bluuebunny/crossref_metadata_2025_split huggingface dataset:
ortho‐Chelating Arenethiolatocopper(I) Complexes as Versatile Catalysts in the Regioselective Cross‐Coupling of Allylic Derivatives with nBuMgI—An Example of Reversed Reactivity of Leaving Groups
Authors: Persson Eva S. M. , van Klaveren Mayra , Grove David M. , Bäckvall Jan E. , van Koten Gerard 
Published: July 2007

AbstractThe regioselectivity in the arenethiolatocopper(I)‐catalyzed crosscoupling reaction of allylic substrates was studied. It was found that allylic acetates gave highly γ‐selective reactions in Et2O at 0°C with slow addition of the Grignard reagent, whereas α‐selective reactions were obtained in THF at −30°C with fast addition of the Grignard reagent. It is proposed that the formation of an intermediate in Et2O, in which the allylic acetate coordinates in a bidentate fashion the arenethiolatocopper(I) catalyst, dramatically increases the reactivity of the leaving group and results in excellent γ‐selectivity. The remarkable observation that an allylic acetate can be made more reactive than an allylic chloride by using the arenethiolatocopper(I) catalyst 1a supports the theory of a bidentate coordination of the substrate to the catalyst through its double bond and acetate oxygen.
The Stereochemistry of Carbenoid Cyclopropanation Reactions
Authors: Stiasny Hans Christian , Hoffmann Reinhard W. 
Published: July 2007

AbstractThe stereochemical course of the intramolecular carbenoid cyclopropanation reaction has been studied for the epimeric carbenoids 12a and 12b. In these reactions the tert‐butyldimethylsilyloxy substituent serves as an internal stereochemical reference point. It was found that 12b cyclizes rapidly at −110°C in a complexation‐assisted concerted process to give the bicyclo[3.1.0]hexane 16. The diastereomer 12a cyclizes more slowly at −100°C to give both 16 and 17; the former is probably formed by a complexation‐assisted carbolithiation pathway.
What Do Ab Initio Calculations Predict for the Structure of Lithiated Azaenolates of Peptides?
Authors: Feigel Martin , Martinek Gisela , Sauer Wolfgang H. B. 
Published: July 2007

AbstractStructures and conformations of the azaenolate lithium salts of amides (formamide, acetamide, and N‐methylacetamide) and of the dipeptide model N‐formylalaninamide were investigated by means of ab initio MO theory. Four possible structures of the lithiated C‐enolates of acetamide were also included in the study. All structures were calculated at the HF/6‐31+G(d) and MP2(fc)/6‐31 + G(d)/HF/6‐31 + G(d) levels; the lithiated azaenolates of formamide were also investigated at higher theoretical levels (up to MP4(fc)/6‐311 + G(d,p)/MP2(fc)/6‐311 + G(d,p)). For the lithiated azaenolates of all amides investigated, the most stable structure contains a four‐membered ring in which the lithium ion is complexed by the oxygen and nitrogen atoms; the substituents attached to the carbon and nitrogen atoms of the azaenolate are in a cis arrangement. The lithiated azaenolates of acetamide are predicted to be more stable than the corresponding C‐enolates. To simulate solvation, calculations on complexes of the lithiated azaenolates of formamide with up to three molecules dimethyl ether were also performed, and all azaenolates of amides were also reoptimized by ab initio reaction‐field calculations. Both solvation models reduce the preference for lithium‐chelated cis structures. The Ramachandran maps of the dilithiated bis(azaenolate) of N‐formylalaninamide (having cis or trans arrangements of the azaenolate substituents) were scanned by MNDO calculations for conformational accessible regions. Thirteen stable structures were subsequently optimized at the HF/6‐31 + G(d) ab initio level. The global minimum resembles a peptide in C7 conformation, but other conformations, not known for peptides, are close in energy. The structures of dimers of the lithiated azaenolates of N‐methylacetamide and of glycinaldehyde were also calculated. The NMR chemical shielding of carbon, nitrogen, and oxygen atoms in all structures were predicted ab initio by using the gauge‐including atomic orbital (GIAO) method.
The “Peroxo Perrhenic Acid” H4Re2O13: An Oxygen‐Rich Metal Peroxide and Oxidation Catalyst
Authors: Herrmann Wolfgang A. , Correia João D. G. , Kühn Fritz E. , Artus Georg R. J. , Romã Carlos C. 
Published: July 2007

