bluuebunny/crossref_metadata_2025_split huggingface dataset:
Discrete Triangle, Square and Hexametallic Alkynyl Cyano‐Bridged Compounds Based on [cis‐Pt(CCR)2(CN)2]2− Building Blocks
Authors: Forniés Juan , Gómez Julio , Lalinde Elena , Moreno M. Teresa 
Published: February 2004

AbstractNovel mixed bis(alkynyl)bis(cyano)platinate(II) species [cis‐Pt(CCR)2(CN)2]2− (1 a: R = tBu, 1 b: R = Ph) have been prepared and their potential as building blocks in the generation of self‐organized systems with a variable molecular architecture has been studied. The reaction of 1 with the ditopic acceptor species [{cis‐Pt(C6F5)2S}2(dppa)] (dppa=diphenylphosphinoacetylene) gave the dianionic cyanide/dppa bridged molecular platinotriangles (NBu4)2[(C6F5)2Pt(μ‐dppa){(μ‐CN)2Pt(CCR)2}Pt(C6F5)2] (2). X‐ray analysis of 2 a confirmed that the “Pt2(C6F5)4(μ‐dppa)” binuclear moiety is connected to the dianionic “Pt(CCR)2(CN)2” unit by two bridging cyanide ligands. Moreover, treatment of 1 with the solvent cationic species [M(cod)(acetone)2]+ afforded heterometallic molecular squares Pt2M2 (M=Rh, Ir) containing cyanide bridges and terminal alkynyl ligands, (NBu4)2[cyclo{[cis‐Pt(CCR)2(μ‐CN)2][M(cod)]}2] (3: M=Rh, 4: M = Ir). The solid‐state structures of phenyl derivatives have been determined by X‐ray crystallography. The terminal alkynyl ligands in these cyanide‐bridged molecular squares 3 and 4 have been used in the assembly of higher multimetallic complexes. Thus, very unusual bis(double‐alkynide)‐cyanide‐bridged hexametallic compounds (NBu4)2[{(C6F5)2Pt(μ‐CCPh)2(μ‐CN)2}2{M(cod)}2] (5 b: M=Rh, 6 b: M = Ir) were easily formed by simple reactions of 3 b and 4 b with two equivalents of [cis‐Pt(C6F5)2(thf)2]. An X‐ray diffraction study on complex 5 b indicated that the derivative was formed by a simultaneous migration of one σ‐alkynyl group from each “Pt(CCPh)2(μ‐CN)2” corner of the square to both “Pt(C6F5)2” units, resulting in bent σ,π‐double‐alkynyl bridging systems. Finally, the novel supramolecular anionic assemblies (NBu4)4[cyclo{[cis‐Pt(CCR)2(μ‐CN)2][SnPh3]}4] 7 have been obtained by self‐assembly of 1 and [SnPh3(acetone)2]+.
Prompt Chemoenzymatic Synthesis of Diverse Complex‐Type Oligosaccharides and Its Application to the Solid‐Phase Synthesis of a Glycopeptide with Asn‐Linked Sialyl‐undeca‐ and Asialo‐nonasaccharides
Authors: Kajihara Yasuhiro , Suzuki Yasuhiro , Yamamoto Naoki , Sasaki Ken , Sakakibara Tohru , Juneja Lekh Raj 
Published: February 2004

