摘要:

AbstractThe broad varieties of organic and organometallic reactions merge into a common unifying mechanism by considering all nucleophiles and electrophiles as electron donors (D) and electron acceptors (A), respectively. Comparison of outer‐sphere and inner‐sphere electron transfers with the aid of Marcus theory provides the thermochemical basis for the generalized free energy relationship for electron transfer (FERET) in Equation (37) and its corollaries in Equations (43) and (44) that have wide predictive applicability to electrophilic aromatic substitutions, olefin additions, organometallic cleavages, etc. The FERET is based on the conversion of the weak nucleophile–electrophile interactions extant in the ubiquitous electron donor—acceptor (EDA) precursor complex [D, A] to the radical ion pair [D⊕, A⊖], for which the free energy change can be evaluated from the charge‐transfer absorption spectra according to Mulliken theory. FERET analysis thus indicates that the charge‐transfer ion pairs [D⊕, A⊖] are energetically equivalent to the transition states for nucleophile/electrophile transformations. The behavior of such ion pairs can be directly observed immediately following the irradiation of the charge‐transfer bands of various EDA complexes with a 25‐ps laser pulse. Such studies confirm the radical ion pair [Arene⊕, NO2] as a viable intermediate in electrophilic aromatic nitration, as presented in the electron‐transfer mechanism between arenes and the nitryl

ISSN:0570-0833    

DOI:10.1002/anie.198812273

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe scientific description of lectins, a class of sugar‐binding proteins distinct from both sugar‐specific enzymes and antibodies, began nearly 100 years ago. The natural ligands of lectins include the sugar moieties of cell glycoproteins, glycolipids, and proteoglycans. Lectins can selectively recognize these potential information‐storing or ‐transferring structural elements and so contribute to regulating biological processes. Plant lectins are now a well‐established means of characterizing and localizing glycoconjugate‐bound sugar determinants. However, work on endogenous lectins is still confronted by many open questions, whose answers require detailed experiments using a suitable model system. Tumors offer an obvious model, particularly since they allow the possible diagnostic and therapeutic uses of lectins to be explored directly. Tumor lectinology employs a broad range of methods, from the chemical synthesis of the sugar components of neoglycoproteins, and their use to seek out cellular sugar receptors with histochemical and cell‐biological techniques, to the biochemical and immunological characterization of the lectins expressed by tumor cells and the use of homology analysis to find further functional domains. The outlook for clinical applications depends on the harmonious integration of these disparate disciplines in tumo

ISSN:0570-0833    

DOI:10.1002/anie.198812671

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractIn both physics and chemistry, increased attention is being paid to metal clusters. One reason for this attitude is furnished by the surprising results that have been obtained from studies of the preparation, structural characterization and physical and chemical properties of the clusters. Whereas investigations of cluster reactivity are at present generally limited to three‐ or four‐membered clusters, successful syntheses of clusters with many more metal atoms have recently been designed. These substances occupy an intermediate position between solid state chemistry and the chemistry of metal complexes. This review presents a versatile method for synthesizing metal clusters: the reaction of complexes of transition metal halides with silylated compounds such as E(SiMe3)2(E = S, Se, Te) and E′R(SiMe3)2(R = Ph, Me, Et; E′ = P, As, Sb). Although some of the compounds thus formed have already been prepared by other routes, the method affords ready access to both small and large transition metal clusters with unusual structures and valence electron concentrations; a variety of reactions in the ligand sphere are also p

ISSN:0570-0833    

DOI:10.1002/anie.198812771

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractHomogeneous catalysis as the major industrial outlet of organometallic basic research has been enjoying great benefit from organotransition metal species that promote bond forming between hydrocarbon fragments. Most of the commercially important processes that serve to produce large‐volume organic feedstock chemicals such as linear α‐olefins (Shell Higher Olefins Process), linear aldehydes (hydroformylation), acetaldehyde (Wacker‐Hoechst), acetic acid (Monsanto), adiponitrile (DuPont hydrocyanation of butadiene) operate at low‐valent metal centers. It is thus hardly surprising that by far the most part of organometallic research during the past few decades has been directed towards an understanding and the improvement of these catalytic reactions as well as towards the related stoichiometric chemistry. As a matter of consequence, our present knowledge on high‐valent organotransition metal compound is comparatively shallow, nor do we know much about the chemical relationship and interconvertability of high and low oxidation states within a given class of compounds. In this article I want to point out some ostensibly challenging perspectives of future organometallic research by describing a novel class of high oxidation state organorhenium compounds as well as by speculating on possible generalizations for other transit

ISSN:0570-0833    

DOI:10.1002/anie.198812971

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe structural characterization of materials and the tailoring of their properties is an important area of chemical research. A new trend in this area is the recourse to lasers both for analytical as well as preparative purposes, exploiting the fact that lasers, by virtue of their properties (sharp energy, spatial and temporal resolution etc.), offer the most precise and selective interaction of energy and matter that we know. Furthermore, photochemical syntheses and material transformations can proceed “cold” and without causing damage to surface structures. Laser chemistry finds application in thin film deposition, in the formation of surface layers, as well as in (structuring) ablation or etching, and in the initiation and enforcing of reactions at surfaces. In the present paper an introduction to these new possibilities on the general basis of molecule‐surface interactions is followed by a brief characterization of lasers suitable for such purposes. Thereafter, four examples are discussed: gas phase deposition from volatile organometallic compounds with photoelectric activation at the surface or photochemical activation of the gaseous species (e.g. by employing molecular beams). In this way (noble) metal contacts can be deposited on various substrates. Instead of surface deposition, nucleation can occur in the gaseous medium, yielding highly disperse powders, e.g. of silicon carbide. Finally, an etching reaction is discussed where the laser does not act as an energy source but as an analytical instrument to provide diagnostic and mechanistic inform

ISSN:0570-0833    

DOI:10.1002/anie.198813141

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198813261

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198813291

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198813301

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198813311

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198813371

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY