A ‘push–pull’ tropylium-fused aminoporphyrazine

Synthesis and Properties of Azahomocorannulenyl Cations and Radicals

Cycloheptatrienyl (tropyl) molecules are representative non-alternant hydrocarbons that offer interesting chemistry because of their unique structures and properties. However, there have been a limited number of polycyclic aromatic tropyl cations and radicals reported in the literature. Herein, we report the synthesis of a series of azahomocorannulene derivatives, where the key reactions are a 1,3-dipolar cycloaddition of polycyclic aromatic azomethine ylides with dibenzotropone and a subsequent palladium-catalyzed cyclization. X-ray diffraction analysis revealed that the obtained azahomocorannulenyl cation and radical adopt planar structures and exhibit unique packing structures. Their electronic and optical properties were investigated experimentally and theoretically to reveal their aromatic character.

Inline monitoring of high ammonia concentrations in methanol with a customized 3D printed flow cell

A novel system for inline monitoring of ammonia (NH3) suitable for methanol is presented. An optical ammonia sensor with a response time t90 of 33 s was combined with a tailor-made, 3D printed flow cell and allowed efficient measurements under continuous flow. The optical sensor includes a fluorescent indicator dye that is physically immobilized into a polyurethane hydrogel. A protective layer made of hydrophobic polyether sulfone (PES) shields the ammonia sensitive material against interfering substances and guarantees long-term stability in methanol. The sensor can be read out via a compact phase fluorimeter. Measurements in continuous flow are enabled by a flow cell manufactured via selective laser melting (SLM) of stainless steel. Stainless steel was chosen for the flow cell due to its good heat transfer properties and relatively good chemical resistance of NH3 in methanol. The measurements were successfully carried out with ammonia concentrations between 0.3 and 5.6 mol L− 1 NH3 in methanol at 25 °C up to 80 °C. Additionally, different flow-rates (0.5–2.0 mL min− 1), varying internal pressure (0.5–2.0 bar) as well as reversibility of the measurements at 25 and 60 °C were studied in detail. The sensor did not degrade indicated by sufficient signal and low drift over a period of two weeks, thus indicating the high potential of the novel set-up for real-time measurements in continuous flow applications.

Graphical abstract

Radical-based regioselective C-H functionalization of electron-deficient heteroarenes: scope, tunability, and predictability

Radical addition processes can be ideally suited for the direct functionalization of heteroaromatic bases, yet these processes are only sparsely used due to the perception of poor or unreliable control of regiochemistry. A systematic investigation of factors affecting the regiochemistry of radical functionalization of heterocycles using alkylsulfinate salts revealed that certain types of substituents exert consistent and additive effects on the regioselectivity of substitution. This allowed us to establish guidelines for predicting regioselectivity on complex π-deficient heteroarenes, including pyridines, pyrimidines, pyridazines, and pyrazines. Since the relative contribution from opposing directing factors was dependent on solvent and pH, it was sometimes possible to tune the regiochemistry to a desired result by modifying reaction conditions. This methodology was applied to the direct, regioselective introduction of isopropyl groups into complex, biologically active molecules, such as diflufenican (44) and nevirapine (45).

Reactivity and synthetic applications of 4,5-dicyanopyridazine: an overview

Despite the poor reputation of electron-deficient pyridazines in intermolecular Hetero Diels-Alder (HDA) reactions, 4,5-dicyanopyridazine (DCP) showed a surprising reactivity as a heterocyclic azadiene in inverse electron-demand HDA processes with different dienophiles. The use of alkenes, alkynes and enamines as 2π electron counterparts afforded dicyanocyclohexa-1,3-dienes and substituted phthalonitriles, respectively, while the use of suitable bis-dienophiles provides a general strategy for the one-pot synthesis of polycyclic carbo- and hetero-cage systems through pericyclic three-step homodomino processes. HDA reactions with heterocyclic dienophiles allowed direct benzoannelation: in particular, pyrrole and indole derivatives were converted to dicyano-indoles and -carbazoles. In addition an unprecedented reactivity of DCP as a very reactive heterocyclic electrophile at the C-4 carbon was also evidenced: by changing the experimental conditions, cyanopyrrolyl- and cyanoindolyl-pyridazines were obtained through reactions of pyrrole and indole systems as carbon nucleophiles in formal S_NAr2 processes where a CN group of DCP acts as leaving group. Thus, careful control of the reaction conditions allows exploitation of both pathways for the synthesis of different classes of heterocyclic derivatives.