Si-Directed regiocontrol in asymmetric Pd-catalyzed allylic alkylations using C1-ammonium enolate nucleophiles

Rhodium and Isothiourea Dual Catalysis: Enantiodivergent Transformation of Terminal Alkynes

A dual rhodium/isothiourea catalytic system was developed for the enantiodivergent transformation of terminal alkynes. Under synergistic rhodium/isothiourea dual catalysis, terminal alkynes can be creatively utilized as precursors for C1-ammonium enolate species, which subsequently participate in [4 + 2] and [2 + 2] annulation reactions with α,β-unsaturated ketimines or ketones, respectively. A wide range of chiral lactams and lactones were obtained in excellent yields and stereoselectivities (up to >20:1 dr, 98% ee).

Cooperative Chiral Lewis Base/Palladium-Catalyzed Asymmetric Syntheses of Methylene-Containing δ-Lactams

We herein report a two-step approach for the enantioselective synthesis of novel chiral δ-lactams. By using a cooperative chiral ITU/achiral Pd-catalyst system, this protocol proceeds via an asymmetric α-allylation of activated aryl esters first, followed by an acid-mediated lactam formation. A variety of differently substituted products could be obtained with usually high levels of enantioselectivities and in reasonable yields (16 examples, up to 98 : 2 er and 73 % yield over two steps). In addition, further utilizations of the products via transformations of the exocyclic double bond were successfully carried out as well.

Enantioselective isothiourea-catalysed reversible Michael addition of aryl esters to 2-benzylidene malononitriles

Catalytic enantioselective transformations usually rely upon optimal enantioselectivity being observed in kinetically controlled reaction processes, with energy differences between diastereoisomeric transition state energies translating to stereoisomeric product ratios. Herein, stereoselectivity resulting from an unusual reversible Michael addition of an aryl ester to 2-benzylidene malononitrile electrophiles using an isothiourea as a Lewis base catalyst is demonstrated. Notably, the basicity of the aryloxide component and reactivity of the isothiourea Lewis base both affect the observed product selectivity, with control studies and crossover experiments indicating the feasibility of a constructive reversible Michael addition from the desired product. When this reversible addition is coupled with a crystallisation-induced diastereomer transformation (CIDT) it allows isolation of products in high yield and stereocontrol (14 examples, up to 95 : 5 dr and 99 : 1 er). Application of this process to gram scale, plus derivatisations to provide further useful products, is demonstrated.

Isothiourea-Catalyzed Enantioselective Functionalisation of Glycine Schiff Base Aryl Esters via 1,6- and 1,4-Additions

The enantioselective α-functionalisation of glycine Schiff base aryl esters through isothiourea catalysis is successfully demonstrated for 1,6-additions to para-quinone methides (21 examples, up to 95:5 dr and 96:4 er) and 1,4-additions to methylene substituted dicarbonyl or disulfonyl Michael acceptors (17 examples, up to 98:2 er). This nucleophilic organocatalysis approach gives access to a range of α-functionalised α-amino acid derivatives and further transformations of the activated aryl ester group provide a straightforward entry to advanced amino acid-based esters, amides or thioesters.

Cooperative Isothiourea/Iridium-Catalyzed Asymmetric Annulation Reactions of Vinyl Aziridines with Pentafluorophenyl Esters

Chiral γ-lactam-containing skeletons are important motifs in bioactive natural products, pharmaceuticals, and bioactive molecules. Herein, we report a general and modular platform to access chiral γ-lactam compounds via an ITU/Ir cooperatively catalyzed [3 + 2] asymmetric annulation reaction of vinyl aziridines with pentafluorophenyl esters. Through the Lewis base and transition metal cooperative catalytic regime, a broad range of optically active γ-lactams were generated in good yields (up to 92%) with high asymmetric induction (up to 98% ee). Furthermore, the utility of this synthetic protocol was also demonstrated by the expedient preparation of diverse enantioenriched architectures.

