19254-79-6Relevant articles and documents
Combined computational and experimental study of substituent effects on the thermodynamics of H2, CO, arene, and alkane addition to iridium
Krogh-Jespersen, Karsten,Czerw, Margaret,Zhu, Keming,Singh, Bharat,Kanzelberger, Mira,Darji, Nitesh,Achord, Patrick D.,Renkema, Kenton B.,Goldman, Alan S.
, p. 10797 - 10809 (2002)
The thermodynamics of small-molecule (H2, arene, alkane, and CO) addition to pincer-ligated iridium complexes of several different configurations (three-coordinate d8, four-coordinate d8, and five-coordinate d6) have been investigated by computational and experimental means. The substituent para to the iridium (Y) has been varied in complexes containing the (Y-PCP)Ir unit (Y-PCP = ν3-1,3,5-C6H2[CH2 PR2]2Y; R = methyl for computations; R = tert-butyl for experiments); substituent effects have been studied for the addition of H2, C-H, and CO to the complexes (Y-PCP)Ir, (Y-PCP)Ir(CO), and (Y-PCP)Ir(H)2. Para substituents on arenes undergoing C-H bond addition to (PCP)Ir or to (PCP)Ir(CO) have also been varied computationally and experimentally. In general, increasing electron donation by the substituent Y in the 16-electron complexes, (Y-PCP)Ir(CO) or (Y-PCP)Ir(H)2, disfavors addition of H-H or C-H bonds, in contradiction to the idea of such additions being oxidative. Addition of CO to the same 16-electron complexes is also disfavored by increased electron donation from Y. By contrast, addition of H-H and C-H bonds or CO to the three-coordinate parent species (Y-PCP)Ir is favored by increased electron donation. In general, the effects of varying Y are markedly similar for H2, C-H, and CO addition. The trends can be fully rationalized in terms of simple molecular orbital interactions but not in terms of concepts related to oxidation, such as charge-transfer or electronegativity differences.
Synthesis and conformational analysis of 2,11-dioxa[3.3]metacyclophanes
Maeda, Hajime,Endo, Shinsuke,Ouchi, Takashi,Mizuno, Kazuhiko,Segi, Masahito
, p. 1357 - 1360 (2017)
Synthesis and conformational analysis of three members of the dioxa[3.3]metacyclophane family were carried out. Variable-temperature 1H NMR spectroscopy was employed to show that three substances exist as equilibrium mixtures of syn and anti conformers in ratios that depend on the nature of arene ring substituent, temperature, and solvent. The findings are explained in terms of thermodynamic parameters and dipole moments of the metacyclophanes.
Participation of benzene hydrogen bonding upon anion binding
In, Sungjae,Seung, Joo Cho,Kyu, Hwan Lee,Kang, Jongmin
, p. 3993 - 3996 (2005)
(Chemical Equation Presented) A m-xylene-bridged imidazolium receptor, 1, has been designed and synthesized. The receptor 1 utilizes two imidazole (C-H)+...anion hydrogen bonds and one benzene hydrogen...anion hydrogen bond. The major driving force of complexation between the receptor 1 and anions comes from two imidazole (C-H) +...anion hydrogen bonds. However, both NMR experiments and ab initio calculations show that the benzene hydrogen attracts the anion guests, clearly indicating benzene (C-H)...anion hydrogen bonding.
Substituents Have a Large Effect on Photochemical Generation of Benzyl Cations and DNA Cross-Linking
Fan, Heli,Sun, Huabing,Peng, Xiaohua
, p. 7671 - 7682 (2018/05/14)
Photoactivated DNA interstrand cross-linking agents have a wide range of biological applications. Recently, several aryl boronates have been reported to induce DNA interstrand cross-link (ICL) formation via carbocations upon photoirradiation. Herein, we synthesized a series of new bifunctional phenyl compounds to test the generality of such a mechanism, and to understand how the chemical structure influences carbocation formation and the DNA cross-linking process. These compounds efficiently form DNA ICLs via generated benzyl cations upon 350 nm irradiation. The DNA cross-linking efficiency and the pathway for carbocation generation depend on both the aromatic substituents and the leaving groups. Bromine as a leaving group facilitates the DNA cross-linking process in comparison with trimethyl ammonium salt. Both electron-donating and -withdrawing substituents induce bathochromic shifts, which favor photoinduced DNA ICL formation. For the bromides, the benzyl cation intermediates were generated through oxidation of the corresponding benzyl radicals. However, for the ammonia salts, the benzyl cations were formed through two pathways: either through oxidation of the benzyl radicals or by direct heterolysis of the C?N bond. Photoinduced C?N homolysis to form benzyl radicals occurred with compounds having donating substituents, whereas direct heterolysis of the C?N bond occurred with those bearing withdrawing substituents. The adducts formed between 1 a and four natural nucleosides were characterized, indicating that the alkylation sites for the photogenerated benzyl cations are dG, dA, and dC.
