bluuebunny/crossref_metadata_2025 huggingface dataset:
Visualization of Tight‐Binding Calculations–The Electronic Structure and Electron Localization of the Si(100) Surface
Authors: Fässler Thomas F. , Häussermann Ulrich , Nesper Reinhard
Published: July 2007

The advantage of computer graphics in the visualization of tight‐binding calculations is highlighted in a model study of the reconstruction of the Si(100) surface. Three different surface models—the unreconstructed surface Si(100)‐(1 × 1), and symmetric and asymmetric pairing of surface atoms Si(100)‐(2 × 1)—are investigated on the basis of density of states (DOS), local (projected) density of states (LDOS) and crystal orbital‐overlap population (COOP) analysis. For the visualization of the real‐space properties of tight‐binding calculations, two‐ and three‐dimensional images of the total (TED) and partial electron densities (PED) are shown. The PED calculated near the Fermi level are compared to densities of HOMOs and LUMOs in molecular systems and used to analyse constant current mode STM images, obtained by applying bias voltages of different sign. They show excellent agreement with STM experiments. The electron‐localization function (ELF) has been shown to describe chemical bonds in molecules and solids surprisingly well. Here, the ELF is calculated for surfaces. In order to visualize the shape of the “dangling” surface bonds and bonds connecting surface atoms, two‐ and three‐dimensional representations of the ELF are discussed. Using the reconstruction of the Si(100) surface as an example, we show that combining methods for extracting information from quantum mechanical calculations, such as PED, TED and ELF, leads to a more comprehensive description of the electronic surface structure. With the help of computer graphics, chemical concepts routinely used for describing local properties of molecules can be transferred very effectively to extended systems.
Ligand‐Field Control and Hydrogen Bonding as Design Elements in the Assembly and Crystallization of Poly(azolyl)borate‐Metal Complexes: Chelate Complexes Versus Coordination Polymers and Symmetrical Versus Distorted Grid Sheets
Authors: Janiak Christoph , Scharmann Tobias G. , Günther Wulf , Girgsdies Frank , Hemling Holger , Hinrichs Winfried , Lentz Dieter
Published: July 2007

The 1‐ and 2‐D coordination polymers [Mn{HB(C2H2N3)3}2(H2O)2].4H2O (11) and [Ni{H2B(CHN4)2}2‐(NH3)2] (13), respectively, and the chelate complex [Ni{H2B(C2H2N3)2}2(H2O)2] · 2H2O (12) were synthesized and structurally characterized. The compounds contain ambidentate poly(azolyl)borato ligands (azolyl = triazolyl or tetrazolyl), which can chelate or bridge metal centers. The metal–ligand structures in 11–13 differ from the known coordination modes of the poly(azolyl)borates towards other metal centers. We describe how a change in the metal and/or the conditions of crystallization affects the ligand‐field stabilization energy and favors one type of nitrogen donor atom over the other for the poly(triazolyl)borato ligands. The crystal structures of 11 and 12 contain additional water of crystallization; this leads to hydrogen‐bonded solvent substructures. In the case of the bis(triazolyl)borato ligands. The crystal structures of 11 and 12 contain additional water of crystallization; this leads to hydrogen‐bonded solvent substructures. In the case of the bis(tetrazolyl)borato ligand the water substructure is shown to function as a “reinforcing bar” that symmetrizes the metal–ligand grid sheet.
Stereodivergent Hetero‐Diels‐Alder Reactions of Chiral 1‐Oxa‐1,3‐butadienes through a Conformational Switch Induced by Lewis Acids
Authors: Tietze Lutz F. , Schneider Christoph , Grote Andrea
Published: July 2007

The stereodivergent asymmetric hetero‐Diels‐Alder reaction of achiral and chiral 1‐oxa‐1,3‐butadienes carrying an oxazolidine moiety with various enol ethers in the presence of different Lewis acids is described as a highly stereoselective and efficient approach to dihydropyrans, which can be used for the synthesis of carbohydrates. In the cycloaddition of the achiral oxabutadiene very good endo/exo selectivity was possible, and with the chiral oxabutadienes excellent 1,6‐asymmetric induction was additionally observed. In the processes a reversal of facial selectivity occurs by changing the Lewis acid, allowing the synthesis of both enantiomers of the dihydropyrans with the same auxiliary. Thus, cycloaddition of 1 to 2 in the presence of Me2 AlCl gives predominantly the endo product 3 (3:4 = 10:1), whereas with SnCl4 the exo product 4 is obtained (3:4 = 1:15). The reaction of 7 and 1 a in the presence of Me2AlCl as promoter nearly exclusively yields the endo‐I adduct 16 a (16 a+17 a:18 a+19 a = > 50:1; 16 a:17 a = 60:1), whereas with TMS‐OTf the endo‐II‐product 17 a was obtained as the main component (16 a+17 a:18 a+19 a = > 50:1; 16 a:17 a = 1:7.9). The use of SnCl4 leads to a mixture of endo and exo, again, however, with excellent induced selectivity. A similarly good induction was obtained with the oxabutadiene 9 containing the new auxiliary 8. Also, other enol others 1 b–g were used, some of which afforded excellent induction. Mechanistic considerations are used to explain the results.
Chirality of Large Random Supramolecular Structures
Authors: Katzenelson Omer , Hel‐Or Hagit Zabrodsky , Avnir David
Published: July 2007

The structural chirality of large, random supramolecular structures, spiral diffusion‐limited aggregates, is analyzed and studied in detail by using a continuous chirality measure. It is found that classical definitions and terminologies of chirality are too restrictive for the description of such complex objects. A refined methodology and a conceptual vocabulary are developed, along with a generalized definition of chirality. Their application is demonstrated in detail on these large structures. The classical definition of chirality, tailored for small, nonrandom species, is a limiting case of the generalized viewpoint we propose.
Tricarbonyl(η6‐1,2‐dioxobenzocyclobutene)chromium(0): Preparation, Nucleophilic Addition Studies and Syntheses of cmplex Polycyclic Systems by Dianionic Oxy‐Cope Rearrangement
Authors: Brands Michael , Wey Hans G. , Bruckmann Joachim , Krüger Carl , Butenschön Holger
Published: July 2007

