Metallacyclopentane intermediates

Dimethyltitanium complex 25, bearing an ethylene and methyl ligands, catalyzed the dimerization of ethylene via a metallacyclopentane intermediate 26 (Eq. 1) [30]. During the dimerization, no insertion of ethylene into the Ti-Me bond was observed due to the perpendicular orientation between methyl and ethylene ligands. This inertness could be attributed to the low oxidation state of 25, i.e. Ti(II). 

Further suggestions included a tetracarbene intermediate7 26 62 depicted as 2 or 3, and a nonconcerted pairwise exchange of alkylidenes via metallacyclopentane intermediates 63... 

The catalytic cycle proposed for the enantioselective ethyhnagnesation involves the chiral zirconocene-ethylene complex (121), formed upon reaction of the dichloride with two equivalents of EtMgCl. Coupling of the alkene substrate leads to the formation of the metallacyclopentane intermediate, which gives the zirconate (122) with a... 

The regiospecificity of the [2 + 2] cycloaddition is usually high, giving 1,3-dispirocyclobutanes, while in thermal reactions the 1,2-dispiro system is preferred (11 1). Steric constraints in the metallacyclopentane intermediate are obviously responsible for the observed regioselec-tivity. ... 

In a Second mechanism the same result is achieved via oxidative addition of tw o olefins to the catalytically active metal center forming a metallacyclopentane intermediate with subsequent fi-H elimination and reductive elimination of the resulting alkyl and hydride unit regenerating the initial metal species3,8. 

The [2 + 2] reaction also occurs for stained olefins 49 and 50, giving only the exo cycloadducts (Scheme 25). It was thought that the reaction proceeds via the exo metallacyclopentane intermediate Y arising from monodentate coordination to the exo face of the bicyclic olefins. The similarity in the electronic spectra of nickel complexes of these olefins to those of the complex of norbomene, which was known to coordinate on the exo face, supports this mechanism. 

In order to prepare very clean unsymmetrical zirconacyclopentadienes, the use of ethene is a prerequisite [14] (Eq. 2.4). An excess of ethene stabilizes the intermediates such as zirconacydopentane 5a and zirconacyclopentene 4. Such a transformation from a metallacyclopentane to a metallacyclopentene was first demonstrated by Erker in the case of the hafnium analogues [15]. 

Mechanistically, it is reasonable to regard metallacyclopentanes as intermediates in the formation of cyclobutane derivatives from two alkene substrates.5 It has been established that nickelacyclopentane not only acts as an intermediate in such a reaction but also as a catalyst.6... 

Intermediate metallacyclopentanes are also implicated in transition metal-catalyzed alkene cycloadditions to form cyclobutanes and the corresponding cycloreversions, e.g. dimerization of norbomadiene (73JA597) and rearrangements of cubane and other cyclo-butanoid hydrocarbons (78JA2573). 

Metallacyclopentanes are of considerable importance as intermediates in catalytic alkene dimerization (see Oligomerization Polymerization by Homogeneous Catalysis). 

Metallacycloalkanes are obtained by several general methods special methods are also known especially for metallacyclopentanes. When a donor atom like phosphorus or arsenic is present adjacent to the metal, the starting compounds must possess two functional centers. They can be generated in different ways. Without doubt metallacyclobutanes and -pentanes play a central role as important intermediates in numerous organometallic reactions. 

Metallacycloalkanes are proven key intermediates in metal-catalyzed cycloadditions and cycloreversions of alkenes. The relationship of some iron metallacyclopentane derivatives with bis(olefin) complexes has been investigated theoretically. Scheme 1 shows a general route from bisalkene... 

Similar to the nickel-catalyzed reactions, metallacycles of palladium are assumed to be intermediates of these conversions. They were isolable in some cases. Thus, 3,3-dimethylcyclopropene with -allyl( ) -cyclopentadienyl)palladium, on cyclizing oxidative addition, formed metallacyclopentanes 20 and metallacyclononanes 21, depending on the reaction conditions. Thermal decomplexation via reductive elimination gave cyclopropane systems and products thereof. ... 

Metallacycles have been suggested as intermediates in many transition-metal catalyzed reactions of olefins, acetylenes, and cyclopropanes. Metallacyclobutane complexes are invoked in olefin and cyclopropane isomerization schemes (Scheme as well as in olefin metathesis schemes. Metallacyclopentane, -pentene, and -pentadiene complexes can all be invoked in olefin and acetylene dimerization and polymerization (Scheme Many of these involve early transition metals and do not include phosphine ligands. 

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