Noncovalent Organocatalysis Research Articles

On‐off Pulsed Ultrasound Promote Asymmetric Noncovalent Organocatalysis “on Water”: An Investigation in Michael Addition and Diels‐Alder Reactions

AbstractPulsed ultrasonic irradiation with on‐off modulation remarkably enhances asymmetric noncovalent organocatalysis “on water”, specifically for Michael addition and Diels‐Alder reactions. This approach effectively addresses challenges associated with solid reactant dispersion encountered in “on‐water” reactions and mitigates overheating issues caused by continuous ultrasound exposure, thereby significantly augmenting reaction rates and stereoselectivities.

Carbene-catalyzed and Pnictogen Bond-assisted Access to PIII-Stereogenic Compounds.

Intermolecular pnictogen bonding (PnB) catalysis has received increased interest in non-covalent organocatalysis. It has been demonstrated that organic electron-deficient pnictogen atoms can act as prospective Lewis acids. Here, we present a catalytic approach for the asymmetric synthesis of chiral PIII compounds by combining intramolecular PnB interactions and carbene catalysis. Our design features a pre-chiral phosphorus molecules bearing two electron-withdrawing benzoyl groups, resulting in the formation of a σ-hole at the P atom. X-ray and non-covalent interaction (NCI) analysis indicate that these phosphorus substrates exhibit intrinsic PnB interactions between the oxygen atom of the formyl group and the phosphorus atom. This induces a conformational locking effect, leading to the crystallization of the phosphorus substrates in a preferred conformation (P212121 chiral group). Under the catalysis of N-heterocyclic carbene, the aldehyde moiety activated by the pnictogen bond selectively reacts with an alcohol to yield the corresponding chiral monoester/phosphorus products with excellent enantioselectivity. This Lewis acidic phosphorus center, aroused by the non-polarized intramolecular pnictogen bond interaction, assists in conformational and selective regulations, providing unique opportunities for catalysis and beyond.

Cooperative Covalent–Noncovalent Organocatalysis of the Knoevenagel Condensation Based on an Amine and Iodonium Salt Mixture

AbstractThe experimentally obtained kinetic data has indicated the existence of a synergetic cocatalytic effect provided by an iodonium salt in the base‐catalyzed Knoevenagel condensation. The diphenyliodonium triflate serving as the halogen bond‐donating Lewis acid provides the higher cocatalytic effect than zinc(II) triflate or triflic acid serving, respectively, as the metal cation‐based Lewis acid and Brønsted acid. Such a cocatalytic effect remains the same for a broad scope of carbonyl compounds covering aldehydes featuring electron‐withdrawing or electron‐donating substituents, as well as ketone involved in the reaction.

Unravelling the Development of Non-Covalent Organocatalysis in India

AbstractThis review is devoted to underpinning the contributions of Indian researchers towards asymmetric organocatalysis. More specifically, a comprehensive compilation of reactions mediated by a wide range of non-covalent catalysis is illustrated. A detailed overview of vividly catalogued asymmetric organic transformations promoted by hydrogen bonding and Brønsted acid catalysis, alongside an assortment of catalysts is provided. Although asymmetric organocatalysis has etched itself in history, we aim to showcase the scientific metamorphosis of Indian research from baby steps to large strides within this field. 1 Introduction2 Non-Covalent Catalysis and Its Various Activation Modes3 Hydrogen-Bonding Catalysis3.1 Urea- and Thiourea-Derived Organocatalysts3.1.1 Thiourea-Derived Organocatalysts3.1.2 Urea-Derived Organocatalysts3.2 Squaramide-Derived Organocatalysts3.2.1 Michael Reactions3.2.2 C-Alkylation Reactions3.2.3 Mannich Reactions3.2.4 [3+2] Cycloaddition Reactions3.3 Cinchona-Alkaloid-Derived Organocatalysts3.3.1 Michael Reactions3.3.2 Aldol Reactions3.3.3 Friedel–Crafts Reactions3.3.4 Vinylogous Alkylation of 4-Methylcoumarins3.3.5 C-Sulfenylation Reactions3.3.6 Peroxyhemiacetalisation of Isochromans3.3.7 Diels–Alder Reactions3.3.8 Cycloaddition Reactions3.3.9 Morita–Baylis–Hilman Reactions4 Brønsted Acid Derived Organocatalysts4.1 Chiral Phosphoric Acid Catalysis4.1.1 Diels–Alder Reactions4.1.2 Addition of Ketimines4.1.3 Annulation of Acyclic Enecarbamates5 Conclusion

Xenon Derivatives as Aerogen Bond-Donating Catalysts for Organic Transformations: A Theoretical Study on the Metaphorical "Spherical Cow in a Vacuum" Provides Insights into Noncovalent Organocatalysis.

