1,3-Dipolar cycloaddition

Pericyclic chemical reaction January 2020 January 2018 Huisgen 1,3-dipolar cycloaddition Ring forming reaction Rolf Huisgen huisgen-1,3-dipolar-cycloaddition 0000018 The 1,3-dipolar cycloaddition is a chemical reaction between a 1,3-dipole and a dipolarophile to form a five-membered ring. The earliest 1,3-dipolar cycloadditions were described in the late 19th century to the early 20th century, following the discovery of 1,3-dipoles. Mechanistic investigation and synthetic application were established in the 1960s, primarily through the work of Rolf Huisgen.Bertrand Guy Wentrup Curt Nitrile Imines: From Matrix Characterization to Stable Compounds Angewandte Chemie International Edition in English 17 March 1994 33 5 527–545 10.1002/anie.199405271Huisgen Rolf 1,3-Dipolar Cycloadditions. Past and Future Angewandte Chemie International Edition in English October 1963 2 10 565–598 10.1002/anie.196305651 Hence, the reaction is sometimes referred to as the Huisgen cycloaddition (this term is often used to specifically describe the 1,3-dipolar cycloaddition between an organic azide and an alkyne to generate 1,2,3-triazole). 1,3-dipolar cycloaddition is an important route to the regio- and stereoselective synthesis of five-membered heterocycles and their ring-opened acyclic derivatives. The dipolarophile is typically an alkene or alkyne, but can be other pi systems. When the dipolarophile is an alkyne, aromatic rings are generally produced. center|x100px Mechanistic overview Originally two proposed mechanisms describe the 1,3-dipolar cycloaddition: first, the concerted pericyclic cycloaddition mechanism, proposed by Rolf Huisgen;Kinetics and Mechanism of 1,3-Dipolar Cycloadditions Rolf Huisgen Angewandte Chemie International Edition 2 11 November 1963 633–645 10.1002/anie.196306331 and second, the stepwise mechanism involving a diradical intermediate, proposed by Firestone.Mechanism of 1,3-dipolar cycloadditions R Firestone Journal of Organic Chemistry 33 6 1968 2285–2290 10.1021/jo01270a023 After much debate, the former proposal is now generally accepted1,3-Dipolar cycloadditions. 76. Concerted nature of 1,3-dipolar cycloadditions and the question of diradical intermediates Rolf Huisgen Journal of Organic Chemistry 41 3 1976 403–419 10.1021/jo00865a001—the 1,3-dipole reacts with the dipolarophile in a concerted, often asynchronous, and symmetry-allowed π4s + π2s fashion through a thermal six-electron Huckel aromatic transition state. However, a few examples exist of a stepwise mechanism for the catalyst-free 1,3-dipolar cycloaddition reactions of thiocarbonyl ylides,First Two-Step 1,2-Dipolar Cycloadditons: Nonstereospecificity G. Mloston E. Langhals Rolf Huisgen J. Am. Chem. Soc. 108 20 1986 6401–66402 10.1021/ja00280a053 1986JAChS.108.6401H and nitrile oxidesAn example of a stepwise mechanism for the catalyst-free 1,3-dipolar cycloaddition between a nitrile oxide and an electron rich alkene Siadati Seyyed Amir Tetrahedron Letters 56 34 2015 4857–4863 10.1016/j.tetlet.2015.06.048 center|x100px|The generic mechanism of a 1,3-dipolar cycloaddition between a dipole and a dipolarophile to give a five-membered heterocycle, through a six-electron transition state. Note that the red curly arrows are conventionally used to denote the reaction process but do not necessarily represent the actual flow of electrons. Pericyclic mechanism Huisgen investigated a series of cycloadditions between the 1,3-dipolar diazo compounds and various dipolarophilic alkenes. The following observations support the concerted pericyclic mechanism, and refute the stepwise diradical or the stepwise polar pathway. Substituent effects: Different substituents on the dipole do not exhibit a large effect on the cycloaddition rate, suggesting that the reaction does not involve a charge-separated intermediate. Solvent effects: Solvent polarity has little effect on the cycloaddition rate, in line with the pericyclic mechanism where polarity does not change much in going from the reactants to the transition state. Stereochemistry: 1,3-dipolar cycloadditions are always stereospecific with respect to the dipolarophile (i.e., cis-alkenes giving syn-products), supporting the concerted pericyclic mechanism in which two sigma bonds are formed simultaneously. Thermodynamic parameters: 1,3-dipolar cycloadditions have an unusually large negative entropy of activation similar to that of the Diels-Alder reaction, suggesting that the transition state is highly ordered, which is a signature of concerted pericyclic reactions. 1,3-Dipole thumb|Structure and nomenclature of all second-row 1,3-dipoles consisting of carbon, nitrogen and oxygen centers. The dipoles are categorized as allyl-type or propargyl/allenyl-type based on the geometry of the central atom. A 1,3-dipole is an organic molecule that can be represented as either an allyl-type or a propargyl/allenyl-type zwitterionic octet/sextet structures. Both types of 1,3-dipoles share four electrons in the π-system over three atoms. The allyl-type is bent whereas the propargyl/allenyl-type is linear in geometry.1,3-Dipolar Cycloadditions. Past and Future Rolf Huisgen Angewandte Chemie International Edition 2 10 1963 565–598 10.1002/anie.196305651 1,3-Dipoles containing higher-row elements such as sulfur or phosphorus are also known, but are utilized less routinely. Resonance structures can be drawn to delocalize both negative and positive charges onto any terminus of a 1,3-dipole (see the scheme below). A more accurate method to describe the electronic distribution on a 1,3-dipole is to assign the major resonance contributor based on experimental or theoretical data, such as dipole moment measurementsMicrowave Spectra of Diazomethane and its Deutero Derivatives A Cox L Thomas J Sheridan Nature 181 1958 1000–1001 4614 10.1038/1811000a0 1958Natur.181.1000C 4245746 or computations.Expansion of molecular orbital wave functions into valence bond wave functions. A simplified procedure. P Hilberty C Leforestier Journal of the American Chemical Society 100 7 1978 2012–2017 10.1021/ja00475a007 1978JAChS.100.2012H For example, diazomethane bears the largest negative character at the