Iminium catalysis

Tandem Friedel-Crafts-Alkylation-Enantioselective-Protonation by Artificial Enzyme Iminium Catalysis

The incorporation of organocatalysts into protein scaffolds holds the promise of overcoming some of the limitations of this powerful catalytic approach. Previously, we showed that incorporation of the non-canonical amino acid para-aminophenylalanine into the non-enzymatic protein scaffold LmrR forms a proficient and enantioselective artificial enzyme (LmrR_pAF) for the Friedel-Crafts alkylation of indoles with enals. The unnatural aniline side-chain is directly involved in catalysis, operating via a well-known organocatalytic iminium-based mechanism. In this study, we show that LmrR_pAF can enantioselectively form tertiary carbon centres not only during C-C bond formation, but also by enantioselective protonation, delivering a proton to one face of a prochiral enamine intermediate. The importance of various side-chains in the pocket of LmrR is distinct from the Friedel-Crafts reaction without enantioselective protonation, and two particularly important residues were probed by exhaustive mutagenesis.

Amine Organocatalysis of Remote, Chemoselective C(sp3)-H Hydroxylation

We introduce an organocatalytic approach for oxaziridinium-mediated C-H hydroxylation that employs secondary amines as catalysts. We also demonstrate the advantages of this operationally simple catalytic strategy for achieving high yielding and highly selective remote hydroxylation of compounds bearing oxidation-sensitive functional groups such as alcohols, ethers, carbamates, and amides. By employing hexafluoroisopropanol as the solvent in the absence of water, a proposed hydrogen bonding effect leads to, among other advantages, as high as ≥99:1 chemoselectivity for remote aliphatic hydroxylation of 2° alcohols, an otherwise unsolved synthetic challenge normally complicated by substantial amounts of alcohol oxidation. Initial studies of the reaction mechanism indicate the formation of an oxaziridinium salt as the active oxidant, and a C-H oxidation step that proceeds in a stereospecific manner via concerted insertion or hydrogen atom transfer/radical rebound. Furthermore, preliminary results indicate that site selectivity can be affected by amine catalyst structure. In the long term, we anticipate that this will enable new strategies for catalyst control of selectivity based on the abundance of catalytic scaffolds that have proliferated over the last twenty years as a result of Nobel Prize-winning discoveries.

Stereoselective 1,2-cis Furanosylations Catalyzed by Phenanthroline

Stereoselective formation of the 1,2-cis furanosidic linkage, a motif of many biologically relevant oligosaccharides and polysaccharides, remains an important synthetic challenge. We herein report a new stereoselective 1,2-cis furanosylation method promoted by phenanthroline catalysts under mild and operationally simple conditions. NMR experiments and density functional theory calculations support an associative mechanism in which the rate-determining step occurs from an inverted displacement of the faster-reacting phenanthrolinium ion intermediate with an alcohol nucleophile. The phenanthroline catalysis system is applicable to a number of furanosyl bromide donors to provide the challenging 1,2-cis substitution products in good yield with high anomeric selectivities. While arabinofuranosyl bromide provides β-1,2-cis products, xylo- and ribofuranosyl bromides favor α-1,2-cis products.

Programmable iodization/deuterolysis sequences of phosphonium ylides to access deuterated benzyl iodides and aromatic aldehydes

Herein, a tunable iodization/deuterolysis protocol for phosphonium ylides by employing D₂O as the deuterium source was designed. Notably, this process could be manipulated by tuning the base, thus leading to two valuable deuterated building blocks - benzyl iodides and aromatic aldehydes with broad substrate scope, good functional group compatibility and excellent deuteration degree. Concise syntheses of a series of deuterated drug analogues have been achieved based on the developed deuteration reaction platform.

