Transient species of fullerenes studied with pulse radiolysis and laser photolysis

The excited triplet of C60 or C70 is generated via either direct excitation by laser light or energy transfer from excited states of solvent to C60 and C70. The cation radical of C60 is produced either via hole transfer from cation radical of CCl4 to C60 or via electron transfer from excited triplet of C60 to CCl4. C60 and C70 could be added to trichloromethyl radical to produce adduct radicals with different mechanisms.     

Reactions of trichloromethyl radicals with organosilicon compounds

The photolysis of carbon tetrachloride in the presence of a number of organosilicon compounds has been investigated in the gas phase. The products obtained from the photolysis experiments were those expected from a chain reaction in which trichloromethyl radicals abstract hydrogen atoms from the organosilane. Arrhenius parameters for hydrogen atom transfer were determined relative to those for trichloromethyl radical combination. The activation energies for the reaction of methyl, trifluoromethyl, and trichloromethyl radicals with organosilicon compounds are compared and the results rationalized in terms of polar effects.     

Ortho effects in organic molecules on electron-impact II– Ortho effects in substituted α,α,α-trichloracetanilides

Elimination of trichloromethyl radical from the molecular ion is the abundant fragment in the mass spectra of most of the thirteen substituted trichloracetanilides studied. Unusual ortho effects of ? OCH₃, ? NO₂ and ? COOH groups are noticed only after the initial ejection of trichloromethyl radical from the molecular ion.     

Palladium‐Catalyzed Atom‐Transfer Radical Cyclization at Remote Unactivated C(sp3)−H Sites: Hydrogen‐Atom Transfer of Hybrid Vinyl Palladium Radical Intermediates

A novel mild, visible‐light‐induced palladium‐catalyzed hydrogen atom translocation/atom‐transfer radical cyclization (HAT/ATRC) cascade has been developed. This protocol involves a 1,5‐HAT process of previously unknown hybrid vinyl palladium radical intermediates, thus leading to iodomethyl carbo‐ and heterocyclic structures.     

Preparation,structure, and reactivity of 1,3,4,6-tetrakis-(isopropylthio)thieno[3,4-c]thiophene-palladium complex with tetracyanoethylene

The reaction of 1,3,4,6-tetrakis(isopropylthio)thieno[3,4-c]thiophene ( 1 ) with the palladium complex Pd₂(dba)₃CHCl₃ (dba = dibenzylideneacetone) and tetracyanoethylene (TCNE) gave a new palladium complex in which two isopropylthio groups of 1 and the double bond of TCNE were trigonally coordinated to palladium. The X-ray analysis revealed the electron donation from palladium to TCNE, leading to a lengthening of the C?C double bond in TCNE and distortion of TCNE from planarity. The radical cation of 1 and the radical anion of TCNE were detected by ESR spectroscopy in methylene dichloride solution of the complex, although the radical content was estimated from the paramagnetic susceptibility to be less than 1%. The reaction of the complex with aniline gave the same product as that in the reaction of the radical cation of 1 with aniline.     

Addition of Tetrachloromethane to Alkenes,Catalyzed by Pt(II) Complexes

Platinum(II) complexes [dichlorobis(triphenylphosphine)platinum(II), dichlorobis(tri-m-tolylphosphine)platinum(II), dichloro(2,9-dimethyl-1,10-N, N′-phenanthroline)platinum(II), etc.] showed catalytic activity in addition of tetrachloromethane across the double bond in 1-hexene, 1-heptene, 1-octene, 1-decene, and cyclohexene. The stability of the platinum catalysts was evaluated by GLC, gas chromatography-mass spectrometry, and ³¹P NMR and IR spectroscopy; the kinetic relationships of the addition reactions were determined. A reaction mechanism involving formation of trichloromethyl radical was suggested. A correlation was revealed for the first time between the catalytic activity of platinum, palladium, and rhodium complexes and the capability of these complexes to generate hexachloroethane.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005, pp. 778–782.Original Russian Text Copyright © 2005 by Zazybin, Khusnutdinova, Osipova, Solomonov.     