terminal nitrogen atom, while hydrazoic acid bears the largest negative character at the internal nitrogen atom. center|300px|Calculated major resonance structures of diazomethane and hydrazoic acid (doi = 10.1021/ja00475a007) Consequently, this ambivalence means that the ends of a 1,3-dipole can be treated as both nucleophilic and electrophilic at the same time. The extent of nucleophilicity and electrophilicity at each end can be evaluated using the frontier molecular orbitals, which can be obtained computationally. In general, the atom that carries the largest orbital coefficient in the HOMO acts as the nucleophile, whereas that in the LUMO acts as the electrophile. The most nucleophilic atom is usually, but not always, the most electron-rich atom.J.F. McGarrity Saul Patai Diazonium and Diazo Groups Basicity, acidity and hydrogen bonding 1 1978 10.1002/9780470771549.ch6 179–230 9780470771549 Direct observation of the methyldiazonium ion in fluorosulfuric acid Daniel Berner John McGarrity Journal of the American Chemical Society 101 11 1979 3135–3136 10.1021/ja00505a059 1979JAChS.101.3135B Untersuchungen an Diazomethanen, VI. Mitteil.: Umsetzung von Diazoäthan mit Methyllithium Eugen Muller Wolfgans Rundel Chemische Berichte 89 4 1956 1065–1071 10.1002/cber.19560890436 In 1,3-dipolar cycloadditions, identity of the dipole-dipolarophile pair determines whether the HOMO or the LUMO character of the 1,3-dipole will dominate (see discussion on frontier molecular orbitals below). Dipolarophile The most commonly used dipolarophiles are alkenes and alkynes. Heteroatom-containing dipolarophiles such as carbonyls and imines can also undergo 1,3-dipolar cycloaddition. Other examples of dipolarophiles include fullerenes and nanotubes, which can undergo 1,3-dipolar cycloaddition with azomethine ylide in the Prato reaction. Solvent effects 1,3-Dipolar cycloadditions experience very little solvent effect because both the reactants and the transition states are generally non-polar. For example, the rate of reaction between phenyl diazomethane and ethyl acrylate or norbornene (see scheme below) changes only slightly upon varying solvents from cyclohexane to methanol.Solvent Dependence of Cycloaddition Rates of Phenyldiazomethane and Activation Parameters Jochen Geittner Rolph Huisgen Hans-Ulrich Reissig Heterocycles 11 1978 109–120 10.3987/S(N)-1978-01-0109 free center|450px|Effect of solvent polarity on 1,3-dipolar cycloaddition reactions(doi:10.3987/S(N)-1978-01-0109.) Lack of solvent effects in 1,3-dipolar cycloaddition is clearly demonstrated in the reaction between enamines and dimethyl diazomalonate (see scheme below).α-Diazocarbonyl compounds and enamines - a dichotomy of reaction paths Rolph Huisgen Hans-Ulrich Reissig Helmut Huber Sabine Voss Tetrahedron Letters 20 32 1979 2987–2990 10.1016/S0040-4039(00)70991-9 The polar reaction, N-cyclopentenyl pyrrolidine nucleophilic addition to the diazo compound, proceeds 1,500 times faster in polar DMSO than in non-polar decalin. On the other hand, a close analog of this reaction, N-cyclohexenyl pyrrolidine 1,3-dipolar cycloaddition to dimethyl diazomalonate, is sped up only 41-fold in DMSO relative to decalin. center|750px|Rate of polar nucleophilic addition reaction versus 1,3-dipolar cycloaddition in decalin and in DMSO (doi:10.1016/S0040-4039(00)70991-9) Frontier molecular orbital theory center|400x1000px|Orbital overlaps in types I, II and III 1,3-dipolar cycloaddition. 1,3-Dipolar cycloadditions are pericyclic reactions, which obey the Dewar-Zimmerman rules and the Woodward–Hoffmann rules. In the Dewar-Zimmerman treatment, the reaction proceeds through a 5-center, zero-node, 6-electron Huckel transition state for this particular molecular orbital diagram. However, each orbital can be randomly assigned a sign to arrive at the same result. In the Woodward–Hoffmann treatment, frontier molecular orbitals (FMO) of the 1,3-dipole and the dipolarophile overlap in the symmetry-allowed π4s + π2s manner. Such orbital overlap can be achieved in three ways: type I, II and III. Orbital energy control of cycloaddition reactivity R Sustmann Pure and Applied Chemistry 40 4 1974 569–593 10.1351/pac197440040569 free The dominant pathway is the one which possesses the smallest HOMO–LUMO gap. Type I The dipole has a high-lying HOMO which overlaps with LUMO of the dipolarophile. A dipole of this class is referred to as a HOMO-controlled dipole or a nucleophilic dipole, which includes azomethine ylide, carbonyl ylide, nitrile ylide, azomethine imine, carbonyl imine and diazoalkane. These dipoles add to electrophilic alkenes readily. Electron-withdrawing groups (EWG) on the dipolarophile would accelerate the reaction by lowering the LUMO, while electron-donating groups (EDG) would decelerate the reaction by raising the HOMO. For example, the reactivity scale of diazomethane against a series of dipolarophiles is shown in the scheme below. Diazomethane reacts with the electron-poor ethyl acrylate more than a million times faster than the electron rich butyl vinyl ether. Kinetics of 1,3-dipolar cycloaddition reactions of diazomethane; A correlation with homo-lumo energies Jochen Geittner Rolf Huisgen Tetrahedron Letters 18 10 1977 881–884 10.1016/S0040-4039(01)92781-9 This type resembles the normal-electron-demand Diels-Alder reaction, in which the diene HOMO combines with the dienophile LUMO. center|450px|doi:10.1016/S0040-4039(01)92781-9 Type II HOMO of the dipole can pair with LUMO of the dipolarophile; alternatively, HOMO of the dipolarophile can pair with LUMO of the dipole. This two-way interaction arises because the energy gap in either direction is similar. A dipole of this class is referred to as a HOMO-LUMO-controlled dipole or an ambiphilic dipole, which includes nitrile imide, nitrone, carbonyl oxide, nitrile oxide, and azide. Any substituent on the dipolarophile would accelerate the reaction by lowering the energy gap between the two interacting orbitals; i.e., an EWG would lower the LUMO while an EDG would raise the HOMO. For example, azides react with various electron-rich and electron-poor dipolarophile with similar reactivities (see reactivity scale