Building Diversity in ortho-Substituted s-Aryltetrazines By Tuning N-Directed Palladium C–H Halogenation: Unsymmetrical Polyhalogenated and Biphenyl s-Aryltetrazines

Molecular engineering of 3-arylated tetrazo[1,2-b]indazoles: divergent synthesis and structure-property relationships

The synthetic scope of 3-arylated tetrazo[1,2-b]indazoles is reported based on a Pd-catalyzed Liebeskind-Srogl cross-coupling reaction followed by an N-cyclisation process. The reactivity of the nitrogen atoms was used to further diversify these N-rich polyaromatic tetrazo[1,2-b]indazoles in a panel of reactions (protonation, selective oxidation, metallations). Selective ortho-C-H activation/functionalization on the heterocycle was also demonstrated with three transition metals (TM = Pd, Ir and Rh). The effects of all these molecular engineering strategies, particularly the N-modifications, on the optical and redox properties of the 3-arylated tetrazoindazoles were studied experimentally and theoretically. This study highlights the diversity of molecular structures and electronic properties offered by the tetrazo[1,2-b]indazole platform.

Advances in the Synthesis of Bioorthogonal Reagents: s-Tetrazines, 1,2,4-Triazines, Cyclooctynes, Heterocycloheptynes, and trans-Cyclooctenes

Aligned with the increasing importance of bioorthogonal chemistry has been an increasing demand for more potent, affordable, multifunctional, and programmable bioorthogonal reagents. More advanced synthetic chemistry techniques, including transition-metal-catalyzed cross-coupling reactions, C-H activation, photoinduced chemistry, and continuous flow chemistry, have been employed in synthesizing novel bioorthogonal reagents for universal purposes. We discuss herein recent developments regarding the synthesis of popular bioorthogonal reagents, with a focus on s-tetrazines, 1,2,4-triazines, trans-cyclooctenes, cyclooctynes, hetero-cycloheptynes, and -trans-cycloheptenes. This review aims to summarize and discuss the most representative synthetic approaches of these reagents and their derivatives that are useful in bioorthogonal chemistry. The preparation of these molecules and their derivatives utilizes both classical approaches as well as the latest organic chemistry methodologies.

Solid-Phase Synthesis of s-Tetrazines

An efficient synthesis of s-tetrazines by solid-phase methods is described. This synthesis route was compatible with different solid-phase resins and linkers and did not require metal catalysts or high temperatures. Monosubstituted tetrazines were routinely synthesized using thiol-promoted chemistry, using dichloromethane as a carbon source, while disubstituted unsymmetrical aryl or alkyl tetrazines were synthesized using readily available nitriles. This efficient approach enabled the synthesis of s-tetrazines in high yields (70-94%), eliminating the classical solution-phase problems of mixtures of symmetrical and unsymmetrical tetrazines, with only a single final purification step required, and paves the way to the rapid synthesis of s-tetrazines with various applications in bioorthogonal chemistry and beyond.

Tetrazo[1,2-b]indazoles: Straightforward Access to Nitrogen-Rich Polyaromatics from s-Tetrazines

The straightforward access to a new class of aza-polyaromatics is reported. Starting from readily available fluorinated s-tetrazine, a cyclization process with azide leads to the formation of an unprecedented tetrazo[1,2-b]indazole or a bis-tetrazo[1,2-b]indazole (cis and trans conformers). Based on the new nitrogen core, further N-directed palladium-catalyzed ortho-C-H bond functionalization allows the introduction of halides or acetates. The physicochemical properties of these compounds were studied by a joint experimental/theoretical approach. The tetrazo[1,2-b]indazoles display solid-state π-stacking, low reduction potential, absorption in the visible range up to the near-infrared, and intense fluorescence, depending on the molecular structure.

