Multi-Functionalization of Polymers by Strain-Promoted Cycloadditions

Highly Efficient and para-Selective C-H Functionalization of Polystyrene Providing a Versatile Platform for Diverse Applications

Postpolymerization modification of polystyrene (PS) can afford numerous value-added materials with different functions and applications, but it has been hampered by the lack of efficient methods. We report herein a highly efficient and para-selective conversion of the C-H bonds of the aromatic ring of PS into diverse functional groups using a combination of thianthrenation and thio-Suzuki-Miyaura coupling reaction. Notably, the thianthrenation efficiency of PS is as high as 99% and the degree of thianthrenation can be conveniently controlled using stoichiometric tuning of the amount of thianthrene-S-oxide added, resulting in 24-99 mol % thianthrenation. In the subsequent thio-Suzuki-Miyaura coupling reaction, 18 functionalized PS containing various functional groups (-CH₂OH, -OMe, -SMe, -OTBS, -CH₃, -NHBoc, -OCOMe, -CHO, -COMe, -Si(Me)₃, etc.) were successfully prepared with a high degree of functionalization (64-99 mol %). The obtained functionalized PS can be readily converted into diverse functional materials, including solid-phase synthesis resins, aggregation-induced emission fluorophores, as well as ionomer binders and ion-exchange membranes for energy conversion devices. This method imparts diverse functionality onto PS with extremely high efficiency and selectivity, providing a versatile platform to transform existing commodity PS plastics into high-performance materials.

Strategies to Achieve a Ferrocene-Based Polymer with Reversible Redox Activity for Chiral Electroanalysis of Nonelectroactive Amino Acids

In the past, various chiral isomers accompanied by electroactive units have been distinguished using electrochemical techniques, which can produce electrochemical signals by themselves. However, it is still difficult to use an electrochemical technique to detect nonelectroactive samples. To address this bottleneck, an electroactive chiral polymer (S,S)-p-CVB-Fc that contains one redox-active ferrocene unit was designed and synthesized in this study. The electroactive polymer can give electrochemical signals as an alternative to the tested chiral samples, regardless of whether the isomers have electroactive units. Then, it was fixed on the surface of a glassy carbon electrode as an electrochemical chiral sensor. When nonelectroactive amino acids including proline, threonine, and alanine were examined by the sensor, clear discrimination in the response of peak current could be observed toward l- and d-isomers at pH 6.5. The peak current ratios (I_L/I_D) for proline and alanine were 1.47 and 1.48, respectively. In contrast, for threonine, the d-isomer exhibited a higher peak current than the l -isomer with a ratio of 2.59. In summary, the results ensure that the current work can enlarge the testing scope of chiral samples in the field of chiral electroanalysis using an electroactive sensor.

Oxidation-Induced "One-Pot" Click Chemistry

Click chemistry has been established rapidly as one of the most valuable methods for the chemical transformation of complex molecules. Due to the rapid rates, clean conversions to the products, and compatibility of the reagents and reaction conditions even in complex settings, it has found applications in many molecule-oriented disciplines. From the vast landscape of click reactions, approaches have emerged in the past decade centered around oxidative processes to generate in situ highly reactive synthons from dormant functionalities. These approaches have led to some of the fastest click reactions know to date. Here, we review the various methods that can be used for such oxidation-induced "one-pot" click chemistry for the transformation of small molecules, materials, and biomolecules. A comprehensive overview is provided of oxidation conditions that induce a click reaction, and oxidation conditions are orthogonal to other click reactions so that sequential "click-oxidation-click" derivatization of molecules can be performed in one pot. Our review of the relevant literature shows that this strategy is emerging as a powerful approach for the preparation of high-performance materials and the generation of complex biomolecules. As such, we expect that oxidation-induced "one-pot" click chemistry will widen in scope substantially in the forthcoming years.

Bioorthogonal Reactions Utilizing Nitrones as Versatile Dipoles in Cycloaddition Reactions

Bioorthogonal chemical reactions have emerged as convenient and rapid methods for incorporating unnatural functionality into living systems. Different prototype reactions have been optimized for use in biological settings. Optimization of 3 + 2 dipolar cycloadditions involving nitrones has resulted in highly efficient reaction conditions for bioorthogonal chemistry. Through substitution at the nitrone carbon or nitrogen atom, stereoelectronic tuning of the reactivity of the dipole has assisted in optimizing reactivity. Nitrones have been shown to react rapidly with cyclooctynes with bimolecular rate constants approaching k₂ = 10² M^-1 s^-1, which are among the fastest bioorthogonal reactions reported (McKay et al. Org. Biomol. Chem. 2012, 10, 3066-3070). Nitrones have also been shown to react with trans-cyclooctenes (TCO) in strain-promoted TCO-nitrone cycloadditions reactions. Copper catalyzed reactions involving alkynes and nitrones have also been optimized for applications in biology. This review provides a comprehensive accounting of the different bioorthogonal reactions that have been developed using nitrones as versatile reactants, and provides some recent examples of applications for probing biological systems.