Bond Energies of Enamines

Energetics of reactive intermediates underlies their reactivity. The availability of these data provides a rational basis for understanding and predicting a chemical reaction. We reported here a comprehensive computational study on the energetics of enamine intermediates that are fundamental in carbonyl chemistry. Accurate density functional theory (DFT) calculations were performed to determine the bond energies of enamines and their derived radical intermediates. These efforts led to the compilation of a database of enamine energetics including a thermodynamic index such as free-energy stability, bond dissociation energy (BDE), and acid dissociation constant (pK _a) as well as a kinetic index such as nucleophilicity and electrophilicity. These data were validated by relating to experimentally determined parameters and their relevance and utility were discussed in the context of modern enamine catalysis. It was found that pK _a values of enamine radical cations correlated well with redox potentials of their parent enamines, the former could be used to rationalize the proton-transfer behavior of enamine radical cations. An analysis of the BDE of enamine radical cations indicated that these species underwent facile β-C-H hydrogen transfer, in line with the known oxidative enamine catalysis. The enamine energetics offers the possibility of a systematic evaluation of the reactivities of enamines and related radicals, which would provide useful guidance in exploring new enamine transformations.

Tailoring Reaction Selectivity by Modulating a Catalytic Diad on a Foldamer Scaffold

Use of a tunable molecular scaffold to align a reactive diad for bifunctional catalysis can reveal relationships between functional group identity and reactivity that might otherwise be impossible to identify. Here we use an α/β-peptide helix to show that an aligned pair of primary amine groups is uniquely competent to catalyze crossed aldol condensations with an aryl aldehyde as the electrophile. Geometrically similar diads in which one amine group is secondary, or both are secondary, are good catalysts for other types of aldol condensations but not those involving an aryl aldehyde. Catalytic efficacy requires β-amino acid residues that are preorganized for helix formation via cyclic constraint. Conventional peptides (exclusively α-amino acid residues) that display the primary amine diad are poor catalysts, which highlights the critical role of the foldamer scaffold.

Differentiating Catalysis in the Dearomative [4 + 2]-Cycloaddition Involving Enals and Heteroaromatic Aldehydes

In this paper, the application of differentiating catalysis in the [4 + 2]-cycloaddition between 2-alkyl-3-formylheteroarenes and α,β-unsaturated aldehydes is described. Within the developed approach, the same aminocatalyst is employed for the independent activation of both starting materials, differentiating their properties via LUMO-lowering and HOMO-rising principles. By the combination of dearomative dienamine activation with iminium ion chemistry high enantio- and diastereoselectivity of the doubly asymmetric process was accomplished. Selected transformations of products were also demonstrated.

GlyConnect-Ugi: site-selective, multi-component glycoprotein conjugations through GlycoDelete expressed glycans

Recently, the GlyConnect-oxime (GC) protein conjugation strategy was developed to provide a site-selective glycan-based conjugation strategy as an extension to the in-house developed GlycoDelete (GD) technology. GD gives access to glycoproteins with single GlcNAc, LacNAc, or LacNAc-Sia type glycans on their N-glycosylation sites. We have previously shown that these glycans provide a unique handle for site-selective conjugation as they provide a short, homogeneous and hydrophilic link to the protein backbone. GC focused on the use of chemical and chemo-enzymatic pathways for conjugation of a single molecule of interest via oxime formation or reductive amination. In the current work, we explore multicomponent reactions (MCR), namely Ugi and Passerini reactions, for GlycoDelete glycan directed, site-specific protein conjugation (MC-GC). The use of the Ugi and Passerini multicomponent reactions holds the potential of introducing multiple groups of interest in a single reaction step while creating a hydrophilic peptide-like linker.

Asymmetric synthesis of chiral imidazolidines by merging copper and visible light-induced photoredox catalysis

A visible light induced copper catalyzed synthesis of decarboxylative radical coupling/cyclization reaction for the synthesis of chiral imidazolidines in high yields and enantioselectivities was reported.

Modulation of π character upon complexation captured by molecular rotation spectra

Two configurations of the furan–CF3Cl complex have been observed by high-resolution rotational spectroscopy. One is characterized by a dominant Cl lone pairs∙∙∙π*aromatic interaction and the other is stabilized by a...

Modulating the Catalytic Activity by the Mechanical Bond: Organocatalysis with Polyamide [2]Rotaxanes bearing a Secondary Amino Function at the Thread

The modulation of the catalytic activity of degenerate succinamide-based [2]rotaxanes by changes at their macrocyclic component is disclosed herein. These systems, bearing an acyclic secondary amine function at the thread...

Recent advances in catalytic oxidative reactions of phenols and naphthalenols

This critical review aims to provide an overview of oxidative phenol and naphthalenol transformations in nature and synthetic chemistry.

