A Silatetragallane—Classical Heterobicyclopentane or closo-Polyhedron?

Classical and nonclassical can be used to describe the bonding in the polyhedral Ga₄Si framework of the silagallanate ion [Me₃SiSi{GaSi(SiMe₃)₃}₃GaSiMe₃]⁻ (the GaSi framework is depicted in the picture). This is the result of density functional calculations that were carried out on model compounds. The cluster was obtained by ultrasonication of gallium and iodine and subsequent reaction with (Me₃Si)₃Li(thf)₃.     

Probing Solid Catalysts under Operating Conditions: Electrons or X‐rays?

Seeing is believing : In light of recent advances, the pros and cons of using electrons and X‐rays for in situ studies of catalysts are analyzed: by using X‐rays the structure of bound reactants at steady state are obtained from extended X‐ray adsorption fine structure spectroscopy (EXAFS) data (see graph), thereby affording mechanistic insights.

     

Nature or Petrochemistry?—Biologically Degradable Materials

Naturally occurring polymers have been utilized for a long time as materials, however, their application as plastics has been restricted because of their limited thermoplastic processability. Recently, the microbial synthesis of polyesters directly from carbohydrate sources has attracted considerable attention. The industrial-scale production of poly(lactic acid) from lactic acid generated by fermentation now provides a renewable resources-based polyester as a commodity plastic for the first time. The biodegradability of a given material is independent of its origin, and biodegradable plastics can equally well be prepared from fossil fuel feedstocks. A consideration of the overall carbon dioxide emissions and consumption of non-renewable resources over the entire life-cycle of a product is not necessarily favorable for plastics based on renewable resources with current technology-in addition to the feedstocks for the synthesis of the polymer materials, the feedstock for generation of the overall energy required for production and processing is decisive.     

Zirconocene Catalysts: Ion‐pairs,Zwitterions, or Weakly Bound Molecules?

The bonding between electron deficient zirconocene complexes such as [(η⁵‐C₅Me₅)₂ZrR]⁺ and the counterions [(C₆F₅)₃BR]^– (R = CH₃, 1 , R = H, 2 ) has been investigated by DFT and extended Hückel calculations. The molecular bonding analysis has been carried out in the light of the structural information on the inter‐ion interactions available for these and other similar electron deficient coordinatively unsaturated zirconocene complexes. It has been shown that the “anion‐cation” interactions in 1 and 2 are different manifestations of the same need to provide electron density to the metal centre. In 1 this is achieved via the ‐C–H σ‐bonds and has been described as a pseudo‐agostic interaction, whereas in 2 electron donation occurs via the phenyl F‐atoms. The study of the covalent component of the bonding between the ionic components has shown that the weak electron donation, though not sufficient to saturate the metal centre, stabilises the unsaturated species and allows it to have a longer life time. It has been inferred that in the homogenous Ziegler‐Natta catalytic process the approach of a strongly coordinating molecule, such as an olefin in the polymerisation, destroys the weak interaction and allows the reaction to proceed whilst the ion‐pairing maintains the counterion in proximity of the complex.     

Formation of cine‐Substitution Products in the Suzuki?Miyaura Cross‐Coupling Reaction Catalyzed by Dinuclear Palladium Complexes

The Suzuki? Miyaura coupling reaction catalyzed by dinuclear palladium complexes gave cine‐substitution products along with ordinary ipso‐substitution products. In the reaction in (D₆)benzene, the ipso position of the cine‐substitution product was highly deuterated. The H/D exchange also occurred in various positions of benzene rings.     

Are Particulate Noble‐Metal Catalysts Metals,Metal Oxides,or Something In‐Between?

A good model? Noble‐metal particulate catalysts often require small amounts of oxygen to obtain optimal activity. However, the structure and stoichiometry of the oxidized metal clusters involved remains obscure, even almost two hundred years after their discovery. A heteropolypalladate salt (see picture; Pd yellow, O red) now offers a view of how oxygen might be incorporated into small noble‐metal clusters.

     

ChemInform Abstract: NF5 — Viable or Not?

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

Antibacterial Natural Products in Medicinal Chemistry—Exodus or Revival?

To create a drug, nature's blueprints often have to be improved through semisynthesis or total synthesis (chemical postevolution). Selected contributions from industrial and academic groups highlight the arduous but rewarding path from natural products to drugs. Principle modification types for natural products are discussed herein, such as decoration, substitution, and degradation. The biological, chemical, and socioeconomic environments of antibacterial research are dealt with in context. Natural products, many from soil organisms, have provided the majority of lead structures for marketed anti-infectives. Surprisingly, numerous "old" classes of antibacterial natural products have never been intensively explored by medicinal chemists. Nevertheless, research on antibacterial natural products is flagging. Apparently, the "old fashioned" natural products no longer fit into modern drug discovery. The handling of natural products is cumbersome, requiring nonstandardized workflows and extended timelines. Revisiting natural products with modern chemistry and target-finding tools from biology (reversed genomics) is one option for their revival.     

