Green Hydrothermal Synthesis of Fluorescent 2,3-Diarylquinoxalines and Large-Scale Computational Comparison to Existing Alternatives

Lophine analogues as fluorophores for selective bioimaging of the endoplasmic reticulum

The design of small-molecule fluorescent probes for labelling the endoplasmic reticulum (ER) revolves around a well-established albeit limited group of structural architectures. Here, we synthesized new fluorescent lophines in one step in high-temperature water (HTW) and explored their application as dyes for selective bioimaging of the ER.

High-temperature water unlocks urea as nitrogen-source towards imidazoles

Urea is a non-toxic, harmless, and abundant bulk organic chemical featuring high nitrogen content. Therefore, urea could be a prime green candidate for introducing nitrogen atoms into organic molecules. In this regard, urea in organic synthesis has been mainly employed as building block, component of solvent systems, catalyst, or for pH adjustment, while uses of urea as NH3-source towards the construction of small organic compounds are scarce. Here, nothing but high-temperature water (HTW) is employed to conduct the aquathermolysis of urea, generating NH3 to propel the Debus-Radzsisweski multicomponent reaction (MCR) towards imidazoles. The approach does neither require additional catalysts nor reaction auxiliaries or volatile organic compounds as solvent. Urea was used as N-source in combination with different 1,2-diketones and aldehydes featuring a variety of functional groups towards 23 lophine analogues (190 °C, 1-3 hours). Moreover, the presented synthesis performs equally or better than classical syntheses when acid-sensitive substrates are employed. The greenness of the synthesis using urea in HTW was assessed through green metrics and compared with syntheses reported in the literature in a large-scale fashion. Overall, the reported syntheses feature E-factor and process mass intensity values in ranges comparable to those of syntheses reported in literature.

2-(Thienyl)quinoxaline derivatives and their application in Ir(III) complexes yielding tuneable deep red emitters

The synthesis and characterisation of eleven different 2-(thienyl)quinoxaline species that incorporate different points of functionality, including at the thiophene or quinoxaline rings, are described. These species display variable fluorescence properties in the visible region (λem = 401-491 nm) depending upon the molecular structures and extent of conjugation. The series of 2-(thienyl)quinoxaline species were then investigated as cyclometalating agents for Ir(III) to yield [Ir(C^N)2(bipy)]PF6 (where C^N = the cyclometalated ligand; bipy = 2,2'-bipyridine). Eight complexes were successfully isolated and fully characterised by an array of spectroscopic and analytical techniques. Two Ir(III) examples were structurally characterised in the solid state using single crystal X-ray diffraction; both structures confirmed the proposed formulations and coordination spheres in each case showing that the thiophene coordinates via a Ir-C bond. The photophysical properties of the complexes revealed that each complex is luminescent under ambient conditions with a range of emission wavelengths observed (665-751 nm) indicating that electronic tuning can be achieved via both the thienyl and quinoxaline moieties.

Urethane functions can reduce metal salts under hydrothermal conditions: synthesis of noble metal nanoparticles on flexible sponges applied in semi-automated organic reduction

We report an additive-free one-pot hydrothermal synthesis of Au, Ag, Pd, and alloy AuPd nanoparticles (NPs) anchored on commercial polyurethane (PU) foams. While unable to reduce the precursor metal salts at room temperature, PU is able to serve as a reducing agent under hydrothermal conditions. The resulting NP@PU sponge materials perform comparably to reported state-of-the-art reduction catalysts, and are additionally very well suited for use in semi-automated synthesis: the NP anchoring is strong enough and the support flexible enough to be used as a 'catalytic sponge' that can be manipulated with a robotic arm, i.e., be repeatedly dipped into and drawn out of solutions, wrung out, and re-soaked.

Synthesis of 2,3-Diarylquinoxaline Carboxylic Acids in High-Temperature Water

Aromatic carboxylic acids are prone to decarboxylate in high-temperature water (HTW). While the decarboxylation kinetics of several aromatic carboxylic acids have been explored, studies on their compatibility with organic syntheses in HTW are scarce. Herein, we report the hydrothermal synthesis (HTS) of 2,3-diarylquinoxaline carboxylic acids from 1,2-diarylketones and 3,4-diaminobenzoic acid. A detailed study of the reaction parameters was performed to identify reaction conditions towards minimal decarboxylation. Thirteen 2,3-diarylquinoxaline-6-carboxylic acids are obtained at temperatures between 150–230 °C within 5–30 minutes. The reported conditions feature comparable performance to those of classic syntheses, avoiding volatile organic solvents, strong acids and toxic catalysts. Decarboxylated quinoxalines arise as side products in variable amounts via direct decarboxylation of the 3,4-diaminobenzoic acid. To completely inhibit the decarboxylation, we show that suitable structural analogues of 3,4-diaminobenzoic acid can act as starting compounds. Thus, ester hydrolysis of methyl 3,4-diaminobenzoate and deprotection of di-Boc-protected 3,4-diminobenzoic can be coupled with the HTS of quinoxaline towards quinoxaline carboxylic acids, while fully avoiding decarboxylated side products.

Geomimetic Hydrothermal Synthesis of Polyimide-Based Covalent Organic Frameworks

Despite its abundance, water is not widely used as a medium for organic reactions. However, under geothermal conditions, water exhibits unique physicochemical properties, such as viscosity and a dielectric constant, and the ionic product become similar to those of common organic solvents. We have synthesized highly crystalline polyimide-based covalent organic frameworks (PICs) under geomimetic hydrothermal conditions. By exploiting triphenylene-2,3,6,7,10,11-hexacarboxylic acid in combination with various aromatic diamines, PICs with various pore dimensions and crystallinities were synthesized. XRD, FT-IR, and DFT calculations revealed that the solubility of the oligomeric intermediates under hydrothermal conditions affected the stacking structures of the crystalline PICs. Furthermore, the synthesized PICs demonstrate promising potential as an anode material in lithium-ion batteries owing to its unique redox-active properties and high surface area.