Computations indicate that cationic and noncharged xenon derivatives should exhibit higher catalytic activity than their iodine-based noncovalent organocatalytic congeners. Perfluorophenyl xenonium(II) is expected to demonstrate the best balance between catalytic activity and chemical stability for use in organocatalysis. Comparing its catalytic activity with that of isoelectronic perfluoroiodobenzene indicates that the high catalytic activity of cationic noncovalent organocatalysts is predominantly attributed to the electrostatic interactions with the reaction substrates, which cause the polarization of ligated species during the reaction progress. In contrast, the electron transfer and covalent contributions to the bonding between the catalyst and substrate have negligible effects. The dominant effect of electrostatic interactions results in a strong negative correlation between the calculated Gibbs free energies of activation for the modeled reactions and the highest potentials of the σ-holes on the central atoms of the catalysts. No such correlation is observed for noncharged catalysts.

Attaching Onto or Inserting Into an Intramolecular Hydrogen Bond: Exploring and Controlling a Chirality-Dependent Dilemma for Alcohols

Prereactive complexes in noncovalent organocatalysis are sensitive to the relative chirality of the binding partners and to hydrogen bond isomerism. Both effects are present when a transiently chiral alcohol docks on a chiral α-hydroxy ester, turning such 1:1 complexes into elementary, non-reactive model systems for chirality induction in the gas phase. With the help of linear infrared and Raman spectroscopy in supersonic jet expansions, conformational preferences are investigated for benzyl alcohol in combination with methyl lactate, also exploring p-chlorination of the alcohol and the achiral homolog methyl glycolate to identify potential London dispersion and chirality effects on the energy sequence. Three of the four combinations prefer barrierless complexation via the hydroxy group of the ester (association). In contrast, the lightest complex predominantly shows insertion into the intramolecular hydrogen bond, such as the analogous lactate and glycolate complexes of methanol. The experimental findings are rationalized with computations, and a uniform helicality induction in the alcohol by the lactate is predicted, independent of insertion into or association with the internal lactate hydrogen bond. p-chlorination of benzyl alcohol has a stabilizing effect on association because the insertion motif prevents a close contact between the chlorine and the hydroxy ester. After simple anharmonicity and substitution corrections, the B3LYP-D3 approach offers a fairly systematic description of the known spectroscopic data on alcohol complexes with α-hydroxy esters.

Synthesis of α-chiral phosphine sulfides via non-covalent organocatalysis

Despite rapid advances in phosphine-containing compound as a privileged ligand for transition metal catalysis, synthesis and coordination chemistry are underexplored for the phosphine sulfide analogs. In this article, we report the enantioselective phospha-Michael addition reaction that leads to α-chiral phosphine sulfide. An independent hydrogen-bonding co-catalyst is found essential to high enantioselectivity (up to 98% yield and 95% enantiomeric excess [ee]). Divergent derivatizations under mild conditions are also attempted for the generic phosphine and phosphine oxide compounds, further emphasizing the bridge role of phosphine sulfide. An X-ray diffraction structure of κ 3 -NNS palladium complex also authenticates the coordination ability of p = S moiety. Combination of NHC and hydrogen bonding donor catalysis α-Chiral phosphine sulfides Downstream derivatizations κ 3 -NNS palladium complex In an effort to diversify the family of phosphine ligands, Li et al. report the orchestration of NHC catalysis and an H-bond donor as a straightforward synthetic strategy to yield α-chiral phosphine sulfides. Preliminary results for downstream derivatization and metal chelation further demonstrate the applications for p = S ligands.

Direct, Asymmetric Synthesis of Carbocycle‐Fused Uracils via [4+2] Cycloadditions: a Noncovalent Organocatalysis Approach

AbstractThe peculiar versatility of remotely enolizable 6‐methyluracil‐5‐carbaldehydes as useful vinylogous pronucleophiles in direct, asymmetric [4+2] cyclizations with suitable nitroolefins has been demonstrated. Under the strategic exploitation of noncovalent bifunctional organocatalysis, a dearomative remote enolization strategy was implemented, to generate oQDM‐type dienolate intermediates that were efficiently and stereoselectively trapped by either aromatic or aliphatic nitroolefins. A series of functionalized, chiral carbocycle‐fused uracils embedding three contiguous stereocenters were thus collected in one step in good yields, with generally good levels of enantioselectivity, and complete diastereocontrol. Furthermore, the ability to provide enantiopure products via simple one‐cycle recrystallizations and the possibility to further functionalize these scaffolds without losing their chiral integrity were demonstrated.magnified image

A robust and tunable halogen bond organocatalyzed 2-deoxyglycosylation involving quantum tunneling

