bluuebunny/crossref_metadata_2025 huggingface dataset:
Silica‐Gel‐Confined Ionic Liquids: A New Attempt for the Development of Supported Nanoliquid Catalysis
Authors: Shi Feng , Zhang Qinghua , Li Dongmei , Deng Youquan
Published: July 2005

A new concept of designing and synthesizing highly dispersed ionic‐liquid catalysts was developed through physical confinement or encapsulation of ionic liquids (with or without metal complex) in a silica‐gel matrix through a sol–gel process. We studied ionic liquids such as EMImBF4, BuMImBF4, DMImBF4, CMImBF4, BuMImPF6, either with or without [Pd(PPh3)2Cl2] and [Rh(PPh3)3Cl], in a silica‐gel matrix (E=ethyl, Bu=butyl M=methyl, D=decyl, C=cetyl and Im=imidazolium). The contents of ionic liquids and loadings of Pd or Rh were 8–53 wt % and 0.1∼0.15 wt %, respectively. Analyses of FT‐Raman spectra showed that abnormal Raman spectra of the confined ionic liquids were observed in comparison with the bulk and pure ionic liquids. EMImBF4 and BuMImBF4 ionic liquids could be completely washed out from the silica‐gel matrix under vigorous reflux conditions, but ionic liquids with larger molecular size, for example, DMImBF4 or CMImBF4, could be confined into the silica‐gel nanopores relatively firmly. These results suggested that the ionic liquids were physically confined or encapsulated into the silica gel. The N2 adsorption measurements indicated that the silica‐gel skeleton was mesoporous with 50–110 Å pore size after the BuMImBF4 ionic liquid was removed completely. Transmission electron microscopy (TEM) and X‐ray diffraction (XRD) analysis showed that the silica‐gel matrix was amorphous and non‐uniformly mesoporous. Carbonylation of aniline and nitrobenzene for synthesis of diphenyl urea, carbonylation of aniline for synthesis of carbamates, and oxime transformation between cyclohexanone oxime and acetone were used as test reactions for these catalysts. Catalytic activities were remarkably enhanced with much lower amounts of ionic liquids needed with respect to bulk ionic‐liquid catalysts or silica‐supported ionic‐liquid catalysts prepared with simple impregnation, in which the ionic liquid may be deposited as a thin layer on the support. Such unusual enhancement in catalytic activities may be attributed to the formation of nanoscale and high‐concentration ionic liquids due to the confinement of the ionic liquid in silica gel; this results in unusual changes in the symmetry and coordination geometry of the ionic liquids.
Rotamer Stability incis‐[Pt(diA)G2] Complexes (diA=Diamine Derivative and G=Guanine Derivative) Mediated by Carrier‐Ligand Amine Stereochemistry as Revealed by Circular Dichroism Spectroscopy
Authors: Benedetti Michele , Marzilli Luigi G. , Natile Giovanni
Published: July 2005

Extensive investigations ofcis‐[Pt(diA)G2] complexes (in which G=a guanine ligand; diA=a single diamine ligand) revealed the types of interactions between the two G ligands and between the G and thecis‐amine substituents when diA is a diamine ligand with substituents on each nitrogen atom being a small hydrogen atom and a bulky group able to slow the rotation about the PtG bond. All these interactions are shown to apply also when diA=dach (1,2‐diaminocyclohexane), even though this chiral primary diamine has only small NH atoms on each side of the coordination plane. However, a slight difference in the stereochemistry of the two protons (one NH has “quasi axial” and the other “quasi equatorial” character) is sufficient to induce a significant change in the relative stabilities of the [Pt(dach)G2]ΔHT andΛHT rotamers (HT=head‐to‐tail). The new results show that at acidic and neutral pH the induction of asymmetry from the dach ligand to the HT rotamers is governed by the G‐to‐G dipole–dipole interaction, which is greater for the six‐membered ring of each guanine leaning towards thecis‐G. Such a “six‐in” canting of the two guanine ligands can be hampered by the steric interaction between the H8 of each guanine and the substituent on thecis‐amine that is on the same side of the coordination plane. Such a repulsion is greater for a “quasi equatorial” NH than for a “quasi axial” NH. Under basic pH conditions, deprotonation of the guanine N1H renders the O6 atom a much better hydrogen‐bond acceptor; therefore, the stability of the HT rotamers is governed by the hydrogen‐bond interaction of guanine O6 and thecis‐amine NH group. Such a guanine O6/NHcis‐amine interaction is stronger for a “quasi axial” than for a “quasi equatorial” NH group. In the head‐to‐head (HH) rotamer, in which the electrostatic repulsion between electron‐rich O6 atoms, both on the same side of the platinum coordination plane, tends to place the six‐membered rings of each guanine further from thecis‐guanine and closer to thecis‐amine, we can expect better NH⋅⋅⋅O6 hydrogen bonding for the “quasi equatorial” NH groups.
The Sustainability of Substitute Family Care for Children Separated from Their Families by War: Evidence from Mozambique
Authors: Charnley Helen
Published: July 2005

This article presents the findings of an empirical study exploring the sustainability of the substitute family in supporting children separated from their families during Mozambique's 16‐year civil conflict. It describes shifts in the boundaries that have defined arrangements for the care of children separated from their normative family care givers and shows that, contrary to received wisdom based on traditional forms of child care, children and substitute families have achieved lasting relationships through new forms of mutual support that typify indigenous coping mechanisms in times of stress.
Study on the Binding of Base‐Mismatched Oligonucleotide d(GCGAGC)2 by Cobalt(III) Complexes
Authors: Chen Huili , Yang Pin , Yuan Caixia , Pu Xiaohua
Published: July 2005

