Fifty Years of π-Conjugated Triaz­enes

Group 11 complexes of a bulky triazene ligand

A new triazene ligand was prepared by the reaction of the bulky aryl azide, TerMesN3, (2,6-bis(2,4,6-trimethylphenyl)phenyl azide), with the bulky N-heterocyclic carbene (NHC), SIPr (N,N'-2,6-bis(diisopropylphenyl)-3,4-dihydroimidazol-2-ylidene). The steric bulk of these two groups leads to perpendicular bonding of the NHC-N3 plane and the aryl group which provides immense steric crowding around the triazene core. The corresponding π-conjugated triazene ligand was utilized as a neutral, monodentate ligand which results in monomeric Cu(I)Cl, Ag(I)OTf, and Au(I)Cl complexes. Instability of the π-conjugated triazene Au(I)Cl complex was observed, even under inert conditions. When dissolved and photolyzed in THF with workup in isopropanol, the compound decomposes forming [(SIPrNH)2Au(I)][Au(I)Cl2] a relatively stable compound to light, moisture, and water - alongside the formation of gold nanoparticles which were characterized using scanning electron microscopy with energy dispersive spectroscopy.

Chemical Probes to Interrogate the Extreme Environment of Mosquito Larval Guts

Mosquito control methods are vital to curtail the spread of life-threatening illnesses, such as dengue fever, malaria, and yellow fever. Vector control technologies must be selective to minimize deleterious effects on our ecosystem. Successful methods that control mosquito larva populations utilize the uniquely high alkaline nature of the midgut. Here, we present novel protected triazabutadienes (pTBD) that are deprotected under basic conditions of the larval midgut, releasing an aryl diazonium ion (ADI) that results in protein modification. The probes contain a bioorthogonal terminal alkyne handle, enabling a selective Cu-click reaction with an azidofluorophore for quantification by SDS PAGE and visualization using fluorescence microscopy. A control TBD, unable to release an ADI, did not label the midgut. We envision our chemical probes will aid in the development of new selective mosquito control methods, thus preventing the spread of mosquito-borne illnesses with minimal impact on other organisms in the ecosystem.

Chemical probes to interrogate the extreme environment of mosquito larval guts

Mosquito control methods are vital for the spread of life-threatening illnesses such as dengue fever, malaria, and yellow fever. Vector control technologies must be selective to minimize deleterious effects to our ecosystem. Successful methods that control mosquito larva populations utilize the uniquely high alkaline nature of the midgut. Here, we present novel protected triazabutadienes (pTBD) which are deprotected under basic conditions of the larval midgut, releasing an aryl diazonium ion (ADI) that results in protein modification. The probes contain a bioorthogonal terminal alkyne handle, enabling a selective Cu-click reaction with an azido-fluorophore for quantification by SDS PAGE and visualization using fluorescence microscopy. A control TBD, unable to release an ADI, did not label the midgut. We envision our chemical probes will aid in the development of new selective mosquito control methods thus preventing the spread of mosquito-borne illnesses with minimal impact on other organisms in the ecosystem.

Carbon-Nitride Popcorn-A Novel Catalyst Prepared by Self-Propagating Combustion of Nitrogen-Rich Triazenes

The interest in development of non-graphitic polymeric carbon nitrides (PCNs), with various C-to-N ratios, having tunable electronic, optical, and chemical properties is rapidly increasing. Here the first self-propagating combustion synthesis methodology for the facile preparation of novel porous PCN materials (PCN3-PCN7) using new nitrogen-rich triazene-based precursors is reported. This methodology is found to be highly precursor dependent, where variations in the terminal functional groups in the newly designed precursors (compounds 3-7) lead to different combustion behaviors, and morphologies of the resulted PCNs. The foam-type highly porous PCN5, generated from self-propagating combustion of 5 is comprehensively characterized and shows a C-to-N ratio of 0.67 (C₃ N_4.45 ). Thermal analyses of PCN5 formulations with ammonium perchlorate (AP) reveal that PCN5 has an excellent catalytic activity in the thermal decomposition of AP. This catalytic activity of PCN5 is further evaluated in a closer-to-application scenario, showing an increase of 18% in the burn rate of AP-Al-HTPB (with 2 wt% of PCN5) solid composite propellant. The newly developed template- and additive-free self-propagating combustion synthetic methodology using specially designed nitrogen-rich precursors should provide a novel platform for the preparation of non-graphitic PCNs with a variety of building block chemistries, morphologies, and properties suitable for a broad range of technologies.