A 1010 Rate Enhancement of Phosphodiester Hydrolysis by a Dinuclear Aminopeptidase—Transition-State Analogues as Substrates?

Streptomyces dizinc aminopeptidase (sAP) shows a specific activity of 33.7 nmol min⁻¹ mg⁻¹ toward the hydrolysis of the transition-state analogue bis-p-nitrophenylphosphate with a rate constant of k_cat/K_m=100 M ⁻¹ s⁻¹ and a first-order rate enhancement of about 10¹⁰, which is much superior to several Zn chemical models and comparable to some phosphodiesterases. This study suggests that sAP can serve as a novel dinuclear model system to provide further insight into dinuclear hydrolysis.     

Noncoordinating Anions—Fact or Fiction? A Survey of Likely Candidates

Is there anything resembling a truly noncoordinating anion? Would it not be great to be able to prepare any crazy, beautiful, or simply useful cationic species that one has in mind, or has detected by mass spectroscopy? In condensed phases the target cation has to be partnered with a suitable counteranion. This is the moment when difficulties arise and many wonderful ideas end in the sink owing to coordination or decomposition of the anion. However, maybe these counteranion problems can be overcome by one of the new weakly coordinating anions (WCAs). Herein is an overview on the available candidates in the quest for the least coordinating anion and a summary of new applications, available starting materials, and general strategies to introduce a WCA into a system. Some of the unusual properties of WCA salts such as high solubility in low dielectric media, pseudo gas‐phase conditions in condensed phases, and the stabilization of weakly bound and low‐charged complexes are rationalized on thermodynamic grounds. Limits of the WCAs, that is, anion coordination and decomposition, are shown and a quantum chemical analysis of all types of WCAs is presented which allows the choice of a particular WCA to be based on quantative data from a wide range of different anions.     

Ligand‐Promoted Borylation of C(sp3)?H Bonds with Palladium(II) Catalysts

A quinoline‐based ligand effectively promotes the palladium‐catalyzed borylation of C(sp³) H bonds. Primary β‐C(sp³) H bonds in carboxylic acid derivatives as well as secondary C(sp³) H bonds in a variety of carbocyclic rings, including cyclopropanes, cyclobutanes, cyclopentanes, cyclohexanes, and cycloheptanes, can thus be borylated. This directed borylation method complements existing iridium(I)‐ and rhodium(I)‐catalyzed C H borylation reactions in terms of scope and operational conditions.     

Amino Acid Derived Macrocycles—An Area Driven by Synthesis or Application?

The synthesis, structure, and physical properties of macrocycles have fascinated chemists for many years. Their inherent properties make them useful in areas as diverse as ion transport across membranes, development of new antibiotics, and catalysis. In this Review, the authors examine the chemistry of macrocycles containing non-peptidic amino acid derived molecules; the analysis is discussed in terms of function, rather than structure or synthesis. It is revealed that the diverse and imaginative structures created by synthetic chemists are not being fully exploited in application-driven endeavors.     

Ligand Reprogramming in Dinuclear Helicate Complexes: A Consequence of Allosteric or Electrostatic Effects?

The ditopic ligand 6,6'-bis(4-methylthiazol-2-yl)-3,3'-([18]crown-6)-2,2'-bipyridine (L(1)) contains both a potentially tetradentate pyridyl-thiazole (py-tz) N-donor chain and an additional "external" crown ether binding site which spans the central 2,2'-bipyridine unit. In polar solvents (MeCN, MeNO(2)) this ligand forms complexes with Zn(II), Cd(II), Hg(II) and Cu(I) ions via coordination of the N donors to the metal ion. Reaction with both Hg(II) and Cu(I) ions results in the self-assembly of dinuclear double-stranded helicate complexes. The ligands are partitioned by rotation about the central py--py bond, such that each can coordinate to both metals as a bis-bidentate donor ligand. With Zn(II) ions a single-stranded mononuclear species is formed in which one ligand coordinates the metal ion in a planar tetradentate fashion. Reaction with Cd(II) ions gives rise to an equilibrium between both the dinuclear double-stranded helicate and the mononuclear species. These complexes can further coordinate s-block metal cations via the remote crown ether O-donor domains; a consequence of which are some remarkable changes in the binding modes of the N-donor domains. Reaction of the Hg(II)- or Cd(II)-containing helicate with either Ba(2+) or Sr(2+) ions effectively reprogrammes the ligand to form only the single-stranded heterobinuclear complexes [MM'(L(1))](4+) (M=Hg(II), Cd(II); M'=Ba(2+), Sr(2+)), where the transition and s-block cations reside in the N- and O-donor sites, respectively. In contrast, the same ions have only a minor structural impact on the Zn(II) species, which already exists as a single-stranded mononuclear complex. Similar reactions with the Cd(II) system result in a shift in equilibrium towards the single-stranded species, the extent of which depends on the size and charge of the s-block cation in question. Reaction of the dicopper(I) double-stranded helicate with Ba(2+) shows that the dinuclear structure still remains intact but the pitch length is significantly increased.