Enantio- and Regioconvergent Nickel-Catalyzed Allylic Substitution of Racemic α- or γ-Silylated Allylic Bromides with Benzylzinc Reagents

An enantio- and regioconvergent nickel-catalyzed benzylation of racemic silylated allylic electrophiles with benzylzinc nucleophiles is reported. The key feature of this method is that the homocoupling pathways of both the nucleophile and the electrophile are minimized. A diverse set of electronically modified benzylzinc reagents was tolerated. The vinylsilane products with allylic stereocenters were formed in moderate to high yields with high enantioselectivities. The regioconvergence is the result of the steering effect of the silyl group.

A Pd-H/Isothiourea Cooperative Catalysis Approach to anti-Aldol Motifs: Enantioselective α-Alkylation of Esters with Oxyallenes

The biological and therapeutic significance of natural products is a powerful impetus for the development of efficient methods to facilitate their construction. Accordingly, and reflecting the prevalence of β-oxy-carbonyl motifs, a sophisticated arsenal of aldol-based strategies has evolved that is contingent on the generation of single enolate isomers. Since this has the potential to compromise efficiency in reagent-based paradigms, direct catalysis-based solutions would be enabling. To complement the array of substrate-based strategies, and regulate enolate geometry at the catalyst level, a direct catalytic alkylation of esters with oxyallenes has been developed. Synergizing metal hydride reactivity with Lewis base catalysis has resulted in a broad reaction scope with useful levels of stereocontrol (up to >99 % ee). Facile derivatization of these ambiphilic linchpins is demonstrated, providing access to high-value vicinal stereocenter-containing motifs, including 1,2-amino alcohols.

Isothiourea-Catalyzed Enantioselective Michael Addition of Malonates to α,β-Unsaturated Aryl Esters

An enantioselective Michael addition of malonates to α,β-unsaturated para-nitrophenyl esters was achieved using the Lewis basic isothiourea HyperBTM, giving excellent levels of product enantioselectivity (up to >99:1 enantiomeric ratio) in good yields and with complete regioselectivity (>20:1 regioselectivity ratio) in the presence of alternative (phenyl ketone and ethyl ester) Michael acceptors. Density functional theory calculations indicate that N-acylation is rate-limiting. This constitutes a rare example of a highly enantioselective addition of simple, readily available malonates to α,β-unsaturated esters.

Recent progress in asymmetric synergistic catalysis - the judicious combination of selected chiral aminocatalysts with achiral metal catalysts

In this review we survey recent synergistic applications of a chiral organocatalyst with an achiral metal to perform stereoselective transformations of synthetic utility (since 2016). The transformations are classified by the modes of reactivity deployed, focussing on organocatalytic activation of carbonyl substrates as chiral nucleophiles via the α-position (e.g., as enamines) and as chiral electrophiles via the β-position (e.g., as iminium ions) combined with complementary activation of their reaction partners by an achiral metal co-catalyst (e.g., Pd or Cu-based). Corresponding radical reactions are also presented in which photocatalysis mediated by achiral metal complexes replaces the metal co-catalyst. Certain privileged structures are revealed and opportunities to develop this exciting field are highlighted.

A critical survey of recent synergistic applications of a chiral organocatalyst with an achiral metal to perform stereoselective transformations of synthetic utility.

Isothiourea-Catalyzed Enantioselective α-Alkylation of Esters via 1,6-Conjugate Addition to para-Quinone Methides

The isothiourea-catalyzed enantioselective 1,6-conjugate addition of para-nitrophenyl esters to 2,6-disubstituted para-quinone methides is reported. para-Nitrophenoxide, generated in situ from initial N-acylation of the isothiourea by the para-nitrophenyl ester, is proposed to facilitate catalyst turnover in this transformation. A range of para-nitrophenyl ester products can be isolated, or derivatized in situ by addition of benzylamine to give amides at up to 99% yield. Although low diastereocontrol is observed, the diastereoisomeric ester products are separable and formed with high enantiocontrol (up to 94:6 er).