AbstractWe describe herein the preparation of 24 pure asparagine‐linked oligosaccharides (Asn‐oligosaccharides) from asparagine‐linked biantennary complex‐type sialylundecasaccharide [(NeuAc‐α‐2,6‐Gal‐β‐1,4‐GlcNAc‐β‐1,2‐Man‐α‐1,6/1,3‐)2‐Man‐β‐1,4‐GlcNAc‐β‐1,4‐GlcNAc‐β‐1‐asparagine, 2] obtained from egg yolk. Our synthetic strategy aimed at adapting branch specific exo‐glycosidases digestion (β‐D‐galactosidase, N‐acetyl‐β‐D‐glucosaminidase and α‐D‐mannosidase) of the individual asialo‐branch after preparation of monosialyloligosaccharides obtained from 2 by acid hydrolysis of NeuAc. In order to perform branch specific exo‐glycosidase digestion, isolation of pure monosialyloligosaccharides obtained was essential. However, isolation of two kinds of monosialyloligosaccharides are difficult by HPLC due to their highly hydrophilic nature. Therefore, we examined chemical protection with hydrophobic protecting (Fmoc and benzyl) groups. These chemical protection enabled us to separate the monosialyloligosaccharides by use of a HPLC column (ODS) on synthetic scales. Using these pure monosialiloligosaccharides enable us to obtain 24 Asn‐linked oligosaccharides (100 mg scale) within a few weeks by branch specific exo‐glycosidase digestions (α‐D‐neuraminidase, β‐D‐galactosidase, N‐acetyl‐β‐D‐glucosaminidase and α‐D‐mannosidase). In addition, solid‐phase synthesis of glycopeptide having Asn‐linked sialyl‐undeca‐ and asialo‐nonasaccharides thus obtained, was also performed on an acid labile HMPA‐PEGA resin.
Efficient Synthesis of S‐Linked Glycopeptides in Aqueous Solution by a Convergent Strategy
Authors: Zhu Xiangming , Schmidt Richard R. 
Published: February 2004

AbstractIn naturally occurring glycopeptides and glycoproteins the glycan residues generally possess N‐ and O‐linkages to the peptide backbone. Here we report the synthesis of the corresponding S‐linked glycopeptides by a convergent strategy to provide compounds which should be quite stable to glycosidases. To this end, peptides that contain β‐bromoalanine and γ‐bromohomoalanine were generated either directly by bromination of serine and homoserine residues, respectively, or by standard ligation of the corresponding amino acids. 1‐Thiosugars of O‐acetyl protected GalNAc, GlcNAc, and lactose were prepared by known procedures. Reaction of the thiosugars with these peptides in an ethyl acetate/water two‐phase system, which contained TBAHS and NaHCO3, or in a one‐phase system that consists of DMF/water and which contains NaHCO3, led to the desired S‐linked glycopeptides cleanly and in almost quantitative yield. This reaction also worked well for O‐unprotected 1‐thiosugars.
Adamantanes, Nortricyclenes, and Dendrimers with Extended Silicon Backbones
Authors: Fischer Roland , Baumgartner Judith , Kickelbick Guido , Hassler Karl , Marschner Christoph 
Published: February 2004

AbstractBy reaction of the hexabromoheptasilane MeSi(SiMe2SiMeBr2)3 (1 a) with H2O, H2S, NH3, and H2NMe the heptasilaadamantanes MeSi(SiMe2SiMeO)3 (4), MeSi(SiMe2SiMeS)3 (5), MeSi(SiMe2SiMeNH)3 (6 a), and MeSi(SiMe2SiMeNMe)3 (6 b), respectively, were prepared in good to moderate yields. Molecular structures of 4, 5, 6 a, and 6 b were determined by X‐ray crystallography. The symmetry of the cages is approximately C3v, and the geometry around the nitrogen atoms is essentially planar. Ab initio SCF/HF calculations with the 6‐31G* basis set confirm these results. Reduction of MeSi(SiMe2SitBuBr2)3 (1 b) with lithium naphthalenide afforded the heptasilanortricyclene MeSi(SiMe2SitBu)3 (7). The 29Si NMR spectrum of 7 consists of three signals with chemical shifts that agree closely with values predicted by ab initio calculations. 29Si INADEQUATE spectra also strongly support the nortricyclene structure. Ab initio SCF/HF calculations were performed for the parent molecule Si7H10, and the ring strain of the cage was estimated as 168.8 kJ mol−1 by using the homodesmic reaction Si7H10 + 3 Si2H6→Si13H28. Compound 1 a also served as the starting material for the preparation of first‐generation dendrimer 2 a by reaction with six equivalents of Ph2MeSiLi. Subsequent protodearylation with HBr and reaction with (Me2PhSi)2SiMeK afforded second‐generation dendrimer 3. All dendrimers were characterized by multinuclear NMR spectroscopy.
Nonswelling Macroporous Synbeads for Improved Efficiency of Solid‐Phase Biotransformations
Authors: Basso Alessandra , Braiuca Paolo , De Martin Luigi , Ebert Cynthia , Gardossi Lucia , Linda Paolo , Verdelli Silvia , Tam Andrea 
Published: February 2004