Enantioselective Syntheses of Yohimbine Alkaloids: Proving Grounds for New Catalytic Transformations

The total synthesis of bioactive alkaloids is an enduring challenge and an indication of the state of the art of chemical synthesis. With the explosion of catalytic asymmetric methods over the past three decades, these compelling targets have been fertile proving grounds for enantioselective bond forming transformations. These activities are summarized herein both to highlight the power and versatility of these methods and to instill future inspiration for new syntheses of these privileged natural products.

Vibrational CD study on the solution phase structures of the MacMillan catalyst and its corresponding iminium ion

We demonstrate that VCD spectroscopy can reveal insights into the conformational preferences of the iminium ion obtained from MacMillan's imidazolidinone catalyst. For both the isolated and in situ generated iminium ion, the comparison of experimental and computed VCD spectra directly confirms that conformer 2b ("Houk-conformer") is the dominant structure in solution. This conclusion is reached without any in-depth interpretation of the spectroscopic data, just by visual comparison of the spectral signatures. For the parent catalyst 1 and its salts 1·HCl and 1·HClO₄, we report a comprehensive analysis of the conformational preferences in two solvents. VCD spectroscopy is subsequently shown to be able to reveal small conformational changes induced by solute-solvent and solute-anion interactions.

Unlocking Asymmetric Michael Additions in an Archetypical Class I Aldolase by Directed Evolution

Class I aldolases catalyze asymmetric aldol addition reactions and have found extensive application in the biocatalytic synthesis of chiral β-hydroxy-carbonyl compounds. However, the usefulness of these powerful enzymes for application in other C-C bond-forming reactions remains thus far unexplored. The redesign of class I aldolases to expand their catalytic repertoire to include non-native carboligation reactions therefore continues to be a major challenge. Here, we report the successful redesign of 2-deoxy-d-ribose-5-phosphate aldolase (DERA) from Escherichia coli, an archetypical class I aldolase, to proficiently catalyze enantioselective Michael additions of nitromethane to α,β-unsaturated aldehydes to yield various pharmaceutically relevant chiral synthons. After 11 rounds of directed evolution, the redesigned DERA enzyme (DERA-MA) carried 12 amino-acid substitutions and had an impressive 190-fold enhancement in catalytic activity compared to the wildtype enzyme. The high catalytic efficiency of DERA-MA for this abiological reaction makes it a proficient "Michaelase" with potential for biocatalytic application. Crystallographic analysis provides a structural context for the evolved activity. Whereas an aldolase acts naturally by activating the enzyme-bound substrate as a nucleophile (enamine-based mechanism), DERA-MA instead acts by activating the enzyme-bound substrate as an electrophile (iminium-based mechanism). This work demonstrates the power of directed evolution to expand the reaction scope of natural aldolases to include asymmetric Michael addition reactions and presents opportunities to explore iminium catalysis with DERA-derived catalysts inspired by developments in the organocatalysis field.

Chiral anthranilic pyrrolidine as custom-made amine catalyst for enantioselective Michael reaction of nitroalkenes with carbonyl compounds

A chiral anthranilic pyrrolidine catalyst as a custom-made amine-catalyst was developed for the enantio- and diastereo selective Michael reaction of nitroalkenes with carbonyl compounds. In particular, a peptide-like catalyst in which an α-amino acid is attached to the anthranilic acid skeleton induced the high stereoselectivity of the reaction with aldehydes. Studies of the reaction mechanism indicated that the catalyst exhibits a divergent stereocontrol in the reaction, namely, steric control by a 2-substituted group on the catalyst and hydrogen-bonding control by a carboxylic acid group on the catalyst.

Recent advances in organocatalytic asymmetric aza-Michael reactions of amines and amides