Redox‐Active Ligand‐Induced Homolytic Bond Activation

Coordination of the novel redox‐active phosphine‐appended aminophenol pincer ligand (PNO^H2) to Pd^II generates a paramagnetic complex with a persistent ligand‐centered radical. The complex undergoes fully reversible single‐electron oxidation and reduction. Homolytic bond activation of diphenyldisulfide by the single‐electron reduced species leads to a ligand‐based mixed‐valent dinuclear palladium complex with a single bridging thiolate ligand. Mechanistic investigations support an unprecedented intramolecular ligand‐to‐disulfide single‐electron transfer process to induce homolytic S? S cleavage, thereby releasing a thiyl (sulfanyl) radical. This could be a new strategy for small‐molecule bond activation.     

Ionization of amino-, thio- and hydroxy-naphtalenes via free (unhindered) electron transfer

The electron transfer from various monosubstituted naphthyl derivatives (naphtols, NpOH; naphtylamines, NpNH2; and thionaphtols, NpSH) to parent n-BuCl radical cations was studied by means of pulse radiolysis. The experiments reveal the synchronous and direct formation of two types of transients: the metastable solute radical cation (NpXH(*+), X = heteroatom) and the corresponding heteroatom-containing radical (NpX(*)) in comparable amounts. This is explained in terms of the free (unhindered) electron transfer in nonpolar solvents, which is a bimolecular process reflecting femtosecond time scale events of intramolecular dynamic motions accompanied by significant changes of the electron distribution within the donor molecule.     

Palladium‐Catalyzed,tert‐Butyllithium‐Mediated Dimerization of Aryl Halides and Its Application in the Atropselective Total Synthesis of Mastigophorene A

A palladium‐catalyzed direct synthesis of symmetric biaryl compounds from aryl halides in the presence of tBuLi is described. In situ lithium–halogen exchange generates the corresponding aryl lithium reagent, which undergoes a homocoupling reaction with a second molecule of the aryl halide in the presence of the palladium catalyst (1 mol %). The reaction takes place at room temperature, is fast (1 h), and affords the corresponding biaryl compounds in good to excellent yields. The application of the method is demonstrated in an efficient asymmetric total synthesis of mastigophorene A. The chiral biaryl axis is constructed with an atropselectivity of 9:1 owing to catalyst‐induced remote point‐to‐axial chirality transfer.     

Palladium‐Catalyzed,tert‐Butyllithium‐Mediated Dimerization of Aryl Halides and Its Application in the Atropselective Total Synthesis of Mastigophorene A

A palladium‐catalyzed direct synthesis of symmetric biaryl compounds from aryl halides in the presence of tBuLi is described. In situ lithium–halogen exchange generates the corresponding aryl lithium reagent, which undergoes a homocoupling reaction with a second molecule of the aryl halide in the presence of the palladium catalyst (1 mol %). The reaction takes place at room temperature, is fast (1 h), and affords the corresponding biaryl compounds in good to excellent yields. The application of the method is demonstrated in an efficient asymmetric total synthesis of mastigophorene A. The chiral biaryl axis is constructed with an atropselectivity of 9:1 owing to catalyst‐induced remote point‐to‐axial chirality transfer.     

Rate constants for the bimolecular self-reactions of ethyl,isopropyl, and tert-butyl radicals in solution. A direct comparison

A competitive method involving the direct measurement of radical concentrations by EPR spectroscopy has been used to show that in solution at 25°C the rate constants for the bimolecular self-reactions of ethyl, isopropyl, tert-butyl, cyclopentyl, and trichloromethyl are all approximately equal, as had been indicated previously by direct measurement of the rate constants for decay of these radicals.     