The tricarbonylchromium complex 3 of 1,2‐dioxobenzocyclobutene (1) is easily prepared by hydrolysis of diacetal 4. Complex 3 stereoselectively undergoes both single and double nucleophilic addition reactions at the keto groups. As a consequence, addition of excess alkenyllithium to 3 gives rise to a dianionic oxy‐Cope rearrangement yielding benzocyclooctenedione complexes with good diastereoselectivity. Trapping of the bis(enolate) intermediate with chlorotrimethylsilane gives bis(enolether) 23. Subsequent addition of two different alkenyllithium derivatives yields the asymmetrically substituted rearrangement product 15. To some extent, dependent on the method of hydrolysis, an intramolecular aldol addition follows the dianionic oxy‐Cope rearrangement. This results in the formation of benzoanellated bicyclo[3.3.0]octane derivatives and, in the case of 1‐cyclopentenyllithium as the alkenylmetal, in the tetraquinane system 19. The trans‐anellation of one of the cyclopentane rings in trans‐20 has been verified by an X‐ray structural analysis. The highly substituted rearrangement product 21/22 is formed only in small yield; in solution the bis(enol) tautomeric form 22 is favored over diketone 21.
Diphosphonio Dihydrophosphetide and 1,2‐Diphospholide Cations
Authors: Jochem Georg , Schmidpeter Alfred , Nöth Heinrich
Published: July 2007

Condensation of a 1,3‐diphosphoniopropenide cation at its reactive 1,3‐positions with a dichlorophosphine in the presence of triethylamine provides a route to 2,4‐diphosphoniodihydrophosphetide cations. An excess of dichlorophosphine in the presence of an additional reducing agent results in a ring expansion and yields 3,5‐diphosphoniodihydro‐1,2‐diphospholide cations. The chlorosubstituted cation derived from PCl3 can be further reduced to the hydrolytically stable 3,5‐diphosphonio‐1,2‐diphospholide cation. It adds halogen to the PP bond and can easily be regained from the halogen adduct. Structural comparison of the 1,2‐diphenyl‐ and 1,2‐dichlorodihydro‐1,2‐diphospholide cation with the 1,2‐diphospholide cation shows three stages of interaction of the C3 and the P2 entities of the ring: no conjugation in the first case, hyperconjugative extension of the allylic system to include the phosphorus atoms in the second case and cyclic π conjugation in the third.
Mechanism of an Alkyl‐Dependent Photochemical Homolysis of the Re–Alkyl Bond in [Re(R)(CO)3(α‐diimine)] Complexes via a Reactive σπ* Excited State
Authors: Rossenaar Brenda D. , Kleverlaan Cornelis J. , Van De Ven Maartje C. E. , Stufkens Derk J. , VlčEk Antonín
Published: July 2007

MLCT excitation of the complexes [Re(R)(CO)3(α‐diimine)] (R = Me, Et, benzyl (Bz); α‐diimine = iPr‐PyCa, R′‐DAB) results in the homolysis of the Re‐R bond leading to the formation of radicals R. and [Re(CO)3(α‐diimine)]. as primary photoproducts. The quantum yield of this photoprocess is dependent on the alkyl group used. For R = Me, the quantum yield is low (10−2) and depends on the temperature and excitation wave‐length, whereas for R = Et and Bz the quantum yield is near unity and independent of T and λexc. The reaction is shown to proceed via a σ(Re‐R)π* excited state that is rapidly (< 20 ps) populated by a nonradiative transition from the optically excited MLCT state. Time‐resolved IR and UV/Vis absorption spectra studied in the ns‐μs and ps‐μs time domains, respectively, show that the σπ* excited state is rather long‐lived (τ ≈ 250 ns) in noncoordinating solvents; the dissociation of the Re‐R bond from this state is strongly accelerated by polar or coordinating solvents (τ< 20 ps). The σπ* excited state is spectroscopically characterized by a (presumably σπ* → MLCT) transition at approximately 500 nm and by CO stretching frequencies closely resembling their ground‐state values. The relative energies of the MLCT and reactive σπ* states, controlled by the nature of the alkyl lig‐and, determine the photoreactivity of the complexes.
Design and Synthesis of a “Starburst”‐Type Nonadiazo Compound and Magnetic Characterization of Its Photoproduct
Authors: Matsuda Kenji , Nakamura Nobuo , Inoue Katsuya , Koga Noboru , Iwamura Hiizu
Published: July 2007

A “starburst”‐type nonadiazo compound was designed and synthesized by using the cyclotrimerization of 3,5‐dibenzylphenyl ethynyl ketone as a key step. The diazo compound was photolyzed in methyltetrahydrofuran solid solution at cryogenic temperatures and analyzed by means of Faraday magnetometry and EPR spectroscopy. While the m‐phenylene‐connected nonacarbene was predicted to have a nonadecet (S = 9) ground state, the magnetic data of the photoproduct was more consistent with a pentadecet (S = 7) species. The result was interpreted in terms of the intramolecular cross‐linking between the carbene centers by the determination of both the amount and the multiplicity of the spin. In the “starburst”‐type polycarbenes with extended branching, the reactive carbene centers are able to approach one another and thus appear to readily recombine. This study highlights one of the limitations of the extension of the carbene network by way of flexible “starburst”‐type structures.
Gas‐Phase Acid‐Induced SN2′ versus SN2 Mechanism in Allylic Alcohols
Authors: Renzi Gabriele , Lombardozzi Antonietta , Dezi Emanuela , Pizzabiocca Adriano , Speranza Maurizio
Published: July 2007

