Mild and room temperature C-C bond forming reactions of nucleoside C-6 arylsulfonates

[reaction: see text] Palladium catalyzed cross coupling of nucleoside arylsulfonates and arylboronic acids has been accomplished under mild conditions and at room temperature. Among three structurally similar ligands that differ in their steric and electronic properties, one yielded an effective catalyst in conjunction with Pd(OAc)2. Of the nucleoside arylsulfonates evaluated, the O6-(2,4,6-trimethylphenyl)sulfonate proved optimal, but other alkyl and alkoxy derivatives were also reasonably reactive. On the other hand, a 2-nitrophenyl and a 2-thienyl derivative were ineffective substrates. PhMe and THP were suitable as solvents, yielding good results in several cases, although reactions of some arylboronic acids were faster in PhMe. In contrast, reactions of arylboronic acids bearing strongly electron-withdrawing groups proceeded more successfully in THP. Interplay between several factors that include substituents on the nucleoside arylsulfonate, ligand substituents, and solvent is responsible for successful cross coupling. Using 31P NMR, an initial investigation has been conducted to study the interaction of Pd(OAc)2 with the ligand. At a 1:1 stoichiometry of ligand and Pd(OAc)2, a predominant species, likely a cyclopalladation product, was obtained. At a 2:1 ratio of ligand and Pd(OAc)2, a different species bearing chemically distinct phosphine ligands was observed. Both complexes display catalytic activity, although the 2:1 species may be superior.     

Silyl-directed, iridium-catalyzed ortho-borylation of arenes. A one-pot ortho-borylation of phenols, arylamines, and alkylarenes

The regioselectivity of the borylation of arenes catalyzed by the combination of 4,4'-di-tert-butylbipyridine (dtbpy) and [Ir(cod)Cl]2 has typically been governed by steric effects. We describe a strategy that makes use of a new substituent for ortho-functionalization to overcome this bias. We show that arenes containing hydrosilyl substituents on an atom attached to the arene ring undergo borylation at the position ortho to the hydrosilyl group. Using iridium-catalyzed formation of silyl ethers and silylamines from silanes and either phenols or arylamines, we have developed the ortho-borylation into a one-pot conversion of free phenols and monoprotected anilines into hydroxy- and amino-substituted organoboron derivatives.     

Turnover numbers, turnover frequencies, and overpotential in molecular catalysis of electrochemical reactions. Cyclic voltammetry and preparative-scale electrolysis

The search for efficient catalysts to face modern energy challenges requires evaluation and comparison through reliable methods. Catalytic current efficiencies may be the combination of many factors besides the intrinsic chemical properties of the catalyst. Defining turnover number and turnover frequency (TOF) as reflecting these intrinsic chemical properties, it is shown that catalysts are not characterized by their TOF and their overpotential (η) as separate parameters but rather that the parameters are linked together by a definite relationship. The log TOF-η relationship can often be linearized, giving rise to a Tafel law, which allows the characterization of the catalyst by the value of the TOF at zero overpotential (TOF(0)). Foot-of-the-wave analysis of the cyclic voltammetric catalytic responses allows the determination of the TOF, log TOF-η relationship, and TOF(0), regardless of the side-phenomena that interfere at high current densities, preventing the expected catalytic current plateau from being reached. Strategies for optimized preparative-scale electrolyses may then be devised on these bases. The validity of this methodology is established on theoretical grounds and checked experimentally with examples taken from the catalytic reduction of CO(2) by iron(0) porphyrins.     

Room temperature borylation of arenes and heteroarenes using stoichiometric amounts of pinacolborane catalyzed by iridium complexes in an inert solvent

Aromatic C-H borylation of arenes and heteroarenes using stoichiometric amounts of pinacolborane was catalyzed by an iridium complex generated from 1/2[Ir(OMe)(COD)]2 and 4,4'-di-tert-butyl-2,2'-bipyridine at room temperature in hexane and afforded the corresponding aryl- and heteroarylboronates in high yields with excellent regioselectivities.     

