bluuebunny/crossref_metadata_2025_split huggingface dataset:
Mechanism of Aromatic Hydroxylation by an Activated FeIVO Core in Tetrahydrobiopterin‐Dependent Hydroxylases
Authors: Bassan Arianna , Blomberg Margareta R. A. , Siegbahn Per E. M. 
Published: September 2003

AbstractThe chemical pathways leading to the hydroxylated aromatic amino acids in phenylalanine and tryptophan hydroxylases have been investigated by means of hybrid density functional theory. In the catalytic core of these non‐heme iron enzymes, dioxygen reacts with the pterin cofactor and is likely to be activated by forming an iron(IV)O complex. The capability of this species to act as a hydroxylating intermediate has been explored. Depending on the protonation state of the ligands of the metal, two different mechanisms are found to be energetically possible for the hydroxylation of phenylalanine and tryptophan by the high‐valent iron–oxo species. With a hydroxo ligand the two‐electron oxidation of the aromatic ring passes through a radical, while an arenium cation is involved when a water replaces the hydroxide. After the attack of the activated oxygen on the substrate, it is also found that a 1,2‐hydride shift (known as an NIH shift) generates a keto intermediate, which can decay to the true product through an intermolecular keto–enol tautomerization. The benzylic hydroxylation of 4‐methylphenylalanine by the FeIVO species has also been investigated according to the rebound mechanism. The computed energetics lead to the conclusion that FeIVO is capable not only of aromatic hydroxylation, but also of benzylic hydroxylation.
Fluorous Biphasic Catalysis: Synthesis and Characterization of Copper(I) and Copper(II) Fluoroponytailed 1,4,7‐Rf‐TACN and 2,2′‐Rf‐Bipyridine Complexes—Their Catalytic Activity in the Oxidation of Hydrocarbons, Olefins, and Alcohols, Including Mechanistic Implications
Authors: Contel María , Izuel Cristina , Laguna Mariano , Villuendas Pedro R. , Alonso Pablo J. , Fish Richard H. 
Published: September 2003

AbstractIn this contribution on fluorous biphasic catalysis (FBC), we present the synthesis and characterization of new copper complexes, and define their role, as precatalysts, in the FBC oxidation of hydrocarbons, olefins, and alcohols. Thus the previously reported, but poorly characterized, fluoroponytailed ligand, 2,2′‐Rf‐bipyridine (Rf=‐(CH2)3C8F17) 2, as well as the new CuII fluoroponytailed carboxylate synthon complex [Cu(C8F17(CH2)2CO2)2] 3, will be addressed. Moreover, the reaction of previously described ligands, 1,4,7‐Rf‐TACN 1, or 2,2′‐Rf‐bipyridine 2 with 3 afforded new perfluoroheptane‐soluble CuII complexes, [Cu(C8F17(CH2)2CO2)2(Rf‐tacn)] 4 and [Cu(C8F17(CH2)2CO2)2(Rf‐bpy)] 5, respectively. The reaction of 1 with [Cu(CH3CN)4]PF6 or [CuCl] provided new CuI complexes, which could be isolated and fully characterized as [Cu(Rf‐tacn)X′]X, in which X=PF6 (6) or X′=Cl (7) (soluble in perfluoroheptane). The CuII and CuI complexes, 4–7, were characterized by elemental analysis, mass spectrometry, and IR, diffuse reflectance UV/Vis, and EPR spectroscopies; complex 7 was also characterized by 1H and 19F{1H} NMR spectroscopy. Complexes 4 and 5, as well as 6 and 7 generated in situ, were evaluated as precatalysts for hydrocarbon and olefin functionalization. The oxidation reactions of these substrates in the presence of the necessary oxidants, tert‐butyl hydroperoxide (TBHP) and oxygen gas, proceeded under FBC conditions for 5, 7, and CuI salts with 2. However, the complexes with ligand 2 could not be recycled, owing to significant ligand dissociation. The CuII complex 4, with the ligand 1, provide the oxidation of 4‐nitrobenzyl alcohol to 4‐nitrobenzaldehyde under single‐phase FBC conditions at 90 °C with TEMPO (2,2,6,6‐tetramethylpiperidinyl‐1‐oxy) and O2; the precatalyst 4, can be utilized for an additional four catalytic cycles without loss of activity. Plausible mechanisms concerning these FBC oxidation reactions will be discussed.
A New Enzyme Model For Enantioselective Esterases Based On Molecularly Imprinted Polymers
Authors: Emgenbroich Marco , Wulff Günter 
Published: September 2003

