Catalysts systems second generation

TiCl catalysts produced by the reduction of TiCl with Al(C2H 2d> subsequentiy treated first with an electron donor (diisoamyl ether), then with TiCl, are highly stereospecific and four to five times more active than d-TiCl (6). These catalysts were a significant advance over the earlier TiCl systems, because removal of atactic polymer was no longer required. They are often referred to as second-generation catalysts. The life of many older slurry process faciUties has been extended by using these catalysts to produce "clean" polymers with very low catalyst residues. 

These catalysts contained promoters to minimise SO2 oxidation. Second-generation systems are based on a combined oxidation catalyst and particulate trap to remove HC and CO, and to alleviate particulate emissions on a continuous basis. The next phase will be the development of advanced catalysts for NO removal under oxidising conditions. Low or 2ero sulfur diesel fuel will be an advantage in overall system development. 

The limitations of the system with regard to substrates and oxidants was attributed to the strong electron-withdrawing character of the perfluorinated chains and the lower steric hindrance in the position adjacent to phenols, in marked contrast to the ferf-butyl groups present in Jacobsen s catalyst, hi view of this, a second generation of fluorinated salen ligands le and If was... 

Alkali-promoted Ru-based catalysts are expected to become the second generation NHs synthesis catalysts [1]. In 1992 the 600 ton/day Ocelot Ammonia Plant started to produce NH3 with promoted Ru catalysts supported on carbon based on the Kellogg Advanced Ammonia Process (KAAP) [2]. The Ru-based catalysts permit milder operating conditions compared with the magnetite-based systems, such as low synthesis pressure (70 -105 bars compared with 150 - 300 bars) and lower synthesis temperatures, while maintaining higher conversion than a conventional system [3]. 

In the course of studying a large nnmber of examples where the side chains of the imidazol- and imidazolidin-2-ylidene were altered, several research groups found that NHCs bearing exclnsively alkyl side chains did not provide catalysts with better characteristics when compared to SIMes- and DVIes-derived systems 14 and 15. While Herrmann and co-workers showed that an unsaturated NHC bearing cyclohexyl wing tips conld be incorporated into a second-generation catalyst that was active in metathesis [20-23], more recent studies showed that similar complexes were either very difficult to prepare or were unstable and showed dramatically decreased catalytic properties [24-26] (complexes 17-19, Fig. 3.4). 

Initial efforts in the ring-dosing metathesis approach were attempted with substrates 34 and 35. However, after employing a variety of catalysts and experimental conditions, no cydized systems (36 or 37) were obtained. Other substrates were prepared to further probe this unexpected failure however, no observable reaction was realized. Model systems later suggested that die dense functionality between C3 and C8 was the culprit for lack of macrocycle formation. Eventually a second generation Cl2-03 RCM (not shown here) approach was developed [26] which yielded mixtures of C12-C13 Z/E isomers that were used in early SAR studies. [26b] However, since the separation of products was so difficult, we did not seriously pursue this route for total synthesis. 

Schrader, W. Eipper, A. Pugh, D.J. Reetz, M.T. Second-Generation MS-Based High-Throughput Screening System for Enantioselective Catalysts and Biocatalysts. Canadian J. Chem. 2002, 80, 626-632. 

The catalysts for xylene isomerization with EB dealkylahon are dominated by MFI zeolite. The de-ethylation reaction is particularly facile over this zeolite. There have been several generations of catalyst technology developed by Mobil, now ExxonMobil [84]. The features in their patents include selectivation and two-catalyst systems in which the catalysts have been optimized separately for deethylation of EB and xylene isomerization [85-87]. The crystallite size used for de-ethylation is significantly larger than in the second catalyst used for xylene isomerization. Advanced MHAI is one example. The Isolene process is offered by Toray and their catalyst also appears to be MFI zeoUte-based, though some patents claim the use of mordenite [88, 89]. The metal function favored in their patents appears to be rhenium [90]. Bimetallic platinum catalysts have also been claimed on a variety of ZSM-type zeolites [91]. There are also EB dealkylation catalysts for the UOP Isomar process [92]. The zeolite claimed in UOP patents is MFI in combination with aluminophosphate binder [93]. 

In order to maximise this interaction, a second generation catalyst with an extended r-system was designed based on an (I )-2-phenyl-1,2-dihydroimidazo[ 1,2a] quinoline (PIQ) core (Fig. 11) [153, 154]. 

To improve these selectivities, Hashimoto studied several catalysts that had been found highly effective for enantioselective C—H insertion reactions. The new catalysts incorporated an additional benzene in the naphthyl system to increase the steric bias of the catalyst. By using the second-generation catalysts in trifluorotoluene as solvent, at 0 °C, and short reaction times gave ee ratios of 68-92%. Lowered reaction temperature generally resulted in reduced chemical yields but did not erode the ee ratio. Tether lengths one smaller or one larger also tended to erode the ee ratio (Scheme 4.73). 

The generation of six-membered ring systems by means of cycloaddition reactions can be divided into two main approaches. The first is the cyclotrimerizationofalkynes utilizing low-valent iron catalyst systems, whereas the second approach is the Diels-Alder (DA) reaction of a diene and a dienophile. The latter reaction can itself be divided into three subclasses DA reactions with normal, neutral and inverse electron demand are known. The electronic structure of the educts dictates the oxidation state of the catalyst system required to perform the diverse classes of DA reactions. Nevertheless, for each subclass examples can be found. 

The first representative 57 of pyrido[2,l-/]-3,l-benzoxazine ring system was synthesized by the ring-closing methathesis of perhydro-3,l-benzox-azine 113 in the presence of the second generation of Grubb s catalyst (06TL3815). 

---