Hybrid Phosphine/Amino-Acid Ligands Built on Phenyl and Ferrocenyl Platforms: Application in the Suzuki Coupling of o-Dibromobenzene with Fluorophenylboronic Acid

We describe the synthesis and characterization of two classes of hybrid phosphino ligands functionalized with amino ester or amino acid groups. These compounds are built either on a rigid planar phenyl platform or on a functionalized - conformationally controlled - rotational ferrocene backbone. Modifications at the -PR₂ phosphino groups (R=aryl and alkyl, with various steric bulk, Ph, Mes, i-Pr, Cy) and at the amino acid/amino ester functions are reported, showing a valuable high modularity. The coordination chemistry of these compounds regarding palladium and gold was investigated, in particular with respect to the coordination mode of the phosphino groups and the preferred interaction with metals for the amino ester and amino acid functions. For all the hybrid ligands, based either on ferrocenyl or phenyl platforms, the (P,N)-chelating effect dominates in solution for coordination to Pd(II), while linear P-Au(I) complexes without interaction with the amino groups are assumed. The investigation of the catalytic activity of these new ligands in the demanding palladium-catalyzed Suzuki-Miyaura coupling of o-dibromoarenes with fluorophenylboronic acid underlined the importance of the amino ester dicyclohexylphosphinoferrocene for avoiding the deleterious homocoupling and arene oligomerization side-reactions that were otherwise observed with the other phosphine ligands.

Alkali halides as nucleophilic reagent sources for N-directed palladium-catalysed ortho-C-H halogenation of s-tetrazines and other heteroaromatics

A general palladium-catalysed selective C-H halogenation reaction is reported, which was successfully achieved for a large variety of functionalized aromatic rings incorporating diverse N-directing groups. By using simple alkali halides of MX type as the nucleophilic reagent source (M = Li, Na, K, Cs and X = I, Br and Cl), and phenyliodanediacetate oxidant, clean C-H-iodination, bromination and chlorination reactions were performed. This general protocol of selective ortho-monohalogenation, which complements but contrasts with the classical methods using electrophilic reagents, is achievable in a short time (30 min) with microwave irradiation assistance. The reaction was extended to substrates bearing N-directing pyridine, pyrimidine, pyrazole, oxazoline, naphtho[1,2-d]thiazole, and azobenzene groups. Notably, the topical and selectivity-challenging s-tetrazine, as a nitrogen-rich heteroaromatic, was successfully halogenated by this protocol.

Derivatization based on tetrazine scaffolds: synthesis of tetrazine derivatives and their biomedical applications

The recent advances in tetrazine scaffold-based derivatizations have been summarized. The advantages and limitations of derivatization methods and applications of the developed tetrazine derivatives in bioorthogonal chemistry have been highlighted.

Coordination Chemistry of a Bis(Tetrazine) Tweezer: A Case of Host-Guest Behavior with Silver Salts

The carbon-carbon cross-coupling of phenyl s-tetrazine (Tz) units at their ortho-phenyl positions allows the formation of constrained bis(tetrazines) with original tweezer structures. In these compounds, the face-to-face positioning of the central tetrazine cores is reinforced by π-stacking of the electron-poor nitrogen-containing heteroaromatic moieties. The resulting tetra-aromatic structure can be used as a weak coordinating ligand with cationic silver. This coordination generates a set of bis(tetrazine)-silver(I) coordination complexes tolerating a large variety of counter anions of various geometries, namely, PF₆⁻, BF₄⁻, SbF₆⁻, ClO₄⁻, NTf₂⁻, and OTf⁻. These compounds were characterized in the solid state by single-crystal X-ray diffraction (XRD) and diffuse reflectance spectroscopy, and in solution by ¹H-NMR, mass spectrometry, electroanalysis, and UV-visible absorption spectrophotometry. The X-ray crystal structure of complexes {[Ag(3)][PF₆]}_∞ (4) and {[Ag(3)][SbF₆]}_∞ (6), where 3 is 3,3′-[(1,1′-biphenyl)-2,2′-diyl]-6,6′-bis(phenyl)-1,2,4,5-tetrazine, revealed the formation of 1D polymeric chains, characterized by an evolution to a large opening of the original tweezer and a coordination of silver(I) via two chelating nitrogen atom and some C=C π-interactions. Electrochemical and UV spectroscopic properties of the original tweezer and of the corresponding silver complexes are reported and compared. ¹H-NMR titrations with AgNTf₂ allowed the determination of the stoichiometry and apparent stability of two solution species, namely [Ag(3)]⁺ and [Ag(3)₂]²⁺, that formed in CDCl₃/CD₃OD 2:1 v/v mixtures.