Synthesis, structural characterization, and catalytic behaviour in Heck coupling of palladium(II) complexes containing pyrimidine-functionalized N-heterocyclic carbenes

[Pd(L1)2(CH3CN)](PF6)2 (L1 = 1-n-butyl-3-(2-pyrimidyl)imidazolylidene, 3) and [Pd(L2)2](PF6)2 (L2 = 1-(2-picolyl)-3-(2-pyrimidyl)imidazolylidene 4), prepared via carbene transfer reactions of [Ag(L1)2]PF6 (1) and [Ag2(L2)2](PF6)2 (2) with palladium salts, respectively, have been fully characterized by 1H and 13C NMR spectroscopy and elemental analysis. The X-ray crystal structures of complexes 1-4 are reported. Complex 3 is an unusual pentacoordinated palladium complex, in which the palladium is coordinated by two imidazolylidene, two pyridine, and one acetonitrile molecule in a square-pyramidal geometry. The apical position is occupied by a pyrimidine nitrogen atom with a relatively long Pd-N distance (2.762(6) angstroms). Complex is a typical square-planar palladium complex with palladium surrounded by two pairs of cis-arranged pyridine and imidazolylidene ligands. The complexes exhibit good catalytic activities in the Heck coupling reaction of aryl bromides and activated aryl chlorides under mild conditions.     

Reaction of 1,2-dibromobenzene with the Si(111)-7x7 surface, a DFT study

Reaction between 1,2-dibromobenzene and the Si(111)-7x7 surface has been studied theoretically on the DFT(B3LYP/6-31G(d)) level. A 12-atom silicon cluster, representing two adatoms and one rest atom of the faulted half of the unit cell, was used to model the silicon surface. The first step of the reaction was a covalent attachment (chemisorption) of an intact 1,2-dibromobenzene molecule to the silicon cluster. Binding energies were calculated to be between 1.04 and 1.14 eV, depending on the orientation of the molecule. A second step of the reaction was the transfer of the Br atom to the silicon cluster. Activation energies for the transfer of the Br atom were calculated to be between 0.4 and 0.6 eV, suggesting that the thermal bromination reaction occurs on a microsecond time scale at room temperature. A third step of the reaction could be the transfer of the second Br atom of the molecule, the desorption of the organic radical, or the change of the adsorption configuration of the radical, depending on the original orientation of the adsorbed intact molecule. A novel, aromatic, two-sigma-bound adsorbed configuration of the C6H4 radical, in which a carbon ring of the radical is perpendicular to the silicon surface, has been introduced to explain previous experimental observations (Surf. Sci. 2004, 561, 11).     

Extraction mechanism for palladium(II) from hydrochloric acid solution with 2-dodecylthiomethylpyridine using a stirred transfer cell.

2-Dodecylthiomethylpyridine (DTP) was newly synthesized to study its extraction properties for precious metals. DTP was a selective extractant for palladium(II) and gold(III) over base metals. The loading test for palladium(II) showed that one palladium ion reacted with one molecule of DTP. The extraction rate of palladium with DTP was measured using a Lewis-type transfer cell at 303 K. The extraction reaction of palladium with DTP has been found to be a first order reaction with respect to palladium ion, DTP, and hydrogen ion concentrations. This reaction is inversely proportional to chloride ion concentration. The rate-determining step was the parallel reactions of DTP with PdCl3(-) and PdCl4(2-) in the aqueous phase.     

Free radical addition of trichloromethyl to caffeine access to C-8 polyhalogenoalkane derivatives and to unexpected 5-trichloromethyl-1,3,7 trimethyl-5,7-dihydrouric acid

The “Electrophilic” trichloromethyl radical was introduced directly into the model purine compound, caffeine, leading to products 2 and 3 (the former being smoothly converted to the corresponding carboxy ester derivative) and to the unexpected C-5 substituted title compound 4.     

