Iminium Salts Research Articles

Redox Cycling with Tellurium. Si-H Bond Activation by a Lewis Superacidic Tellurenyl Cation.

The C,N-chelated aryltellurenyl triflate [2-(tBuNCH)C6H4Te][OTf] (1) activates the Si-H bonds in the tertiary silanes R3SiH via Umpolung of H- to H+ to give rise to the iminium salts (tBuN(H)CH)C6H4TeSiR3][OTf] (2R, R = Et, Ph (elusive) and R = Si(CH3)3 isolated; OTf = O3SCF3) comprising Te-Si bonds, which are capable of generating silyl triflates, R3SiOTf Under attack of a second equivalent of 1 The unprecedented Si-H activation was utilized in main group redox catalysis using p-quinones, which were converted into (silylated) hydroquinones.

Ring Contraction by Rearrangement of Sterically Congested Cyclic (Amino)(aryl)carbenes.

The rearrangement of sterically congested cyclic (amino)(aryl)carbenes (CAArCs) by the reaction of related iminium salts with potassium bis(trimethylsilyl)amide is reported, allowing for forming benzocyclobutanimines via a ring contraction process. Mechanistic studies by theoretical calculations indicate that the formation of conjugated ketenimines as intermediates could be considered, in which steric hindrance caused by N-alkyl motifs of CAArCs plays an important role in promoting the ring-opening by the cleavage of C-N bond.

Cyclic (Alkyl)(Amino)Carbene-Iminoboryl Compounds with Three Formal Oxidation States.

BN/CC isosterism is an effective strategy to build hybrid functional molecules with unique properties. In contrast to the alkynyl iminium salts derived from cyclic (alkyl)(amino)carbenes (CAACs) that feature only one reversible reduction wave, the isoelectronic cationic CAAC-iminoboryl adducts could be singly and doubly reduced smoothly. Both the resultant neutral radical and anionic azaborataallenes bear NBC-mixed allenic structures. The former radical has a high spin-density of 0.55e at CCAAC carbon, yet exhibits formal boron-centered radical reactivity. The latter azaborataallenes feature the nucleophilic CCAAC center and polar N(δ-)═B(δ+)═C(δ-) unit, and readily undergo nucleophilic substitution, isocyanide insertion, dipolar addition and cycloaddition reactions etc. The N-substituents have been shown to have a significant influence on the solid-state structure, thermal stability, and reactivity of azaborataallenes. This work showcases the allenic BN-unsaturated species as versatile building blocks in organic synthesis.

CBr4 as a Mild Oxidant-Enabled Oxidation of a sp3 C-H Bond: A Facile Synthesis of the Persistent Iminium Salts of Tetrahydroisoquinolines.

Using CBr4 as a mild oxidant, the direct C-H bond oxidation of N-aryltetrahydroisoquinolines was achieved, giving a series of the corresponding iminium salts in high yields under metal- and photo-free reaction conditions. This reaction is superior in high yields and good functional group tolerance, and the late-stage derivatization showed that these iminium salts can readily be applied to the synthesis of the functionalized THIQs.

Intramolecular trapping of an iminium salt: rapid construction of quindoline derivatives.

Construction of the pyridine ring is a practical and streamline way to construct a variety of quindoline derivatives. We have developed a novel method for synthesis of quindoline derivatives by means of intramolecular ring-closure reactions of 3-N-methylphenylindoles via an iminium salt intermediate. This practical method has the advantages of a short reaction time, operational simplicity, and nearly quantitative yields; and it can be used for the rapid synthesis of a variety of valuable quindoline derivatives.

Stereoselective mechanochemical synthesis of thiomalonate Michael adducts via iminium catalysis by chiral primary amines.