below). K1.3-Dipolare Cycloadditionen, XXXII. Kinetik der Additionen organischer Azide an CC-Mehrfachbindungen Rolf Huisgen Gunter Szeimies Leander Mobius Chemische Berichte 100 8 1967 2494–2507 10.1002/cber.19671000806 center|550px|doi:10.1021/ja01016a011 Type III The dipole has a low-lying LUMO which overlaps with HOMO of the dipolarophile (indicated by red dashed lines in the diagram). A dipole of this class is referred to as a LUMO-controlled dipole or an electrophilic dipole, which includes nitrous oxide and ozone. EWGs on the dipolarophile decelerate the reaction, while EDGs accelerate the reaction. For example, ozone reacts with the electron-rich 2-methylpropene about 100,000 times faster than the electron-poor tetrachloroethene (see reactivity scale below). Rates of ozone-olefin reactions in carbon tetrachloride solutions D. G. Williamson R. J. Cvetanovic Journal of the American Chemical Society 1968 3668–3672 10.1021/ja01016a011 90 14 1968JAChS..90.3668W This type resembles the inverse electron-demand Diels-Alder reaction, in which the diene LUMO combines with the dienophile HOMO. center|550px|doi:10.1021/ja01016a011 Reactivity Concerted processes such as the 1,3-cycloaddition require a highly ordered transition state (high negative entropy of activation) and only moderate enthalpy requirements. Using competition reaction experiments, relative rates of addition for different cycloaddition reactions have been found to offer general findings on factors in reactivity. Conjugation, especially with aromatic groups, increases the rate of reaction by stabilization of the transition state. During the transition, the two sigma bonds are being formed at different rates, which may generate partial charges in the transition state that can be stabilized by charge distribution into conjugated substituents. More polarizable dipolarophiles are more reactive because diffuse electron clouds are better suited to initiate the flow of electrons. Dipolarophiles with high angular strain are more reactive due to increased energy of the ground state. Increased steric hindrance in the transition state as a result of unhindered reactants dramatically lowers the reaction rate. Hetero-dipolarophiles add more slowly, if at all, compared to C,C-diapolarophiles due to a lower gain in sigma bond energy to offset the loss of a pi bond during the transition state. Isomerism of the dipolarophile affects reaction rate due to sterics. trans-isomers are more reactive (trans-stilbene will add diphenyl(nitrile imide) 27 times faster than cis-stilbene) because during the reaction, the 120° bond angle shrinks to 109°, bringing eclipsing cis-substituents towards each other for increased steric clash. 300px|See Huisgen reference 10.1002/anie.196306331.|center Stereospecificity 1,3-dipolar cycloadditions usually result in retention of configuration with respect to both the 1,3-dipole and the dipolarophile. Such high degree of stereospecificity is a strong support for the concerted over the stepwise reaction mechanisms. As mentioned before, many examples show that the reactions were stepwise, thus, presenting partial or no stereospecificity. With respect to dipolarophile cis-Substituents on the dipolarophilic alkene end up cis, and trans-substituents end up trans in the resulting five-membered cyclic compound (see scheme below). The Stereospecificity of Diazomethane Cycloadditions Werner Bihlmaier Jochen Geittner Rolf Huisgen Hans-Ulrich ReissigP Heterocycles 10 1978 147–152 http://www.heterocycles.jp/newlibrary/libraries/journal/10/1 10.3987/S-1978-01-0147 free 11 June 2019 3 December 2012 https://web.archive.org/web/20190611171654/http://www.heterocycles.jp/newlibrary/libraries/journal/10/1 dead center|600px|doi:10.3987/S-1978-01-0147 With respect to dipole Generally, the stereochemistry of the dipole is not of major concern because only few dipoles could form stereogenic centers, and resonance structures allow bond rotation which scrambles the stereochemistry. However, the study of azomethine ylides has verified that cycloaddition is also stereospecific with respect to the dipole component. Diastereopure azomethine ylides are generated by electrocyclic ring opening of aziridines, and then rapidly trapped with strong dipolarophiles before bond rotation can take place (see scheme below). Stereospecific Conversion of cis-trans Isomeric Aziridines to Open-Chain Azomethine Ylides Rolf Huisgen Wolfgang Scheer Helmut Huber Journal of the American Chemical Society 89 7 1967 1753–1755 10.1021/ja00983a052 1967JAChS..89.1753H Conrotatory ring opening of cyanostilbene oxides to carbonyl ylides Alexander Dahmen Helmut Hamberger Rolf Huisgen Volker Markowski Journal of the Chemical Society D: Chemical Communications 1971 1192–1194 10.1039/C29710001192 19 If weaker dipolarophiles are used, bonds in the dipole have the chance to rotate, resulting in impaired cycloaddition stereospecificity. These results altogether confirm that 1,3-dipolar cycloaddition is stereospecific, giving retention of both the 1,3-dipole and the dipolarophile. center|700px|doi:10.1021/ja00983a052 Diastereoselectivity When two or more stereocenters are generated during the reaction, diastereomeric transition states and products can be obtained. In the Diels-Alder cycloaddition, the endo diastereoselectivity due to secondary orbital interactions is usually observed. In 1,3-dipolar cycloadditions, however, two forces influence the diastereoselectivity: the attractive π-interaction (resembling secondary orbital interactions in the Diels-Alder cycloaddition) and the repulsive steric interaction. Unfortunately, these two forces often cancel each other, causing poor diastereoselection in 1,3-dipolar cycloaddition. Examples of substrate-controlled diastereoselective 1,3-dipolar cycloadditions are shown below. First is the reaction between benzonitrile N-benzylide and methyl acrylate. In the transition state, the phenyl and the methyl ester groups stack to give the cis-substitution as the exclusive final pyrroline product. This favorable π-interaction offsets the steric repulsion between the phenyl and the methyl ester groups. Photocycloaddition of arylazirenes with electron-deficient