Oxocarbon Acids and their Derivatives in Biological and Medicinal Chemistry

The biological and medicinal chemistry of the oxocarbon acids 2,3- dihydroxycycloprop-2-en-1-one (deltic acid), 3,4-dihydroxycyclobut-3-ene-1,2-dione (squaric acid), 4,5-dihydroxy-4-cyclopentene-1,2,3-trione (croconic acid), 5,6-dihydroxycyclohex- 5-ene-1,2,3,4-tetrone (rhodizonic acid) and their derivatives is reviewed and their key chemical properties and reactions are discussed. Applications of these compounds as potential bioisosteres in biological and medicinal chemistry are examined. Reviewed areas include cell imaging, bioconjugation reactions, antiviral, antibacterial, anticancer, enzyme inhibition, and receptor pharmacology.

A Physical Organic Chemistry Approach to Developing Cyclopropenium-Based Energy Storage Materials for Redox Flow Batteries

ConspectusRedox flow batteries (RFBs) represent a promising modality for electrical energy storage. In these systems, energy is stored via paired redox reactions of molecules on opposite sides of an electrochemical cell. Thus, a central objective for the field is to design molecules with the optimal combination of properties to serve as energy storage materials in RFBs. The ideal molecules should undergo reversible redox reactions at relatively high potentials (for the molecule that is oxidized during battery charging, called the catholyte) or low potentials (for the species that is reduced during battery charging, called the anolyte). Furthermore, anolytes and catholytes must be highly soluble in the electrolyte solution and stable to extended electrochemical cycling in all battery-relevant redox states. The ideal candidates would undergo more than one reversible electron transfer event. Finally, the optimal structures should be resistant to crossover through a selective separator in order to maintain isolation of the two sides of the cell. This Account describes our design and optimization of organic molecules for this application. We first provide background for the metrics and experiments used to characterize anolytes/catholytes and to progress them toward deployment in flow batteries. We then use our studies of aminocyclopropenium-based catholytes to illustrate this workflow and approach.We identified tris(dimethylamino) cyclopropenium hexafluorophosphate as a first-generation catholyte for nonaqueous RFBs based on literature reports from the 1970s describing its reversible chemical and electrochemical oxidation. Cyclic voltammetry and electrochemical cycling experiments in acetonitrile/LiPF₆ confirmed that this molecule undergoes oxidation at relatively high potential (0.86 V versus ferrocene/ferrocenium) and exhibits moderate stability toward charge-discharge cycling. Replacing the methyl groups with isopropyl substituents led to enhanced cycling stability but poor solubility of the radical dication (<0.1 M in acetonitrile). Solubility was optimized using quantitative structure-property relationship modeling, which predicted derivatives with ≥10-fold enhanced solubility. Cyclopropeniums with 300-500 mV higher redox potentials were identified by replacing one of the dialkylamino substituents with a less electron-donating thioalkyl or aryl group. Multielectron catholytes were developed by creating hybrid structures that contain a di(amino) cyclopropenium conjugated with a phenothiazine moeity. Finally, oligomeric tris(amino) cyclopropeniums were designed as crossover resistant catholytes. Optimization of their solubility enabled the deployment of these oligomers in high concentration asymmetric redox flow batteries with energy densities that are comparable to the state-of-the-art commercial aqueous inorganic systems.

Tris(dialkylamino)cyclopropenium dialkylphosphate ionic liquids as lubricants

Six new tris(dialkylamino)cyclopropenium dialkylphosphate ionic liquids (ILs), [C₃(NR₂)₃]BEHP (NR₂ = NEt₂, NBuMe, NPr₂, NBu₂, NHex₂, NDec₂; BEHP = bis(2-ethylhexyl)phosphate), were prepared and characterised as potential lubricants. Thermophysical and thermochemical properties of these ILs were investigated, namely: viscosity, density, conductivity, miscibility, thermal stability and phase transitions. Miscibility studies indicated that [C₃(NEt₂)₃]BEHP would not be suitable due to its water solubility and hexane immiscibility. [C₃(NDec₂)₃]BEHP was not investigated as a lubricant due to its low purity (the chloride salt of this cation is also hexane miscible). Of the other four, [C₃(NHex₂)₃]BEHP was found to exhibit significantly less wear for pin-on-disk test conditions than the standard phosphonium [P_6,6,6,14]BEHP IL. The amount of wear was found to generally decrease with increasing molecular weight.

