Exploring the potential of the vitamin B12 derivative azidocobalamin to undergo Huisgen 1,3-dipolar azide-alkyne cycloaddition reactions

There is considerable interest in using the metalloprotein cofactor vitamin B12 as a vehicle to deliver drugs and diagnostic agents into mammalian or bacterial cells by exploiting the B12-specific active uptake pathways. Conjugation of the cargo via the β-axial site or the 5'-OH of the ribose of the nucleotide are the most desirable sites, to maximise intracellular uptake. Herein we show the potential of conjugation at the beta-azido ligand of the vitamin B12 derivative azidocobalamin via a click-type azide-alkyne 1,3-dipolar cycloaddition (Huisgen cycloaddition) reaction. Reacting azidocobalamin with dimethyl acetylenedicarboxylate at 40 °C results in essentially stoichiometric conversion of azidocobalamin to the corresponding triazolato complex. The stability of the complex as a function of pH and in the presence of cyanide were investigated. The complex is stable in pD 7.0 phosphate buffer for 24 h. The rate of beta-axial ligand substitution was found to be one order of magnitude slower for the triazolatocobalamin complex compared with azidocobalamin.

Macrocyclic Copper(II) Complexes as Catalysts for Electrochemically Mediated Atom Transfer

Copper-catalyzed electrochemical atom transfer radical addition (eATRA) is a new method for the creation of new C-C bonds under mild conditions. In this work, we have explored the reactivity of an analogous series of N4 macrocyclic CuII complexes as eATRA precatalysts, which are primed by reduction to their monovalent oxidation state. These complexes were fully characterized structurally, spectroscopically, and electrochemically. A spectrum of radical activation reactivity was found across the series [CuI(Me4cyclen)(NCMe)]+ (Me4cyclen = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane), [CuI(Me4cyclam)(NCMe)]+ (Me4cyclam = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), and [CuI(Me2py2clen)(NCMe)]+ (Me2py2clen = 3,7-dimethyl-3,7-diaza-1,5(2,6)-dipyridinacyclo-octaphane). The rate of radical production by [Cu(Me2py2clen)(NCMe)]+ was modest, but rapid radical capture to form the organocopper complex [CuI(Me2py2clen)(CH2CN)] led to a dramatic acceleration in catalysis, greater than seen in any comparable Cu complex, but this led to rapid radical self-termination instead of radical addition.

Azido-mediated intermolecular interactions of transition metal complexes

The coordinated azido ligand has a variety of ways to establish intermolecular contacts whose nature is computationally analysed in this work on dimers of the [N3-Hg(CF3)] complex with different interactions involving only N⋯N contacts, or with an additional Hg⋯N contact. The applied tools include the molecular electrostatic map of the monomer, an energy decomposition analysis (EDA), a topological AIM analysis of the electron density and the study of NCI (non-covalent interactions) isosurfaces. The interactions between two azido ligands are found to be weakly stabilizing (by 0.2 to 2.7 kcal mol-1), topology-dependent and require dispersion forces to complement orbital and electrostatic stabilization. Those interactions are supplemented by the formation of simultaneous Hg⋯N secondary interactions by about -1 kcal mol-1, and by the ability of the monomer to simultaneously interact with several neighbours in the crystal structure.

Isostructural Series of Ni(II), Pd(II), Pt(II), and Au(III) Azido Complexes with a N^C^N Pincer Ligand to Elucidate Trends in the iClick Reaction Kinetics and Structural Parameters of the Triazolato Products

An isoelectronic and isostructural series of cyclometalated azido complexes [M(N3)(dpb)] with M = Ni(II), Pd(II), Pt(II), and Au(III) based on the N^C^N pincer ligand 1,3-di(2-pyridyl)phenide (dpb) was characterized by X-ray diffraction analysis and investigated for reactivity in the iClick reaction with a wide range of internal and terminal alkynes by using 1H and 19F NMR spectroscopy. Reaction rate constants were found to increase with greater charge density in the order Ni(II) > Pd(II) > Pt(II) > Au(III). Terminal alkynes R-C≡C-R' with strongly electron-withdrawing groups R and R' exhibited faster kinetics than those with electron-donating substituents in the order CF3 > ketone > ester > H > phenyl ≫ amide, while R = CH3 resulted in complete loss of reactivity. Four symmetrical triazolato complexes [M(triazolatoCOOCH3,COOCH3)(dpb)] with M = Ni(II), Pd(II), Pt(II), and Au(III) as well as four nonsymmetrically substituted triazolato complexes [Pt(triazolatoR,R')(dpb)] originating from terminal and internal alkynes were shown by X-ray crystal structure analysis to exclusively feature N2-coordination of the five-membered ring ligand. However, the Pt(II) triazolato complexes exist as a mixture of N1- and N2-coordinated species in solution. Torsion angles between the mean planes of the N^C^N pincer and the triazolato ligand increase from a nearly coplanar to a perpendicular arrangement when going from Au(III)/Pt(II)/Pd(II) to Ni(II), while different substituents R and R' on the alkyne have no influence on the torsion angle and were rationalized by DFT calculations. Finally, a carbohydrate derivative obtained by glucuronic acid conjugation to methyl propiolate demonstrates the facile biofunctionalization of metal complexes via the iClick reaction.