AbstractThe rhenium oxides Re2O7 and ReO3 react with hydrogen peroxide solutions yielding peroxo complexes that efficiently catalyze the oxidation of olefins, aromatics, and certain organometallic compounds. In contrast, related oxides of molybdenum (MoO3) and vanadium (V2O5) do not activate H2O2 under comparable conditions. The active rhenium peroxo complex was isolated from the system Re2O7/H2O2: the crystalline red‐orange, explosive compound of formula H4Re2O13 is the most oxygen rich rhenium compound isolated to date. Its structure resembles a “peroxo perrhenic acid”. The binuclear compound could be isolated in the form of a diglyme adduct, structurally defined as two corner‐sharing pentagonal bipyramids with apical oxo and aquo ligands; the equatorial positions are occupied by the bridging oxygen and by n2‐peroxo groups (two [O2]2− ligands per rhenium). In contrast to the known complex [CH3ReO(O2)2].H2O, the new peroxo species [O{ReO(O2)2.H2O}2] decomposes hydrolytically during the catalytic cycle and can thus not compete in terms of catalytic activity in oxidation reactions involving H2O2. Hydrolysis yields “perrhenic acid” Re2O7.2H2O, the diglyme adduct of which compound was also characterized by means of an X‐ray diffraction analysis.
The Effect of Microsolvation on E2 and SN2 Reactions: Theoretical Study of the Model System F− + C2H5F + nHF
Authors: Matthias Bickelhaupt F. , Baerends Evert Jan , Anibbering Nico M. M. 
Published: July 2007

AbstractThe model reaction system F− +C2H5F + nHF (n = 0–4) has been investigated by use of a density‐functional method, in order to achieve a qualitative understanding of the effect of solvation on the E2 and SN2 reactions. Two characteristic effects already occur upon monosolvation: a) the activation energies of the E2 and SN2 pathways increase significantly and even become positive, because reactants are more strongly solvated than transition states; b) the SN2 transition state is stabilized much more and becomes lower in energy than the anti‐E2 transition state. This agrees with general experience from gas‐ and condensed‐phase experiments. The solvation is analyzed from two complementary viewpoints: a) as an interaction between solvent molecules and the F−/C2H5F reaction system; b) as an interaction between the [F−, nHF] solvated base and the C2H5F substrate. The extent to which condensed‐phase characteristics can be modeled by this microsolvation approach is discussed.
A Novel Route to Racemic and Nonracemic Products of the Ugi Reaction: Synthesis of Ugi′s Labile α‐Adducts from Iminoaziridines and Carboxylic Acids, and their Transformations
Authors: Quast Helmut , Aldenkortt Sven 
Published: July 2007

AbstractIminoaziridines (11) are highly reactive synthetic equivalents for three of the four components in the Ugi four‐component condensation. Thus, iminoaziridines react rapidly with carboxylic acids at temperatures as low as –20°C to afford α‐amino isoimides (14), which are identical to the elusive α‐adducts of isocyanides in the Ugi reaction. 1,4‐Migration of the acyl group (O → α‐N) in 14 furnishes the α‐acylamino amides 15. Very little, if any, racemisation is observed when carboxylic acids react with nonracemic iminoaziridines [(R)‐11 a,c], which are readily available. Mumm rearrangement by O → N‐acyl 1,3‐migration to afford α‐amino imides (16 e, f) competes if the O → α‐N 1,4‐shift is slowed down by steric hindrance. The latter acyl shift is catalysed by carboxylic acids while the former is not. The iminoaziridines (R)‐11 a,c react quantitatively and without racemisation with hydrazoic acid to produce the 5‐aminoalkyltetrazoles (R)‐21 a,c.
Nucleophilic Addition of Secondary Phosphines to Cationic Dienyl Tricarbonyliron Complexes: A novel Route to Optically Active Phosphines
Authors: Englert Ulli , Ganter Beate , Käser Markus , Klinkhammer Elke , Wagner Trixie , Salzer Albrecht 
Published: July 2007