AbstractAn efficient enzyme model exhibiting enantioselective esterase activity was prepared by using molecular imprinting techniques. The enantiomerically pure phosphonic monoesters 4 L and 5 L were synthesized as stable transition‐state analogues. They were used as templates connected by stoichiometric noncovalent interactions to two equivalents of the amidinium binding site monomer 1. After polymerization and removal of the template, the polymers were efficient catalysts for the hydrolysis of certain nonactivated amino acid phenylesters (2 L, 2 D, 3 L, 3 D) depending on the template used. Imprinted catalyst IP4 (imprinted with 4 L) enhanced the hydrolysis of the corresponding substrate 2 L by a factor of 325 relative to that of a buffered solution. Relative to a control polymer containing the same functionalities, prepared without template 4 L, the enhancement was still about 80‐fold, showing the highest imprinting effect up to now. In cross‐selectivity experiments a strong substrate selectivity of higher than three was found despite small differences in the structure of the substrate and template. Plots of initial velocities of the hydrolysis versus substrate concentration showed typical Michaelis–Menten kinetics with saturation behavior. From these curves, the Michaelis constant KM and the catalytic constant kcat can be calculated. The enantioselectivity shown in these values is most interesting. The ratio of the catalytic efficiency kcat/KM, between the hydrolysis of 2 L‐ and 2 D‐substrate with IP4, is 1.65. This enantioselectivity derives from both selective binding of the substrate (KML/KMD=0.82), and from selective formation of the transition state (kcatL/kcatD=1.36). Thus, these catalysts give good catalysis as well as high imprinting and substrate selectivity. Strong competitive inhibition is caused by the template used in imprinting. This behavior is also quite similar to the behavior of natural enzymes, for which these catalysts are good models.
Diastereoselective Conjugate Addition to Chiral α,β‐Unsaturated Carbonyl Systems in Aqueous Media: An Enantioselective Entry to α‐ and γ‐Hydroxy Acids and α‐Amino Acids
Authors: Suárez Rosa M. , Pérez Sestelo José , Sarandeses Luis A. 
Published: September 2003

AbstractThe stereoselectivity of the ultrasonically induced zinc—‐copper conjugate addition of iodides to chiral α,β‐unsaturated carbonyl systems under aqueous conditions was studied. Alkyl iodides add diastereoselectively to methylenedioxolanone 1 and methyleneoxazolidinone 2 to afford the 1,4‐addition products in good yields (38–95 %) and with high diastereomeric excess (44–90 % de). The 1,4‐addition to chiral γ,δ‐dioxolanyl‐α,β‐unsaturated esters 3–5 also proceeds with good yields (51–99 %). The diastereoselectivity is dependent on the geometry of the olefin: the Z isomer 3 gives high diastereoselectivity, while the reactions with the E isomer 4 are nonstereoselective. The reaction proceeds with excellent chemoselectivity and allows the use of iodides bearing ester, hydroxy, and amino groups. Since the 1,4‐addition products can be readily hydrolyzed, this methodology constitutes a novel entry for the enantioselective synthesis of α‐ and γ‐hydroxy acids and α‐amino acids in aqueous media. The results obtained support the radical mechanism proposed by Luche, and represent one of the few examples of a radical stereoselective conjugate addition in aqueous medium.
Waste‐Free and Facile Solid‐State Protection of Diamines, Anthranilic Acid, Diols, and Polyols with Phenylboronic Acid
Authors: Kaupp Gerd , Naimi‐Jamal M. Reza , Stepanenko Vladimir 
Published: September 2003

