Adapting N‐Heterocyclic Carbene/Azide Coupling Chemistry for Polymer Synthesis: Enabling Access to Aromatic Polytriazenes

A click by any other name : Coupling bis(N‐heterocyclic carbene)s with bis(azide)s afforded a novel class of conjugated polytriazenes. These polymers were rendered electrically conductive upon doping, and fluorene‐containing variants exhibited luminescence. This adaptation of N‐heterocyclic carbene (NHC)/azide coupling chemistry to polymer synthesis reveals the potential of NHCs as building blocks for accessing polymers having useful electronic properties.

     

Oxidative Enantioselective α‐Fluorination of Aliphatic Aldehydes Enabled by N‐Heterocyclic Carbene Catalysis

Described is the first study on oxidative enantioselective α‐fluorination of simple aliphatic aldehydes enabled by N‐heterocyclic carbene catalysis. N‐fluorobis(phenyl)sulfonimide serves as a an oxidant and as an “F” source. The C? F bond formation occurs directly at the α position of simple aliphatic aldehydes, thus overcoming nontrivial challenges, such as competitive difluorination and nonfluorination, and proceeds with high to excellent enantioselectivities.     

Neoglycopolymers Based on 4‐Vinyl‐1,2,3‐Triazole Monomers Prepared by Click Chemistry

The synthesis of a new glycomonomer based on mannose, prepared via CuAAC, is reported. The resulting 1,2,3‐triazole linkage between mannose and the polymer backbone ensures the formation of highly stable glycopolymers, which will not undergo hydrolysis. The monomer 2′‐(4‐vinyl‐[1,2,3]‐triazol‐1‐yl)ethyl‐O‐α‐D ‐mannopyranoside was polymerized in the presence of a RAFT agent – 3‐benzylsulfanylthiocarbonylsulfanyl propionic acid – to yield well‐defined polymers with molecular weights up to 51 500 g mol^?1 and a PDI of 1.16. The resulting polymer was employed as a macroRAFT agent in the polymerization of NIPAAm in order to generate thermo‐responsive block copolymers, which undergo reversible micelle formation at elevated temperatures. The rapid interaction between the polymers prepared and ConA confirms the high affinity of these structures to proteins. While the linear glycopolymers already undergo a fast complexation with ConA, the reported rates have found to be exceeded by the micellar glycopolymer structure presented in the current contribution.

     

A Dual‐Response [2]Rotaxane Based on a 1,2,3‐Triazole Ring as a Novel Recognition Station

Two novel multilevel switchable [2]rotaxanes containing an ammonium and a triazole station have been constructed by a Cu^I‐catalyzed azide–alkyne cycloaddition reaction. The macrocycle of [2]rotaxane containing a C6‐chain bridge between the two hydrogen bonding stations exhibits high selectivity for the ammonium cation in the protonated form. Interestingly, the macrocycle is able to interact with the two recognition stations when the bridge between them is shortened. Upon deprotonation of both [2]rotaxanes, the macrocycle moves towards the triazole recognition site due to the hydrogen‐bond interaction between the triazole nitrogen atoms and the amide groups in the macrocycle. Upon addition of chloride anion, the conformation of [2]rotaxane is changed because of the cooperative recognition of the chloride anion by a favorable hydrogen‐bond donor from both the macrocycle isophthalamide and thread triazole CH proton.     

N‐Heterocyclic Carbene Catalyzed Oxidative Coupling of Alkenes/ α‐Bromoacetophenones with Aldehydes: A Facile Entry to α,β‐Epoxy Ketones

A novel, N‐heterocyclic carbene (NHC) catalyzed direct oxidative coupling of styrenes with aldehydes has been described for the synthesis of α,β‐epoxy ketones in good yields. This unprecedented regioselective oxidative coupling employs NBS/DBU/DMSO (DBU=1,8‐diazabicyclo [5.4. 0] undec‐7‐ene, DMSO=dimethylsulfoxide, NBS=N‐bromosuccinimide) as an oxidative system at ambient conditions. Additionally, first NHC‐catalyzed Darzens reaction of α‐bromoketones and aldehydes under mild reaction conditions has also been described. Interestingly, mechanistic studies have revealed the preferred reactivity of NHC with alkene/α‐bromoketone rather than aldehydes, thus proceeding via the ketodeoxy Breslow intermediate.     

Catalysis with N‐Heterocyclic Carbenes under Oxidative Conditions

This Concept article discusses the potential of oxidative carbene catalysis in synthesis and comprehensively covers pioneering studies as well as recent developments. Oxidative carbene catalysis can be conducted by using inorganic and organic oxidants. Applications in cascade processes, in enantioselective catalysis, and also in natural product synthesis are discussed.     