Catalytic enantioselective synthesis of 1,4-dihydropyridines via the addition of C(1)-ammonium enolates to pyridinium salts

The regio- and stereoselective addition of C(1)-ammonium enolates - generated in situ from aryl esters and the isothiourea catalyst (R)-BTM - to pyridinium salts bearing an electron withdrawing substituent in the 3-position allows the synthesis of a range of enantioenriched 1,4-dihydropyridines. This represents the first organocatalytic approach to pyridine dearomatisation using pronucleophiles at the carboxylic acid oxidation level. Optimisation studies revealed a significant solvent dependency upon product enantioselectivity, with only toluene providing significant asymmetric induction. Using DABCO as a base also proved beneficial for product enantioselectivity, while investigations into the nature of the counterion showed that co-ordinating bromide or chloride substrates led to higher product er than the corresponding tetrafluoroborate or hexafluorophosphate. The scope and limitations of this process are developed, with enantioselective addition to 3-cyano- or 3-sulfonylpyridinium salts giving the corresponding 1,4-dihydropyridines (15 examples, up to 95 : 5 dr and 98 : 2 er).

Enantio- and Regioconvergent Nickel-Catalyzed C(sp3 )-C(sp3 ) Cross-Coupling of Allylic Electrophiles Steered by a Silyl Group

A two‐step sequence for the enantio‐ and diastereoselective synthesis of exclusively alkyl‐substituted acyclic allylic systems with a stereocenter in the allylic position is reported. The asymmetric induction and the site selectivity are controlled in an enantio‐ and regioconvergent nickel‐catalyzed C(sp³)−C(sp³) cross‐coupling of regioisomeric mixtures of racemic α‐/γ‐silylated allylic halides and primary alkylzinc reagents. The silyl group steers the allylic displacement towards the formation of the vinylsilane regioisomer, and the resulting C(sp²)−Si bond serves as a linchpin for the installation of various C(sp³) substituents in a subsequent step.

Recent Advances in Enantioselective Pd-Catalyzed Allylic Substitution: From Design to Applications

This Review compiles the evolution, mechanistic understanding, and more recent advances in enantioselective Pd-catalyzed allylic substitution and decarboxylative and oxidative allylic substitutions. For each reaction, the catalytic data, as well as examples of their application to the synthesis of more complex molecules, are collected. Sections in which we discuss key mechanistic aspects for high selectivity and a comparison with other metals (with advantages and disadvantages) are also included. For Pd-catalyzed asymmetric allylic substitution, the catalytic data are grouped according to the type of nucleophile employed. Because of the prominent position of the use of stabilized carbon nucleophiles and heteronucleophiles, many chiral ligands have been developed. To better compare the results, they are presented grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions are mainly promoted by PHOX or Trost ligands, which justifies organizing this section in chronological order. For asymmetric oxidative allylic substitution the results are grouped according to the type of nucleophile used.

Generation and Reactivity of C(1)-Ammonium Enolates by Using Isothiourea Catalysis

C(1)-Ammonium enolates are powerful, catalytically generated synthetic intermediates applied in the enantioselective α-functionalisation of carboxylic acid derivatives. This minireview describes the recent developments in the generation and application of C(1)-ammonium enolates from various precursors (carboxylic acids, anhydrides, acyl imidazoles, aryl esters, α-diazoketones, alkyl halides) using isothiourea Lewis base organocatalysts. Their synthetic utility in intra- and intermolecular enantioselective C-C and C-X bond forming processes on reaction with various electrophiles will be showcased utilising two distinct catalyst turnover approaches.