The binding of [Co(phen)2(HPIP)]Cl3 and [Co(phen)2(DPQ)]Cl3 to the oligonucleotide d(GCGAGC)2 containing two sheared G:A mispairs has been studied by NMR spectroscopy for the first time. For both the complexes, a considerable number of intermolecular NOEs were observed in NOESY spectra of the cobalt‐complex‐bound hexanucleotide. The results suggest that [Co(phen)2(HPIP)]Cl3, with HPIP, intercalates between the G3:A4 base pairs from the minor groove and extends to the major groove, and [Co(phen)2 (DPQ)]3+ binds to the terminal G5:C6 region from two directions. 31P NMR spectroscopy indicates that [Co(phen)2(HPIP)]Cl3 binding induces a change in the phosphate backbone in the region of the mismatched base pairs, while [Co(phen)2 (DPQ)]3+ does not cause obvious changes in the backbone. Melting experiments indicate that [Co(phen)2(HPIP)]Cl3 stabilizes the double‐strand structure, while [Co(phen)2(DPQ)]Cl3 cause the duplex to untwist. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Adsorption of Zn2+ and Cd2+ from Aqueous Solution onto a Carbon Sorbent Containing a Pyrimidine–Polyamine Conjugate as Ion Receptor
Authors: García‐Martín Javier , López‐Garzón Rafael , Godino‐Salido Mari Luz , Cuesta‐Martos Rafael , Gutiérrez‐Valero Maria Dolores , Arranz‐Mascarós Paloma , Stoeckli‐Evans Helen
Published: July 2005

The compound N‐(4‐amino‐1‐methyl‐5‐nitroso‐6‐oxo‐1,6‐di‐hydropyrimidin‐2‐yl)‐N'‐[bis(2‐aminoethyl)]ethylenedi‐amine (HL) was synthesised and structurally characterised by analytical methods and 1H, 13C and 15N NMR spectroscopy. The protonation of HL and its coordinating ability towards Zn2+ and Cd2+ ions in water solution were also studied by potentiometric methods and UV/Visible and 1H‐13C NMR spectroscopy. The molecular structures of [H3L]2+[CdI4]2–·2H2O and [(Cl3Zn)(μ‐Cl){Zn(μ2‐HL)}]2·2.5H2O were solved by single‐crystal X‐ray diffraction methods. On the basis of these data, the performance of HL anchored to activated carbon (AC) was evaluated as a molecular receptor for Zn2+ and Cd2+ metal ions in aqueous solution. This was done by a comparative study of the adsorption capacity of the adsorbents AC alone and HL anchored on the AC (AC‐HL) for these metal ions and by a structural study of the AC‐HL, AC‐HL‐Zn2+ and AC‐HL‐Cd2+ solid phases by XPS and IR spectroscopy. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Influence of 2‐Aminopyridine on the Formation of Molybdates under Hydrothermal Conditions
Authors: Pavani Katikaneani , Ramanan Arunachalam
Published: July 2005

The hydrothermal reaction of an aqueous ammonium heptamolybdate solution with first‐row transition metal salts in the presence of 2‐aminopyridine at around 180 °C and under autogenous pressure yields several molybdates, such as the 1D chains in (C5N2H7)4Mo8O26 (1), zero‐dimensional (C5N2H7)6Mo7O24·3H2O (2), and the first synthesis of Lindgrenite [Cu3Mo2O8(OH)2 (3)] The structures of these materials were established by single‐crystal and powder X‐ray diffraction techniques. The crystal structures of wolframite‐based MnMoO4 (4) and ZnMoO4 (5) were established by powder X‐ray diffraction. The role of 2‐aminopyridine (2‐ampy) in the formation of the different solids under hydrothermal conditions is discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Identification of Copper(II) Binding Sites in the Aminoglycosidic Antibiotic Neomycin B
Authors: Jeżowska‐Bojczuk Małgorzata , Szczepanik Wojciech , Mangani Stefano , Gaggelli Elena , Gaggelli Nicola , Valensin Gianni
Published: July 2005

Protonation and copper(II) coordination properties of neomycin B were studied in solution by potentiometry, NMR, UV/Vis, CD, and EPR spectroscopy, XAS and mass spectrometry. Mono‐ and dinuclear complexes were found depending on the metal‐to‐ligand molar ratio. Neomycin B anchors CuIIions above pH 5.0 with an NH2group from ring B. Simultaneously, the second amino group of the same ring and the hydroxyl group of ring A complete the binding set of donors. With an increase in pH the remaining –NH3+functional groups in the neomycin B molecule are deprotonated without affecting the complexation pattern. However, these groups, particularly the ones located in theD‐ring of the antibiotic, may coordinate the second copper(II) ion when the metal is present in excess. We have proved this process with the use of potentiometry, CD and especially mass spectrometry. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Copper, Cobalt and Platinum Complexes with Dithiothiophene‐Based Ligands
Authors: Belo Dulce , Figueira Mauro J. , Mendonça Joana , Santos Isabel C. , Almeida Manuel , Henriques Rui T. , Duarte Maria Teresa , Rovira Concepció , Veciana Jaume
Published: July 2005

The preparation and characterisation of new thiophenedithiolate complexes of Cu, Co and Pt with the ligands α‐tpdt and tpdt (α‐tpdt = 2,3‐thiophenedithiolate, tpdt = 3,4‐thiophenedithiolate) is reported. The Co and Pt complexes present regular square‐planar coordination geometry and low oxidation potentials when compared with complexes with simpler ligands, but their monoanionic state is found to be rather unstable. The Co(tpdt)2 complex can be isolated both in the monoanionic and dianionic states as a stable compound but the Co(α‐tpdt)2 and the Pt complexes can be obtained only in the dianionic state, while their monoanionic state is unstable. The Co and Pt complexes can, however, be easily oxidised to the neutral state, giving a fine‐powdered microcrystalline material without high electrical conductivity. With Cu only a less frequent geometry based on a tetrametallic CuI cluster and three ligands is observed as [Cu4(α‐tpdt)3]2– and [Cu4(tpdt)3]2–. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
The Reductive Elimination of Methane from ansa‐Hydrido(methyl)metallocenes of Molybdenum and Tungsten: Application of Hammond’s Postulate to Two‐State Reactions
Authors: Carreón‐Macedo José‐Luis , Harvey Jeremy N. , Poli Rinaldo
Published: July 2005