C-H Bond Arylation of Pyrazoles at the β-Position: General Conditions and Computational Elucidation for a High Regioselectivity

Direct arylation of most five-membered ring heterocycles are generally easily accessible and strongly favored at the α-position using classical palladium-catalysis. Conversely, regioselective functionalization of such heterocycles at the concurrent β-position remains currently very challenging. Herein, we report general conditions for regioselective direct arylation at the β-position of pyrazoles, while C-H α-position is free. By using aryl bromides as the aryl source and a judicious choice of solvent, the arylation reaction of variously N-substituted pyrazoles simply proceeds via β-C-H bond functionalization. The β-regioselectivity is promoted by a ligand-free palladium catalyst and a simple base without oxidant or further additive, and tolerates a variety of substituents on the bromoarene. DFT calculations revealed that a protic solvent such as 2-ethoxyethan-1-ol significantly enhances the acidity of the proton at β-position of the pyrazoles and thus favors this direct β-C-H bond arylation. This selective pyrazoles β-C-H bond arylation was successfully applied for the straightforward building of π-extended poly(hetero)aromatic structures via further Pd-catalyzed combined α-C-H intermolecular and intramolecular C-H bond arylation in an overall highly atom-economical process.

Catalytic C-H Functionalization of Unreactive Furan Cores in Bio-Derived Platform Chemicals

C-H functionalization is one of the most convenient and powerful tools in the arsenal of modern chemistry, deservedly nominated as the "Holy Grail" of organic synthesis. A frequent disadvantage of this method is the need for harsh reaction conditions to carry out transformations of inert C-H bonds, which limits the possibility of its use for modifying less stable substrates. Biomass-derived furan platform chemicals, which have a relatively unstable aromatic furan core and highly reactive side chain substituents, are extremely promising and valuable organic molecules that are currently widely used in a variety of research and industrial fields. The high sensitivity of furan derivatives to acids, strong oxidants, and high temperatures significantly limits the use of classical methods of C-H functionalization for their modification. New methods of catalytic functionalization of non-reactive furan cores are urgently required to obtain a new generation of materials with controlled properties and potentially bioactive substances.

1,2,4,5-Tetrazine derivatives as components and precursors of photo- and electroactive materials

Synthetic approaches to 3,6-disubstituted-1,2,4,5-tetrazine systems are analyzed, and their properties attractive to practical applications in photo- and electroactive materials are overviewed.

Emerging Concepts in Carbon Nitride Organic Photocatalysis

Carbon nitrides encompass a class of transition-metal-free materials possessing numerous advantages such as low cost (few Euros per gram), high chemical stability, broad tunability of redox potentials and optical bandgap, recyclability, and a high absorption coefficient (>10⁵  cm^-1 ), which make them highly attractive for application in photoredox catalysis. In this Review, we classify carbon nitrides based on their unique properties, structure, and redox potentials. We summarize recently emerging concepts in heterogeneous carbon nitride photocatalysis, with an emphasis on the synthesis of organic compounds: 1) Illumination-Driven Electron Accumulation in Semiconductors and Exploitation (IDEASE); 2) singlet-triplet intersystem crossing in carbon nitride excited states and related energy transfer; 3) architectures of flow photoreactors; and 4) dual metal/carbon nitride photocatalysis. The objective of this Review is to provide a detailed overview regarding innovative research in carbon nitride photocatalysis focusing on these topics.