A first demonstration of the existence of the concerted SN2′ mechanism in the gas phase was obtained by establishing the regioselectivity of the attack of a neutral nucleophile, such as MeOH, on several allylic oxonium ions. These were generated in the gas phase by the reaction of radiolytically formed GA+ acids (GA+CnH+5 (n = 1, 2), iC3H+7, and (CH3)2F+) with trans‐ (1) and cis‐2‐buten‐1‐ol (2) as well as with 1‐buten‐3‐ol (3). Firm evidence in favor of the concerted SN2′ pathway accompanying the classical SN2 one in these systems was obtained after careful evaluation of the extent of conceivable intramolecular isomerization both of the primary oxonium ions from GA+ attack on 1–3 before nucleophilic displacement by MeOH and of their substituted intermediates before neutralization. The intermediacy of free allylic ions in the nucleophilic substitution was ruled out by generating the ions by protonation of 1,3‐butadiene and by investigating their behavior in exactly the same media employed in the substitution reactions. The regioselectivity of MeOH with the ionic substrates investigated showed the occurrence of nearly equally extensive SN2′ and SN2 pathways in the oxonium ions from 1 (SN2′ (57 ± 2%) and SN2 (43 ± 2%)) and 3 (SN2′ (54 ± 2%) and SN2 (46 ± 2%)), whereas, with 2, the SN2 (66 ± 2%) reaction prevailed over the SN2′ one (34 ± 2%). The role of intrinsic structural factors in determining the SN2′/SN2 branching in the selected oxonium ions is discussed.
The photochemical Formation of Fulvene from Benzene via Prefulvene–a Theoretical Study
Authors: Dreyer Jens , Klessinger Martin
Published: July 2007

Results of semiempirical MN‐DOC‐CI and ab initio CASSCF calculations reveal that fulvene is a primary product of the photolysis of benzene. The photochemical step is assumed to lead to prefulvene, and both a synchronous conversion and a two‐step process via bicyclo[3.1.0]hexa‐1,3‐diene (isofulvene) could be ruled out as possible pathways from prefulvene to benzene. The most probable mechanism for the photochemical isomerization of benzene to fulvene involves the intermediate structures prefulvene and 1,3‐cyclopentadienylcarbene and has to compete with the almost barrierless formation of benzvalene and rearomatization to benzene. The short‐lived intermediates are rather flexible structures with negligible barriers to inversion at the radical center in the threemembered ring of prefulvene and prebenzvalene and to rotation around the exocyclic single bond in 1,3‐cyclopentadienylcarbene.
NMR Chemical‐Shift Anomaly and Bonding in Piano‐Stool Carbonyl and Related Complexes–an Ab Initio ECP/DFT Study
Authors: Kaupp Martin
Published: July 2007

The origin of the unusually large carbonyl 13C shifts and of unusual periodic trends in four‐legged piano‐stool complexes [M(n5‐C5H5)(CO)4]− (MTi, Zr, Hf) and in related species has been investigated by using a combination of ab initio effective‐core potentials (ECPs) and density‐functional theory (DFT). The ECP/SOS‐DFPT(IGLO) calculations indicate a considerable reduction in the anisotropy of the 13C(CO) chemical shift tensors compared to terminal carbonyl ligands in “normal” complexes. This is due to large paramagnetic contributions from metal d AO type (dz2, dxy) orbitals to the parallel component, σ33, of the shielding tensors of the carbonyl carbon atoms. The neutral d4 Group 5 and 6 complexes [M(n5‐C5H5)(CO)4] (MV, Nb, Ta) and [M(n5‐C5H5)(CO)3CH3] (MCr, Mo, W) exhibit successively smaller but still significant paramagnetic d‐orbital contributions to σ33, consistent with the observed less dramatic deshielding. The three‐legged d6 piano‐stool complexes [M(n5‐C5H5)(CO)3] (MMn, Tc, Re) do not exhibit these reductions of the shielding anisotropy, but have carbonyl 13C shift tensors comparable to regular octahedral carbonyl complexes. The special situation for the four‐legged complexes is related to the presence of high‐lying occupied metal d orbitals, and particularly to the favorable spatial arrangement of these d orbitals with respect to the carbonyl ligands. Bent‐sandwich d2 complexes like [Zr(n5‐C5H5)2(CO)2] exhibit comparable deshielding contributions from an occupied metal d orbital. For similar reasons, the 17O resonances for these piano‐stool and bent‐sandwich complexes are also predicted to be at unusually high frequencies, with low shift anisotropy. NMR shifts for the (n5‐C5H5)‐ligand atoms and the structures of the complexes are also discussed.
Self‐Assembled Monolayers on Gold Nanoparticles
Authors: Badia Antonella , Singh Shanti , Demers Linette , Cuccia Louis , Brown G. Ronald , Lennox R. Bruce
Published: July 2007

Self‐assembled monolayers (SAMs) of n‐alkanethiolates on gold, silver, and copper have been intensively studied both as model organic surfaces and as modulators of metal surface properties. Sensitivity restrictions imposed by monolayer coverage and the low surface area of planar metal substrates, however, limit the characterization of these films in molecular terms to surface enhancement techniques. As a result, key aspects such as film dynamics and alkyl chain ordering remain ill‐defined. The characterization of the thermal behaviour of SAMs is important not only for the design of stable, well‐ordered organic superlattices, but also for the fundamental understanding of the factors that drive molecular interactions in two dimensions. Phase properties in SAMs have been addressed here through the synthesis of gold nanoparticles of 20–30 Å in diameter and fully covered with alkylthiol chains. These thiolmodified gold nanoparticles with large surface areas have enabled the monolayer film structure to be uniquely characterized by transmission FT‐IR spectroscopy, NMR spectroscopy, and differential scanning calorimetry. Our studies reveal that for long‐chain thiols (≥ C16), the alkyl chains exist predominantly in an extended, all‐trans ordered conformation at 25°C. Furthermore, calorimetry, variable temperature transmission FT‐IR spectroscopy, and solid‐state 13C NMR studies have established that a cooperative chain melting process occurs in these alkylated metal colloids. How this arises is not immediately evident, given the relation between the extended chain conformation and the geometry of the spherical nanoparticles. Transmission electron microscopy (TEM) reveals that adjacent gold particles are separated by approximately one chain length; this suggests that chain ordering arises from an interdigitation of chains on neighboring particles. The thermotropic behavior is sensitive to the alkyl chain length and chain packing density. The alkylated nanoparticles can thus serve as a highly dispersed analogue to the much‐studied planar SAMs.
A Modified Cyclodextrin with a Fully Encapsulated Dansyl Group: Self‐Inclusion in the Solid State and in Solution
Authors: Corradini Roberto , Dossena Arnaldo , Marchelli Rosangela , Panagia Anna , Sartor Giorgio , Saviano Michele , Lombardi Angela , Pavone Vincenzo
Published: July 2007

