an intriguing low-valent allylpalladium pathway that features tandem operation of Pd(0/II/I)-Pd(0/II/I/II) cycles. Specifically, we propose that (i) the photoredox/Pd system enables the in situ generation of allyl radicals from low-valent Pd(I)-allyl species, and (ii) effective interception of the fleeting allyl radical by the chiral Pd(I)-allyl species results in the formation of an enantioenriched product. Notably, the cooperation of the two pathways highlights the bifunctional role of Pd(I)-allyl species in the generation and interception of transient allyl radicals. Moreover, the mechanism implies divergent substrate-activation modes in this homocoupling reaction, suggesting a theoretical possibility for cross-coupling. Combined, the current study offers a novel mechanistic hypothesis for photoredox/Pd dual catalysis and highlights the use of low-valent allylpalladium as a means to efficiently intercept radicals for selective asymmetric bond constructions.

Chelation enables selectivity control in enantioconvergent Suzuki–Miyaura cross-couplings on acyclic allylic systems

Asymmetric Suzuki–Miyaura cross-couplings with aryl boronic acids and allylic electrophiles are a powerful method to convert racemic mixtures into enantioenriched products. Currently, enantioconvergent allylic arylations are limited to substrates that are symmetrical about the allylic unit, and the absence of strategies to control regio-, E/Z- and enantioselectivity in acyclic allylic systems is a major restriction. Here, using a system capable of either conjugate addition or allylic arylation, we have discovered the structural features and experimental conditions that allow an acyclic system to undergo chemo- and regioselective, enantioconvergent allylic Suzuki–Miyaura-type arylation. A wide variety of boronic acid coupling partners can be used, and both alkyl and aromatic substituents are tolerated on the allylic unit so that a wide variety of structures can be obtained. Preliminary mechanistic studies reveal that the chelating ability of the ester group is crucial to obtaining high regio- and enantioselectivity. Using this method, we were able to synthesize the natural products (S)-curcumene and (S)-4,7-dimethyl-1-tetralone and the clinically used antidepressant sertraline (Zoloft).

Z-Retentive Asymmetric Allylic Substitution Reactions of Aldimine Esters under Ru/Cu Dual Catalysis.

Ru/Cu dual catalysis has been applied for Z-retentive asymmetric allylic substitution reactions of aldimine esters. This reaction provides an enantioselective synthesis of chiral Z-olefins in high yields (up to 91% yield) with excellent enantioselectivity (up to 98% ee) under mild conditions. The previously unreacted trisubstituted allylic electrophiles under Ir catalytic system are found to be compatible, affording the stereoretentive products in either Z- or E-form. Both linear and branched allylic electrophiles are suitable substrates with excellent reaction outcomes. Notably, Ru and Cu complexes are added in one-pot and simplifies the manipulation of this protocol and self-sorting phenomena could be observed in this Ru/Cu dual catalytic system.

Direct Regioselective Reductive Allylation of Imines: Application to Synthesis of Oxazines with Halogenated Reagent.

Oxazines are an important class of compounds in oxazine ligands and medical chemistry. Here, we describe a linear-selective allylation of imines with allyl electrophiles via cross-electrophile coupling reactions, followed by cyclization with halogenated reagents, providing a new strategy to afford oxazine compounds with a tetrasubstituted carbon center. Mechanistic studies indicate that α-amino carbanion, generated by successive single-electron transfer processes, is a key intermediate for nucleophile attack on π-allylpalladium in photoredox/palladium catalysis.

Cooperative Bimetallic Catalysis via One-Metal/Two-Ligands: Mechanistic Insights of Polyfluoroarylation-Allylation of Diazo Compounds.