Acceleration of CO2 insertion into metal hydrides: Ligand, Lewis acid, and solvent effects on reaction kinetics
Heimann, Jessica E.,Bernskoetter, Wesley H.,Hazari, Nilay,Mayer, James M.
, p. 6629 - 6638 (2018/08/24)
The insertion of CO2 into metal hydrides and the microscopic reverse decarboxylation of metal formates are important elementary steps in catalytic cycles for both CO2 hydrogenation to formic acid and methanol as well as formic acid and methanol dehydrogenation. Here, we use rapid mixing stopped-flow techniques to study the kinetics and mechanism of CO2 insertion into transition metal hydrides. The investigation finds that the most effective method to accelerate the rate of CO2 insertion into a metal hydride can be dependent on the nature of the rate-determining transition state (TS). We demonstrate that for an innersphere CO2 insertion reaction, which is proposed to have a direct interaction between CO2 and the metal in the rate-determining TS, the rate of insertion increases as the ancillary ligand becomes more electron rich or less sterically bulky. There is, however, no rate enhancement from Lewis acids (LA). In comparison, we establish that for an outersphere CO2 insertion, proposed to proceed with no interaction between CO2 and the metal in the rate-determining TS, there is a dramatic LA effect. Furthermore, for both inner- and outersphere reactions, we show that there is a small solvent effect on the rate of CO2 insertion. Solvents that have higher acceptor numbers generally lead to faster CO2 insertion. Our results provide an experimental method to determine the pathway for CO2 insertion and offer guidance for rate enhancement in CO2 reduction catalysis.
Cytotoxic agents comprising new tomaymycin derivatives
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Page/Page column 25, (2008/06/13)
The present invention is related to new tomaymycin derivatives of formula (I), their process of preparation and their therapeutic uses.
Energy transfer labels with mechanically linked fluorophores
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, (2008/06/13)
Mechanically linked energy transfer labels comprising at least one donor fluorophore, at least one acceptor fluorophore, and at least one support member, wherein steric interactions between the donor fluorophore(s), the acceptor fluorophore(s), and/or the support member(s) induce non-covalent association between the fluorophores and the support member(s), thereby forming a three-dimensional macromolecular structure which mechanically links the donor fluorophore(s) and the acceptor fluorophore(s). Fluorescence resonance energy transfer (FRET) occurs from donor fluorophore to acceptor fluorophore through space. No direct connectivity with covalent bonds exists between the fluorophores. Instead, mechanical barriers hold the donor/acceptor fluorophores in place during the FRET process.
Optimization of a synthetic arginine receptor. Systematic tuning of noncovalent interactions
Rensing,Arendt,Springer,Grawe,Schrader
, p. 5814 - 5821 (2007/10/03)
The simple arginine binder 1 could be optimized by strengthening π-cation as well as electrostatic interactions. Electron-donating or -withdrawing substituents in the 5-position provide experimental evidence for π-cation interactions, because binding energies increase by up to 0.6 kcal/mol due to a single benzene-guanidinium interaction. Even more effective is the introduction of a third phosphonate functionality at the correct distance, so that the guanidinium cation is recognized by optimal electrostatic and hydrogen bond interactions. Monte Carlo simulations and NOESY experiments confirm the expected complex geometries. The optimized host molecule 8 binds arginine half an order of magnitude more efficiently than the parent molecule.
Modular Approach to the Accelerated Convergent Growth of Laser Dye-Labeled Poly(aryl ether) Dendrimers Using a Novel Hypermonomer
Gilat, Sylvain L.,Adronov, Alex,Frechet, Jean M. J.
, p. 7474 - 7484 (2007/10/03)
The synthesis of novel dendrimers functionalized with laser dyes both at the periphery and at the core, along with all relevant model compounds necessary for accurate photophysical studies, is described. The utilized synthetic strategy involves a modular approach in which a variety of peripheral and core moieties can be placed on a dendritic structure bearing electrophilic peripheral groups and a nucleophilic core. Specifically, the target macromolecules required functionalization with the laser dyes coumarin 2 (periphery) and coumarin 343 (core) due to the possibility of energy transfer from the former to the latter dye. In addition, the preparation of a novel, highly soluble and reactive hypermonomer utilized in the rapid and efficient synthesis of high-generation dye-labeled dendrimers and model compounds is outlined.
