Metallacyclic systems

The above mentioned conversion of 3,3-dimethylcyclopropene proceeded via 5-(2,2 -bipyridyl)-3,3,7,7-tetramethyl-tra 5-5-nickelatricyclo[4.1.0.0 ]heptane (9) when 2,2 -bipy-ridyl(cycloocta-l,5-diene)nickel was used. Upon thermal or oxidative decomplexation, the metallacyclic system 9 gave 3,3,6,6-tetramethyl-tra j-tricyclo[3.1.0.0 ]hexane. ... 

Indeed, methyl rw-6,6-dimethyl-2-(triphenylphosphanyl)-2-nickelabicyclo[3.1.0]hexane-3-carboxylate (13) [and the corresponding ethenebis(dimethylphosphane) complex] was isolated in 65% yield from the conversion of (/7 -methylacrylate)bis(triphenylphosphane)nickel and 3,3-dimethylcyclopropene. The former complex catalyzes the cotrimerization of dimethylcyclo-propene with methyl acrylate with almost identical activity as earlier observed with an in situ catalyst obtained by mixing bis(cycloocta-l,5-diene)nickel and triphenylphosphane (1 1). Thus, this metallacyclic system can be regarded as an intermediate in the co-cyclotrimerization reaction. ... 

As A ° of the corresponding Nd(III) system differs likewise notably from the A ° values of two adducts, [(MeaSi)2N]aNd L (see Table 25), weaker agostic interactions appear to play a role in [(MeaSi)2N]aLn systems, too. In case of the complex [(MeaSi)2N]3UH, agostic U C interactions are most likely to facilitate its well-studied dehydrogenation into the metallacyclic system [ (Me3Si)2N]3UCH2SiMc2NSiMe3, schematically [136] ... 

Treatment of the bis-alkynide Zr(CCPh)Cp 2 with Zr(r 4-C4Hg)Cp 2 gives a compound with a delocalised metallacyclic system with, to a first approximation, two a,ir-alkynyl units.42 The single electron reduction of Ti(CCPh)Cp2 is reversible and gives an anion which although stable at -30°C, loses Cp at 25°C.43... 

Metallacycles containing metal-heteroatom bond in their ring system 98YGK171. 

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). 

A metallacylic complex has been isolated from the reaction of PhN02 with CO using the Pd11-o-phenanthroline system known to produce PhNCO or carbamates catalytically. Reactions of this metallacycle have been studied and its role in the catalytic reaction discussed.574... 

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... 

R1 = ph R2 = Me) with the double hydrocarbonyl-bridged Cp2Zr(p-C=CPh)(p-CPh—CMeJAlMe 2 complex exhibiting a planar-tetracoordinate carbon atom within the central metallacyclic ring system. Adapted by the authors. 

Second Test System Thermomorphic Metallacycle Catalysts. 76... 

This catalyst system was the first to utilize both terminal alkynes and olefins in the intramolecular reaction. Although a mechanistic rationale for the observed stereoselectivity was not offered, the formation of the single stereoisomer 26 may be rationalized through the diastereotopic binding of the rhodium complex to the diene moiety (Scheme 12.3). This facial selective binding of the initial ene-diene would then lead to the formation the metallacycle III, which ultimately isomerizes and reductively eliminates to afford the product [14]. 

Displacement of the chelate carbonyl from palladium by ethene has never been observed in model studies, which accounts for the virtual absence of double ethene insertions in actual copolymerisation reactions. Indeed, (5-chelate opening is actually brought about by CO to generate a six-membered metallacycle (y-chelate), while p-chelates of catalytically active systems generally react with CO to yield carbonyl acyl complexes, even at very low temperature. For the systems investigated by Bianchini [5e, f], the activation barriers for the conversion of the P-chelates... 

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