Metal/ligand in situ assembly is often crucial for tailoring the reactivity & selectivity in transition metal catalysis. Although cooperative bimetallic catalysis has been applied as a powerful strategy in new reaction development, cooperative catalysis via a single metal/two ligands is still underdeveloped, since it is rather challenging to harness the distinct reactivity profiles of the species generated by self-assembly of a single metal precursor with a mixture of different ligands. Herein, we report a catalytic system composed of a single metal/two ligands for a three-component reaction of polyfluoroarene, α-diazo ester, and allylic electrophile, leading to highly efficient construction of densely functionalized quaternary carbon centers, that are otherwise hardly accessible. Mechanistic studies suggest this reaction follows a cooperative bimetallic pathway via two catalysts with distinct reactivity profiles, which are assembled in situ from a single metal precursor and two ligands and work in concert to escort the transformation.

Cooperative Bimetallic Catalysis via One‐Metal/Two‐Ligands: Mechanistic Insights of Polyfluoroarylation‐Allylation of Diazo Compounds

AbstractMetal/ligand in situ assembly is crucial for tailoring the reactivity & selectivity in transition metal catalysis. Cooperative catalysis via a single metal/two ligands is still underdeveloped, since it is rather challenging to harness the distinct reactivity profiles of the species generated by self‐assembly of a single metal precursor with a mixture of different ligands. Herein, we report a catalytic system composed of a single metal/two ligands for a three‐component reaction of polyfluoroarene, α‐diazo ester, and allylic electrophile, leading to highly efficient construction of densely functionalized quaternary carbon centers, that are otherwise hardly accessible. Mechanistic studies suggest this reaction follows a cooperative bimetallic pathway via two catalysts with distinct reactivity profiles, which are assembled in situ from a single metal precursor and two ligands and work in concert to escort the transformation.

Iridium-Catalyzed Enantioselective Formal α-Allylic Alkylation of Acrylonitrile.

A highly enantioselective formal α-allylic alkylation of acrylonitrile is developed using 4-cyano-3-oxotetrahydrothiophene (c-THT) as a safe and easy-to-handle surrogate of acrylonitrile. This two-step process consists of an Ir(I)/(P,olefin)-catalyzed branched-selective allylic alkylation using easily accessible branched rac-allylic alcohols as the allylic electrophile followed by retro-Dieckmann/retro-Michael fragmentation and is shown to be applicable for the enantioselective synthesis of α-allylic acrylates as well as α-allylic acrolein.

Regioselective Formal β-Allylation of Carbonyl Compounds Enabled by Cooperative Nickel and Photoredox Catalysis.

The Tsuji-Trost reaction between carbonyl compounds and allylic precursors has been widely used in the synthesis of natural products and pharmaceutical compounds. As the α-C-H bond is far more acidic than the β-C-H bond, carbonyl compounds undergo highly regioselective allylation at the α-position and their β-allylation is therefore highly challenging. This innate α-reactivity conversely hampers diversity, especially if the corresponding β-allylation product is targeted. Herein, we present a formal intermolecular β-C-C bond formation reaction of a broad range of aldehydes and ketones with different allyl electrophiles through cooperative nickel and photoredox catalysis. β-Selectivity is achieved via initial transformation of the aldehydes and ketones to their corresponding silyl enol ethers. The overall transformation features mild conditions, excellent regioselectivity, wide functional group tolerance and high reaction efficiency. The introduced facile and regioselective β-allylation of carbonyl compounds proceeding through cooperative catalysis allows the preparation of valuable building blocks that are difficult to access from aldehydes and ketones using existing methodology.

Regioselective Formal β‐Allylation of Carbonyl Compounds Enabled by Cooperative Nickel and Photoredox Catalysis

AbstractThe Tsuji–Trost reaction between carbonyl compounds and allylic precursors has been widely used in the synthesis of natural products and pharmaceutical compounds. As the α‐C−H bond is far more acidic than the β‐C−H bond, carbonyl compounds undergo highly regioselective allylation at the α‐position and their β‐allylation is therefore highly challenging. This innate α‐reactivity conversely hampers diversity, especially if the corresponding β‐allylation product is targeted. Herein, we present a formal intermolecular β‐C−C bond formation reaction of a broad range of aldehydes and ketones with different allyl electrophiles through cooperative nickel and photoredox catalysis. β‐Selectivity is achieved via initial transformation of the aldehydes and ketones to their corresponding silyl enol ethers. The overall transformation features mild conditions, excellent regioselectivity, wide functional group tolerance and high reaction efficiency. The introduced facile and regioselective β‐allylation of carbonyl compounds proceeding through cooperative catalysis allows the preparation of valuable building blocks that are difficult to access from aldehydes and ketones using existing methodology.