The energetic profile of the methane reductive elimination from a selected number of hydrido(methyl)molybdenocene and ‐tungstenocene derivatives has been calculated by DFT methods. The calculations were carried out for the CH2(C5H4)2M (a‐M), SiH2(C5H4)2M (a‐H2Si–M), and SiMe2(C5Me4)2M (a‐Me2Si–M*) ansa‐metallocene systems for M = Mo, W. They include the full optimization of minima [the hydrido(methyl) starting complexes, M(H)(CH3), the intermediate methane complexes, M(CH4), and the metallocene products in the singlet and triplet configurations, (3M and 1M)], transition states (for the methyl hydride reductive elimination, M–TSins, and for the hydrogen exchange, M–TSexch), and the minimum energy crossing point (M–MECP) leading from the singlet methane complexes to the corresponding triplet metallocenes. The results are compared with those previously obtained for the simpler (C5H5)2M (Cp2M) systems (J. C. Green, J. N. Harvey, and R. Poli, J. Chem. Soc., Dalton Trans. 2002, 1861). The calculated energy profiles, notably the relative energies of M–TSins and M–MECP, are in agreement with available experimental observations for the a‐Me2Si–M* systems. The comparison of the energies and geometries of the rate‐determining M–TSins and M–MECP structures with those of the thermodynamically relevant minima for the various systems show the applicability of Hammond’s postulate to two‐state reactions. However, one notable exception serves to show that the principle is only quantitatively reliable when all the potential energy surfaces for the set of analogous reactions have similar shapes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Synthesis and Reactivity of 1,2‐ and 1,3‐Diphosphanes that Contain Four Chiral Rhenium Fragments: Architecturally Novel Tetrametallo‐DMPE and ‐DMPP Species that are Unprivileged Ligands for Enantioselective Catalysis
Authors: Kromm Klemenz , Eichenseher Sandra , Prommesberger Markus , Hampel Frank , Gladysz J. A.
Published: July 2005

Reactions of enantiopure (S)‐[(η5‐C5H5)Re(NO)(PPh3)(=CH2)]+ PF6– [(S)‐2] and PH2CH2(CH2)nCH2PH2 (0.5 equiv.) give (SReSRe)‐[(η5‐C5H5)Re(NO)(PPh3){CH2PH2CH2(CH2)nCH2PH2CH2}(Ph3P)(ON)Re(η5‐C5H5)]2+ 2PF6– [n = 0/1, (SReSRe)‐3/4; 65–62/77–58 %]. Reaction of racemic 2 (BF4– salt) and PH2(CH2)2PH2 (0.5 equiv.) gives the meso and rac diastereomers of 3 (BF4– salts) in 28 % and 38 % yields after crystallization. Treatments of (SReSRe)‐3/4 with tBuOK and then (S)‐2 give the tetrarhenium complexes (SReSReSReSRe)[{(η5‐C5H5)Re(NO)(PPh3)(CH2)}2{PHCH2(CH2)nCH2PH}{(CH2)(Ph3P)(ON)Re(η5‐C5H5)}2]2+ 2PF6– [n = 0/1, (SReSReSReSRe)‐7/8; 89–88/98–87 %]. The crystal structure of(SReSReSReSRe)‐7 is determined and its conformation analyzed. Reactions of (SReSReSReSRe)‐7/8 and tBuOK give air‐sensitive diphosphanes (SReSReSReSRe)‐{(η5‐C5H5)Re(NO)(PPh3)(CH2)}2{PCH2(CH2)nCH2P}{(CH2)(Ph3P)(ON)Re(η5‐C5H5)}2 [n = 0/1, (SReSReSReSRe)‐9/10; 92/62 %]. Additions of (a) PhIO give the corresponding dioxides (72/62 %), and (b) [Rh(NBD)2]+ PF6– give the corresponding chelates [(P–P)Rh(NBD)]+ PF6– (75/82 %) (NBD = norbornadiene). These catalyze hydrogenations of protected dehydroamino acids and hydrosilylations of propiophenone with only modest enantioselectivities. Similar results are obtained when (SReSReSReSRe)‐9/10 are applied in rhodium‐catalyzed conjugate additions of aryl boronic acids, or palladium‐catalyzed allylic alkylations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Synthetic and Theoretical Aspects of New Dimroth Rearrangement of6‐Aminopyran‐2‐ones to 6‐Hydroxypyridin‐2‐ones via Carbamoyl Ketenes
Authors: Subbotina Julia O. , Fabian Walter M. F. , Tarasov Evgeniy V. , Volkova Natalia N. , Bakulev Vasiliy A.
Published: July 2005

Two alternative directions for thermal transformation of 6‐amino‐4‐oxopyrano[3,4‐d] [1,2,3]thiadiazoles1, leading either to 6‐hydroxy‐4‐oxo‐[1,2,3]thiadiazolo[4,5‐c]pyridines2or 2‐cyano‐2‐(1,2,3‐thiadiazol‐5‐yl)acetamide4b, were observed. The first one represents a new, Dimroth‐type rearrangement and proceeds by thermal opening of the pyrane ring, followed by the simultaneous rotational isomerization of the ketene intermediate7(s‐cis) to7(s‐trans) and its recyclization onto the amido group to form the pyridin‐2‐one cycle.The first step of the rearrangement has a calculated [B3LYP/6‐31G(d)] activation barrier of 24–34 kcal/mol, the involvement of amines reduces it by ca. 5 kcal/mol [PCM‐B3LYP/6‐31G(d), DMSO]. In contrast, the recyclization step onto the amido group is calculated to occur essentially barrierless (ΔE< 2 kcal/mol). The influence of substituents, in particular of those capable of intramolecular hydrogen bonding, on the preferred reaction path was also studied. The alternative ring opening of the pyrane cycle was calculated to be at least 5 kcal/mol [B3LYP/6‐31G(d)] less favorable than the Dimroth rearrangement. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
The P‐Stereocontrolled Synthesis of PO/PS‐Chimeric Oligonucleotides by Incorporation of Dinucleoside Phosphorothioates Bearing an O‐4‐Nitrophenyl Phosphorothioate Protecting Group
Authors: Wozniak Lucyna A. , Góra Marcin , Bukowiecka‐Matusiak Małgorzata , Mourgues Sophie , Pratviel Geneviève , Meunier Bernard , Stec Wojciech J.
Published: July 2005