AbstractAn application of novel, highly porous nonswelling resins (Synbeads) for enzymatic catalysis on solid supports is reported. These new resins combine easy handling of the beads, chemical stability, improved accessibility of proteins and higher productivity relative to swelling polymers. The present study demonstrates that the resin porosity greatly affects the efficiency in solid‐phase biotransformations and that Synbead resins are valuable alternatives to swelling polymers for solid‐phase chemistry and biocatalysis. The present study investigates the influence of key parameters, such as porosity and reactive functional‐group density, on the reaction efficiency.
Dendrimers as Ligands: An Investigation into the Stability and Kinetics of Zn2+ Complexation by Dendrimers with 1,4,8,11‐Tetraazacyclotetradecane (Cyclam) Cores
Authors: Saudan Christophe , Balzani Vincenzo , Gorka Marius , Lee Sang‐Kyu , van Heyst Jeroen , Maestri Mauro , Ceroni Paola , Vicinelli Veronica , Vögtle Fritz 
Published: February 2004

AbstractWe have investigated the complexation of Zn2+ with 1,4,8,11‐tetrakis(naphthylmethyl) cyclam (1; cyclam=1,4,8,11‐tetraazacyclotetradecane) and with two dendrimers consisting of a cyclam core with four dimethoxybenzene and eight naphthyl appendages (2), and twelve dimethoxybenzene and sixteen naphthyl appendages (3). An important, common feature of model compound 1 and dendrimers 2 and 3 is that their potentially fluorescent naphthyl units are quenched by exciplex formation with the cyclam nitrogen atoms. Complexation with Zn2+, however, prevents exciplex formation and results in the appearance of an intense naphthyl fluorescence signal that can be used for monitoring the complexation process. Luminescence titration, together with competition experiments and 1H NMR titration, have shown that 1:1 and 1:2 (metal/ligand) complexes are formed in the cases of 2 and 3, whereas model compound 1 gives only a 1:1 complex. We have also investigated the 1:1 complexation kinetics by the stopped‐flow technique. In the case of 1, a second‐order process (k1=44×105 M−1 s−1) is followed by two consecutive first‐order steps (k2=0.53 s−1 and k3=0.10 s−1). For 2, a slower second‐order process (k1=4.9×105 M−1 s−1) is followed by a slow first‐order step (k2=0.40 s−1). In the case of 3, only a very slow second‐order process was observed (k1=1.2×105 M−1 s−1). The different metal–ion incorporation rates for model compound 1 and dendrimers 2 and 3 have been discussed in terms of conformational changes of the dendron subunits affecting the chelating properties of the cyclam core. This work reports the first kinetic study on metal–ion coordination by dendrimers with a well‐defined coordination site.
Mechanism of Dioxygen Activation in 2‐Oxoglutarate‐Dependent Enzymes: A Hybrid DFT Study
Authors: Borowski Tomasz , Bassan Arianna , Siegbahn Per E. M. 
Published: February 2004

AbstractThe reaction mechanism for dioxygen activation in 2‐oxoglutarate‐dependent enzymes has been studied by means of hybrid density functional theory. The results reported here support a mechanism in which all chemical transformations take place on a quintet potential‐energy surface. More specifically, the activated dioxygen species attacks the carbonyl group of the co‐substrate producing the FeII–persuccinate–CO2 complex, which readily releases the carbon dioxide molecule. The step in which the FeII–peracid–CO2 complex is formed is found to be rate‐limiting and irreversible. Subsequent heterolysis of the OO bond in the FeII–persuccinate complex proceeds in two one‐electron steps and produces the high‐valent iron–oxo species FeIVO, which is most likely to be responsible for oxidative reactions catalyzed by 2‐oxoglutarate‐dependent enzymes. The concerted pathway for simultaneous OO and CC bond cleavage on the septet potential‐energy surface is found to be less favorable. The relative stability of different forms of the active iron–oxo species is assessed, and the quintet five‐coordinate complex is found to be most stable.
A Critical Evaluation of the Factors Determining the Effect of Intramolecular Hydrogen Bonding on the OH Bond Dissociation Enthalpy of Catechol and of Flavonoid Antioxidants
Authors: Lucarini Marco , Pedulli Gian Franco , Guerra Maurizio 
Published: February 2004