Highly-fluorinated Triaminocyclopropenium Ionic Liquids

A series of highly‐fluorinated triaminocyclopropenium salts, with up to six fluorous groups, were prepared and their properties as ionic liquids investigated. Reaction of pentachlorocyclopropane or tetrachlorocyclopropene with bis(2,2,2‐trifluoroethyl)amine, HN(CH₂CF₃)₂, occurs in the presence of a trialkylamine, NR₃, to give cations with two fluorinated amino groups, [C₃(N(CH₂CF₃)₂)₂(NR₂)]⁺ (R=Et, Pr, Bu, Hex), with traces of [C₃(N(CH₂CF₃)₂)₃]⁺. Use of appropriate reagent ratios and reaction times and subsequent addition of a dialkylamine, HNR'R”, gives cations with one fluorinated amino group, [C₃(N(CH₂CF₃)₂)(NR₂)(NR'R”)]⁺ ((NR₂)(NR'R”)=(NBu₂)₂, (NEt₂)(NPr₂), (NBu₂)(NBuMe)). These cations were isolated as chloride salts and some of these were converted to bistriflamide, dicyanamide and triflate salts to provide ionic liquids. These salts were characterised by thermal (DSC and TGA) measurements and miscibility/solubility properties (determined in a range of solvents). Ionic liquids (ILs) were also characterised by density, viscosity and conductivity measurements where possible. X‐ray diffraction studies of chloride salts showed the formation of fluorous regions and more hydrophilic ionic regions in the solid state.

Recent advances in the organocatalytic applications of cyclopropene- and cyclopropenium-based small molecules

The development of novel small molecule-based catalysts for organic transformations has increased noticeably in the last two decades. A very recent addition to this particular research area is cyclopropene- and cyclopropenium-based catalysts. At one point in time, particularly in the mid-20^th century, much attention was focused on the structural aspects and physical properties of cyclopropene-based compounds. However, a paradigm shift was observed in the late 20^th century, and the focus shifted to the synthetic utility of these compounds. In fact, a wide range of cyclopropene derivatives have been found serving as valuable synthons for the construction of carbocycles, heterocycles and other useful organic compounds. In the last few years, the catalytic applications of cyclopropene/cyclopropenium-based compounds have been uncovered and many synthetic protocols have been developed using cyclopropene-based compounds as organocatalysts. Therefore, the main objective of this review is to highlight recent developments in the catalytic applications of cyclopropene-based small molecules in different areas of organocatalysis such as phase-transfer catalysis (PTC), Brønsted base catalysis, hydrogen-bond donor catalysis, nucleophilic carbene catalysis, and electrophotocatalysis developed within the past two decades.

pKa Scale for Cyclopropenium Ions with Applications in CO2 Capture

Molecular acid-base properties are core to understanding chemical systems and the prediction of reactivity. This axiom holds for cyclopropenium ions in terms of their broad use as (organo)catalysts, ligands, redox-flow batteries, and applications in materials sciences. In view of this significant status, and with it, the critical importance of acidity, we disclose in this report the first comprehensive computational study of the pK_a values of cyclopropenium ions employing a subset of 70 structurally diverse cyclopropenium derivatives, density functional computations, and Hammett linear free-energy relationships. Capitalizing upon these computed findings, and with an eye toward greenhouse gas trapping, we further document the timely use of a cyclopropenium-cyclopropenylidene coupled platform for CO₂ capture and light-triggered release.

A Series of Discrete Dichloride Dihydrates: Characterisation and Symmetry Effects

A series of three discrete dichloride dihydrates [Cl₂ (H₂ O)₂ ]^2- have been isolated with different triaminocyclopropenium (TAC) cations and with different crystallographic symmetries. The cluster exhibits D_2h symmetry with the tris(dimethylamino)cyclopropenium cation [C₃ (NMe₂ )₃ ]⁺ , C_2h symmetry with the fluorinated cation [C₃ (N(CH₂ CF₃ )₂ )(NBu₂ )₂ ]⁺ (containing two 2,2,2-trifluoroethyl substituents) and C_2v symmetry with the more fluorinated [C₃ (N(CH₂ CF₃ )₂ )₂ (NBu₂ )]⁺ cation. The effect of symmetry on the infrared spectra of the dichloride ion-pair clusters, as well as deuterated analogues, has been investigated. The D_2h - and C_2h -symmetric clusters each exhibit two stretching bands in the infrared at 3427 and 3368 cm^-1 for D_2h symmetry and 3444 and 3392 cm^-1 for C_2h symmetry, whereas the C_2v -symmetric cluster exhibits three bands at 3475, 3426 and 3373 cm^-1 . Computational studies were carried out on a [Cl₂ (H₂ O)₂ ]^2- cluster with C_2v symmetry to aid the infrared band assignments.