olefins Albert Padwa Joel Smolanoff Journal of the American Chemical Society 1971 548–550 10.1021/ja00731a056 93 2 1971JAChS..93..548P Second is the reaction between nitrone and dihydrofuran. The exo-selectivity is achieved to minimize steric repulsion. A Synthesis of dl-isoretronecanol Takashi Iwashita Takenori Kusumi Hiroshi Kakisawa Chemistry Letters 8 1979 1337–1340 https://www.jstage.jst.go.jp/article/cl1972/8/11/8_11_1337/_article 10.1246/cl.1979.1337 11 subscription 4 July 2018 17 December 2012 https://web.archive.org/web/20180704213036/https://www.jstage.jst.go.jp/article/cl1972/8/11/8_11_1337/_article dead Last is the intramolecular azomethine ylide reaction with alkene. The diastereoselectivity is controlled by the formation of a less strained cis-fused ring system. A short and stereospecific synthesis of (±)-α-lycorane Chia-Lin Wang William Ripka Pat Confalone Tetrahedron Letters 1984 4613–4616 10.1016/S0040-4039(01)91213-4 25 41 center|400px|doi:10.1021/ja00731a056 Directed 1,3-dipolar cycloaddition Trajectory of the cycloaddition can be controlled to achieve a diastereoselective reaction. For example, metals can chelate to the dipolarophile and the incoming dipole and direct the cycloaddition selectively on one face. The example below shows addition of nitrile oxide to an enantiomerically pure allyl alcohol in the presence of a magnesium ion. The most stable conformation of the alkene places the hydroxyl group above the plane of the alkene. The magnesium then chelates to the hydroxyl group and the oxygen atom of nitrile oxide. The cycloaddition thus comes from the top face selectively. Metal-Assisted Stereocontrol of 1,3-Dipolar Cycloaddition Reactions Shuji Kanemasa Synlett 2002 1371–1387 10.1055/s-2002-33506 2002 9 center|450px Such diastereodirection has been applied in the synthesis of epothilones. Stereoselective Syntheses of Epothilones A and B via Directed Nitrile Oxide Cycloaddition. Jeffrey Bode Erick Carreira Journal of the American Chemical Society 2011 3611–3612 10.1021/ja0155635 11472140 123 15 center|600px Regioselectivity For asymmetric dipole-dipolarophile pairs, two regioisomeric products are possible. Both electronic/stereoelectronic and steric factors contribute to the regioselectivity of 1,3-dipolar cycloadditions. Vsevolod V. Rostovtsev Luke G. Green Valery V. Fokin K. Barry Sharpless A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed Regioselective Ligation of Azides and Terminal Alkynes 2002 Angewandte Chemie International Edition 41 14 2596–22599 10.1002/1521-3773(20020715)41:14<2596::AID-ANIE2596>3.0.CO;2-4 12203546 Electronic/stereoelectronic effect The dominant electronic interaction is the combination between the largest HOMO and the largest LUMO. Therefore, regioselectivity is governed by the atoms that bear the largest orbital HOMO and LUMO coefficients. Geometries of nitrilium betaines. The clarification of apparently anomalous reactions of 1,3-dipoles Pierluigi Caramella K.N. Houk Journal of the American Chemical Society 1976 6397–6399 10.1021/ja00436a062 98 20 1976JAChS..98.6397C A derivation of the shapes and energies of the molecular orbitals of 1,3-dipoles. Geometry optimizations of these species by MINDO/2 and MINDO/3 Pierluigi Caramella Ruth W. Gandour Janet A. Hall Cynthia G. Deville K. N. Houk Journal of the American Chemical Society 1977 385–392 10.1021/ja00444a013 99 2 1977JAChS..99..385C For example, consider the cycloaddition of diazomethane to three dipolarophiles: methyl acrylate, styrene or methyl cinnamate. The carbon of diazomethane bears the largest HOMO, while the end olefinic carbons of methyl acrylate and styrene bear the largest LUMO. Hence, cycloaddition gives the substitution at the C-3 position regioselectively. For methyl cinnamate, the two substituents (Ph v.s. COOMe) compete at withdrawing electrons from the alkene. The carboxyl is the better electron-withdrawing group, causing the β-carbon to be most electrophilic. Thus, cycloaddition yields the carboxyl group on C-3 and the phenyl group on C-4 regioselectively. center|400px|doi:10.1021/ja00444a013 and doi:10.1021/ja00436a062 Steric effect Steric effects can either cooperate or compete with the aforementioned electronic effects. Sometimes steric effects completely outweighs the electronic preference, giving the opposite regioisomer exclusively. Kinetics and Mechanism of 1,3-Dipolar Cycloadditions Rolf Huisgen Angewandte Chemie International Edition 2 11 November 1963 633–645 10.1002/anie.196306331 For example, diazomethane generally adds to methyl acrylate to give 3-carboxyl pyrazoline. However, by putting more steric demands into the system, we start to observe the isomeric 4-carboxyl pyrazolines. The ratio of these two regioisomers depends on the steric demands. At the extreme, increasing the size from hydrogen to t-butyl shifts the regioselectivity from 100% 3-carboxyl to 100% 4-carboxyl substitution. Padwa Albert 1,3-Dipolar Cycloaddition Chemistry General Heterocyclic Chemistry Series 1 1983 Wiley-Interscience United States of America 978-0-471-08364-1 141–145PhD Thesis thesis Koszinowski J. 1980 center|1000px|0-471-08364-X. and Koszinowski, J. (1980) (PhD Thesis) Synthetic applications 1,3-dipolar cycloadditions are important ways toward the synthesis of many important 5-membered heterocycles such as triazoles, furans, isoxazoles, pyrrolidines, and others. Additionally, some cycloadducts can be cleaved to reveal the linear skeleton, providing another route toward the synthesis of aliphatic compounds. These reactions are tremendously useful also because they are stereospecific, diastereoselective and regioselective. Several examples are provided below. Nitrile oxides 1,3-dipolar cycloaddition with nitrile oxides is a widely used masked-aldol reaction. Cycloaddition between a nitrile oxide and an alkene yields the cyclic isoxazoline product, whereas the reaction with an alkyne yields the isoxazole. Both isoxazolines and isoxazoles can be cleaved by hydrogenation to reveal aldol-type β-hydroxycarbonyl or Claisen-type β-dicarbonyl products, respectively. Nitrile oxide-alkyne cycloaddition followed by hydrogenation was utilized in the synthesis of Miyakolide