AbstractNew experimental results on the determination of the bond dissociation enthalpy (BDE) value of 3,5‐di‐tert‐butylcatechol, a model compound for flavonoid antioxidants, by the EPR radical equilibration technique are reported. By measurement of the equilibrium constant for the reaction between 3,5‐di‐tert‐butylcatechol and the 2,6‐di‐tert‐butyl‐4‐methylphenoxyl radical, in UV irradiated isooctane solutions at different temperatures, it has been shown that the thermodynamic parameters for this reaction are ΔH° = −2.8±0.1 kcal mol−1 and ΔS° = +1.3±0.2 cal mol−1 K−1. This demonstrates that the entropic variations in the hydrogen exchange reaction between phenols and the corresponding phenoxyl radicals are also negligible when one of the reacting species is a polyphenol and that the EPR radical equilibration technique also allows the determination of the OH BDEs in intramolecularly hydrogen‐bonded polyphenols.The BDE of 3,5‐di‐tert‐butylcatechol (78.2 kcal mol−1) was determined to be identical to that of α‐tocopherol. Through use of the group additivity rule, this piece of data was also used to calculate the strength of the intramolecular hydrogen bond between the hydroxyl proton and the oxygen radical centre in the corresponding semiquinone radical (5.6 kcal mol−1), which is responsible both for the excellent antioxidant properties of catechols and for the BDE of catechol (81.8 kcal mol−1). These values are in poor agreement with those predicted by DFT calculations reported in the literature (9.5 kcal mol−1 and 77.6 kcal mol−1, respectively). Extensive theoretical calculations indicate that the BDE of catechol is reproduced well (81.6 kcal mol−1) by use of diffuse functions on oxygen and the CCSD method.
The Diphosphate Monoanion in the Gas Phase: A Joint Mass Spectrometric and Theoretical Study
Authors: Ricci Andreina , Pepi Federico , Stefano Marco Di , Rosi Marzio 
Published: February 2004

AbstractH3P2O7− ions were obtained in an electrospray ion source of a Fourier transform ion cyclotron resonance (ESI/FTICR) mass spectrometer from a CH3CN/H2O (1:1) pyrophosphoric acid solution and in the ionic source of a triple quadrupole (TQ) mass spectrometer from the chemical ionisation (CI) of pyrophosphoric acid introduced by a thermostatically controlled direct insertion probe. The ions were structurally characterised by mass spectrometric techniques and theoretical calculations. Consistent with collisionally activated dissociation (CAD) mass spectrometric results, theoretical calculations identified the linear diphosphate anion (I) as the most stable isomer on the H3P2O7− potential energy surface. The joint application of mass spectrometric techniques and theoretical methods provided information on the dissociative processes of diphosphate anions in the gas phase. Finally, this study provides an insight into the structures and stabilities of the [H3PO4⋅⋅⋅PO3]−, [HP2O6⋅⋅⋅H2O]− and [H2PO4⋅⋅⋅HPO3]− clusters and allows the stability and structure of the dimetaphosphate anion, HP2O6−, to be investigated at the B3LYP6‐31+G* and CCSD(T) levels of theory.
Aided Self‐Assembly of Porphyrin Nanoaggregates into Ring‐Shaped Architectures
Authors: Lensen Marga C. , Takazawa Ken , Elemans Johannes A. A. W. , Jeukens Cecile R. L. P. N. , Christianen Peter C. M. , Maan Jan C. , Rowan Alan E. , Nolte Roeland J. M. 
Published: February 2004

AbstractThe formation of micrometer‐sized, highly ordered porphyrin rings on surfaces has been investigated. The porphyrin‐based nanoarchitectures are formed by deposition from evaporating solutions through a surface dewetting process which can be tuned by variations in the substitution pattern of the molecules used, the coating of the surface and the conditions under which the evaporation takes place. Control over the combined self‐assembly and surface dewetting results in nanorings possessing a defined internal architecture. The ordering of the molecules within the rings has been studied by a variety of microscopy techniques (TEM, AFM, fluorescence microscopy) and the exact ordering of the porphyrins within the rings has been quantified.
Straightforward Synthesis of (R)‐(−)‐Kjellmanianone
Authors: Christoffers Jens , Werner Thomas , Frey Wolfgang , Baro Angelika 
Published: February 2004

