Reversible Photothermal Isomerization of Carborane-Fused Azaborole to Borirane: Synthesis and Reactivity of Carbene-Stabilized Carborane-Fused Borirane

A fully reversible photothermal isomerization between carborane-fused trigonal-planar azaborole (dark-purple) and tetrahedral borirane (pale-yellow) has been observed, leading to the isolation and structural characterization of the first example of carborane-fused borirane. DFT calculations indicate that the azaborole is thermodynamically more stable than the borirane by 11.2 kcal mol⁻¹, and the energy barrier for the thermal conversion from azaborole to borirane is 35.5 kcal mol⁻¹. The reactivity studies show that the B−C(cage) bond in borirane can be broken in the reaction with CuCl, HCl, or elemental sulfur.     

Tandem [4 + 2]/[2 + 2] cycloaddition of o-carboryne with enynes: facile construction of carborane-fused tricyclics

o-Carboryne (1,2-dehydro-o-carborane) is a very useful synthon for the synthesis of a variety of carborane-functionalized molecules. With 1-Li-2-OTf-o-C₂B₁₀H₁₀ as the precursor, o-carboryne undergoes an efficient [4 + 2] cycloaddition with various conjugated enynes, followed by a subsequent [2 + 2] cycloaddition at room temperature, generating a series of carborane-fused tricyclo[6.4.0.0^2,7]dodeca-2,12-dienes in moderate to high isolated yields. This reaction is compatible with many functional groups and has a broad substrate scope. A reactive carborane-fused 1,2-cyclohexadiene intermediate is involved, which is supported by experimental results and DFT calculations. This protocol offers a convenient strategy for the construction of complex carborane-functionalized tricyclics.

An unprecedented tandem [4 + 2]/[2 + 2] cycloaddition of o-carboryne with enynes has been disclosed for the efficient synthesis of various carborane-fused tricyclics, in which a reactive carborane-fused 1,2-cyclohexadiene intermediate is involved.     

Functional monomers and polymers. LIX. Properties and function of Cu(II) complexes of the copolymers having pendant sulfide and imidazolyl groups

The Cu(II) complexes of copolymers having pendant sulfide and imidazolyl groups were prepared by a free radical copolymerization of ethylvinylsulfide with vinylimidazole, and their properties and function were studied spectrophotometrically in comparison with those of poly[4(5)-vinylimidazole]. The complexes were found to be effective as catalysts for the oxidation of hydroquinone. Visible and ESR spectra of the Cu(II)-copolymer complexes were similar to those of the Cu(II)-homopolymer complexes, while the catalytic activity for the oxidation was different between these complex systems. A rapid reaction followed by a slow reaction, particularly at high ethylvinylsulfide content in the copolymers, was observed in the Cu(II)-copolymer complex systems, but a continuous reaction proportional to the reaction time was observed in the Cu(II)-homopolymer complex systems. The reoxidation rate of Cu(I) to Cu(II) complex, which was little affected by the concentration of imidazolyl group, decreased with a rise of the ethylvinylsulfide content in the copolymer. It was suggested that the sulfur atom of the sulfide group was weakly coordinated to Cu(II) but strongly to Cu(I), and an electron transfer reaction from substrate to the Cu(II) complex was increased, while reoxidation reaction of the Cu(I) complex was decreased in the copolymer complex systems.     

Oxygen activation by the noncoupled binuclear copper site in peptidylglycine alpha-hydroxylating monooxygenase. Reaction mechanism and role of the noncoupled nature of the active site

Reaction thermodynamics and potential energy surfaces are calculated using density functional methods to investigate possible reactive Cu/O(2) species for H-atom abstraction in peptidylglycine alpha-hydroxylating monooxygenase (PHM), which has a noncoupled binuclear Cu active site. Two possible mononuclear Cu/O(2) species have been evaluated, the 2-electron reduced Cu(II)(M)-OOH intermediate and the 1-electron reduced side-on Cu(II)(M)-superoxo intermediate, which could form with comparable thermodynamics at the catalytic Cu(M) site. The substrate H-atom abstraction reaction by the Cu(II)(M)-OOH intermediate is found to be thermodynamically accessible due to the contribution of the methionine ligand, but with a high activation barrier ( approximately 37 kcal/mol, at a 3.0-A active site/substrate distance), arguing against the Cu(II)(M)-OOH species as the reactive Cu/O(2) intermediate in PHM. In contrast, H-atom abstraction from substrate by the side-on Cu(II)(M)-superoxo intermediate is a nearly isoenergetic process with a low reaction barrier at a comparable active site/substrate distance ( approximately 14 kcal/mol), suggesting that side-on Cu(II)(M)-superoxo is the reactive species in PHM. The differential reactivities of the Cu(II)(M)-OOH and Cu(II)(M)-superoxo species correlate to their different frontier molecular orbitals involved in the H-atom abstraction reaction. After the H-atom abstraction, a reasonable pathway for substrate hydroxylation involves a "water-assisted" direct OH transfer to the substrate radical, which generates a high-energy Cu(II)(M)-oxyl species. This provides the necessary driving force for intramolecular electron transfer from the Cu(H) site to complete the reaction in PHM. The differential reactivity pattern between the Cu(II)(M)-OOH and Cu(II)(M)-superoxo intermediates provides insight into the role of the noncoupled nature of PHM and dopamine beta-monooxygenase active sites, as compared to the coupled binuclear Cu active sites in hemocyanin, tyrosinase, and catechol oxidase, in O(2) activation.     

