Halo-1,2,3-triazolium Salts as Halogen Bond Donors for the Activation of Imines in Dihydropyridinone Synthesis

Halogen Bond Catalysis: A Physical Chemistry Perspective

As important noncovalent interactions, halogen bonds have been widely used in material science, supramolecular chemistry, medicinal chemistry, organocatalysis, and other fields. In the past 15 years, halogen bond catalysis has become a developed field in organocatalysis for the catalysts' advantages of being environmentally friendly, inexpensive, and recyclable. Halogen bonds can induce various organic reactions, and halogen bond catalysis has become a powerful alternative to the fully explored hydrogen bond catalysis. From a physical chemistry view, this perspective provides an overview of the latest progress and key examples of halogen bond catalysis via activation of the lone pair systems of organic functional group, π systems, and metal complexes. The research progresses in halogen bond catalysis by our group were also introduced.

Iodine(I)-based and iodine(III)-based halogen bond catalysis on the Friedel-Crafts reaction: a theoretical study

Halogen bond catalysis, especially iodine derivatives catalysis, has attracted increasing attention in recent years owing to the advantages of relatively cheap, stable, green, easy to handle, and favorable catalytic activity. To obtain insights into the catalytic mechanism and activity of halogen bond donor catalysts, iodine(I)-based and iodine(III)-based halogen bond catalysis on the Friedel-Crafts reaction were investigated in this study. The entire reaction contains several key steps: carbon-carbon bond coupling, proton transfer, hydroxyl departure, indole addition, and deprotonation process. According to the energetic span model, iodine(III)-based donor catalysts exhibit higher catalytic activity than iodine(I)-based catalysts and double cationic catalysts are more potent than single cationic ones. For halogen bond catalysis, the Gibbs energy barriers have linear relation to the electron density at the halogen bond critical points. Furthermore, the Gibbs energy barriers are also linearly related to the integral charge values of the increased region of electron density outside the oxygen atom of reactants. Therefore, the stronger halogen bond results in lower Gibbs energy barrier, and the stronger polarization further benefits the halogen bond catalysis.

Balancing Activity and Stability in Halogen-Bonding Catalysis: Iodopyridinium-Catalyzed One-Pot Synthesis of 2,3-Dihydropyridinones

A one-pot cascade reaction for 2,3-dihydropyridinone synthesis was accomplished with 3-fluoro-2-iodo-1-methylpyridinium triflate as the halogen bond catalyst. The desired [4+2] cycloaddition products, bearing aryl, heteroaryl, alkyl, and alicyclic substituents, were successfully furnished in 28-99% yields. Mechanistic investigations proved that a strong halogen-bonding interaction forged between the iodopyridinium catalyst and imine intermediate was essential to dynamically masking the vulnerable C-I bond on the catalyst and accelerating the following aza-Diels-Alder reaction.

Unravelling the Mechanism and Governing Factors in Lewis Acid and Non-Covalent Diels-Alder Catalysis: Different Perspectives

In the current literature, many non-covalent interaction (NCI) donors have been proposed that can potentially catalyze Diels-Alder (DA) reactions. In this study, a detailed analysis of the governing factors in Lewis acid and non-covalent catalysis of three types of DA reactions was carried out, for which we selected a set of hydrogen-, halogen-, chalcogen-, and pnictogen-bond donors. We found that the more stable the NCI donor-dienophile complex, the larger the reduction in DA activation energy. We also showed that for active catalysts, a significant part of the stabilization was caused by orbital interactions, though electrostatic interactions dominated. Traditionally, DA catalysis was attributed to improved orbital interactions between the diene and dienophile. Recently, Vermeeren and co-workers applied the activation strain model (ASM) of reactivity, combined with the Ziegler-Rauk-type energy decomposition analysis (EDA), to catalyzed DA reactions in which energy contributions for the uncatalyzed and catalyzed reaction were compared at a consistent geometry. They concluded that reduced Pauli repulsion energy, and not enhanced orbital interaction energy, was responsible for the catalysis. However, when the degree of asynchronicity of the reaction is altered to a large extent, as is the case for our studied hetero-DA reactions, the ASM should be employed with caution. We therefore proposed an alternative and complementary approach, in which EDA values for the catalyzed transition-state geometry, with the catalyst present or deleted, can be compared one to one, directly measuring the effect of the catalyst on the physical factors governing the DA catalysis. We discovered that enhanced orbital interactions are often the main driver for catalysis and that Pauli repulsion plays a varying role.