Syntheses and crystal structure of 4-[(pyridin-3-yl)diazen-yl]morpholine and 1-[(pyridin-3-yl)diazen-yl]-1,2,3,4-tetra-hydro-quinoline

Two new heterocyclic 1,2,3-triazenes were synthesized by diazo-tation of 3-amino-pyridine following respectively by coupling with morpholine or 1,2,3,4-tetra-hydro-quinoline. 4-[(Pyridin-3-yl)diazen-yl]morpholine (I), C9H12N4O, has monoclinic P21/c symmetry at 100 K, while 1-[(pyridin-3-yl)diazen-yl]-1,2,3,4-tetra-hydro-quinoline (II), C14H14N4, has monoclinic P21/n symmetry at 100 K. These 1,2,3-triazene derivatives were synthesized by the organic medium method by coupling reactions of 3-amino-pyridine with morpholine and 1,2,3,4-tetra-hydro-quinoline, respectively, and characterized by 1H NMR, 13C NMR, IR, mass spectrometry, and single-crystal X-ray diffraction. The mol-ecule of compound I consists of pyridine and morpholine rings connected by an azo moiety (-N=N-). In the mol-ecule of II, the pyridine ring and the 1,2,3,4-tetra-hydro-quinoline unit are also connected by an azo moiety. The double- and single-bond distances in the triazene chain are comparable for the two compounds. In both crystal structures, the mol-ecules are connected by C-H⋯N inter-actions, forming infinite chains for I and layers parallel to the bc plane for II.

Photochromic Fentanyl Derivatives for Controlled μ-Opioid Receptor Activation

Photoswitchable ligands as biological tools provide an opportunity to explore the kinetics and dynamics of the clinically relevant μ-opioid receptor. These ligands can potentially activate or deactivate the receptor when desired by using light. Spatial and temporal control of biological activity allows for application in a diverse range of biological investigations. Photoswitchable ligands have been developed in this work, modelled on the known agonist fentanyl, with the aim of expanding the current "toolbox" of fentanyl photoswitchable ligands. In doing so, ligands have been developed that change geometry (isomerize) upon exposure to light, with varying photophysical and biochemical properties. This variation in properties could be valuable in further studying the functional significance of the μ-opioid receptor.

Direct Intracellular Delivery of Benzene Diazonium Ions As Observed by Increased Tyrosine Phosphorylation

A challenge within the field of bioconjugation is developing probes to uncover novel information on proteins and other biomolecules. Intracellular delivery of these probes offers the promise of giving relevant context to this information, and these probes can serve as hypothesis-generating tools within complex systems. Leveraging the utility of triazabutadiene chemistry, herein, we discuss the development of a probe that undergoes reduction-mediated deprotection to rapidly deliver a benzene diazonium ion (BDI) into cells. The intracellular BDI resulted in an increase in global tyrosine phosphorylation levels. Seeing phosphatase dysregulation as a potential source of this increase, a tyrosine phosphatase (PTP1B) was tested and shown to be both inhibited and covalently modified by the BDI. In addition to the expected azobenzene formation at tyrosine side chains, key reactive histidine residues were also modified.

Protein Modification via Mild Photochemical Isomerization of Triazenes to Release Aryl Diazonium Ions

This work describes the development of phenyl diazenyl piperidine triazene derivatives that can be activated to release aryl diazonium ions for labeling of proteins using light. These probes show marked bench stability at room temperature and can be photoisomerized via low-intensity UVA irradiation at physiological pH. Upon isomerization, the triazenes are rendered more basic and readily protonate to release reactive aryl diazonium ions. It was discovered that the intensity and duration of the UV light was essential to the observed diazonium ion reactivity in competition with the traditionally observed photolytic radical pathways. The combination of their synthetic efficiency coupled with their overall stability makes triazenes an attractive candidate for use in bioconjugation applications. Bioorthogonal handles on the triazenes are used to demonstrate the ease by which proteins can be modified.

Constructing fused N-heterocycles from unprotected mesoionic N-heterocyclic olefins and organic azides via diazo transfer

Mesoionic N-heterocyclic olefins (mNHOs) were first reported last year and their reactivity remains largely unexplored. Herein we report the reaction of unprotected mNHOs and organic azides as a novel synthetic route to a variety of pyrazolo[3,4-d][1,2,3]triazoles, an important structural motif in drug candidates and energetic materials. The only byproduct aniline can be easily recycled and converted back to the starting organic azide, in compliance with the green chemistry principle. The reaction mechanism has been explored through experimental and computational studies.

