bluuebunny/crossref_metadata_2025_split huggingface dataset:
New efficient copper fluoride‐based catalyst for enantioselective hydrosilylation of ketones in aerobic conditions
Authors: Courmarcel James , Mostefaï Naouël , Sirol Sabine , Choppin Sabine , Riant Olivier 
Published: July 2004

AbstractA new copper(II) fluoride–chiral diphosphines catalytic system was developed. This one is very efficient and selective for the hydrosilylation of several substituted or unsubstituted aromatic ketones in so far as moderate to excellent enantioselectivities were obtained. An oxygen acceleration effect was observed that led us to propose a practical protocol with a low amount of catalyst.
A family of thioxanthato ruthenium and osmium aryls
Authors: Chattopadhyay Swarup , Panda Bikash Kumar , Ghosh Kaushik , Chakravorty Animesh 
Published: July 2004

AbstractThe title complexes of type M(RL2)(PPh3)2(CO)(S2CSEt) (2a: M = Ru; 2b: M = Os) have been synthesized in excellent yields by reacting M(RL1)(PPh3)2(CO)X (1a: M = Ru, × = Cl; 1b: M = Os, × = Br) with potassium ethyl thioxanthate and have been characterized with the help of spectral and electrochemical data. The RL2 ligand in 2 is the imine‐phenol tautomer of N‐C6H4R(p)‐4‐methylsalicylaldimine (R = Me, MeO, Cl) coordinated at the carbanionic‐C2 atom only while RL1 in 1 is the iminium‐phenolato tautomer chelated via carbanionic‐C2 and phenolato‐O atoms. The synthetic reaction is thus attended with tautomerization of the Schiff base ligand. It is also associated with a rotation of the ligand by ˜180° around the M–C bond in order to exclude steric repulsion. These features have been revealed by structure determination of 2a (R = Me). The metallated aldimine ring is found to be highly noncoplanar (dihedral angle ˜40°) with the thioxanthate chelate ring due to steric repulsion originating from the relatively large size of the sulfur atom. This phenomenon, which is absent in both the precursor 1 (R = Me) and in the carboxylate analogue Ru(MeL2)(PPh3)2(CO)(O2CMe), 7, has distinctive effects on bond parameters of 2a (R = Me). Thus the two Ru–P bonds in 2a (R = Me) differ in length by as much as 0.06 Å. The thioxanthate 2 is thermodynamically more stable than the precursor 1 as well as the carboxylate 7. Accordingly, both of these are irreversibly transformed to 2a (R = Me) upon treatment with thioxanthate.
Toward new coordination environments and molecular architectures using F8BINOL, an electron‐poor isostere of BINOL
Authors: Pandiaraju Subramanian , Lough Alan , Yudin Andrei K. 
Published: July 2004

AbstractStructural study of the Ti(OiPr)4/F8BINOL interaction demonstrates that F8BINOL, an isostere of BINOL with significant electronic perturbation, leads to novel complexes with cubic Ti4O4cores and bridging, rather than commonly observed chelating, arrangements of the diolate ligands.
Density‐dependent photochemical branching ratio in supercritical CO2: Photodissociation and isomerization of diiodomethane
Authors: Grimm Christian , Kling Matthias , Schroeder Jörg , Troe Jürgen , Zerbs Jochen , Schroeder Jörg , Troe Jürgen 
Published: July 2004

