Cyclopentadienyl titanium derivatives

Kagan et al. were the first to report the corresponding enantioselective catalytic hydrogenation using chiral metallocene derivatives [94, 95]. By using menthyl- and neomenthyl-substituted cyclopentadienyl titanium derivatives in the presence of activators (Scheme 6.5) [96], these authors observed low ee-values (7-14.9%) for the catalytic hydrogenation of 2-phenyl-l-butene into 2-phenylbutane. In contrast, no enantiomeric excess was obtained with the corresponding zirconocene derivatives. 

Data on the bis(cyclopentadienyl)titanium derivatives are also listed in Table IV. 

Another ionic cyclopentadienyl titanium derivative tested for antitumor properties, the five-coordinate mono(cyclopentadienyl)titanium(IV) bis(dithiolene) chelate XLVIII ), belongs to the type M [(C5Hs)TiX4]- in which the cyclopentadienyl titanium moiety represents the anionic part of the complex salt. 

After treatment with the indenyl and tetrahydroindenyl titanium complexes XXXV-XXXVII and the mono(cyclopentadienyl) titanium derivatives XXXVIII-XL phenomena comparable with those observed after application of substituted titanocene derivatives occurred. When only one cyclopentadienyl ring was exchanged by the equally Ti -bound tetrahydroindenyl ligand (XXXVI), the optimum cure rate amounted to 73% whereas, in the case of the bis(indenyl) and bis(tetrahydro-... 

In unsymmetrical 2-alkenylbis(cyclopentadienyl)titanium(III) derivatives, the -attached ally residue is also attacked by aldehydes at the more highly substituted position (Section 1.3.3.8.2.I.2.). 

The breakthrough was achieved with the creation of dialkoxy(tj5-cyclopentadienyl)titanium(IV) chlorides derived from sterically hindered chiral alcohols11 35,35a 36. Allyl derivatives 8 and 10, prepared in situ from complexes 7 and 9, have sufficient stability and reactivity. 

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. 

Dialkylbis(cyclopentadienyl)silanes can be used in a similar manner, as in the following preparation of a titanium derivative (R = Me) (96) ... 

The polymerization of a-olefins, accelerated by homogeneous Ziegler-Natta catalysts derived from bis(cyclopentadienyl)titanium(IV) or analogous zirconium compounds and aluminum alkyls, occurs simultaneously with... 

The moisture- and air-sensitive dark-green titanium derivative (dec. > 180°) was also obtained in 54% yield when bis[cyclopentadienyl]diphenyltitanium in heptane and elemental tellurium were refluxed for 12 days. The compound was purified by recrystallization from dry toluene3. The activation energy of the ring inversion of the metallocycle was determined by H-NMR spectroscopy to be 51kJ mol-1. 

The bis(rj-cyclopentadienyl)titanium(III) aryl complexes form interesting dinitrogen derivatives (Section III,B) the 1-methyl, 17-allyl complex (Tj-C5H5)2Ti(i73-C4H7) is the precursor to a particularly effective homogeneous olefin hydrogenation catalyst (Section VI). Attempts to prepare bis(i7-cyclopentadienyl)titanium(III) monoalkyl complexes were unsuccessful (40). 

The formation of the 17-electron paramagnetic vanadium complex is not surprising in view of the known corresponding carbonyl complex, however the 16-electron titanium derivative is unexpected in view of the ready formation of the 18-electron biscarbonyl and bistrifluorophos-phine metal complexes containing the 5-cyclopentadienyl ligand. The solid-state structure of the PF3 adduct of bis[2,4-dimethyl-(pentadienyl)]titanium has recently been determined (111) and is shown in Fig. 20. The corresponding vanadium complex is isomorphous. The metal-PF3 distances are 2.326(Ti) and 2.275(V) A. 

From cyclopentadienyl titanium complexes 49 and di-p-tolylcarbodiimide, a product 50, derived from a reductive coupling reaction, is obtained. ... 

CpTiCls is conveniently prepared by treatment of TiCLi with CpSiMes (eqnation 23)7 The synthesis and reactivity bis(cyclopentadienyl)titanium throngh the mid- 90s has been extensively reviewed by Bechaus np7 The synthesis of chiral titanocene derivatives has been reviewed by Halterman. ... 

Considerable work in recent years has been devoted to the synthesis of ring-substituted derivatives of bis( ) -cyclopentadienyl)titanium dUialides. The ring orientation in... 

The reactivity of such compounds has been studied in detail 201-212). Substitution of the 7r-cyclopentadienyl group has been investigated for monocyclopentadienyl halogen and alkoxytitanium derivatives, dicyclo-pentadienyltitanium halides and alkoxides 201-211), and for (77-a llyl) (77-cyclopentadienyl)palladium 212a). The stereochemistry of these complexes has been confirmed unequivocally by physical methods including X-ray studies. The structures of (77-allyl)(7T-cyclopentadienyl)palladium 213) and of (77-cyclopentadienyl)titanium trichloride 214) are shown. 

Probably substitution of the cyclopentadienyl ligand on alcoholysis and hydrolysis, and its transfer in reactions of di- and monocyclopentadienyl-titanium derivatives with FeClj or iron(II) acetylacetonate, follows a mechanism common for 77-ligand exchange in other metal 77-complexes (see later). 

The examples outlined in this chapter show that carbohydrates are efficient stereodifferentiating auxiliaries, which offer possibilities for stereochemical discrimination in a wide variety of chemical reactions. Interesting chiral products are accessible, including chiral carbo- and heterocycles, a- and 3-amino acid derivatives, 3-lactams, branched carbonyl compounds and amines. Owing to the immense material published since the time of the earlier review articles on carbohydrates in asymmetric synthesis [9,10], the examples discussed in this chapter necessarily focused on the use of carbohydrates as auxiliaries covalently linked to and cleavable from the substrate. Given the scope of this chapter, a discussion of other interesting asymmetric reactions has not been permitted — for example, reactions in which carbohydrate-derived Lewis acids, such as cyclopentadienyl titanium carbohydrate complexes, exhibit stereocontrol in aldol reactions [180]. Similarly, processes in which in situ glycosylation induces reactivity and stereodifferentiation — for example, in Mannich reactions of imines [181] — have also been excluded from this discussion. 

Bis(r -cyclopentadienyl)titanium pentasulfide was prepared first by Samuel and Schmidt from TiCp2Cl2 (Cp = cyclopentadienyl) and polysulftde salts. It can also be prepared from the reaction of TiCp2(CO)2 or TiCp2(CH2R)2 with elemental sulfur. The method presented here is a modification of Kopf s procedure and is equally applicable to the Cp, MeCp, and i-PrCp ((-Pr = isopropyl) derivatives. This reaction has been successfully conducted at 40 times... 

Metallocene derivatives are the most extensively studied class of homogeneous catalysts for the polymerization of olefins. Less saturated and less hindered mono-Gp group 4 metal species of the type [Cp MR2]+ (Cp denotes Cp or a substituted cyclopentadienyl ring) may also behave as useful catalysts or initiators for olefin polymerization. Catalysts for syndiospecific polymerization of styrene based on mono-Cp titanium derivatives with different substituents on the Gp ligand and with various types of tetraphenylborates have been examined. A good relationship between the... 

Following these synthetic methods a wide number of mono-Cp -amido titanium derivatives have been prepared. An overview of examples organized according to the nature of the cyclopentadienyl-type ligand (Gp, Cp, Ind, Flu) and the bridging group (Si, C, B) connecting the Cp-type and the amido functionalities is presented. 

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