The study presents a novel approach utilizing iminium salt activation and mild enolization of thioesters, offering an efficient and rapid synthesis of Michael adducts with promising stereoselectivity and marking a significant advancement in mechanocatalysis. The stereoselective addition of bisthiomalonates 1-4 to cyclic enones and 4-chlorobenzylideneacetone proceeds stereoselectively under iminium activation conditions secured by chiral primary amines, in contrast to oxo-esters as observed in dibenzyl malonate addition. Mild enolization of thioesters allows for the generation of Michael adducts with good yields and stereoselectivities. Reactions in a ball mill afford product formation with similar efficacy to solution-phase reactions but with slightly reduced enantioselectivity, yet they yield products in just one hour compared to 24 or even 168 hours in solution-based reactions. It is noteworthy that this represents one of the early reports on the application of iminium catalysis using first-generation chiral amines under mechanochemical conditions, along with the utilization of easily enolizable thioesters as nucleophiles in this transformation.

Oxidation Chemistry of Bicarbonate and Peroxybicarbonate: Implications for Carbonate Management in Energy Storage.

Carbonate formation presents a major challenge to energy storage applications based on low-temperature CO2 electrolysis and recyclable metal-air batteries. While direct electrochemical oxidation of (bi)carbonate represents a straightforward route for carbonate management, knowledge of the feasibility and mechanisms of direct oxidation is presently lacking. Herein, we report the isolation and characterization of the bis(triphenylphosphine)iminium salts of bicarbonate and peroxybicarbonate, thus enabling the examination of their oxidation chemistry. Infrared spectroelectrochemistry combined with time-resolved infrared spectroscopy reveals that the photoinduced oxidation of HCO3- by an Ir(III) photoreagent results in the generation of the short-lived bicarbonate radical in less than 50 ns. The highly acidic bicarbonate radical undergoes proton transfer with HCO3- to furnish the carbonate radical anion and H2CO3, leading to the eventual release of CO2 and H2O, thus accounting for the appearance of H2O and CO2 in both electrochemical and photochemical oxidation experiments. The back reaction of the carbonate radical subsequently oxidizes the Ir(II) photoreagent, leading to carbonate. In the absence of this back reaction, dimerization of the carbonate radical provides entry into peroxybicarbonate, which we show undergoes facile oxidation to O2 and CO2. Together, the results reported identify tangible pathways for the design of catalysts for the management of carbonate in energy storage applications.

2-[2,6-Diisopropylphenyl]-4-phenyl-5H-5,9b[1′,2′]-benzonaphtho[1,2-b]pyrrol-2-ium Tetrafluoroborate

A novel α,β-unsaturated iminium salt (3) incorporated into a rigid dibenzobarrelene backbone was synthesized by heating N-(anthracen-9-ylmethyl)-2,6-diisopropylaniline (2) and 3-phenyl-2-propynal in THF in the presence of excess amounts of magnesium sulfate and 0.5 equivalents of an HBF4-Et2O complex. The molecular structure of 3 was characterized unambiguously by NMR spectroscopy and single-crystal X-ray diffraction (SCXRD) analyses. Compound 3 exhibits yellow luminescence in CH2Cl2 (λem = 516 nm) and in the solid state (λem = 517 nm) with relatively high to moderate quantum yields (ΦF(CH2Cl2) = 0.63; ΦF(solid) = 0.34).

Recent Advances in Catalytic and Technology-Driven Radical Addition to N,N-Disubstituted Iminium Species.

Recently, radical chemistry has grown exponentially in the toolbox of organic synthetic chemists. Upon the (re)introduction of modern catalytic and technology-driven strategies, the implementation of highly reactive radical species is currently facilitated while expanding the scope of numerous synthetic methodologies. In this context, this review intends to cover the recent advances in radical-based transformations of N,N-disubstituted iminium substrates that encompass unique reactivities with respect to imines or protonated iminium salts. In particular, we have focused on the literature concerning the dipole type substrates, such as nitrones or azomethine imines, together with the chemistry of N+-X- (X = O, NR) azaarenium dipoles, which proved to be very versatile platforms in that field of research. The N-alkylazaarenium salts were been considered, which demonstrated specific reactivity profiles in radical chemistry.

