Synergistic Strategies in Aminocatalysis

Organocatalytic Enantioselective Nucleophilic Substitution of Furfurals with Alcohols via a Remote Chiral Control

We describe herein the direct organocatalytic asymmetric nucleophilic substitution reaction of 2-benzylfurfurals with benzyl alcohols and allylic alcohol in the absence of any metal catalysts and ligands. A series of γ-regioselective products were obtained in high yields and up to 99% enantiomeric excess.

Organocatalytic Packed-Bed Reactors for the Enantioselective Flow Synthesis of Quaternary Isotetronic Acids by Direct Aldol Reactions of Pyruvates

The utilization of the homogeneous (S)-2-pyrrolidine-tetrazole organocatalyst (Ley catalyst) in the self-condensation of ethyl pyruvate and cross-aldol reactions of ethyl pyruvate donor with non-enolizable pyruvate acceptors, namely the sterically hindered ethyl 3-methyl-2-oxobutyrate or the highly electrophilic methyl 3,3,3-trifluoropyruvate, is described as the key enantioselective step toward the synthesis of the corresponding biologically relevant isotetronic acids featuring a quaternary carbon functionalized with ester and alkyl groups. The transition from homogeneous to heterogeneous flow conditions is also investigated, detailing the fabrication and operation of packed-bed reactors filled with a silica-supported version of the pyrrolidine-tetrazole catalyst (SBA-15 as the matrix).

Enantioselective Cascade Reactions of Aminocatalytic Dienamines and Trienamines Initiated by a Cycloaddition Reaction

Cycloadditions are widely accepted as a group of reactions that rapidly generate molecular complexity. Being highly atom economic and often predictable, these reactions can generate up to four stereogenic centers and two C-C (or C-X) bonds in one reaction step. During the last two decades, asymmetric aminocatalysis has shown to be a successful strategy for controlling stereoselectivity and enabling reactivity of cycloaddition reactions. By increasing the conjugation of the carbonyl species employed, dienamines and trienamines can be catalytically formed. Not only can these facilitate the cycloaddition, often accompanied by high levels of stereocontrol, but they also leave a residual enamine or carbonyl (by hydrolysis) in the cycloadduct. This residual functionality can engage in further intramolecular reactions generating complex cyclic systems in a one-pot cascade manner. In this regard, asymmetric aminocatalysis can add another layer of complexity to the already complex nature of cycloadditions. In this review, we will present the general concept of such reactivity patterns of dienamines and trienamines, and hereafter showcase examples in the literature. We aspire that the chemical community can use these concepts to design new enantioselective aminocatalytic cascade reactions to access enantioenriched, complex compounds, and perhaps use these in complex molecule synthesis.

Enantioselective Preparation of Cyclopentene-Based Amino Acids with a Quaternary Carbon Center

Azlactone is an important starting material for synthesizing amino acids containing a quaternary α-carbon. In this study, we have developed a sequential "one-pot" procedure involving an enantioselective spirocyclization reaction followed by acidic azlactone opening, which led to amino acid derivatives. The key step of this procedure is a spirocyclization between propargylated azlactones and enals by using a cooperative catalytic approach that combines chiral secondary amine and achiral Pd(0) complexes. The final acid opening of the azlactone motif allows isolation of the corresponding amino acid derivatives as major diastereoisomers in yields ranging from 37% to 70% with enantioselectivities of 85-97% ee. These synthesized amino acid derivatives hold great potential in the pharmaceutical and bioactive compound industries. Moreover, the final amino acid products with a cyclopentene moiety can be further derivatized, opening up even more possibilities for their application.

Visible-Light-Promoted Enantioselective α-Amidation of Aldehydes by Harnessing Organo-Iron Dual Catalysis

The strategic integration of organocatalysis with transition-metal catalysis to achieve otherwise unattainable stereoselective transformations may serve as a powerful synthetic tool. Herein, we present a synthetically versatile α-amidation of aldehydes by leveraging dual iron and chiral enamine catalysis in an enantioselective manner (up to >99:1 er). Experimental and computational studies have led us to propose a new mechanistic platform, wherein visible-light-promoted LMCT generates [Fe(II)Cl3-], which effectively activates dioxazolones to form an iron-acylnitrenoid radical that inserts into chiral enamine intermediates.

Organocatalysis as an enabling tool for enantioselective ring-opening reactions of cyclopropanes

The rich reactivity profile of cyclopropanes has been extensively explored to trigger new organic transformations that enable unusual disconnective approaches to synthesize molecular motifs that are not easily reached through conventional reactions. In particular, the chemistry of cyclopropanes has received special attention in the last decade, with multiple new approaches that capitalize on the use of organocatalysis for the activation of the cyclopropane scaffold. This situation has also opened the possibility of developing enantioselective variants of many reactions that until now were only carried out in an enantiospecific or diastereoselective manner. Our group has been particularly active in this field, focusing more specifically on the use of aminocatalysis and Brønsted acid catalysis as major organocatalytic activation manifolds to trigger new unprecedented transformations involving cyclopropanes that add to the current toolbox of general methodologies available to organic chemists for the enantioselective synthesis of chiral compounds.

