摘要:

AbstractThe power of cycloadditions for ring formation derives from the potential for chemo‐, regio‐, diastereo‐, and enantioselectivity. Trimethylenemethane (TMM) and its equivalents offer the possibility of extending such benefits to the synthesis of five‐membered rings. Three types of conjunctive reagents appear to be promising candidates: (1) selected 4‐alky‐lidene‐4,5‐dihydro‐3H‐pyrazoles, (2) 2‐[(trimethylsilyl)methyl]allyl esters and halides, and (3) alkylidenecyclopropanes. Thermal reactions, especially intramolecular ones, effect the cycloaddition of the pyrazoles to olefins–especially electron‐poor olefins. The latter two precursors require a catalyst, notably a palladium complex, to unlock the synthetic potential. With the 2‐[(trimethylsilyl)methyl]allyl esters, a general chemo‐, regio‐, and diastereoselective cycloaddition with typical Diels‐Alder dienophiles forms methylenecyclopentanes in both inter‐ and intramolecular fashion. A tin analogue allows extension of this cycloaddition to aldehydes, and, in a related sequence, to ketones and imines to form 3‐methylenete‐trahydrofurans and 3‐methylenepyrrolidines with excellent diastereoselectivity. The 2‐[(tri‐methylsilyl)methyl]allyl esters also serve to effect net methylenecyclopentane annulation to cyclic enols. Such adducts can further undergo three‐carbon intercalation or combined ring contraction‐spiroannulation simply by the use of either a nucleophilic or electrophilic trigger. Alkylidenecyclopropanes undergo cycloadditions to both electron‐rich and electron‐poor olefins when stimulated by metal catalysts, especially palladium. A complementary regiochemistry to the approach based upon silicon conjunctive reagents is observed. The availability of these cycloadditions provides

ISSN:0570-0833    

DOI:10.1002/anie.198600013

出版商:Hüthig&Wepf Verlag    

年代:1986

数据来源: WILEY

摘要:

AbstractNumerous analogies between organic and inorganic chemistry have emerged in recent years. The most prominent example is the isolobal relationship. Many reactions have shown that metal‐metal double and triple bonds exhibit a pattern of reactivity similar to that of alkenes and alkynes. In compounds containing a σ2π4triple bond between molybdenum and tungsten atoms, the MM bond order can be increased from three to four by reductive elimination or decreased from three to two or one by oxidative addition. Complexes with MM bonds can be used to prepare clusters or can serve as catalysts. In this review relationships between structure (electronic and stereochemical) and reactivity that are characteristic for modern inorganic chemistry are di

ISSN:0570-0833    

DOI:10.1002/anie.198600211

出版商:Hüthig&Wepf Verlag    

年代:1986

数据来源: WILEY

摘要:

AbstractThis article sets out to describe and account for the chemical and physical consequences of the presence of gross disorder in solids. Knowledge of the structure of such disordered materials is an obvious prerequisite to a further understanding of other properties and behavior, and our current knowledge of the structure of various noncrystalline systems is discussed together with the experimental techniques which need to be employed in order to obtain such information. The so‐called glass transition, which takes place as a liquid is supercooled below the crystallization temperature, is discussed in terms of the various models which have been proposed to account for this phenomenon. The effect of noncrystallinity on electronic properties is also discussed, and we highlight new developments in the understanding of electron localization and transport processes. Finally, two applications of amorphous solids are considered in some detail: optical fibers for use in communication networks and “superionic” glasses for possible use in solid‐state ba

ISSN:0570-0833    

DOI:10.1002/anie.198600311

出版商:Hüthig&Wepf Verlag    

年代:1986

数据来源: WILEY

摘要:

AbstractInfluenza is one of the great plagues which is not yet under control. The reason for this is the immense variability of the infecting agent, the influenza A viruses. These viruses behave like a chameleon: they adapt very rapidly to varying environments. New strains are “synthesized,” which can escape the immune response of the host, cross species barriers, and become highly pathogenic. We are beginning to understand the molecular background of this extraordinarily high variability. The genome of influenza A viruses consists of eight single‐stranded RNA segments, each of which constitutes a gene. The total base sequence of the eight RNA segments of several strains is known. If a suitable organism becomes doubly infected with two different influenza A strains, each of the RNA segments behaves like a chromosome. This means that by reassortment of the 16 RNA segments, 28−2 = 254 new combinations (= reassortants) are theoretically possible, each having different properties. Furthermore, mutations in the various RNA segments are relatively easily tolerated. Another great problem resides in the enormous reservoir of different influenza A viruses in the animal kingdom, especially in feral waterbirds. In these birds the avian influenza A viruses normally cause at most mild symptoms, and therefore these viruses are distributed over and between continents. Bearing this in mind it appears to be necessary to develop new ideas as to how to overcome this great

ISSN:0570-0833    

DOI:10.1002/anie.198600471

出版商:Hüthig&Wepf Verlag    

年代:1986

数据来源: WILEY

摘要:

AbstractThe last two decades have seen a dramatic development in the study of metal‐metal multiple bonds, particular successes being recorded in the field of organometallic chemistry. Syntheses designed to produce novel transition metal complexes with single, double, triple and quadruple metal‐metal bonds occupy a most important place in such research, as also do reactivity studies. A striving to establish general principles has provided much of the motivation for such work, but one less obvious goal—the commercial application of the catalytic properties of metal‐metal multiple bonding systems, in the medium and long term—should not be overlooked. All aspects of the investigations of metal‐metal multiple bonds also apply to a particular class of compound that has, however, enjoyed little lime‐light and thus deserves the present review: complexes with multiple bonds between transition metals and substituent‐free (“bare”) main group elements. Although based mostly on accidental discoveries, the few noteworthy examples are now beginning to unfold general concepts of synthesis that are capable of being extended and thus are deserving of exploitation in preparative chemistry. The availability of further structural patterns exhibiting multiple bonds between transition metals and ligand‐free main group elements might enable preparative organometallic chemistry to expand in a completely new direction (for instance by the stabilizing or activation of small molecules at the metal complex). This essay discusses the chemistry of complexes of bare carbon, nitrogen, and oxygen ligands (carbido‐, nitrido‐, and oxo‐complexes) and their relationships to higher homologues from both a synthetic and

ISSN:0570-0833    

DOI:10.1002/anie.198600561

出版商:Hüthig&Wepf Verlag    

年代:1986

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198600771

出版商:Hüthig&Wepf Verlag    

年代:1986

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198600781

出版商:Hüthig&Wepf Verlag    

年代:1986

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198600791

出版商:Hüthig&Wepf Verlag    

年代:1986

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198600801

出版商:Hüthig&Wepf Verlag    

年代:1986

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198600811

出版商:Hüthig&Wepf Verlag    

年代:1986

数据来源: WILEY