Halonium, chalconium, and pnictonium salts as noncovalent organocatalysts: a computational study on relative catalytic activity

This theoretical study sheds light on the relative catalytic activity of pnictonium, chalconium, and halonium salts in reactions involving elimination of chloride and electrophilic activation of a carbonyl group. DFT calculations indicate that for cationic aromatic onium salts, values of the electrostatic potential on heteroatom σ-holes gradually increase from pnictogen- to halogen-containing species. The higher values of the potential on the halogen atoms of halonium salts result in the overall higher catalytic activity of these species, but in the case of pnictonium and chalconium cations, weak interactions from the side groups provide an additional stabilization effect on the reaction transition states. Based upon quantum-chemical calculations, the catalytic activity of phosphonium(V) and arsenonium(V) salts is expected to be too low to obtain effective noncovalent organocatalytic compounds, whereas stibonium(V), telluronium(IV) and iodonium(III) salts exhibit higher potential in application as noncovalent organocatalysts.

Multifunctional Catalysts in the Asymmetric Mannich Reaction of Malononitrile with N-Phosphinoylimines: Coactivation by Halogen Bonding versus Hydrogen Bonding

A multifunctional (noncovalent) catalyst containing halogen-bond donor, hydrogen-bond donor, and Lewis basic sites was developed and applied in an enantioselective Mannich reaction between malononitrile and diphenylphosphinoyl-protected aldimine affording products in high yields (up to 98%) and moderate to high enantiomeric purities (ee up to 89%). Typically, noncovalent catalysts rely on several weak interactions to activate the substrate, with one or two of these giving the most notable contribution to activation. In this instance, instead of the initially proposed coactivation by halogen bonding, it was revealed that hydrogen bonding plays a key role in determining the enantioselectivity.

Supramolecular networks by imine halogen bonding

Halogen bonding of neutral donors using imine groups of porous organic cage compounds as acceptors leads to the formation of halogen-bonded frameworks. We report the use of two different imine cages, in combination with three electron-poor halogen bond donors. Four resulting solid-state structures elucidated by single-crystal X-ray analysis are presented and analysed for the first time by plane-wave DFT calculations and QTAIM-analyses of the entire unit cells, demonstrating the formation of halogen bonds within the networks. The supramolecular frameworks can be obtained either from solution or mechanochemically by liquid-assisted grinding.

Diaryliodoniums as Hybrid Hydrogen- and Halogen-Bond-Donating Organocatalysts for the Groebke-Blackburn-Bienaymé Reaction

Dibenziodolium and diphenyliodonium triflates display high catalytic activity for the multicomponent reaction that leads to a series of imidazopyridines. Density functional theory (DFT) calculations indicate that both the salts can play the role of hybrid hydrogen- and halogen-bond-donating organocatalysts, which electrophilically activate the carbonyl and imine groups during the reaction process. The ortho-H atoms in the vicinal position to the I atom play a dual role: forming additional noncovalent bonds with the ligated substrate and increasing the maximum electrostatic potential on the σ-hole at the iodine atom owing to the effects of polarization. Dibenziodolium triflate exhibits higher catalytic activity, and the results obtained from ¹H nuclear magnetic resonance (NMR) titrations, in conjunction with those from DFT calculations, indicate that this could be explained in terms of the additional energy required for the rotation of the phenyl ring in the diphenyliodonium cation during ligation of the substrate.

Fluoride-Mediated Nucleophilic Aromatic Amination of Chloro-1H-1,2,3-triazolium Salts

We report a fluoride-mediated nucleophilic aromatic amination of chloro-1H-1,2,3-triazolium salts with aliphatic amines. The reaction proceeded under mild reaction conditions to provide amino-1,2,3-triazolium salts with various functional groups, which can be utilized for further transformations. Moreover, it was found that an amino-1,2,3-triazolium salt was transformed via deprotonation into the N-heterocyclic imine (NHI), which exhibited the excellent catalytic activity for the cyanosilylation of acetophenone with trimethylsilyl cyanide.