Coordination-synchronized trans-cis photoisomerization of bipyridylazobenzene driven by a Cu(II)/Cu(I) redox change

The trans-cis photoisomerization behavior of azobenzene-bipyridine ligand (dmpAB) was synchronized with coordination of the bipyridine moiety to copper. The coordination reaction can be reversibly controlled with reversible redox reaction of copper, to afford [Cu(dmpAB)(2)](+) in Cu(I) state and free dmpAB in Cu(II) state. UV irradiations to Cu(I) and Cu(II) samples form trans-rich and cis-rich compositions, respectively. The results enable us to control the trans-cis isomerization reversibly through Cu(II)/Cu(I) redox and a single UV light.     

Catalytic signal amplification using a Heck reaction. An example in the fluorescence sensing of CuII

Catalytic signal enhancement using an organometallic reaction is demonstrated. The reactivity of a Heck cross-coupling reaction that creates a fluorophore is modulated by the addition of a polyazacyclam inhibitor. The inhibitor will complex with Cu(II), which restores the activity of the Pd(II). The addition of Cu(II) therefore leads to the generation of fluorescence, thereby creating a very sensitive assay for Cu(II). The rate of the Heck reaction is followed by monitoring emission as a function of time. The rate is proportional to the Cu(II) concentration and correlates to the affinity of the inhibitor to various metals. This strategy represents a general technique that can be exploited with other catalytic organometallic reactions.     

Oxidations by copper metalloenzymes and some biomimetic approaches

This paper illustrates the various aspects of the reactivity of the Cu(II)–Cu(I) system in biological systems, with one example of an enzymatic reaction in which Cu(II) alone is oxidizing enough to carry out the reaction (superoxide dismutase), one example in which a Cu(II)-bound peroxo intermediate is the active species (tyrosinase) and the examples of galactose oxidase and copper amine oxidases in which Cu(II) is associated with a redox active organic cofactor. In some cases, we will show some illustrations of biomimetic approaches developed in our laboratories, aimed at a better understanding of reaction mechanisms and at an original design of new catalysts with potential applications in synthetic chemistry. Some comments are given concerning the respective features of copper and iron.     

Coordination and metalation bifunctionality of Cu with 5,10,15,20-tetra(4-pyridyl)porphyrin: toward a mixed-valence two-dimensional coordination network

We investigated the coordination self-assembly and metalation reaction of Cu with 5,10,15,20-tetra(4-pyridyl)porphyrin (2HTPyP) on a Au(111) surface by means of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. 2HTPyP was found to interact with Cu through both the peripheral pyridyl groups and the porphyrin core. Pairs of pyridyl groups from neighboring molecules coordinate Cu(0) atoms, which leads to the formation of a supramolecular metal-organic coordination network. The network formation occurs at room temperature; annealing at 450 K enhances the process. The interaction of Cu with the porphyrin core is more complex. At room temperature, formation of an initial complex Cu(0)-2HTPyP is observed. Annealing at 450 K activates an intramolecular redox reaction, by which the coordinated Cu(0) is oxidized to Cu(II) and the complex Cu(II)TPyP is formed. The coordination network consists then of Cu(II) complexes linked by Cu(0) atoms; that is, it represents a mixed-valence two-dimensional coordination network consisting of an ordered array of Cu(II) and Cu(0) centers. Above 520 K, the network degrades and the Cu atoms in the linking positions diffuse into the substrate, while the Cu(II)TPyP complexes form a close-packed structure that is stabilized by weak intermolecular interactions. Density functional theory investigations show that the reaction with Cu(0) proceeds via formation of an initial complex between metal atom and porphyrin followed by formation of Cu(II) porphyrin within the course of the reaction. The activation barrier of the rate limiting step was found to be 24-37 kcal mol(-1) depending on the method used. In addition, linear coordination of a Cu atom by two CuTPyP molecules is favorable according to gas-phase calculations.     