Divergent Synthesis of Monosubstituted and Unsymmetrical 3,6-Disubstituted Tetrazines from Carboxylic Ester Precursors

Since tetrazines are important tools to the field of bioorthogonal chemistry, there is a need for new approaches to synthesize unsymmetrical and 3-monosubstituted tetrazines. Described here is a general, one-pot method for converting (3-methyloxetan-3-yl)methyl carboxylic esters into 3-thiomethyltetrazines. These versatile intermediates were applied to the synthesis of unsymmetrical tetrazines through Pd-catalyzed cross-coupling and in the first catalytic thioether reduction to access monosubstituted tetrazines. This method enables the development of new tetrazine compounds possessing a favorable combination of kinetics, small size, and hydrophilicity. It was applied to a broad range of aliphatic and aromatic ester precursors and to the synthesis of heterocycles including BODIPY fluorophores and biotin. In addition, a series of tetrazine probes for monoacylglycerol lipase (MAGL) were synthesized and the most reactive one was applied to the labeling of endogenous MAGL in live cells.

Directing Group Assisted Unsymmetrical Multiple Functionalization of Arene C-H Bonds

Multiple C-H bond functionalizations promptly install diverse groups on the molecular framework and consequently fabricate complex molecular entities. This review briefly surveys the conceptual development of directing group assisted unsymmetrical multiple functionalization of arene C(sp² )-H bonds, which is exceedingly appealing and highly important.

Sulfoximine-Assisted Unsymmetrical Twofold C-H Functionalization of Arenes

An unprecedented ruthenium (Ru)-catalyzed twofold unsymmetrical annulation of 3-O/N-allyl benzoic acid derivatives with isocyanates for the construction of dihydro-furan/indole-fused phthalimide scaffolds is discussed. This double-unsymmetrical functionalization of both o,o'-C-H bonds of arene moiety is explicitly viable under the influence of methylphenyl sulfoximine directing group constructing three different [C-C/C-C(O)/N-C(O)] bonds under a single catalytic system. A broad scope with all six-carbon-substituted arene motifs, control experiments, and gram-scale synthesis make the synthetic model viable and significant.

Correction: Iridium-catalyzed C-H amidation of s-tetrazines

Correction for 'Iridium-catalyzed C-H amidation of s-tetrazines' by Huan Xiong et al., Chem. Commun., 2020, DOI: 10.1039/d0cc01647k.

Iridium-catalyzed C–H amidation of s-tetrazines

Synthesis of ortho-amino substituted s-tetrazines by iridium-catalyzed C–H activation for bioconjugation and DNA-encoded library.

Synthesis and structural characterisation of bulky heptaaromatic (hetero)aryl o-substituted s-aryltetrazines

Hexaphenylbenzene analogs with an electron-poor tetrazine core are synthesized in two high-yield steps from diphenyl-s-tetrazine. Crystal packing of these unique non-planar heptaaromatics is analyzed in details.

C-H Halogenation of Pyridyl Sulfides Avoiding the Sulfur Oxidation: A Direct Catalytic Access to Sulfanyl Polyhalides and Polyaromatics

Palladium-catalyzed oxidative C-H halogenation and acetoxylation reactions of S-unprotected sulfides, selectively directed by pyridinyl groups, allows the formation of C-X bonds (X = I, Br, Cl, OAc) by using simple halosuccinimide or phenyliodine diacetate (PIDA) oxidants. The undesired formation of sulfoxides and/or sulfones, which are usually observed under oxidative conditions, is fully obviated. Under the solvent-dependent conditions that we proposed, sulfide C-H functionalization is achieved in less than 1 h without any direct electrophilic halogenation at the pyridine moiety. N-Directed ortho-C-H activation of aryl also facilitates dibromination reactions which are hardly accessible with sulfone and sulfoxide counterparts because of their higher structural rigidity. This general method gives a straightforward access to polyhalide sulfides, without an organosulfur reduction step or protection-deprotection sequence. Polyhalide sulfides are valuable synthons that give a practical entry to new constrained polyaromatic and biphenyl sulfides, including synthetically challenging unsymmetrical examples.