Porous Salts as Platforms for Heterogeneous Catalysis

The preparation of a new class of reactive porous solids, prepared via straightforward salt metathesis reactions, is described here. Reaction of the dimethylammonium salt of a magnesium-based porous coordination cage with the chloride salt of [Cr^II Cl(Me₄ cyclam)]⁺ affords a porous solid with concomitant removal of dimethylammonium chloride. The salt consists of the ions combined in the expected ratio based on their charge as confirmed by UV-vis and X-ray photoelectron spectroscopies, ion chromatography (IC), and inductively coupled plasma mass spectrometry (ICP-MS). The porous salt boasts a Brunauer-Emmett-Teller (BET) surface area of 213 m²  g^-1 . Single crystal X-ray diffraction reveals the chromium(II) cations in the structure reside in the interstitial space between porous cages. Importantly, the chromium(II) centers, previously shown to react with O₂ to afford reactive chromium(III)-superoxide adducts, are still accessible in the solid state as confirmed by UV-vis spectroscopy. The site-isolated reactive centers have competence toward hydrogen atom abstraction chemistry and display significantly increased stability and reactivity as compared to dissolved ions.

Development of photoactivable phenanthroline-based manganese(I) CO-Releasing molecules (PhotoCORMs) active against ESKAPE bacteria and bacterial biofilms

The synthesis and biological evaluation of a series of phenanthroline-based visible-light-activated manganese(I) carbon-monoxide-releasing molecules (PhotoCORMs) against ESKAPE bacteria and bacterial biofilms is reported. Four carbonyl compounds of general formula fac-[Mn(N^∧N)(CO)₃(L)] have been synthesized and characterized. Despite being thermally stable in the absence of light, these PhotoCORMs readily release CO upon blue (435-450 nm) LED light irradiation as confirmed by spectrophotometric CO releasing experiments (Mb Assay). The antibacterial activity of the four PhotoCORMs has been investigated against a panel of ESKAPE bacteria. The compounds 1-3 were found to be effective antibacterials at low concentrations against multidrug-resistant Klebsiella pneumoniae and Acinetobacter baumannii when photoactivated with blue-light. In addition, the PhotoCORMs 1-2 were found to inhibit the formation of Klebsiella pneumoniae and Acinetobacter baumannii bacterial biofilms at low concentrations (MIC = 4-8 μg/mL), turning out to be promising candidates to combat antimicrobial resistance. The antibacterial and biofilm inhibitory effect of the PhotoCORMs is plausibly due to the release of CO as well as the formation of phenanthroline photo-by-products as revealed by spectroscopy and microbiology experiments.

An Application Exploiting Aurophilic Bonding and iClick to Produce White Light Emitting Materials

The paper focuses on exploiting aurophilic bonding to produce white light emitting materials. Inorganic Click (iClick) is employed to link two or four Au(I) metal ions through a triazolate bridge. Depending on the choice of phosphine ligand (PEt₃ or PPh₃), dinuclear Au₂-FO or tetranuclear Au₄-FO complexes can be controllably synthesized (FO = 2-(9,9-dioctylfluoreneyl-)). The iClick products Au₂-FO and Au₄-FO are characterized by combustion analysis and multinuclear NMR, TOCSY 1D, ¹H-¹³C gHMBC, and ¹H-¹³C gHSQC. In addition, the photophysical properties of Au₂-FO and Au₄-FO were examined in THF solution. Transient absorption spectroscopy was employed to elucidate the excited state features of the gold compounds. Solution processed OLEDs were fabricated and characterized, which gave white light electroluminescence with CIE coordinates (0.34, 0.36), as seen referenced to CIE standard illuminant D65 (0.31, 0.32). TDDFT computational analysis of Au₂-FO and Au₄-FO reveals the origin of light emission. In the case of Au₄-FO, direct excitation leads to increased aurophilic bonding in the excited state, and as a result the emission profile is broadened to cover a larger region of the visible spectrum, thus giving white light emission. Designing molecules that can access or increase aurophilic bonding in the excited state provides another tool for fine-tuning the emission profiles of gold complexes.