as illustrated in the figure below. Synthesis and Absolute Stereochemical Assignment of (+)-Miyakolide David Evans David Ripin David Halstead Kevin Campos Journal of the American Chemical Society 121 29 1999 6816–6826 10.1021/ja990789h 1999JAChS.121.6816E 900px|center Carbonyl ylides 1,3-dipolar cycloaddition reactions have emerged as powerful tools in the synthesis of complex cyclic scaffolds and molecules for medicinal, biological, and mechanistic studies. Among them, [3+2] cycloaddition reactions involving carbonyl ylides have extensively been employed to generate oxygen-containing five-membered cyclic molecules.Synthetic Reactions of M=C and M=N Bonds: Ylide Formation, Rearrangement, and 1,3-Dipolar Cycloaddition; Hiyama, T. W., J., Ed.; Elsevier, 2007; Vol. 11. Preparation of carbonyl ylides for 1,3-dipolar cycloaddition reactions Ylides are regarded as positively charged heteroatoms connected to negatively charged carbon atoms, which include ylides of sulfonium, thiocarbonyl, oxonium, nitrogen, and carbonyl.10.1021/cr00003a001 Ylide formation from the reaction of carbenes and carbenoids with heteroatom lone pairs Chemical Reviews 91 3 263–309 1991 Padwa Albert. Hornbuckle Susan F. Several methods exist for generating carbonyl ylides, which are necessary intermediates for generating oxygen-containing five-membered ring structures, for [3+2] cycloaddition reactions. Synthesis of carbonyl ylides from diazomethane derivatives by photocatalysis One of the earliest examples of carbonyl ylide synthesis involves photocatalysis.10.1021/ja00356a044 Structure of an electronically stabilized carbonyl ylide Journal of the American Chemical Society 105 18 5929–5930 1983 Janulis Eugene P. Arduengo Anthony J. 1983JAChS.105.5929J Photolysis of diazotetrakis(trifluoromethyl)cyclopentadiene* (DTTC) in the presence of tetramethylurea can generate the carbonyl ylide by an intermolecular nucleophilic attack and subsequent aromatization of the DTTC moiety. This was isolated and characterized by X-ray crystallography due to the stability imparted by aromaticity, electron withdrawing trifluoromethyl groups, and the electron donating dimethylamine groups. Stable carbonyl ylide dipoles can then be used in [3+2] cycloaddition reactions with dipolarophiles. center|Scheme 1. Photolysis of DTTC in the presence of tetramethylurea. Modified from Janulis, E. P.; Arduengo, A. J. J. Am. Chem. Soc. 1983, 105, 5929. Another early example of carbonyl ylide synthesis by photocatalysis was reported by Olah et al.Prakash, G. K. S.; Ellis, R. W.; Felberg, J. D.; Olah, G. A. Formaldehyde 0-Methylide, [CH2=O+-CH2: The Parent Carbonyl Ylide] J Am Chem Soc 1986, 108, 1341. Dideuteriodiazomethane was photolysed in the presence of formaldehyde to generate the dideuterioformaldehyde carbonyl ylide. center|Scheme 2. Photolysis of dideuteriodiazomethane with formaldehyde. Modified from Prakash, G. K. S.; Ellis, R. W.; Felberg, J. D.; Olah, G. A. J Am Chem Soc 1986, 108, 1341. Synthesis of carbonyl ylides from hydroxypyrones by proton transfer Carbonyl ylides can be synthesized by acid catalysis of hydroxy-3-pyrones in the absence of a metal catalyst.Sammes, P. G.; Street, L. J. Intra molecular Cyclo additions with Oxido pyrylium Ylides J. Chem. Soc., Chem. Commun. 1982, 1056. An initial tautomerization occurs, followed by elimination of the leaving group to aromatize the pyrone ring and to generate the carbonyl ylide. A cycloaddition reaction with a dipolarophile lastly forms the oxacycle. This approach is less widely employed due to its limited utility and requirement for pyrone skeletons. center|Scheme 3. Acid-catalyzed synthesis of carbonyl ylides from hydroxy-3-pyrones. Modified from Sammes, P. G.; Street, L. J. J. Chem. Soc., Chem. Commun. 1982, 1056. 5-hydroxy-4-pyrones can also be used to synthesize carbonyl ylides by an intramolecular hydrogen transfer.Garst, M. E.; McBride, B. J.; Douglass III, J. G. Intramolecular cycloadditions with 2-(ω-alkenyl)-5-hydroxy-4-pyrones Tetrahedron Lett. 1983, 24, 1675. After hydrogen transfer, the carbonyl ylide can then react with dipolarophiles to form oxygen-containing rings. center|Scheme 4. Intramolecular hydrogen transfer-mediated synthesis of carbonyl ylides from 5-hydroxy-4-pyrones. Modified from Garst, M. E.; McBride, B. J.; Douglass III, J. G. Tetrahedron Lett. 1983, 24, 1675. Synthesis of α-halocarbonyl ylides from dihalocarbenes Dihalocarbenes have also been employed to generate carbonyl ylides, exploiting the electron withdrawing nature of dihalocarbenes.http://pubs.acs.org/doi/pdf/10.1021/jo00212a009 10.1021/jo00212a009 Dichlorocarbene from flash vacuum pyrolysis of trimethyl(trichloromethyl)silane. Possible observation of 1,1-dichloro-3-phenylcarbonyl ylide 1985 Gisch John F. Landgrebe John A. The Journal of Organic Chemistry 50 12 2050–2054 subscription10.1021/jo00172a015 Dibromocarbonyl ylides. Deoxygenation of aldehydes and ketones by dibromocarbene The Journal of Organic Chemistry 48 24 4519–4523 1983 Huan Zhenwei Landgrebe John A. Peterson Kimberly10.1021/jo00400a009 Halogenated carbonyl ylides in the reactions of mercurial dihalocarbene precursors with substituted benzaldehydes The Journal of Organic Chemistry 43 6 1071–1076 1978 Martin Charles W. Lund Paul R. Rapp Erich Landgrebe John A. Both phenyl(trichloromethyl)mercury and phenyl(tribromomethyl)mercury are sources dichlorocarbenes and dibromocarbenes, respectively. The carbonyl ylide can be generated upon reaction of the dihalocarbenes with ketones or aldehydes. However, the synthesis of α-halocarbonyl ylides can also undesirably lead to the loss of carbon monoxide and the generation of the deoxygenation product. center|Scheme 5. α-Halocarbonyl ylide synthesis through dihalocarbene intermediates. Modified from Padwa, A.; Hornbuckle, S. F. Chem Rev 1991, 91, 263. Synthesis of carbonyl ylides from diazomethane derivatives by metal catalysis A universal approach for generating carbonyl ylides involves metal catalysis of α-diazocarbonyl compounds, generally in the presence of dicopper or dirhodium catalysts.Hodgson, D. M.; Bruckl, T.; Glen, R.; Labande, A. H.; Selden, D. A.; Dossetter, A. G.; Redgrave, A. J. Catalytic enantioselective intermolecular cycloadditions of 2-diazo-3,6-diketoester-derived carbonyl ylides with alkene dipolarophiles Proceedings of the National Academy of Sciences of the United States of America 2004, 101, 5450. After release of nitrogen gas and conversion to the metallocarbene, an intermolecular reaction with a carbonyl group can generate the carbonyl ylide. Subsequent cycloaddition reaction with an alkene or alkyne dipolarophile can afford oxygen-containing five-membered rings. Popular catalysts that give modest yields towards synthesizing oxacycles include Rh2(OAc)4 and Cu(acac)2.10.1021/jo00102a037 Intramolecular Cycloaddition of Isomunchnone Dipoles to Heteroaromatic .pi.