Asymmetric [4+2] Annulation of C1 Ammonium Enolates with Copper-Allenylidenes

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.

A Desilylative Approach to Alkyl Substituted C(1)-Ammonium Enolates: Application in Enantioselective [2+2] Cycloadditions

The catalytic generation of C(1)-ammonium enolates from the corresponding α-silyl-α-alkyl substituted carboxylic acids using the isothiourea HyperBTM is reported. This desilylative approach grants access to α-unsubstituted and α-alkyl substituted C(1)-ammonium enolates, which are typically difficult to access through traditional methods reliant upon deprotonation. The scope and limitations of this process is established in enantioselective [2+2]-cycloaddition processes with perfluoroalkylketones (31 examples, up to 96 % yield and >99 : 1 er), as well as selective [2+2]-cycloaddition with trifluoromethyl enones (4 examples, up to 75 % yield and >99 : 1 er). Preliminary mechanistic studies indicate this process proceeds through an initial kinetic resolution of an in situ prepared (±)-α-silyl-α-alkyl substituted anhydride, while the reaction process exhibits overall pseudo zero-order kinetics.

Cooperative Palladium/Isothiourea Catalyzed Enantioselective Formal (3+2) Cycloaddition of Vinylcyclopropanes and α,β-Unsaturated Esters

A protocol for the enantioselective synthesis of substituted vinylcyclopentanes has been realised using cooperative palladium and isothiourea catalysis. Treatment of vinylcyclopropanes with Pd(PPh3 )4 generates a zwitterionic π-allyl palladium intermediate that intercepts a catalytically generated α,β-unsaturated acyl ammonium species prepared from the corresponding α,β-unsaturated para-nitrophenyl ester and the isothiourea (R)-BTM. Intermolecular formal (3+2) cycloaddition between these reactive intermediates generates functionalised cyclopentanes in generally good yields and excellent diastereo- and enantiocontrol (up to >95 : 5 dr, 97 : 3 er), with the use of LiCl as an additive proving essential for optimal stereocontrol. To the best of our knowledge a dual transition metal/organocatalytic process involving α,β-unsaturated acyl ammonium intermediates has not been demonstrated previously.

Ru-NHC-Catalyzed Asymmetric Hydrogenation of 2-Quinolones to Chiral 3,4-Dihydro-2-Quinolones

Direct enantioselective hydrogenation of unsaturated compounds to generate chiral three-dimensional motifs is one of the most straightforward and important approaches in synthetic chemistry. We realized the Ru(II)-NHC-catalyzed asymmetric hydrogenation of 2-quinolones under mild reaction conditions. Alkyl-, aryl- and halogen-substituted optically active dihydro-2-quinolones were obtained in high yields with moderate to excellent enantioselectivities. The reaction provides an efficient and atom-economic pathway to construct simple chiral 3,4-dihydro-2-quinolones. The desired products could be further reduced to tetrahydroquinolines and octahydroquinolones.

Enantioselective Synthesis of α-Aryl-β2 -Amino-Esters by Cooperative Isothiourea and Brønsted Acid Catalysis

The synthesis of α-aryl-β2 -amino esters through enantioselective aminomethylation of an arylacetic acid ester in high yields and enantioselectivity via cooperative isothiourea and Brønsted acid catalysis is demonstrated. The scope and limitations of this process are explored (25 examples, up to 94 % yield and 96:4 er), with applications to the synthesis of (S)-Venlafaxine⋅HCl and (S)-Nakinadine B. Mechanistic studies are consistent with a C(1)-ammonium enolate pathway being followed rather than an alternative dynamic kinetic resolution process. Control studies indicate that (i) a linear effect between catalyst and product er is observed; (ii) an acyl ammonium ion can be used as a precatalyst; (iii) reversible isothiourea addition to an in situ generated iminium ion leads to an off-cycle intermediate that can be used as a productive precatalyst.

Synthesis of Tetra-Substituted Trifluoromethyl-3,1-Benzoxazines by Transition-Metal-Catalyzed Decarboxylative Cyclization of N-Benzoyl Benzoxazinones

Efficient synthesis of N,O-heterocyclic tetra-substituted trifluoromethyl-3,1-benzoxazines via a transition-metal-catalyzed decarboxylative intramolecular cyclization was achieved. The decarboxylation of N-benzoyl trifluoromethyl-benzoxazinones generated the amide oxygen nucleophile, allowing a selective internal C1 -attack on Pd- or Cu-coordinated zwitterions, affording medicinally attractive tetra-substituted vinyl- or ethynyl-trifluoromethyl-3,1-benzoxazines. This protocol can be applied to the synthesis of perfluoroalkyl- and non-fluorinated 3,1-benzoxazines.

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.

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.

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.

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.

Traversing Steric Limitations by Cooperative Lewis Base/Palladium Catalysis: An Enantioselective Synthesis of α-Branched Esters Using 2-Substituted Allyl Electrophiles

Cooperative catalysis enables the direct enantioselective α-allylation of linear prochiral esters with 2-substituted allyl electrophiles. Critical to the successful development of the method was the recognition that metal-centered reactivity and the source of enantiocontrol are independent. This feature is unique to simultaneous catalysis events and permits logical tuning of the supporting ligands without compromising enantioselectivity.

Efficient Synthesis of N-Alkylated 4-Pyridones by Copper-Catalyzed Intermolecular Asymmetric Propargylic Amination

Copper-catalyzed intermolecular asymmetric propargylic amination with 4-hydroxypyridines has been realized for the first time. In the presence of Cu complex derived from Pybox ligand, the N-propargylated 4-pyridones were obtained under mild reaction conditions with up to 99 % yield and 95 % ee. The Pybox ligand bearing a 4-F-phenyl substituent plays a key role for the high enantioselectivity. The products can be easily transformed to the core structure of quinolizidine alkaloids.