Recent Progress on Synthesis of Functionalized 1,5-Disubstituted Triazoles

Immediately after the invention of 'Click Chemistry' in 2002, the regioselective 1,2,3- triazole scaffolds resulted from respective organic azides and terminal alkynes under Cu(I) catalysis have been well recognized as the functional heterocyclic core at the centre of modern organic chemistry, medicinal chemistry, and material sciences. This CuAAC reaction has several notable features including excellent regioselectivity, high-to-excellent yields, easy to execute, short reaction time, modular in nature, mild condition, readily available starting materials, etc. Moreover, the resulting regioselective triazoles can serve as amide bond isosteres, a privileged functional group in drug discovery and development. More than hundreds of reviews had been devoted to the 'Click Chemistry' in special reference to 1,4-disubstituted triazoles, while only little efforts were made for an opposite regioisomer i.e., 1,5-disubstituted triazole. Herein, we have presented various classical approaches for an expeditious synthesis of a wide range of biologically relevant 1,5- disubstituted 1,2,3-triazole analogues. The syntheses of such a class of diversly functionalized triazoles have emerged as a crucial investigation in the domain of chemistry and biology. This tutorial review covers the literature assessment on the development of various synthetic protocols for the functionalized 1,5-disubstituted triazoles reported during the last 12 years.

Growing Impact of Intramolecular Click Chemistry in Organic Synthesis

Click Chemistry, a modular, rapid, and one of the most reliable tool for the regioselective 1,2,3-triazole forming [3+2] reaction of organic azide and terimal alkyne is widely explored in various emerging domains of research ranging from chemical biology to catalysis and medicinal chemistry to material science. This regioselective reaction from a diverse range of azido-alkyne scaffolds has been well performed in both intermolecular as well as intramolecular fashions. In comparison to the intermolecular metal (Cu/Ru/Ni) variant of 'Click Chemistry', the intramolecular click tool is little addressed. The intramolecular click chemistry is exemplified as a mordern tool of cyclization which involves metal-catalyzed (CuAAC/RuAAC) cyclization, organo-catalyzed cyclization, and thermal-induced topochemical reaction. Thus, we report herein the recent approaches on intramolecular azide-alkyne cycloaddition 'Click Chemistry' with their wide-spread emerging applications in the developement of a diverse range of molecules including fused-heterocycles, well-defined peptidomemics, and macrocyclic architectures of various notable features.

Side‐Chain Control of Topochemical Polymer Single Crystals with Tunable Elastic Modulus

Topochemical polymerizations hold the promise of producing high molecular weight and stereoregular single crystalline polymers by first aligning monomers before polymerization. However, monomer modifications often alter the crystal packing and result in non-reactive polymorphs. Here, we report a systematic study on the side chain functionalization of the bis(indandione) derivative system that can be polymerized under visible light. Precisely engineered side chains help organize the monomer crystals in a one-dimensional fashion to maintain the topochemical reactivity. By optimizing the side chain length and end group of monomers, the elastic modulus of the resulting polymer single crystals can also be greatly enhanced. Lastly, using ultrasonication, insoluble polymer single crystals can be processed into free-standing and robust polymer thin films. This work provides new insights on the molecular design of topochemical reactions and paves the way for future applications of this fascinating family of materials.

Topochemical Postulates: Are They Relevant for Topochemical Reactions Occurring at Elevated Temperatures?

A rigid inositol-derived monomer functionalized with azide and alkyne as the complementary reactive groups (CRGs) crystallized as three distinct polymorphs I-III. Despite the unsuitable orientation of CRGs in the crystals for complete polymerization, all the three polymorphs underwent regiospecific and quantitative topochemical azide-alkyne cycloaddition (TAAC) polymerization upon heating to yield three different polymorphs of 1,2,3-triazol-1,4-diyl-linked-poly-neo-inositol. The molecules in these polymorphs exploit the weak intermolecular interactions, free space in the crystal lattice, and heat energy for their large and cooperative molecular motion to attain a transient reactive orientation, ultimately leading to the regiospecific TAAC reaction yielding distinct crystalline polymers. This study cautions that the overreliance on topochemical postulates for the prediction of topochemical reactivity at high temperatures could be misleading.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Topochemical polymerizations for the solid-state synthesis of organic polymers

Topochemical polymerizations are solid-state reactions driven by the alignment of monomers in the crystalline state. The molecular confinement in the monomer crystal lattice offers precise control over the tacticity, packing and crystallinity of the polymer formed in the topochemical reaction. As topochemical reactions occur under solvent- and catalyst-free conditions, giving products in high yield and selectivity/specificity that do not require tedious chromatographic purification, topochemical polymerizations are highly attractive over traditional solution-phase polymer synthesis. By this method, polymers having sophisticated structures and desired topologies can be availed. Often, such ordered packing confers attractive properties to the topochemically-synthesized polymers. Diverse categories of topochemical polymerizations are known, such as polymerizations via [2+2], [4+4], [4+2], and [3+2] cycloadditions, and polymerization of diynes, triynes, dienes, trienes, and quinodimethanes, each of which proceed under suitable stimuli like heat, light or pressure. Each class of these reactions requires a unique packing arrangement of the corresponding monomers for the smooth reaction and produces polymers with distinct properties. This review is penned with the intent of bringing all the types of topochemical polymerizations into a single platform and communicating the versatility of these lattice-controlled polymerizations. We present a brief history of the development of each category and comprehensively review the topochemical synthesis of fully-organic polymers reported in the last twenty years, particularly in crystals. We mainly focus on the various molecular designs and crystal engineering strategies adopted to align monomers in a suitable orientation for polymerization. Finally, we analyze the current challenges and future perspectives in this research field.

