Ti catalyst

Al—Ti Catalyst for cis-l,4-PoIyisoprene. Of the many catalysts that polymerize isoprene, four have attained commercial importance. One is a coordination catalyst based on an aluminum alkyl and a vanadium salt which produces /n j -l,4-polyisoprene. A second is a lithium alkyl which produces 90% i7j -l,4-polyisoprene. Very high (99%) i7j -l,4-polyisoprene is produced with coordination catalysts consisting of a combination of titanium tetrachloride, TiCl, plus a trialkyl aluminum, R Al, or a combination of TiCl with an alane (aluminum hydride derivative) (86—88). 

Not only has the Ti precursor been investigated, but also the structure of the molecular sieve has been heavily investigated. Thus we now have an array of silica and silica-alumina molecular sieve supported Ti catalysts. These include Ti on amorphous Si02,10,11 Ti on a variety of Si02 mixed oxides,12 Ti-0 (titanium-beta),13"17 Ti-MCM-48,18 Ti-MCM-41,19 Ti-HMS,18 titanium-... 

Why are transition metals well suited for catalysis of this process Certainly the electrophilicity of cationic metal centers is important, as is the relative weakness of transition-metal-carbon bonds. However, similar electrophilicities and bond strengths could be found among main-group cations as well. A key to the effectiveness of Ti catalysts is the presence of two metal-based acceptor orbitals. In effect, two such orbitals are needed to choreograph the reversal of net charge flow at the two alkene carbons as the intermediate alkene complex moves through the transition state toward the final product. 

Preparation of the catalyst can be accomplished under mild conditions without stirring, heating, or cooling, and allyl addition can also be conducted more conveniently using 10 mol% of a 2 1 BINOL/Ti catalyst system at room temperature.91... 

The self-assembly of a chiral Ti catalyst can be achieved by using the achiral precursor Ti(OPr )4 and two different chiral diol components, (R)-BINOL and (R,R)-TADDOL, in a molar ratio of 1 1 1. The components of less basic (R)-BINOL and the relatively more basic (R,R)-TADDOL assemble with Ti(OPr )4 in a molar ratio of 1 1 1, yielding chiral titanium catalyst 118 in the reaction system. In the asymmetric catalysis of the carbonyl-ene reaction, 118 is not only the most enantioselective catalyst but also the most stable and the exclusively formed species in the reaction system. 

Scheme 8-44. General scheme for Ti-catalyst asymmetric glyoxylate-ene reactions. Reprinted with permission by Wiley-VCH Verlag GmbH, Ref. 89.

The idea of enantioselective activation was first reported by Mikami and Matsukawa111 for carbonyl-ene reactions. Using an additional catalytic amount of (R)-BINOL or (/ )-5.5 -dichloro-4,4, 6,fi -tctramcthyl biphenyl as the chiral activator, (R)-ene products were obtained in high ee when a catalyst system consisting of rac-BINOL and Ti(OPri)4 was employed for the enantioselective carbonyl ene reaction of glyoxylate (Scheme 8-54). Amazingly, racemic BINOL can also be used in this system as an activator for the (R)-BINOL-Ti catalyst, affording an enhanced level of enantioselectivity (96% ee). 

Sato, Naito, and coworkers/Iida, Igarashi, and coworkers—promoting effect of Re incorporation to Pt/Ti and Ir/Ti catalysts—formate mechanism proposed. 

The activity of Ti catalysts in SSP depends on the kind of stabilizer fed into the reactor. In the production of PET, phosphorous-containing chemicals are commonly added as stabilizers. These products improve the thermal stability, particularly in processing, which results in reduced degradation and discoloration and are therefore of importance with respect to quality. Such materials are added during the production of the prepolymer. These stabilizers are mainly based on phosphoric or phosphonic (phosphorous) acids or their esters. 

Ti Catalyst temperature at top of a zone T, Temperature of air at air inlet Wi Weight fraction of coke on catalyst... 