Metal-catalyzed B–H acylmethylation of pyridylcarboranes: access to carborane-fused indoliziniums and quinoliziniums

Metal-catalyzed mono-acylmethylation of pyridylcarboranes has been realized using α-carbonyl sulfoxonium ylides as a coupling partner. The reaction features high efficiency, excellent site-selectivity and good functional group tolerance. In the presence of pyridyl and enolizable acylmethyl groups, a post-coordination mode has been proposed and validated by in situ high resolution mass spectroscopy (HRMS) to rationalize the unique mono-substitution. Post-functionalization at the newly incorporated alkyl site provides additional utility of this method, including the construction of carborane-fused indoliziniums and quinoliziniums. We believe that these mono-alkylated carboranes, together with their post-functionalized derivatives, may find applications in luminescent materials and drug discovery in the near future.

Metal-catalyzed selective mono-acylmethylation of pyridylcarboranes has been realized, which provides further utility to construct carborane-fused indoliziniums and quinoliziniums.     

Cu(II) catalyzed reaction between phenyl hydrazine and toluidine blue—dual role of acid

The detailed kinetics of Cu(II) catalyzed reduction of toluidine blue (TB⁺) by phenyl hydrazine (Pz) in aqueous solution is studied. Toluidine white (TBH) and the diazonium ions are the main products of the reaction. The diazonium ion further decomposes to phenol (PhOH) and nitrogen. At low concentrations of acid, H⁺ ion autocatalyzes the uncatalyzed reaction and hampers the Cu(II) catalyzed reaction. At high concentrations, H⁺ hinders both the uncatalyzed and Cu(II) catalyzed reactions. Cu(II) catalyzed had stoichiometry similar to the uncatalyzed reaction, Pz+2 TB⁺+H₂O=PhOH+2 TBH+2 H⁺+N₂. Cu(II) catalyzed reaction occurs possibly through ternary complex formation between the unprotonated toluidine blue and phenyl hydrazine and catalyst. The rate coefficient for the Cu(II) catalyzed reaction is 2.1×10⁴ M⁻² s⁻¹. A detailed 13‐step mechanistic scheme for the Cu(II) catalyzed reaction is proposed, which is supported by simulations. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 271–276, 1999     

Electron transfer reactions between copper(II) porphyrin complexes and various oxidizing reagents in acetonitrile

Homogeneous electron transfer reactions of the Cu(II) complexes of 5,10,15,20-tetraphenylporphyrin (TPP) and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) with various oxidizing reagents were spectrophotometrically investigated in acetonitrile. The reaction products were confirmed to be the pi-cation radicals of the corresponding Cu(II)-porphyrin complexes on the basis of the electronic spectra and the redox potentials of the complexes. The rate of the electron transfer reaction between the Cu(II)-porphyrin complex and solvated Cu(2+) was determined as a function of the water concentration under the pseudo first-order conditions where Cu(2+) is in large excess over the Cu(II)-porphyrin complex. The decrease in the pseudo first-order rate constant with increasing the water concentration was attributed to the stepwise displacement of acetonitrile in [Cu(AN)(6)](2+)(AN = acetonitrile) by water, and it was concluded that only the Cu(2+) species fully solvated by acetonitrile, [Cu(AN)(6)](2+), possesses sufficiently high redox potential for the oxidation of Cu(ii)-OEP and Cu(ii)-TPP. The reactions of the Cu(II)-porphyrin complexes with other oxidizing reagents such as [Ni(tacn)(2)](3+)(tacn = 1,4,7-triazacyclononane) and [Ru(bpy)(3)](3+)(bpy = 2,2'-bipyridine) were too fast to be followed by a conventional stopped-flow technique. Marcus cross relation for the outer-sphere electron transfer reaction was used to estimate the rate constants of the electron self-exchange reaction between Cu(II)-porphyrin and its pi-cation radical: log(k/M(-1) s(-1))= 9.5 +/- 0.5 for TPP and log(k/M(-1) s(-1))= 11.1 +/- 0.5 for OEP at 25.0 degrees C. Such large electron self-exchange rate constants are typical for the porphyrin-centered redox reactions for which very small inner- and outer-sphere reorganization energies are required.     