The synthesis of protected model dinucleoside (3',5')‐O‐aryl phosphorothioates, their separation into pure diastereomers, and their successful incorporation into oligonucleotides followed by stereospecific deprotection of the O‐aryl phosphorothioate function with oximate ion (inversion) enables the preparation of chimeric PO/PS‐oligonucleotides with a predetermined sense of P‐chirality at each internucleotide phosphorothioate position. The absolute configuration at the phosphorus of the internucleotide O‐aryl phosphorothioate in “dimeric building blocks” has been assigned. The 3'‐terminal SP‐phosphorothioate linkages effectively protect such chimeric constructs from degradation by human plasma exonuclease (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Flexible Stereo‐ and Regioselective Synthesis of myo‐Inositol Phosphates(Part 2): Via Nonsymmetrical Conduritol B Derivatives
Authors: Podeschwa Michael A. L. , Plettenburg Oliver , Altenbach Hans‐Josef
Published: July 2005

A practical route is described for the preparation of myo‐inositol phosphates. Optically pure compounds can be prepared, in both forms, from p‐benzoquinone by enzymatic resolution of a diacetoxyconduritol key intermediate. Monosubstituted inositol derivatives can be obtained by breaking the C2 symmetry of conduritol B derivatives. A wide variety of myo‐inositol phosphates can be synthesized by combining the previously reported symmetrical approach with this new nonsymmetrical approach. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
A Soluble and Reusable Colorimetric Sensor Based on the Covalent Attachment of a Triarylpentenedione to Poly(ethylene glycol)
Authors: Alvaro Mercedes , Aprile Carmela , García Hermenegildo , Peris Encarna
Published: July 2005

3‐(4‐Aminophenyl)‐1,5‐diphenylpent‐2‐ene‐1,5‐dione (PDO) is a colorimetric sensor of Lewis acid cations that has been covalently attached to a poly(ethylene glycol) (PEG) scaffold. The resulting PDO‐PEG is soluble in water and other organic solvents and acts as a colorimetric sensor of Lewis acid cations by undergoing cyclization into the triarylpyrylium form. In water, PDO‐PEG also undergoes the colorimetric change in the presence of Brönsted acids, which may limit its usefulness. This modified PEG is recoverable by precipitation from cold ethanol and reusable after reopening of the triarylpyrylium heterocycle to the active pentenedione form. The testing‐recovery‐reuse cycle has been performed five times without noticeable differences in the response. In contrast to PDO‐PEG, PDO on its own is insoluble in water and remains completely soluble in ethanol. This solubility behavior of PDO makes the molecule unsuitable for use as a reusable and recoverable sensor. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
New Insights into the Reactivity of Nitrogen Dioxide with Substituted Phenols: A Solvent Effect
Authors: Astolfi Paola , Panagiotaki Maria , Greci Lucedio
Published: July 2005

Various alkyl‐substituted phenols react readily with nitrogen dioxide (·NO2) in different solvents at room temperature. In all cases nitration is the major reaction and leads to the formation of mono‐ and dinitrophenols and 4‐nitrocyclohexa‐2,5‐dienones from 2,4,6‐tri‐substituted phenols. Oxidation, dimerisation and, in one case, nitrosation are also observed. The reaction pathway followed changes according to the solvent and to the nature and the number of substituents on the phenolic ring. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
The First Chemical Synthesis of UDP[6‐3H]‐α‐D‐galactofuranose
Authors: Mariño Karina , Marino Carla , Lima Carlos , Baldoni Luciana , de Lederkremer Rosa M.
Published: July 2005

Galactofuranose metabolism is a good target for the development of novel chemotherapeutic agents for the treatment of some microbial infections. This is a valid objective because galactofuranose is absent in mammals. Two enzymes are involved in the biosynthesis of molecules containing galactofuranose: a mutase, which catalyzes the interconversion of UDP‐Galp and UDP‐Galf, and D‐galactofuranosyltransferases. The mechanism of action of the mutase and its inhibition is currently being investigated, whereas studies on the galactofuranosyltransferases have been hampered by the lack of a labeled galactofuranose nucleotide. In the present work we describe the chemical synthesis of UDP‐α‐D‐[6‐3H]Galf and we prove its effectiveness for incorporation of radioactive galactofuranose into a natural acceptor. This is the first report on the chemical synthesis of a labeled donor of galactofuranose with the potential for studying the galactofuranosyltransferases independently from the UDP‐Galp mutase. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Enantioselective Synthesis of α‐Alkylmalates as the Pharmacophoric Group of Several Natural Alkaloids and Glycosides
Authors: El Bialy Serry A. A. , Braun Holger , Tietze Lutz F.
Published: July 2005

A general enantioselective synthesis of α‐alkylmalates found in Cephalotaxus and Orchidaceae species is described. This preparation is based upon Seebach’s procedure for the alkylation of D‐malic acid with self‐regeneration of stereogenic centers. Reaction of the dioxolanone 8a with LiHMDS and allyl halides as well as benzyl bromide gave 10 and 14, respectively, which could be transformed into enantiopure α‐alkylmalates 13, 15, 18 and 19. In addition, enantiopure furans of type 20 have been prepared. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Diastereoselective Reactions of the Tiglic Acid Functionality Mediated by Oxazolidine Chiral Auxiliaries: A Mechanistic Comparison of DMD andm‐CPBA Epoxidations versus Singlet Oxygen and PTAD Ene Reactions
Authors: Pastor Aurelia , Adam Waldemar , Wirth Thomas , Tóth Gábor
Published: July 2005