Palladium-catalyzed annulative allylic alkylation for regioselective construction of indole-fused medium-sized cyclic ethers

A new palladium-catalyzed annulative allylic alkylation (AAA) reaction of 2-(indol-2-yl)phenols with dual allylic electrophiles such as isobutylene dicarbonate and butene dicarbonate is described, leading to the regioselective synthesis of tetracyclic medium-sized cyclic ethers possessing a bridged aryl-indole scaffold, namely, benzo[2,3]oxocino[4,5-b]indoles and benzo[2,3]oxepino[4,5-b]indoles, in good to excellent yields. This protocol demonstrates a broad substrate scope, good compatibility with substituents and high regioselectivity, providing a catalytic and flexible method for creating bridged aryl-indole skeletons.

Enantio- and Regioconvergent Synthesis of γ-Stereogenic Vinyl Germanes and Their Use as Masked Vinyl Halides.

A nickel-catalyzed enantio- and regioconvergent alkylation of regioisomeric mixtures of racemic germylated allylic electrophiles with alkyl nucleophiles is reported. Key to success is a newly developed hept-4-yl-substituted Pybox ligand that enables accessing various chiral γ-germyl α-alkyl allylic building blocks in excellent yields and enantioselectivities. The reason for the regioconvergence is the steering effect of the bulky germyl group. The resulting vinyl germanes can be easily halodegermylated without racemization of the allylic stereocenter to afford synthetically valuable γ-stereogenic vinyl halides.

Iridium‐Catalyzed Chemo‐, Diastereo‐, and Enantioselective Allyl‐Allyl Coupling: Accessing All Four Stereoisomers of (E)‐1‐Boryl‐Substituted 1,5‐Dienes by Chirality Pairing

AbstractHere, we report a highly chemo‐, diastereo‐, and enantioselective allyl‐allyl coupling between branched allyl alcohols and α‐silyl‐substituted allylboronate esters, catalyzed by a chiral iridium complex. The α‐silyl‐substituted allylboronate esters can be chemoselectively coupled with allyl electrophiles, affording a diverse set of enantioenriched (E)‐1‐boryl‐substituted 1,5‐dienes in good yields, with excellent stereoselectivity. By permuting the chiral iridium catalysts and the substrates, we efficiently and selectively obtained all four stereoisomers bearing two consecutive chiral centers. Mechanistic studies via density functional theory calculations revealed the origins of the diastereo‐ and chemoselectivities, indicating the pivotal roles of the steric interaction, the β‐silicon effect, and a rapid desilylation process. Additional synthetic modifications for preparing a variety of enantioenriched compounds containing contiguous chiral centers are also included.

Iridium-Catalyzed Chemo-, Diastereo-, and Enantioselective Allyl-Allyl Coupling: Accessing All Four Stereoisomers of (E)-1-Boryl-Substituted 1,5-Dienes by Chirality Pairing.

Here, we report a highly chemo-, diastereo-, and enantioselective allyl-allyl coupling between branched allyl alcohols and α-silyl-substituted allylboronate esters, catalyzed by a chiral iridium complex. The α-silyl-substituted allylboronate esters can be chemoselectively coupled with allyl electrophiles, affording a diverse set of enantioenriched (E)-1-boryl-substituted 1,5-dienes in good yields, with excellent stereoselectivity. By permuting the chiral iridium catalysts and the substrates, we efficiently and selectively obtained all four stereoisomers bearing two consecutive chiral centers. Mechanistic studies via density functional theory calculations revealed the origins of the diastereo- and chemoselectivities, indicating the pivotal roles of the steric interaction, the β-silicon effect, and a rapid desilylation process. Additional synthetic modifications for preparing a variety of enantioenriched compounds containing contiguous chiral centers are also included.