Mesoionic and N-Heterocyclic Carbenes Coordinated N+ Center: Experimental and Computational Analysis

N-Heterocyclic carbenes, carbocyclic carbenes, remote N-heterocyclic carbenes and N-heterocyclic silylenes are known to form L→N⁺ coordination bonds. However, mesoionic carbenes (MICs) are not reported to form coordination bonds with cationic nitrogen. Herein, synthesis and quantum chemical studies were performed on 1,2,3-triazol-5-ylidene stabilized N⁺ center. Six compounds with MIC→N⁺ ←NHC were synthesized. Density functional theory calculations and energy decomposition analysis were carried out to explore the bonding situation between MIC and N⁺ center. The C→N⁺ bond lengths were in the range of 1.295-1.342 Å and bond dissociation energies were <400 kcal/mol. Natural bond orbital analysis supported the presence of excess electron density (>3 electrons) at the N⁺ center. The computational and X-ray diffraction analysis results confirmed the presence of divalent N^I character of center nitrogen and MIC→N⁺ ←NHC coordination interactions.

Predicting the catalytic activity of azolium-based halogen bond donors: an experimentally-verified theoretical study

This report demonstrates the successful application of electrostatic surface potential distribution analysis for evaluating the relative catalytic activity of a series of azolium-based halogen bond donors. A strong correlation (R² > 0.97) was observed between the positive electrostatic potential of the σ-hole on the halogen atom and the Gibbs free energy of activation of the model reactions (i.e., halogen abstraction and carbonyl activation). The predictive ability of the applied approach was confirmed experimentally. It was also determined that the catalytic activity of azolium-based halogen bond donors was generally governed by the structure of the azolium cycle, whereas the substituents on the heterocycle had a limited impact on the activity. Ultimately, this study highlighted four of the most promising azolium halogen bond donors, which are expected to exhibit high catalytic activity.

The Chloroazaphosphatrane Motif for Halogen Bonding in Solution

Chloroazaphosphatranes, the corresponding halogenophosphonium cations of the Verkade superbases, were evaluated as a new motif for halogen bonding (XB). Their modulable synthesis allowed for synthetizing chloroazaphosphatranes with various substituents on the nitrogen atoms. The binding constants determined from NMR titration experiments for Cl^-, Br^-, I^-, AcO^-, and CN^- anions are comparable to those obtained with conventional iodine-based monodentate XB receptors. Remarkably, the protonated azaphosphatrane counterparts display no affinity for anions under the same conditions. The strength of the XB interaction is, to some extent, related to the basicity of the corresponding Verkade superbase. The halogen bonding abilities of this new class of halogen donor motif were also revealed by the Δδ(³¹P) NMR shift observed in CD₂Cl₂ solution in the presence of triethylphosphine oxide (TEPO). Thus, chloroazaphosphatranes constitute a new class of halogen bond donors, expanding the repertory of XB motifs mainly based on C_Ar-I bonds.

Enantioselective Michael addition to vinyl phosphonates via hydrogen bond-enhanced halogen bond catalysis

An asymmetric Michael addition of malononitrile to vinyl phosphonates was accomplished by hydrogen bond-enhanced bifunctional halogen bond (XB) catalysis. NMR titration experiments were used to demonstrate that halogen bonding, with the support of hydrogen-bonding, played a key role in the activation of the Michael acceptors through the phosphonate group. This is the first example of the use of XBs for the activation of organophosphorus compounds in synthesis. In addition, the iodo-perfluorophenyl group proved to be a better directing unit than different iodo- and nitro-substituted phenyl groups. The developed approach afforded products with up to excellent yields and diastereoselectivities and up to good enantioselectivities.

Supramolecular Halogen Bonds in Asymmetric Catalysis

Halogen bonding has received a significant increase in attention in the past 20 years. An important part of this interest has centered on catalytic applications of halogen bonding. Halogen bond (XB) catalysis is still a developing field in organocatalysis, although XB catalysis has outgrown its proof of concept phase. The start of this year witnessed the publication of the first example of a purely XB-based enantioselective catalytic reaction. While the selectivity can be improved upon, there are already plenty of examples in which halogen bonds, among other interactions, play a crucial role in the outcome of highly enantioselective reactions. This paper will give an overview of the current state of the use of XBs in catalytic stereoselective processes.

