Regiospecific functionalization of methyl C-H bonds of alkyl groups in reagents with heteroatom functionality

We report the regiospecific, rhodium-catalyzed borylation of saturated terminal C-H bonds in molecules with existing functionality. Moderate to good yields were obtained with the organic substrate in excess and as limiting reagent. The borylations of trialkylamines, protected alcohols, protected ketones, and fluoroalkanes occurred regiospecifically at the methyl group that is least sterically hindered. Reactions were also conducted that probed electronic effects on the alkyl borylation. These reactions showed that the borylation occurred preferentially at the methyl group that is most electron-deficient. Methods to conduct tandem borylation of C-H bonds and conversion of the resulting boronate esters to alcohols, alkylarenes, and alkyltrifluoroborates were also developed.     

Intermolecular and intramolecular, platinum-catalyzed, acceptorless dehydrogenative coupling of hydrosilanes with aryl and aliphatic methyl C-H bonds

Intermolecular acceptorless dehydrogenative coupling of silanes with arene C-H bonds and intramolecular coupling of silanes with aryl and alkyl C-H bonds occur in good yield in the presence of 5 mol % of TpMe2PtMe2H (TpMe2 = hydridotris(3,5-dimethylpyrazolyl)borate) and related platinum(IV) complexes. The intermolecular reactions of arenes occurred with both trialkyl and dialkylaryl silanes. Intramolecular reactions of dialkylsilylalkylarenes occurred at aryl C-H bonds, and reactions of tributylsilane or dibutylphenylsilane occurred intramolecularly at the aliphatic, primary C-H bond. The reactions of arenes occurred preferentially at the least sterically hindered C-H bonds and preferentially with more electron-poor arenes. Crossover experiments and the lack of reactivity of the arylsilanes with H2 imply that the dehydrogenative silylation of arenes can be irreversible, even in a closed reaction vessel.     

Chemisorbed benzoate-to-benzene conversion via phenyl radicals on Cu(110): kinetic observation of conformational effects

The adsorption and decomposition of benzoic acid on the Cu(110) surface has been investigated using temperature-programmed reaction (TPR) spectroscopy and scanning tunneling microscopy (STM). The benzoate species is found to exist in two conformations: a phase containing upright species at monolayer saturation and a phase containing many tilted species at lower coverages. Thermal decomposition begins to occur near 500 K, yielding benzene and CO2. It is found that phenyl radicals, generated preferentially from the tilted benzoate species, efficiently abstract H atoms from undecomposed benzoate species to produce benzene in a rate-controlling process with an activation energy of about 29 kcal/mol. Using deuterium atom substitution at the 4-C position on the benzoate ring, it is found that the hydrogen abstraction reaction is selective for 2-,3- and 5-,6-C-H bonds. This observation indicates that the mobile phenyl radical is surface bound and preferentially attacks C-H bonds which are nearest the Cu surface binding the benzoate species, either as an upright species or as a tilted species.     

Insertion reactions of dimethylsilylene: relative reactivity towards oxygenz.sbnd;hydrogen,siliconhydrogen,and siliconz.sbnd;alkoxy bonds

Photochemically-generated dimethylsilylene is found in competition experiments to insert preferentially into oxygenhydrogen bonds of alcohols compared to either siliconhydrogen bonds of silanes or siliconoxygen bonds of alkoxysilanes. This selectivity for OH bonds compared to SiH bonds is quite high in tetrahydrofuran and in dilute hydrocarbon solutions. However, it decreases in more concentrated hydrocarbon solutions. These effects are discussed in terms of aggregation of alcohols, hydrogen bonding, and solvent-mediated dimethylsilylene reactivity.     

Propane σ‐Complexes on PdO(101): Spectroscopic Evidence of the Selective Coordination and Activation of Primary C?H Bonds

Achieving selective C H bond cleavage is critical for developing catalytic processes that transform small alkanes to value‐added products. The present study clarifies the molecular‐level origin for an exceptionally strong preference for propane to dissociate on the crystalline PdO(101) surface via primary C H bond cleavage. Using reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations, we show that adsorbed propane σ‐complexes preferentially adopt geometries on PdO(101) in which only primary C H bonds datively interact with the surface Pd atoms at low propane coverages and are thus activated under typical catalytic reaction conditions. We show that a propane molecule achieves maximum stability on PdO(101) by adopting a bidentate geometry in which a H Pd dative bond forms at each CH₃ group. These results demonstrate that structural registry between the molecule and surface can strongly influence the selectivity of a metal oxide surface in activating alkane C H bonds.     

