Third generation catalyst properties

Carbene ligands can replace phosphines dne to similar electronic properties. The development of NHC design concepts featnring different substituents and backbones eventually culminated in the most prominent derivative, the SIMes (SDVtes = A, A -bis[2,4,6-(trimethyl)phenyl]imidazolidin-2-ylidene) ligand that is nsed in the second and later also third generation catalysts (complexes 72, 73, 74b and 74c in Fig. 3.28) [105, 109, 114,116],... 

Aminations of five-membered heterocyclic halides, such as furans and thiophenes, are limited. These substrates are particularly electron-rich. As a result, oxidative addition of the heteroaryl halide and reductive elimination of the amine are slower than for simple aryl halides (see Sections 4.7.1 and 4.7.3). In addition, the amine products can be air-sensitive and require special conditions for their isolation. Nevertheless, Watanabe has reported examples of successful couplings between diarylamines and bromothiophenes [126]. Triaryl-amines are important for materials applications because of their redox properties, and these particular triarylamines should be especially susceptible to electrochemical oxidation. Chart 1 shows the products formed from the amination of bromothiophenes and the associated yields. As can be seen, 3-bromothiophene reacted in higher yields than 2-bromothiophene, but the yields were more variable with substituted bromothiophenes. In some cases, acceptable yields for double additions to dibromothiophenes were achieved. These reactions all employed a third-generation catalyst (vide infra), containing a combination of Pd(OAc)2 and P(tBu)3. The yields for reactions of these substrates were much higher in the presence of this catalyst than they were in the presence of arylphosphine ligands. 

The property envelope of polyolefins has been expanded by the direct synthesis, in the reactor, of novel alloys and blends with a balance of properties not possible ftom conventional, third-generation catalyst polymerization. Such advances in Ziegler-Natta polymerization technology show that polyolefin-based resins can continue to provide the most cost-effective solutions for a wide and ever-increasing range of plications in the global plastics market. 

Ziegler-Nai ta catalyst development did not stop with the introduction of the high yield, high isotactic third generation catalyst process. The supersetive catalyst has provided the basis for a versatility in product and process development never previously thought possible. This system meets the four major objectives vital for the control of polyolefin properties. 

Since the first catalyst (7) used in the organocatalytic Diels-Alder reaction, several modificahons have been made to improve its catalytic properties. In 2002 the commonly called MacMillan second-generation catalyst (25) was used in the addihon of indoles to enals [10]. A few years later, MacMillan s third-generation catalyst (26) was used for the enantioselective transfer hydrogenation of enals (Figure 33.3) [11]. 

There are multiple requirements for a modern commercial polypropylene production catalyst catalyst productivity, polymer stereoregularity and molecular veight control, and powder morphology have to be balanced and maximized. The research and development carried out on the TiCl3 catalyst is now embodied in the third generation SB 12 catalyst, a product combining a unique set of the most desirable properties. The performance of the SB 12 catalyst in typical polymerization conditions is detailled. The basic influence of the intrinsic catalyst properties on both polymer properties and production process is discussed. 

Solvay has been pionnering high performance titanium trichloride catalyst since 1970. In 1977, this work led to the first definite Jump in industrial PP catalyst performance through the commercial production of the second generation catalyst affording both higher productivity and stereospecificity. The continuous research effort to improve and extend the catalyst properties has now led to the new SB 12, third generation 11013. product embodies the latest development in the field and... 

The effectiveness of the third generation, superactive Ziegler-Natta catalyst is dependent on reproducible dispersion of the active sites on the support as well as the chemical and physical properties of the support itself. The five basic indispensable requirements for this superactive catalyst are ... 

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