AbstractThe photodissociation of diiodomethane following excitation at 305 nm in supercritical CO2 was investigated by femtosecond pump‐probe absorption spectroscopy at pressures between 10 and 100 MPa. As in liquid solution, transient absorption signals in the wavelength range from 350 to 450 nm generally show an instantaneous peak, followed by a fast initial decay (200–300 fs) and a subsequent rise on a 10‐ps timescale. The initial fast decay time is found to be linearly dependent on viscosity, suggesting that dissociative motion on the repulsive surface is damped by solvent friction. Both amplitude and formation rate of the rising component, which is assigned to formation of iso‐diiodomethane within the solvent cage, increase with increasing pressure. Spectral narrowing of the transient absorption band indicates vibrational cooling of hot isomer by energy transfer to CO2 in about 20–40 ps. Immediately after excitation, this band shows absorption anisotropy for about 3 ps. The anisotropy decay rate increases from 3·1011 s−1 to 2·1012 s−1 as the pressure is lowered from 80 to 10 MPa. It is tentatively assigned to rotational relaxation of “hot” CH2I radicals generated in ultrafast photodissociation of the parent molecule. The observed density dependence of formation rate and relative yield of iso‐diiodomethane are described in terms of a simple kinetic model.
Remarkable asymmetric induction from remote stereocenters in conjugate addition chemistry for the synthesis of alkyl‐branched α, ω‐diaminoazelates
Authors: Cluzeau Jérôme , Lubell William D. 
Published: July 2004

AbstractConjugate addition of aryl Grignard reagents to (2S, 5E, 8S)‐di‐tert‐butyl 4‐oxo‐2,8‐bis‐[N‐(PhF)amino]non‐5‐enedioate (6, PhF = 9‐(9‐phenyl‐fluorenyl)) in THF proceeded with complete chemoselectivity and >9:1 stereoselectivity to provide predominantly (2S,6S,8S)‐6‐aryl 4‐oxo‐2,8‐diamino‐azelates7. In the presence of magnesium dibromide, diastereoselectivity in the addition of PhMgBr to enone6was improved to 15:1 in favor of the 6S‐isomer. Although lower chemoselectivity and stereoselectivity were obtained from the analogous reaction of6with isopropyl magnesium bromide in the absence of MgBr2, both were improved significantly when the addition reaction was performed in the presence of MgBr2. In contrast, the corresponding higher‐order cyanocuprates reacted with low diastereoselectivity on6by a 1,4‐addition pathway. In an attempt to identify the source of the high selectivity in the conjugate addition chemistry with6and Grignard reagents, the syntheses of enones12and13provided model systems in which one of the two amino carboxylate moieties of6was replaced by a branched alkyl substituent. Conjugate addition reactions on12and13demonstrated that chemoselectivity with Grignard reagents in the 1,4‐addition reaction was contingent on the presence of an amino carboxylate moiety near the ketone of the enone system. Furthermore, because diastereoselectivity with Grignard reagents was significantly lower in additions to amino enones12and13relative to diamino enone6, the presence of both amino carboxylate moieties has been highlighted as an important factor for remarkable asymmetric induction in the conjugate addition of Grignard reagents.
Enantioselective tandem formation and [2,3]‐sigmatropic rearrangement of cyclic propargylic oxonium ylides catalyzed by dirhodium(II) tetrakis[N‐phthaloyl‐(S)‐tert‐leucinate]
Authors: Tsutsui Hideyuki , Matsuura Maya , Makino Kazuishi , Nakamura Seiichi , Nakajima Makoto , Kitagaki Shinji , Hashimoto Shunichi 
Published: July 2004

AbstractThis paper describes the first examples of catalyst‐dependent asymmetric induction in the periselective [2,3]‐sigmatropic rearrangement of propargylic oxonium ylides generated by catalytic diazo decomposition. Catalysis of α‐diazo‐ß‐keto esters 4a—g using dirhodium(II) tetrakis[N‐phthaloyl‐(S)‐tert‐leucinate] in toluene has led to the virtually exclusive formation of benzofuran‐3‐ones bearing an allenic group with up to 79% ee.
Collapse of transmissivity in a triple barrier
Authors: Lefebvre Roland , Atabek Osman , Lefebvre Roland , Martin Manuel 
Published: July 2004