AbstractPhenylboronic acid (2) reacts quantitatively by ball‐milling in the solid state with o‐phenylendiamine, 1,8‐diaminonaphthalene, anthranilic acid, pyrocatechol, pyrogallol, pinacol, bicyclic cis‐diols, mannitol, and inositol to form the five‐ or six‐membered cyclic phenylboronic amides or esters. Catalysts or other auxiliaries are strictly excluded as they are not required and would have to be removed after the reactions. These varied model reactions provide pure protected products without the necessity of further purifying workup and the potential for protection chemistry is demonstrated. Some of the reactions can also be quantitatively performed if stoichiometric mixtures of the reactants are co‐ground or co‐milled and heated to appropriate temperatures either below the eutectics or above the melting points. The temperatures are much higher in the latter case. Similar reactions in solution suffer from less than 100 % yield of the mostly sensitive compounds that are difficult to purify and thus create much waste. The hydrolysis (deprotection) conditions of the products are rather mild in most cases. Therefore, this particularly easy access to heteroboroles, heteroborolanes, heteroborinones, heteroborines, and heteroborinines is highly valuable for their more frequent use in protective syntheses.
Soluble Carbon Nanotubes
Authors: Tasis Dimitrios , Tagmatarchis Nikos , Georgakilas Vasilios , Prato Maurizio 
Published: September 2003

AbstractCarbon nanotubes have attracted great interdisciplinary interest because of their unique structure and properties. However, carbon‐nanotube research is challenged by several problems, such as: i) mass production of material, ii) control of length, diameter, and chirality, and iii) manipulation for use in diverse technological fields. Issues regarding the synthesis and purification as well as the functionalization and solubilization of carbon nanotubes are relevant topics in this rapidly growing field. In this paper, covalent and noncovalent approaches to functionalized and solubilized nanotubes are examined in detail, with particular emphasis on the change of properties that accompany the chemical modification.
Computation of Aromatic C3N4 Networks and Synthesis of the Molecular Precursor N(C3N3)3Cl6
Authors: Vodak David T. , Kim Kibum , Iordanidis Lykourgos , Rasmussen Paul G. , Matzger Adam J. , Yaghi Omar M. 
Published: September 2003

AbstractThe successful synthesis and structural characterization of molecules that represent segments of extended solids is a valuable strategy for learning metric and stereochemical characteristics of those solids. This approach has been useful in cases in which the solids are particularly difficult to crystallize and thus their atomic connectivity and overall structures become difficult to deduce with X‐ray diffraction techniques. One such class of materials is the covalently linked CxNy extended solids, where molecular analogues remain largely absent. In particular, structures of C3N4 solids are controversial. This report illustrates the utility of a simple molecule, N(C3N3)3Cl6, in answering the question of whether triazine based C3N4 phases are layered or instead they adopt 3D structures. Here, we present density functional calculations that clearly demonstrate the lower stability of graphitic C3N4 relative to 3D analogues.
From Central to Helical Chirality: Synthesis of P and M Enantiomers of [5]Helicenequinones and Bisquinones from (SS)‐2‐(p‐Tolylsulfinyl)‐ 1,4‐benzoquinone
Authors: Carreño M. Carmen , García‐Cerrada Susana , Urbano Antonio 
Published: September 2003

AbstractThe reaction of 1,4‐divinyl‐1,3‐cyclohexadiene, 5,8‐dimethoxy‐ or tert‐butyldimethylsilyloxy‐3‐vinyl‐1,2‐dihydrophenanthrene or 6‐vinyl‐7,8‐dihydro‐1,4‐phenanthrenequinone with an excess of enantiopure (SS)‐2‐(p‐tolylsulfinyl)‐1,4‐benzoquinone (2) led to the direct formation of enantioenriched dihydro[5]helicenequinones or bisquinones (50→98 % ee). A domino Diels–Alder cycloaddition/sulfoxide elimination/partial aromatization process occurs, being the absolute configuration of the final helicene defined in the aromatization step. Both M and P helimers are accessible through a stepwise enantiodivergent process if the pentacyclic dihydroaromatic intermediate resulting in the two first steps is aromatized in the presence of (±)‐2, DDQ, CAN or DBU.
Solvatochromic Azamethine Dyes for Probing the Polarity of Gold‐Cluster‐Functionalized Silica Particles
Authors: Spange Stefan , Kunzmann Dagmar , Sens Rüdiger , Roth Isabelle , Seifert Andreas , Thiel Werner R. 
Published: September 2003