Free energy and dynamics of electron-transfer reactions in a room temperature ionic liquid

Reaction free energetics and dynamics of unimolecular electron-transfer processes in ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI+PF6-) are investigated via molecular dynamics computer simulations employing a model diatomic solute and compared with those in aprotic acetonitrile. Using the free energy perturbation method, diabatic free energy curves relevant to charge separation and recombination processes are studied over a wide range of the reaction coordinate. The diabatic curves are found to vary with the solute charge distribution, especially in EMI+PF6-. Nevertheless, if the free energy of reaction is not that substantial, the Marcus free energy relationship holds reasonably well, provided that the reorganization free energy averaged between the reactant and product states is employed. The effective polarity, measured as solvation-induced stabilization of dipolar solutes, is higher for EMI+PF6- than for acetonitrile, consonant with many solvatochromic measurements. Thus, in the normal regime, activation barriers for charge separation and recombination reactions are, respectively, lower and higher in EMI+PF6- than in acetonitrile. The influence of solvent dynamics on reaction kinetics through modulations of activation, deactivation, and barrier crossing is analyzed. Even though overall solvent relaxation dynamics in EMI+PF6- are considerably slower than those in acetonitrile, the deviation of the rate constant from the transition state theory predictions is found to be small for both solvents. Implications of this finding for other reactions in ionic liquids are briefly discussed.     

Recyclable fluorous organocatalysts promoted three-component reactions of pyruvate,aldehyde and amine at room temperature

A new fluorous imine carbothioate has been prepared as an organocatalyst for the synthesis of pyrrol-2-ones via the cyclo-condensation reaction of aldehydes, amines, and pyruvate at room temperature. The fluorous catalyst can be easily recovered from the reaction mixture by simple fluorous solid-phase extraction (F-SPE) and used for next run reaction without further purification.     

Direct aminoalkylation of arenes, heteroarenes, and alkenes via Ni-catalyzed Negishi cross-coupling reactions

A room-temperature Ni-catalyzed cross-coupling of aryl, heteroaryl, and alkenyl electrophiles with aminoalkylzinc bromides, readily available from the corresponding aminoalkyl chlorides via Grignard reagents, was developed. The reaction allows a convenient one-step preparation of various aminoalkyl products, including piperidine and tropane derivatives. Such functionalized amine moieties are widely present in various biologically active molecules. Aryl, heteroaryl, and alkenyl iodides, bromides, chlorides and triflates are suitable electrophiles. A short total synthesis of two natural products, (±)-galipinine and (±)-cusparine, is also reported.     

Hamiltonian dynamics of chemical reactions. Consecutive first-order reactions and reactions possessing one autocatalytic intermediate

A recently proposed Hamiltonian approach to phenomenological chemical kinetics [T. Georgian and G.L. Findley, Int. J. Quantum Chem., Quantum Biol. Symp. 10 , 331 (1983); T. Georgian, J.M. Halpin, and G.L. Findley, Int. J. Quantum Chem., Quantum Biol. Symp. 11 , 347 (1984)] is applied to all consecutive first-order, single-step reactions, and to all reactions possessing one autocatalytic intermediate. The reaction Hamiltonians presented are shown to be consistent with the phenomenological rate equations and the relationship between reaction form and the form of the reaction potential is discussed. In particular, we show: (1) that the interaction between consecutive reactions manifests itself as a coupling term in the reaction potential, a term which may be eliminated via transition to “normal reaction coordinates” for the chemical system; and (2) that coupled sets of autocatalytic reactions give rise to coupling terms in the reaction Hamiltonian which are characteristic of the reaction mechanism.     

Sequential C-F activation and borylation of fluoropyridines via intermediate Rh(I) fluoropyridyl complexes: a multinuclear NMR investigation

The C-F bond activation of fluoropyridines by [Rh(SiPh3)(PMe3)3] afforded Rh(I) fluoropyridyl complexes of the type [Rh(Ar(F))(PMe3)3] with concomitant formation of fluorotriphenylsilane; subsequent treatment with bis-catecholatodiboron yielded fac-[Rh(Bcat)3(PMe3)3] and the free fluoropyridyl boronate esters (Ar(F)Bcat).     