Electrochemical Asymmetric Radical Functionalization of Aldehydes Enabled by a Redox Shuttle

Aminocatalysis is a well-established tool that enables the production of enantioenriched compounds under mild conditions. Its versatility is underscored by its seamless integration with various synthetic approaches. While the combination of aminocatalysis with metal catalysis, photochemistry, and stoichiometric oxidants has been extensively explored, its synergy with electrochemical activation remains largely unexplored. Herein, we present the successful merger of electrochemistry and aminocatalysis to perform SOMO-type transformations, expanding the toolkit for asymmetric electrochemical synthesis. The methodology harnesses electricity to drive the oxidation of catalytically generated enamines, which ultimately partake in enantioselective radical processes, leading to α-alkylated aldehydes. Crucially, mechanistic studies highlight how this electrochemical strategy is enabled by the use of a redox shuttle, 4,4'-dimethoxybiphenyl, to prevent catalyst degradation and furnishing the coveted compounds in good yield and high enantioselectivity.

An Artificial Enzyme for Asymmetric Nitrocyclopropanation of α,β-Unsaturated Aldehydes-Design and Evolution

The introduction of an abiological catalytic group into the binding pocket of a protein host allows for the expansion of enzyme chemistries. Here, we report the generation of an artificial enzyme by genetic encoding of a non-canonical amino acid that contains a secondary amine side chain. The non-canonical amino acid and the binding pocket function synergistically to catalyze the asymmetric nitrocyclopropanation of α,β-unsaturated aldehydes by the iminium activation mechanism. The designer enzyme was evolved to an optimal variant that catalyzes the reaction at high conversions with high diastereo- and enantioselectivity. This work demonstrates the application of genetic code expansion in enzyme design and expands the scope of enzyme-catalyzed abiological reactions.

Enantioselective Transformations by "1 + x" Synergistic Catalysis with Chiral Primary Amines

ConspectusSynergistic catalysis is a powerful tool that involves two or more distinctive catalytic systems to activate reaction partners simultaneously, thereby expanding the reactivity space of individual catalysis. As an established catalytic strategy, organocatalysis has found numerous applications in enantioselective transformations under rather mild conditions. Recently, the introduction of other catalytic systems has significantly expanded the reaction space of typical organocatalysis. In this regard, aminocatalysis is a prototypical example of synergistic catalysis. The combination of aminocatalyst and transition metal could be traced back to the early days of organocatalysis and has now been well explored as an enabling catalytic strategy. Particularly, the acid-base properties of aminocatalysis can be significantly expanded to include usually electrophiles generated in situ via metal-catalyzed cycles. Later on, aminocatalyst has also been exploited in synergistically combining with photochemical and electrochemical processes to facilitate redox transformations. However, synergistically combining one type of aminocatalyst with many different catalytic systems remains a great challenge. One of the most daunting challenges is the compatibility of aminocatalysts in coexistence with other catalytic species. As nucleophilic species, aminocatalysts may also bind with metal, which leads to mutual inhibition or even quenching of the individual catalytic activity. In addition, oxidative stability of aminocatalyst is also a non-neglectable issue, which causes difficulties in exploring oxidative enamine transformations.In 2007, we developed a vicinal diamine type of chiral primary aminocatalysts. This class of primary aminocatalysts was developed and evolved as functional and mechanistic mimics to the natural aldolase and has been widely applied in a number of enamine/iminium ion-based transformations. By following a "1 + x" synergistic strategy, the chiral primary amine catalysts were found to work synergistically or cooperatively with a number of transition metal catalysts, such as Pd, Rh, Ag, Co, and Cu, or other organocatalysts, such as B(C6F5)3, ketone, selenium, and iodide. Photocatalysis and electrochemical processes can also be incorporated to work together with the chiral primary amine catalysts. The 1 + x catalytic strategy enabled us to execute unexploited transformations by fine-tuning the acid-base and redox properties of the enamine intermediates and to achieve effective reaction and stereocontrol beyond the reach individually. During these efforts, an unprecedented excited-state chemistry of enamine was uncovered to make possible an effective deracemization process. In this Account, we describe our recent efforts since 2015 in exploring synergistic chiral primary amine catalysis, and the content is categorized according to the type of synergistic partner such that in each section the developed synergistic catalysis, reaction scopes, and mechanistic features are presented and discussed.

Asymmetric Organocatalysis—A Powerful Technology Platform for Academia and Industry: Pregabalin as a Case Study

Enantioselective organocatalysis has quickly established itself as the third pillar of asymmetric catalysis. It is a powerful technology platform, and it has a tremendous impact in both academic and industrial settings. By focusing on pregabalin, as a case study, this Perspective aims to show how a process amenable to industry of a simple chiral molecule can be tackled in several different ways using organocatalysis.