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.

Bioinspired Functionalization of Carbonyl Compounds Enabled by Metal Chelated Bifunctional Ligands

In Nature, enzymatic reactions proceed through exceptionally ordered transition states giving rise to extraordinary levels of stereoselection. In those reactions, the active site of the enzyme plays crucial roles - through one position, it holds the substrate in the proximity to the reaction epicentre that facilitates both the reactivity and stereoselectivity of the chemical process. Inspired by this natural phenomenon, synthetic chemists have designed bifunctional ligands that not only coordinate to a metal centre but also preassociate with an organic substrate, for example aldehyde and ketone, and exerts stereodirecting influence to accelerate the attack of the incoming reacting partner from a particular enantiotopic face. The chief goal of the current review is to give an overview of the recently developed approaches enabled by privileged bio-inspired bifunctional ligands that not only bind to the metal catalyst but also activates carbonyl substrates via organocatalysis, thereby easing in the new bond forming step. As carbonyl α-functionalizations are dominated by enamine and enolate chemistry, the current review primarily focusses on enamine- and enolate-metal catalysis by bifunctional ligands. Thus, developments based on traditional cooperative catalysis occurring through two directly coupled but independent catalytic cycles of an organocatalyst and a metal catalyst are not covered.

Conjugated Microporous Polymers Based on Ferrocene Units as Highly Efficient Electrodes for Energy Storage

This work describes the facile designing of three conjugated microporous polymers incorporated based on the ferrocene (FC) unit with 1,4-bis(4,6-diamino-s-triazin-2-yl)benzene (PDAT), tris(4-aminophenyl)amine (TPA-NH₂), and tetrakis(4-aminophenyl)ethane (TPE-NH₂) to form PDAT-FC, TPA-FC, and TPE-FC CMPs from Schiff base reaction of 1,1'-diacetylferrocene monomer with these three aryl amines, respectively, for efficient supercapacitor electrodes. PDAT-FC and TPA-FC CMPs samples featured higher surface area values of approximately 502 and 701 m² g^-1, in addition to their possession of both micropores and mesopores. In particular, the TPA-FC CMP electrode achieved more extended discharge time compared with the other two FC CMPs, demonstrating good capacitive performance with a specific capacitance of 129 F g^-1 and capacitance retention value of 96% next 5000 cycles. This feature of TPA-FC CMP is attributed to the presence of redox-active triphenylamine and ferrocene units in its backbone, in addition to a high surface area and good porosity that facilitates the redox process and provides rapid kinetics.

Ferrocenium Ions as Catalysts: Decomposition Studies and Counteranion Influence on Catalytic Activity

Catalyst decomposition has a negative effect on catalytic activity, and knowledge of decomposition pathways can assist with catalyst development. Ferrocenium cations have been employed as catalysts in a number of organic transformations, and we investigated the stability of a number of ferrocenium salts in solution. The observed rate decomposition constants for [Fc]Cl, [Fc]PF6, [Fc]BF4, [Fc]CSA [Fc = ferrocenium, CSA = camphor-10-sulfonate (β)], [AcFc]SbF6, (AcFc = acetylated ferrocene), and [FcB(OH)2]SbF6 [FcB(OH)2 = ferrocenylboronic acid] were determined in CH2Cl2 solution by time-resolved UV-vis spectroscopy. The rate decomposition constants depended on the nature of the counterion, with [Fc]Cl being the most stable complex in solution. The decomposition rate constants dropped by roughly an order of magnitude in most cases when the experiments were performed in nitrogenated solvent, demonstrating that the decomposition is mainly an oxidative process. The cosolvent HFIP (1,1,1,3,3,3-hexafluoropropan-2-ol) slowed the decomposition of the ferrocenium cations as well. Many catalytic or stoichiometric reactions of ferrocenium cations are performed with alcohols; we determined that hexan-1-ol is decomposed over the course of 16 hours, but not oxidized in the presence of a ferrocenium cation. Finally, the different ferrocenium cations were employed in a test reaction to determine catalytic activity. The nucleophilic substitution of hydroxyl groups in a tertiary propargylic alcohol by an alcohol is catalyzed by all complexes, and, again, a counterion dependency of the catalytic activity was observed. Also, HFIP increases the catalytic activity of the ferrocenium cations. The research has importance in the development of ferrocenium-based catalyst systems, because changes in the counterion as well as the architecture of the ferrocenium cation have an influence on stability and catalytic activity.