Nitrogen-containing scaffolds are ubiquitous in nature and constitute an important class of building blocks in organic synthesis. The asymmetric aza-Michael reaction (aza-MR) alone or in tandem with other organic reaction(s) is an important synthetic tool to form new C-N bond(s) leading to developing new libraries of diverse types of bioactive nitrogen compounds. The synthesis and application of a variety of organocatalysts for accomplishing highly useful organic syntheses without causing environmental pollution in compliance with 'Green Chemistry" has been a landmark development in the recent past. Application of many of these organocatalysts has been extended to asymmetric aza-MR during the last two decades. The present article overviews the literature published during the last 10 years concerning the asymmetric aza-MR of amines and amides catalysed by organocatalysts. Both types of the organocatalysts, i.e., those acting through non-covalent interactions and those working through covalent bond formation have been applied for the asymmetric aza-MR. Thus, the review includes the examples wherein cinchona alkaloids, squaramides, chiral amines, phase-transfer catalysts and chiral bifunctional thioureas have been used, which activate the substrates through hydrogen bond formation. Most of these reactions are accompanied by high yields and enantiomeric excesses. On the other hand, N-heterocyclic carbenes and chiral pyrrolidine derivatives acting through covalent bond formation such as the iminium ions with the substrates have also been included. Wherever possible, a comparison has been made between the efficacies of various organocatalysts in asymmetric aza-MR.

Chiral Brønsted acid-controlled intermolecular asymmetric [2 + 2] photocycloadditions

Control over the stereochemistry of excited-state photoreactions remains a significant challenge in organic synthesis. Recently, it has become recognized that the photophysical properties of simple organic substrates can be altered upon coordination to Lewis acid catalysts, and that these changes can be exploited in the design of highly enantioselective catalytic photoreactions. Chromophore activation strategies, wherein simple organic substrates are activated towards photoexcitation upon binding to a Lewis acid catalyst, rank among the most successful asymmetric photoreactions. Herein, we show that chiral Brønsted acids can also catalyze asymmetric excited-state photoreactions by chromophore activation. This principle is demonstrated in the context of a highly enantio- and diastereoselective [2+2] photocycloaddition catalyzed by a chiral phosphoramide organocatalyst. Notably, the cyclobutane products arising from this method feature a trans-cis stereochemistry that is complementary to other enantioselective catalytic [2+2] photocycloadditions reported to date.

Lewis acids have recently been shown to enable stereocontrol in photochemical cycloadditions, a difficult task due to the reactivity of excited-state compounds. Here the authors show that chiral Brønsted acids are competent chromophore activators in [2+2] cycloadditions, forming diastereomers disfavored in similar Lewis acid catalyzed photochemical reactions.

Direct synthesis of p-methyl benzaldehyde from acetaldehyde via an organic amine-catalyzed dehydrogenation mechanism

p-Methyl benzaldehyde (p-MBA) is a class of key chemical intermediates of pharmaceuticals. Conventional industrial processes for p-MBA production involve the consecutive photochlorination, amination, and acid hydrolysis of petroleum-derived p-xylene, while producing vast pollutants and waste water. Herein, we report a direct, green route for selective synthesis of p-MBA from acetaldehyde using a diphenyl prolinol trimethylsilyl ether catalyst. The optimized p-MBA selectivity is up to 90% at an acetaldehyde conversion as high as 99.8%. Intermediate structure and ¹⁸O-isotope data revealed that the conversion of acetaldehyde to p-methylcyclohexadienal intermediates proceeds in an enamine-iminium intermediate mechanism. Then, controlled experiments and D-isotope results indicated that the dehydrogenation of p-methylcyclohexadienal to p-MBA and H₂ is catalyzed by the same amines through an iminium intermediate. This is an example that metal-free amines catalyze the dehydrogenation (releasing H₂), rather than using metals or stoichiometric oxidants.

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A direct route to produce p-methyl benzaldehyde from biomass-derived acetaldehyde

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Revealing the reaction kinetics and mechanism under reaction conditions

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An example of an organic amine-catalyzed dehydrogenation-aromatization reaction

Computational Investigation on the Origin of Atroposelectivity for the Cinchona Alkaloid Primary Amine-Catalyzed Vinylogous Desymmetrization of N-(2-t-Butylphenyl)maleimides

Mechanistic studies clarifying how chiral primary amines control the stereochemistry of vinylogous processes are rare. We report a density functional theory (DFT) computational study for the comprehension of the reaction mechanism of the vinylogous atroposelective desymmetrization of N-(2-t-butylaryl)maleimide catalyzed by 9-amino(9-deoxy)epi-quinine. Our results illustrate how the origin of the atroposelectivity was realized by the catalyst through steric and dispersion interactions. The role of N-Boc-l-Ph-glycine was crucial for the formation of a closed transition-state geometry and the activation of both reaction partners.