A monofunctionalized β‐cyclodextrin containing a dansyl moiety, 6‐ deoxy‐ 6 ‐N‐ (N′‐ (5‐ dimethylamino ‐ 1 ‐ naphthalenesulfonyl)diaminoethane) ‐ β‐cyclodextrin (CD‐en‐DNS,2), was synthesized and its crystal structure determined. It was shown that the dansyl group is fully encapsulated within the cyclodextrin cavity, with the dimethylamino and sulfonyl groups emerging from opposite sides. The shape of the cavity is considerably flattened, since O(4)–O(4) distances parallel to the naphtalene ring were found to be longer than the others. The conformation of the diaminoethane linker was found to be determined by the inclusion of the dansyl group and by a hydrogen bond between the sulfonamide NH and one of the O(6)–H groups on the cyclodextrin rim. The self‐inclusion features of the aromatic moiety were found to be consistent with the solution data:1H NMR ROESY spectra suggested that the orientation of the dansyl moiety observed in the solid state was retained in aqueous solution; the circular dichroism spectrum was consistent with an axial complexation model. Fluorescence spectra showed that the inclusion of the dansyl group in the cyclodextrin cavity considerably increases the quantum yield: time‐resolved fluorescence experiments showed the presence of a long‐lifetime component (16.1 ns), which was attributed to the included fluorophore. The ability of2to act as a fluorescence sensor was evaluated by the addition of several guests of different shape: fluorescence intensity was lowered, especially upon addition of adamantanecarboxylic acid. All the data obtained were consistent with the model of the in‐out movement of the dansyl group from the self‐included conformation observed in the solid state to a position more exposed to the bulk solvent. Copper(II) was shown to enhance the difference in the fluorescence of2in the presence of guests by additional static quenching.
Synthesis, Structure, and Stereoselective Reaction of a Chiral Hydroxy‐Stabilized Metal‐Free Enolate
Authors: Reetz Manfred T. , Hütte Stephan , Goddard Richard , Robyr Chantal
Published: July 2007

The reaction of acetophenone with tetrabutylammonium hydroxide affords the tetrabutylammonium enolate of phenyl (2‐hydroxy‐2‐phenyl)propyl ketone. The crystal structure of this chiral enolate shows intramolecular hydrogen bonding between the hydroxyl group and the enolate oxygen atom. Furthermore, the α‐methylene units of the ammonium counterion form hydrogen bonds to the basic enolate C and O atoms and to the O atom of the hydroxy group. This three‐point bonding occurs selectively on the Re,Re side, a phenomenon which may be responsible for the direction of diastereo‐selectivity in the epoxide‐forming reaction of the enolate with N‐bromosuccinimide.
A New Experimental Method to Determine the Mutual Orientation of Helices in Coiled‐Coil Proteins: Structural Information about the Dimeric Interface of cJun, cFos, GCN4, and gp41
Authors: Kazmierski Wieslaw M. , McDermed John , Aulabaugh Ann
Published: July 2007

Disruption of protein dimers interacting by a leucine zipper motif represents a new potential pharmaceutical target. However, structural information concerning the exact nature of the interacting helices is usually not available. Towards this end, we have developed a disulfide‐trapping approach capable of distinguishing between the ad and gd modes of dimerization (Fig. 1), thus providing information useful in the design of small molecules that interfere with helix–helix interactions. We designed and synthesized nine cysteine‐substituted peptide fragments: GCN 4(g), GCN 4(a), GCN 4(d), cFos(g), cFos(a), cFos(d), cJun(g), cJun(a), and cJun(d), and evaluated the covalent crosslinking rates for them and their binary mixtures. Neither homogeneous cJun nor cFos dimerized and crosslinked, but their binary mixtures did with t1/2 of formation a > d > g, indicating to cFos–cJun heterodimerization according to ad mode (Fig. 1 a). Similarly, GCN 4 dimerized and crosslinked in the ad fashion; this result was in excellent agreement with the published X‐ray structure. Next, we investigated the mode of gp 41 dimerization, which appears critical for HIV‐1 replication The gp41 cysteinesubstituted fragments gp 41(g), gp 41(a), and gp 41(d) also dimerized and crosslinked, but with a different order of t1/2 of formation g > d > a, thus providing evidence that gp41 dimerizes in the gd mode (Fig. 1 b). Thus, the crosslinking experiments allow rapid elucidation of structural details of macromolecular interactions in aqueous media. These findings should prove useful in the design of compounds that inhibit macromolecular association.
Synthesis of Methyl 5′‐Thio‐α‐isomaltoside via an Acyclic Monothioacetal and its Behavior toward Glucoamylase
Authors: Hashimoto Hironobu , Kawanishi Masashi , Yuasa Hideya
Published: July 2007

Methyl 5′‐thio‐α‐isomaltoside (1), which contains the ring‐sulfur analogue of the nonreducing glucoside of isomaltose, was synthesized from gentiobiose through a novel ring opening‐recyclization approach. The nonreducing glucoside of per‐O‐benzylated phenyl 1‐thio‐β‐gentiobioside underwent O‐5′–C‐1′ bond cleavage with dimethyl‐boron bromide and thiolacetic acid to give the acyclic monothioacetal 4 with the 1‐thioglucopyranoside at the reducing end intact. The HO‐5′ group in 4 was inverted by a standard oxidation‐reduction process with good efficiency. Recyclization under Mitsunobu condition allowed C‐5′–S‐1′ bond formation with inversion of configuration at C‐5′, to give 1 after functional group interconversion. TLC analysis showed that 1, unlike isomaltose, was not hydrolyzed by glucoamylase from Rhizopus niveus. A fluorometric assay confirmed that the dissociation constant (Kd) for 1 with the enzyme was 39 mM at 20°C, which is comparable with that for isomaltose. A binding assay involving fluorescence titration of the enzyme‐1 complex with gluconolactone indicated that the disaccharide 1 was bound to the catalytic and noncatalytic subsites. Since isomaltose is known to bind only to the noncatalytic subsites, this result indicates a relatively high affinity of the 5‐thioglucose moiety for the catalytic subsite.
[Lithium tert‐butylperoxide]12: Crystal Structure of an Aggregated Oxenoid
Authors: Boche Gernot , Möbus Konrad , Harms Klaus , Lohrenz John C. W. , Marsch Michael
Published: July 2007