Bridge-Clamp Bis(tetrazine)s with [N]8 π-Stacking Interactions and Azido-s-Aryl Tetrazines: Two Classes of Doubly Clickable Tetrazines

Click chemistry at a tetrazine core is useful for bioorthogonal labeling and crosslinking. Introduced here are two new classes of doubly clickable s-aryl tetrazines synthesized by Cu-catalyzed cross-coupling. Homocoupling of o-brominated s-aryl tetrazines leads to bis(tetrazine)s structurally characterized by tetrazine cores arranged face-to-face. [N]₈ π-stacking interactions are essential to the conformation. Upon inverse electron demand Diels-Alder (iEDDA) cycloaddition, the bis(tetrazine)s produce a unique staple structure. The o-azidation of s-aryl tetrazines introduces a second proximal intermolecular clickable function that leads to double click chemistry opportunities. The stepwise introduction of fluorophores and then iEDDA cycloaddition, including bioconjugation to antibodies, was achieved on this class of tetrazines. This method extends to (thio)etherification, phosphination, trifluoromethylation and the introduction of various bioactive nitrogen-based heterocycles.

Installation of Minimal Tetrazines through Silver-Mediated Liebeskind-Srogl Coupling with Arylboronic Acids

Described is a general method for the installation of a minimal 6-methyltetrazin-3-yl group via the first example of a Ag-mediated Liebeskind-Srogl cross-coupling. The attachment of bioorthogonal tetrazines on complex molecules typically relies on linkers that can negatively impact the physiochemical properties of conjugates. Cross-coupling with arylboronic acids and a new reagent, 3-((p-biphenyl-4-ylmethyl)thio)-6-methyltetrazine (b-Tz), proceeds under mild, PdCl₂(dppf)-catalyzed conditions to introduce minimal, linker-free tetrazine functionality. Safety considerations guided our design of b-Tz which can be prepared on decagram scale without handling hydrazine and without forming volatile, high-nitrogen tetrazine byproducts. Replacing conventional Cu(I) salts used in Liebeskind-Srogl cross-coupling with a Ag₂O mediator resulted in higher yields across a broad library of aryl and heteroaryl boronic acids and provides improved access to a fluorogenic tetrazine-BODIPY conjugate. A covalent probe for MAGL incorporating 6-methyltetrazinyl functionality was synthesized in high yield and labeled endogenous MAGL in live cells. This new Ag-mediated cross-coupling method using b-Tz is anticipated to find additional applications for directly introducing the tetrazine subunit to complex substrates.

One-Pot Twofold Unsymmetrical C-Si Bond 2,6-Bifunctionalization of Arenes via Sequential [1,4]-Csp2 to O-Silyl Migration

Twofold unsymmetrical C-Si bond bifunctionalization of 2,6-di(trimethylsilyl) benzyl alcohols has been achieved in one pot via sequential [1,4]-Csp² to O-silyl migration. The hydroxyl group functions as an "on-off-on" switch to control two successive silyl migrations, and 4,7-dimethyl-o-phenanthroline ligand favors cleavage of the endocyclic C-Si bond. Diverse Csp³/Csp³ or Csp²/Csp³ electrophiles can be installed at the 2- and 6-positions. This approach was used to chemoselectively functionalize the three C-Si bonds of 2,4,6-tri(trimethylsilyl) benzyl alcohol, transforming it into isochroman derivatives. The approach even works as a five-component reaction to construct complex symmetric structures.

Organocatalytic and Scalable Syntheses of Unsymmetrical 1,2,4,5-Tetrazines by Thiol-Containing Promotors

Despite the growing application of tetrazine bioorthogonal chemistry, it is still challenging to access tetrazines conveniently from easily available materials. Described here is the de novo formation of tetrazine from nitriles and hydrazine hydrate using a broad array of thiol-containing catalysts, including peptides. Using this facile methodology, the syntheses of 14 unsymmetric tetrazines, containing a range of reactive functional groups, on the gram scale were achieved with satisfactory yields. Using tetrazine methylphosphonate as a building block, a highly efficient Horner-Wadsworth-Emmons reaction was developed for further derivatization under mild reaction conditions. Tetrazine probes with diverse functions can be scalably produced in yields of 87-93 %. This methodology may facilitate the widespread application of tetrazine bioorthogonal chemistry.