Carbenes halogen complexes

Abstract The manuscript describes the methods that are most often used in the preparation of N-heterocyclic carbene (NHC) complexes. These methods include (1) insertion of a metal into the C = C bond of bis(imidazolidin-2-ylidene) olefins (2) use of carbene adducts or protected forms of free NHC carbenes (3) use of preformed, isolated free carbenes (4) deprotonation of an azolium salt with a base (5) transmetallation from an Ag-NHC complex prepared from direct reaction of an imidazolium precursor and Ag20 and (6) oxidative addition via activation of the C2 - X (X = Me, halogen, H) of an imidazolium cation. 

The treatment of Sc-23 or Sc-24 with CgFsI yielded a reversed addition with formation of Me3SiCH2l and metal-aryl complexes. Unfortunately, the complexes mentioned above could not cleave even the weakest earbon-halogen bonds by addition across the M-Cga bene bond. However, the lithium carbene ate complex 64 led to the alkylation of the NHC by Ph3CClwith subsequent liberation of Ph3CCH2SiMe3 and formation of bimetallic Sc complexes 23 (Scheme 6.12). ... 

Addition of halogens and pseudohalogens to the cyclopropylthiocarbene chromium complexes 122 affords the 1,4-dihalo-1-phenylthio-l-alkenes 123 stereoselectively [65]. Electrophilic halogen is likely to activate the carbene complexes, followed by the homo-Michael addition of halide anion. (Scheme 44)... 

For alcohol additions, mono- and dicarbene complexes have been synthesized, for palladium, platinum (487-489), and gold (475, 490- 493), from methanol, ethanol, or ethanethiol (494) additions to the isocyanide precursor. The stability of the gold(I) carbenes can be gauged from reactions with halogens which gave the oxidative addition product without loss of the carbene ligand (492). 

C-halogen bond activation via nickel carbenes, 8, 24 with Ni(0) complexes, 8, 47 Aryl glycines, from Grignard reagents, 9, 67-68 (j-Aryl groups... 

The second example shows that such a reaction need not always lead to an uncharged system. If, instead of thiocarbene complexes, amino-carbene complexes are treated with hydrogen halides, products of saltlike character are isolable (66). One finds the halogen at the metal and the hydrogen at the removed carbene ligand, and one obtains iminium halo-genopentacarbonylmetalates ... 

Of subsequent interest to us was the question of whether a third kind of ligand in the starting carbene complex could influence the orientation of the halogen in the eventual carbyne complex. Therefore, we first treated cis-tetracarbonyl[methoxy(methyl) carbene trimethylphosphine-, -arsine-,... 

The photoreactions characteristic for the halogenated phenols and anilines have shown to occur in most members of the series of chlorophenoxy and halophenylurea derivative pesticides, and in some other related biocides as well. Photohydrolysis is a frequently encountered process, but carbene formation has been demonstrated in several cases too. The phototransformation mechanisms of these molecules, however, are frequently complex, owing to the greater complexity of the molecules in question, and homolytic cleavage steps have been found to contribute in several cases. 

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