Design and Synthesis of a Chiral Halogen-Bond Donor with a Sp3-Hybridized Carbon-Iodine Moiety in a Chiral Fluorobissulfonyl Scaffold

The first example of a chiral halogen-bond donor with a sp³-hybridized carbon–iodine moiety in a fluorobissulfonyl scaffold is described. The binaphthyl backbone was designed as a chiral source and the chiral halogen-bond donor (R)-1 was synthesized from (R)-1,1′-binaphthol in 11 steps. An NMR titration experiment demonstrated that (R)-1 worked as a halogen-bond donor. The Mukaiyama aldol reaction and quinoline reduction were examined using (R)-1 as a catalyst to evaluate the asymmetric induction.

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.

Halogen bonding (HB) catalysis is rapidly gaining momentum, however, cases of XB activation for challenging bonds formation are rare. Here, the authors show a robust XB catalyzed 2-deoxyglycosylation with broad scope and featuring a quantum tunneling phenomenon in the proton transfer rate determining step.

Phosphine Oxides as Spectroscopic Halogen Bond Descriptors: IR and NMR Correlations with Interatomic Distances and Complexation Energy

An extensive series of 128 halogen-bonded complexes formed by trimethylphosphine oxide and various F-, Cl-, Br-, I- and At-containing molecules, ranging in energy from 0 to 124 kJ/mol, is studied by DFT calculations in vacuum. The results reveal correlations between R–X⋅⋅⋅O=PMe₃ halogen bond energy ΔE, X⋅⋅⋅O distance r, halogen’s σ-hole size, QTAIM parameters at halogen bond critical point and changes of spectroscopic parameters of phosphine oxide upon complexation, such as ³¹P NMR chemical shift, ΔδP, and P=O stretching frequency, Δν. Some of the correlations are halogen-specific, i.e., different for F, Cl, Br, I and At, such as ΔE(r), while others are general, i.e., fulfilled for the whole set of complexes at once, such as ΔE(ΔδP). The proposed correlations could be used to estimate the halogen bond properties in disordered media (liquids, solutions, polymers, glasses) from the corresponding NMR and IR spectra.

Preorganization: A Powerful Tool in Intermolecular Halogen Bonding in Solution

Preorganization is a powerful tool in supramolecular chemistry which has been utilized successfully in intra- and intermolecular halogen bonding. In previous work, we had developed a bidentate bis(iodobenzimidazolium)-based halogen bond donor which featured a central trifluoromethyl substituent. This compound showed a markedly increased catalytic activity compared to unsubstituted bis(iodoimidazolium)-based Lewis acids, which could be explained either by electronic effects (the electron withdrawal by the fluorinated substituent) or by preorganization (the hindered rotation of the halogen bonding moieties). Herein, we systematically investigate the origin of this increased Lewis acidity via a comparison of the two types of compounds and their respective derivatives with or without the central trifluoromethyl group. Calorimetric measurements of halide complexations indicated that preorganization is the main reason for the higher halogen bonding strength. The performance of the catalysts in a series of benchmark reactions corroborates this finding.

Halogen-bonding-induced diverse aggregation of 4,5-diiodo-1,2,3-triazolium salts with different anions

The synthesis of 4,5-diiodo-1,3-dimesityl-1,2,3-triazolium salts with different anions have been developed. These triazolium salts show diverse aggregation via halogen bonding between C-I bonds and anions. Triazolium with halide anions exists as a tetramer with saddle conformation. Triazolium tetrafluoroborate exists as a trimer with Chinese lantern shape conformation. Triazolium trifluoroacetate and acetate exist as dimers, respectively, while the former shows boat conformation and the latter forms rectangle conformation. Triazolium salts form a linear polymer with polyiodide.

An Aliphatic Bischler-Napieralski Reaction: Dihydropyridones by Cyclocarbonylation of 3-Allylimidazolidin-4-ones

The N-chloroformylimidazolidinone derivative of enantiopure l-alanine was deprotonated to form an enolate and functionalized with a series of allylic halides. Treatment of the resulting carbamoyl chlorides with potassium iodide led to cyclization of the allylic substituent onto the carbonyl group in an intramolecular aliphatic Friedel-Crafts-type acylation that corresponds to an aliphatic Bischler-Napieralski reaction. The product 3,4-dihydropyridinones were amenable to further functionalization, and finally hydrolysis, to deliver a series of enantio-enriched pipecolic acid derivatives.