Self-Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

Aminocyclopropenium ions have raised much attention as organocatalysts and redox active polymers. However, the self-assembly of amphiphilic aminocyclopropenium ions remains challenging. The first deltic ionic liquid crystals based on aminocyclopropenium ions have been developed. Differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction provided insight into the unique self-assembly and nanosegregation of these liquid crystals. While the combination of small headgroups with linear p-alkoxyphenyl units led to bilayer-type smectic mesophases, wedge-shaped units resulted in columnar mesophases. Upon increasing the size and polyphilicity of the aminocyclopropenium headgroup, a lamellar phase was formed.

Fluoride Ionic Liquids in Salts of Ethylmethylimidazolium and Substituted Cyclopropenium Cation Families

A series of solvent-stabilized ionic liquid fluorides were prepared, [EMIM]F.nCH3COOH n = 1. 0, 1.6, 2.1, 2.4, and 3.2, either via exchange from the chloride salt using KF or AgF, or by neutralization of the hydroxide salt using HF. Azeotrope drying was used to remove water. Their viscosity, conductivity and density properties were determined. A diethanol solvate of the triaminocyclopropenium salt [C3(NPr2)3]F was found to be stable and its viscosity, conductivity and density properties were also determined. The monoethanol solvate, however, was found to be unstable with trace water present. Intramolecular stabilization of fluoride was achieved by using OH functionalized cations: [C3(NEt2)2N(CH2CH2OH)2]F, [C3(N(CH2CH2OH)2)3]F, and [Me3NCH2CH2OH]F.H2O, The first of these is an ionic liquid at ambient temperature and has a TGA mass loss onset at 175°C, indicating a useful range of liquid state stability.

Synthesis and Properties of Triaminocyclopropenium Cation Based Ionic Liquids as Hypergolic Fluids

A novel family of hydrophobic triaminocyclopropenium cation based ionic liquids have been synthesized, and their structures and physicochemical properties characterized by NMR and IR spectroscopy, elemental analysis, differential scanning calorimetry, and hypergolic tests. The experimental results showed that all of these ionic liquids exhibited the expected hypergolic reactivity with the oxidizer white fuming nitric acid. Among them, the hypergolic ionic liquid based on the cyanoimidazolylborohydride anion showed excellent integrated properties, including high decomposition temperature (194 °C), high density (0.95 g cm^-3 ), moderate viscosity (44 MPa s), ultrafast ignition delay time (6 ms), and high specific impulse (301.9 s); this demonstrates its potential as an environmentally friendly alternative to toxic hydrazine derivatives.

Synthesis and physical properties of tris(dialkylamino)cyclopropenium dicyanamide ionic liquids

The synthesis and properties of 16 tris(dialkylamino)cyclopropenium (TDAC) cations with the dicyanamide (DCA) anion, [N(CN)₂]⁻, are described. D_3h- and C_3h-symmetric cations ([C₃(NR₂)₃]DCA (R = Me, Et, Pr, Bu, Pent, Hex, Dec) and [C₃(NRMe)₃]DCA (R = Bu, St), respectively) were synthesised by reaction of C₃Cl₅H with the corresponding amine. Reaction of the alkoxydiaminocyclopropenium salt [C₃(NEt₂)₂(OMe)]⁺ with amines led to a series of C_2v-symmetric salts [C₃(NEt₂)₂(NR₂)]DCA (R = Me, Bu, Hex) and two C_s-symmetric salts and [C₃(NEt₂)₂(NRMe)]DCA (R = Me, Bu). Similarly, [C₃(NMe₂)₂(OMe)]⁺, was used to prepare the C_s-symmetric salts [C₃(NMe₂)₂(NRMe)]DCA (R = Pr, Bu). In addition to characterisation by NMR, mass spectrometry and microanalysis, the salts were characterised by DSC, TGA, density, viscosity, conductivity and miscibility/solubility studies. Comparisons have been made with similar series of bistriflimide (NTf₂⁻) salts that have been previously reported to see whether the same trends are observed with a different anion.

An investigation of symmetry (D_3h, C_3h, C_2v and C_s) and alkyl chain length (6–60 alkyl carbon atoms) effects on the physical properties of peralkylated triaminocyclopropenium dicyanamide salts.