Metallacycles reaction with

R=Me the connectivity is TiCp2(/U-S3)(/U-S4)NMe. This different behavior between the SyNH and SyNMe systems cannot be attributed to either recognizable steric or electronic reasons. By contrast, the reaction with RN(/t-S2)2NR (R=Me, n-Oct) in hexane at 20 °C gave not the expected seven-membered ring compound but a six-membered metallacycle, TiCpyl/f-SylNR [36] (Scheme 5). 

An example of the application of the metallacycle reaction to the synthesis of natural products is given by the synthesis of an intermediate for grandisol starting with isoprene (example 11, Table I). 

Carbon dioxide instead of aldehydes can be involved in Ni(0)-promoted reductive coupling reactions (Equations (76) and (77) Scheme 90).434,434a 434c A stoichiometric amount of Ni(COD)2/DBU reacts with C02 and dienes, alkynes, or allenes to afford a metallacycle intermediate. This metallacycle reacts with organozinc compounds or aldehydes in one-pot to give carboxylic acid derivatives. As shown in Scheme 90, double carboxylation occurs in the presence of dimethylzinc, where the stereochemical outcome is opposite to that of the reaction with diphenylzinc. 

The first metal-catalyzed [4 +2]-reaction of tethered dienes with 7r-systems was reported by Wender and Jenkins using alkynes initially as the two-carbon component.21 This study was based on the earlier observation by Wender and Ihle that in the [4 + 4]-cycloaddition of bis-dienes a competing side-reaction is the [4 + 2]-cycloaddition of the diene with a mono-ene portion of a second diene. The extension of this reaction to the synthesis of seven-membered rings by trapping the metallacycloheptadiene with CO, a formal [4 + 2 + l]-cycloaddition, has been shown in preliminary studies to be feasible. For example, tethered diene-yne 160 can be converted to cycloheptadienone 163 in an Rh(l)-catalyzed [4 + 2 + l]-reaction with CO, albeit the [4 + 2]- and [2 + 2 + l]-reaction products dominate (Equation (29)). The mechanistic scheme (Scheme 44) illustrates the possible metallacyclic intermediates leading to the observed products and provided the conceptual basis for the realization of three novel reaction types ([4 + 2], [2 + 2 + 1], and [4 + 2 + 1 ]).1... 

Coordination of Ni(0) to the alkyne gives a n complex, which can be written in its Ni(II) resonance form. Coordination and insertion of another alkyne forms the new C6-C7 bond and gives a nickelacyclopenta-diene. Maleimide may react with the metallacycle by coordination, insertion, and reductive elimination to give a cyclohexadiene. Alternatively, [4+2] cycloaddition to the metallacycle followed by retro [4+1] cycloaddtion to expel Ni(0) gives the same cyclohexadiene. The cyclohexadiene can undergo Diels-Alder reaction with another equivalent of maleimide to give the observed product. 

Initially alkynes were polymerised by trial and error with the use of Ziegler type recipes and the mechanism for these reactions may well be an insertion type mechanism. Undefined metathesis catalysts of ETM complexes were known to give poly-acetylene in their reaction with alkynes (acetylene) [45] and metallacycles were proposed as intermediates. Since the introduction of well-defined catalysts far better results have been obtained. The mechanism for this reaction is shown in Figure 16.24 [46], The conductive polymers obtained are soluble materials that can be treated and deposited as solutions on a surface. 

Metal ion discharge, chemical identity of adsorbed intermediates, 38 19-20 Metal ion-exchanged zeolites, 31 13 Metallacycle mechanism, 41 323-324 Metallathiabenzenes formation of, 42 421 reaction with hydrogen gas, 42 420 Metallic catalysts, 27 3 see also specific catalysts... 

Attempt to prepare Jt-complexes of triafulvenes and related methylene cyclopro-parenes285,427 428 directly by ligand exchange reaction with transition metal complexes resulted in metal insertion into the sigma bond, forming metallacyclic complexes. Thus reaction of the electron-poor triafulvene l,2-diphenyl-3-dicyanomethylenecyclopropene with (ethylene)bis(triphenylphosphine)platinum in refluxing benzene gave two crystalline products whose platinacyclobutene structure was confirmed by X-ray structure analysis (equation 364)429. 

In metal peroxide chemistry, the heterolytic or homolytic nature of catalytic oxidation seems to be strongly dependent on the heterolytic or homolytic dissociation mode of the peroxide intermediate, for which the triangular coordination mode of the peroxide moiety of the metal appears to be a key feature. Heterolytic oxidations require attainable coordination sites on the metal, involve strained metallacyclic reaction intermediates, and are highly selective. In contrast, homolytic oxidations involve bimolecular radical processes with no metal-substrate interactions and are less selective. In the important field of palladium oxidation chemistry, hydroperoxo... 

Only one isolated example of formation of thioketene and thioaldehyde dimolybdenum complexes has been reported507, but in contrast, zirconocene thioacetaldehyde and thiobenzaldehyde complexes, introduced by Buchwald, have been the subject of several studies508, including their application in the synthesis of novel antimony thiametallacy-cles 148509 (equation 156). Compound 146 was generated in situ and, after reaction with alkynes, gave rise to metallacycles 147 which, by treatment with SbCR, afforded 148. 

Treating the sterically encumbered zirconocene ZrCFfn-CslliiBi F with "BuLi in the presence of ethene also provides a structurally characterised zirconocyclo-pentane Zr(C4H8)(r -C5H3Bu2)2 18. The reversibility of metallacycle formation was demonstrated by its reaction with diphenylacetylene to provide the zirconacyclo-pentene 19 incorporating the alkyne and one equivalent of ethene 22... 

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