The development of noncovalent halogen bonding (XB) catalysis is rapidly gaining traction, as isolated reports documented better performance than the well-established hydrogen bonding thiourea catalysis. However, convincing cases allowing XB activation to be competitive in challenging bond formations are lacking. Herein, we report a robust XB catalyzed 2-deoxyglycosylation, featuring a biomimetic reaction network indicative of dynamic XB activation. Benchmarking studies uncovered an improved substrate tolerance compared to thiourea-catalyzed protocols. Kinetic investigations reveal an autoinductive sigmoidal kinetic profile, supporting an in situ amplification of a XB dependent active catalytic species. Kinetic isotopic effect measurements further support quantum tunneling in the rate determining step. Furthermore, we demonstrate XB catalysis tunability via a halogen swapping strategy, facilitating 2-deoxyribosylations of D-ribals. This protocol showcases the clear emergence of XB catalysis as a versatile activation mode in noncovalent organocatalysis, and as an important addition to the catalytic toolbox of chemical glycosylations.

Cover Picture: Bidentate Chiral Bis(imidazolium)‐Based Halogen‐Bond Donors: Synthesis and Applications in Enantioselective Recognition and Catalysis (Angew. Chem. Int. Ed. 17/2020)

Halogen bonding is by now established in noncovalent organocatalysis, but enantioselective applications are still very rare. In their Communication on page 6806, S. M. Huber et al. show that chiral bidentate halogen bond donors are able to differentiate the enantiomers of a diamine via NMR spectroscopy and that they can also be used to induce moderate enantioselectivity in a Mukaiyama aldol reaction. This is the first case in which enantioinduction was achieved with a “pure” halogen bond donor.

Carbonyl Activation by Selenium- and Tellurium-Based Chalcogen Bonding in a Michael Addition Reaction.

In the last years the use of chalcogen bonding—the noncovalent interaction involving electrophilic chalcogen centers—in noncovalent organocatalysis has received increased interest, particularly regarding the use of intermolecular Lewis acids. Herein, we present the first use of tellurium‐based catalysts for the activation of a carbonyl compound (and only the second such activation by chalcogen bonding in general). As benchmark reaction, the Michael‐type addition between trans‐crotonophenone and 1‐methylindole (and its derivatives) was investigated in the presence of various catalyst candidates. Whereas non‐chalcogen‐bonding reference compounds were inactive, strong rate accelerations of up to 1000 could be achieved by bidentate triazolium‐based chalcogen bond donors, with product yields of >90 % within 2 h of reaction time. Organotellurium derivatives were markedly more active than their selenium and sulphur analogues and non‐coordinating counterions like BArF 4 provide the strongest dicationic catalysts.

Catalysis with Pnictogen, Chalcogen, and Halogen Bonds.

Halogen‐ and chalcogen‐based σ‐hole interactions have recently received increased interest in non‐covalent organocatalysis. However, the closely related pnictogen bonds have been neglected. In this study, we introduce conceptually simple, neutral, and monodentate pnictogen‐bonding catalysts. Solution and in silico binding studies, together with high catalytic activity in chloride abstraction reactions, yield compelling evidence for operational pnictogen bonds. The depth of the σ holes is easily varied with different substituents. Comparison with homologous halogen‐ and chalcogen‐bonding catalysts shows an increase in activity from main group VII to V and from row 3 to 5 in the periodic table. Pnictogen bonds from antimony thus emerged as by far the best among the elements covered, a finding that provides most intriguing perspectives for future applications in catalysis and beyond.

Non-Covalent Organocatalyzed Domino Reactions Involving Oxindoles: Recent Advances.

The ubiquitous presence of spirooxindole architectures with several functionalities and stereogenic centers in bioactive molecules has been appealing for the development of novel methodologies seeking their preparation in high yields and selectivities. Expansion and refinement in the field of asymmetric organocatalysis have made possible the development of straightforward strategies that address these two requisites. In this review, we illustrate the current state-of-the-art in the field of spirooxindole synthesis through the use of non-covalent organocatalysis. We aim to provide a concise overview of very recent methods that allow to the isolation of unique, densely and diversified spirocyclic oxindole derivatives with high structural diversity via the use of cascade, tandem and domino processes.

ChemInform Abstract: Organocatalysis by Networks of Cooperative Hydrogen Bonds: Enantioselective Direct Mannich Addition to Preformed Arylideneureas.

The concept of noncovalent organocatalysis by means of networks of cooperative hydrogen bonds (NCHB organocatalysis) has been explored. Arylideneureas were chosen as ideal substrates because of their powerful donor-acceptor properties. We have examined their uncatalyzed, direct Mannich reaction with acetoacetates in comparison with that catalyzed by a number of salan derivatives capable of providing a network of cooperative hydrogen bonds. Catalyst D [(R,R)-N,N'-bis(salicyl)cyclohexane-1,2-diamine] was found to drive the above direct Mannich reaction in an enantioselective manner, thereby allowing the synthesis of several Biginelli dihydropyrimidinones with high enantioselectivity. DFT calculations (B3LYP-D-PCM/6-31+G*//B3LYP/6-31+G*) revealed that the NCHB organocatalyst lowers the energy barrier of the reaction. The NCHB organocatalysts appear to function as biomimetic catalysts.