Reductive activation of arenes 22. Reactions of the terephthalonitrile radical anion and dianion with α,ω-dibromoalkanes. New evidence for the charge transfer complex as a key intermediate in the reactions of the dianion

The major products of reactions of the terephthalonitrile radical anion with α,ω-dibromoalkanes Br(CH₂)_nBr (n = 3–5) were 4-(ω-bromoalkyl)benzonitriles. Analogous reactions of the terephthalonitrile dianion mainly yielded α,ω-bis(4-cyanophenyl)alkanes. Both transformations are convenient one-step routes to otherwise not easily accessible compounds that are valuable as versatile building blocks. The results of alkylation allow one to suggest that reactions of the dianion with intermediate 4-(ω-bromoalkyl)benzonitriles proceed more rapidly than those with the starting α,ω-dibromoalkanes. This was confirmed by competitive reactions of the dianion with 4-(ω-bromoalkyl)benzonitriles and the corresponding alkyl bromides. To explain such a ratio of the reaction rates, a mechanism was proposed for the reaction of the dianion with 4-(ω-bromoalkyl)benzonitriles. According to this mechanism, a charge transfer complex is a key reaction intermediate. Dedicated to the memory of Academician N. N. Vorozhtsov on the 100th anniversary of his birth. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1069–1077, June, 2007.     

Early reactions of light-induced protochlorophyllide and chlorophyllide transformations analyzed in vivo at room temperature with a diode array spectrofluorometer

The steps of protochlorophyllide (Pchlide) photoreduction and subsequent chlorophyllide (Chlide) transformations which occur in the seconds to minutes time-scale were studied using a diode array spectrofluorometer in dark-grown barley leaves. The intensity of the excitation light was varied between 3 and 2,500 micromol m(-2) s(-1) and a series of fluorescence spectra were recorded at room temperature in the seconds and minutes time scales. In certain experiments, 77-K emission spectra were measured with the same equipment. The high quality of the spectra allowed us to run spectral resolution studies which proved the occurrence, at room temperature, of multiple Pchlide and Chlide forms found previously in 77-K spectra. The comparison of the 77-K and room-temperature spectra showed that the fluorescence yields of the nonphotoactive 633-nm Pchlide form and of the Chlide product emitting at 678 nm were temperature independent. The fluorescence intensity of aggregated NADPH-pigment-POR complexes (photoactive 656-nm Pchlide and 693-nm Chlide forms) were strongly increased at 77 K, while that of the NADP(+)-Chlide-POR (684-686-nm Chlide form) was much less affected by temperature. Information was obtained also about the dynamics of the transformation of pigment forms in the light at different photon densities. At low light intensities, the phototransformation of the 642-644-nm Pchlide form was faster than that of the 654-656-nm form. The relative amplitudes of Gaussian components related to different Chlide forms found after exposure to a constant amount of photons strongly depended on the light intensity used. Strong quenching of all Chlide components occurred upon prolonged exposure to high intensity light. These effects are discussed by considering the interconversion processes between different forms of the pigment-protein complexes, their relative fluorescence yields and energy migration processes.     

Kinetics, selectivity, and mechanism of the reactions of arenes with adamantyl carbocations in sulfuric acid

Data on the kinetics, selectivity, kinetic isotope effect, and the effect of the acidity of the medium on the rate of the reactions of benzene and alkylbenzenes in sulfuric acid (59–78 wt.% H₂SO₄) solutions of 1-adamantanol at 30 °C indicate that the direct reagents are the adamantyl carbocations (Ad⁺) that alkylate the arenes. The ortho positions of the benzene ring are not accessible on account of steric hindrances. The rate of attack by the Ad⁺ cation on the accessible para and meta positions of the ring is controlled by the formation of a σ complex. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 1, pp. 14–18, January–February, 2006.     