AbstractOne generally expects that the transmissivity through a symmetric arrangement of barriers and wells should go to unity when the energy of the incident particle coincides with a resonance energy of the system. It has been noticed before that for a symmetric triple barrier, there are conditions that lead to a merging of the two lowest unit transmissivity peaks, accompanied by a collapse of the transmissivity profile. This feature has been interpreted as meaning that there is a reduction of the number of resonances under these conditions. We reanalyze this situation with the help of a model that involves the same number of bound and continuum states, but which possesses an analytic solution. Because the transition probability in this case is expressed entirely in terms of resonance parameters (positions and widths), it is possible to calculate with the expression derived from the model a profile that agrees completely with that calculated with the transfer matrix method. The resonance parameters are obtained from the poles of the transition amplitude. This shows that the merging of the peaks and the collapse of the transmissivity occurs without a reduction in the number of resonances.
Light‐induced electron spin polarization in the ground state of water‐soluble copper porphyrins
Authors: Rozenshtein Vladimir , Berg Alexander , Levanon Haim , Krueger Uwe , Stehlik Dietmar , Kandrashkin Yuri , Van Der Est Art 
Published: July 2004

AbstractLight‐induced spin‐polarized transient EPR spectra are reported for several water‐soluble copper porphyrins. The spectra are assigned to the doublet ground state, with emissive spin polarization resulting from photoexcitation and subsequent electronic relaxation. In contrast to other systems for which polarization of a doublet ground state has been observed, the exchange interactions in the copper porphyrins are strong and the geometry is fixed. It is proposed that intersystem crossing from the photoexcited trip‐doublet to the trip‐quartet state can lead to net polarization of the spin system and that this polarization is maintained during electronic decay, possibly via charge‐transfer and exciplex states. The intensity of the observed spin polarization is essentially independent of the molecular orientation in the external field, but is strongly dependent on the nature of the charged peripheral groups. Possible reasons for this behavior are discussed.
Experimental and DFT calculated structure of the diruthenium(2.5) complex [(Me3TACN)Ru(μ‐Cl)3Ru(Me3TACN)](PF6)2
Authors: Kaim Wolfgang , Titze Christoph , Klein Axel , Knödler Axel , Zalis Stanislav 
Published: July 2004

AbstractThe diruthenium(2.5) complex [(Me3TACN)Ru(μ‐Cl)3Ru(Me3TACN)]‐(PF6)2, Me3TACN = 1,4,7‐trimethyl‐1,4,7‐triazacyclononane, has been crystallized for structural characterization. The results are reproduced by density functional theory (DFT) calculations and confirm the sensitivity of the central Ru(μ‐Cl)3Ru core to contacts between the Cl bridging atoms and the co‐ligands. The singly occupied MO is characterized as a σ* MO involving the metal dz2 orbitals and a small halide contribution by DFT calculations and EPR.
Structure and bonding of ArClF: Intermolecular potential surface
Authors: Prosmiti Rita , Villarreal Pablo , Delgado‐Barrio Gerardo 
Published: July 2004

AbstractThe ground‐state potential surface of the Ar‐ClF complex is determined by restricted coupled‐cluster with single and double excitations and a noniterative perturbation treatment of triple excitation (RCCSD(T)) calculations, using augmented correlation consistent basis sets supplemented with an additional set of midbond functions. Three local minima are found for the Ar‐ClF, which correspond to linear Ar‐Cl‐F and Ar‐F‐Cl configurations and a near T‐shaped structure. The well depths and the equilibrium distances are 283.62 cm−1 at 3.84 Å and 128.96 cm−1 at 4.31 Å for the linear ones and 173.13 cm−1 at 3.58 Å for the one at θ = 107.5°.Bound‐state calculations are carried out and the linear Ar‐Cl‐F structure is found to be the most stable one, in accordance with the experiment. Dissociation energies, intermolecular distances, and vibrational frequencies are computed and compared with available experimental data.
Temperature‐dependent nonlinear effects and catalyst evolution in the asymmetric addition of diethylzinc to benzaldehyde
Authors: Balsells Jaume , Costa Anna M. , Walsh Patrick J. 
Published: July 2004