-Systems The Journal of Organic Chemistry 59 23 7072–7084 1994 Padwa Albert Hertzog Donald L. Nadler William R.Hamaguchi, M.; Ibata, T. New Type of Mesoionic System. 1,3-Dipolar Cycloaddition of Isomunchnon With Ethylenic Compounds https://web.archive.org/web/20200411024251/https://www.jstage.jst.go.jp/article/cl1972/4/5/4_5_499/_pdf 11 April 2020 Chem Lett 1975, 499. center|Scheme 6. Metal-catalyzed synthesis of carbonyl ylides. Reproduced from Hodgson, D. M.; Bruckl, T.; Glen, R.; Labande, A. H.; Selden, D. A.; Dossetter, A. G.; Redgrave, A. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5450. Mechanism of the 1,3-dipolar cycloaddition reaction mediated by metal catalysis of diazocarbonyl compounds The universality and extensive use of 1,3-dipolar cycloaddition reactions mediated by metal catalysis of diazocarbonyl molecules, for synthesizing oxygen-containing five-membered rings, has spurred significant interest into its mechanism. Several groups have investigated the mechanism to expand the scope of synthetic molecules with respect to regio- and stereo-selectivity. However, due to the high turn over frequencies of these reactions, the intermediates and mechanism remains elusive. The generally accepted mechanism, developed by characterization of stable ruthenium-carbenoid complexes10.1002/chem.19960020311 Aryloxycarbonylcarbene Complexes of Bis(oxazolinyl)pyridineruthenium as Active Intermediates in Asymmetric Catalytic Cyclopropanations Chemistry - A European Journal 2 3 303–306 1996 Park Soon-Bong Sakata Naoya Nishiyama Hisao and rhodium metallocarbenes,10.1021/ja016928o 11697986 A Stable Dirhodium Tetracarboxylate Carbenoid: Crystal Structure, Bonding Analysis, and Catalysis Journal of the American Chemical Society 123 45 11318–11319 2001 Snyder James P. Padwa Albert Stengel Thomas Arduengo Anthony J. Jockisch Alexander Kim Hyo-Joong 2001JAChS.12311318S involves an initial formation of a metal-carbenoid complex from the diazo compound. Elimination of nitrogen gas then affords a metallocarbene. An intramolecular nucleophilic attack by the carbonyl oxygen regenerates the metal catalyst and forms the carbonyl ylide. The carbonyl ylide can then react with an alkene or alkyne, such as dimethyl acetylenedicarboxylate (DMAD) to generate the oxacycle. center|Scheme 7. Accepted mechanism of the 1,3-dipolar cycloaddition reaction mediated by metal catalysis (example dirhodium catalyst) of diazocarbonyl compounds. Modified from M. Hodgson, D.; H. Labande, A.; Muthusamy, S. In Organic Reactions; John Wiley & Sons, Inc.: 2004. However, it is uncertain whether the metallocarbene intermediate generates the carbonyl ylide. In some cases, metallocarbenes can also react directly with dipolarophiles.Hodgson, D. M.; Pierard, F. Y. T. M.; Stupple, P. A. Catalytic enantioselective rearrangements and cycloadditions involving ylides from diazo compounds Chem Soc Rev 2001, 30, 50. In these cases, the metallocarbene, such as the dirhodium(II)tetracarboxylate carbene, is stabilized through hyperconjugative metal enolate-type interactions.10.1002/adsc.200303092 Theoretical Studies on Diastereo- and Enantioselective Rhodium-Catalyzed Cyclization of Diazo Compoundvia Intramolecular C—H Bond Insertion Advanced Synthesis & Catalysis 345 910 1159–1171 2003 Yoshikai Naohiko Nakamura Eiichi10.1021/ja017823o 12059244 Mechanism of C−H Bond Activation/C−C Bond Formation Reaction between Diazo Compound and Alkane Catalyzed by Dirhodium Tetracarboxylate Journal of the American Chemical Society 124 24 7181–7192 2002 Nakamura Eiichi Yoshikai Naohiko Yamanaka Masahiro 2002JAChS.124.7181N Costantino, G.; Rovito, R.; Macchiarulo, A.; Pellicciari, R. Structure of metal–carbenoid intermediates derived from the dirhodium(II)tetracarboxylate mediated decomposition of α-diazocarbonyl compounds: a DFT study J Mol Struc-Theochem 2002, 581, 111.M. Hodgson, D.; H. Labande, A.; Muthusamy, S. In Organic Reactions; John Wiley & Sons, Inc.: 2004. Subsequent 1,3-dipolar cycloaddition reaction occurs through a transient metal-complexed carbonyl ylide. Therefore, a persistent metallocarbene can influence the stereoselectivity and regioselectivity of the 1,3-dipolar cycloaddition reaction based on the stereochemistry and size of the metal ligands. center|The dirhodium(II)tetracarboxylate metallocarbene stabilized by πC-Rh→πC=O hyperconjugation. Modified from M. Hodgson, D.; H. Labande, A.; Muthusamy, S. In Organic Reactions; John Wiley & Sons, Inc.: 2004. The mechanism of the 1,3-dipolar cycloaddition reaction between the carbonyl ylide dipole and alkynyl or alkenyl dipolarophiles has been extensively investigated with respect to regioselectivity and stereoselectivity. As symmetric dipolarophiles have one orientation for cycloaddition, only one regioisomer, but multiple stereoisomers can be obtained. On the contrary, unsymmetric dipolarophiles can have multiple regioisomers and stereoisomers. These regioisomers and stereoisomers may be predicted based on frontier molecular orbital (FMO) theory, steric interactions, and stereoelectronic interactions.10.1021/jo051743b 16356001 Efficient Catalytic Effects of Lewis Acids in the 1,3-Dipolar Cycloaddition Reactions of Carbonyl Ylides with Imines The Journal of Organic Chemistry 70 26 10782–10791 2005 Suga Hiroyuki Ebiura Yasutaka Fukushima Kazuaki Kakehi Akikazu Baba Toshihide10.1021/ja00164a034 Tandem cyclization-cycloaddition reaction of rhodium carbenoids. Scope and mechanistic details of the process Journal of