Alkynes as Privileged Synthons in Selected Organic Name Reactions

BACKGROUND

Alkynes are actually basic chemicals, serving as privileged synthons for planning new organic reactions for assemblage of a reactive motif, which easily undergoes a further desirable transformation. Name reactions, in organic chemistry are referred to those reactions which are well-recognized and reached to such status for being called as their explorers, discoverers or developers. Alkynes have been used in various name reactions. In this review, we try to underscore the applications of alkynes as privileged synthons in prevalent name reactions such as Huisgen 1,3-dipolar cycloaddtion via Click reaction, Sonogashira reaction, and Hetero Diels-Alder reaction.

OBJECTIVE

In this review, we try to underscore the applications of alkynes as privileged synthons in the formation of heterocycles, focused on the selected reactions of alkynes as a synthon or impending utilization in synthetic organic chemistry, which have reached such high status for being included in the list of name reactions in organic chemistry.

CONCLUSION

Alkynes (including acetylene) are an unsaturated hydrocarbon bearing one or more triple C-C bond. Remarkably, alkynes and their derivatives are frequently being used as molecular scaffolds for planning new organic reactions and installing reactive functional group for further reaction. It is worth mentioning that in general, the terminal alkynes are more useful and more frequently being used in the art of organic synthesis. Remarkably, alkynes have found different applications in pharmacology, nanotechnology, as well as being known as appropriate starting precursors for the total synthesis of natural products and biologically active complex compounds. They are predominantly applied in various name reactions such as Sonogashira, Glaser reaction, Friedel-crafts reaction, Castro-Stephens coupling, Huisgen 1.3-dipolar cycloaddtion reaction via Click reaction, Sonogashira reaction, hetero-Diels-Alder reaction. In this review, we tried to impress the readers by presenting selected name reactions, which use the alkynes as either stating materials or precursors. We disclosed the applications of alkynes as a privileged synthons in several popular reactions, which reached to such high status being classified as name reactions. They are thriving and well known and established name reactions in organic chemistry such as Regioselective, 1,3-dipolar Huisgen cycloaddtion reaction via Click reaction, Sonogashira reaction and Diels-Alder reaction.

A novel Pt(iv) mono azido mono triazolato complex evolves azidyl radicals following irradiation with visible light

A novel Pt^IV azido triazolato complex exists as an equilibrium between two species in d₃-MeCN and evolves azide radicals (but not hydroxide radicals) when irradiated with visible light.

The platinum(iv) azido complex trans,trans,trans-[Pt^IV(N₃)₂(OH)₂(py)₂] (1) undergoes cycloaddition with 1,4-diphenyl-2-butyne-1,4-dione (2) under mild, catalyst-free conditions, affording a number of mono and bis click products. The major mono click product (3) exists in MeCN as an equilibrium mixture between two species; 3a and 3b rapidly interconvert through nucleophilic attack of the axial Pt–OH group at the adjacent Ph–CO group. The kinetic and thermodynamic parameters for this interconversion have been measured by selective saturation-transfer NMR spectroscopic experiments and are consistent with cyclisation at the Pt centre. Complex 3b was also characterised by X-ray crystallography. Visible light irradiation (440–480 nm) of 3 in d₃-MeCN produces azidyl radicals (N₃˙), as demonstrated by EPR spin-trapping with DMPO; no generation of hydroxyl radicals was observed. ¹H–¹⁹⁵Pt HMBC NMR confirmed that the photoproducts were Pt^IV rather than Pt^II species, and HPLC was consistent with these being [3–N₃]⁺ species; no facile photoejection of the triazolato ligand was observed, consistent with MS/MS fragmentation of 3. When 3 was irradiated in the presence of 5′-GMP, no 5′-GMP photoproducts were observed, suggesting that complex 3 is likely to exhibit significantly simplified biological activity (release of azidyl radicals but not DNA binding) compared with complex 1.