2,2‐Dimethyloxazolidines have been utilized as chiral auxiliaries for the diastereoselective functionalization of the optically active tiglic acid derivatives (S)‐1 by means of epoxidation with DMD or m‐CPBA and ene reactions with 1O2 or PTAD. In the DMD and m‐CPBA epoxidations, high diastereoselectivities but opposite senses of diastereomer selection were observed. In contrast, the stereochemistry of the 1O2 and PTAD ene reactions depended on the size of the attacking enophile: whereas essentially perfect diastereoselectivity was obtained with PTAD, much lower stereoselection was observed with 1O2. The stereochemical results for the DMD and m‐CPBA epoxidations and the PTAD ene reaction are explained in terms of the energy differences for the corresponding diastereomeric transition states, dictated by steric and electronic effects. The PTAD ene reaction for these tiglic acids (S)‐1 provides, after removal of the chiral auxiliaries, an attractive synthetic route for optically active β‐amino acid derivatives.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Synthesis of Rigid Receptors Based on Triphenylene Ketals
Authors: Schopohl Matthias C. , Faust Andreas , Mirk Daniela , Fröhlich Roland , Kataeva Olga , Waldvogel Siegfried R.
Published: July 2005

The synthesis of rigid receptors based on triphenylene ketals, including some improved procedures, is described in detail. Since chemical transformations are strongly influenced by the rigid character and steric bulkiness of the receptors, the construction of a subunit allowing quicker synthetic development is also reported. The preparation of these receptor structures involves an oxidative trimerization and a subsequent repeated isomerization procedure. The scope for the attachment of sterically demanding affinity groups on the receptor scaffold and the corresponding subunit is discussed, and the molecular structures and available space for molecular recognition of the chirally modified systems are outlined by some crystal structures. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Liquid Crystals Based on Hypervalent Sulfur Fluorides: Exploring the Steric Effects of ortho‐Fluorine Substituents
Authors: Kirsch Peer , Hahn Alexander
Published: July 2005

Liquid crystals that have ortho‐fluorinated pentafluoro‐λ6‐sulfanyl terminal groups are the most polar materials that are still compatible with current active matrix LCD technology. The synthesis of the first examples of this class of substance has been achieved by direct fluorination. The physico‐chemical characterization, supported by DFT calculations, revealed some surprising features of these new compounds: the bulky SF5 group is easily deformed and responds in a quite unusual way to the steric strain exerted by its ortho substituents. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Novel Chiral (Salen)MnIII Complexes Containing a Calix[4]arene Unit as Catalysts for Enantioselective Epoxidation Reactions of (Z)‐Aryl Alkenes
Authors: Amato Maria E. , Ballistreri Francesco P. , Pappalardo Andrea , Tomaselli Gaetano A. , Toscano Rosa M. , Williams David J.
Published: July 2005

New asymmetric (salen)MnIII and UO2 complexes containing a calix[4]arene unit in the ligand framework were synthesized. The UO2 complexes were characterized by 1H‐, 13C‐, 2D TOCSY and T‐ROESY NMR spectroscopy. Furthermore, the structure of one UO2 complex was determined by single‐crystal X‐ray analysis. The data showed that UO2 complexes, which can be considered in first approximation models of the Mn=O oxidant active species, possess a chiral pocket and adopt relevant conformations for the selectivity of the oxygen transfer process. Epoxidation data of model alkenes with the MnIII complexes showed moderate ee values and were not conclusive in indicating that the calix[4]arene unit might be able to influence the selectivity by a molecular recognition mechanism. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Asymmetric γ‐Deprotonation and Substitution Reactions of (Z)‐1,3‐Diphenyl‐1‐propenyl N,N‐Diisopropylcarbamate
Authors: Reuber Jenny , Fröhlich Roland , Hoppe Dieter
Published: July 2005

(Z)‐1,3‐Diphenyl‐1‐propenyl N,N‐diisopropylcarbamate (6d) is deprotonated by n‐butyllithium/(–)‐sparteine (5) with a high degree of enantiotopic differentiation in the γ‐position to form the enantiomerically enriched allyllithium derivative 7d. Despite of its high mesomeric stabilization it shows a high degree of configurational stability. Trapping with carbonyl electrophiles proceeds exclusively in a syn‐SE' substitution as shown by direct assignments and stereochemical correlations. Thus, the involvement of a η3 complex is suggested. After transfer to the analogous allyltitanates 16 enantiomerically and diastereomerically pure homoaldol products 17 are furnished. Evidence for a competing retro‐homoaldol reaction is reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Multiple Vinylation of Tribenzotriquinacenes and Fenestrindanes at Their Aromatic Peripheries by Use of Nájera’s Catalyst
Authors: Cao Xiao‐Ping , Barth Dieter , Kuck Dietmar
Published: July 2005

Highly efficient six‐ and eightfold Heck cross‐coupling reactions were carried out with two related convex‐concave and saddle‐shaped hydrocarbons bearing several ortho‐dibromo functionalities at the molecular periphery, viz. the hexabromotribenzotriquinacene 3a and the octabromofenestrindane 4a. In contrast to conventional palladium catalysts, use of Nájera’s oxime‐derived palladacycle catalyst 7 effected exhaustive vinylation with styrene or methyl acrylate in high yield and without concomitant formation of partial cross coupling or reduction products. The hexavinyltribenzotriquinacenes and octavinylfenestrindanes thus obtained are of interest as potential building blocks for novel organic networks. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
p‐Nitrobenzyloxycarbonyl (pNZ) as a Temporary Nα‐Protecting Group in Orthogonal Solid‐Phase Peptide Synthesis – Avoiding Diketopiperazine and Aspartimide Formation
Authors: Isidro‐Llobet Albert , Guasch‐Camell Judit , Álvarez Mercedes , Albericio Fernando
Published: July 2005