In further developments Sandrock et al. [1, 75] optimized the amonnt of Ti catalyst needed to effectively increase the rates of absorption/desorption and introdnced... 

G. Sandrock, K. Gross, G. Thomas, Effect of Ti-catalyst content on the reversible hydrogen storage properties of the sodium alanate , J. Alloys Compd. 339 (2002) 299-308. 

S. Isobe, T. Ichikawa, N. Hanada, H.Y. Leng, M. Fichtner, O. Fuhr, H. Fujii, Effect of Ti catalyst with different chemical form on li-N-H hydrogen storage properties , J. Alloys Compd. 404-406 (2005) 439 42. 

Co complexes, Buchwald reported the Ti-catalyzed carbonylative coupling of enynes-the so-called Pauson-Khand-type reaction [28]-and realized the first such catalytic and enantioselective reaction using a chiral Ti complex [29]. Here, a variety of enynes were transformed into bicyclic cyclopentenones with good to high ee-values however, several steps were required to prepare the chiral Ti catalyst, while the low-valent complex proved to be so unstable that it had to be treated under oxygen-free conditions in a glove box. 

The preparation of the heptanorbornyl silsesquioxane trisilanol 5 has been reported by Maschmeyer et al It is formed in admixture with the corresponding tetrasilanol. The norbornyl-substituted species still await further exploration as precursors in metallasilsesquioxane chemistry. The same applies for the isobutyl and isooctyl derivatives 6 and 7, respectively, which have been propagated in the patent literature by Lichtenhan and Abbenhuis. The application of high-speed experimentation techniques to optimize the preparation of silsesquioxanes as precursors for Ti catalysts has been reported by Maschmeyer et al ... 

The asymmetric hetero-Diels-Alder reaction of aldehydes with Danishefsky s diene catalyzed by Ti catalysts generated from a library of 13 chiral ligands or activators has also been reported (Scheme 8.18). The catalyst library contains 104 members. The Ti catalysts bearing L, L , L, and J are found to have a remarkable effect on both enantioselectivity (76.7-95.7% ee) and yield (63-100%). On the other hand, ligands bearing sterically demanding substituents at the 3,3 -positions are found to be detrimental to the reaction. The optimized catalysts, both L /Ti/L and L /Ti/L , are the most efficient for the reaction of a variety of aldehydes, including aromatic, olefinic, and aliphatic derivatives. 

Reaction 5.45 is at least partly hypothetical. Evidence that the Cl does react with the Na component of the alanate to form NaCl was found by means of X-ray diffraction (XRD), but the final form of the Ti catalyst is not clear [68]. Ti is probably metallic in the form of an alloy or intermetallic compound (e.g. with Al) rather than elemental. Another possibility is that the transition metal dopant (e.g. Ti) actually does not act as a classic surface catalyst on NaAlH4, but rather enters the entire Na sublattice as a variable valence species to produce vacancies and lattice distortions, thus aiding the necessary short-range diffusion of Na and Al atoms [69]. Ti, derived from the decomposition of TiCU during ball-milling, seems to also promote the decomposition of LiAlH4 and the release of H2 [70]. In order to understand the role of the catalyst, Sandrock et al. performed detailed desorption kinetics studies (forward reactions, both steps, of the reaction) as a function of temperature and catalyst level [71] (Figure 5.39). 

In Tables 2.3-1 and 2.3-2 the products formed by Ti- and Ni-catalysts are compared with those formed by Cr catalysts. Ni- and Ti-catalysts show an opposite behavior in the fff/ffc-isomer distribution of 1,5,9-cyclododecatrienes and in 2 1- vs. 

Cr-catalyst can perhaps pair the electrons of the half-filled d orbitals in two ways, leading to comparable control like in Ni- or Ti-catalysts. It can be assumed, that HOss/LUmCTI)- or HOM/LUss-interactions (Ni) may lead to the observed consequences. 

Controlling Factors in Homo neous Tiansistion-Metal Catalysis Table 2.3-1. Cyclotrimerization of butadiene by means of Ni-, Ct- or Ti-catalysts ... 

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