The X‐ray crystal structure of the dodecameric lithium tert‐butylperoxide [2]12 is the first of an alkali or alkaline earth peroxide. It shows the lithium ion bridging the two oxygen atoms of the peroxide unit and a slight lenghtening of the O–O bond, in agreement with quantum‐chemical calculations. A calculation for the model reaction of MeLi with LiOOH to give MeOLi and LiOH reveals the importance of Li bridging the O–O bond in the transition state of this reaction, as similarly discussed for many oxidation reactions of (transition‐) metal peroxides. Preliminary theoretical studies of the O–O bond length (and thus of the oxenoid character) as a function of the aggregation of 2 disclose that increasing aggregation leads to stabilization of the charge at the anionic oxygen atom and thus to a reduction of the O–O bond length (oxenoid character). Related considerations of the effect of aggregation should also be valid for other lithium (organometallic) compounds and their structure and reactivity as well as other properties.
Stereoselective Preparation of Polyfunctional Cyclopentane Derivatives by Radical Nickel‐ or Palladium‐Catalyzed Carbozincations
Authors: Stadtmüller Heinz , Vaupel Andrea , Tucker Charles E. , Stüdemann Thomas , Knochel Paul
Published: July 2007

The reaction of 5‐hexenyl iodides with diethylzinc (2 equiv) and catalytic amounts of a PdII or NiII complex like PdCl2(dppf), PdCl2(MeCN)2, or Ni(acac)2 results in an efficient ring closure (THF, RT, 2–12 h) affording cyclopentylmethylzinc iodides, which, after transmetalation with CuCN·2LiCl, can be further functionalized by treatment with a range of electrophiles like allylic halides, acyl chlorides, enones, nitroolefins, ethyl propynoate, and alkynyl halides to yield polyfunctional cyclopentane derivatives. The ring closures occur via radical intermediates, and the stereochemistry of the products can be explained according to the rules for radical cyclizations developed by Beckwith. The preparation of several di‐ and trisubstituted cyclopentanes has been achieved with high stereoselectivity. Tandem ring closures can be performed to construct bicyclic or tricyclic ring systems. Cyclizations of iodo‐ethylenic and acetylenic esters and ketones can be accomplished, although the high reactivity of acetylenic ketones leads to unexpected cyclization products. The synthetic utility of this method has been demonstrated by an enantioselective synthesis of (+)‐methyl epijasmonate and (‐)‐methyl cucurbate.
Gas‐Phase Experiments Aimed at Probing the Existence of the Elusive Water Oxide Molecule
Authors: Schröder Detlef , Schalley Christoph A. , Schwarz Helmut , Goldberg Norman , Hrûsák Jan
Published: July 2007

Collisional activation, charge reversal, and six different neutralization ‐ reionization mass spectrometric experiments with [H2,O2]·+ radical cations and [H2,O2]·‐ radical anions were performed in order to probe the predicated existence of neutral water oxide, H2OO, the long sought after tautomer of hydrogen peroxide, HOOH. The experiments together with ab initio calculations indicate that H2OO is a local minimum on the [H2,O2] potential‐energy surface, and the elusive molecule seems to be formed as a transient upon neutralization of the corresponding radical cation H2OO·+ in the gas phase.
Oxidative Degradation of Polychlorinated Phenols Catalyzed by Metallosulfophthalocyanines
Authors: Sorokin Alexander , Meunier Bernard
Published: July 2007

2,4,6‐trichlorophenol (TCP) is oxidized by potassium monopersulfate or hydrogen peroxide in the presence of iron or manganese tetrasulfonatophthalocyanines (FePcS or MnPcS) to yield not only the corresponding 2,6‐dichloro‐1,4‐benzoquinone but also ring‐cleavage products. Catalytic oxidation of the TCP ring by hydrogen peroxide is more efficient than by potassium monopersulfate, despite a slower substrate conversion, suggesting that different mechanisms are involved for these two catalytic systems: a metal‐oxo mechanism for FePcS/KHSO5 and a metal‐peroxo mechanism for FePcS/H2O2. Eight different final oxidation products and four quinone intermediates have been identified in the oxidation of TCP by the FePcS/H2O2 catalytic system. Chloromaleic acid is the main product of the oxidative ring cleavage. An iron‐peroxo complex PcS‐FeOOH is probably the active species responsible for the epoxidation of 2,6‐dichloro‐1,4‐benzoquinone and the C–C bond cleavage of 3,5‐dichloro‐2‐hydroxy‐1,4‐benzoquinone ring, both intermediates generated during the catalytic TCP degradation. The oxidation of pentachlorophenol (PCP) is also catalyzed by FePcS or MnPcS with KHSO5 or H2O2.
Novel Heterocumulenes: Bisiminopropadienes and Linear Ketenimines
Authors: Wolf Reinhard , Stadtmüller Stefan , Wong Ming Wah , Wentrup Curt , Barbieux‐Flammang Monique , Flammang Robert
Published: July 2007