Propane σ‐Complexes on PdO(101): Spectroscopic Evidence of the Selective Coordination and Activation of Primary CH Bonds

Achieving selective C? H bond cleavage is critical for developing catalytic processes that transform small alkanes to value‐added products. The present study clarifies the molecular‐level origin for an exceptionally strong preference for propane to dissociate on the crystalline PdO(101) surface via primary C? H bond cleavage. Using reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations, we show that adsorbed propane σ‐complexes preferentially adopt geometries on PdO(101) in which only primary C? H bonds datively interact with the surface Pd atoms at low propane coverages and are thus activated under typical catalytic reaction conditions. We show that a propane molecule achieves maximum stability on PdO(101) by adopting a bidentate geometry in which a H? Pd dative bond forms at each CH₃ group. These results demonstrate that structural registry between the molecule and surface can strongly influence the selectivity of a metal oxide surface in activating alkane C? H bonds.     

On the collision-activated fragmentation of proferrioxamines: Evidence for a succinimide-mediated mechanism

The fragmentation mechanism of acyclic proferrioxamines has been studied by tandem mass spectrometry in a triple stage quadrupole mass analyzer by using activation in the collision cell as well as in the high pressure region prior to the first mass analyzer. The data suggest that proferrioxamines fragment preferentially at the hydroxamate bonds via cyclic rearrangement to succinimide derivatives. This pattern was observed most clearly for the peracetyl derivatives, in which the influence of terminal functional groups was masked. Free amino or carboxylic acid functions may modify this basic fragmentation pattern. Using hydrogen-deuterium exchange, we also were able to show that the hydrogen atoms that are “recruited” in the formation of ammonium ions are “acidic” ones from elsewhere in the molecule or the matrix. At the same time, this rules out that they originate from “activated” methylene groups, as previously proposed     

Calculation of number and free energy of the conformers of linear alkanes with medium and long chains. Implications for catalysis

A method for calculating the number of rotamers of a linear alkane and of the number of rotamers with a given number of gauche conformations along the chain as a function of the total number of atoms in the chain, using general equations, is presented. A graphical method for generating individual rotamers was applied to the homologs up to decane, which has 1134 rotamers. The steric energies calculated by molecular mechanics (MM2 force field) were used as measures of the heat of formation for the coiled conformations relative to the anti conformer for each molecule, whereas the statistical entropy differences were calculated for classes of coiled rotamers grouped by the number of gauche bonds and steric energy. The free energy values calculated from these components show that already at 400 K hexane exists preferentially in conformations containing gauche bonds. For larger chains the free energy advantage for the coiled chains increases very steeply. The implications for the question of reactions of linear alkanes occurring on the surface or inside the channels of small- and medium-pore zeolites are briefly examined.     

DFT Predictions on Structures and Stabilities of Eleven-vertex nido-and cioso-Heteroboranes

Based on the octadecahedron of eleven-vertex closo-borane,the eleven-vertex closo-heteroborane was suggested with nonmetallic atoms instead of the different nonequivalent boron,and the stabilities were predicted at G96PW91/6-31+G(3d,2p) level.The small heteroatoms,C,N,O,preferentially occupy vertex 2 with the absolutely lowest relative energy to form the high stabilization closo-heteroboranes.They cap four-membered rings to satisfy the geometrical demand of short B-Z bonds.The electron attractions from the vicinal boron atoms make the frameworks shrink.Differently,Si and Ge preferentially substitute for boron at vertex 1 with six tight B-Z bonds and form stabilized molecules.P,As,S,and Se tend to occupy vertex 4 and the optimized structures belong to the nido configurations,in contrast to high electronegative heteroatoms,S and Se transfer less negative charges to framework and the electropositive heteroatoms,Si and Ge transfer more negative charges to framework to form the delocalization structures.The HOMO-LUMO gaps show that most of predicted clusters possess chemical stabilities.The substitutions of heteroatoms for boron atoms in eleven-vertex closo-hcteroboranes are consistent with the topological charge stabilization rule proposed by Gimarc.     

Die β,γ-Spaltung von Carbonylver-Bindungen und derivaten als Allgemeingültige Hauptabbau-Reaktion IM Massenspektrometer

Aliphatic compounds with polarized double bonds (I) suffer in the mass spectrometer preferentially cleavage of C? C bonds in β,γ-position to the functional group. The hetero-atom containing fragment produced in this manner must be formulated with a conjugated double bond (IV). It is the result of a reciprocal hydrogen rearrangement (I → II → III → IV). Deuterium labelling, high resolution and metastable ions indicate that this ion (IV) can be degradated further by McLafferty rearrangement.     

The muonium bond

Muonium is a light isotope of hydrogen, so light that in strong hydrogen bonds its zero-point energy is close to or above the energy barrier; this favours the formation of symmetrical muonium bonds. In water, muons will exist as Mu(H₂O)₆⁺ in contrast to H(H₂O)₄⁺; the stability of this species will slow the exchange of Mu and H. With alkenes and alkynes, muons will preferentially form non-classical complexes because of their lower zero-point energy in weak bonds: in liquids, a muon will attach two molecules via a symmetrical muonium bond.     