AbstractAzamethine dyes of the merocyanine type [4‐(N,N‐di‐n‐butylamino)‐2‐methylphenyl]{2,4‐di‐keto‐3‐[N′‐(n‐hexyl)]‐5‐cyano‐6‐methyl‐3‐pyridinio}‐1‐azamethine (1) and [4‐(N,N‐diethylamino)‐2‐(N′‐tert‐butylcarboxy)‐amidophenyl]‐{2,4‐diketo‐3‐[N″‐(n‐hexyl)]‐5‐cyano‐6‐methyl‐3‐pyridinio}‐1‐azamethine (2) have been used as surface‐polarity indicators for gold‐cluster‐functionalized silica particles. Their UV/Vis absorption maxima range from about λ=600 to 700 nm as a function of solvent polarity and are clearly separated from the surface plasmon UV/Vis absorption band of gold (λ≈520–540 nm). Solvatochromism of both dyes has been investigated in 26 solvents of different polarity. The positive solvatochromic band shifts of 1 and 2 can be well expressed in terms of the empirical Kamlet–Taft solvent polarity parameters α and π*. They are mainly sensitive to the dipolarity/polarizability (π* term; 70–75 %) and HBD (hydrogen‐bond donating) acidity (α term) of the solvent. Both dyes adsorb readily on functionalized silica samples from solutions in 1,2‐dichloroethane or cyclohexane. The surface polarities of gold‐cluster‐functionalized silica particles, with and without co‐adsorbed L‐cysteine and poly(ethylenimine), have been investigated by using these solvatochromic dyes. The specific interaction of dye 2 with cysteine has been examined independently by quantum‐chemical calculations by using the AM1 and PM3 methods.
Employing the Structural Diversity of Nature: Development of Modular Dipeptide‐Analogue Ligands for Ruthenium‐Catalyzed Enantioselective Transfer Hydrogenation of Ketones
Authors: Pastor Isidro M. , Västilä Patrik , Adolfsson Hans 
Published: September 2003

AbstractA library of novel dipeptide‐analogue ligands based on the combination of tert‐butoxycarbonyl(N‐Boc)‐protected α‐amino acids and chiral vicinal amino alcohols were prepared. These highly modular ligands were combined with [{RuCl2(p‐cymene)}2] and the resulting metal complexes were screened as catalysts for the enantioselective reduction of acetophenone under transfer hydrogenation conditions using 2‐propanol as the hydrogen donor. Excellent enantioselectivity of 1‐phenylethanol (up to 98 % ee) was achieved with several of the novel catalysts. Although most of the ligands contained two stereocenters, it was demonstrated that the absolute configuration of the product alcohol was determined by the configuration of the amino acid part of the ligand. Employing ligands based on L‐amino acids generated S‐configured products, and catalysts based on D‐amino acids favored the formation of the R‐configured alcohol. The combination N‐Boc‐L‐alanine and (R)‐phenylglycinol (Boc‐L‐Ab) or its enantiomer (N‐Boc‐D‐alanine and (S)‐phenylglycinol, Boc‐D‐Aa) proved to be the best ligands for the reduction process. Transfer hydrogenation of a number of aryl alkyl ketones were evaluated and excellent enantioselectivity, up to 96 % ee, was obtained.
How Nucleophilic Are Diazo Compounds?
Authors: Bug Thorsten , Hartnagel Manfred , Schlierf Clemens , Mayr Herbert 
Published: September 2003