Flash vacuum thermolysis (FVT) of suitably substituted isoxazol‐5(4H)‐ones 7–9 leads to three different types of ketenimines, namely, the isoxazolonoketenimines 2, the novel bisiminopropadienes RNCCCNR (5), and the C‐cyanoketenimines 14, all characterized by a combination of FVT/matrix isolation/IR spectroscopy and FVT/MS. An unusual, linear CCN‐C backbone in ketenimines 2g and 2h is revealed by their exceptional spectroscopic properties as well as an X‐ray crystal structure of 2g, and confirmed by density functional calculations (B3LYP/6–31 G*); these compounds are best described as resonance hybrids of ketenimines and isonitrile ylides R2C‐CN‐R'. The identification of the highly reactive bisiminopropadienes 5 is supported by the observed shifts in the IR bands of the 15N and 13C isotopomers as well as theoretical calculations, tert‐Butyl‐substituted isoxazolones 7e and 7f, and 8i form the expected ketenimines 2, which then undergo elimination of isobutene and CO2 to generate C‐cyanoketenimines 14 and 14i. N‐Phenyl‐dicyanoketenimine 32 is also described.
Repetitive Strategy for Exponential Growth of Hydroxy‐Functionalized Dendrons
Authors: Klopsch Rainer , Schlüter A.‐Dieter , Franke Peter
Published: July 2007

The synthesis of dendrons with orthogonally protected functional groups, which allows the construction of dendrons in an exponential fashion, is described. The sequence has been carried out on a gram scale for the first, second, and fourth generation. The dendrons have a relatively loose and flexible structure, which should make further growth feasible. Owing to the dendrons' adjustable and broadly applicable substitution pattern, they are the first representatives of a future construction kit for the systematic synthesis of layered dendrimers. The coupling step involves urethane formation.
A General Scheme Based on Empirical Increments for the Prediction of Hydrogen‐Bond Associations of Nucleobases and of Synthetic Host–Guest complexes
Authors: Sartorius Joachim , Schneider Hans‐Jörg
Published: July 2007

Association energies ΔGt in chloroform, in part also in carbon tetrachloride, were determined by NMR titrations of suitably substituted nucleosides and several synthetic analogues. Based on these and on many literature data, two simple free energy increments were derived describing the ΔGt values of 58 complexes within 1.8 kJ mol‐1. With chloroform as solvent the increment for the primary interaction between donor and acceptor is 7.9 kJ mol‐1, for the secondary one 2.9 kJ mol‐1, irrespective of whether the latter is attractive or repulsive. Addition of only 1% methanol to CCl4 led to a decrease in association constants by a factor of 25. Calorimetric titrations of G‐C nucleoside derivatives in CCl4 showed substantial contributions from G dimers, in line with NMR titrations, and surprisingly small decreases in entropy. Preliminary NOE measurements allowed us to single out some of the possible association modes; they are also in line with expected self‐ and triple‐association modes of the nucleobases. These modes are generally in accord with nucleobase associations predicted by MM calculations in the literature, which in turn agree with predictions based solely on the increments derived in the present work.
Mineralization of Gold Nanoparticles in a Block Copolymer Microemulsion
Authors: Spatz Joachim P. , Mößmer Stefan , Möller Martin
Published: July 2007

Controlled mineralization of gold nanoparticles has been performed in a microemulsion of polystyrene‐block‐poly(2‐vinylpyridine). The starting point was the formation of a thermodynamically stable dispersion of HAuCl4 in inverse micelles of the block copolymer in toluene, which became metastable when the gold was reduced. Kinetic control of the transformation allowed the following stages of the mineralization/coagulation process to be stabilized: 1) one gold particle per micelle, 2) aggregated micelles containing two or three gold particles, and 3) a state in which empty micelles coexist with larger polymer‐stabilized gold particles. Distinctive variations in the spectra were observed depending on the particle size and whether two particles had formed a couple with orientation‐dependent dipolar interactions.
A Combined Spectroscopic, Photophysical and Theoretical (DFT) Study of the Electronically Excited Inorganometallic Complexes [Ru(E)(E′)(CO)2(iPr–DAB)] (ECl, Me, SnPh3, PbPh3; E′GePh3, SnR3, PbR3 (RMe, Ph); iPr–DAB=N,N'‐diisopropyl‐1,4‐diaza‐1,3‐butadiene): Evidence of an Exceptionally Long‐Lived 3σπ* Excited State for [Ru(SnPh3)2(CO)2(iPr–DAB)]
Authors: Aarnts Maxim P. , Stufkens Derk J. , Wilms Maikel P. , Baerends Evert Jan , Vlček Antonín , Clark Ian P. , George Michael W. , Turner James J.
Published: July 2007

The photophysical properties of the metal–metal bonded complexes [Ru‐(E)(E′)(CO)2(iPr–DAB)] (ECl, E′SnPh3, PbPh3; EMe, E′SnPh3, PbPh3; ESnPh3, E′SnMe3, SnPh3, GePh3; EPbPh3, E′PbMe3, PbPh3, GePh3; iPr–DAB=N,N'‐diisopropyl‐1,4‐diaza‐1,3‐butadiene) have been studied. According to time‐resolved emission, UV/vis and IR spectra, combined with density functional (DFT) MO calculations, the lowest excited state has triplet metal‐to‐ligand charge‐transfer (3MLCT), triplet halide‐to‐ligand charge‐transfer (3XLCT) or 3σ(E–Ru–E)π* character, depending on the nature and combination of the ligands E and E′. The 3σ(E–Ru–E′)π* state is a bound state whose lifetime is strongly influenced by the ligands E and E′. An exceptionally long lifetime (264 μs at 80 K) is observed for the 3σ(Sn–Ru–Sn)π* state of the symmetrically substituted [Ru(SnPh3)2(CO)2(iPr–DAB)] complex. The reason for this long lifetime is the fact that the excited state of this complex is hardly distorted with respect to the ground state, owing to the delocalised character of the σ(Sn–Ru–Sn) bonding orbital, which mixes strongly with the π* orbital of the iPr–DAB ligand. This delocalisation is also responsible for the unusually high oscillator strength of the σ → π* electronic transition in the visible spectral region.
Molecular Recognition by Hydrogen Bonding in Polyelectrolyte Multilayers
Authors: Laschewsky André , Wischerhoff Erik , Denzinger Steffen , Ringsdorf Helmut , Delcorte Arnaud , Bertrand Patrick
Published: July 2007