AbstractSeveral efficient and highly enantioselective titanium‐based catalysts promote the asymmetric addition of alkyl groups to aldehydes. The catalysts involved in these reactions have been found not to exhibit nonlinear behavior. We have been able to identify a series of chiral bis(sulfonamide)‐based titanium catalysts that do show nonlinear behavior. Monitoring of the product ee's (eep's) at different conversions and different temperatures has led us to propose that nonlinear behavior is caused by an autoinduction process rather than by catalyst aggregation. Evidence to support this hypothesis includes eep's that change with the extent of conversion. We have found that when enantiopure ligand was used, the eepincreases with conversion from 72% to 80%. However, use of non‐enantiopure ligand results in eep's thatdecreasewith conversion. These results are rationalized by different rates of alkoxide exchange processes between like and unlike titanium‐alkoxide containing complexes.
New electronic and magnetic properties of monolayers of thiols on gold
Authors: Carmeli Itai , Naaman Ron , Leitus Gregory , Reich Shimon , Vager Zeev 
Published: July 2004

AbstractNew electronic and magnetic properties are induced by the adsorption of close‐packed monolayers of thiols on gold. In layers made from chiral molecules, unexpectedly large electronic dichroism is observed, which manifests itself as spin‐specific electron transmission. For many thiolated molecules self‐assembled on gold, a surprisingly large ferromagnetism is observed. All the observations can be rationalized by assuming orbital ferromagnetism of the organic thin layer. This is a new type of magnetism, induced by the formation of a closed packed layer of organic molecules on metal. The adsorption results in charge transfer between the substrate and the adsorbed layer, which is the origin of this magnetism.
Enantioselective addition Of diketene to aldehydes promoted by chiral schiff base—titanium alkoxide complex. Application to asymmetric synthesis of potential inhibitors of HMG coenzyme reductase
Authors: Hayashi Masahiko , Yoshimoto Kazuya , Hirata Naohito , Tanaka Kiyoshi , Oguni Nobuki , Harada Katsumasa , Matsushita Akio , Kawachi Yasuhiro , Sasaki Hiroshi 
Published: July 2004

AbstractHighly enantioselective addition of diketene to aldehydes was achieved by using novel Schiff base—titanium alkoxide complexes. Up to 92% ee of 5‐hydroxy‐3‐oxoesters was obtained. This procedure provides an efficient method for the asymmetric synthesis of potential inhibitors of HMG coenzyme reductase.
Ring expansion and isomerization in methyl indene and methylene indene radicals. Quantum chemical and transition‐state theory calculations
Authors: Dubnikova Faina , Lifshitz Assa 
Published: July 2004

AbstractRing expansion and isomerization in 1‐ and 2‐methylene indene radicals and isomerizations among the three isomers of methyl indene were studied by the Becke three‐parameter hybrid method with Lee‐Yang‐Parr correlation functional approximation (B3LYP). Structure, energy, and frequency calculations were carried out with the Dunning correlation consistent polarized doubleζ, (cc‐pVDZ) basis set. In contrast to methyl cyclopentadiene, transition states for ring expansion starting from the molecule methyl indene could not be located. The potential energy surfaces for ring expansion in methylene indene radicals consist of several intermediates and transition states and involve two principal pathways. One pathway is associated with cleavage of the five‐membered ring adjacent to the methylene group and formation of a six‐membered ring. In the second pathway, the transition states of the first stage consist of a newly‐formed three‐membered ring fused to the original cyclopentadiene ring. In all cases, the reaction pathways leading to ring expansion include an intermediate that, via an additional transition state, produces an α‐ or (β‐hydronaphthalene radical. The latter, by a fast H‐atom ejection, forms naphthalene. The structure, energetics, and additional parameters on the potential energy surfaces are shown. Several transition states and intermediates are common to both ring expansion and isomerization, so that there are competing parallel pathways that determine the final distribution among the isomerization and ring expansion products. Potential energy surfaces for interisomerization among the various isomers of methyl indene were calculated, and several reaction pathways were suggested.
DIOS‐MSEED: A chip‐based method for measurement of enantiomeric excess by kinetic resolution/mass spectrometry
Authors: Shen Zhouxin , Yao Sulan , Crowell John E. , Siuzdak Gary , Finn M. G. 
Published: July 2004