p‐Nitrobenzyloxycarbonyl (pNZ) was used as a temporary protecting group for α‐amino functionalities in solid‐phase peptide synthesis. The corresponding derivatives are readily synthesized solids that perform well on solid phase. The pNZ moiety is orthogonal with the most common protecting groups used in peptide chemistry, and is removed under neutral conditions in the presence of catalytic amounts of acid. The use of pNZ derivatives in conjunction with Fmoc chemistry circumvents typical side reactions associated with the use of piperidine, such as DKP and aspartimide formation. The flexibility of pNZ can be exploited for the preparation of libraries of small organic molecules. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Preparation of Cyclopropane Analogues of the Natural AntibioticTAN 1057 A/B
Authors: Kordes Markus , Brands Michael , Es‐Sayed Mazen , de Meijere Armin
Published: July 2005

The two new analogues 2 and 3 of the natural dipeptide antibiotic TAN 1057 A/B (1), both with cyclopropyl containing β‐amino acid side chains, were prepared. The synthesis of the corresponding β‐amino acids in appropriately protected form (4‐Z3 and 5‐Z2, respectively) from the correspondingly protected (hydroxyethyl)‐ (7a) and (hydroxymethyl)‐substituted cyclopropylideneacetate (7b), prepared in two steps each from homoallyl and allyl benzyl ether, respectively, was achieved in nine and seven steps, respectively, with overall yields of 17 (4‐Z3) and 9.3 % (5‐Z2), respectively. Coupling with the N‐methyldihydropyrimidinone 22 and deprotection gave the compounds 2 and 3, which turned out to be active against methicillin resistant strains of Staphylococcus aureus, albeit to a lesser extent than TAN 1057 itself. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Introduction of Aromatic and Heteroaromatic Groups in the 2‐ and 8‐Positions of the Tröger’s Base Core by Suzuki, Stille and Negishi Cross‐Coupling Reactions – A Comparative Study
Authors: Solano Carlos , Svensson Daniel , Olomi Zarina , Jensen Jacob , Wendt Ola F. , Wärnmark Kenneth
Published: July 2005

A comparative study on the bis(functionalization) of the Tröger’s base core by aromatic and heteroaromatic groups using palladium‐catalyzed cross‐coupling reactions is presented. Three different reactions, the Suzuki, Stille, and Negishi couplings, were investigated using Tröger’s base analogues equally substituted in the 2,8‐positions with (HO)2B, Bu3Sn and ZnCl groups, respectively, as the metallated component. Six aryl halides with different electronic and steric properties were employed as coupling partners. The presence of the bulky and electron‐rich phosphane P(tBu)3 as co‐catalyst was found to play an important role. In addition, the palladium source, [Pd(PPh3)4] or [Pd2(dba)3], was also found to be an important factor for the yield of the reactions. The Suzuki coupling was found to be the best method in general, giving excellent yields for most aryl halides, whereas the Stille and Negishi couplings gave moderate to good yields. Finally, the crystal structures of the 4‐nitrophenyl‐ and the 2‐pyridyl‐appended analogues of Tröger’s base, 7d and 7f, are presented in the Supporting Information. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Solid‐Phase Synthesis of Cyclic C‐Glycoside/Amino Acid Hybrids by Carbamate Coupling Chemistry and On‐Support Cyclization
Authors: Katajisto Johanna , Lönnberg Harri
Published: July 2005

A solid‐supported synthesis of cyclic C‐glycoside/amino acid conjugates is described. For this purpose, N‐(tert‐butoxycarbonyl)‐[6‐O‐(p‐nitrophenoxycarbonyl)‐2,3,4‐tri‐O‐(p‐toluoyl)‐β‐D‐glycopyranosyl]methylamines derived from galactose and glucose were prepared and used as activated monomers together with appropriately protected amino acids. The solid phase assembly of the linear precursor was conducted by alternating peptide and carbamate coupling, using glutamate immobilized through its γ‐carboxy function to a SCAL linker as a handle. On‐support cyclization followed by cleavage from the support gave the desired conjugates. Removal of the p‐toluoyl protections was carried out in solution phase by methoxide ion‐catalyzed transesterification in methanol. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Mapping single‐molecule SERRS from Langmuir–Blodgett monolayers on nanostructured silver island films
Authors: Goulet Paul J. G. , Pieczonka Nicholas P. W. , Aroca Ricardo F.
Published: July 2005

To explore the breakdown of ensemble averaging as the single molecule regime is approached, spatial mapping of surface‐enhanced resonance Raman scattering (SERRS) intensities was employed in the study of mixed dye–fatty acid Langmuir–Blodgett (LB) monolayers deposited on nanostructured Ag island films. By variation of the ratio of the two components in the films, the effects of dye concentration, on both SERRS spectra and LB monolayer architecture, were explored down to the single‐molecule level. Insight was gained into the nature of areas of intense local electromagnetic field strength (i.e. ‘hot spots’) that provide enormous enhancement of Raman signals, and their wavenumber of occurrence on nanostructured Ag island films. These enhancing films were further characterized by UV–visible surface plasmon absorbance and atomic force microscopy. The target analyte employed in this work, n‐pentyl‐5‐salicylimidoperylene, was dispersed in monolayers of arachidic acid on Ag nanostructured films for single point and 2D mapping SERRS experiments. The optical characterization of this dye was completed with solution‐phase molecular absorption and fluorescence, in addition to monolayer resonance Raman scattering (RRS). Single‐molecule SERRS mapping results suggest that electromagnetic hot spots on Ag island films are in fact highly localized on the nanoscale, corresponding to relatively few molecular sites, and hence also stress the rarity of coincidence between hot spots and single isolated molecules as a key consideration in ultrasensitive SERRS measurements of this type. Copyright © 2005 John Wiley & Sons, Ltd.
Surface‐enhanced hyper Raman scattering in silver colloidal solutions
Authors: Kneipp Harald , Kneipp Katrin
Published: July 2005