ChemInform Abstract: Non‐covalent Organocatalysis in Asymmetric Oxidative C(sp3)—H Bond Functionalization — Broadening C—H Bond Coupling Reactions

AbstractReview: [19 refs.

Organocatalysis by Networks of Cooperative Hydrogen Bonds: Enantioselective Direct Mannich Addition to Preformed Arylideneureas.

The concept of noncovalent organocatalysis by means of networks of cooperative hydrogen bonds (NCHB organocatalysis) has been explored. Arylideneureas were chosen as ideal substrates because of their powerful donor-acceptor properties. We have examined their uncatalyzed, direct Mannich reaction with acetoacetates in comparison with that catalyzed by a number of salan derivatives capable of providing a network of cooperative hydrogen bonds. Catalyst D [(R,R)-N,N'-bis(salicyl)cyclohexane-1,2-diamine] was found to drive the above direct Mannich reaction in an enantioselective manner, thereby allowing the synthesis of several Biginelli dihydropyrimidinones with high enantioselectivity. DFT calculations (B3LYP-D-PCM/6-31+G*//B3LYP/6-31+G*) revealed that the NCHB organocatalyst lowers the energy barrier of the reaction. The NCHB organocatalysts appear to function as biomimetic catalysts.

Organocatalysis by Networks of Cooperative Hydrogen Bonds: Enantioselective Direct Mannich Addition to Preformed Arylideneureas

AbstractThe concept of noncovalent organocatalysis by means of networks of cooperative hydrogen bonds (NCHB organocatalysis) has been explored. Arylideneureas were chosen as ideal substrates because of their powerful donor–acceptor properties. We have examined their uncatalyzed, direct Mannich reaction with acetoacetates in comparison with that catalyzed by a number of salan derivatives capable of providing a network of cooperative hydrogen bonds. Catalyst D [(R,R)‐N,N′‐bis(salicyl)cyclohexane‐1,2‐diamine] was found to drive the above direct Mannich reaction in an enantioselective manner, thereby allowing the synthesis of several Biginelli dihydropyrimidinones with high enantioselectivity. DFT calculations (B3LYP‐D‐PCM/6‐31+G*//B3LYP/6‐31+G*) revealed that the NCHB organocatalyst lowers the energy barrier of the reaction. The NCHB organocatalysts appear to function as biomimetic catalysts.

Non-covalent organocatalysis in asymmetric oxidative C(sp3)–H bond functionalization – broadening C–H bond coupling reactions

In this highlight, the design of cooperative non-covalent organocatalysis for the development of novel asymmetric oxidative C–H bond functionalization reactions is presented.

Iterative Coupling of Two Different Enones by Nitromethane Using Bifunctional Thiourea Organocatalysts. Stereocontrolled Assembly of Cyclic and Acyclic Structures.

An organocatalytic iterative assembly line has been developed in which nitromethane was sequentially coupled with two different enones using a combination of pseudoenantiomeric cinchona-based thiourea catalysts. Application of unsaturated aldehydes and ketones in the second step of the iterative sequence allows the construction of cyclic syn-ketols and acyclic compounds with multiple contiguous stereocenters. The combination of the multifunctional substrates and ambident electrophiles rendered some organocatalytic transformations possible that have not yet been realized in bifunctional noncovalent organocatalysis.

Unprecedented Hydrophobic Amplification in Noncovalent Organocatalysis “on Water”: Hydrophobic Chiral Squaramide Catalyzed Michael Addition of Malonates to Nitroalkenes

In this study, water was demonstrated to be an exceptionally efficient reaction medium for the noncovalent, hydrogen-bonding-promoted enantioselective Michael addition of malonates to diverse nitroolefins using cinchona-based squaramide catalysts. A significant increase in the reaction rate was observed when the reaction was performed “on water” rather than in the conventional organic solvents, because of the hydrophobic hydration effect. This hydrophobic amplification was significantly dependent upon the hydrophobicity of the C3-substituent (vinyl or ethyl) of cinchona-based catalysts. Thus, the use of more hydrophobic catalyst with an ethyl group at the C3-position, even a highly challenging Michael donor such as dimethyl methylmalonate was also smoothly converted to the desired adduct. Furthermore, because of the remarkable rate acceleration under “on water” conditions, the catalyst loading also significantly decreased. Thus, in the case of β-ketoesters, even 0.01 mol % of catalyst loading was enough t...