Visible-Light-Mediated Aerobic Tandem Dehydrogenative Povarov/Aromatization Reaction: Synthesis of Isocryptolepines

A metal‐free, photoinduced aerobic tandem amine dehydrogenation/Povarov cyclization/aromatization reaction between N‐aryl glycine esters and indoles leads to tetracyclic 11H‐indolo[3,2‐c]quinolines under mild conditions and with high yields. The reaction can be performed by using molecular iodine along with visible light, or by combining an organic photoredox catalyst with a halide anion. Mechanistic studies reveal that product formation occurs through a combination of radical‐mediated oxidation steps with an iminium ion or N‐haloiminium ion [4+2]‐cycloaddition, and the N‐heterocyclic products constitute new analogues of the antiplasmodial natural alkaloid isocryptolepine.

Lewis acid–base synergistic catalysis of cationic halogen-bonding-donors with nucleophilic counter anions

1,3,4-Triaryl-5-iodotriazolium iodides have been developed as halogen-bonding based bifunctional catalysts for simultaneous activation of nucleophiles and electrophiles.

Frontiers in Halogen and Chalcogen-Bond Donor Organocatalysis

Non-covalent molecular interactions on the basis of halogen and chalcogen bonding represent a promising, powerful catalytic activation mode. However, these "unusual" non-covalent interactions are typically employed in the solid state and scarcely exploited in catalysis. In recent years, an increased interest in halogen and chalcogen bonding has been awaken, as they provide profound characteristics that make them an appealing alternative to the well-explored hydrogen bonding. Being particularly relevant in the binding of "soft" substrates, the similar strength to hydrogen bonding interactions and its higher directionality allows for solution-phase applications with halogen and chalcogen bonding as the key interaction. In this mini-review, the special features, state-of-the-art and key examples of these so-called σ-hole interactions in the field of organocatalysis are presented.

Water/Alkali-Catalyzed Reactions of Azides with 2-Cyanothioacetamides. Eco-Friendly Synthesis of Monocyclic and Bicyclic 1,2,3-Thiadiazole-4-carbimidamides and 5-Amino-1,2,3-triazole-4-carbothioamides

The reactions of thioamides with azides in water were studied. It was reliably shown that the reaction of 2-cyanothioacetamides 1 with various types of azides 2 in water in the presence of alkali presents an efficient, general, one-step, atom-economic, and eco-friendly method for the synthesis of 1,2,3-thiadiazol-4-carbimidamides 5 and 1,2,3-triazole-4-carbothioamides 4. This method can be extended to the one-pot reaction of sulfonyl chlorides and 6-chloropyrimidines 2'o with sodium azide, leading to final products in higher yields, that is, avoiding the isolation of unsafe sulfonyl azides. The method was furthermore applied to the reaction of N,N'-bis-(2-cyanothiocarbonyl)pyrazine 1h with sulfonyl azides to afford bicyclic 1,2,3-thiadiazoles 8 and 1,2,3-triazoles 9 connected via a 1,1'-piperazinyl linker. 2-Cyanothioacetamides 1 were also shown to react with aromatic azides in water in the presence of alkali to afford 1-aryl-5-amino-1,2,3-triazole-4-carbothioamides 11. In contrast to aromatic azides and similarly to sulfonyl azides, 6-azidopyrimidine-2,4-diones 2o-q react with cyanothioacetamides to form N-pyrimidin-6-yl-5-dialkylamino-1,2,3-thiadiazole-4-N-l-carbimidamides 12. A mechanism was proposed to rationalize the role of water in changing the reactivity of azides toward 2-cyanothioacetamides.

Tunable chiral triazole-based halogen bond donors: assessment of donor strength in solution with nitrogen-containing acceptors

Strong halogen bond (XB) donors are needed for the activation of neutral substrates. We demonstrate that XB donor properties of iodo-triazoles can be significantly enhanced by quaternization in combination with varying the counterion and aromatic substituent, exemplified by association constants with quinuclidine as high as 1.1 × 104 M-1.