Organocatalytic Strategies for the Development of the Enantioselective Inverse-electron-demand Hetero-Diels-Alder Reaction

Cycloaddition reactions, in particular Diels-Alder reactions, have attracted a lot of attention from organic chemists since they represent one of the most powerful methodologies for the construction of carbon-carbon bonds. In particular, inverse-electron-demand hetero-Diels-Alder reactions have been an important breakthrough for the synthesis of heterocyclic compounds. Among all their variants, the organocatalytic enantioselective version has been widely explored since the asymmetric construction of diversely functionalized scaffolds under reaction conditions encompassed within the green chemistry field is of great interest. In this review, a profound revision on the latest advances on the organocatalytic asymmetric inverse-electron demand hetero-Diels-Alder reaction is shown.

Regiodivergent Organocatalytic Reactions

Organocatalysts are abundantly used for various transformations, particularly to obtain highly enantio- and diastereomeric pure products by controlling the stereochemistry. These applications of organocatalysts have been the topic of several reviews. Organocatalysts have emerged as one of the very essential areas of research due to their mild reaction conditions, cost-effective nature, non-toxicity, and environmentally benign approach that obviates the need for transition metal catalysts and other toxic reagents. Various types of organocatalysts including amine catalysts, Brønsted acids, and Lewis bases such as N-heterocyclic carbene (NHC) catalysts, cinchona alkaloids, 4-dimethylaminopyridine (DMAP), and hydrogen bond-donating catalysts, have gained renewed interest because of their regioselectivity. In this review, we present recent advances in regiodivergent reactions that are governed by organocatalysts. Additionally, we briefly discuss the reaction pathways of achieving regiodivergent products by changes in conditions such as solvents, additives, or the temperature.

Allenic phosphonium borate zwitterions via a phosphonium allenylidene intermediate

We describe the synthesis of alkynyl phosphanes of the type R₂P-C[triple bond, length as m-dash]C-C(OCH₃)Ph₂ (R = Ph, Cy) and investigate their transformation to geminally substituted phosphonium borato-allene zwitterions upon their reaction with B(C₆F₅)₃. The mechanism for this transformation was studied experimentally and by density functional theory computations (DFT), suggesting the intermediacy of an unsaturated 3-coordinate phosphonium electrophile akin to a methylene phosphonium cation.

Green Chemistry Meets Asymmetric Organocatalysis: A Critical Overview on Catalysts Synthesis

Can green chemistry be the right reading key to let organocatalyst design take a step forward towards sustainable catalysis? What if the intriguing chemistry promoted by more engineered organocatalysts was carried on by using renewable and naturally occurring molecular scaffolds, or at least synthetic catalysts more respectful towards the principles of green chemistry? Within the frame of these questions, this Review will tackle the most commonly occurring organic chiral catalysts from the perspective of their synthesis rather than their employment in chemical methodologies or processes. A classification of the catalyst scaffolds based on their E factor will be provided, and the global E factor (EG factor) will be proposed as a new green chemistry metric to consider, also, the synthetic route to the catalyst within a given organocatalytic process.

Organocatalytic Enantiospecific Total Synthesis of Butenolides

Biologically important, chiral natural products of butenolides, (−)-blastmycinolactol, (+)-blastmycinone, (−)-NFX-2, (+)-antimycinone, lipid metabolites, (+)-ancepsenolide, (+)-homoancepsenolide, mosquito larvicidal butenolide and their analogues were synthesized in very good yields in a sequential one-pot manner by using an organocatalytic reductive coupling and palladium-mediated reductive deoxygenation or organocatalytic reductive coupling and silica-mediated reductive deamination as the key steps.

Unlocking Iminium Catalysis in Artificial Enzymes to Create a Friedel-Crafts Alkylase

The construction and engineering of artificial enzymes consisting of abiological catalytic moieties incorporated into protein scaffolds is a promising strategy to realize non-natural mechanisms in biocatalysis. Here, we show that incorporation of the noncanonical amino acid para-aminophenylalanine (pAF) into the nonenzymatic protein scaffold LmrR creates a proficient and stereoselective artificial enzyme (LmrR_pAF) for the vinylogous Friedel-Crafts alkylation between α,β-unsaturated aldehydes and indoles. pAF acts as a catalytic residue, activating enal substrates toward conjugate addition via the formation of intermediate iminium ion species, while the protein scaffold provides rate acceleration and stereoinduction. Improved LmrR_pAF variants were identified by low-throughput directed evolution advised by alanine-scanning to obtain a triple mutant that provided higher yields and enantioselectivities for a range of aliphatic enals and substituted indoles. Analysis of Michaelis-Menten kinetics of LmrR_pAF and evolved mutants reveals that different activities emerge via evolutionary pathways that diverge from one another and specialize catalytic reactivity. Translating this iminium-based catalytic mechanism into an enzymatic context will enable many more biocatalytic transformations inspired by organocatalysis.

