摘要:

AbstractThe rather interesting question why the „anionic”︁ carbenoids1a, nitrenoids1band oxenoids1creact so easily with nucleophiles is answered by experimental investigations of their structure and by means of quantumchemical calculations. It turns out that the bond to the leaving group (C(N,O)‐X is weakened (elongated), and that the energy of the σ*C(N,O)‐Xorbital is lowered. This allows for a more facile substitution of X by a nucleophile. The substitution of X−is further supported by the met

ISSN:1022-1360    

DOI:10.1002/masy.19981340103

出版商:WILEY‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe strong inhibiting effect of trace quantities of selected compounds on the reaction between ultra fine particles of magnesium and alkyl halides in THF strongly suggests that the generally accepted linear mechanism should be replaced by a chain mechanism. The reactivity of batteries of free radical clocks centered on an aliphatic skeleton displays unexpected leaving group effects. Such effects could be rationalized according to a variety of mechanistic schemes. The main ones are: 1) coexistence of polar and electron transfer routes, 2) participation of solvated electrons in the mechanism, 3) gradient of reactivity starting from the magnesium surface to the bulk solution. A series of experiments aiming at a choice between these possibilities is described.

ISSN:1022-1360    

DOI:10.1002/masy.19981340104

出版商:WILEY‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractElectron transfer from an alkali metal to a suitable complexant for the cation can yield crystalline ionic solids that contain the complexed cation and either an alkali metal anion (alkalide) or a trapped electron (electride). The nature and properties of electrides are emphasized in this paper. When the organic complexant contains aromatic groups, the anionic species is an aromatic radical anion. Preliminary work on the addition of alkali metals to LOGEAs (large organic globular electron acceptors) is described and strategies for the synthesis of mixed electride‐anion compounds are considere

ISSN:1022-1360    

DOI:10.1002/masy.19981340105

出版商:WILEY‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractPhotozymes are novel water‐soluble polymers made by the copolymerization of mixtures of hydrophobic and hydrophilic monomers, some of which contain chromophores capable of absorbing light and transmitting the excitation energy to selected traps by means of the antenna effect. The interactions between these groups and water force the polymer to adopt a hypercoiled conformation with hydrophobic pockets similar to those in the catalytic sites of natural enzymes. Hydrophobic organic compounds in the water solution will seek out and localize themselves in these regions, where they are subjected to electronic energy transfer from the light‐excited antenna chromophores. The chemical reactions which occur are often different and more specific than in the case of photoreactions in common organic solvents. In a number of cases the reactions appear to proceed by an electron transfer mechanism. This paper summarizes recent results on the dechlorination of chlorinated aromatic and aliphatic compounds, and laser studies of multiphoton processes in aromatic compounds such as 9‐(acetoxymethyl)phenanthrene

ISSN:1022-1360    

DOI:10.1002/masy.19981340106

出版商:WILEY‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractSingle electron transfer SET induced usually by organometallic complexes of transition metals or alkali metal complexes of arenes e.g. naphthalene is common in organic reactions. In this paper unusual two‐electron transfer to suitable acceptor molecule mediated by recently discovered alkali metal supramolecular complexes in aprotic etheral solvents is presented. The capability of these solutions of transferring two electrons to a suitable organic substrate and formation of corresponding carbanions opens new door in synthetic chemistry. The carbanions are interesting synthons in the many organic reactions and catalysts in anionic polymerization processes. Reactions of some vinyl compounds, alkyl and silyl halides, as well as ring‐opening polymerization of β‐lactones and oxiranes are dis