Reversible dioxygen binding and aromatic hydroxylation in O2-reactions with substituted xylyl dinuclear copper(I) complexes: Syntheses and low-temperature kinetic/thermodynamic and spectroscopic investigations of a copper monooxygenase model system
Karlin, Kenneth D.,Nasir, M. Sarwar,Cohen, Brett I.,Cruse, Richard W.,Kaderli, Susan,Zuberbühler, Andreas D.
, p. 1324 - 1336 (2007/10/02)
The binding and subsequent reactivity of dioxygen (O2) upon binding to copper ion centers is of fundamental interest in chemical and biological processes. We provide here a detailed account of the reaction of O2 with dicopper(I) complexes, involving O2-reversible binding, followed by the stoichiometric aromatic hydroxylation of the ligand. Thus, tricoordinated dicopper(I) complexes [Cu2(R-XYL)]2+ (R = H, MeO, t-Bu, F, CN, NO2; 1a-f) possess dinucleating meta-substituted xylylene ligands with two chelating tridentate bis[2-(2-pyridyl)ethyl]amine (PY2) moieties and a 5-R substituent. Upon reaction with O2, dioxygen adducts [Cu2(R-XYL)(O2)]2+ (2a,c-f) form reversibly, and these subsequently yield 2-xylylene-hydroxylated products [Cu2(R-XYL-O-)(OH)]2+ (3a-f), which are phenoxo- and hydroxo-bridged copper(II) complexes. The products 3 have been characterized via the X-ray structure of the parent complex 3a, and by their UV-visible, infrared, and room-temperature magnetic properties. Incorporation of the O-atom from dioxygen into the phenolic products has been proven by isotopic labeling experiments, except in the case of 3f, where workup results in an exchange reaction causing loss of the oxygen label. In reactions of O2 with 1 in dichloromethane at room temperature, 10-25% yields of unhydroxylated complexes [Cu2(R-XYL)(OH)]3+ (5) are obtained. A stopped-flow kinetics study of O2 reactions of 1 in CH2Cl2 demonstrates that [Cu2(R-XYL)(O2)]2+ (2a,c-f) complexes form reversibly, proceeding via the reaction 1 + O2 ? 2 (K1 = k1/k-1); this is followed by the irreversible reaction 2 → 3 (k2). Analysis of temperature-dependent data which is accompanied by spectrophotometric monitoring yields both kinetic and thermodynamic parameters for R = H, t-Bu, F, and NO2. Dioxygen binding to 1 occurs in a single observable step with low activation enthalpies (6-29 kJ mol-1) and large, negative activation entropies (-66 to -167 J K-1 mol-1). The remote R-substituent has a significant effect on the dioxygen-binding process and this is explained in terms of its multistep nature. Strong binding (K1) occurs at low temperature (e.g. -80 °C), and thermodynamic parameters indicate a large enthalpic contribution (ΔH° = -52 to -74 kJ mol-1), but room-temperature stabilities of the dioxygen adducts are precluded by very large unfavorable entropies (ΔS° = -156 to -250 J K-1 mol-1). Electron-releasing R-substituents cause a small but significant enhancement of k2, the hydroxylation step, consistent with a mechanism involving electrophilic attack of the Cu2O2 intermediate 2 upon the xylyl aromatic ring. The influence of substituent upon the various rates of reaction allows for stabilization (～minutes), allowing the bench-top observation of 2d,e,f using UV-visible spectroscopy at -80 °C. "Vacuum-cycling" experiments can be carried out on 1f/2f, i.e., the repetitive oxygenation of 1f at -80 °C, followed by removal of O2 from 2f by application of a vacuum. Dicopper(I) complexes I have been characterized by 1H and 13C NMR spectroscopy, along with analogs in which an ethyl group has been placed in the 5-position of the pyridyl ring donor groups, i.e., [CuI2(R-XYL-(5-Et-PY))]2+ (1g, R = H; 1h, R = NO2). Variable-temperature 1H NMR spectroscopic studies provide clues as to why [Cu2(MeO-XYL)]2+ (1b) does not oxygenate (i.e., bind O2 and/or hydroxylate) at low temperature, the conclusion being that significant interactions of the coordinately unsaturated copper(I) ion(s) with the chelated methoxybenzene group result in conformations unsuitable for O2-reactivity. The biological implications of the biomimetic chemistry described here are discussed, as a system effecting oxidative C-H functionalization using O2 under mild conditions and as a monooxygenase model system for tyrosinase (phenol o-monooxygenase), with its dinuclear active site.