Revealing the Similarities of α,β-Unsaturated Iminiums and Acylazoliums in Organocatalysis

The secondary amine-catalyzed reactions proceeding via α,β-unsaturated iminiums and the N-heterocyclic carbene (NHC)-catalyzed transformations taking place via α,β-unsaturated acylazoliums are the two widely used electrophilic intermediates in organocatalysis. Over the last two decades, these two intermediates are extensively utilized for the enantioselective construction of valuable molecules. Both intermediates are generated by the covalent binding of catalysts to the substrates leading to LUMO activation of α,β-unsaturated carbonyls. A variety of soft nucleophiles are known to add to the α,β-unsaturated iminiums and acylazoliums in a conjugate fashion, and in many cases, striking similarity in reactivity has been observed. Having said this, there are few cases where these intermediates exhibit difference in reactivity. This Minireview is aimed at highlighting the resemblances in reactivity between α,β-unsaturated iminiums and acylazoliums thereby shedding light on the unnoticed parallels of the two intermediates in organocatalysis.

Diastereodivergent synthesis of 4-oxocyclohexanecarbaldehydes by using the modularly designed organocatalysts upon switching on their iminium catalysis

The cinchona thiourea moiety in the self-assembled modularly designed organocatalysts (MDOs) switches off the iminium catalysis of these catalysts. In this study, it was found that the inhibited iminium catalysis could be switched on by using an appropriate weak acid and that, once the iminium catalysis was switched on, these catalysts could be applied for the highly stereoselective and diastereodivergent synthesis of 4-oxocyclohexanecarbaldehydes via a domino reaction between ketones and α,β-unsaturated aldehydes.

The Pauli Repulsion-Lowering Concept in Catalysis

Organic chemistry has undoubtedly had a profound impact on humanity. Day in and day out, we find ourselves constantly surrounded by organic compounds. Pharmaceuticals, plastics, fuels, cosmetics, detergents, and agrochemicals, to name a few, are all synthesized by organic reactions. Very often, these reactions require a catalyst in order to proceed in a timely and selective manner. Lewis acids and organocatalysts are commonly employed to catalyze organic reactions and are considered to enhance the frontier molecular orbital (FMO) interactions. A vast number of textbooks and primary literature sources suggest that the binding of a Lewis acid or an iminium catalyst to a reactant (R1) stabilizes its LUMO and leads to a smaller HOMO(R2)-LUMO(R1) energy gap with the other reactant (R2), thus resulting in a faster reaction. This forms the basis for the so-called LUMO-lowering catalysis concept. Despite the simplicity and popularity of FMO theory, a number of deficiencies have emerged over the years, as a consequence of these FMOs not being the operative factor in the catalysis. LUMO-lowering catalysis is ultimately incomplete and is not always operative in catalyzed organic reactions. Our groups have recently undertaken a concerted effort to generate a unified framework to rationalize and predict chemical reactivity using a causal model that is rooted in quantum mechanics. In this Account, we propose the concept of Pauli repulsion-lowering catalysis to understand the catalysis in fundamental processes in organic chemistry. Our findings emerge from state-of-the-art computational methods, namely, the activation strain model (ASM) of reactivity in conjunction with quantitative Kohn-Sham molecular orbital theory (KS-MO) and a matching energy decomposition analysis (EDA). The binding of the catalyst to the substrate not only leads to a stabilization of its LUMO but also induces a significant reduction of the two-orbital, four-electron Pauli repulsion involving the key molecular orbitals of both reactants. This repulsion-lowering originates, for the textbook Lewis acid-catalyzed Diels-Alder reaction, from the catalyst polarizing the occupied π orbital of the dienophile away from the carbon atoms that form new bonds with the diene. This polarization of the occupied dienophile π orbital reduces the occupied orbital overlap with the diene and constitutes the ultimate physical factor responsible for the acceleration of the catalyzed process as compared to the analogous uncatalyzed reaction. We show that this physical mechanism is generally applicable regardless of the type of reaction (Diels-Alder and Michael addition reactions) and the way the catalyst is bonded to the reactants (i.e., from pure covalent or dative bonds to weaker hydrogen or halogen bonds). We envisage that the insights emerging from our analysis will guide future experimental developments toward the design of more efficient catalytic transformations.