AbstractSecondary phosphines such as HPPh2 and to the cationic iron dienyl complex [η5‐(1R)‐ethylnopadienyl)Fe‐(CO)3]+ (1) by nucleophilic addition. The phosphonium salt initially formed is readily deprotonated to yield an optically active tertiary phosphine [(n4‐(1 R)‐ethylnopadienePPh2)Fe(CO)3] (2b). A similar reaction also occurs with [C6H7Fe‐(CO)3]+ (3) and [C7H9Fe(CO)3]+ (4) to give [(C6H7PPh2)Fe(CO)3] (5) and [(C7H9PPh2)Fe(CO)3] (6) in good yields. The mechanism of formation of these novel phosphines is discussed. Complex 2 b crystallizes in the space group P212121 (no. 19); 5 crystallizes in the space group P21/c (no. 14). Like other monodentate optically active phosphines, 2 b is capable of coordinating to transition metal complexes. It forms palladium complexes on reaction with [{μ‐chloro(allyl)palladium}2] as well as with [{μ‐chloro[(N,N‐dimethylamino‐kN‐2‐methyl)phenyl‐kC]palladium}2] (11). The latter reaction product crystallizes in the space group P31 (no. 144).
Coordination and Coupling of OH‐Functionalized C2 Units at a Single Metal Center: The Synthesis of Alkynyl(Vinylidene), Alkynyl(Enyne), Bis(Alkynyl)Hydrido, Enynyl, and Hexapentaene Rhodium Complexes from Propargylic Alcohols as Precursors
Authors: Werner Helmut , Wiedemann Ralf , Mahr Norbert , Steinert Paul , Wolf Justin 
Published: July 2007

AbstractThe reaction of [Rh(η3‐C3H5)‐(PiPr3)2] (1) with HCCCH(Ph)OH yields the alkynyl(vinylidene) complex trans‐[Rh{CCCH(Ph)OH}{CCH–CH(Ph)OH}(PiPr3)2] (2), while from 1 and HCCCPh2OH the alkynyl‐(enyne)metal derivative trans‐[Rh(CCCPh2OH){n2‐(E)‐Ph2(OH)CCCCHCH–CPh2OH} (PiPr3)2] (3) is obtained. On treatment with 1‐alkyn‐3‐ols HCCCR2OH (R = Me, Ph, iPr), the highly reactive π‐benzyl compound [Rh(n3‐CH2Ph)(PiPr3)2] (4) yields the five‐coordinate complexes [RhH(CCCR2OH)2(PiPr3)2] (5–7) of which those with R = Me and Ph can be converted to the alkynyl(vinylidene)metal isomers trans‐[Rh(CCCR2OH)(CCH–CR2OH)‐(PiPr3)2] (8, 9). Compounds 8 and 9 react with L′ = CO and isocyanides by migration of the alkynyl ligand to the vinylidene unit to give the enynylrhodium(I) complexes trans‐[Rh{n1‐(Z)‐C(CCCR2OH)CH–CR2OH}(L′)(PiPr3)2] (10, 11: L′ = CO; 12–15: L′ = CNR′). Cleavage of the Rh–C s̀‐bond of 10 with CF3‐CO2H affords trans‐[Rh(n1‐O2CCF3)‐(CO)(PiPr3)2] (16) and the enyne (E)‐Me2(OH)CCCCHCH–CMe2OH (17). Compounds 5–7 react with L′ = CO and isocyanides to give the octahedral 1:1 adducts [RhH(CCCR2OH)2(L′)(P‐iPr3)2] (18–20 and 24–27), of which the CO derivatives 18–20 readily eliminate HCCCR2OH to yield trans‐[Rh‐(CCCR2OH)(CO)(PiPr3)2] (21–23). On treatment of 6 or 9 (R = Ph) with Al2O3 in the presence of chloride ions, besides trans‐[RhCl(CCCPh2)(P‐iPr3)2] (28) the hexapentaenerhodium(I) complex trans‐[RhCl(n2‐Ph2CCCCCCPh2)(PiPr3)2] (29) is formed. The kinetically preferred isomer trans‐[RhCl(n2‐Ph2CCCCCCPh2)(P‐iPr3)2] (33) has been prepared from [RhCl(PiPr3)2]2 and Ph2CCCCCCPh2; it rearranges smoothly at room temperature to the thermodynamically more stable isomer 29. The molecular structures of 7 and 29 have been determined.
Se2NBr3, Se2NCl5, Se2NCl‐6: New Nitride Halides of Selenium(III) and Selenium(IV)
Authors: Lau Carsten , Neumüller Bernhard , Vyboishchikov Sergei F. , Frenking Gernot , Dehnicke Kurt , Hiller Wolfgang , Herker Martin 
Published: July 2007