Asymmetric Reactions Enabled by Cooperative Enantioselective Amino- and Lewis Acid Catalysis

Organocatalysis-the branch of catalysis featuring small organic molecules as the catalysts-has, in the last decade, become of central importance in the field of asymmetric catalysis, so much that it is now comparable to metal catalysis and biocatalysis. Organocatalysis is rationalized and classified by a number of so-called activation modes, based on the formation of a covalent or not-covalent intermediate between the organocatalyst and the organic substrate. Among all the organocatalytic activation modes, enamine and iminium catalysis are widely used for the practical preparation of valuable products and intermediates, both in academic and industrial contexts. In both cases, chiral amines are employed as catalysts. Enamine activation mode is generally employed in the reaction with electrophiles, while nucleophiles require the iminium activation mode. Commonly, in both modes, the reaction occurs through well-organized transitions states. A large variety of partners can react with enamines and iminium ions, due to their sufficient nucleophilicity and electrophilicity, respectively. However, despite the success, organocatalysis still suffers from narrow scopes and applications. Multicatalysis is a possible solution for these drawbacks because the two different catalysts can synergistically activate the substrates, with a simultaneous activation of the two different reaction partners. In particular, in this review we will summarize the reported processes featuring Lewis acid catalysis and organocatalytic activation modes synergically acting and not interfering with each other. We will focus our attention on the description of processes in which good results cannot be achieved independently by organocatalysis or Lewis acid catalysis. In these examples of cooperative dual catalysis, a number of new organic transformations have been developed. The review will focus on the possible strategies, the choice of the Lewis acid and the catalytic cycles involved in the effective reported combination. Additionally, some important key points regarding the rationale for the effective combinations will be also included. π-Activation of organic substrates by Lewis acids, via formation of electrophilic intermediates, and their reaction with enamines will be also discussed in this review.

Modulation of catalyst enantioselectivity through reversible assembly of supramolecular helices

The enantioselectivity displayed by a supramolecular copper catalyst involved in successive reactions can be modulated through reversible assembly of the helices supporting the catalytic centres.

Self-Assembly of Amphiphilic Dipeptide with Homo- and Heterochiral Centers and Their Application in Asymmetric Aldol Reaction

Chiral self-assembly has drawn increasing interest in supramolecular chemistry. Here, we have designed amphiphilic l-Pro-l-Glu and l-Pro-d-Glu dipeptides and investigated their chiral self-assembly as well as asymmetric catalytic performance to disclose the synergistic effect of two stereogenic centers in the self-assembly and catalysis. It was found that both of the diastereomeric dipeptides can easily self-assemble into organogels with nanofibers. When these nanofibers were used as a catalyst for the asymmetric aldol reactions, enhanced enantioselectivity was obtained compared with their molecular state. Moreover, the L-L isomer assemblies showed higher enantioselectivity than the L-D isomer. It was revealed that both the supramolecular chirality of the nanofiber and the chiral catalytic site of l-proline played important roles in the asymmetric catalysis. In addition, the synergistic effect of two homochiral centers led to more efficient supramolecular catalysis that the L-L assemblies showed high yields (up to 97%), anti-diastereoselectivity (up to 99%), and excellent enantioselectivity (up to >99%).

