Monocyclopentadienyl titanium

Monocyclopentadienyl titanium derivatives are the most active precursors for catalysts possessing high syndiospecific polymerisation activity for styrene and ring-substituted styrenes. The polymerisation activity of biscyclopentadie-nyl titanium compounds activated with methylaluminoxane is lower than that of other soluble titanium-based catalysts [73]. 

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). 

A selection of structures together with the most relevant polymerization results is reported in Table 2 (monocyclopentadienyl titanium complexes) and Table 3 (zirconocenes). 

Eirth VA, Stephan WD. Monocyclopentadienyl—titanium aryloxide sulfide eomplexes. Inorg Chem 1998 37(18) 4726-31. 

The first hydroaminations by this mechanism were reported by Bergman with zircono-cene complexes and by Livinghouse with monocyclopentadienyl titanium and zirconium complexes. Bergman reported the intermolecular addition of a hindered aniline to an alkyne. The hindrance of the aniline was important to prevent formation of stable dimeric complexes containing bridging imido groups. Livinghouse reported intramolecular reactions that occurred at lower temperatures over shorter times. The intramolecularity of this process allows the [2+2] cycloaddition of the imido complex with the alkyne to be faster than the dimerization. 

Monocyclopentadienyl and (monoindenyl)titanium trichlorides are generating interest as catalyst precursors for syndiospecific polymerization of styrene (112-127). A class of highly active olefin pol5unerization catalysts based on a (monocyclopentadienyl)titanium dimethyl cation [CpTi(CH3)2l+, generated hy... 

In spite of the high activities shown for BD polymerization by homogeneous catalyst systems, amongst these only a few have been found to be active for isoprene polymerization. MAO-activated titanium complexes produce 1,4-polyisoprene polymer with a prevalently cis microstructure (>94%). Recently, Miyazawa et al. have shown that monocyclopentadienyl titanium complexes activated by MAO can promote isoprene polymerization, giving polymers with narrow molecular weight distributions (Mw/Afn < 2) and cis contents of up to 92% with small amounts of 1,2- and 3,4-enchained structures also present. ... 

From both the experimental and theoretical points of view, the most thoroughly studied catalytic systems are undoubtedly allylnickel(II) systems and monocyclopentadienyl titanium complexes. In the case of the nickel systems, chain growth proceeding by BD insertion into the allyl-transition metal bond was proven directly by NMR spectroscopy for both 1,4-trans- and 1,4-cis-regulating catalysts. In this case, the proposed mechanism for stereoregulation suggests that the cis-trans... 

FIGURE 17.16 Possible orientations of the coordinated monomer and the allylic end group of the growing polymer chain around the metal center in diene polymerization with monocyclopentadienyl titanium catalyst systems (a) endo-endo (prone-prone) (b) endo-exo (prone-supine) (c) exo-endo (supine-prone) (d) exo-exo (supine-supine). (Reprinted with permission from Peluso, A. Improta, R. Zambelli, A. Organometallics 2000,19, 411-419. Copyright 2000 American Chemical Society.)... 

Thus, the reaction between CpTiX3 and HN=C BuR (or LiN=C BuR) resulted in a new class of monocyclopentadienyl titanium catalysts for olefin polymerization [CpTi(N=C BuR)X2] [37] ... 

Use as Ligand in Ziegler-Natta Polymerization of Ethylene in Solution. A medium pressure process for the polymerization of ethylene has been developed in the presence of a catalytic system involving a monocyclopentadienyl titanium species, containing TIPSthiolate as the heteroligand and two activable ligands (2C1 or 2Me), associated with an ionic activator such as triphenyl-carbenium tetrakis(pentafluorophenyl)borate. ... 

Several cationic titanium and zirconium monocyclopentadienyl derivatives, [Cp MR2]+ (M = Ti, Zr), have been synthesized by the reaction of MR3Cp ... 