Functional polyanions were prepared by copolymerization of sulfopropyl acrylate and sulfopropyl methacrylate with monomers bearing triaminopyrimidine or barbituric acid functionalities, respectively. Functionalized polyelectrolyte multilayers were assembled from these copolymers by stepwise alternating adsorption with poly(choline methacrylate). These multilayers are suited for molecular recognition of substrates that are complementary to the functional groups incorporated. Thus, multilayers containing triaminopyrimidine moieties selectively bind barbituric acid, and vice versa, when exposed to solutions of the 1:1 complex of barbituric acid and triaminopyrimidine. The molecular recognition process was monitored by UV/Vis spectroscopy, time‐of‐flight secondary ion mass spectroscopy (ToF‐SIMS), and photoelectron spectroscopy (XPS). Remarkably, after successful recognition and binding of the complementary substrates to the multilayers, the stepwise layering could be continued.
Total syntheses of the slime mold alkaloid arcyriacyanin A
Authors: Brenner Michael , Mayer Guido , Terpin Andreas , Steglich Wolfgang
Published: July 2007

Arcyriacyanin A (1) has been synthesized by three different routes. In the first synthesis the bisbromomagnesium salt of 2,4′‐biindole (5) was treated with dibromomaleimide (6) to yield arcyriacyanin A (1). The second approach used an intramolecular Heck reaction for the cyclization of a 4‐(triflyloxy)arcyriarubin 8 to N‐methylarcyriacyanin A (2). Thirdly, compound 2 was obtained by a domino Heck reaction between 3‐bromo‐4‐[1‐(tert‐butoxycarbonyl)indol‐3‐yl]‐1‐methylmaleimide (9) and 4‐bromoindole (10). The N‐methyl derivative 2 could be transformed into arcyriacyanin A (1) by standard methods.
The Oxide Anion Accelerated Retro‐Diels‐Alder Reaction
Authors: Bunnage Mark E. , Nicolaou K. C.
Published: July 2007

The widespread application of the retro‐Diels‐Alder reaction in synthesis has been hampered by the high temperatures usually required to effect cycloreversion. The discovery of the anionic oxy‐Cope reaction was followed by predictions that the accelerating effect of the oxide anion should also be observed with other pericyclic reactions. Recently, such predictions have been confirmed for the retro‐Diels‐Alder reaction, which often proceeds rapidly at room temperature by oxide anion rate acceleration. Such mild retro‐Diels‐Alder reactions have now been employed in the synthesis of a range of molecular targets, including temperature‐sensitive enediynes.
Heterosupramolecular Chemistry: Self‐Assembly of an Electron Donor (TiO2 Nanocrystallite)‐Acceptor (Viologen) Complex
Authors: Cusack Lucy , Rao S. Nagaraja , Fitzmaurice Donald
Published: July 2007

A TiO2 nanocrystallite has been modified to recognise and selectively bind, by complementary hydrogen bonding, a uracil substrate incorporating a viologen moiety. Band‐gap excitation of the self‐assembled donor (TiO2 nanocrystallite)‐acceptor (viologen) complex results in electron transfer. Some implications of these findings for the self‐assembly of functional nanostructures containing both condensed phase and molecular components are considered.
Hexabromotricyclobutabenzene and Hexabromohexaradialene: Their Nickel‐Mediated One‐Pot Syntheses and Crystal Structure
Authors: Stanger Amnon , Ashkenazi Nissan , Boese Roland , Bläser Dieter , Stellberg Peter
Published: July 2007

The reaction of hexakis(dibromomethyl)benzene with [(Bu3P)2‐Ni(COD)] (COD = 1,5‐cyclooctadiene) in DMF at 65–70°C yielded a mixture of the title compounds. The mixture was separated by column chromatography to yield hexabromotricyclobutabenzene (3 a) and hexabromohexaradialene (4) in 24 and 16% yields, respectively. 1H and 13C NMR spectroscopy suggest that 3 is obtained as the syn‐all‐trans isomer 3 a, and the symmetric anti‐all‐trans isomer 3 b is not obtained at all. The X‐ray structures of 3 a and 4 are reported. The hexaradialene 4 has a chair conformation, and deviates from planarity by 43.6°. Heat or radical impurities cause the clean transformation of 3 a to 4.
Terrylenimides: New NIR Fluorescent Dyes
Authors: Holtrup Frank O. , R. J. Müller Gert , Quante Heribert , De Feyter Steven , De Schryver Frans C. , Müllen Klaus
Published: July 2007

Terrylenimides 3 and 4 represent a new class of blue colorants, exhibiting absorption maxima at 650 to 700 nm and fluorescence emissions in the NIR region (673 to 750 nm). The terrylenimides were synthesized by means of various organometallic coupling reactions, catalyzed by transition metal complexes (Nio, Pdo) and starting from the aromatic bromides, boronic acids, or organotin compounds. The terrylenimides have all the properties expected of excellent fluorescent dyes: high extinction coefficients, high fluorescence quantum yields, and very good thermal, chemical, and photochemical stabilities. Owing to its extended π system, 3 can reversibly accept four negative charges. By varying the substituents, 3 and 4 can be modified to serve either as soluble dyes or as insoluble pigments.
Evidence for DirecttransInsertion in a Hydrido‐Olefin Rhodium Complex—Free Nitrogen as a Trap in a Migratory Insertion Process
Authors: Vigalok Arkadi , Kraatz Heinz‐Bernhard , Konstantinovsky Leonid , Milstein David
Published: July 2007

