View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by MURAL - Maynooth University Research Archive Library MICROREVIEW DOI: 10.1002/ejoc.201101823 Nitrile Oxide/Alkyne Cycloadditions–ACredible Platform for Synthesis of Bioinspired Molecules by Metal-Free Molecular Clicking Frances Heaney*[a] Keywords: Bioorganic chemistry / Click chemistry / 1,3-Dipolar cycloaddition / Strain-promoted cycloaddition / Nucleic acids / Polymers / Alkynes The need for precise and flexible synthetic methodology to veloped beyond the original triazole-forming trick and azides underpin modern research in chemical biology and materials are no longer the only dipoles pursued as click cycloaddition science has fuelled a resurgence of interest in Huisgen 1,3- partners. This article reviews some of the complications of dipolar cycloaddition chemistry. Of late, the in vogue chemis- the CuAAC reaction and evaluates the potential of nitrile ox- try for the assembly of complex biological molecules and spe- ide/alkyne cycloaddition (NOAC) as a covalent conjugation cialist materials has been the copper-catalysed azide alkyne tool. With a focus on applications in nucleic acid chemistry cycloaddition (CuAAC) reaction. However, in certain circum- and materials science it presents the case for a prominent stances aversion to the copper catalyst flaws this approach position for nitrile oxides in the catalyst-free bioconjugation and alternatives have been sought. Click chemistry has de- toolbox. 1. Introduction materials science. Publications have increased exponentially, totalling about 5000 to date, including almost 200 review [1] Transformations classified as “click reactions” en- articles and a specialist monograph.[2] Entire issues of compass those allowing rapid and reliable creation of di- QSAR & Combinatorial Chemistry [2007, 26 (11–12)], Mac- verse chemical entities simply by joining molecular pieces romolecular Rapid Communications [2008, 29 (12–13)] and as easily as one might click a buckle. To be in this illustrious Chemical Society Reviews [2010, 39 (4)] have been dedicated club, reactions must be wide in scope, easily performed un- to click chemistry; most recently Accounts of Chemical Re- der mild conditions, insensitive to oxygen and water and search has published a themed issue on bioorthogonal proceed in a benign solvent or under solvent-free condi- chemistry [2011, 44 (9)]. tions. They must use easily accessible reagents and display orthogonality with other common reactants. The reactions ought to be spontaneous, with a thermodynamic driving 2. The Click Toolbox force greater than 20 kcalmol–1 and result in almost com- The click toolbox is ever expanding and practitioners can plete conversion of reactants to a single product without choose their tool to reflect a preference for selectivity or the need for extensive purification. sensitivity. Favoured reactions include nucleophilic opening During the last decade these privileged reactions have be- of spring-loaded electrophiles[3a] and nucleophilic addition come valued in industry and academia; applications trans- to carbonyl substrates yielding oximes and hydrazones,[3b] a verse biomedical research, drug design, biotechnology and variety of thiol chemistries[4] and cycloaddition reactions including the Diels–Alder reaction.[5] However, the wide- [a] Department of Chemistry, National University of Ireland, spread adoption of the copper(I)-catalysed azide/alkyne cy- Maynooth, Co. Kildare, Republic of Ireland Fax: +353-1-7083815 cloaddition – the so-called CuAAC reaction – has made it E-mail: [email protected] almost synonymous with the term “click chemistry”. Frances Heaney studied Chemistry and Biochemistry at Queen’s University, Belfast. After graduating with an Hons BSc degree she remained in Belfast. In 1990 she attained a PhD in chemistry under the supervision of Professor Ron Grigg. She spent two years as a post-doctoral researcher in Trinity College, Dublin, before being appointed to the staff at the National University of Ireland Galway. She subsequently moved to the National University of Ireland Maynooth, where she is now senior lecturer. Eur. J. Org. Chem. 2012, 3043–3058 © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3043 MICROREVIEW F. Heaney 3. The Gold Standard CuAAC Reaction tions free of cytotoxicity; practical solutions include con- ducting the reaction under air-free conditions or with CuI- [1,6] In its modern genesis, the triazole-forming CuAAC stabilizing ligands (e.g., TBTA, BTTES or BTTAA).