Preferentially linear connection of gold nanoparticles in derivatization with phosphorothioate oligonucleotides

A gold nanosphere in water is considered to attain special stability in derivatization like an artificial atom when the octet rule is satisfied by forming four covalent bonds with two 5'-phosphorothioate-modified oligonucleotide molecules. Owing to this, the hybridization of two mutually complementary gold-bound oligonucleotides makes gold nanospheres preferentially connected linearly by duplexes to produce strands like linear artificial molecules. We have then fixated the linear strands of DNA-linked gold nanospheres by reducing Ag(+) ion clusters immobilized around duplexes to show the absorption spectrum of silver-coated artificial-molecular nanorods.     

Dehydroxylation of Glycerol on Pt Surfaces: Ab Initio Molecular Dynamics Study

Glycerol is an important raw material in the chemical industry, and dehydroxylation of glycerol would produce 1, 2-propanediol and 1, 3-propanediol. Here we studied glycerol dehydroxylation with ab initio molecular dynamics simulations on Pt(111) and Pt(211) surfaces at 453 K. The free energies obtained on Pt show that dehydroxylation is more likely to occur at the terminal carbon than the central carbon, and 1, 2-propanediol would be produced preferentially, which is consistent with the selectivity observed experimentally. We found a linear relationship between the free energy barrier and the difference of average distances between O atoms at the initial state and transition state. Although a high correlation between the stability of gaseous glycerol and the number of formed hydrogen bonds is determined from density functional theory calculations, the hydrogen bonds formed within surface structures play a negligible role in determining the free energy barriers of dehydroxylation.     

Biodegradation of a synthetic co-polyester by aerobic mesophilic microorganisms

The aerobic biological degradation of the synthetic aliphatic-aromatic co-polyester Ecoflex™ (BASF) by 29 strains of enzyme-producing soil bacteria, fungi and yeasts was investigated at moderate environmental conditions. Previous studies had shown that these materials could be degraded but these studies were done under thermophilic conditions. In this paper, a screening procedure was developed to assess the biodegradability of the co-polyester at ambient environmental conditions and to investigate the mechanism of biodegradation. Results showed that the aliphatic-aromatic co-polyester could be degraded by a number of different microorganisms. However, after 21 days exposure to even the most promising cultures of pure microorganisms, only partial degradation of the Ecoflex™ was accomplished and only a few samples showed visible signs of degradation as loosely defined by the mechanical weakening of the films. Weight loss was not as obvious as the visual degradation and suggested broader types of microbial attack. The bacteria studied preferentially degraded the bonds between aliphatic components of the copolymer and the rate of biodegradation of oligomers was appreciably faster than that for the polymer chains. Using GC-MS techniques, degradation intermediates were identified to be the monomers of the co-polyester. Gel permeation chromatography results suggested exo-enzyme type degradation, where the microbes hydrolysed the ester bonds at the termini of the polymeric chains preferentially.     

Electronic structure of aqueous borohydride: a potential hydrogen storage medium

Borohydride salts have been considered as good prospects for transportable hydrogen storage materials, with molecular hydrogen released via hydrolysis. We examine details of the hydration of sodium borohydride by the combination of X-ray absorption spectroscopy and first principles' theory. Compared to solid sodium borohydride, the aqueous sample exhibits an uncharacteristically narrow absorption feature that is shifted to lower energy, and ascribed to the formation of dihydrogen bonds between borohydride and water that weaken the boron-hydrogen covalent bonds. Water also acts to localize the highly excited molecular orbitals of borohydride, causing transitions to excited states with p character to become more intense and a sharp feature, uncharacteristic of tetrahedral molecules, to emerge. The simulations indicate that water preferentially associates with borohydride on the tetrahedral corners and edges.     

A new cross-linking route via the unusual collision kinematics of hyperthermal protons in unsaturated hydrocarbons: the case of poly(trans-isoprene)

The bombardment of a approximately 18 nm film of poly(trans-isoprene) molecules (approximately 300,000 carbon atoms per molecule) with a mass-separated proton beam at 10 eV has been shown to induce highly efficient cross-linking of several macromolecules per incident proton. In this new method using physical means to conduct synthetic chemistry, the hyperthermal protons preferentially cleave C-H bonds because of their unusual kinematics in hydrocarbons, and the carbon radicals thus created initiate a polymerization chain reaction of the isoprene C[double bond, length as m-dash]C bonds. The method produces ultrathin polymeric films via cross-linking with a dry process having high chemical selectivity and reactivity but requiring no thermal cost and no chemical additives, conditions that are commonly required in the fabrication of microelectronics and photonics.     