We report surface‐enhanced hyper Raman scattering (SEHRS) from molecules in silver colloidal solutions using high repetition rate mode‐locked picosecond excitation at 1064 nm. SEHRS was is studied under non‐resonant and resonant conditions by choosing adenine and crystal violet as test molecules. Compared with its SERS spectrum, the SEHRS spectrum of adenine displays strong ‘new’ lines, which, in agreement with theory, in most cases also show up in the infrared absorption spectrum. We also explore the capability of SEHRS for trace detection and demonstrate detection and identification of crystal violet based on its SEHRS signals down to nanomolar concentrations. Copyright © 2005 John Wiley & Sons, Ltd.
Electronic effects in SERS by liquid water
Authors: Chen Yan‐Xia , Otto Andreas
Published: July 2005

By exploiting the attenuated total reflection (ATR)–Otto configuration, surface‐enhanced Raman scattering of water at monocrystalline copper and silver electrodes in 0.1 M Na2SO4 was observed at cathodic electrode potentials with hydrogen evolution at a laser wavelength of 647.1 nm. The proposed enhancement model is based on the condition that the upshift of the Fermi level of the electrode and the absorption of one laser photon allow an electron of the electrode at the Fermi level to reach the lower level of the energy band of delocalized electrons in liquid water. The electron interaction with the water molecules in the first hydration shell of cations in the Helmholtz plane is so‐called impulse scattering, which is the short‐time, non‐resonant excursion to the anionic ground state of molecules known for a long time in low‐energy electron scattering from free molecules. Copyright © 2005 John Wiley & Sons, Ltd.
Surface‐enhanced resonance Raman scattering using pulsed and continuous‐wave laser excitation
Authors: Littleford Rachael E. , Cunningham Dale , Matousek Pavel , Towrie Michael , Parker Anthony W. , Khan Imran , McComb David , Ewen Smith W.
Published: July 2005

Pulsed and continuous‐wave (CW) lasers were compared as excitation sources for surface‐enhanced resonance Raman scattering (SERRS). CW excitation provided SERRS spectra with a greater signal‐to‐noise ratio and more sensitive detection by a factor of ∼50 compared with the high peak power, low repetition rate pulsed configuration used. The SERRS intensity using a pulsed laser produced a non‐linear response with respect to changes in power of the laser. At powers of less than ∼0.012 mW, the absolute intensity under the peaks of the CW and pulsed SERRS spectra converged, suggesting that lower peak power, high repetition rate systems may be more effective excitation sources for SERRS. Transmission electron microscopy of pulsed laser‐irradiated silver particles showed significant sample damage and morphological changes. This problem was overcome with the use of a recirculating large‐volume flow cell system, providing a fresh sample for each measurement. A picosecond‐resolved time delay experiment found that SERRS intensity decreased by ∼60% when exposed to a 400 nm pump pulse and probed with a 529 nm pulse. As the time delay between pump and probe increased the system recovered gradually to ∼60% of the original SERRS intensity after 50 ps, where it remained constant. This suggests that the surface bonding between the silver and the dye is significantly perturbed, with some nanoscale diffusion occurring of the dye away from the metal surface. Hence chemical enhancement is temporarily prevented and electromagnetic enhancement is reduced as a function of 1/r3 as the dye moves away from the surface. Additionally, transient heating of the colloidal particles caused by the pulsed laser may also lead to plasmon shifts and changes in absorption intensity. Other factors such as surface annealing or decomposition of the silver particle or dye due the extreme temperature conditions may account for the permanent loss in SERRS intensity. Copyright © 2005 John Wiley & Sons, Ltd.
Surface‐enhanced Raman scattering from oxazine 720 adsorbed on scratched gold films
Authors: Brolo Alexandre G. , Addison Christopher J.
Published: July 2005

Surface‐enhanced Raman scattering (SERS) and surface‐enhanced resonance Raman scattering (SERRS) from oxazine 720 (oxa) dye adsorbed on scratched gold films are reported. The SERS‐active surface was prepared by performing a series of scratches in a 100 nm thick gold film deposited in glass. Atomic force microscopic imaging revealed a sub‐structure within the scratches containing a set of parallel gold wires of different sizes and shapes. The 1‐D order imposed by the parallelism between these wires is responsible for an interesting polarization effect observed in forward scattering experiments. It is shown that the maximum enhanced signal is observed when the polarization of the incident field is perpendicular to the direction of the scratches. This polarization discrimination may be useful in the design of SERS applications in chemical sensing and optical switching. Moreover, we also show that these scratched gold surfaces can be used as ordinary SERS substrates for experiments in backscattering using a common Raman microscope in non‐resonance conditions with the excitation energy. This was accomplished by obtaining the electrochemical SERS of oxa in situ (under electrochemical control). The potential dependence of the SERS from oxa adsorbed on scratched Au is compared with previous results obtained with Ag electrodes. Copyright © 2005 John Wiley & Sons, Ltd.
Surface‐enhanced Raman spectroscopy with ultraviolet excitation
Authors: Lin Xu‐Feng , Ren Bin , Yang Zhi‐Lin , Liu Guo‐Kun , Tian Zhong‐Qun
Published: July 2005

Surface‐enhanced Raman scattering (SERS) spectroscopy excited with a UV laser was successfully developed and the UV–SER spectra of various adsorbates, including pyridine and SCN−, on different transition metal electrodes were obtained. The experimental requirements for obtaining UV–SERS in an electrochemical system are given. The surface enhancement factor of a roughened Rh electrode covered with thiocyanate as a model molecule was estimated to be about two orders of magnitude in the UV region, consistent with our preliminary theoretical calculation based on the electromagnetic model. The investigation of SERS in the UV region will improve the understanding of the SERS enhancement mechanism and broaden the research field of SERS in areas such as surface science and the life sciences. Copyright © 2005 John Wiley & Sons, Ltd.
Investigations of chemisorption and reaction at non‐aqueous electrochemical interfaces by in situ surface‐enhanced Raman spectroscopy
Authors: Cao Peigen , Sun Yuhua , Gu Renao
Published: July 2005