AbstractThe determination of enantiomeric excess by kinetic resolution mass spectrometry has been implemented with the Desorption/Ionization On Silicon (DIOS) MS technique. Measurements can thereby be made much more rapidly than was previously possible, bringing this general methodology for screening asymmetric catalysts closer to true high‐throughput status.
On the collapse and restoration of condensates in n dimensions in the mean‐field approximation
Authors: Cederbaum Lorenz S. , Moiseyev Nimrod , Cederbaum Lorenz S. , Moiseyev Nimrod 
Published: July 2004

AbstractThe mean‐field equation of N bosons in an external potential interacting via a short‐range δ‐function potential is studied. This equation is formally equivalent to the nonlinear Schrödinger equation. Virial theorems are derived and the number of possible solutions is investigated for various dimensions n. The unboundedness from below of the underlying mean‐field operator for attractive nonlinearity and the related collapse of the wave function are studied in n dimensions. Within the symmetry preserving mean‐field approximation, an ansatz to avoid the collapse by modifying the spherically symmetric external potential is discussed and analyzed. Other suggestions to stabilize the mean‐field condensate are briefly mentioned. Illustrative numerical examples of the collapse and its restoration are given. Detailed conclusions are drawn.
Reaction of Aryl Iodides with (PCP)Pd(II)—Alkyl and Aryl Complexes: Mechanistic Aspects of Carbon—Carbon Bond Formation
Authors: Kraatz Heinz‐Bernhard , van der Boom Milko E. , Ben‐David Yehoshoa , Milstein David 
Published: July 2004

AbstractThe reaction of the PCP‐type complex Pd(Me){2,6‐(iPr2PCH2)2C6H3}(3) with phenyl iodide results in the formation of Pd(I){2,6‐(iPr2PCH2)2C6H3} (5), methyl iodide, toluene, and biphenyl. Formation of Pd(Ph){2,6‐(iPr2PCH2)2C6H3}(4) is observed during the reaction by 31P NMR. Reaction of 4 with aryl iodides results in the formation of 5 and Ph–Ph, Ph–Ar, and Ar–Ar, products indicative of a radical reaction. Under pseudo‐first‐order conditions, the rates of the reactions follow the order p‐OMe > p‐Me > H > p‐NO2 > m‐Cl. The reaction is likely to involve electron transfer from 4 to the aryl iodide followed by fast decomposition of a postulated radical cation [Pd(Ph){2,6‐(iPr2PCH2)2C6H3}]+. (4+.) to give a phenyl radical and [Pd{2,6‐(iPr2PCH2)2C6H3}]+ (6+). Facile decomposition of the aryl iodide radical anion generates an aryl radical and I−. Recombination of aryl radicals gives rise to mixed biaryls, and 6+ combines with I− to give 5.
Syntheses of inherently chiral monosulfinyltrithiacalix[4]arenes by the oxidation of one of the bridging sulfurs of a tetrathiacalix[4]arene fixed in the 1,3‐alternate conformation
Authors: Morohashi Naoya , Naito Ryoichiro , Iki Nobuhiko , Miyano Sotaro 
Published: July 2004

AbstractA new synthetic strategy for the construction of an inherently chiral, sulfur‐bridged calix[4]arene molecular framework was presented by oxidation of one of the four epithio bonds of a tetrathiacalix[4]arene derivative of 1,3‐alternate conformation: Tetra(carboxymethyl) ether of p‐tert‐butyltetrathiacalix[4]arene of 1,3‐alternate conformation (7) was converted to the tetra(l‐menthoxycarbonylmethyl) ether (8). Oxidation of the tetra l‐menthyl ester 8 by treatment with NaBO3 (1.1 molar equiv.) afforded a pair of diastereomers of the monosulfinyl derivatives (9) in 58% yield, which were readily separable by silica‐gel column chromatography. Removal of the l‐menthyl moieties followed by methyl esterification gave enantiomerically pure samples of inherently chiral (‐)‐ and (+)‐monosulfinyltrithiacalix[4]arene derivatives ((‐)‐10 and (+)‐10) in 62 and 56% yield, respectively.
Photochemical applications of phase‐modulus interdependencies
Authors: Englman Robert , Englman Robert , Yahalom Asher 
Published: July 2004

