Titanium complexes cyclopentadienyl

There has been significant interest in substituting cobalt for other transition metals giving rise to what are known as Pauson-Khand type reactions.60 In 1996, Buchwald and co-workers showed that a titanium -cyclopentadienyl complex (45) could be used sub-substoichiometrically in the reaction (46->47 - Scheme 18).61,62... 

A new interesting development is the discovery and tailoring by Kaminsky of Z.N. type systems composed of titanium cyclopentadienyl complexes, and aluminoxanes obtained by controlled hydrolysis of aluminum alkyls (see [8] and papers of the corresponding symposium held at Akron in June 1986). These soluble and extremely active systems (for ethylene) are even claimed to exhibit a good stereoselectivity in propylene polymerization under homogeneous conditions (probably due to the hindered environment provided by the alumoxane structure). 

The dissociation of coordinatively sufficient organometallic complexes in solution. For instance, for the system based on cyclopentadienyl complexes of titanium the active centers of catalytic polymerization (C Hj) -TiR]+ are caused by the following process (178, 179) ... 

While the majority of group 4B metal carbonyl complexes contain 7r-bonded hydrocarbon ligands, most notably 17-cyclopentadienyl, recent studies by Wreford and co-workers have led to the identification and isolation of three novel phosphine-stabilized titanium carbonyl complexes (12,13). 

Two other functionally substituted 17-cyclopentadienyl titanium dicarbonyl complexes prepared by Rausch and co-workers include the vinyl Cp compound (i7-C5H4CH=CH2)CpTi(CO)2 (81) and the carbomethoxy Cp compound (rj-C5H4C02Me)CpTi(C0)2 (82). Both were synthesized via the aluminum-induced reductive carbonylation of the corresponding dichloride derivatives. 

Covalently bonded chiral auxiliaries readily induce high stereoselectivity for propionate enolates, while the case of acetate enolates has proved to be difficult. Alkylation of carbonyl compound with a novel cyclopentadienyl titanium carbohydrate complex has been found to give high stereoselectivity,44 and a variety of ft-hydroxyl carboxylic acids are accessible with 90-95% optical yields. This compound was also tested in enantioselective aldol reactions. Transmetalation of the relatively stable lithium enolate of t-butyl acetate with chloro(cyclopentadienyl)-bis(l,2 5,6-di-<9-isopropylidene-a-D-glucofuranose-3-0-yl)titanate provided the titanium enolate 66. Reaction of 66 with aldehydes gave -hydroxy esters in high ee (Scheme 3-23). 

Titanium(IV) complexes, 25 97-101 Titanium(IV) compounds six-coordinated, 25 90 Titanium(IV) cyclopentadienyls, 25 110, 111—112t... 

Figures 5-41 and 5-42 compare CpTiCp (centroid) bond angles in titanium cyclopentadienyl dichloride complexes from PM3 and BP/ 6-31G calculations, respectively, with experimental values for these and other compounds dealt with in this section from X-ray crystallography. Due to practical limitations, the data used for comparison with the density functional calculations are a subset of that used in comparison with PM3. Both models perform well in separating those systems where the cyclopentadienyl rings are spread far apart from those where they are closer together.

Titanium cyclopentadienyl derivatives form an extensive series of dinitrogen complexes. However, when the titanium-containing precursors are bound to polymers there is no reactivity with dinitrogen comparable to that observed for the analogous derivatives in solution, and fixation is, at best, marginal (202, 220). 

Most titanium(IV) alkyls tend to be reduced by aluminum alkyls in a complicated sequence of reactions accompanied by evolution of alkane and alkene. The catalytic activity of the bis(cyclopentadienyl)titanium-aluminum complexes is associated with the titanium alkyl. Hence, it is very interesting to investigate the mechanism of any reductive reaction. In order to study side reactions in the absence of polymerization, highly alkylated systems completely free of halogen are preferred. Moreover, reduction takes place much faster, the higher the alkyl-group content of the added aluminum alkyl. 

Two communications on propene polymerization by non-metallocene catalysts that include DFT/MM calculations have been recently published [60, 61]. They deal with group 4 bidentate non-cyclopentadienyl complexes. In the first communication [60], the topic addressed is the fact that a C2-symmetric precatalyst of titanium leads to a syndiotactic polymer, contrary to observations of metallocene catalysts. The chirality at the metal center is found to play a key role in the stereocontrol of the process. The second communication [61] addresses the fact that a C2-symmetric precatalyst of zirconium very similar to the previous one produces an isotactic polymer, finds out that it is due to a complicated concourse of synergic steric and electronic effects, and emphasizes the key role that serendipity still plays in the design of new catalysts. 

Table 3. Preparation of tt-Cyclopentadienyl Complexes of Titanium, Zirconium, and Hafnium ...

The formation and properties of many other complexes are reported in the literature. These include a cyclopentadienyl-titanium(lll) complex with maleic hydrazide (81IC2084), a nickel(ll) and cobalt (II) complex of... 

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