the American Chemical Society 112 8 3100–3109 1990 Padwa Albert Fryxell Glen E. Zhi Lin 1990JAChS.112.3100P center|Scheme 9. Products of the 1,3-dipolar cycloaddition reaction between carbonyl ylide dipoles and alkenyl or alkynyl dipolarophiles. Modified from M. Hodgson, D.; H. Labande, A.; Muthusamy, S. In Organic Reactions; John Wiley & Sons, Inc.: 2004. Regioselectivity of the 1,3-dipolar cycloaddition reaction mediated by metal catalysis of diazocarbonyl compounds Regioselectivity of 1,3-dipolar cycloaddition reactions between carbonyl ylide dipoles and alkynyl or alkenyl dipolarophiles is essential for generating molecules with defined regiochemistry. FMO theory and analysis of the HOMO–LUMO energy gaps between the dipole and dipolarophile can rationalize and predict the regioselectivity of experimental outcomes.10.1021/ja00803a017 Frontier molecular orbitals of 1,3 dipoles and dipolarophiles Journal of the American Chemical Society 95 22 7287–7301 1973 Houk K. N. Sims Joyner. Duke R. E. Strozier R. W. George John K. 1973JAChS..95.7287H 10.1021/ja00525a006 Theoretical studies of the structures and reactions of substituted carbonyl ylides Journal of the American Chemical Society 102 5 1504–1512 1980 Houk K. N. Rondan Nelson G. Santiago Cielo Gallo Catherine J. Gandour Ruth Wells Griffin Gary W. 1980JAChS.102.1504H The HOMOs and LUMOs can belong to either the dipole or dipolarophile, for which HOMOdipole-LUMOdipolarophile or HOMOdipolarophile-LUMOdipole interactions can exist. Overlap of the orbitals with the largest coefficients can ultimately rationalize and predict results. center|Scheme 10. diagram of the molecular orbital interactions of HOMOdipole-LUMOdipolarophile or HOMOdipolarophile-LUMOdipole between a carbonyl ylide dipole and alkenyl dipolarophile. The archetypal regioselectivity of the 1,3-dipolar cycloaddition reaction mediated by carbonyl ylide dipoles has been examined by Padwa and coworkers.10.1021/cr950022h 11848752 Cascade Processes of Metallo Carbenoids Chemical Reviews 96 1 223–270 1996 Padwa Albert Weingarten M. David Using a Rh2(OAc)4 catalyst in benzene, diazodione underwent a 1,3-dipolar cycloaddition reaction with methyl propiolate and methyl propargyl ether. The reaction with methyl propiolate affords two regioisomers with the major resulting from the HOMOdipole-LUMOdipolarophile interaction, which has the largest coefficients on the carbon proximal to the carbonyl group of the carbonyl ylide and on the methyl propiolate terminal alkyne carbon. The reaction with methyl propargyl ether affords one regioisomer resulting from the HOMOdipolarophile-LUMOdipole interaction, which has largest coefficients on the carbon distal to the carbonyl group of the carbonyl ylide and on the methyl propargyl ether terminal alkyne carbon. center|Scheme 11. Regioselectivity and molecular orbital interactions of the 1,3-dipolar cycloaddition reaction between a diazodione and methyl propiolate or methyl propargyl ether. Modified from Padwa, A.; Weingarten, M. D. Chem Rev 1996, 96, 223. Regioselectivities of 1,3-dipolar cycloaddition reactions mediated by metal catalysis of diazocarbonyl compounds may also be influenced by the metal through formation of stable metallocarbenes.10.1021/jo951576n Ligand-Induced Selectivity in the Rhodium(II)-Catalyzed Reactions of α-Diazo Carbonyl Compounds† The Journal of Organic Chemistry 61 63–72 1996 Padwa Albert Austin David J. Stabilization of the metallocarbene, via metal enolate-type interactions, will prevent the formation of carbonyl ylides, resulting in a direct reaction between the metallocarbene dipole and an alkynyl or alkenyl dipolarophile (see image of The dirhodium(II)tetracarboxylate metallocarbene stabilized by πC-Rh→πC=O hyperconjugation.). In this situation, the metal ligands will influence the regioselectivity and stereoselectivity of the 1,3-dipolar cycloaddition reaction. Stereoselectivity and asymmetric induction of the 1,3-dipolar cycloaddition reaction mediated by metal catalysis of diazocarbonyl compounds The stereoselectivity of 1,3-dipolar cycloaddition reactions between carbonyl ylide dipoles and alkenyl dipolarophiles has also been closely examined. For alkynyl dipolarophiles, stereoselectivity is not an issue as relatively planar sp2 carbons are formed, while regioselectivity must be considered (see image of the Products of the 1,3-Dipolar Cycloaddition Reaction Between Carbonyl Ylide Dipoles and Alkenyl or Alkynyl Dipolarophiles). However, for alkenyl dipolarophiles, both regioselectivity and stereoselectivity must be considered as sp3 carbons are generated in the product species. 1,3-dipolar cycloaddition reactions between carbonyl ylide dipoles and alkenyl dipolarophiles can generate diastereomeric products. The exo product is characterized with dipolarophile substituents being cis to the ether bridge of the oxacycle. The endo product is characterized with the dipolarophile substituents being trans to the ether bridge of the oxacycle. Both products can be generated through pericyclic transitions states involving concerted synchronous or concerted asynchronous processes. One early example conferred stereoselectivity in terms of endo and exo products with metal catalysts and Lewis acids.Suga, H.; Kakehi, A.; Ito, S.; Inoue, K.; Ishida, H.; Ibata, T. Stereocontrol in a Ytterbium Triflate-Catalyzed 1,3-Dipolar Cycloaddition Reaction of Carbonyl Ylide with N-Substituted Maleimides and Dimethyl Fumarate B Chem Soc Jpn 2001, 74, 1115. Reactions with just the metal catalyst Rh2(OAc)4 prefer the exo product while reactions with the additional Lewis acid Yb(OTf)3 prefer the endo product. The endo selectivity observed for Lewis acid cycloaddition reactions is attributed to the optimized orbital overlap of the carbonyl π systems between the dipolarophile coordinated by Yb(Otf)3 (LUMO) and the dipole (HOMO). After many investigations, two primary approaches for influencing the stereoselectivity of carbonyl ylide cycloadditions have been developed that exploit the chirality of metal catalysts and Lewis acids. center|Facial Selectivity of the 1,3-Dipolar Cycloaddition Reaction using a Metal Catalyst and Lewis Acid center|Rationale