[28] The [7] reaction, discovered at the end of the 19th century and last two, tris(triazolylmethyl)amines, are especially attract- [8] studied extensively by Huisgen in the 1960s, has been ut- ive; they confer improved kinetics and bio-benign status on terly transformed. It is now known that the poor kinetics the CuAAC reaction. Finally, “ligandless” click ligation or and selectivity of the classic thermal reaction can be over- labelling of unprotected RNA substrates has been facili- come by the addition of a catalytic amount of copper. The tated by judicious choice of reaction solvent or alkyne part- CuAAC reaction is the most championed click reaction, ner.[29] with applications in supramolecular[9] materials[10] and polymer chemistry.[11] It has also been central to the synthe- sis of drug candidates[12] and nucleoside analogues;[13] other 3.3. Ruthenium as a Replacement for the Copper Catalyst bioinspired applications include protein engineering,[2] bio- logical imaging[14] and biomimetic modelling.[15] Ruthenium-catalysed azide/alkyne cycloadditions (RuAACs), with regiochemical complementarity to the Cu- promoted version have been developed[27b,30] (Scheme 1). II 3.1. Drawbacks to Copper-Catalysed Azide/Alkyne Several Ru complexes are catalytically active, and whilst Cycloadditions Cp*RuCl(PPh3)2 is the most widely adopted the nature of the reacting azide informs the choice of catalyst.[31] In some Despite its status as the quintessential click reaction, the instances sluggish reactivity can be overcome by microwave CuAAC is not ubiquitously attractive.[16] It requires op- irradiation.[31b] The reaction is compatible with both ter- timisation of a number of reagents and the need for the minal and internal alkynes, although with the latter there Cu(1) catalyst can be limiting. Toxicity is problematic for can be some compromise in regioselectivity.[30b,30c] Applica- reactivity in living systems, in tissue engineering applica- tions have been reported in materials[32] and medicinal tions and in development of therapeutics.[17] The cytotoxic chemistry,[33] as well as in the preparation of nucleoside and copper ions[18] can cause oxidative damage including DNA sugar derivatives, peptide surrogates and aminoacyl-tRNA degradation and polysaccharide or protein denaturation.[19] analogues.[34] Catalytic copper ions are incompatible with some synthetic targets, such as those incorporating chelating or Fischer carbene moieties.[20] In other cases, product entrapment of the catalyst ions has necessitated the use of stoichiometric amounts of copper and has resulted in polymeric materials with inferior properties to those formed by classical thermal polymerisation methods.[21] Trace copper is especially prob- lematic in the pharmaceutical sector, where levels must re- main below 15 ppm.[22] Catalyst impurities impact nega- tively on enzyme-linked immunosorbent assays (ELIZA), Scheme 1. Catalyst-controlled regioselective cycloadditions of az- [30b] on the biochemical properties of labelled phages and the ides and terminal alkynes: i) Cp*RuCl(PPh3)2, PhH, reflux; luminescence characteristics of quantum-dot-functionalised ii) sodium ascorbate, CuSO4, tBuOH, H2O, room temp. biomolecules.[23] 3.4. Alkynes with Enhanced Reactivities – SPAAC 3.2. General Solutions to the Problems of Residual Copper The poor kinetics of the CuAAC reaction at the concen- A palette of tools designed to circumvent problems asso- trations desirable for bioconjugation have been addressed ciated with the presence of residual copper is emerging. In by the development of strain-promoted azide/alkyne cyclo- limited circumstances the reactions might proceed satisfac- addition (SPAAC) chemistry. The significant reduction in torily in the absence of copper (e.g. with “activated” sub- the activation barrier makes the requirement for a catalyst strates[24]) or with the application of specialised experimen- redundant and, depending on the application, SPAAC reac- tal techniques (e.g. microcontact printing[25]). However, tions can compete with the best CuAAC protocols.[23b] Nu- more general solutions are desirable. One approach involves merous cyclooctyne probes have been developed (Figure 1). extensive washing to eliminate residual copper ions;[26] an- Incremental improvements on first-generation cyclooc- other relies on catalyst remediation (e.g., sequestration by tynes[23b,35] have yielded azacyclooctynes with enhanced oxide-capped metallic iron nanoparticles, FexOy@Fe).[22] aqueous solubilities, as well as α-fluorinated and α,α-difluo- Alternatively, judicious choice of copper source can facili- rinated versions with improved kinetics.[20a,36] Diarylcy- tate removal by simple filtration [e.g., CuI-zeolites, bench- clooctynes, generated by classical or photochemical ap- [37] stable copper-in-charcoal or copper nitride (Cu3N) nano- proaches, their aza analogues [azadibenzocyclooctynes particles supported on a superparamagnetic silica micro- (ADIBOs)],[38] including some carrying fluorescent