Titanium-based catalyst systems

Reetz et al. reported that a chiral Ti complex prepared from TiCL). and the dilithium salt of (S)-BINOL promoted the aldol reaction of 3-mefhylbutanal with KSA 48 with only poor enantioselectivity (60%, 8% ee) [115 b]. After this pioneering work, the titanium-based catalyst system has been intensively improved to attain an efficient catalytic cycle and high stereoselectivity [147-155]. 

The relatively weaker Lewis acidic titanium complexes require the use of a stronger nucleophile than allylsilanes, and tributylallyltin (6.81) is the most common aUylating agent employed when using titanium-based catalyst systems. In 1993, Umani-Ronchi and Keck published related results using BINOL/titanium derived catalysts. In the Umani-Ronchi system, BINOL is employed, in combination with TiCl2 (0 Pr)2 and shown to work weU with aliphatic... 

Cocatalysts, such as diethylzinc and triethylboron, can be used to alter the molecular-weight distribution of the polymer (89). The same effect can also be had by varying the transition metal in the catalyst chromium-based catalyst systems produce polyethylenes with intermediate or broad molecular-weight distributions, but titanium catalysts tend to give rather narrow molecular-weight distributions. 

Transition metal catalysis plays a key role in the polyolefin industry. The discovery by Ziegler and Natta of the coordination polymerization of ethylene, propylene, and other non-polar a-olefins using titanium-based catalysts, revolutionized the industry. These catalysts, along with titanium- and zirconium-based metallocene systems and aluminum cocatalysts, are still the workhorse in the manufacture of commodity polyolefin materials such as polyethylene and polypropylene [3-6],... 

Mikami et al X developed a palladium-based catalyst system 22, capable of forming quaternary centers via a carbonyl-ene reaction. This is one of the few recent examples of a carbonyl-ene reaction that uses a ketone rather than an aldehyde and affords 84% yields in the case of the five-membered ring (23, n = 1) to near quantitative yields for the six-membered ring (23, n = 2), with 96% ee or better in both cases (Equation (12)). This catalyst system also shows selectivity when other ene partners are used, giving synl anti ratios equal to or better than the titanium systems and affords linear products with complete ( )-selectivity. 

There has recently been much work in this area using Ru-based catalysts, particularly with porphyrin-based catalysts, following the work by Sharpless et al. on asynunetric epoxidation of allylic alcohols by a titanium-based tartrate system. There are reviews on asymmetric epoxidations catalysed by chiral Ru porphyrins [5, 18]. 

In parallel with the search for catalytic systems, an impressive amount of results in the field of enantioselective allylation has emerged (equation 8). The pioneering work of Marshall and Tang, using a chiral (acyloxy)borane 6 (CAB) system57, was followed by titanium-based catalysts 758 and 859-62 leading to various homoallylic alcohols with enantiomeric excess up to 98%. 

Many ot-olefins were polymerized by the Ziegler-Natta catalysts to yield high polymers and many such polymers were found to be stereospecific and crystalline. Polymerizations of a-olefins of the general structure of CH2 = CH — (CH2) — R, where x is 0-3 and R denotes CH3, CH-(CH3)2, C(CH3)3, or CsHs, can be catalyzed by vanadium trichloride/triethyl aluminum [80]. The conversions are fairly high, though higher crystallinity can be obtained with titanium-based catalysts [81]. Addition of Lewis bases, such as ( 4119)20, (C4H9)3N, or ( 4119)3 , to the catalyst system further increases crystallinity [82]. 

Table 12.5 collects literature values for r and rir2 for different catalyst systems. With vanadium-based catalysts, a correct evaluation of rir2 is hindered by the remarkable presence of regioirregularities. One could say, however, that short comonomer sequences are obtained. With heterogeneous titanium-based catalysts, blocky copolymers are prepared. A correct determination of r r2 is, however, hindered by the multisite nature of the catalyst, that is, by the presence of (many) different polymer fractions resulting from many catalyst sites with different rir2 values that could in principle be responsible for an apparent higher blockiness. 

The structure and composition of copolymers of ethylene and styrene are dependent on the catalyst system used and on the molar ratio of transition metal to cocatalyst (typically MAO). Polymers containing phenylethylene units bridging polyethylene sequences, that is isolated styrene units surrounded by PE segments, are generally formed with titanium-based catalysts. ... 

Wang, J. Liu, Z. Wang, D. Guo, D. Syndiospecifie polymerization of styrene based on dichlorobis(l,3-diketonato)titanium/methylaluminoxane catalyst system Effects of substituents on catalyst activity. Polym. Int. 2000,49,1665-1669. 

The Nobel Prize in Chemistry was awarded to Karl Ziegler and Giulio Natta in 1963 for their research in developing olefin polymerization catalysts primarily based on titanium compounds and aluminum alkyl compounds required for the catalyst initiation process. However, by 1963 the details on the discovery of the Cr-based catalyst system in which the chromium compound was supported on amorphous silica were widely published and commercially important for the manufacture of HDPE. In the view of this author, the 1963 Nobel Prize awarded in chemistry should also have included two additional scientists, John P. Hogan and Robert L. Banks from Phillips Petroleum. 

Since the previous titanium-based catalyst was not able to perform a-olefin polymerization, it was obviously interesting to investigate other soluble active systems to further study the problem of stereoregulation. That was done by Zambelli [Makromol Chemie, 112, 160 (1968)] with the complex obtained from VCI4 and AIR2CI (Fig. 21) in the presence of either an excess of A1 derivative or a Lewis base ligand, e.g. anisole. 

Although hydroamination of allenes can be easily achieved with group 4 and group 5 metal catalysts, the stereoselectivity of these systems is rather limited. Several attempts to perform asymmetric hydroamination/cyclization of aminoallenes employing chiral aminoalcohols [260, 261] and sulfonamide alcohols [262] as chiral proligands for titanium- and tantalum-based catalyst systems have produced vinyl pyrrolidines with low selectivities only. While the titanium catalysts were... 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

Similar to IFP s Dimersol process, the Alphabutol process uses a Ziegler-Natta type soluble catalyst based on a titanium complex, with triethyl aluminum as a co-catalyst. This soluble catalyst system avoids the isomerization of 1-butene to 2-butene and thus eliminates the need for removing the isomers from the 1-butene. The process is composed of four sections reaction, co-catalyst injection, catalyst removal, and distillation. Reaction takes place at 50—55°C and 2.4—2.8 MPa (350—400 psig) for 5—6 h. The catalyst is continuously fed to the reactor ethylene conversion is about 80—85% per pass with a selectivity to 1-butene of 93%. The catalyst is removed by vaporizing Hquid withdrawn from the reactor in two steps classical exchanger and thin-film evaporator. The purity of the butene produced with this technology is 99.90%. IFP has Hcensed this technology in areas where there is no local supply of 1-butene from other sources, such as Saudi Arabia and the Far East. 

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