AbstractThe kinetics of the reactions of benzhydryl cations with eight diazo compounds 1 a–g were investigated photometrically in dichloromethane. The nucleophilicity parameters N and slope parameters s of these diazo compounds were derived from the equation log k (20 °C)=s (E+N) and compared with the nucleophilicities of other π systems (alkenes, arenes, silyl enol ethers, silyl ketene acetals). It is shown that the nucleophilic reactivities of diazo compounds cover more than ten orders of magnitude, being comparable to that of styrene on the low reactivity end and to that of enamines on the high reactivity end. The rate‐determining step of these reactions is the electrophilic attack at the diazo‐carbon atom to yield diazonium ions, which rapidly lose nitrogen.
Highly Substituted Azulene Dyes as Multifunctional NLO and Electron‐Transfer Compounds
Authors: Lambert Christoph , Nöll Gilbert , Zabel Manfred , Hampel Frank , Schmälzlin Elmar , Bräuchle Christoph , Meerholz Klaus 
Published: September 2003

AbstractTwo highly substituted azulene derivatives were synthesised by Pd‐mediated dimerisation from the corresponding tolan species. One azulene derivative (2) has donor functionalities (dianisylaminophenyl and dianisylamino) in the 1‐, 2‐, 3‐ and 6‐positions, while the other (1) has donors (dianisylaminophenyl) in the 2‐ and 6‐positions and acceptors (nitrophenyl) in the 1‐ and 3‐positions. Each azulene derivative shows strong bond length alternation in the solid state, determined by X‐ray crystal analysis, and an intense CT band around 450–500 nm in its UV/Vis spectrum. The first‐order hyperpolarisability of 1 and of 2 was measured by hyper‐Rayleigh scattering and is about that of disperse red DR1. Both azulene derivatives show multiple oxidation processes. The intramolecular adiabatic ET behaviour of the mixed valence radical cations of 1 and of 2 was investigated by UV/Vis/NIR spectroelectrochemistry. The intervalence‐CT band of 1+ could be analysed by the Generalised Mulliken–Hush theory, which yields an electronic coupling V=1140 cm−1 for the optically induced adiabatic hole transfer.
Synthesis, X‐ray Crystal Structure, UV/Visible Linear and Nonlinear (Optical Limiting) Spectral Properties of Symmetrical and Unsymmetrical Porphyrazines with Annulated 1,2,5‐Thiadiazole and 1,4‐Diamyloxybenzene Moieties
Authors: Donzello Maria Pia , Ercolani Claudio , Gaberkorn Anna A. , Kudrik Evgenij V. , Meneghetti Moreno , Marcolongo Gabriele , Rizzoli Corrado , Stuzhin Pavel A. 
Published: September 2003

AbstractCo‐cyclization of 1,2,5‐thiadiazole‐3,4‐dicarbonitrile and 3,6‐diamyloxyphthalodinitrile in the presence of magnesium or lithium amylate in amyl alcohol leads to mixtures containing the Mg derivatives of the symmetrical species tetrakis(1,2,5‐thiadiazolo)porphyrazine, (S4)PzH2, and tetrakis(1,4‐diamyloxybenzo)porphyrazine, (A4)PzH2, and the low‐symmetry macrocycles bearing peripheral 1,2,5‐thiadiazole and 1,4‐diamyloxybenzene rings in the ratio 1:3, 2:2 (cis and trans), and 3:1, that is, (SA3)PzH2, (S2A2)PzH2, (SASA)PzH2, and (S3A)PzH2, respectively. The basic Mg materials were converted to the corresponding free‐base macrocycles by treatment with CF3COOH. The species were separated from the mixtures by chromatography, either as Mg complexes or demetalated materials. With results on (S4)PzH2 and (SA3)PzH2 in hand, including crystallographic work on the latter, a general chemical physical investigation has been carried out of all the symmetrical and unsymmetrical free‐base macrocycles. The structures of the species (S2A2)PzH2 and (A4)PzH2. were elucidated by single‐crystal X‐ray crystallography. The effect of the progressive variation of the macrocyclic structure along the series, from the symmetrical (S4)PzH2 to its symmetrical partner (A4)PzH2 via the low‐symmetry 3:1, 2:2 (cis and trans), and 1:3 macrocycles, was studied by IR, 1H NMR, and UV/Vis linear and nonlinear (optical limiting) measurements. The results are interpreted on the basis of intra‐ and intermolecular interactions between the electron‐deficient 1,2,5‐thiadiazole and the electron‐donating 1,4‐diamyloxybenzene moieties.
Molybdenum Amido Complexes with Single MoN Bonds: Synthesis, Structure, and Reactivity
Authors: Morales Dolores , Pérez Julio , Riera Lucía , Riera Víctor , Miguel Daniel , Mosquera Marta E. G. , García‐Granda Santiago 
Published: September 2003