Among titanium-based precursors, monocyclopentadienyl compounds of the type CpTiCl3 or Cp TiCl3 activated by MAO or B(C6F5)3 showed the best performance, although several substituted mono-Cp or indenyl derivative and Cp-free compounds as Ti(CH2Ph)4 and Ti(OR)4 (R = alkyl, aryl) are quite active as well. In short, practically any soluble titanium compound can be used as precatalyst.154-158... 

Density functional theory studies arene chromium tricarbonyls, 5, 255 beryllium monocyclopentadienyls, 2, 75 chromium carbonyls, 5, 228 in computational chemistry, 1, 663 Cp-amido titanium complexes, 4, 464—465 diiron carbonyl complexes, 6, 222 manganese carbonyls, 5, 763 molybdenum hexacarbonyl, 5, 392 and multiconfiguration techniques, 1, 649 neutral, cationic, anionic chromium carbonyls, 5, 203-204 nickel rj2-alkene complexes, 8, 134—135 palladium NHC complexes, 8, 234 Deoxygenative coupling, carbonyls to olefins, 11, 40 (+)-4,5-Deoxyneodolabelline, via ring-closing diene metathesis, 11, 219... 

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

Monocyclopentadienyl complexes have also been used as active polymerization catalysts. Indeed, titanium monocyclopentadienyl metallocenes—the so-called constrained geometry catalysts —have been focal points of Dow s activity in this area. A typical example of such a catalyst is shown by the structure 6.24. 

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. 

Diene complexes of the so-called constrained geometry monocyclopentadienyl-amido titanium complexes have also been prepared. Interest in these molecules stems from their utility as catalyst precursors in olefin polymerization... 

Monocyclopentadienyl complexes of titanium (CpTiXs) perform poorly as catalysts for ethylene or propylene polymerization, but in the presence of MAO, they polymerize styrene to stereo- and regioregular syndiotactic polystyrene, a crystalline material with very high melting point (273 °C) and glass transition temperature (100°C). In this case, the active polymerizing species is a complex (Figure 8). Each styrene monomer inserts in a secondary manner and the stereoregularity is maintained by the conformation of the last inserted unit (chain-end control). 

In 1985, Ishihara [10] at Indemitsu, Japan, reported that monocyclopentadienyl, indenyl, or fluorenyl titanium complexes, with different substituents on the Cp derivatives, were highly active for styrene polymerization, producing syndiotactic polystyrene with high melting temperature. Ishihara smdied early transition metals from group 3 or 4 systems based on late transition metals such as Ni were inactive for styrene homo- or copolymerization. Monocyclopentadienyl complexes, also... 

Monocyclopentadienyl complex of trimethoxy titanium with a borate activator as metallocene catalyst to polymerize styrene in aqueous emulsions in presence of surfactants 138... 

A variety of Group 4 metal complexes, in combination with common olefin polymerization activators, have been evaluated as potential catalysts for syn-diospecific polymerization of styrene (for reviews, see Refs. 114, 115, 123, and 426). Monocyclopentadienyl and monoindenyl titanocenes generally exhibit the highest activities (eq. 5) (112-127). Curiously, half-sandwich titanium-trifluoride-based catalysts are more active than their trichloride analogues (124,427,428). The polymerization mechanism for sPS formation is under debate. Kinetic studies and spectroscopic investigations of the catalytic systems suggest a cationic Ti(III) complex as the active species (123). 

Indeed, Ishihara and co-workers succeeded in the first preparation of sPS through activation of a transition metal complex with MAO (36,37). Typically, Group IV metallocene complexes have been used as catalysts for the polymerization of sPS. Of these, the monocyclopentadienyl-type complexes of titanium have been found to give the highest pol5nnerization activity based on transition metal (38,39). Subsequent to the development of MAO as the sPS cocatalyst, it has been foimd that highly electrophilic activators, such as the tetrakis(pentafluorophenyl) borate type, can be used as cocatalysts for the production of sPS (40,41). 

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