β-Sheet to Helical-Sheet Evolution Induced by Topochemical Polymerization: Cross-α-Amyloid-like Packing in a Pseudoprotein with Gly-Phe-Gly Repeats

Protein-mimics are of great interest for their structure, stability, and properties. We are interested in the synthesis of protein-mimics containing triazole linkages as peptide-bond surrogate by topochemical azide-alkyne cycloaddition (TAAC) polymerization of azide- and alkyne-modified peptides. The rationally designed dipeptide N₃ -CH₂ CO-Phe-NHCH₂ CCH (1) crystallized in a parallel β-sheet arrangement and are head-to-tail aligned in a direction perpendicular to the β-sheet-direction. Upon heating, crystals of 1 underwent single-crystal-to-single-crystal polymerization forming a triazole-linked pseudoprotein with Gly-Phe-Gly repeats. During TAAC polymerization, the pseudoprotein evolved as helical chains. These helical chains are laterally assembled by backbone hydrogen bonding in a direction perpendicular to the helical axis to form helical sheets. This interesting helical-sheet orientation in the crystal resembles the cross-α-amyloids, where α-helices are arranged laterally as sheets.

Topochemical synthesis of different polymorphs of polymers as a paradigm for tuning properties of polymers

Different packing is a mechanism through which nature can produce materials of different properties from the same basic units. There is great interest in constructing different forms of the same polymer by utilising different packing. Common solution-synthesized polymers are amorphous and their post-synthesis crystallization into different topologies is almost impossible. Here we show solid-state polymerization of different reactive polymorphs of a monomer pre-organized in different topologies. Trimorphs of a dipeptide monomer pack in a head-to-tail fashion, placing the azide and alkyne of adjacent monomers in proximity. On heating, these crystals undergo a topochemical azide-alkyne cycloaddition reaction yielding triazole-linked polymer in three different crystalline states; one with antiparallel arrangement of polymer chains, another with parallelly oriented chains, and a third form containing a 1:1 blend of two different conformers aligned in parallel. This approach of exploiting different polymorphs of a monomer for topochemical polymerization to yield polymorphs of polymers is promising for future research.

Topochemical Azide-Alkyne Cycloaddition Reaction

Topochemical reactions are solid-state reactions that transpire under the strict control of molecular packing in the crystal lattice. Due to this lattice control, these reactions generate products in a regio-/stereospecific manner and in very high yields. In a broader sense, topochemical reactions mimic nature's way of carrying out reactions in a confined environment of enzymes giving specific products. Apart from their remarkable specificity, topochemical reactions have many other interesting features that make these reactions more attractive than solution-phase reactions. Solution-phase reactions necessitate the use of reactants, reagents, catalysts, and solvents and often give products along with varying amounts of byproducts, necessitating complex workup and chromatographic purification using various chemicals. These inevitable chemical wastes from solution-state reactions could be avoided by topochemical reactions, as they are solvent-free and catalyst-free and often do not require any chromatographic purification in view of their specificity and high yielding nature. Also the confinement offered by the crystal lattice gives products that are not possible by solution-phase reactions. Another interesting feature of topochemical reactions is the possibility of formation of products in an ordered (crystalline) form, which imparts interesting properties. Thus, topochemical reactions have control not only at the molecular level (regio-/stereospecificity) but also at the supramolecular level (packing). Many topochemical reactions happen in single-crystal-to-single-crystal (SCSC) fashion, and crystal structure analysis of such reactions often gives mechanistic insights and knowledge about the geometrical criteria required for the reaction. Despite all these attractive features, reactions that can be done topochemically are limited. There is tremendous interest in the development of new categories of topochemical reactions and strategies to achieve reactivity in crystals. In this Account, we will summarize our attempts to develop topochemical azide-alkyne cycloaddition (TAAC) reactions. We have used hydrogen-bonding as the main noncovalent interaction for aligning azide-and-alkyne-substituted derivatives of various biomolecules in orientations suitable for their proximity-driven cycloaddition reaction in crystals. Overall, three major classes of biomolecules; carbohydrates, nucleosides, and peptides were successfully exploited for their TAAC reactions using conventional O-H···O, N-H···O, and N-H···N hydrogen bonds as supramolecular glues for controlling their assembly in crystals. The crystals of these monomers underwent TAAC reaction either spontaneously at room temperature or under heating yielding triazole-linked biopolymer mimics. The ordered packing of product molecules imparted special properties to the products formed. The legendary "cream of the crop" azide-alkyne click reaction has diverse applications in the areas of bioconjugation, material science, polymer synthesis, and so forth. Belonging to the same genre, TAAC is a novel metal-free approach for making the triazole-linked products employing the ordered crystal/gel as a reaction medium. In brief, our studies suggest that TAAC reaction can be implemented in diverse molecular categories and has high potential to develop into a field with practical applications.