AbstractReactions of the complex [MoCl(η3‐C3H4‐Me‐2)(CO)2(phen)] (1) (phen=1,10‐phenanthroline) with potassium arylamides were used to synthesize the amido complexes [Mo(N(R)Ar)(η3‐C3H4‐Me‐2)(CO)2(phen)] (R=H, Ar=Ph, 2 a; R=H, Ar=p‐tolyl, 2 b; R=Me, Ar=Ph; 2 c). For 2 b the MoN(amido) bond length (2.105(4) Å) is consistent with it being a single bond, with which the metal attains an 18‐electron configuration. The reaction of 2 b with HOTf affords the amino complex [Mo(η3‐C3H4‐Me‐2)(NH2(p‐tol))(CO)2(phen)]OTf (3‐OTf). Treatment of 3‐OTf with nBuLi or KN(SiMe3)2 regenerates 2 b. The new amido complexes react with CS2, arylisothiocyanates and maleic anhydride. A single product corresponding to the formal insertion of the electrophile into the MoN(amido) bond is obtained in each case. For maleic anhydride, ring opening accompanied the formation of the insertion product. The reaction of 2 b with maleimide affords [Mo(η3‐C3H4‐Me‐2){NC(O)CHCHC(O)}(CO)2(phen)] (7), which results from simple acid–base metathesis. The reaction of 2 b with (p‐tol)NCO affords [{Mo(η3‐C3H4‐Me‐2)(CO)2(phen)}2(η2‐MoO4)] (8), which corresponds to oxidation of one third of the metal atoms to MoVI. Complex 8 was also obtained in the reactions of 2 b with CO2 or the lactide 3,6‐dimethyl‐1,4‐dioxane‐2,5‐dione. The structures of the compounds 2 b, 3‐OTf, [Mo(η3‐C3H4‐Me‐2){SC(S)(N(H)Ph)}(CO)2(phen)] (4), [Mo(η3‐C3H4‐Me‐2){SC(N(p‐tol))(NH(p‐tol))}(CO)2(phen)] (5 a), and [Mo(η3‐C3H4‐Me‐2){OC(O)CHCHC(O)(NH(p‐tol))}(CO)2(phen)] (6), 7, and 8 (both the free complex and its N,N′‐di(p‐tolyl)urea adduct) were determined by X‐ray diffraction.
Li3[ScN2]: The First Nitridoscandate(III)—Tetrahedral Sc Coordination and Unusual MX2 Framework
Authors: Niewa Rainer , Zherebtsov Dmitri A. , Leoni Stefano 
Published: September 2003

AbstractLi3[ScN2] was prepared from Li3N with Sc or ScN in a nitrogen atmosphere at 1020 K as a light yellow powder with an optical band gap of about 2.9 eV. The crystal structure was refined based on X‐ray and neutron powder diffraction data (Ia$\bar 3$, Z=16, X‐ray diffraction: Rprofile=0.078, RBragg=0.070; Neutron diffraction: Rprofile=0.077, RBragg=0.074; Rietfeld: a=1003.940(8) pm, Guinier: a=1004.50(3) pm). Li3[ScN2] is an isotype of Li3[AlN2] and Li3[GaN2] and crystallizes in an ordered superstructure of the Li2O structure type, leading to a three‐dimensional framework of all‐vertex‐sharing tetrahedra ${^{3}_{\infty }}$[ScN${_{4/2}^{3-}}$]. Li is displaced from the center of a tetrahedron of N atoms in the direction of one trigonal face. Li3[ScN2] decomposes above 1050 K to form ScN and Li3N. Calculations of the periodic nodal surface (PNS) and of the electron localization function (ELF) support the picture of a covalent ScN network separated from isolated Li cations, whereby scandium d orbitals are involved in the chemical bonding.