AbstractA direct route to enantiomerically pure (−)‐kjellmanianone is reported. The synthesis involves a cerium‐catalyzed α‐hydroxylation and an enzyme‐catalyzed procedure to resolve tertiary alcohols at key stages. The intermediate β‐oxo ester was α‐hydroxylated to give good yields of racemic kjellmanianone. The resolution of the racemic material was achieved by enzymatic saponification, followed by a chemical decarboxylation sequence to give enantiopure (−)‐kjellmanianone with 99 % ee. Bromination then afforded the (−)‐bromo derivative, whose X‐ray structure provided evidence for the R configuration of (−)‐kjellmanianone.
The Control of the Nitrogen Inversion in Alkyl‐Substituted Diaziridines
Authors: Trapp O. , Schurig V. , Kostyanovsky R. G. 
Published: February 2004

AbstractFor the first time the nitrogen inversion barriers in 3,3‐unsubstituted trans‐diaziridines, such as 1,2‐di‐tert‐butyldiaziridine (1) and 1,2‐di‐n‐butyldiaziridine (2) were determined. Enantioselective stopped‐flow multidimensional gas chromatography was used to investigate the enantiomerization barrier of 1 between 126.2 and 171.0 °C (ΔG${{{\ne}\hfill \atop {\rm gas}\hfill}}$(150.7 °C) = 135.8±0.2 kJ mol−1, ΔH${{{\ne}\hfill \atop {\rm gas}\hfill}}$ = 116.1±2.5 kJ mol−1, ΔS${{{\ne}\hfill \atop {\rm gas}\hfill}}$ = −46±2 J K−1 mol−1). The separation of the enantiomers has been achieved in presence of the chiral stationary phase (CSP) Chirasil‐β‐Dex with a high separation factor (α = 1.44 at 80 °C). In a complementary approach, the enantiomerization barriers of 1,2‐di‐tert‐butyldiaziridine (1), 1,2‐di‐n‐butyldiaziridine (2), 1‐n‐butyl‐3,3‐dimethyldiaziridine (3), and 1,2,3,3‐tetramethyldiaziridine (4) were determined for comparison by enantioselective dynamic chromatography (DGC) and computer simulation of the dynamic elution profiles. The enantiomerization barrier of 2 was shown to be the highest among the nonsterically hindered diaziridines studied so far, whereas 1 exhibited the highest value found for strained nitrogen‐containing rings, that is, aziridines, diaziridines and oxaziridines.
Binding N2, N2H2, N2H4, and NH3 to Transition‐Metal Sulfur Sites: Modeling Potential Intermediates of Biological N2 Fixation
Authors: Sellmann Dieter , Hille A. , Rösler A. , Heinemann F. W. , Moll M. , Brehm G. , Schneider S. , Reiher M. , Hess B. A. , Bauer W. 
Published: February 2004