Reduction of the hydrido chloride complex [Rh(H)Cl{CH3C(CH2CH2‐P(tBu)2)2}] (4) with NaH under a nitrogen atmosphere results in formation of two products: the dinitrogen complex [Rh(N2)‐{CH3C(CH2P(tBu)2)2}](2)and the unusual low‐valent hydrido‐olefin complex, [RhH{CH2C(CH2CH2P(tBu)2)2}](3). In the presence of N2, complexes2and3are in equilibrium in solution;2is about 2.9kcalmol−1more stable than3+ N2. Both complexes co‐crystallize in the solid state; they occupy the same crystallographic site in the crystal lattice (P2‐(1)/c;Z= 4;a= 12.173(2),b= 14.121 (3),c= 15.367 (3); α = 90, β = 106.50(3), γ = 90°). The mechanism of the reversible interconversion of2and3has been studied in detail. Complex3undergoes rapid olefin insertion/β‐hydrogen elimination processes. The insertion rates were measured at different temperatures by saturation transfer NMR experiments, providing evidence for a highly organized late transition state (δS≠≈︂ – 40 e.u.), which can be caused by a concerted “transmigration”. This theoretically unfavorable process is assisted by a distortion from the ideal square‐planar configuration, including a decrease of the P‐Rh‐P angle and some bias of the double bond toward the hydride as indicated by the X‐ray crystal structure of3. Under a nitrogen atmosphere, the intermediate formed upon olefin insertion is slowly trapped by free dinitrogen to form complex2. The dinitrogen dissociation from2was found to be the rate‐determining step for the overall interconversion of2and3(δG≠298= 24.1 kcalmol−1).
Nickel Complexes of 21‐Oxaporphyrin and 21, 23‐Dioxaporphyrin
Authors: Chmielewski Piotr J. , Latos‐Grażyński Lechoslaw , Olmstead Marilyn M. , Balch Alan L.
Published: July 2007

The nickel(I) and nickel(II) complexes of 5,20‐bis(p‐tolyl)‐10, 15‐diphenyl‐21‐oxaporphyrin (ODTDPPH) and 5,10,15,20‐tetraphenyl‐21, 23‐dioxaporphyrin (O2TPP) have been investigated. These oxa analogues of 5,10,15,20‐tetraarylporphyrin, where one or two pyrrole rings are replaced by a furan moiety, have been synthesized by condensation of the respective precursors, namely 2,5‐bis(arylhydroxymethyl)furan, pyrrole, and arylaldehyde. Insertion of nickel(II) into ODTDPPH or O2TPP yielded high‐spin five‐ and six‐coordinate ([(ODTDPP)NiIICl] and [(O2TPP)NiIICl2]) complexes, which can be reduced with moderate reducing reagents. The EPR spectra of [(ODTDPP)NiI] and [(O2TPP)NiICl] revealed the NiI oxa(dioxa)porphyrin rather than a NiI anion radical electronic structure. In the structures of [(ODTDPP)NiIICl], [(O2TPP)NiIICl2], and [(ODTDPP)NiI], determined by X‐ray diffraction, the furan ring is planar and coordinates in the η1 fashion through the trigonal oxygen atom; the nickel ion lies in the furan plane for the latter two complexes, but slightly outside it in [(ODTDPP)NiIICl]. The Ni–N and Ni–O bond lengths decrease upon reduction of high‐spin five‐coordinate [(ODTDPP)NiIICl] to four‐coordinate [(ODTDPP)NiI]. The pattern of downfield pyrrole resonances in 1H NMR spectra of [(ODTDPP)NiIICl] and [(O2TPP)‐NiIICl2] has been established. The downfield positions of furan resonances are unusual for NiII heteroporphyrins; they have been accounted for by the nearly in‐plane coordination of the furan moiety as opposed to the side‐on coordination found for thiophene‐ or selenophene‐containing heteroporphyrins. An example of ion‐pair formation, [(O2TPPH)2][NiIICl4], was produced from [(O2TPP)NiIICl2] by acidification with HCl.
New Aspects of the Photophysical Properties of 1′H‐2′,3′‐Dimethoxycarbonylspiro[fluoren‐9,1′‐pyrrolo[2,1‐a]quinoline]
Authors: Andreis Christine , Dürr Heinz , Wintgens Véronique , Valat Pierre , Kossanyi Jean
Published: July 2007

In this paper we describe photophysical and mechanistic studies of spiro[fluorene‐9,1′‐pyrrolo[2,1‐a]quinolines] (DHI) 1–4 and of their styryl homologues 5–11 (styryl DHI). DHIs 1–11 have low fluorescence quantum yields (approximately ϕf = 10‐3). The position of the fluorescence emission band is temperature‐dependent. The ring‐opened betaines 1′–11′ do not fluoresce at all. DHIs 1–11 exhibit phosphorescence, which is hypsochromically shifted by NO2 substitution of the fluorene ring. The triplet can be populated only by sensitization (with benzophenone as sensitizer) and, therefore, the reaction that occurs under direct excitation of the molecule involves the singlet excited state only. On irradiation under time‐resolved conditions, two transients were seen to be formed in the picosecond and microsecond domains. Slightly tilted educt or product‐like geometries are attributed to these transients. An energy diagram is proposed for the photoreaction of the model compound spiro[pyrroloquinoline] 1 → 1′, which takes into account the singlet pathway and the different transients of ca. 100 ps lifetimes formed from the singlet excited state of 1–11.
Electronic Structure and Linear and Nonlinear Optical Properties of Symmetrical and Unsymmetrical Squaraine Dyes
Authors: Meyers Fabienne , Chen Chin‐Ti , Marder Seth R. , Brédas Jean‐Luc , Meyers F.
Published: July 2007

In this theoretical study, we investigate the molecular and electronic structures of symmetrical and unsymmetrical squaraine dyes. Such dyes can be represented by the formula D‐A‐D, where D is a donor group and A an acceptor moiety. We analyze the evolution in geometric structure that results from changing both donor substituents simultaneously or from varying only one donor group to produce an asymmetrical system. The changes in geometric and electronic structures are compared and found to be consistent. The trends in linear and nonlinear optical properties, and in particular in second‐order polarizabilities, are investigated in several ways. The two‐state model appears to be inadequate in describing the second‐order polarizability, β. Consequently, we are unable to deduce simple structure/property relationships that might help in the design of quadrupolar compounds for nonlinear optics. Finally, a series of unsymmetrical squaraines with OH substitution and enol‐ketone isomerism are investigated; the calculated nonlinear properties follow a similar trend to the experimental results: the OH substitution and isomerization contribute to increasing the ground‐state polarization.