Platinum(iv) azido complexes undergo copper-free click reactions with alkynes

We report our investigations into the first examples of copper-free 1,3-dipolar cycloaddition (click) reactions of electrophiles with a PtIV azido complex. The Pt-IV azido complex trans, trans, trans-[PtIV(py)2(N3)2(OH)2] (1) was reactive towards dimethyl acetylenedicarboxylate (DMAD) (2), diethyl acetylenedicarboxylate DEACD (3), N-[(1R,8S,9s)-bicyclo[6.1.0]non-4-yn-9-ylmethyloxycarbonyl]-1,8-diamino-3,6-dioxaoctane (BCN) (11) and dibenzocyclooctyne-amine (DBCO) (12) resulting in formation of the corresponding mono (a) and bis-substituted (b) complexes. Complexes of 2 undergo further reactions between the Pt centre and the carbonyl group to form 2a' and 2b'. This is not seen for the products of the corresponding PtII azido complex trans-[Pt(py)2(N3)2] with acetylene 2. Novel complexes 2a', 2b', 11a and 11b have been characterised by multinuclear NMR, IR and UV-vis spectroscopy and ESI-MS. These reactions represent new synthetic routes to novel Pt(iv) complexes.

Catalyst-free room-temperature iClick reaction of molybdenum(ii) and tungsten(ii) azide complexes with electron-poor alkynes: structural preferences and kinetic studies

Two isostructural and isoelectronic group VI azide complexes of the general formula [M(η³-allyl)(N₃)(bpy)(CO)₂] with M = Mo, W and bpy = 2,2'-bipyridine were prepared and fully characterized, including X-ray structure analysis. Both reacted smoothly with electron-poor alkynes such as dimethyl acetylenedicarboxylate (DMAD) and 4,4,4-trifluoro-2-butynoic acid ethyl ester in a catalyst-free room-temperature iClick [3 + 2] cycloaddition reaction. Reaction with phenyl(trifluoromethyl)acetylene, on the other hand, did not lead to any product formation. X-ray structures of the four triazolate complexes isolated showed the monodentate ligand to be N2-coordinated in all cases, which requires a 1,2-shift of the nitrogen from the terminal azide to the triazolate cycloaddition product. On the other hand, a ¹⁹F NMR spectroscopic study of the reaction of the fluorinated alkyne with the tungsten azide complex at 27 °C allowed detection of the N1-coordinated intermediate. With this method, the second-order rate constant was determined as (7.3 ± 0.1) × 10^-2 M^-1 s^-1, which compares favorably with that of first-generation compounds such as difluorocyclooctyne (DIFO) used in the strain-promoted azide-alkyne cycloaddition (SPAAC). In contrast, the reaction of the molybdenum analogue was too fast to be studied with NMR methods. Alternatively, solution IR studies revealed pseudo-first order rate constants of 0.4 to 6.5 × 10^-3 s^-1, which increased in the order of Mo > W and F₃C-C[triple bond, length as m-dash]C-COOEt > DMAD.

A combined experimental and computational study on supramolecular assemblies in hetero-tetranuclear nickel(ii)-cadmium(ii) complexes with N2O4-donor compartmental Schiff bases

Two new hetero-tetranuclear nickel(ii)/cadmium(ii) complexes, a cubane [(CH₃CO₂)₂Ni₂(L¹)₂Cd₂(NCS)₂] (1) and a linear tetramer [(DMSO)₂NiL²Cd(NCS)(μ_1,3-SCN)Ni(DMSO)L²Cd(NCS)₂] (2) {where H₂L¹ = N,N'-bis(3-methoxysalicylidene)propane-1,3-diamine and H₂L² = N,N'-bis(3-ethoxysalicylidene)propane-1,3-diamine are potential octadentate compartmental Schiff bases}, were synthesized and characterized. The structures of both complexes were confirmed by single crystal X-ray diffraction studies. Complex 1 contained a Ni₂Cd₂O₄ cubane core, whereas complex 2 featured an end-to-end thiocyanate-bridged tetranuclear moiety. Furthermore, complex 1 showed C-HH-C interactions, whereas a unique Sπ interaction was observed in complex 2. Theoretical studies were performed using several computational tools such as NBO and AIM analyses. Both complexes showed photoluminescence in DMSO medium at room temperature upon irradiation with ultraviolet light. The lifetimes of the excited states were ∼27 ns.

Probing the factors that influence the conformation of a guanidinato ligand in [(η5-C5Me5)M(NN)X] (NN = chelating N,N′,N′′-tri(o-substituted aryl)guanidinate(1−); X = chloro, azido and triazolato)

The conformational difference illustrated is ascribed to a subtle repulsive interaction between the o-Cl substituent of two proximal aryl rings in the guanidinate ligand.

