Use of Co-catalysts

The early Phillips catalysts only needed to be activated and reduced to generate active centers, but several patents since have described the use of co-catalysts to promote the production of LLDPE. For example, a typical catalyst that had been modified with titanium, activated in air and reduced in caibon monoxide was then further activated by addition of triethylboron prior to operation. This procedure led to the production of linear low density polyethylene, LLDPE, which had a density of 0.9726 g cm, directly, without the need for the addition of an a-olefin to the pure ethylene feed. Olefins were produced in situ and these were incorporated into the polyethylene. A second catdyst was made using silica with a very high pore volume, modified with titanium, activated in air, and finally reduced with caibon monoxide at 350°C. This catalyst was then treated with triethylboron before use. LLDPE polymers with densities in the range 0.890 to 0.915 were obtained from a feedstock of ethylene and hexene-1, but the addition of some hydrogen to the gas stream was required to limit the length of the polymer chain. 

Higher-molecular-weight HDPE polymers, with broad molecular weight distributions, can be made using an air-activated titanium-modified catalyst with 

It has been possible to introduce maity different polyethylene varieties with Phillips catalysts by modifying the silica surface or using co-catalysts, which will continue to allow a much wider use of chromium catalysts in the future. 

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Improvements in the rate of the condensation reaction have been claimed with the use of co-catalysts such as an ionisable sulphur compound and by pre-irradiation with actinic light. ... 

Wang et al. have discovered that ultrafme Ni powder in the presence of Cul, PPh3, and KOH promotes coupling of terminal alkynes with aryl and alkenyl iodides in high yields [65], Recent developments have shown, moreover, that the use of co-catalysts (Cu, Zn, Al, etc.) to facilitate the formation of the acetylides is not always required and that cross-coupling reactions of acetylenes and aryl halides can be performed successfully with Pd-based catalysts alone, even with difficult substrates [48, 66]... 

The second development was the use of promoter salts (e.g., alkali, phospho-nium or ammonium salts) to stabilize the activated complex in the catalyst system [41b] and the use of co-catalysts with rhodium, such as base catalyst metals Ti, Zr,... 

Remarkably, C—H oxygenations can also be performed with O2 as terminal oxidant, with and without the use of co-catalysts. Variations of the catalytic conditions result in efficient protocols for introducing C—O bonds in allylic (24), aromatic (25), benzylic (27), and aliphatic positions (28) with high atom economy. 

The use of a catalyst such as cadmium oxide increases the yield of dibasic acids to about 51% of theoretical. The composition of the mixed acids is about 75% C-11 and 25% C-12 dibasic acids (73). Reaction of undecylenic acid with carbon monoxide using a triphenylphosphine—rhodium complex as catalyst gives 11-formylundecanoic acid, which, upon reaction with oxygen in the presence of Co(II) salts, gives 1,12-dodecanedioic acid in 70% yield (74). 

The use of CO is complicated by the fact that two forms of adsorption—linear and bridged—have been shown by infrared (IR) spectroscopy to occur on most metal surfaces. For both forms, the molecule usually remains intact (i.e., no dissociation occurs). In the linear form the carbon end is attached to one metal atom, while in the bridged form it is attached to two metal atoms. Hence, if independent IR studies on an identical catalyst, identically reduced, show that all of the CO is either in the linear or the bricked form, then the measurement of CO isotherms can be used to determine metal dispersions. A metal for which CO cannot be used is nickel, due to the rapid formation of nickel carbonyl on clean nickel surfaces. Although CO has a relatively low boiling point, at vet) low metal concentrations (e.g., 0.1% Rh) the amount of CO adsorbed on the support can be as much as 25% of that on the metal a procedure has been developed to accurately correct for this. Also, CO dissociates on some metal surfaces (e.g., W and Mo), on which the method cannot be used. 

Antibody-catalyzed aldol condensation was demonstrated in a [bmim][PFg] solvent system by Kitazume and co-workers (Fig. 17). They tested recyclable use of antibody catalyst in the solvent system and, very interestingly, found that the chemical yield was increased for the second cycle (89%) over the initial run (21%). 

A recent stndy (13,27) describes the use of Co-Si-TUD-1 for the liquid-phase oxidation of cyclohexane. Several other metals were tested as well. TBHP (tert-butyl hydroperoxide) was used as an oxidant and the reactions were carried out at 70°C. Oxidation of cyclohexane was carried out using 20 ml of a mixture of cyclohexane, 35mol% TBHP and 1 g of chlorobenzene as internal standard, in combination with the catalyst (0.1 mmol of active metal pretreated overnight at 180°C). Identification of the products was carried out using GC-MS. The concentration of carboxylic side products was determined by GC analysis from separate samples after conversion into the respective methyl esters. Evolution and consumption of molecular oxygen was monitored volumetrically with an attached gas burette. All mass balances were 92% or better. 

Combined use of Co(acac)2 and DiBAlH also gives selective reduction for a,(3-unsaturated ketones, esters, and amides.112 Another reagent combination that selectively reduces the carbon-carbon double bond is Wilkinson s catalyst and triethylsilane. The initial product is the enol silyl ether.113... 

Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

Cyanoaminecobaltate(II) catalysts (/, p. 150) were initially studied in relation to the well-known activity of Co(CN)53 (/, p. 106). Use of such catalysts with optically active amines (1,2-propanediamine and N.N-dimethyl-1,2-propanediamine), thought to be bridged in complexes such as [(CN4)Co-amine-Co(CN4)]4 , led to asymmetric hydrogenation of atro-pate [Eq. (55)] to a 7% ee (309). 

The first published report on the use of this catalyst for the cross-metathesis of functionalised acyclic alkenes was by Blechert and co-workers towards the end of 1996 [37]. This report was also noteworthy for its use of polymer-bound alkenes in the cross-metathesis reaction. Tritylpolystyrene-bound AT-Boc N-al-lylglycinol 18 was successfully cross-metathesised with both unfunctionalised alkenes and unsaturated esters (Eq. 17) (Table 1). 

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