A Survey of Strain-Promoted Azide-Alkyne Cycloaddition in Polymer Chemistry

Highly efficient reactions that enable the assembly of molecules into complex structures have driven extensive progress in synthetic chemistry. In particular, reactions that occur under mild conditions and in benign solvents, while producing no by-products and rapidly reach completion are attracting significant attention. Amongst these, the strain-promoted azide-alkyne cycloaddition, involving various cyclooctyne derivatives reacting with azide-bearing molecules, has gained extensive popularity in organic synthesis and bioorthogonal chemistry. This reaction has also recently gained momentum in polymer chemistry, where it has been used to decorate, link, crosslink, and even prepare polymer chains. This survey highlights key achievements in the use of this reaction to produce a variety of polymeric constructs for disparate applications.

Strained alkyne polymers capable of SPAAC via ring-opening metathesis polymerization

We present a strategy that combines the attractive traits of chain-growth polymerization and strain-promoted azide–alkyne cycloaddition chemistry for the production of functional polymers.

Strain-Promoted Oxidation of Methylenecyclopropane Derivatives using N-Hydroxyphthalimide and Molecular Oxygen in the Dark

The hydroperoxidation of alkylidenecyclopropanes and other strained alkenes using an N-hydroxylamine and molecular oxygen occurred in the absence of catalyst, initiator, or light. The oxidation reaction proceeds through a radical pathway that is initiated by autoxidation of the alkene substrate. The hydroperoxides were converted to their corresponding alcohols and ketones under mild conditions.

Nitrone-Modified Gold Nanoparticles: Synthesis, Characterization, and Their Potential as 18F-Labeled Positron Emission Tomography Probes via I-SPANC

A novel bioorthogonal gold nanoparticle (AuNP) template displaying interfacial nitrone functional groups for bioorthogonal interfacial strain-promoted alkyne-nitrone cycloaddition reactions has been synthesized. These nitrone-AuNPs were characterized in detail using 1H nuclear magnetic resonance spectroscopy, transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy, and a nanoparticle raw formula was calculated. The ability to control the conjugation of molecules of interest at the molecular level onto the nitrone-AuNP template allowed us to create a novel methodology for the synthesis of AuNP-based radiolabeled probes.

The Platin-X series: activation, targeting, and delivery

Anticancer platinum (Pt) complexes have long been considered to be one of the biggest success stories in the history of medicinal inorganic chemistry. Yet there remains the hunt for the "magic bullet" which can satisfy the requirements of an effective chemotherapeutic drug formulation. Pt(iv) complexes are kinetically more inert than the Pt(ii) congeners and offer the opportunity to append additional functional groups/ligands for prodrug activation, tumor targeting, or drug delivery. The ultimate aim of functionalization is to enhance the tumor selective action and attenuate systemic toxicity of the drugs. Moreover, an increase in cellular accumulation to surmount the resistance of the tumor against the drugs is also of paramount importance in drug development and discovery. In this review, we will address the attempts made in our lab to develop Pt(iv) prodrugs that can be activated and delivered using targeted nanotechnology-based delivery platforms.

Post-synthetic functionalization of a polysulfone scaffold with hydrazone-linked functionality

The synthesis, characterization, and post-synthetic functionalization of a readily functionalized step-growth linear polymer derived from divinyl sulfone (DVS) and tert-butylcarbazate (TBC) is presented.

Iodosylbenzene-Pseudohalide-Based Initiators for Radical Polymerization

Iodosylbenzene reacts with various (pseudo)halides (trimethylsilyl azide or isocyanate or potassium azide, cyanate, and bromide) to yield unstable hypervalent iodine(III) compounds, PhIX₂ (X = (pseudo)halide), that undergo rapid homolysis of the hypervalent I-X bonds and generate (pseudo)halide radicals, which can initiate the polymerization of styrene, (meth)acrylates, and vinyl esters. Polymers are formed containing (pseudo)halide functionalities at the α-chain end but, depending on the termination mechanism and the occurrence of transfer of (pseudo)halide groups from the initiator to the propagating radicals, also at the ω-chain end. With slowly polymerizing monomers (styrene and methyl methacrylate) and initiators that were generated rapidly at high concentrations and were especially unstable, the reactions proceeded via a "dead-end" polymerization mechanism, and only low to moderate monomer conversions were attained. When the initiator was generated more slowly and continuously throughout the polymerization (using the combination of iodosylbenzene with the poorly soluble potassium (pseudo)halide salts), typically higher conversions and higher molecular weights were reached. The presence of (pseudo)halide functionalities in the polymers was proved by elemental analysis, IR, and NMR spectroscopy. The azide-containing polymers underwent click-type coupling reactions with dialkynes, while the (iso)cyanate-containing polymers reacted with diamines to afford high-molecular-weight polymers with triazole- and urea-type interchain links, respectively.

