Ferrocene-titanium complexes

In other examples, compounds in which a metal atom is already coordinated in a molecule can be used as a comonomer in an addition polymerization. Two examples involve the ferrocenes discussed in Chapter 6. The vinyl ferrocene molecule is shown in 7.14, and a similar vinyl manganese complex in 7.15.30 An alternative approach involves condensation polymerization. For example, if the R group in the ferrocene unit shown in 7.16 contains a hydroxyl group, it can be copolymerized with a diacid chloride. If it is an acid chloride, it can be copolymerized with a diamine. (This type of polymer is called a heteroannular chain if only one of the rings in the repeat unit is in the backbone, the polymer is called homoannular.)7 Similarly, the titanium complex shown in 7.17 is copolymerized with diacids or diols.30 Numerous other examples involving ferrocenes are discussed in Chapter 6. 

Redox active ancillary ligands were first used to control lactide polymerizations using titanium complexes [51]. Very recently, Diaconescu and co-workers have reported derivatives of salen complexes incorporating a ferrocene unit which can be oxidized and reduced to switch the polymerization OFF and ON , respectively (Fig. 11) [52, 53],... 

This method has been extended to the reaction of PhCsCZnBr with aldehydes in the presence of stoichiometric amounts of the lithium alcoholate of 1.14 (R = Me). Divinylzincs (RCH=CH)2Zn react under similar conditions, or also in the presence of other lithium alcoholates [650]. Under precise experimental conditions, PhMgBr adds to aliphatic or aromatic aldehydes in the presence of zinc salts and 1.14 (R = n-Bu). The enantiomeric excesses in these additions are higher than 75%. Diaiylzincs react with aldehydes in the presence of aminoalcohols bearing a ferrocene skeleton 2.47 with a very high enantioselectivity [651, 652], Schiff bases have also been used as catalysts in such reactions [367], as have some titanium complexes (see below) [559,653,654,655],... 

A variable-temperature NMR spectroscopic study of the titanium(IV) complex 43 also indicated free rotation of the five-membered rings, but, as in the ferrocene derivative 38 allowed the determination of the activation barrier for the phenyl ring rotation (AG (-90 °C) = 9.8 0.5 kcal mol1). 

Several preparations of silylated cyclopentadienylmetal complexes involve the formation of a triorganosilylcyclopentadienyl anion (by treatment of a silylated cyclopentadiene with an alkali metal in tetrahy-drofuran or by metalation with n-butyllithium), followed by reaction with metal chlorides. This type of reaction has been used for the synthesis of silylated ferrocenes (41, 43, 58, 83, 84, 103, 107, 116, 135, 142, 171, 172), cobaltocenes (135), nickelocene (135), titanium cyclopen-tadienyls (46, 145), and cyclopentadienylmanganese tricarbonyl (30) [Eqs. (19) and (20)]. It is remarkable that Si—C5H6 bonds are not... 

A number of additional cyclizations involving alkynes have been reported. For instance, it has been shown that indoles may also be accessed from 2-bromo- or 2-chloroanilines, as illustrated by the regioselective preparation of the carbinol 373 in the presence of the ferrocene 374 (Equation 104) <20040L4129>, whereas a one-pot sequence featuring titanium catalyzed hydroamination of 2-chloroanilines with acetylenes, followed by intramolecular Heck cyclization in the presence of an imidazol-2-ylidene palladium complex, has also been reported <2004CC2824>. A set of aryl-2-indolyl carbinols have been prepared in high enantiomeric purity by palladium-catalyzed annulation of... 

Long-range electron transfer is postulated to occur from ferrocene to tris(bipyridine)iron(lll) constructed within the pores of a NaY zeoUte. The iron bipyridine complex is too large to move throughout the faujasite pores to the surface, thus requiring the long-range transfer. The asyimnetric catalyst, titanium tartrate, has been prepared inside NaY and used as an immobilized catalyst for the epoxidation of cinnamyl alcohol. ... 

Some cyclopentadienyltitanium complexes react with FeClj or iron acetylacetonate with transfer of the cyclopentadienyl ligand from titanium to iron. The yield of ferrocene also depends on the other ligands on titanium. The yield increases with decreasing number of electron-attractive halogen atoms in the molecule (see Table IV). 

All the 3d elements have been obtained in neutral molecules, and, with the exceptions of the manganese and titanium compounds discussed below, they appear to have the same structure and essentially the same bonding as in ferrocene however, only ferrocene is air-stable, the others being sensitive to destruction or oxidation by air the stability order is Ni > Co > V > Cr. It is still uncertain whether (/z5-C5H5)2Ti actually exists. Apparently, there are at least two substances, one metastable relative to the other, of stoichiometry C10H10Ti, as well as various derivatives such as H2 and N2 complexes. The chemistry is complex and knowledge of structures virtually non-existent as yet9 (see also Section 25-A-4). 

The titanium (or zirconium) present in these complexes is in an oxidation state of +4 and therefore has 0 valence electrons. Each Cp contributes 6e each, making a total of 12e. Each Cf contributes 2e each. Thus, Cp2ZrCl2 and Cp2TiCl2 are 16e compounds and therefore are more reactive than ferrocene. Further the bent structure of metallocenes allows additional ligands to approach the metal ion. 

Enantioselective TMS cyanohydrin formation with aldehydes can be achieved using a variety of chirally-modified titanium catalytic systems chiral modifiers include Schiff bases, including salen ligands and sulphoximines. A chirally-modified yttrium complex has been employed for the same purpose (equation 8) here, the chiral modifier was a ferrocene-derived 1,3-diketone. This general area has been reviewed recently . 

The name of metallocenes is derived from ferrocene, which is a complex compound of iron with dicyclopentadiene. The metallocene complexes are new types of Ziegler-Natta catalysts. The general formula of these catalysts is identified as Cp MeCl, where Cp denotes the cyclopentadiene ring or its derivatives. Me is metal atom, which the most often is zirconium (Zr), hafhium (Hf), titanium (Ti), scandium (Sc), thorium (Th) or another rare earth element. 

Metallocene chemistry is presently a very promising field of inorganic chemistry. E.g., metallocene complexes of titanium and zirconium play an important role in the production of polymers. The role of ferrocene derivatives as effective nucleophilic catalysts in kinetic resolutions of many useful... 

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