Enthalpies of Transfer of Tetraalkylammonium Bromides and CsBr from water to Aqueous DMF at 298.15 K

Enthalpies of transfer of tetraalkylammonium bromides and CsBr from water to aqueous DMF mixtures are reported and analyzed in terms of a new solvation theory. It was found that a previous equation could not reproduce these data over the whole range of solvent compositions. Using a new solvation theory to model the enthalpies of transfer shows excellent agreement between experimental and calculated values over the entire range of solvent compositions. The analyses show that tetrapropylammonium bromide, Pr₄NBr, and tetrapentylammonium bromide, Pen₄NBr, are preferentially solvated by water; in contrast tetrabutylammonium bromide, Bu₄NBr, is preferentially solvated by DMF. The solvation of tetramethylammonium bromide, Me₄NBr, and cesium bromide, CsBr, is random. The extent to which the tetraalkylammonium bromides disrupt solvent–solvent bonds increases systematically in going from Me₄NBr to Pen₄NBr.     

Aromatic Trifluoromethylselenolation via Pd-catalyzed C−H functionalization

The synthesis of trifluoromethylselenolated aromatic molecules via an auxiliary-assisted, palladium catalyzed, C−H bonds functionalization with trifluoromethyl tolueneselenosulfonate as reagent is described. The mono- or bis-products can be preferentially formed. Some mechanistic investigations were realized to better understand the reaction. This methodology was also extended to fluoroalkylselenyl groups.     

The effect of aqueous solutions of trimethylamine-N-oxide on pressure induced modifications of hydrophobic interactions

To understand the mechanism of protein protection by the osmolyte trimethylamine-N-oxide (TMAO) at high pressure, using molecular dynamics (MD) simulations, solvation of hydrophobic group is probed in aqueous solutions of TMAO over a wide range of pressures relevant to protein denaturation. The hydrophobic solute considered in this study is neopentane which is a considerably large molecule. The concentrations of TMAO range from 0 to 4 M and for each TMAO concentration, simulations are performed at five different pressures ranging from 1 atm to 8000 atm. Potentials of mean force are calculated and the relative stability of solvent-separated state over the associated state of hydrophobic solute are estimated. Results suggest that high pressure reduces association of hydrophobic solutes. From computations of site-site radial distribution function followed by analysis of coordination number, it is found that water molecules are tightly packed around the nonpolar particle at high pressure and the hydration number increases with increasing pressure. On the other hand, neopentane interacts preferentially with TMAO over water and although hydration of neopentane reduces in presence of this osmolyte, TMAO does not show any tendency to prevent the pressure-induced dispersion of neopentane moieties. It is also observed that TMAO molecules prefer a side-on orientation near the neopentane surface, allowing its oxygen atom to form favorable hydrogen bonds with water while maintaining some hydrophobic contacts with neopentane. Analysis of hydrogen-bond properties and solvation characteristics of TMAO reveals that TMAO can form hydrogen bonds with water and it reduces the identical nearest neighbor water molecules caused by high hydrostatic pressures. Moreover, TMAO enhances life-time of water-water hydrogen bonds and makes these hydrogen bonds more attractive. Implication of these results for counteracting effect of TMAO against protein denaturation at high pressures are discussed.     

Interaction of aromatic units of amino acids with guanidinium cation: The interplay of π···π, XH···π, and M+···π contacts

Complexes formed by guanidinium cation and a pair of aromatic molecules among benzene, phenol, or indole have been computationally studied to determine the characteristics of the cation···π interaction in ternary systems modeling amino acid side chains. Guanidinium coordinates to the aromatic units preferentially in the following order: indole, phenol, and benzene. Complexes containing two different aromatic units show an intermediate behavior between that observed for complexes with only one kind of aromatic unit. Most stable structures correspond to doubly‐T shaped arrangements with the two aromatic units coordinating guanidinium by its NH₂ groups. Other structures with only one aromatic unit coordinated to guanidinium, such as T‐shaped or parallel‐stacked ones, are less favorable but still showing significant stabilization. In indole and phenol complexes, the formation of hydrogen bonds between the aromatic molecules introduces extra stabilization in T‐shaped structures. Three body effects are small and repulsive in doubly T‐shaped minima. Only when hydrogen bonds involving the aromatic molecules are formed in T‐shaped structures a cooperative effect can be observed. In most complexes the interaction is controlled by electrostatics, with induction and dispersion also contributing significantly depending on the nature and orientation of the aromatic species forming the complex. Although the stability in these systems is mainly controlled by the intensity of the interaction between guanidinium and the aromatic molecules coordinated to it, interactions between aromatic molecules can modulate the characteristics of the complex, especially when hydrogen bonds are formed. © 2014 Wiley Periodicals, Inc.