Crystal structure and spectroscopic characterization of a cobalt(II) tetraazamacrocycle: completing a series of first-row transition-metal complexes

The tetraazamacrocyclic ligand 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (TMC) has been used to bind a variety of first-row transition metals but to date the crystal structure of the cobalt(II) complex has been missing from this series. The missing cobalt complex chlorido(1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane-κ⁴N)cobalt(II) chloride dihydrate, [CoCl(C₁₄H₃₂N₄)]Cl·2H₂O or [Co^IICl(TMC)]Cl·2H₂O, crystallizes as a purple crystal. This species adopts a distorted square-pyramidal geometry in which the TMC ligand assumes the trans-I configuration and the chloride ion binds in the syn-methyl pocket of the ligand. The Co^II ion adopts an S = 3/2 spin state, as measured by the Evans NMR method, and UV-visible spectroscopic studies indicate that the title hydrated salt is stable in solution. Density functional theory (DFT) studies reveal that the geometric parameters of [Co^IICl(TMC)]Cl·2H₂O are sensitive to the cobalt spin state and correctly predict a change in spin state upon a minor perturbation to the ligand environment.

Organogold oligomers: exploiting iClick and aurophilic cluster formation to prepare solution stable Au4 repeating units

Cluster formation via multiple gold–gold bonds provides sufficient thermodynamic driving force to overcome entropic penalties to link multiple units and create solution stable organogold oligomers.

Au-iClick mirrors the mechanism of copper catalyzed azide–alkyne cycloaddition (CuAAC)

Isolated digold-triazolate products and first-order kinetic profiles for Au–acetylide/azide reactants in iClick provide compelling support for two copper ions in CuAAC.

Amino acid bioconjugation via iClick reaction of an oxanorbornadiene-masked alkyne with a Mn(I)(bpy)(CO)3-coordinated azide

The catalyst-free room temperature iClick reaction of an unsymmetrically 2,3-disubstituted oxanorbornadiene (OND) as a "masked" alkyne equivalent with [Mn(N3)(bpy(CH3,CH3))(CO)3] leads to isolation of a phenylalanine ester bioconjugate, in which the model amino acid is linked to the metal moiety via a N-2-coordinated triazolate formed in a cycloaddition-retro-Diels-Alder (crDA) reaction sequence, in a novel approach to bioorthogonal coupling reactions based on metal-centered reactivity.

Towards a better understanding of magnetic exchange mediated by hydrogen bonds in Mn(iii)/Fe(iii) salen-type supramolecular dimers

Detailed investigations of the magnetic coupling and magnetic anisotropy in a series of Schiff base salen-type Fe(iii) and Mn(iii) complexes, based on SQUID experiments and DFT calculations, are reported.

Strain-promoted azide-alkyne cycloaddition with ruthenium(II)-azido complexes

The reactivity of an exemplary ruthenium(II)-azido complex towards non-activated, electron-deficient, and towards strain-activated alkynes at room temperature and low millimolar azide and alkyne concentrations has been investigated. Non-activated terminal and internal alkynes failed to react under such conditions, even under copper(I) catalysis conditions. In contrast, as expected, rapid cycloaddition was observed with electron-deficient dimethyl acetylenedicarboxylate (DMAD) as the dipolarophile. Since DMAD and related propargylic esters are excellent Michael acceptors and thus unsuitable for biological applications, we investigated the reactivity of the azido complex towards cycloaddition with derivatives of cyclooctyne (OCT), bicyclo[6.1.0]non-4-yne (BCN), and azadibenzocyclooctyne (ADIBO). While no reaction could be observed in the case of the less strained cyclooctyne OCT, the highly strained cyclooctynes BCN and ADIBO readily reacted with the azido complex, providing the corresponding stable triazolato complexes, which were amenable to purification by conventional silica gel column chromatography. An X-ray crystal structure of an ADIBO cycloadduct was obtained and verified that the formed 1,2,3-triazolato ligand coordinates the metal center through the central N2 atom. Importantly, the determined second-order rate constant for the ADIBO cycloaddition with the azido complex (k2=6.9 × 10(-2) M(-1) s(-1)) is comparable to the rate determined for the ADIBO cycloaddition with organic benzyl azide (k2=4.0 × 10(-1) M(-1) s(-1)). Our results demonstrate that it is possible to transfer the concept of strain-promoted azide-alkyne cycloaddition (SPAAC) from purely organic azides to metal-coordinated azido ligands. The favorable reaction kinetics for the ADIBO-azido-ligand cycloaddition and the well-proven bioorthogonality of strain-activated alkynes should pave the way for applications in living biological systems.

Inorganic click (iClick) synthesis of heterotrinuclear Pt(II)/Au(I)2 complexes

Metal-azide-metal-acetylide cycloaddition (iClick) reactions to synthesize heterotrimetallics and an unexpected novel tetranuclear gold(I) complex, are described. In addition, a discussion regarding the connection between traditional azide-alkyne cycloaddition reactions and iClick is presented focusing on applications towards linking multiple metal ions.