AbstractSe2NBr3, a planar molecule shaped liked dividers, was formed by the reaction of SeBr4 with N(SiMe3)3, which reduced the selenium to the trivalent state. Under the same conditions, Se2NCl5 was obtained by the reaction of SeCl4 with N(SiMe3)3, retaining the tetravalent state of selenium. The molecule is formed by two edge‐sharing pseudotrigonal bipyramids with selenium as their central atoms. Se2NCl5 reacts with PPh4Cl to form the Se2NCl‐6 ion by the addition of a Cl‐ ion to only one of the two selenium atoms, resulting in pseudooctahedral coordination of this atom. Ab initio calculations for all the compounds reported confirm the experimentally determined structures.
Polystyrene–Poly(propylene imine) Dendrimers: Synthesis, Characterization, and Association Behavior of a New Class of Amphiphiles
Authors: van Hest J. C. M. , Delnoye D. A. P. , Baars M. W. P. L. , Elissen‐Román C. , van Genderen M. H. P. , Meijer E. W. 
Published: July 2007

AbstractA new class of amphiphilic macromolecules has successfully been synthesized by creating well‐defined diblock copolymers of polystyrene (PS, Mn3.2 × 103, Mw/Mn=1.04) with poly‐(propylene imine) dendrimers. A poly‐styrene core molecule with a primary amine end‐group was prepared by a quantitative three‐step modification procedure of acid‐functionalized polystyrene. On this core molecule, five different generations from PS‐dendr‐NH2 up to PS‐dendr‐(NH2)32 were constructed in high yields. The molecular structure of the block co‐polymers was analyzed in detail with NMR and IR spectroscopy and electrospray mass spectrometry. With conductivity measurements and monolayer pressure–area isotherm determinations, we observed generation‐dependent amphiphilic behavior. Dynamic light scattering and transmission electron microscopy showed that amphiphile geometry had a distinct effect on aggregation behavior; this is in qualitative agreement with Israelachvili's theory. Critical association concentrations determined with the pyrene probe luminescence technique were as low as 5 × 10‐7 M. The amphiphiles presented here, therefore, show a resemblance to traditional block copolymers with regard to size and stability, and are similar to surfactants with respect to tuning of the shape.
Helical Complexes and Beyond
Authors: Williams Alan 
Published: July 2007

AbstractHelical complexes have been much studied as examples of self‐assembled supramolecular units. Their structural features are analysed in terms of the metal ion, the binding site on the ligand and the bridging group linking these sites. Understanding these elements allows the controlled assembly of multicomponent systems. Preliminary results show the complexes to have high stability, arising from the inertness of the self‐assembled species.
Organic Fluorine Hardly Ever Accepts Hydrogen Bonds
Authors: Dunitz Jack D. , Taylor Robin 
Published: July 2007

AbstractStatistical analysis of structural data and detailed inspection of individual crystal structures culled from the Cambridge Structural Database and the Brookhaven Protein Data Bank show that covalently bound fluorine (in contrast to anionic fluoride) hardly ever acts as a hydrogen‐bond acceptor. The weakness of covalently bound fluorine as hydrogen‐bond acceptor is backed by results of new molecular orbital calculations on model systems using ab initio intermolecular perturbation theory (IMPT), and is in accord with results of other physicochemical studies and with the physical properties of fluorinated organic compounds. Factors influencing the strength of hydrogen bonding in extended systems are discussed.
The Importance of Magnetic Coupling Through Atoms with Large Spin Densities—Structure and Magnetic Properties of meso‐Tetrakis‐(4′‐tert‐Butylphenyl)Porphinatomanganese(III) Hexacyanobutadienide, [MnIIITtBuPP]+[C4(CN)6].‐
Authors: Sugiura Ken‐Ichi , Arif Atta M. , Rittenberg Durrell K. , Schweizer Jacques , Öhrstrom Lars , Epstein Arthur J. , Miller Joel S. 
Published: July 2007

