Tetrafluorination of Aromatic Azide Yields a Highly Efficient Staudinger Reaction: Kinetics and Biolabeling

Trimethylsilanol Cleaves Stable Azaylides As Revealed by Unfolding of Robust "Staudinger" Single-Chain Nanoparticles

Herein, we disclose a unique and selective reagent for the cleavage of stable azaylides prepared by the nonhydrolysis Staudinger reaction, enabling the on-demand unfolding of robust single-chain nanoparticles (SCNPs). SCNPs with promising use in catalysis, nanomedicine, and sensing are obtained through intrachain folding of discrete synthetic polymer chains. The unfolding of SCNPs involving reversible interactions triggered by a variety of external stimuli (e.g., pH, temperature, light, and redox potential) or substances (e.g., competitive reagents, solvents, and anions) is well known. Conversely, methods for the unfolding (i.e., intrachain disassembly) of SCNPs with stronger covalent interactions are scarce. We show that trimethylsilanol (Me3SiOH) triggers the efficient unfolding of robust "Staudinger" SCNPs with stable azaylide (-N=P-) moieties as intrachain cross-linking units showing exceptional stability toward water, air, and CS2, a standard reagent for azaylides. As a consequence, Me3SiOH arises as a rare, exceptional, and valuable reagent for the cleavage of stable azaylides prepared by the nonhydrolysis Staudinger reaction.

Transformation of Highly Hydrophobic Triarylphosphines into Amphiphiles via Staudinger Reaction with Hydrophilic Trichlorophenyl Azide

Owing to its hydrophobic properties and reactivity, triarylphosphines (PAr3 ) are promising precursors for the development of new amphiphiles. However, an efficient and reliable synthetic method for amphiphiles based on highly hydrophobic PAr3 is still required. Herein, a straightforward transformation of highly hydrophobic PAr3 into amphiphiles via the Staudinger reaction is reported. By simply mixing PAr3 and a hydrophilic trichlorophenyl azide containing two hydrophilic chains, amphiphiles bearing a N=P bond (i. e., an azaylide moiety) were quantitatively formed. The obtained azaylide-based amphiphiles were remarkably water-soluble, enabling their spontaneous self-assembly into 2 nm-sized micelles composed of 4-5 molecules in water with a low critical micelle concentration (up to 0.05 mM or less) due to the effective intermolecular interactions among the hydrophobic surfaces. Although the azaylide moiety is easily hydrolyzed in the presence of water, the azaylide in the amphiphiles displayed notable stability in water even at 60 h, which stems from the LUMO modulation induced by the presence of three electron-withdrawing chloro groups and two twisted alkoxycarbonyl groups, according to DFT calculations. An amphiphile having a large hydrophobic surface solubilized various hydrophobic organic dyes through efficient intermolecular interactions, resulting in the dyes exhibiting either monomer or excimer emissions in water.

Recent Advances in Bioorthogonal Ligation and Bioconjugation

The desire to create biomolecules modified with functionalities that go beyond nature's toolbox has resulted in the development of biocompatible and selective methodologies and reagents, each with different scope and limitations. In this overview, we highlight recent advances in the field of bioconjugation from 2016 to 2023. First, (metal-mediated) protein functionalization by exploiting the specific reactivity of amino acids will be discussed, followed by novel bioorthogonal reagents for bioconjugation of modified biomolecules.

A Universal and Modular Scaffold for Heparanase Activatable Probes and Drugs

Heparanase (HPSE) is an endo-β-glucuronidase involved in extracellular matrix remodeling in rapidly healing tissues, most cancers and inflammation, and viral infection. Its importance as a therapeutic target warrants further study, but such is hampered by a lack of research tools. To expand the toolkits for probing HPSE enzymatic activity, we report the design of a substrate scaffold for HPSE comprised of a disaccharide substrate appended with a linker, capable of carrying cargo until being cleaved by HPSE. Here exemplified as a fluorogenic, coumarin-based imaging probe, this scaffold can potentially expand the availability of HPSE-responsive imaging or drug delivery tools using a variety of imaging moieties or other cargo. We show that electronic tuning of the scaffold provides a robust response to HPSE while simplifying the structural requirements of the attached cargo. Molecular docking and modeling suggest a productive probe/HPSE binding mode. These results further support the hypothesis that the reactivity of these HPSE-responsive probes is predominantly influenced by the electron density of the aglycone. This universal HPSE-activatable scaffold will greatly facilitate future development of HPSE-responsive probes and drugs.

