摘要:

AbstractTartaric acid esters with lipophilic alcohol moieties can discriminate between enantiomeric ammonium ions such as norephedrinium ions. Quantitative results were obtained by partitioning the components between water and 1, 2‐dichloroethane. The stereoselectivity was characterized by the free energy difference of the partition process (ΔΔG). Diamond‐lattice sectors were used to construct models of the (unstable) lipophilic ester/ammonium salt complexes from X‐ray structures of the individual components (esters and ammonium salts). These models can be used to interpret the effect of structure and configuration of the alcohol moiety on the stereoselectivity and enantioselectivity towards 1‐phenyl‐2‐amino‐1‐propanol (no

ISSN:0570-0833    

DOI:10.1002/anie.198911473

出版商:Hüthig&Wepf Verlag    

年代:1989

数据来源: WILEY

摘要:

AbstractManganese is an essential trace element, forming the active sites of a number of metalloproteins. Several metalloproteins contain two or more manganese ions per subunit. The structural properties of these enzymes and the experimental evidence for their proposed structures are described. Parallel to the efforts of biochemists, who are seeking to understand the function of these enzymes on a molecular level, inorganic chemists have been investigating the coordination chemistry of bi‐ and polynuclear complexes of manganese which contain O, N donor atoms and a variety of bridging O, N ligands. A large number of such complexes have been synthesized, their X‐ray structures determined and their magnetic and spectroscopic properties studied in detail. In some instances the electronic and spectroscopic properties of these model compounds are amazingly similar to those of the biomolecules. This has led to a deeper understanding of the structure and sometimes of the function of the metalloproteins. Research on manganese metalloproteins with polynuclear active sites represents a fascinating example of interdisciplinary cooperation between physicists, biochemists and inorganic chemi

ISSN:0570-0833    

DOI:10.1002/anie.198911531

出版商:Hüthig&Wepf Verlag    

年代:1989

数据来源: WILEY

摘要:

AbstractThe history of ring systems in organic chemistry parallels their synthetic accessibility. Transition‐metal‐catalyzed cyclizations offer a new opportunity to create carbo‐ and heterocyclic compounds with great facility. Among these methods, allylic alkylations catalyzed by palladium have proven unusually productive because of the extraordinary chemo‐, regio‐, and diastereoselectivity and the continuing possibility for the development of enantioselectivity. The rules for ring closure differ from those for non‐transition‐metal‐catalyzed reactions. A major benefit is the ability to generate medium (eight‐, nine‐, ten‐, and eleven‐membered) and large rings in preference to normal (five‐, six‐ and seven‐membered) rings. With the appropriate substrate, efficient macrocyclizations are possible under conditions of normal concentrations. A second major benefit derives from the complementary stereochemistry of the metalcatalyzed substitution (net retention of configuration) compared to non‐metal‐catalyzed reactions (inversion of configuration). Further, the requirement for the substrate to conform to the transition‐metal template may impose a stereochemical preference in the intermediate that ultimately translates into the thermodynamically less stable organic product regardless of the stereochemistry of the starting material. While more work has focused on carbocyclic synthesis, the possibilities for heterocyclic synthesis are just beginning to be tapped. In addition to forming heterocycles by CC bond formation, use of a heteroatom as a nucleophile has already proven effective for oxygen and nitrogen, with other nucleophiles awaiting investigation. New dimensions for cyclization via allylic alkylation arise by generating the requisite π‐allylpalladium intermediates by methods other than palladium(0)‐initiated allylic ionizations. In addition, metals other than palladium will clearly expand the

ISSN:0570-0833    

DOI:10.1002/anie.198911731

出版商:Hüthig&Wepf Verlag    

年代:1989

数据来源: WILEY

摘要:

AbstractAlthough sunlight induced photochemistry must have occurred on the planet Earth for billions of years, the chemical changes caused by light have attracted systematic scientific scrutiny only relatively recently. How did scientists first conceive the idea that the interaction of materials with light could not only cause physical phenomena, but could also alter their chemical nature? When sunlight began to be employed as a heat source for distillation, the eventual discovery of photochemical reactions was assured. One can envision three types of changes that would have aroused the curiosity of laboratory chemists: color changes; the evolution of gas bubbles (oxygen in photosynthesis); and the precipitation of a photoproduct less soluble than its precursor. Less predictable was the observation that sunlight caused crystalline santonin to burst because it is converted into a product with a different crystal lattice. In the course of the eighteenth and nineteenth centuries a variety of photochemical reactions, some observed by chance, others uncovered in carefully planned studies, ultimately led to a major systematic investigation that established photochemistry as a viable branch of chemistry.

ISSN:0570-0833    

DOI:10.1002/anie.198911931

出版商:Hüthig&Wepf Verlag    

年代:1989

数据来源: WILEY

摘要:

AbstractUntil recently, the production of gallium arsenide, indium phosphide, and related compound semiconductors has fallen in the domain of the materials scientist and the electrical engineer. By clever use of classical chemistry, exemplified by the thermal reaction of Me3Ga and AsH3, it is possible to make semiconductors on a commercial scale. However, there are some drawbacks associated with the existing methodology, including the environmental and health hazards of handling pyrophoric and toxic starting materials as well as stoichiometry control problems and undesirable side reactions. Can the synthetic inorganic and organometallic chemist play a useful role in this important area by designing and developing new reagents for the production of semiconductor materials? We believe the answer is yes, and in this article we discuss a new approach to the preparation of GaAs and InP thin films based on single‐source precursors. These compounds feature strong σ‐bonding between the group III and V elements, together with substituents that are capable of facile thermal elimination. The III/V precursors are more stable toward air and moisture and considerably less toxic than either adducts or mixtures of group III and V compo

ISSN:0570-0833    

DOI:10.1002/anie.198912081

出版商:Hüthig&Wepf Verlag    

年代:1989

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198912161

出版商:Hüthig&Wepf Verlag    

年代:1989

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198912181

出版商:Hüthig&Wepf Verlag    

年代:1989

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198912191

出版商:Hüthig&Wepf Verlag    

年代:1989

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198912211

出版商:Hüthig&Wepf Verlag    

年代:1989

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198912221

出版商:Hüthig&Wepf Verlag    

年代:1989

数据来源: WILEY