The development of room temperature phosphorescence into a new technique for chemical determinations: Part 2. Analytical considerations of room temperature phosphorimetry

The rapidly developing technique of room temperature phosphorimetry is discussed from a practical analytical standpoint in this second part of the review. Basic factors concerning the technique such as methods of sample preparation, special instrumentation employed, and quantitative capability are presented, together with a listing of the variety of organic compounds reported to display room temperature phosphorescence. Potential applications of room temperature phosphorimetry and the advantages of sensitivity and selectivity afforded by this technique are discussed.     

High temperature reactions of an aryl-alkyl phosphine, an exceptionally efficient melt stabiliser for polyethylene

Bis(diphenylphosphino)-2,2-dimethylpropane (PMP) is a highly efficient melt stabiliser of polyethylene. This aryl-alkyl phosphine hinders the degradation of the polymer during processing even in small concentrations and in combination with a phenolic antioxidant its consumption rate is considerably slower than that of phosphites and phosphonites. In this study the reactions of PMP were studied at temperatures corresponding to those used for the processing of polyethylene in order to explore the processing stabilisation mechanism of this additive. Thermal and thermo-oxidative stability were determined by DSC and TGA, respectively by heating PMP in argon and oxygen at 200 and 240 °C. Reactions with peroxy, carbon-centred and oxy radicals, as well as with hydroperoxide were investigated at 200 °C. Reaction products were identified by FT-IR and solution-state NMR spectroscopy. The results revealed that the phosphine studied has sufficient thermal- and thermo-oxidative stability under the processing conditions of polyethylene. It oxidises easily with any oxidising agent including molecular oxygen of air. Consequently, PMP does not only decompose hydroperoxide groups and react with oxy macroradicals during the processing of polyethylene, as claimed by most references on phosphorous antioxidants, but it can also hinder the formation of peroxy macroradicals, i.e., the initiation reaction of thermo-oxidative degradation.     

High temperature study of the reactions of O and OH with NH3

Mixtures of NH₃ and N₂O dilute in Ar were heated behind incident shock waves in the temperature range 1750–2060 K. A cw ring dye laser, tuned to the center of an OH absorption line in the ultraviolet, was used to monitor OH concentration profiles by absorption spectroscopy. Infrared emission was used to follow N₂O (at 4.5 μm) and NH₃ (at 10.5 μm) concentration—time histories. The early-time NH₃ and OH concentration profiles were sensitive to the rate constants of the reactions leading to the following best-fit expressions for k₂ and k₃:k₂ = 10^13.34±0.3 exp(?4470/T) and k₃ = 10^13.91±0.2 exp(-4230/T) cm³ mol^?1 s^?1. The results of this study combined with previous low-temperature data suggest a significant non-Arrhenius behavior for both k₂ and k₃.     

High catalytic activity of a new Ag phosphorus ylide complex supported on montmorillonite: synthesis,characterization, and application for room temperature nitro reduction

In this work, the phosphorus ylide, [PPh₃CHC(O)CH₂Cl], was reacted with AgNO₃ to give the [Ag{C(H)PPh₃C(O)CH₂Cl}₂]⁺NO₃ ^? as the product. Then, it was supported on the modified montmorillonite nanoclay to prepare a new catalyst for the reduction reaction. The structure and morphology of the nanoclay catalyst were characterized by FT-IR, X-ray powder diffraction, scanning electron microscopy, energy-dispersive X-ray analysis and transmission electron microscopy techniques; also, the content of silver was obtained by inductively coupled plasma analyzer. This composition was exploited to study its catalytic activity in the reduction in aromatic nitro compounds; it displayed the high catalytic activity. Factors such as catalyst amount, solvent, temperature and reaction time were all systematically investigated to elucidate their effects on the yield of catalytic reduction in nitroarenes. This catalytic system exhibited high activity toward aromatic nitro compounds under mild conditions. The catalyst was reused five times without any significant loss in its catalytic activity.