Supramolecular Control of Reactivity toward Hydrolysis of 7-Diethylaminocoumarin Schiff Bases by Cucurbit[7]uril Encapsulation

A series of aromatic Schiff bases, featuring 7-diethylamino-coumarin and with five different substituents at an adjacent phenyl ring, were synthesized and characterized. With the aim of assessing the stability of these dyes in acidic medium, their hydrolysis reactions were kinetically studied in the absence and presence of the macrocycle cucurbit[7]uril (CB[7]). Our results are consistent with a model containing three different forms of substrates (un-, mono-, and diprotonated) and three parallel reaction pathways. The pK a values and the rate constants were estimated and discussed in terms of the presence of a hydroxyl group at the ortho position and electron-releasing groups on the phenyl ring of the dyes. The kinetic study in the presence of CB[7] led to two different behaviors. Promotion of the reaction by CB[7] was observed for the hydrolysis of the Schiff bases containing only one coordination site toward the macrocycle. Conversely, an inhibitor effect was observed for the hydrolysis of a Schiff base with two coordination sites toward CB[7]. The latter effect could be explained with a model as a function of a prototropic tautomeric equilibrium and the formation of a 2:1 host/guest complex, which prevents the attack of water. Therefore, the kinetic results demonstrated a supramolecular control of the macrocycle toward the reactivity and stability of 7-diethylaminocoumarin Schiff bases in acidic medium.

Catalytic Asymmetric Disulfuration by a Chiral Bulky Three-Component Lewis Acid-Base

A three-component Lewis acid-base (Lewis trio) involving a bulky chiral primary amine, B(C₆ F₅ )₃ and a bulky tertiary amine has been developed as an effective enamine catalyst for enantioselective disulfuration reactions. The bulky tertiary amine was found to activate a bulky primary-tertiary diamine-borane Lewis pair for enamine catalysis via frustrated interaction. The resulted chiral bulky Lewis trio (BLT) allows for the construction of chiral disulfides via direct disulfuration with β-ketocarbonyls or α-branched aldehydes in a practical and highly stereocontrolled manner.

Asymmetric 1,4-Michael Addition in Aqueous Medium Using Hydrophobic Chiral Organocatalysts

Organic transformations exclusively in water as an environmentally friendly and safe medium have drawn significant interest in the recent years. Moreover, transition metal-free synthesis of enantiopure molecules in water will have a great deal of attention as the system will mimic the natural enzymatic reactions. In this work, a new set of proline-derived hydrophobic organocatalysts have been synthesized and utilized for asymmetric Michael reactions in water as the sole reaction medium. Among the various catalysts screened, the catalyst 1 is indeed efficient for stereoselective 1,4-conjugated Michael additions (dr: >97:3, ee up to >99.9%) resulting in high chemical yields (up to 95%) in a very short reaction time (1 h) at room temperature. This methodology provides a robust, green, and convenient protocol and can thus be an important addition to the arsenal of the asymmetric Michael addition reaction. Upon successful implementation, the present strategy also led to the formation of an optically active octahydroindole, the key component found in many natural products.

Application of Acrolein Imines to Organic Synthesis, Biofunctional Studies, and Clinical Practice

N-alkyl unsaturated imines derived from acrolein, a toxin produced during oxidative stress, and biogenic alkyl amines occur naturally and are considered biologically relevant compounds. However, despite the recent conceptual and technological advances in organic synthesis, research on the new reactivity of these compounds is lacking. This personal account discusses research on the reactivity that has been overlooked in acrolein imines, including the discovery of new methods to synthesize biologically active compounds, the determination of new functions of relevant imines and their precursors, i. e., aldehydes and amines, and the application of these methods for clinical diagnosis.