Stereodivergent Carbon-Carbon Bond Formation between Iminium and Enolate Intermediates by Synergistic Organocatalysis

We report here a stereodivergent method for the Michael addition of aryl acetic acid esters to α,β-unsaturated aldehydes catalyzed by a combination of a chiral pyrrolidine and a chiral Lewis base. This reaction proceeds through a synergistic catalytic cycle which consists of one cycle leading to a chiral iminium electrophile and a second cycle generating a nucleophilic chiral enolate for the construction of a carbon-carbon bond. By varying the combinations of catalyst enantiomers, all four stereoisomers of the products with two vicinal stereocenters are accessible with high enantio- and diastereoselectivity. The products of the Michael addition, 1,5-aldehyde esters, can be readily transformed into a variety of other valuable enantioenriched structures, including those bearing three contiguous stereocenters in an acyclic system, thus providing an efficient route to an array of structural and stereochemical diversity.

Exploring the Scope of Tandem Palladium and Isothiourea Relay Catalysis for the Synthesis of α-Amino Acid Derivatives

The scope and limitations of a tandem N-allylation/[2,3]-rearrangement protocol are investigated through the incorporation of a variety of functional groups within an allylic phosphate precursor. This method uses readily accessible N,N-dimethylglycine aryl esters and functionalized allylic phosphates, forming quaternary ammonium salts in situ in the presence of a palladium catalyst. Subsequent enantioselective [2,3]-sigmatropic rearrangement, promoted by the chiral isothiourea tetramisole, generates α-amino acid derivatives with two contiguous stereocenters. The incorporation of electron-withdrawing ester and amide groups gave the best results, furnishing the desired products in moderate to good yields (29–70%), with low diastereocontrol (typically 60:40 dr) but high enantioselectivity (up to 90:10 er). These results indicate that substrate–catalyst interactions in the proposed transition state are sensitive to the substitution pattern of the substrates.

Tertiary Amine Lewis Base Catalysis in Combination with Transition Metal Catalysis

The cooperation between two orthogonal catalytic events during the formation of carbon-carbon and carbon-heteroatom bonds has emerged as an effective strategy for enantioselective chemical synthesis. In recent years, a number of pioneering investigations have described useful chemical synthesis methods whereby the reactivity or nucleophile-electrophile combinations can be fine-tuned or extended far beyond the effect and influence of a single catalyst. The recognition of this has had profound implications for the development cooperative catalysis as a field and has provided a foundation for the development of broadly useful chemical synthesis methods. This chapter focuses on the combination of tertiary amine Lewis base and transition metal catalysts, which the authors hope will simulate further developments and advances.

Base-free Enantioselective C(1)-Ammonium Enolate Catalysis Exploiting Aryloxides: A Synthetic and Mechanistic Study

An isothiourea-catalyzed enantioselective Michael addition of aryl ester pronucleophiles to vinyl bis-sulfones via C(1)-ammonium enolate intermediates has been developed. This operationally simple method allows the base-free functionalization of aryl esters to form α-functionalized products containing two contiguous tertiary stereogenic centres in excellent yield and stereoselectivity (all ≥99:1 er). Key to the success of this methodology is the multifunctional role of the aryloxide, which operates as a leaving group, Brønsted base, Brønsted acid and Lewis base within the catalytic cycle. Comprehensive mechanistic studies, including variable time normalization analysis (VTNA) and isotopologue competition experiments, have been carried out. These studies have identified (i) orders of all reactants; (ii) a turnover-limiting Michael addition step, (iii) product inhibition, (iv) the catalyst resting state and (v) catalyst deactivation through protonation.

A Regio- and Stereodivergent Synthesis of Homoallylic Amines by a One-Pot Cooperative-Catalysis-Based Allylic Alkylation/Hofmann Rearrangement Strategy

Herein, we report a modular synthetic route to linear and branched homoallylic amines that operates through a sequential one-pot Lewis base/transition-metal catalyzed allylic alkylation/Hofmann rearrangement strategy. This protocol is operationally trivial, proceeds from simple and easily prepared substrates and catalysts, and enables all aspects of regio- and stereoselectivity to be controlled through a conserved experimental protocol. Overall, the high levels of enantio-, regio-, and diastereoselectivity obtained, in concert with the ability to access orthogonally protected or free amines, render this a straightforward and effective approach for the preparation of useful enantioenriched homoallylic amines. We have also demonstrated the utility of the products in the context of pharmaceutical synthesis.

