Catalytic synthesis, types

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

Borabenzene complexes of cobalt such as Co(C5H5BPh)(COD) (51) and its 5-ethyl analog show the same type of catalysis but improved activity and chemoselectivity (77). Thus, 51 as the catalyst precursor gave the hitherto best results in the catalytic synthesis of the valuable 2-vinylpyridine from C2H2 and CH2=CHCN (120°C, 51 bar, 2 hours, turnover number 2164) (77,101). Furthermore, this catalyst for the first time allowed the synthesis of pyridine from C2H2 and HCN under mild conditions (110°C, 23 bar, 60 minutes, turnover number 103) (77). 

The assembly consists of three fritted jacketed columns (each 2 cm wide) including two top columns (type A) for reagent synthesis a catalytic column (type B) into which the reagent columns feed and a scavenger resin column (type C) below the catalytic column. (Adapted from Hafez et ah, 2001)... 

The asymmetric aldol reaction is one of the most important topics in modern catalytic synthesis [54]. The products, namely />-hydroxy carbonyl compounds, have a broad range of applications and play a key role in the production of pharmaceuticals [55], Since the discovery of the catalytic asymmetric aldol reaction with enolsi-lanes by Mukaiyama et al. [56], steady improvements of the metal-catalyzed asymmetric aldol reaction have been made by many groups [57]. For this type of aldol reaction a series of chiral metal catalysts which act as Lewis acids activating the aldol acceptor have been shown to be quite efficient. It was recently shown by the Shibasaki group that the asymmetric metal-catalyzed aldol reaction can be also performed with unmodified ketones [57a], During the last few years, several new concepts have been developed which are based on use of organocatalysts [58], Enolates and unmodified ketones can be used as aldol donors. 

The silatropic ene pathway, i.e. direct silyl transfer from an enol silyl ether to an aldehyde, might be a possible mechanism in the Mukaiyama aldol-type reaction. Indeed, ab initio calculations show the silatropic ene pathway, involving the cyclic (boat and chair) transition states for the BHs-promoted aldol reaction of the trihydro-silyl enol ether derived from acetaldehyde with formaldehyde, to be favored [94], We recently reported the possible intervention of a silatropic ene pathway in the asymmetric catalytic aldol-type reaction of silyl enol ethers of thioesters [95]. The chloro and amino compounds thus obtained are useful intermediates in the synthesis of carnitine and GABOB (Sch. 34) [96]. 

The complex Rh(H)(CO)2(PPh3)2 can be used in the catalytic synthesis of n-pentanal from an alkene having one less carbon. Propose a mechanism for this process. Give an appropriate designation for each type of reaction step (such as oxidative addition or alkyl migration) and identify the catalytic species. 

The first process of this type utilized gases as raw materials and produced carbon blacks for rubber with only moderate reinforcement properties at a maximum yield of 30%. The changeover to liquid and melting raw materials increased the yield of carbon black for rubber to 40 to 70% and enabled a much broader range of raw of products to be produced, so that this type of raw material is now almost exclusively used e.g. aromatic compound-rich viscous residues from oil refineries, coking plants, catalytic crackers and steam crackers for the production of ethene and from the catalytic synthesis of petrol. 

The type of phase-transfer catalyst plays a key role in the phase-transfer catalytic synthesis of l-bromo-1-chlorocyclopropanes, which are formed in good yields and with high selectivity if the reaction of dibromochloromethane with an alkene is performed using a crown ether (dibenzo-18-crown-6, " 3,5-di-fer/-butylbenzo-15-crown-5, " " 3,3, 5,5 -tetra-tert-butyldiben-zo-lS-crown-b ) or tetramethylammonium chloride.For the specific effect of the tetra-methylammonium chloride on the dichlorocyclopropanation of unconjugated dienes, see Section I.2.I.4.2.I.2., and some electrophilic alkenes, see Section I.2.I.4.2.I.8.2. The reason why these catalysts exhibit peculiar properties is not clear,other crown ethers behave like typical phase-transfer catalysts (Table 25). " ... 

Another important development in the area of catalytic Pauson-Khand type cy-clizations has been the discovery of other transition metal carbonyl complexes which are capable of effecting the catalytic synthesis of cyclopentenones. Two recent reports from Murai and Mitsudo detailed a Ru3(CO)i2-catalyzed enyne cyclocarbonylation, Eqs. (10) and (11) [34,35]. While this protocol allowed for the cyclization of a variety of l,6-enynes,the cyclizations of terminal alkynes as well as 1,7-enynes were problematic. The feasibility of using Cp2Ti(CO)2 as a catalyst for the intramolecular Pauson-Khand type cyclization of a variety of 1,6-and 1,7-enynes (vide infra) has also been demonstrated [36]. Based on the wide array of transition metals that are capable of effecting stoichiometric Pauson-Khand type cyclizations (vide supra), the development of more catalytic systems is to be expected this should greatly facilitate the search for catalytic asymmetric variants. 

Catalytic biphasic reactions, including their aqueous variants, are widely used for the catalytic synthesis of organic products. Catalysts, various types of water-soluble ligands, synthetic uses, industrial applications, and their advantages over conventional homogeneously catalyzed reactions are well documented (e.g. [1-3]). Table 1 presents few examples of reactions performed in an aqueous gas/liquid-liquid medium with a homogeneous complex catalyst soluted either in the aqueous phase or organic phase. 

Intramolecular addition of alcohols across alkenes was established as one of the first examples of this type, and represents a classic approach to the catalytic synthesis of oxygenated heterocycles [12]. A first reaction to this end was described in 1975, with the conversion of 2-allyl phenols 1 to their corresponding dihydrobenzofuranes 2. 

Merck has discovered a more efficient catalytic synthesis for sitagliptin, a chiral (3-amino acid derivative that is the active ingredient in their new treatment for type 2 diabetes, Januvia . This revolutionary synthesis creates 220 pounds less waste for each pound of sitagliptin manufactured, and increases the overall yield by nearly 50%. Over the lifetime of Januvia , Merck expects to eliminate the formation of at least 330 million pounds of waste, including nearly 110 million pounds of aqueous waste. 

Nitroalkanes have been rarely used as substrates for catalytic reactions with carbon monoxide. This is mainly due to the fact that they usually react with transition metal complexes in their aci form [1]. Thus their reactivity is seldom comparable to the one shown by aromatic nitro derivatives. The catalytic synthesis of oximes by carbonylation of nitroalkanes is a rare example not only of their use in this type of catalysis, but also of the formation of these derivatives. 

North and coworkers recently described an efficient and wide-scope catalytic synthesis of both heterocycles of type A and C (Figure 17) by variation of the reaction temperature [138]. In an initial screening phase, a bimetallic Al(salen) complex was used as a Lewis acid activator in conjxmction with Bu4NBr as nucleophilic cocatalyst at 70 °C in a sealed tube using 2-7 equivalent of CS2 and propylene oxide as substrate. A reaction temperature of 50 °C was found to be optimal considering both the chemical yield of the process as well as the chemoselectivity for the l,3-oxathiolane-2-thione product (96%, ratio A/C 8.0) using only 1.8 equivalent of CS2-... 

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