The mechanism of the modified Ullmann reaction

The copper-mediated aromatic nucleophilic substitution reactions developed by Fritz Ullmann and Irma Goldberg required stoichiometric amounts of copper and very high reaction temperatures. Recently, it was found that addition of relatively cheap ligands (diamines, aminoalcohols, diketones, diols) made these reactions truly catalytic, with catalyst amounts as low as 1 mol% or even lower. Since these catalysts are homogeneous, it has opened up the possibility to investigate the mechanism of these modified Ullmann reactions. Most authors agree that Cu(I) is the true catalyst even though Cu(0) and Cu(II) catalysts have also shown to be active. It should be noted however that Cu(I) is capable of reversible disproportionation into Cu(0) and Cu(II). In the first step, the nucleophile displaces the halide in the LnCu(I)X complex forming LnCu(I)ZR (Z = O, NR′, S). Quite a number of mechanisms have been proposed for the actual reaction of this complex with the aryl halide: 1. Oxidative addition of ArX forming a Cu(III) intermediate followed by reductive elimination; 2. Sigma bond metathesis; in this mechanism copper remains in the Cu(II) oxidation state; 3. Single electron transfer (SET) in which a radical anion of the aryl halide is formed (Cu(I)/Cu(II)); 4. Iodine atom transfer (IAT) to give the aryl radical (Cu(I)/Cu(II)); 5. π-complexation of the aryl halide with the Cu(I) complex, which is thought to enable the nucleophilic substitution reaction. Initially, the radical type mechanisms 3 and 4 where discounted based on the fact that radical clock-type experiments with ortho-allyl aryl halides failed to give the cyclised products. However, a recent DFT study by Houk, Buchwald and co-workers shows that the modified Ullmann reaction between aryl iodide and amines or primary alcohols proceeds either via an SET or an IAT mechanism. Van Koten has shown that stalled aminations can be rejuvenated by the addition of Cu(0), which serves to reduce the formed Cu(II) to Cu(I); this also corroborates a Cu(I)/Cu(II) mechanism. Thus the use of radical clock type experiments in these metal catalysed reactions is not reliable. DFT calculations from Hartwig seem to confirm a Cu(I)/Cu(III) type mechanism for the amidation (Goldberg) reaction, although not all possible mechanisms were calculated.     

Catalytic determination of silver(I) by extractive flow injection analysis

A new catalytic method for the determination of silver(I) was developed based on a metal exchange reaction between ethylenediaminetetraacetatomercury(II) (Hg(II)-EDTA) in the aqueous phase and bis(diethyldithiocarbamato)copper(II) (Cu(II)-DDTC) in the organic phase. This exchange reaction (Cu(II)-DDTC(org)+Hg(II)-EDTA-->Hg(II)-DDTC(org)+Cu(II)-EDTA, where org denotes the organic phase) was observed to proceed slowly and the Cu(II)-DDTC complex transferred quantitatively to Hg(II)-complex in the organic phase in the equilibrium state. In this system, silver(I) acts as the catalyst and can be determined by measuring the decrease in the absorbance of the Cu(II)-DDTC complex (lambda(max)=435 nm). The reaction was applied to the extractive flow injection analysis of silver(I). The present method allows the determination of silver(I) at 10(-7) mol dm(-3) level with the sampling frequency of 30 h(-1). The relative standard deviation of 0.28% (n=10) was obtained at 4.0x10(-7) mol dm(-3) of silver(I).     

Intervalence charge-transfer system by 1D assembly of new mixed-valence octanuclear CuI/CuII/CuIII cluster units

The reaction of Cu(Hm-dtc)(2), Br(2), and CuBr(2) yielded a new mixed-valence octanuclear Cu(I)/Cu(II)/Cu(III) cluster, encapsulating a Br anion in the center of the cluster cage. The octanuclear cluster units form a 1D assemblage, which induces intervalence charge-transfer transitions from Cu(II) ions to Cu(III) ions between the clusters.     

Copper(II)-Catalyzed Decomposition of Hydrogen Peroxide: Catalyst Activation by Halide Ions

Summary.  The catalytic action of Cu(II) on the decomposition of H₂O₂ in near-neutrality aqueous solutions is activated by halide ions. The activation energies amount to 113±7 (parent reaction) and 69.9±1.4 (chloride-activated reaction) kJ · mol⁻¹. Free-radical chain mechanisms are proposed for both the parent reaction and the halide-activated reaction. The catalyst activation caused by halide ions is explained in terms of coordination of halide ligands by both Cu(II) and Cu(I), the coordination causing a higher stabilization of Cu(I) than of Cu(II). At low concentrations, Br⁻ causes an inhibition of the Cu(II)/H₂O₂ reaction. This is explained in terms of an increase of the rate of termination of the chain reaction due to the scavenging effect of OH radicals caused by Br⁻. Received March 27, 2001. Accepted (revised) May 25, 2001.     