Surface‐enhanced Raman scattering (SERS) spectroscopy has been successfully extended to transition metal/non‐aqueous electrochemical interfaces recently in our laboratory. Some interesting and new findings are presented with the objective of illustrating the virtues of SERS as a versatile technique to examine not only aqueous but also non‐aqueous interfaces. Emphasis is placed on highly catalytic roughened platinum metal surfaces. Static interfacial structure information of Pt (Ag, Au)/acetonitrile was extracted from the relevant SERS spectral features of the surface species, including interfacial trace water, the production of dissociation reactions of acetonitrile (CN−), and some effects on these spectra exerted by interfacial inorganic ions. The simultaneous competitive adsorption and reorganization processes between these interfacial species and some intentionally added small organic molecules were also systematically investigated. The important aspect of the corrosion inhibition mechanism relevant to non‐aqueous media was then probed based on the success of observing SERS from non‐aqueous interfaces. These included mainly widely used N‐ and S‐containing inhibitors for iron, nickel, silver and gold. Finally, the archetypal surface catalytic oxidation mechanism of C1 molecules (formic acid and methanol) on Pt group metals was investigated by using SERS combined with traditional electrochemical techniques. The non‐linear vibrational Stark tuning effect of the poisoning intermediate CO and its oxidation processes are also described in comparison with experimental observations from aqueous solution and other metal surfaces. Copyright © 2005 John Wiley & Sons, Ltd.
Some anomalies of the SER spectra of symmetrical molecules adsorbed on transition metal substrates: consideration by the dipole–quadrupole SERS theory
Authors: Polubotko A. M.
Published: July 2005

The SERS spectra of pyridine, pyrazine and benzene, adsorbed on Ni and some other transition metals are analyzed from the point of view of the dipole–quadrupole SERS theory. There are some anomalies in the regularities of these spectra with respect to those with the molecules adsorbed on Ag, Cu and Au. The breathing mode may not be the most enhanced. Although quadrupole interactions were not found precisely in the spectra of pyridine and pyrazine, they manifest in the SERS spectra of benzene adsorbed on silica‐supported Ni. From the analysis of the enhancement coefficient and some another points, we consider that these anomalies may be associated with the larger influence of the dipole interaction or a smaller role of the quadrupole interactions for the considered molecules and metals and also with some features of the complex dielectric constants in the above systems. Further, a strong influence of the minor quadrupole moments on the enhancement factor of strongly distorted benzene was found. This result is very important, because it indicates the complexity of the SERS dipole–quadrupole theory. Copyright © 2005 John Wiley & Sons, Ltd.
Surface‐enhanced Raman spectroscopy of bacteria: the effect of excitation wavelength and chemical modification of the colloidal milieu
Authors: Zeiri L. , Efrima S.
Published: July 2005

Surface‐enhanced Raman spectroscopy (SERS) of bacteria infused with silver or gold colloids reflects the different biochemicals in the immediate, nano‐vicinity of the particles. In particular we demonstrate high sensitivity to flavins, important coenzymes usually associated with the cell wall. Varying the type of colloid, the laser excitation wavelength and the chemical protocols enables us to observe spectral signatures from different biochemical components of the microorganism. Specifically, we can switch between normal Raman, resonance Raman, and SERS, altering the relative spectral contributions of the various cell constituents. Copyright © 2005 John Wiley & Sons, Ltd.
Density functional theory study of surface‐enhanced Raman scattering spectra of pyridine adsorbed on noble and transition metal surfaces
Authors: Wu De‐Yin , Duan Sai , Ren Bin , Tian Zhong‐Qun
Published: July 2005

Pyridine as a typical model molecule has been most extensively studied by SERS for three decades. However, there has not been a satisfactory explanation for the dramatic change of the relative intensity of the two main bands assigned to the total symmetric modes at 1008 and 1035 cm−1 when pyridine was adsorbed on silver and many other metals. We performed a hybrid density functional theory study of the binding interaction and the Raman spectral properties of pyridine–metal clusters. The calculated Raman scattering factors of total symmetric modes for the pyridine–metal complexes indicate that the bonding interaction influences the relative Raman intensity of the vibrational modes. The surface bonding mechanism was analyzed on the basis of molecular orbital theory. It was also shown that the binding interaction will lead to geometric changes and result in metal‐dependent surface Raman spectra. This theoretical approach could be helpful in using SERS to study surface chemistry, in particular to extract valuable information about surface bonding and adsorption through calculating and analyzing the relative intensity of SERS spectra. Copyright © 2005 John Wiley & Sons, Ltd.
Vibrational characterization of E102 food additive by Raman and surface‐enhanced Raman spectroscopy and theoretical studies
Authors: Peica N. , Pavel I. , Cîntǎ Pînzaru S. , Rastogi V. K. , Kiefer W.
Published: July 2005

The well‐known food dye E102 could be detected in aqueous solutions by means of regular Raman and surface‐enhanced Raman scattering (SERS) spectroscopy at micromolar and nanomolar levels, respectively. The changes observed in the profile of the band at 1365 cm−1, characteristic of the ν(NN) mode of the azo chromophore group, allowed us to establish the present species in the Raman and SERS solutions at different concentrations or pH values. Protonation at the azo group of the molecule was detected for acidic pH values <3. In the pH range 3–8, no changes in the molecular species were observed. Density functional theory (DFT)‐calculated geometries, harmonic vibrational modes and Raman scattering activities for E102 were in good agreement with the experimental data, a complete vibrational assignment being proposed. A strong chemical interaction of E102 with colloidal particles was evidenced and an adsorbed geometry was proposed. Copyright © 2005 John Wiley & Sons, Ltd.