Abstract[MnIIITtBuPP]+[C4(CN)6].‐ · 5PhMe [MnIIITtBuPP = meso‐tetrakis‐(4′‐tert‐butylphenyl)porphinatomanganese(III)] has been prepared and structurally and magnetically characterized. The uniform, linear‐chain (1‐D) coordination polymer comprises alternating cations and anions. The bond lengths in planar ion [C4(CN)6].‐]'‐ are 1.377(10) (CC–CC), 1.418(7) (C–CCC), 1.414 (C–CN), 1.457 (C–CNMn), 1.150 (CN), and 1.134 Å (C = NMn). The Mn–N–C angle is 172.3(4)°, and the intrachain Mn — Mn separation is 10.685 Å. Each [C4(CN)6].‐]' unit is bonded to two MnIII atoms through the interior nitrogen atoms in a trans‐μ2‐N‐σ manner with N–Mn bond lengths of 2.353 Å. The ṽCN absorptions are at 2217 (w, br) and 2190 (m) cm−1. Above 50 K the magnetic susceptibility of [MnIIITtBuPP]+[C4(CN)6].‐ can be fitted to the Curie‐Weiss expression, χ∝1(T – θ), with an effective θ of ‐13 K. This is consistent with weak antiferromagnetic coupling, which is in contrast to the effective θ of +67 K for the uniform chain [MnIIIOEP]+[C4(CN)6].‐ [OEP = octaethylporphinato]. Here, the [C4(CN)6].‐'‐ units are bonded to the MnIII centers through endo CN nitrogen atoms in a similar trans‐μ2 manner. Density functional theory MO calculations reveal that the spin density of the CN nitrogen atom bound to [MnIIITtBuPP]+ (0.019 μBÅ−3) is significantly lower than that of the N atom bound to [MnIIIOEP]+ (0.102 μBÅ−3). This is consistent with the reduced spin coupling observed for [MnIIITtBuPP]+[C4(CN)6].‐with respect to [MnIIITtBuPP]+[C4(CN)6].‐, as evidenced by the lower θ value. The different orientations of the [C4(CN)6].‐ units—almost perpendicular (84.72°) for [MnIIITtBuPP]+[C4(CN)6].‐ and substantially tilted (32.1°) for [MnIIIOEP]+ [C4(CN)6].‐ may also contribute to the poorer overlap and weaker spin coupling. Hence, binding between sites with large spin densities is needed to stabilize strong ferromagnetic coupling.
Design of Sequence‐Specific DNA‐Binding Ligands
Authors: Nielsen Peter E. 
Published: July 2007

AbstractDouble‐stranded DNA can be viewed as a multifunctional, modular receptor that can be read sequence‐selectively in a digital way (base pair per base pair) by a complementary, similarly modular ligand. This principle has been exploited in several approaches to design sequence‐specific DNA‐binding ligands, such as triplex‐forming oligonucleotides, peptide nucleic acids and minor groove binding polyamides.
Synthesis of (R)‐Sulforaphane Using [cpru(R, R)‐CHIRAPHOS)]+ as Chiral Auxiliary
Authors: Schenk Wolfdieter A. , Dürr Michael 
Published: July 2007

AbstractA new enantioselective (80% ee) synthesis of (R)‐sulforaphane and its epimer (S)‐sulforaphane is described, which makes use of the pseudo‐tetrahedral complex fragment [CpRu‐(CHIRAPHOS)]+ as a chiral auxiliary. Reaction of the chloride complexes [CpRu(L–L)Cl] [L–L = 1,‐bis(diphenylphosphino)ethane (dppe), (2S,3S)‐and (2R,3R)‐bis(diphenylphosphino)‐butane ((S,S)‐ and (R, R)‐CHIRAPHOS, respectively)] with phthalimidobutyl methyl sulfide gives the thioether complexes [CpRu(L–L)(MeSC4H8NPhth)]PF6. Oxygen transfer from dimethyldioxirane (DMD) produces the corresponding sulfoxide complexes in high yield and high diastereoselectivity. Cleavage of the phthaloyl group with aqueous hydrazine and subsequent reaction with thiophosgene yields the sulforaphane complexes [CpRu(L–L)(MeS(O)C4H8NCS)]PF6. Treatment of these with sodium iodide finally liberates the sulforaphane without noticeable racemization.
Reiterative Synthesis of trans‐Fused Polytetrahydropyrans Using the Oxiranyl Anion
Authors: Mori Yuji 
Published: July 2007