Synthesis of Azaylide-Based Amphiphiles by the Staudinger Reaction

Catalyst- and reagent-free reactions are powerful tools creating various functional molecules and materials. However, such chemical bonds are usually hydrolysable or require specific functional groups, which limits their use in aqueous media. Herein, we report the development of new amphiphiles through the Staudinger reaction. Simple mixing of chlorinated aryl azide with a hydrophilic moiety and various triarylphosphines (PAr3) gave rise to azaylide-based amphiphiles NPAr3, rapidly and quantitatively. The obtained NPAr3 formed ca. 2 nm-sized spherical aggregates (NPAr3)_n in water. The hydrolysis of NPAr3 was significantly suppressed as compared with those of non-chlorinated amphiphiles nNPAr3. Computational studies revealed that the stability is mainly governed by the decrease in LUMO around the phosphorus atom owing to the o-substituted halogen groups. Furthermore, hydrophobic dyes such as Nile red and BODIPY were encapsulated by the spherical aggregates (NPAr3)_n in water.

Bioorthogonal Reactions of Triarylphosphines and Related Analogues

Bioorthogonal phosphines were introduced in the context of the Staudinger ligation over 20 years ago. Since that time, phosphine probes have been used in myriad applications to tag azide-functionalized biomolecules. The Staudinger ligation also paved the way for the development of other phosphorus-based chemistries, many of which are widely employed in biological experiments. Several reviews have highlighted early achievements in the design and application of bioorthogonal phosphines. This review summarizes more recent advances in the field. We discuss innovations in classic Staudinger-like transformations that have enabled new biological pursuits. We also highlight relative newcomers to the bioorthogonal stage, including the cyclopropenone-phosphine ligation and the phospha-Michael reaction. The review concludes with chemoselective reactions involving phosphite and phosphonite ligations. For each transformation, we describe the overall mechanism and scope. We also showcase efforts to fine-tune the reagents for specific functions. We further describe recent applications of the chemistries in biological settings. Collectively, these examples underscore the versatility and breadth of bioorthogonal phosphine reagents.

π-Extended Coumarins Derived with Nonhydrolyzable Iminophosphoranes as Two-Photon-Excited Fluorophores

Novel coumarin-iminophosphorane (IPP) fluorophores that have stable resonance contributions from aza-ylides were formed by using the nonhydrolysis Staudinger reaction. The N═P formation reaction kinetics obey the conventional Staudinger reaction. The absorption and emission profiles of the coumarin-IPP derivatives can be fine-tuned: an electron-donating group at PPh₃ enhances absorption and fluorescence, whereas an electron-withdrawing group at C-3 drives absorption and emission peaks toward blue-light wavelengths. Two-photon adsorption, accompanied by anti-Stokes fluorescence, is achieved under near-infrared femtosecond laser excitation.

Multifluorinated Aryl Azides for the Development of Improved H2 S Probes, and Fast Strain-promoted Azide-Alkyne Cycloaddition and Staudinger Reactions

The development of advanced bioorthogonal reactions for detection and labeling of biomolecules is significant in chemical biology. Recently, researchers have found that multifluorinated aryl azides hold great potential for the development of improved bioorthogonal reactions. The fluorine atom can be a perfect substituent group because of its properties of excellent electronegativity and small steric hindrance. In this Minireview, we discuss recent developments of improved hydrogen sulfide (H₂ S) fluorescence probes, fast strain-promoted azide-alkyne cycloaddition (SPAAC) and nonhydrolysis Staudinger reactions based on the use of multifluorinated aryl azides. Additionally, kinetic studies and biological applications of these reactions are also presented.