Development of an Environmentally Friendly Extraction Method Using Smartphone-Based Digital Images for the Determination of Total Sulfonamides in Meat Samples

Sulfonamides (SAs) are low-cost synthetic antimicrobials widely used in veterinary and human medicine to treat diseases and prevent infections. However, long periods of exposure to sulfonamides can cause adverse effects such as allergic reactions. This work aims to study dispersive solid-liquid microextraction as an alternative method for extracting total SAs in meat samples. The procedure uses a colorimetric reaction based on the formation of a pink compound (imine salt) to determine total sulfonamides (sulfamethazine, sulfadimethoxine, sulfathiazole) with digital measurements. A linear response was observed between 33-233 μg kg-1 for total sulfonamides, and the coefficient of variation (n = 11; 67 μg kg-1 of total SAs) and the limit of detection were estimated to be 0.63% and 10 μg kg-1, respectively. For a 750 mg meat sample, 0.11 mg of 4-dimethylaminocinnamaldehyde, 2.60 mg of sodium dodecyl sulfate, and 275 μL of 1-butanol were consumed per sample, and consequently, generating only 335 μL of residue. Besides this, addition-recovery tests were performed, resulting in a 71-100% recovery range, indicating the trueness of the proposed method.

Construction, photophysical properties, structure-activity relationship, and applications of fluorescein analogs

The investigation on the construction strategy, relationship between the structure and photophysical properties of fluorescein analogs plays an important role in the rational design of specific fluorescent dyes. In this work, a fluorescein analog Fc-1, containing a reactive aldehyde group was prepared using 2′, 7′-dichlorofluorescein (DCF) as the raw material and Vilsmeier–Haack reaction mechanism. Meanwhile, two more fluorescein analogs Fc-2 and Fc-3 were synthesized based on Fc-1 and the Knoevenagel reaction mechanism. The results of Fc-1 indicated that the yield of Fc-1 was higher than 95% after a step of the low-temperature hydrolysis (40 °C) from an intermediate imine salt DCF-3. The fluorescence quantum yield (Φ) of Fc-1 was relatively high in polar protic solvents, even in H2O (Φ = 0.25). Fc-1 had large Stokes shifts in different solvents (≧ 123 nm), which was indentified by the Gaussian calculation of Fc-1. Meanwhile, Fc-1 was also used as a ratio and colorimetric fluorescent probe for H2S in vitro, and the dianion ([Fc-1-2H]2-) mechanism for detecting H2S was proposed by nuclear magnetic resonance (NMR), pH titration, and Gaussian calculation. Additionally, Fc-1 can be used in leukocyte classification because of its relatively high Φ in water, cell permeability, and cytotoxicity. All the data of dyes Fc-2 and Fc-3 indicated that the derivatization of the carboxyl group for Fc-2 did not affect the photophysical properties of Fc-2. Both of them had relatively large Stokes shifts and long emission wavelengths. The large Stokes shift of Fc-2 and Fc-3 was due to the distribution of electron clouds in the excited states of Fc-2 and Fc-3. The results of pH titration proved that dyes Fc-2 and Fc-3 were sensitive to the changes in pH and exhibited their potentials as pH probes. Thus, the strategy of constructing fluorescent dyes based on Vilsmeier–Haack and Knoevenagel reactions and the relationship between structure and properties will be beneficial to developing more dyes with large Stokes shift.