ISSN:1022-1360    

DOI:10.1002/masy.19981340107

出版商:WILEY‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractWith C1‐, C2‐ or Cs‐symmetric metallocenes, different intermediates and types of copolymers can be obtained from randomly distributed to alternating structures. Substitution of the Cp‐ring in [Me2C‐(tert‐Bu Cp)(Flu)]ZrCl2yields ethene/norbornene copolymers with an alternating structure, because the rigid norbornene can only be inserted from the open side of the metallocene. By variation of the polymerization parameters, copolymers with glass transition temperatures above 180°C and molecular weights>100 000 are synthesized. By supporting different metallocenes on a silica/methylaluminoxane (MAO) carrier the deactivation reaction under electron and hydrogen transfer can be suppressed. This is proved for different Al/Zr ratios when trimethylaluminum (TMA) is used as cocatalyst by the lack of methane evolution by metallocenes and by near independence of the polymerization activity on the prereaction time, after reaching maximum activity. Aluminumalkyls and MAO leach Cp2ZrCl2from the carrier, the leached metallocene is only active in polymerization b

ISSN:1022-1360    

DOI:10.1002/masy.19981340108

出版商:WILEY‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractUsing kinetic, steric and thermodynamic results it is argued that the rate‐determining step in some aliphatic, nucleophilic substitutions is the transfer of an electro

ISSN:1022-1360    

DOI:10.1002/masy.19981340109

出版商:WILEY‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractCertain hydrocarbons, preferrable strained ones, can be cleaved via electron transfer processes whereby lithium dust or lithium 4,4′‐di‐tert‐butylbiphenylide (LiDBB) have been used as electron transfer reagents. For the scission of a C‐C σ‐bond the presence of a π‐system in the neighborhood (α or β) is an abso

ISSN:1022-1360    

DOI:10.1002/masy.19981340110

出版商:WILEY‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractPart I: Reactions of 3‐substituted 1‐azabicyclo[1.1.0]butanes (1aand1b) with dichlorocarbene proceed in each case via nitrogen ylide intermediates with concomitant ring opening to afford the corresponding dihaloimines (2aand2b, respectively).Part II: Vinylidènecarbene‐cycloalkyne equilibria have been studied in three polycarbocyclic systems, i.e.,10a‐10b,20a‐20b, and27a‐27b. In all three systems, only the vinylidenecarbene intermediate can be trappedin situby cyclohexene. The results of site‐specific13C labeling studies that employ9‐C(8)‐13CBr2(enriched with 4 atom‐%13C),19‐C(4)‐13CBr2(enriched with 5 atom‐%13C), and23‐C(4)‐13CBr2(enriched with 10 atom‐%13C) also support this conclusion and suggest that cycloalkyne intermediates, if formed in these systems, must rearrange very rapidly to the corresponding (thermodynamically favored) vinylidenecarbenes.Part III: Base‐promoted reactions ofN‐tosyl‐ andN‐benzhydrylazetidin‐3‐ones (i.e.,30aand30b, respectively) with diethyl diazomethylphosphonate (DAMP), when performed in the presence of cyclohexene, in each case afford exclusively the product that results viain situtra

ISSN:1022-1360    

DOI:10.1002/masy.19981340111

出版商:WILEY‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe participation of electron transfer (ET) processes in atom transfer radical polymerization (ATRP) is discussed. Inner sphere electron transfer process (atom transfer) is the inherent part of ATRP. It includes the homolytic cleavage of C‐halogen bond in dormant macromolecular alkyl halides species by the Cu(I) activator with the rate constant kact≈︁100±1M−1s−1. This process is reversible and the deactivation of growing radicals by X‐Cu(II) species is very fast with the rate constant of deactivation kact≈︁107±1M−1s−1. However, in addition to the atom transfer process (inner sphere ET), outer sphere ET may also occur. Growing radicals can either be oxidized by Cu(II) species to carbocations or reduced by Cu(I) species to carbanions. These reactions were observed in the polymerization of p‐methoxystyrene and initiation with 2‐bromomalonates, respectively. In order to avoid these side reactions the catalyst structure should be fine tuned for the particular monomer and the range of targeted molecular weights. In addition, some side reactions can be avoided by slow addition of initiator (e.g. for benzhydryl halides), catalyst (for malonates and CCl4as initiators) and by reducing p

ISSN:1022-1360    

DOI:10.1002/masy.19981340112

出版商:WILEY‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY