Cyclopentadienyl-type ruthenium

From the mechanistic point of view, the observed competitive reactions can be explained by considering two different pathways (Scheme 114). The intermediacy of ruthenacyclopentadiene 453 or biscarbenoid 452, formed from the reaction of a diyne and a ruthenium(ll) complex, is postulated in the proposed mechanism. Cyclopropanation of the alkene starts with the formation of ruthenacyclobutane 456, which leads to the generation of the vinylcarbene 457. Then, the second cyclopropanation occurs to afford the biscyclopropyl product 458. Insertion of the alkene 459 into the ruthenacyclopentadiene 453 affords the ruthenacycloheptadiene 454. The subsequent reductive elimination gives the cyclotrimerization product 455. The selectivity toward the bis-cyclopropyl product 458 is improved with an increasing order of haptotropic flexibility of the cyclopentadienyl-type ligand. 

While ATRP of methyl acrylate was reported only for the copper catalyst system [290-292] methyl meth(acrylate) was also polymerized with copper [290,293 295], ruthenium/aluminum alkoxide [296,297], iron [298,299] and nickel [300 303] eatalyst systems (Table 9). Thereby, it must be noted that in principle, the ruthenium-based system proposed by Sawamoto et al. requires the addition of Lewis acids, e.g., Al(0- -Pr)3 [297]. Recent investigations showed, that the half-metallocene -type ruthenium(II) chloride Ru(Ind)Cl(PPh3)2 (Ind = indenyl) led to a fast and well controlled polymerization even without the addition of Al(0- -Pr)3, whereas in case of a polymerization with Ru(Cp)Cl(PPh3)2 (Cp = cyclopentadienyl), the addition of Al(0-z-Pr)3 is necessary. The activity of Ru(II)-catalysts decreases in the order Ru(Ind)Cl(PPh3)2 > RuCl2(PPh3)2 > Ru(Cp)Cl(PPh3)2 [304]. 

The successful acylation of ferrocene set off a vigorous research effort that has resulted in the establishment of the present field of metallocene chemistry, ir- Cyclopentadienyl compounds of ruthenium, osmium, manganese, vanadium, and chromium also exhibit certain aromatic-type reactions in varying degrees. These metallocenes thus represent a new class of heterocyclic compounds in which transition metals, akin to nitrogen, oxygen, and sulfur in classical heterocycles, not only are an integral part of the structure but actually participate directly in many reactions. 

The type of ruthenium catalysts, as shown in Figure 1.8, however with cyclopentadienyl ligands instead of cyclohexyl ligands,... 

Ferrocene is only one of a large number of compounds of transition metals with the cyclopentadienyl anion. Other metals that form sandwich-type structures similar to ferrocene include nickel, titanium, cobalt, ruthenium, zirconium, and osmium. The stability of metallocenes varies greatly with the metal and its oxidation state ferrocene, ruthenocene, and osmocene are particularly stable because in each the metal achieves the electronic configuration of an inert gas. Almost the ultimate in resistance to oxidative attack is reached in (C5H5)2Co , cobalticinium ion, which can be recovered from boiling aqua regia (a mixture of concentrated nitric and hydrochloric acids named for its ability to dissolve platinum and gold). In cobalticinium ion, the metal has the 18 outer-shell electrons characteristic of krypton. 

Compound 127 is a useful starting material in preparing a variety of arene cyclopentadienyl ruthenium complexes of type 125 by... 

This type of sandwich complex, first reported with iron as the central metal, has now become widespread with ruthenium as well. Early routes to ruthenium complexes were modeled on iron chemistry, and used the AICI3-catalyzed exchange of a cyclopentadienyl ligand for an arene in ruthenocene, or reaction of CpRu(CO)2Cl with AlCb/arene. These methods are less successful with ruthenium than with iron, however, owing to the greater stability of ruthenocene. A mixture of arene, pentamethylcyclopentadiene, and RuCls in the Zn reduction method gives good yields of the mixed-sandwich cations. A... 

To date, two types of metallocene complexes have been used in the polymerization of olefins at CNT surfaces by PFT a ruthenium alkylidene complex anchored by two routes onto SWCNTs via a pyrene derivative spacer [94], and zirconium complexes of general formula [(R)2ZrCl2], R being a derivative of cyclopentadienyl or indenyl, anchored onto CNTs modified with MAO [97]. 

Although benezene and benzene type complexes are much less common than those with cyclopentadienyl ligands, a number of such complexes are known for chromium and ruthenium. The bis benzene complex (// -C6H6)2Cr is photochemi-... 

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