Multifunctionalization of Unactivated Cyclic Ketones via Synergistic Catalysis of Copper and Diarylamine: Access to Cyclic α-Enaminone

A multifunctionalization of unactivated cyclic ketones via synergistic catalysis of copper and diarylamine for the direct synthesis of cyclic α-enaminone is reported for the first time. This reaction goes through oxidative α-amination, followed by a desaturation, and features mild reaction conditions, a broad substrate scope, and great functional group tolerance.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Combinations of Aminocatalysts and Metal Catalysts: A Powerful Cooperative Approach in Selective Organic Synthesis

The cooperation and interplay between organic and metal catalyst systems is of utmost importance in nature and chemical synthesis. Here innovative and selective cooperative catalyst systems can be designed by combining two catalysts that complement rather than inhibit one another. This refined strategy can permit chemical transformations unmanageable by either of the catalysts alone. This review summarizes innovations and developments in selective organic synthesis that have used cooperative dual catalysis by combining simple aminocatalysts with metal catalysts. Considerable efforts have been devoted to this fruitful field. This emerging area employs the different activation modes of amine and metal catalysts as a platform to address challenging reactions. Here, aminocatalysis (e.g., enamine activation catalysis, iminium activation catalysis, single occupied molecular orbital (SOMO) activation catalysis, and photoredox activation catalysis) is employed to activate unreactive carbonyl substrates. The transition metal catalyst complements by activating a variety of substrates through a range of interactions (e.g., electrophilic π-allyl complex formation, Lewis acid activation, allenylidene complex formation, photoredox activation, C-H activation, etc.), and thereby novel concepts within catalysis are created. The inclusion of heterogeneous catalysis strategies allows for "green" chemistry development, catalyst recyclability, and the more eco-friendly synthesis of valuable compounds.

Redox-mediated reactions of vinylferrocene: toward redox auxiliaries

Chemical redox reactions have been exploited to transform unreactive vinylferrocene into a powerful dienophile for the Diels-Alder reaction and reactive substrate for thiol addition reactions upon conversion to its ferrocenium state. We have further investigated the ability of these reactions to facilitate redox-auxiliary-like reactivity by further hydrogenolyisis of the Diels-Alder adduct to the corresponding cyclopentane derivative.

Macrocyclic Se4N2[7,7]ferrocenophane and Se2N[10]ferrocenophane containing benzyl unit: synthesis, complexation, crystal structures, electrochemical and optical properties

Se₄N₂[7,7]ferrocenophane containing benzyl unit shows switch-on third-order NLO responses to Cu²⁺ and Hg²⁺ due to the formation of delocalized π-conjugated systems.

Reaction control in heterogeneous catalysis using montmorillonite: switching between acid-catalysed and red-ox processes

For a montmorillonite clay modified with a super-acid (CF₃SO₃H), two different modes of behaviour can take place simply by a judicious choice of reaction conditions.

Reversible modulation of the activity of thiourea catalysts with anions: a simple approach to switchable asymmetric catalysis

A simple and straightforward approach to switchable asymmetric catalysis is presented, based on the interactions of thiourea catalysts with anions.

1,1′-Diacetyloctamethylferrocene: an overlooked and overdue synthon leading to the facile synthesis of an octamethylferrocenophane

Surprisingly easy access to versatile synthons: high yielding acylation, chloroformylation and intramolecular cyclisation reaction to a ferrocenophane.

Selection of two optional covalent bonds by electric stimuli: dual catalytic switching of redox-active copper

Two types of redox functionality were selected for covalent binding on a carbon electrode, responding to an electric potential applied to a dual-active copper catalyst.

Artificial switchable catalysts

Catalysis is key to the effective and efficient transformation of readily available building blocks into high value functional molecules and materials. For many years research in this field has largely focussed on the invention of new catalysts and the optimization of their performance to achieve high conversions and/or selectivities. However, inspired by Nature, chemists are beginning to turn their attention to the development of catalysts whose activity in different chemical processes can be switched by an external stimulus. Potential applications include using the states of multiple switchable catalysts to control sequences of transformations, producing different products from a pool of building blocks according to the order and type of stimuli applied. Here we outline the state-of-art in artificial switchable catalysis, classifying systems according to the trigger used to achieve control over the catalytic activity and stereochemical or other structural outcomes of the reaction.

Highly modular dipeptide-like organocatalysts for direct asymmetric aldol reactions in brine

The dipeptide-like organocatalysts have been developed for asymmetric aldol reactions in brine to achieve high yields and enantioselectivities with 1 mol% catalyst-loading.