AbstractAfter an accolade to Joshua Jortner, we trace the influences of his chemistry background in his physics writings. On the way, we note the richness of physics in principles (or large‐scale laws) and the fact‐orientedness of chemistry.Next we turn to a recent laser‐induced electron‐detachment experiment and utilize analytic properties of the developing wave packet in the (complex) time domain, studied by us previously, to relate the phase of the optimized laser field to its intensity. It is suggested that these results can be used to reduce the labor of pulse optimization in phase‐intensity controlled reaction dynamics. Phase‐intensity inter‐dependencies are also established in simulated results (obtained with the electron nuclear dynamics END‐algorithm) for photoexcited hydrogen molecules.
1,2‐Bond shift of the Fe(CO)3 moiety in the 1,5‐O‐heterocyclization of (1,3‐dienyl)‐tricarbonyliron diols: Application to the synthesis of novel phosphocholines
Authors: Braun Alain , Lellouche Jean‐Paul (Moshe) 
Published: July 2004

AbstractA novel concept of intramolecular cationic 1,5‐O‐heterocyclization of ψ‐exo‐/ψ‐endo‐(1,3‐dienyl)tricarbonyliron diols has been demonstrated to occur under acid catalysis with a quite unusual 1,2‐migration of the complexation site. This cationic cyclization has been shown to be stereoselective when involving homochiral precursor diols. It allowed straightforward access to new racemic and optically active conformationally locked alkylphosphocholines as potential anticancer agents.
Surface tension from molecular dynamics simulation: Adsorption at the gas‐liquid interface
Authors: Mucha Martin , Hrobárik Tomáš , Jungwirth Pavel 
Published: July 2004

AbstractThe gas‐liquid interface of binary mixtures of argon and krypton is investigated using molecular dynamics simulations. Surface tension and local pressure profiles are extracted from the simulations and cast into individual molecular contributions. The surface excess of argon is rationalized in terms of the relative strength of van der Waals interactions.
Asymmetric synthesis of the (2S,4S,6S)‐2,4,6‐trimethylnonyl subunit of siphonarienes
Authors: Diaz David D. , Crisóstomo Fernando R. P. , Martín Víctor S. 
Published: July 2004

AbstractThe paper describes an asymmetric approach to the synthesis of the (2S,4S,6S)‐2,4,6‐trimethylnonyl segment of siphonarienes. The key step used is a newly developed methodology to obtain sec‐dialkyl acetylenes based on the intramolecular hydride transfer from a secondary γ‐benzyloxy group with well‐defined absolute stereochemistry, ensured by a Sharpless asymmetric epoxidation reaction, to a cation generated by Lewis acid treatment of a tertiary Co2(CO)6‐complexed propargylic alcohol.
Planar chiral ferrocenes as ligands in the vanadium‐catalyzed asymmetric epoxidation of allylic alcohols
Authors: Bolm Carsten , Kühn Toralf 
Published: July 2004

AbstractStarting from either (S)‐4‐tert‐butyl‐2‐ferrocenyl‐2‐oxazoline or (S)‐S‐ferrocenyl‐S‐4‐tolyl sulfoxide, new planar chiral hydroxamic acids have been synthesized employing the directed ortho‐metalation strategy. The ferrocene‐derived hydroxamic acids are capable of serving as ligands in the vanadium‐catalyzed asymmetric epoxidation of allylic alcohols, furnishing the epoxide of 2‐methyl‐3‐phenylprop‐2‐en‐1‐ol with up to 34% ee.