AbstractIn the quest for low‐molecular‐weight metal sulfur complexes that bind nitrogenase‐relevant small molecules and can serve as model complexes for nitrogenase, compounds with the [Ru(PiPr3)(‘N2Me2S2’)] fragment were found (‘N2Me2S2’2−=1,2‐ethanediamine‐N,N′‐dimethyl‐N,N′‐bis(2‐benzenethiolate)2−). This fragment enabled the synthesis of a first series of chiral metal sulfur complexes, [Ru(L)(PiPr3)(‘N2Me2S2’)] with L=N2, N2H2, N2H4, and NH3, that meet the biological constraint of forming under mild conditions. The reaction of [Ru(NCCH3)(PiPr3)(‘N2Me2S2’)] (1) with NH3 gave the ammonia complex [Ru(NH3)(PiPr3)(‘N2Me2S2’)] (4), which readily exchanged NH3 for N2 to yield the mononuclear dinitrogen complex [Ru(N2)(PiPr3)(‘N2Me2S2’)] (2) in almost quantitative yield. Complex 2, obtained by this new efficient synthesis, was the starting material for the synthesis of dinuclear (R,R)‐ and (S,S)‐[μ‐N2{Ru(PiPr3)(‘N2Me2S2’)}2] ((R,R)‐/(S,S)‐3). (Both 2 and 3 have been reported previously.) The as‐yet inexplicable behavior of complex 3 to form also the R,S isomer in solution has been revealed by DFT calculations and 2D NMR spectroscopy studies. The reaction of 1 or 2 with anhydrous hydrazine yielded the hydrazine complex [Ru(N2H4)(PiPr3)(‘N2Me2S2’)] (6), which is a highly reactive intermediate. Disproportionation of 6 resulted in the formation of mononuclear diazene complexes, the ammonia complex 4, and finally the dinuclear diazene complex [μ‐N2H2{Ru(PiPr3)(‘N2Me2S2’)}2] (5). Dinuclear complex 5 could also be obtained directly in an independent synthesis from 1 and N2H2, which was generated in situ by acidolysis of K2N2(CO2)2. Treatment of 6 with CH2Cl2, however, formed a chloromethylated diazene species [{Ru(PiPr3)(‘N2Me2S2’)}‐μ‐N2H2{Ru(Cl)(‘N2Me2S2CH2Cl’)}] (9) (‘N2Me2S2CH2Cl’2− =1,2‐ethanediamine‐N,N′‐dimethyl‐N‐(2‐benzenethiolate)1−‐N′‐(2‐benzenechloromethylthioether)1−]. The molecular structures of 4, 5, and 9 were determined by X‐ray crystal structure analysis, and the labile N2H4 complex 6 was characterized by NMR spectroscopy.
Molecular Structure, Spectroscopy and Matrix Photochemistry of Fluorocarbonyl Iodide, FC(O)I
Authors: Chiappero M. S. , Argüello G. A. , Garcia P. , Pernice H. , Willner H. , Oberhammer H. , Peterson K. A. , Francisco J. S. 
Published: February 2004

AbstractThe molecular structure of FC(O)I has been determined by gas electron diffraction. High‐level ab initio methods, including coupled‐cluster and the new correlation‐consistent basis sets for fourth row elements, have been used to calculate the structure of FC(O)I. A comprehensive vibrational spectroscopic study (both IR and Raman) complemented by high‐level calculations has also been performed. Furthermore, UV, mass, and NMR spectra have been recorded for FC(O)I. The matrix photochemistry of FC(O)I has been studied with a low‐pressure mercury lamp and with a high‐pressure xenon lamp in combination with interference and cut‐off filters. UV photolysis revealed the formation of the OC⋅⋅⋅IF and OC⋅⋅⋅FI complexes and further photolysis of these complexes at λ>320 nm resulted in a re‐formation of FC(O)I. The structural conformation of the complexes has been characterized by comparing shifts in their CO and IF vibrational modes with respect to those of the free species. The structures, vibrational properties, and stability of the complexes were analyzed with the aid of coupled‐cluster ab initio calculations.
Iodine(III)‐Mediated Preparations of Nitrogen‐Containing Sulfur Derivatives: Dramatic Influence of the Sulfur Oxidation State
Authors: Leca Dominique , Song Kai , Amatore Muriel , Fensterbank Louis , Lacôte Emmanuel , Malacria Max 
Published: February 2004

AbstractReaction of sulfonamides with iodosobenzene leads to phenyliodinanes. A new catalysis reaction of the decomposition of these products in the presence of sulfoxides that allows the smooth synthesis of sulfoximines has been evidenced and studied: copper(II) salts were used to prepare compounds 4 a–j and 5 b, d, f, j, k from the corresponding, easily prepared, sulfoxides. The reactions proceed with retention of configuration at the sulfur center, and copper(II) triflate is the best candidate for the catalyst for the imination. Switching from sulfonamides to sulfinamides in the preparation of the starting iodinanes completely alters the reaction pathway: iodinanes are no longer accessible, and sulfonimidates 7 a–j are obtained instead. This behavior can be rationalized by the increase in pKa brought about by the removal of one oxygen atom from the sulfur center. Sulfonimidates are interesting molecules with varied applications. Optimization of their one‐pot synthesis has been achieved by carrying out the reaction in acetonitrile. The stereochemical study has shown that the transformation proceeds with global retention of the configuration at the sulfur center, albeit with erosion of the enantiomeric purity. A model accounting for this outcome is proposed. In addition, the presence of oxidized sulfonamide by‐products has been explained, and this latter pathway becomes the sole one when alcohol is replaced by water. Good yields of the oxidized products are obtained.