银配合物与联氨定向反应的研究I

本文研究了银的不同配合物与N2H4的反应, 得出微量的Cu^2^+不仅能加快反应速度, 能有效地促进N2H4按反应(4)进行四电子定向反应的比率, 而且N2H4按四电子定向的反应率随银配合物稳4Ag^+(AgL^q^±2)+N2H4 4Ag+N2+4H^+(8L^{q±(-1)]/2) (4)Ag^+(AgL^q^±2)+N2H4 Ag+1/2N2+NH3+H^+(+2L[q±(-1)]/2) (5)定常数的增大而降低的结论。无Cu^2^+时, N2H4的四电子反应率与银配合物的logβ2成线性关系; Cu^2^+存在时, N2H4单电子反应率的对数与1/logβ2呈线性关系。     

Copper(II) determination by means of a thermometric method based on its catalytic effect on the hydrazine-hydrogen peroxide reaction

A new catalytic-thermometric method for Cu(II) determination at ppb levels has been established based on the hydrazine-hydrogen peroxide redox reaction. The reaction rate is obtained from the temperature-time curve and shows two linear response zones, between 15–200 ppb and 0.1–0.9 ppm, with a relative standard deviation of 2.0% and 2.2%, respectively. Only 20 ppm of Pb(II) and Fe(III), 1 ppm of Mn(II) and 5 ppb of Co(II) interfere. Interferences of Pb(II) and Fe(III) can be eliminated by the use of maskings. The proposed method can be applied to determine Cu(II) in several samples. In the present paper, this method has been applied to determine Cu(II) in wine.     

Reaction of a copper(II)-nitrosyl complex with hydrogen peroxide: putative formation of a copper(I)-peroxynitrite intermediate

The reaction of a Cu(II)-nitrosyl complex (1) with hydrogen peroxide at -20 °C in acetonitrile results in the formation of the corresponding Cu(I)-peroxynitrite intermediate. The reduction of the Cu(II) center was monitored by UV-visible spectroscopic studies. Formation of the peroxynitrite intermediate has been confirmed by its characteristic phenol ring nitration reaction as well as isolation of corresponding Cu(I)-nitrate (2). On air oxidation, 2 resulted in the corresponding Cu(II)-nitrate (3). Thus, these results demonstrate a possible decomposition pathway for H(2)O(2) and NO through the formation of a peroxynitrite intermediate in biological systems.     

The rotating ring-disk electrochemistry of the copper(II) complex of thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) forms an electroactive Cu(II) complex in aqueous solution. Rotating ring-disk electrochemistry reveals oxidation at the disk electrode and reduction at the ring electrode. The plot of limiting current vs. square root of rotation frequency deviates from the Levich equation, indicating both preceding and following chemical reactions. The reaction following the oxidation is a multiple-electron ECE-type of process that has been seen before in Cu(II)–peptide electrochemistry. The preceding reaction is unusual. The deviation from diffusion-controlled behavior is more pronounced at higher initial concentration of Cu(II) and peptide. We propose that a non-electroactive dimer, Cu(II)₂–TRH₂, is in a slow equilibrium with the electroactive Cu(II)–TRH. Simulation of the RRDE behavior of the postulated Cu(II)–TRH system has succeeded in matching experimental data. Capillary electrophoresis indicates that there is a negative charge on the dimer. It is suggested that a hydroxo-bridge may link the two Cu(II) centers. Calculations verify that bi-nuclear Cu(II)₂–TRH₂ complexes are possible.     

An unusual dimeric structure of a Cu(I) bis(thiosemicarbazone) complex: implications for the mechanism of hypoxic selectivity of the Cu(II) derivatives

The electrochemical reduction of Cu(II) bis(thiosemicarbazone) complexes [Cu(II)(btsc)] is accompanied by protonation to give an unstable Cu(I) intermediate [Cu(btscH(2))](+). The nature of this intermediate was probed by reaction of bis(thiosemicarbazone) ligands with a Cu(I) precursor which gave a novel helical dimeric dicationic complex. The dependence of these reactions on the ligand backbone substituents is discussed together with their possible relevance to the use of Cu(II) bis(thiosemicarbazone) ligands as hypoxic selective imaging and therapeutic agents.