Synthetic Applications of Flexible SNO-OCT Strained Alkynes and Their Use in Postpolymerization Modifications

SNO-OCTs are eight-membered heterocyclic alkynes that have fast rates of reactivity with 1,3-dipoles. In contrast to many other reported cycloalkynes, SNO-OCTs contain multiple sites for derivatization, display stability under a variety of common reaction conditions, and offer the opportunity for strain-induced ring-opening following the initial reaction of the alkyne moiety. In this paper, we describe how the unique features of SNO-OCTs can be employed to modify an oxime-bearing styrene copolymer and introduce an array of polar functionalities into the polymer. This can be achieved through both the addition of SNO-OCT to the polymer, as well as in the subsequent opening of the sulfamate ring once it has been installed in the polymer.

Catalyst-free site-specific surface modifications of nanocrystalline diamond films via microchannel cantilever spotting

Microchannel cantilever spotting is combined with a copper-free click chemistry ligation to achieve the patterning of nanocrystalline diamond films.

The emergence of oxime click chemistry and its utility in polymer science

The synthesis of new, highly functional and dynamic polymeric materials has risen dramatically since the introduction of click chemistry in 2001.

The Missing C1-C5 Cycloaromatization Reaction: Triplet State Antiaromaticity Relief and Self-Terminating Photorelease of Formaldehyde for Synthesis of Fulvenes from Enynes

The last missing example of the four archetypical cycloaromatizations of enediynes and enynes was discovered by combining a twisted alkene excited state with a new self-terminating path for intramolecular conversion of diradicals into closed-shell products. Photoexcitation of aromatic enynes to a twisted alkene triplet state creates a unique stereoelectronic situation, which is facilitated by the relief of excited state antiaromaticity of the benzene ring. This enables the usually unfavorable 5-endo-trig cyclization and merges it with 5-exo-dig closure. The 1,4-diradical product of the C1-C5 cyclization undergoes internal H atom transfer that is coupled with the fragmentation of an exocyclic C-C bond. This sequence provides efficient access to benzofulvenes from enynes and expands the utility of self-terminating aromatizing enyne cascades to photochemical reactions. The key feature of this self-terminating reaction is that, despite the involvement of radical species in the key cyclization step, no external radical sources or quenchers are needed to provide the products. In these cascades, both radical centers are formed transiently and converted to the closed-shell products via intramolecular H-transfer and C-C bond fragmentation. Furthermore, incorporating C-C bond cleavage into the photochemical self-terminating cyclizations of enynes opens a new way for the use of alkenes as alkyne equivalents in organic synthesis.

Fluorogenic Strain-Promoted Alkyne-Diazo Cycloadditions

Fluorogenic reactions, in which non- or weakly fluorescent reagents produce highly fluorescent products, are attractive for detecting a broad range of compounds in the fields of bioconjugation and material sciences. Herein, we report that a dibenzocyclooctyne derivative modified with a cyclopropenone moiety (Fl-DIBO) can undergo fast strain-promoted cycloaddition reactions under catalyst-free conditions with azides, nitrones, nitrile oxides, as well as mono- and disubstituted diazo-derivatives. Although the reaction with nitrile oxides, nitrones, and disubstituted diazo compounds gave cycloadducts with low quantum yield, monosubstituted diazo reagents produced 1H-pyrazole derivatives that exhibited an approximately 160-fold fluorescence enhancement over Fl-DIBO combined with a greater than 10,000-fold increase in brightness. Concluding from quantum chemical calculations, fluorescence quenching of 3H-pyrazoles, which are formed by reaction with disubstituted diazo-derivatives, is likely due to the presence of energetically low-lying (n,π*) states. The fluorogenic probe Fl-DIBO was successfully employed for the labeling of diazo-tagged proteins without detectable background signal. Diazo-derivatives are emerging as attractive reporters for the labeling of biomolecules, and the studies presented herein demonstrate that Fl-DIBO can be employed for visualizing such biomolecules without the need for probe washout.