Hydroboration of Arynes with N‐Heterocyclic Carbene Boranes

Arynes were generated in situ from ortho‐silyl aryl triflates and fluoride ions in the presence of stable N‐heterocyclic carbene boranes (NHC? BH₃). Spontaneous hydroboration ensued to provide stable B‐aryl‐substituted NHC‐boranes (NHC? BH₂Ar). The reaction shows good scope in terms of both the NHC‐borane and aryne components and provides direct access to mono‐ and disubstituted NHC‐boranes. The formation of unusual ortho regioisomers in the hydroboration of arynes with an electron‐withdrawing group supports a hydroboration process with hydride‐transfer character.     

5‐(1,2,3‐Triazol‐1‐yl)tetrazole Derivatives of an Azidotetrazole via Click Chemistry

N? C bonded (non‐bridged) 5‐(1,2,3‐triazol‐1‐yl)tetrazoles were synthesized by the Cu^I‐catalyzed 1,3‐dipolar azide–alkyne cycloaddition click reaction using 5‐azido‐N‐(propan‐2‐ylidene)‐1H‐tetrazole ( 1 ). For example, the click reaction of 1 in the presence of CuSO₄?5 H₂O and Na ascorbate at 65–70 °C for 48 h in CH₃CN/H₂O co‐solvent was found to be limited to only terminal alkynes that have electron‐withdrawing groups, CF₃C?CH ( 2 a ) and SF₅C?CH ( 2 b ), giving rise to isopropylidene‐[5‐(4‐trifluoromethyl‐1,2,3‐triazol‐1‐yl)tetrazol‐1‐yl]amine ( 3 a ) and isopropylidene‐[5‐(4‐pentafluorosulfanyl‐1,2,3‐triazol‐1‐yl)tetrazol‐1‐yl]amine ( 3 b ) in 47 % and 66 % yields, respectively. When carried out under conditions using CuI and 2,6‐lutidine as catalysts at 0 °C for 13 h in CHCl₃, the click reaction was versatile toward alkynes even those having electron‐donating groups. Properties of new products were determined and compared with those of 1 . Heats of formation, detonation pressures, detonation velocities and impact sensitivities are reported for these new 5‐(1,2,3‐triazol‐1‐yl)tetrazoles.     

Calixarene‐Based Chemosensors by Means of Click Chemistry

Click chemistry, a new strategy for organic chemistry, has been widely used in the chemical modification of calixarenes because of its reliability, specificity, biocompatibility, and efficiency. Click‐derived triazoles also play a critical role in sensing ions and molecules. This in‐depth review provides an overview of calixarene‐based chemosensors that incorporate click‐derived triazoles, and their three characteristics (chromogenic, fluorescence, and wettability) are reviewed.     

An N‐Heterocyclic Carbene with a Saturated Backbone and Spatially‐Defined Steric Impact

The synthesis and coordination chemistry of a saturated analogue of a “bulky‐yet‐flexible” N‐heterocyclic carbene (NHC) ligand are described. “SIPaul” is a 4,5‐dihydroimidazol‐2‐ylidene ligand with unsymmetrical aryl N‐substituents, and is one of the growing class of “bulky‐yet‐flexible” NHCs that are sufficiently bulky to stabilize catalytic intermediates, but sufficiently flexible that they do not inhibit productive chemistry at the central metal atom. Here, the synthesis of SIPaul.HCl and its complexes with copper, silver, iridium, palladium, and nickel, and its selenourea, are reported. The steric impact of the ligand is quantified using percent buried volume (% V_bur), whereas the electronic properties are probed and quantified using the Tolman Electronic Parameter (TEP) and δ_Se of the corresponding selenourea. This work shows that despite the often very different performance of saturated versus unsaturated carbenes in catalysis, the effect of backbone saturation on measurable properties is very small.     

Cyclometalated Mono‐ and Dinuclear IrIII Complexes with “Click”‐Derived Triazoles and Mesoionic Carbenes

Orthometalation at Ir^III centers is usually facile, and such orthometalated complexes often display intriguing electronic and catalytic properties. By using a central phenyl ring as C?H activation sites, we present here mono‐ and dinuclear Ir^III complexes with “click”‐derived 1,2,3‐triazole and 1,2,3‐triazol‐5‐ylidene ligands, in which the wingtip phenyl groups in the aforementioned ligands are additionally orthometalated and bind as carbanionic donors to the Ir^III centers. Structural characterization of the complexes reveal a piano stool‐type of coordination around the metal centers with the “click”‐derived ligands bound either with C^N or C^C donor sets to the Ir^III centers. Furthermore, whereas bond localization is observed within the 1,2,3‐triazole ligands, a more delocalized situation is found in their 1,2,3‐triazol‐5‐ylidene counterparts. All complexes were subjected to catalytic tests for the transfer hydrogenation of benzaldehyde and acetophenone. The dinuclear complexes turned out to be more active than their mononuclear counterparts. We present here the first examples of stable, isomer‐pure, dinuclear cyclometalated Ir^III complexes with poly‐mesoionic‐carbene ligands.     