Rate constants for the combination of trichloromethyl radicals and the electron-transfer reaction between hydroxymethyl radicals and carbon tetrachloride in solution

The rate constant for the bimolecular combination of trichloromethyl radicals in methanol is determined as a function of temperature by steady-state kinetics and electron-spin resonance with modulated radical initiation. The rate constant is in accord with the Smoluchowski equation and indicates a diffusion-controlled radical termination. The temperature dependence of the rate constant for the electron-transfer reaction between hydroxymethyl radicals and carbon tetrachloride in methanol is determined, and is found to disagree with predictions of the Marcus theory. This disagreement is tentatively ascribed to the structure of the transition state.     

Coherent Vibrational and Dissociation Dynamics of Polyatomic Radical Cations

The ultrafast dynamics of polyatomic radical cations contribute to important processes including energy transfer in photovoltaics, electron transfer in photocatalysis, radiation-induced DNA damage, and chemical reactions in the upper atmosphere and space. Probing these dynamics in the gas phase is challenging due to the rapid dissociation of polyatomic radical cations following electron removal, which arises from excess electronic excitation of the molecule during the ionization process. This Concept article introduces the reader to how the pump-probe technique of femtosecond time-resolved mass spectrometry (FTRMS) can overcome this challenge to capture coherent vibrational dynamics on the femtosecond timescale in polyatomic radical cations and enable the analysis of their dissociation pathways. Examples of FTRMS applied to three families of polyatomic radical cations are discussed.     

Radical Cations of Dithia- and Tetraselenaradialenes with Restricted Delocalization of the Unpaired Electron

The radical cations of the permethylated dithia[6]radialene 1 and the tetraselena[8]radialene 2 have been investigated by ESR- and ENDOR-spectroscopy. It is shown that in the radical cation of 1 the unpaired electron is delocalized only in one half of the molecule. The magnitude of the (33)S coupling constant suggests a restricted localization within the 2,3-dithiatetramethyl-butadiene unit. We ascribe the slow electron transfer from one part of the molecule to the other to steric effects. The analysis of the ESR spectrum of 2(*)()(+)() shows that only two of the four selenium atoms are involved. The delocalization of the unpaired electron is restricted to only one of the divinyl diselenide moieties with a slow electron transfer rate to the other one.     

Complexation and chemical transformations in the ternary system silver-carbon tetrachloride-mesogenic cyanobiphenyl at low temperatures

The possibility of cryoformation and chemical transformations of new metallomesogenic complexes, exemplified by co-condensates of silver with carbon tetrachloride in the matrix of mesogenic 4-n-pentyl-4′-cyanobiphenyl, has been studied by EPR. Four types of paramagnetic products were detected in this system at low temperatures: biligand π-complexes of silver atoms with cyanobiphenyl molecules; σ-complexes of silver atoms with the cyanobiphenyl molecule stabilized by an additional electron acceptor ligand CCl₄; trichloromethyl radical adducts of silver chloride — AgCl·CCl ₃ ^? and silver nanoclusters formed as a result of aggregation of silver atoms. Thermal stability of these particles was examined and a possible mechanism of their chemical transformations was proposed.     

Amphiphilic diblock star polymer catalysts via atom transfer radical polymerization

Diblock star polymers were synthesized via atom transfer radical polymerization from a palladium porphyrin macroinitiator. The arms of the star polymers had an amphiphilic design, with the central Pd-porphyrin surrounded by a relatively hydrophobic block of poly(butyl acrylate) and terminated by a hydrophilic block of poly(oligoethyleneglycol monomethylether monomethacrylate). The size of both the interior and exterior blocks of the polymer arms were tuned over a wide range of molecular weights with the exterior block used to solubilize the stars in polar media. The star polymers showed enhanced reactivity in the oxidation of 2-furaldehyde relative to a small molecule porphyrin, suggesting that the polymer backbone aids with catalytic turnover. Oxygen diffusion studies indicate that the polymer backbone shields the porphyrin excited state from oxygen quenching. Shielding is independent of molecular weight and polymer composition, but it is not pronounced enough to retard the rate of singlet oxygen generation under preparative photooxidation conditions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4939–4951, 2006