Branched Polyhedral Oligomeric Silsesquioxane Nanoparticles Prepared via Strain-Promoted 1,3-Dipolar Cycloadditions

Conjugation of small organic molecules and polymers to polyhedral oligosilsesquioxane (POSS) cores results in novel hybrid materials with unique physical characteristics. We report here an approach in which star-shaped organic-inorganic scaffolds bearing eight cyclooctyne moieties can be rapidly functionalized via strain-promoted azide-alkyne cycloaddition (SPAAC) to synthesize a series of nearly monodisperse branched core-shell nanoparticles with hydrophobic POSS cores and hydrophilic arms. We established that SPAAC is a robust method for POSS core octafunctionalization with the reaction rate constant of 1.9 × 10(-2) M(-1) s(-1). Functionalization with poly(ethylene glycol) (PEG) azide, fluorescein azide, and unprotected lactose azide gave conjugates which represent different classes of compounds: polymer conjugates, fluorescent dots, and bioconjugates. These resulting hybrid compounds were preliminarily tested for their ability to self-assemble in solution and at the air-water interface. We observed the formation of robust smooth Langmuir monolayers with diverse morphologies. We found that polar lactose moieties are completely submerged into the subphase whereas the relatively hydrophobic fluorescein arms had extended conformation at the interface, and PEG arms were partially submerged. Finally, we observed the formation of stable micelles with sizes between 70 and 160 nm in aqueous solutions with size and morphology of the structures dependent on the molecular weight and the type of the peripheral hydrophilic moieties.

Hydrogen-Bonded Multifunctional Supramolecular Copolymers in Water

We have investigated the self-assembly in water of molecules having a single hydrophobic bis-urea domain linked to different hydrophilic functional side chains, i.e., bioactive peptidic residues and fluorescent cyanine dyes. By using a combination of spectroscopy, scattering, and microscopy techniques, we show that each one of these molecules can individually produce well-defined nanostructures such as twisted ribbons, two-dimensional plates, or branched fibers. Interestingly, when these monomers of different functionalities are mixed in an equimolar ratio, supramolecular copolymers are preferred to narcissistic segregation. Radiation scattering and imaging techniques demonstrate that one of the molecular units dictates the formation of a preferential nanostructure, and optical spectroscopies reveal the alternated nature of the copolymerization process. This work illustrates how social self-sorting in H-bond supramolecular polymers can give straightforward access to multifunctional supramolecular copolymers.

Kinugasa reactions in water: from green chemistry to bioorthogonal labelling

The Kinugasa reaction has become an efficient method for the direct synthesis of β-lactams from substituted nitrones and copper(I) acetylides. In recent years, the reaction scope has been expanded to include the use of water as the solvent, and with micelle-promoted [3+2] cycloadditions followed by rearrangement furnishing high yields of β-lactams. The high yields of stable products under aqueous conditions render the modified Kinugasa reaction amenable to metabolic labelling and bioorthogonal applications. Herein, the development of methods for use of the Kinugasa reaction in aqueous media is reviewed, with emphasis on its potential use as a bioorthogonal coupling strategy.

The utilization of post-synthetic modification in opto-electronic polymers: an effective complementary approach but not a competitive one to the traditional direct polymerization process

By presenting some typical examples, the recent progress of opto-electronic polymers is reviewed, which were only accessible from the post-synthetic modification strategy.

Exploring strain-promoted 1,3-dipolar cycloadditions of end functionalized polymers

Strain-promoted 1,3-dipolar cycloaddition of cyclooctynes with 1,3-dipoles such as azides, nitrones, and nitrile oxides, are of interest for the functionalization of polymers. In this study, we have explored the use of a 4-dibenzocyclooctynol (DIBO)-containing chain transfer agent in reversible addition-fragmentation chain transfer polymerizations. The controlled radical polymerization resulted in well-defined DIBO-terminating polymers that could be modified by 1,3-dipolar cycloadditions using nitrones, nitrile oxides, and azides having a hydrophilic moiety. The self-assembly properties of the resulting block copolymers have been examined. The versatility of the methodology was further demonstrated by the controlled preparation of gold nanoparticles coated with the DIBO-containing polymers to produce materials that can be further modified by strain-promoted cycloadditions.

Copper-free click-chemistry platform to functionalize cisplatin prodrugs

The ability to rationally design and construct a platform technology to develop new platinum(IV) [Pt(IV)] prodrugs with functionalities for installation of targeting moieties, delivery systems, fluorescent reporters from a single precursor with the ability to release biologically active cisplatin by using well-defined chemistry is critical for discovering new platinum-based therapeutics. With limited numbers of possibilities considering the sensitivity of Pt(IV) centers, we used a strain-promoted azide-alkyne cycloaddition approach to provide a platform, in which new functionalities can easily be installed on cisplatin prodrugs from a single Pt(IV) precursor. The ability of this platform to be incorporated in nanodelivery vehicle and conjugation to fluorescent reporters were also investigated.