Computational Comparison of the Stability of Iminium Ions and Salts from Enals and Pyrrolidine Derivatives (Aminocatalysts)

AbstractThe energies of CH2=CH−CH=N+R2+HNR*2→CH2=CH−CH=N+R*2+HNR2 reactions (exchange of propenal between two secondary amines) and of similar equilibria with cinnamaldehyde have been calculated and compared. Iminium ions from pyrrolidines with substituents that can help stabilize the positive charge are especially stable in the gas phase, as expected, whereas in very polar solvents the predicted order of stability (of their iminium ions) is: O‐tert‐butyldiphenylsilylprolinol>pyrrolidine>O‐methylprolinol>2‐tert‐butylpyrrolidine>Jørgensen‐Hayashi catalyst>2‐tritylpyrrolidine>N,N‐dimethylprolinamide>trimethylsilyl prolinate>3‐triflamidopyrrolidine>methyl prolinate≫MacMillan‐1 catalyst>MacMillan‐2 catalyst. When ion pairs such as iminium tetrafluoroborates, in CHCl3, are compared, the order is similar. These data can be used to predict which iminium salts may predominate when two or more secondary amines and appropriate acids are added to conjugated carbonyl compounds.

S-Shaped Fused Azacorannulene Dimer: Structural and Redox Properties

S-Shaped Fused Azacorannulene Dimer: Structural and Redox Properties

One pot synthesis of propargylamines by three component amine-aldehyde-acetylene (A3) coupling catalyzed by neutral Ag(I) and Au(I) and cationic Pd(II) and Ni(II) complexes of a pincer N-heterocyclic carbene

The amine−aldehyde−acetylene (A3) coupling yielding propargylamines was successfully catalyzed by a series of neutral silver and gold complexes, [2,6-bis-(1-(1S)-menthyl-3-methylene-imidazol-2-ylidene)-pyridine]M2Cl2 [M = Ag (2), Au (3)], and cationic palladium and nickel complexes {[2,6-bis-(1-(1S)-menthyl-3-methylene-imidazol-2-ylidene)-pyridine]MCl}X [M = Pd, X = Cl (4), BF4 (5); M = Ni, X = Cl (6), BF4 (7)], stabilized over a new (1R)-(−)-menthyl substituted bis(imidazole) based [CcarbeneNCcarbene] type pincer ligand. The Au(I) (3), Pd(II) (4) and Ni(II) (6) complexes were conveniently synthesized by transmetallation reaction from the Ag(I) analog (2), which was obtained from the N-heterocyclic carbene ligand precursor (1) by treatment with Ag2O. The anion exchange reaction of Pd(II) (4) and Ni(II) (6) with AgBF4 yielded the corresponding Pd(II) (5) and Ni(II) (7) complexes respectively. Mass spectrometry studies suggested the formation of a catalytically active metal acetylide species namely, [NHC]M2(CCPh)Cl [M = Ag (2a), Au (3a)] and {[NHC]M(CCPh)}X [M = Pd, X = Cl (4a), BF4 (5a); M = Ni, X = Cl (6a), BF4 (7a)] (where NHC = 2,6-bis-(1-(1S)-menthyl-3-methylene-imidazol-2-ylidene)-pyridine). which mediates the formation of the propargylamine with a protonated Schiff imine salt produced in situ from an aldehyde and an amine in the single pot reaction.

Iminium-substituted 2,3-dihydroisoxazoles: synthesis from acetylenic iminium salts and nitrones, and some transformations

Abstract Iminium-functionalized 2,3-dihydroisoxazoles (4-isoxazolines) were prepared in high yields by [3+2] cycloaddition of N-methyl-C-phenylnitrone and acetylenic iminium salts. Conversion of the very hygroscopic cycloadducts into neutral compounds was achieved by hydrolysis or hydride reduction of the iminium functional group. By a thermal rearrangement, the cycloadducts were converted into 2,3-dihydro-1H-imidazolium salts. With C,N-diphenylnitrone as the reaction partner, the expected cycloadduct, a [(2,3-dihydroisoxazol-4-yl)-1-(phenyl)methylene]-N,N-dimethylammonium triflate, could be prepared at −75°C and was characterized by NMR. This product was found to be thermally unstable above −30°C, being converted into an inseparable product mixture. Basic hydrolysis of this mixture afforded a structurally interesting acyclic enaminone/half-aminal, which is likely derived from a 2,3-dihydro-1,3-oxazole.