Electrophilic Azides for Materials Synthesis and Chemical Biology

Organic azides are involved in a variety of useful transformations, including nitrene chemistry, reactions with nucleophiles and electrophiles, and cycloadditions. The 1,3-dipolar cycloadditions of azides constitute a major class of highly reliable and versatile reactions, as shown by the development and rapid adoption of click chemistry and bioorthogonal chemistry. Metal-catalyzed azide-alkyne cycloaddition (Cu/RuAAC), the prototypical click reaction, has found wide utility in pharmaceutical, biomedical, and materials sciences. The strain-promoted, or distortion-accelerated, azide-alkyne cycloaddition eliminates the need for a metal catalyst.In the azide-mediated 1,3-dipolar cycloaddition reactions, azides are ambiphilic, i.e., HOMO-LUMO-controlled dipoles where both the HOMO and LUMO interact strongly with the dipolarophile. Azide-alkyne cycloaddition proceeds primarily through the HOMO_azide-LUMO_{dipolarophile} interaction, and electron-deficient dipolarophiles react more readily. The inverse-electron-demand reaction, involving the LUMO_azide-HOMO_{dipolarophile} interaction, is less common because of the low stability of electron-deficient azides such as acyl, sulfonyl, and phosphoryl azides. Nevertheless, there have been reports since the 1960s showing enhanced reaction kinetics between electron-poor azides and electron-rich dipolarophiles. Our laboratory has developed the use of perfluoroaryl azides (PFAAs), a class of stable electron-deficient azides, as nitrene precursors and for reactions with nucleophiles and electron-rich dipolarophiles. Perfluorination on the aryl ring also facilitates the synthesis of PFAAs and quantitative analysis of the products by ¹⁹F NMR spectroscopy.In this Account, we summarize key reactions involving electrophilic azides and applications of these reactions in materials synthesis and chemical biology. These electron-deficient azides exhibit unique reactivity toward nucleophiles and electron-rich or strained dipolarophiles, in some cases leading to new transformations that do not require any catalysts or products that are impossible to obtain from the nonelectrophilic azides. We highlight work from our laboratories on reactions of PFAAs with enamines, enolates, thioacids, and phosphines. In the reactions of PFAAs with enamines or enolates, the triazole or triazoline cycloaddition products undergo further rearrangement to give amidines or amides as the final products at rates of up to 10⁵ times faster than their non-fluorinated anlogues. Computational investigations by the distortion/interaction activation strain model reveal that perfluorination lowers the LUMO of the aryl azide as well as the overall activation energy of the reaction by decreasing the distortion energies of the reactants to reach the transition states. The PFAA-enamine reaction can be carried out in a one-pot fashion using readily available starting materials of aldehyde and amine, making the reaction especially attractive, for example, in the functionalization of nanomaterials and derivatization of antibiotics for the preparation of theranostic nanodrugs. Similar fast kinetics was also observed for the PPAA-mediated Staudinger reaction, which proceeds at 10⁴ times higher rate than the classic Staudinger ligation, giving stable phosphoimines in high yields. The reaction is biorthogonal, allowing cell-surface labeling with minimal background noise.

Sequential conjugation methods based on triazole formation and related reactions using azides

The recent remarkable progress in azide chemistry has realized sequential conjugation methods with selective 1,2,3-triazole formation. On the basis of the diverse reactivities of azides and azidophiles, including terminal alkynes and cyclooctynes, various selective reactions to furnish triazoles and a wide range of platform molecules, such as diynes, diazides, triynes, and triazides, have been developed so far for bis- and tris(triazole) syntheses. This review highlights recent transformations involving selective triazole formation, allowing the efficient preparation of unsymmetric bis- and tris(triazole)s using diverse platform molecules.

Tetrafluoroaryl azide as an N-terminal capping group for click-to-dissolve diphenylalanine hydrogels

The synthesis of a bioorthogonal-responsive low molecular weight diphenylalanine (PhePhe)-based hydrogel that is capped with a 4-azido-2,3,5,6-tetrafluorobenzyl carbamate self-immolative linker is reported. The hydrogelator (AzF₄-PhePhe) generates a stable hydrogel at 0.1 wt%, and rapidly reacts with the bioorthogonal reagent trans-cyclooctene (TCO), inducing a gel-to-solution transition. The critical gel concentration is five-fold lower than our previously synthesized non-fluorinated hydrogelator (Az-PhePhe), and the minimum concentration of TCO required for visible gel-to-solution transition in 24 hours is 1 mM. Doxorubicin can be encapsulated in the hydrogel and TCO-triggered dissolution results in 76% and 89% release after 10 and 24 hours, respectively. Compared with our non-substituted aryl azide capping group used for Az-PhePhe, the tetrafluorinated aryl azide group improves the stability of the hydrogel in unbuffered water at a lower critical gel concentration, while improving sensitivity towards the bioorthogonal reagent TCO.

A tetrafluoroaryl azide group attached to diphenylalanine via a carbamate linker provides a strong and stable hydrogel that undergoes a gel-to-solution transition following a rapid bioorthogonal 1,3,-dipolar cycloaddition.

Tetra-fluorinated aromatic azide for highly efficient bioconjugation in living cells

We developed a fast strain-promoted azide–alkyne cycloaddition reaction (SPAAC) by tetra-fluorinated aromatic azide with a kinetic constant of 3.60 M⁻¹ s⁻¹, which is among the fastest SPAAC ligations reported so far. We successfully employed the reaction for covalent labelling of proteins with high efficiency and for bioimaging of mitochondria in living cells. The reaction could be a generally useful toolbox for chemical biology and biomaterials.

A fast strain-promoted azide–alkyne cycloaddition based on tetra-fluorinated aromatic azide was developed and applied to label proteins and living cells with high efficiency.

The one-pot nonhydrolysis Staudinger reaction and Staudinger or SPAAC ligation

The one-pot nonhydrolysis Staudinger reaction and Staudinger or SPAAC ligation were used for producing a FRET-based dyad in living cells as a proof-of-concept study.