Metallacyclopentanes

Metallacyclopentanes, 3,4-dimethylene-synthesis, 1, 669 Metallacyclopentan-2-ones synthesis, 1, 669 Metallacyclopentenes synthesis, 1, 670 Metallafluorenes synthesis, 1, 671 Metallaindanes synthesis, 1, 670 Metallaindenes synthesis, 1, 670, 671 Metallaxanthenes synthesis, 1, 671 Metalloporphyrins anions, 4, 398 demetallation, 4, 389... 

Reactions of cyclic tetrasulfides containing a heavier group 14 element, Tbt(Ar)MS4 (M=Ge, Ar=Mes or Tip M=Sn, Ar=Tip), with diphenyl diazomethane gives l,2,3,5-tetrathia-4-metallacyclohexanes, l,2,4,5-tetrathia-3-metallacyclohexanes, and l,2,4-trithia-3-metallacyclopentanes (Scheme 54) [52, 118-120],... 

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). 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

Since 8-elimination reactions are often rapid and reversible, it is surprising that no examples of metallacyclobutane to metallacyclopentane ring expansion reactions according to equation (6) have been found. 

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]. 

An important aspect of the carbomagnesation of six-membered and larger heterocycles is the exclusive intermediacy of metallacyclopentanes, in which the C—Zr bond is formed a to the heterocycle C—X bond (Scheme 6.2). Whether the regioselectivity in the zircona-... 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

Species 111 formed by trans-addition of e.g. ClCHj—Cl to Pt(II)d metallacyclopentanes show the effect of hgand constraint on the selectivity of reductive elimination. Complexes in which L is a monodentate phosphine decompose preferentially... 

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... 

Cyclobutanes were also obtained from metallacyclopentanes by simple thermal decomposition, by treatment with other nucleophiles (rather than alkenes) such as phosphanes, nucleophilic solvents or by reaction with oxygen. Byproducts of these reactions are the respective alkenes or linear dimers. The extent of the formation of byproducts depends on the temperature of the decomposition, on the solvent and the nucleophile and on the coordination number of the metal. 

Table 3. Examples of Cyclobutanes from Other Metallacyclopentanes...

Mononuclear metallacyclopentanes were produced from metallacyclopentanes containing two metals in the ring, which were obtained by dialkylation of di- -carbonylbis(tjs-cyclopentadi-enyl)cobaltate and diiodopropane. Reaction of this complex with iodine gave cyclobutanone (14) in 50% yield.137... 

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

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). 

Although much effort has been devoted to decarbonylation of cyclopropylcarbonyl metal complexes (vide supra), only (cyclopentadienyl)dicarbonyliron (Fp) derivatives have been successfully decarbonylated either photochemically22,24 or using Wilkinson s rhodium catalyst [(PPh3)2RhCl]2 (equation 35). Further decarbonylation by irradiation led to metallacyclopentane formation, whereas thermal decomposition resulted in the formation of the corresponding Cp(CO)Fe(allyl) complexes. 

Thus, solution and solid-state structural studies of such fluoroolefin compounds have helped to formulate and refine theories of metal-carbon bonding. Studies of the reactivity of metal-fluoroolefin compounds have also provided useful models and predictions for hydrocarbon systems. For example, the oxidative cyclization of fluoroolefins within the coordination sphere of a metal to give metallacyclopentane compounds was discovered many years before the importance of the corresponding reaction of hydrocarbon olefins was realized (3). 

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