Reactions of 1-trifluoromethyl-prop-2-yne1-iminium salts with 2- and 3-aminopyridines

Abstract 4-CF3-pyrido[1,2-a]pyrimidin-5-ium triflates could be prepared in fair to high yields from acetylenic iminium salts R-C≡C–C(CF3)=N+Me2·CF3SO3 − and 2-aminopyridinium triflate under thermal conditions. Their reactions with protic heteronucleophiles, leading to 4-CF3, 4-XR-substituted 1,4-dihydropyrido[1,2-a]pyrimidin-5-ium salts, have been studied and reversible addition as well as subsequent opening of the pyrimidine ring have been observed. The cyclocondensation of a 3-phenyl-propyniminium triflate and 3-aminoyridinium triflate required strong thermal activation and yielded a 4-CF3-and a 4-CF3,5-CH3-[1,7]naphthyridine. A 1-CF3-benzo[f][1,7]naphthyridine was obtained from the same propyniminium salt and 3-aminoquinolinium triflate.

C2-Selective, Functional-Group-Divergent Amination of Pyrimidines by Enthalpy-Controlled Nucleophilic Functionalization.

Synthesis of heteroaryl amines has been an important topic in organic chemistry because of their importance in small-molecule discovery. In particular, 2-aminopyrimidines represent a highly privileged structural motif that is prevalent in bioactive molecules, but a general strategy to introduce the pyrimidine C2-N bonds via direct functionalization is elusive. Here we describe a synthetic platform for site-selective C-H functionalization that affords pyrimidinyl iminium salt intermediates, which then can be transformed into various amine products in situ. Mechanism-based reagent design allowed for the C2-selective amination of pyrimidines, opening the new scope of site-selective heteroaryl C-H functionalization. Our method is compatible with a broad range of pyrimidines with sensitive functional groups and can access complex aminopyrimidines with high selectivity.

Annulated carbamates are precursors for the ring contraction of the adamantane framework.

We report a protocol for the one-pot two-step synthesis of noradamantane methylene amines. The first step is the triflic acid-promoted decarboxylation of adamantane carbamates, which causes rearrangement of the adamantane framework to form noradamantane iminium salts, which are reduced to amines in the second separate step.

On the Protonation and Deuteration of Per‐Arylated Amines**

AbstractThe protonation of certain triarylamines and two N‐arylcarbazoles with trifluoromethanesulfonic acid yields, besides of N‐protonated species with ammonium salt structure, C‐substituted iminium salts with a cylohexa‐1,2‐ or 1,4‐diene structure also. The last mentioned compounds could be detected in some cases only by deuteration experiments. Whereas the C‐substituted iminium salts are formed with triphenylamine and 1‐(N,N‐diphenyl)in a very low extent, they are formed with other naphthylamine and carbazole derivates in higher yields. In accordance with this finding, all per‐aryl substituted amines studied could be transformed by reaction with deuterated trifluormethanesulfonic acid into C‐deuterated species with different deuterium content.

Modifications and hybrids of 1,2,3,4-tetrahydropyridinium salts and their antiprotozoal potencies

The antiprotozoal activity of 1-benzyltetrahydropyridin-4-yliden iminium salts is reported. This paper describes the preparation of a series of analogs from dihydropyridines or dihydrothiopyrans as educts. The new compounds were investigated for their activity against Plasmodium falciparum NF54, a causative organism of Malaria tropica and Trypanosoma brucei rhodesiense, the causative organism of Human African Trypanosomiasis (sleeping sickness). Several structure–activity relationships were detected. Both the substituents in ring positions 1 and 4 of the tetrahydropyridinium moiety had a strong impact on the antiprotozoal activities as well as on the cytotoxicity of compounds against L-6 cells (rat skeletal myoblasts). All new compounds were characterized using FT-IR spectroscopy, HRMS, and NMR spectroscopy.Graphic abstract