An enantioselective synthesis of α-alkylated pyrroles via cooperative isothiourea/palladium catalysis

Herein we describe the direct enantioselective Lewis base/Pd catalysed α-allylation of pyrrole acetic acid esters. This provides high isolated yields of highly enantioenriched products and exhibits broad reaction scope with respect to both reaction partners. The products can be readily elaborated in a manner which points towards potential applications in target directed synthesis.

Chiral AuI - and AuIII -Isothiourea Complexes: Synthesis, Characterization and Application

During an investigation into the potential union of Lewis basic isothiourea organocatalysis and gold catalysis, the formation of gold-isothiourea complexes was observed. These novel gold complexes were formed in high yield and were found to be air- and moisture stable. A series of neutral and cationic chiral gold(I) and gold(III) complexes bearing enantiopure isothiourea ligands was therefore synthesized and fully characterized. The steric and electronic properties of the isothiourea ligands was assessed through calculation of their percent buried volume and the synthesis and analysis of novel iridium(I)-isothiourea carbonyl complexes. The novel gold(I)- and gold(III)-isothiourea complexes have been applied in preliminary catalytic and biological studies, and display promising preliminary levels of catalytic activity and potency towards cancerous cell lines and clinically relevant enzymes.

Evaluating aryl esters as bench-stable C(1)-ammonium enolate precursors in catalytic, enantioselective Michael addition–lactonisations

An evaluation of a range of aryl, alkyl and vinyl esters as prospective C(1)-ammonium enolate precursors in enantioselective Michael addition–lactonisation processes with (E)-trifluoromethylenones using isothiourea catalysis is reported.

Enantioselective α-Allylation of Aryl Acetic Acid Esters via C1-Ammonium Enolate Nucleophiles: Identification of a Broadly Effective Palladium Catalyst for Electron-Deficient Electrophiles

We have identified a generally effective Pd catalyst for the highly enantioselective cooperative Lewis base/Pd-catalyzed α-allylation of aryl acetic esters using electron-deficient electrophiles. Changing between aldehyde, ketone, ester, and amide substituents at the terminus of intermediate cationic π-(allyl)Pd species affects both the efficiency of the reaction and, in the case of amides, control over the stereochemistry of the product alkene, as a function of the ligand. Tris[tri(2-thienyl)phosphino]Pd(0) serves as a broadly effective catalyst and overcomes these challenges to provide a general, high-yielding, and operationally simple C(sp³)-C(sp³) bond-forming method that gives products with high levels of enantioselectivity.

Enantioselective α-Allylation of Acyclic Esters Using B(pin)-Substituted Electrophiles: Independent Regulation of Stereocontrol Elements through Cooperative Pd/Lewis Base Catalysis

Cooperation between a Lewis base and Pd catalyst enables the direct enantioselective α-functionalization of aryl and vinyl acetic acid esters using a bifunctional B(pin)-substituted electrophile. Critical to the success of this method was the recognition that both catalysts could control the necessary stereochemical aspects; the Lewis base catalyst controls the enantioselectivity of the reaction, whereas the Pd catalyst regulates alkenyl-B(pin) configuration. This is the first example of using cooperative catalysis to control both stereochemical features during Pd-catalyzed allylic alkylation.

Enantioselective α-Benzylation of Acyclic Esters Using π-Extended Electrophiles

The first asymmetric cooperative Lewis base/palladium catalyzed benzylic alkylation of acyclic esters is reported. This reaction proceeds via stereodefined C1-ammonium enolate nucleophiles. Critical to its success was the identification of benzylic phosphate electrophiles, which were uniquely reactive. Alkylated products were obtained with very high levels of enantioselectivity, and this method has been applied toward the synthesis of the thrombin inhibitor DX-9065a.