Direct Esterification of Aromatic Aldehydes with Tetraphenylphosphonium Bromide under Oxidative N‐Heterocyclic Carbene Catalysis

An unconventional reagent, tetraphenylphosphonium bromide, was employed as a phenyl source in the direct transformation of aromatic aldehydes to the corresponding phenyl esters under oxidative N‐heterocyclic carbene (NHC) catalysis. The phenyl esters were obtained in moderate yields under mild and organocatalytic conditions.     

Enantioselective Synthesis of Tertiary Propargylic Alcohols under N‐Heterocyclic Carbene Catalysis

A straightforward procedure to carry out the enantioselective benzoin reaction between aldehydes and ynones by employing a chiral N‐heterocyclic carbene (NHC) as catalyst was developed. Under the optimized reaction conditions, these ynones undergo a clean and selective 1,2‐addition with the catalytically generated Breslow intermediate, not observing any byproduct arising from competitive Stetter‐type reactivity. This procedure allows the preparation of tertiary alkynyl carbinols as highly enantioenriched materials, which have the remarkable potential to be used as chiral building blocks in organic synthesis.     

Protecting‐Group‐Free One‐Pot Synthesis of Glycoconjugates Directly from Reducing Sugars

The conversion of sugars into glycomimetics typically involves multiple protecting‐group manipulations. The development of methodology allowing the direct aqueous conversion of free sugars into glycosides, and mimics of oligosaccharides and glycoconjugates in a high‐yielding and stereoselective process is highly desirable. The combined use of 2‐azido‐1,3‐dimethylimidazolinium hexafluorophosphate and the Cu‐catalyzed Huisgen cycloaddition allowed the synthesis of a range of glycoconjugates in a one‐step reaction directly from reducing sugars under aqueous conditions. The reaction, which is completely stereoselective, may be applied to the convergent synthesis of triazole‐linked glycosides, oligosaccharides, and glycopeptides. The procedure provides a method for the one‐pot aqueous ligation of oligosaccharides and peptides bearing alkyne side chains.     

Chiroptical Probing of Lanthanide‐Directed Self‐Assembly Formation Using btp Ligands Formed in One‐Pot Diazo‐Transfer/Deprotection Click Reaction from Chiral Amines

A series of enantiomeric 2,6‐bis(1,2,3‐triazol‐4‐yl)pyridines (btp)‐containing ligands was synthesized by a one‐pot two‐step copper‐catalyzed amine/alkyne click reaction. The Eu^III‐ and Tb^III‐directed self‐assembly formation of these ligands was studied in CH₃CN by monitoring their various photophysical properties, including their emerging circular dichroism and circularly polarized luminescence. The global analysis of the former enabled the determination of both the stoichiometry and the stability constants of the various chiral supramolecular species in solution.     

Facet‐Dependent Catalytic Activity of Cu2O Nanocrystals in the One‐Pot Synthesis of 1,2,3‐Triazoles by Multicomponent Click Reactions

We report the highly facet‐dependent catalytic activity of Cu₂O nanocubes, octahedra, and rhombic dodecahedra for the multicomponent direct synthesis of 1,2,3‐triazoles from the reaction of alkynes, organic halides, and NaN₃. The catalytic activities of clean surfactant‐removed Cu₂O nanocrystals with the same total surface area were compared. Rhombic dodecahedral Cu₂O nanocrystals bounded by {110} facets were much more catalytically active than Cu₂O octahedra exposing {111} facets, whereas Cu₂O nanocubes displayed the slowest catalytic activity. The superior catalytic activity of Cu₂O rhombic dodecahedra is attributed to the fully exposed surface Cu atoms on the {110} facet. A large series of 1,4‐disubstituted 1,2,3‐triazoles have been synthesized in excellent yields with high regioselectivity under green conditions by using these rhombic dodecahedral Cu₂O catalysts, including the synthesis of rufinamide, an antiepileptic drug, demonstrating the potential of these nanocrystals as promising heterogeneous catalysts for other important coupling reactions.     

Synthesis of Thermally Stable Energetic 1,2,3‐Triazole Derivatives

Various thermally stable energetic polynitro‐aryl‐1,2,3‐triazoles have been synthesized through Cu‐catalyzed [3+2] cycloaddition reactions between their corresponding azides and alkynes, followed by nitration. These compounds were characterized by analytical and spectroscopic methods and the solid‐state structures of most of these compounds have been determined by using X‐ray diffraction techniques. Most of the polynitro‐bearing triazole derivatives decomposed within the range 142–319 °C and their heats of formation and crystal densities were determined from computational studies. By using the Kamlet–Jacobs empirical relation, their detonation velocities and pressures were calculated from their heats of formation and crystal densities. Most of these newly synthesized compounds exhibited high positive heats of formation, good thermal stabilities, reasonable densities, and acceptable detonation properties that were comparable to those of TNT.