AbstractThe oxiranyl anion is a very unstable nucleophilic epoxide. This previously uncommon nucleophile has now been used for the synthesis of polycyclic ethers. Alkylation of an oxiranyl anion, a three‐carbon building block, with a triflate derived from tetrahydropyran‐methanol followed by acid‐catalyzed 6‐endo cyclization provides a trans‐fused bicyclic compound. Reiterative application of this procedure enables the rapid construction of trans‐fused polytetrahydropyrans.
Synthesis of Glycodendrimers by Modification of Poly(propylene imine) Dendrimers
Authors: Ashton Peter R. , Boyd Sue E. , Brown Christopher L. , Nepogodiev Sergey A. , Meijer E. W. , Peerlings H. W. I. , Peerlings H. W. I. , Stoddart J. Fraser 
Published: July 2007

AbstractThe use of preformed poly‐(propylene imine) dendrimers (DAB‐dendr‐(NH2)x) for the rapid and facile construction of high molecular weight carbohydrate‐coated dendrimers (glyco‐dendrimers) is presented. An efficient attachment of spacer‐armed derivatives of D‐galactose and lactose to the primary amino end groups of DAB‐dendr‐(NH2)x has been achieved by means of amide bond formation, using the N‐hydroxysuccinimide coupling procedure. Acetate protecting groups have been employed in order to avoid side reactions at the coupling stage. Deacetylation leads to the target glycodendrimers. The reactivity of all the available DAB‐dendr‐(NH2)x (generations 1–5) has been investigated and a series of homologous carbohydrate‐coated dendrimers have been synthesized. In addition, the attachment of larger saccharide moieties has been demonstrated by the condensation of a trisgalactoside cluster with DAB‐dendr‐(NH2)x carrying both four and eight primary amino groups. The regularity of the glycodendrimers has been proven by NMR spectroscopy, and the molecular weights of the low‐generation carbohydrate‐coated dendrimers have been determined by mass spectrometry. Modifications of DAB‐dendr‐(NH2)x with biologically active carbohydrates affords a new and simple approach to high molecular weight compounds that may be considered as neoglycoconjugates with perfectly symmetrical structures and that offer much promise as multivalent ligands involved in carbohydrate‐protein interactions.
Synthetic Carbohydrate‐Containing Dendrimers
Authors: Jayaraman Narayanaswamy , Nepogodiev Sergey A. , Stoddart J. Fraser 
Published: July 2007

AbstractDendrimers coated with carbohydrates on their exterior surfaces have been constructed by using both convergent and divergent synthetic routes. Alternatively, cluster glycosides in the form of highly branched oligosaccharides can serve as dendritic wedges in the subsequent elaboration of fully carbohydrate dendrimers. It is anticipated that these novel saccharide‐containing polymers, which are highly branched and water‐soluble, will find applications of a biological nature as well as in the context of new materials.
The Phosphodiester Groups of d(TpT)− and d(TpG)‐ Coordinate to Platinum(II) in N,N‐Dimethylformamide
Authors: Kozelka Jiří , Barre Gilles 
Published: July 2007

AbstractThe propensity of the phospho‐diester group to bind platinum(II) was investigated by examining the reactions of the dinucleotide salts NH4{d(TpT)} and NH4{d(TpG)} with the complexes [Pt‐(NO3)(NH3)3]NO3 (1) and cis‐[Pt(NO3)2‐(NH3)2] (2) in N,N‐dimethylformamide. d(TpT)‐ coordinates to the platinum atom of both 1 and 2 through either of the two nonbridging phosphoryl oxygen atoms, as revealed by 31P NMR spectroscopy. Conversely, d(TpG)‐ binds to 1 through the guanine N7 atom. With 2, d(TpG)‐ forms monodentate adducts bound to N7(G), as well as two distinct macrochelates, in which N7(G) and one of the nonbridging phosphoryl oxygens are crosslinked by platinum. We suggest that guanine‐phosphoryl macrochelate complexes play a role as intermediates in rearrangements of platinum monoadducts formed with DNA.