Suzuki Coupling of Protected Aryl Diazonium Ions: Expanding the Knowledge of Triazabutadiene Compatible Reactions

Aryl diazonium ions are important in synthesis and chemical biology, and the acid-labile triazabutadiene can protect this handle for future use. We report a Suzuki coupling strategy that is compatible with the triazabutadiene scaffold, expanding the scope of synthetically available triazabutadienes. Shown herein, the triazabutadiene scaffold remains intact and reactive after coupling, as demonstrated by releasing the aryl diazonium ion to label a tyrosine-rich model protein.

Crystal structure and spectroscopic properties of (E)-1,3-dimethyl-2-[3-(4-nitro-phen-yl)triaz-2-enyl-idene]-2,3-di-hydro-1H-imidazole

The title compound (E)-1,3-dimethyl-2-[3-(4-nitro-phen-yl)triaz-2-enyl-idene]-2,3-di-hydro-1H-imidazole, C11H12N6O2, has monoclinic (C2/c) symmetry at 100 K. This triazene derivative was synthesized by the coupling reaction of 1,3-di-methyl-imidazolium iodide with 1-azido-4-nitro benzene in the presence of sodium hydride (60% in mineral oil) and characterized by 1H NMR, 13C NMR, IR, mass spectrometry, and single-crystal X-ray diffraction. The mol-ecule consists of six-membered and five-membered rings, which are connected by a triazene moiety (-N=N-N-). In the solid-state, the mol-ecule is found to be planar due to conjugation throughout the mol-ecule. The extended structure shows two layers of mol-ecules, which present weak inter-molecular inter-actions that facilitate the stacked arrangement of the mol-ecules forming the extended structure. Furthermore, there are several weak pseudo-cyclical inter-actions between the nitro oxygen atoms and symmetry-adjacent H atoms, which help to arrange the mol-ecules.

Vinyl and Alkynyl Triazenes: Synthesis, Reactivity, and Applications

Aromatic compounds containing triazenyl groups (N₃ RR') have a profound impact on synthetic organic and medicinal chemistry. In contrast, the chemistry of vinyl and alkynyl triazenes was a largely uncharted territory until recently. The situation has changed over the last five years, and it has become apparent that vinyl and alkynyl triazenes are highly interesting compounds with a unique reactivity. The electron-donating properties of the triazenyl group provide alkynyl triazenes with an ynamide-like reactivity, which can be exploited in reactions of the triple bond. Vinyl triazenes, on the other hand, can be used for electrophilic vinylation reactions. The foundation for this new triazene chemistry are synthetic pathways which allow preparing vinyl and alkynyl triazenes in few steps from readily available starting materials. In this Minireview, we summarize recent developments in this area.

New Insights in Frustrated Lewis Pair Chemistry with Azides

The geminal frustrated Lewis pair (FLP) tBu2 PCH2 BPh2 (1) reacts with phenyl-, mesityl-, and tert-butyl azide affording, respectively, six, five, and four-membered rings as isolable products. DFT calculations revealed that the formation of all products proceeds via the six-membered ring structure, which is thermally stable with an N-phenyl group, but rearranges when sterically more encumbered Mes-N3 and tBu-N3 are used. The reaction of 1 with Me3 Si-N3 is believed to follow the same course, yet subsequent N2 elimination occurs to afford a four-membered heterocycle (5), which can be considered as a formal FLP-trimethylsilylnitrene adduct. Compound 5 reacts with hydrochloric acid or tetramethylammonium fluoride and showed frustrated Lewis pair reactivity towards phenylisocyanate.

Metal-free cross-coupling of π-conjugated triazenes with unactivated arenes via photoactivation

A metal-free synthesis of biaryl compounds was achieved under photo-irradiation, which represents a milder alternative for cross-coupling reactions.

Modifications of amino acids using arenediazonium salts

Aryl transfer reactions from arenediazonium salts have started to make their impact in chemical biology with initial forays in the arena of arylative modifications and bio-conjugations of amino acids, peptides and proteins.

Protecting Triazabutadienes To Afford Acid Resistance

Recent work on triazabutadienes has shown that they have the ability to release aryl diazonium ions under exceptionally mild acidic conditions. There are instances that require that this release be prevented or minimized. Accordingly, a base-labile protection strategy for the triazabutadiene is presented. It affords enhanced synthetic and practical utility of the triazabutadiene. The effects of steric and electronic factors in the rate of removal are discussed, and the triazabutadiene protection is shown to be compatible with the traditional acid-labile protection strategy used in solid phase peptide synthesis.

PIDA–I2 mediated direct vicinal difunctionalization of olefins: iodoazidation, iodoetherification and iodoacyloxylation

I⁺ (IOAc) was produced from the combination of phenyliodine diacetate (PIDA) and iodine.