for the Endo Selectivity of the 1,3-Dipolar Cycloaddition Reaction with a Lewis Acid The first approach employs chiral metal catalysts to modulate the endo and exo stereoselectivity. The chiral catalysts, in particular Rh2[(S)-DOSP]4 and Rh2[(S)-BPTV]4 can induce modest asymmetric induction and was used to synthesize the antifungal agent pseudolaric acid A.10.1002/anie.200602056 16906616 Total Synthesis of Pseudolaric Acid A Angewandte Chemie International Edition 45 37 6197–6201 2006 Geng Zhe Chen Bin Chiu Pauline This is a result of the chiral metal catalyst remaining associated with the carbonyl ylide during the cycloaddition, which confers facial selectivity. However, the exact mechanisms are not yet fully understood. center|Asymmetric induction of the 1,3-dipolar cycloaddition reaction with chiral metal catalysts The second approach employs a chiral Lewis acid catalyst to induce facial stereoselectivity after the generation of the carbonyl ylide using an achiral metal catalyst.10.1021/jo049007f 15624905 Chiral 2,6-Bis(oxazolinyl)pyridine−Rare Earth Metal Complexes as Catalysts for Highly Enantioselective 1,3-Dipolar Cycloaddition Reactions of 2-Benzopyrylium-4-olates The Journal of Organic Chemistry 70 1 47–56 2005 Suga Hiroyuki Inoue Kei Inoue Shuichi Kakehi Akikazu Shiro Motoo The chiral Lewis acid catalyst is believed to coordinate to the dipolarophile, which lowers the LUMO of the dipolarophile while also leading to enantioselectivity. center|Asymmetric induction of the 1,3-dipolar cycloaddition reaction with chiral Lewis acid catalysts Azomethine ylides Azomethine ylide 1,3-Dipolar cycloaddition between an azomethine ylide and an alkene furnishes an azacyclic structure, such as pyrrolidine. This strategy has been applied to the synthesis of spirotryprostatin A. Concise, Asymmetric Total Synthesis of Spirotryprostatin A Tomoyuki Onishi Paul Sebahar Robert Williams Organic Letters 5 17 2003 3135–3137 10.1021/ol0351910 12917000 900px|center Ozone Ozonolysis is a very important organic reaction. Alkenes and alkynes can be cleaved by ozonolysis to give aldehyde, ketone or carboxylic acid products. Biological applications The 1,3-dipolar cycloaddition between organic azides and terminal alkynes (i.e., the Huisgen cycloaddition) has been widely utilized for bioconjugation. Copper catalysis The Huisgen reaction generally does not proceed readily under mild conditions. Meldal et al. and Sharpless et al. independently developed a copper(I)-catalyzed version of the Huisgen reaction, CuAAC (for Copper-catalyzed Azide-Alkyne Cycloaddition), which proceeds very readily in mild, including physiological, conditions (neutral pH, room temperature and water solution). Peptidotriazoles on Solid Phase: [1,2,3]-Triazoles by Regiospecific Copper(I)-Catalyzed 1,3-Dipolar Cycloadditions of Terminal Alkynes to Azides Christian Tornoe Caspar Christensen Morten Meldal Journal of Organic Chemistry 2002 3057–3064 10.1021/jo011148j 11975567 67 9 11957672 A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed Regioselective Ligation of Azides and Terminal Alkynes Vsevolod Rostovtsev Luke Green Valery Fokin Barry K. Sharpless Angewandte Chemie International Edition 2002 41 14 2596–2599 10.1002/1521-3773(20020715)41:14<2596::AID-ANIE2596>3.0.CO;2-4 12203546 This reaction is also bioorthogonal: azides and alkynes are both generally absent from biological systems and therefore these functionalities can be chemoselectively reacted even in the cellular context. They also do not react with other functional groups found in nature, so they do not perturb biological systems. The reaction is so versatile that it is termed the "Click" chemistry. Although copper(I) is toxic, many protective ligands have been developed to both reduce cytotoxicity and improve rate of CuAAC, allowing it to be used in in vivo studies. Increasing the Efficacy of Bioorthogonal Click Reactions for Bioconjugation: A Comparative Study Christen Besanceney-Webler Hao Jiang Tianqing Zheng Lei Feng David Soriano del Amo Wei Wang Liana M. Klivansky Florence L. Marlow Yi Liu Peng Wu Angewandte Chemie International Edition 2011 50 35 8051–8056 10.1002/anie.201101817 21761519 3465470 350px|center For example, Bertozzi et al. reported the metabolic incorporation of azide-functionalized saccharides into the glycan of the cell membrane, and subsequent labeling with fluorophore-alkyne conjugate. The result is that the cell membrane is fluorescently labeled, and can therefore be imaged using a fluorescence microscope. Targeted metabolic labeling of yeast N-glycans with unnatural sugars Mark Breidenbach Jennifer Gallagher David King Brian Smart Peng Wu Carolyn Bertozzi Proceedings of the National Academy of Sciences of the United States of America 2010 107 3988–3993 10.1073/pnas.0911247107 9 20142501 2840165 2010PNAS..107.3988B free center|800px Strain-promoted cycloaddition To avoid toxicity of copper(I), Bertozzi et al. developed the strain-promoted azide-alkyne cycloaddition (SPAAC) between organic azide and strained cyclooctyne. The angle distortion of the cyclooctyne helps to speed up the reaction by both reducing the activation strain and enhancing the interactions, thereby enabling it to be used in physiological conditions without the need for the catalyst. A Strain-Promoted [3 + 2] Azide−Alkyne Cycloaddition for Covalent Modification of Biomolecules in Living Systems Nicholas Agard Jennifer Prescher Carolyn Bertozzi Journal of the American Chemical Society 2004 126 46 15046–15047 10.1021/ja044996f 15547999 2004JAChS.12615046A 350px|center For instance, Ting et al. introduced an azido functionality onto specific proteins on the cell surface using a ligase enzyme. The azide-tagged protein is then labeled with cyclooctyne-fluorophore conjugate to yield a fluorescently labeled protein. Redirecting lipoic acid ligase for cell surface protein labeling with small-molecule probes Marta Fernandez-Suarez Hemanta Baruah Laura Martinez-Hernandez Kathleen Xie Jeremy Baskin Carolyn Bertozzi Alice Ting Nature Biotechnology 2007 25 12 1483–1487 10.1038/nbt1355 18059260 2654346 center|650px References Category:Nitrogen heterocycle forming reactions Category:Cycloadditions