New Cocatalysts

Keys to the high polymerization activities of single-site catalysts are the cocatalysts. MAO is most commonly used and is synthesized by controlled hydrolysis of trimethyl aluminum. Other bulky anionic complexes which show a weak coordination, such as borates, also play an increasingly important role. One function of the cocatalysts is to form a cationic metallocene and an anionic cocatalyst species. Another function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds, even at -60°C (69). Excess MAO leads to the dialkylated species, as shown by NMR measurements. For the active site to form, it is necessary that at least one alkyl group be bonded to the metallocene (70). 

Other alkylalumoxanes (e.g., ethylalumoxane or isobutylalumoxane) were also used as cocatalysts instead of MAO (67) but show a much lower polymerization activity. The combination of zirconocenes with MAO is evidently optimal. The three-dimensional structure plays a role (there is a rough similarity to enzymes)—changing either the metal or the alkyl groups leads to lower activity, as does changing the amino acids in enzymes. 

Alkylalumoxanes are prepared by a variety of methods, with the preferred being the reaction of trimethylaluminum with water (ice), a violent, highly exothermic reaction best carried out at low temperatures in an inert solvent (71, 72). Instead of water, hydrolysis of A12(S04)3- 16 H20 can be used. In both cases the reaction is evidenced by the evolution of methane during hydrolysis  

Unfortunately, this reaction is a gross oversimplification of the series of reaction steps that occur during the hydrolysis reaction. Hydrolysis has been shown to proceed via the formation of an alkylaluminum-water complex, which subsequently eliminates methane to form a dimethylaluminum hydroxide complex. This rapidly associates to give dimers or larger oligomers in solution. In the case of f-butylalumoxane, some of the intermediate species have been isolated and characterized structurally (73-76). 

These can associate, resulting in coordination of the unsaturated aluminum atoms. There are tri- and tetracoordinated aluminum atoms, of which the trivalent ones show extreme Lewis acidity. Four of the A1403(CH3)6 units can form a cage structure resembling a half-open dodecahedron (Ali6Oi2 (CH3)24) (Fig. 7) (77). 

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One of the most efficient methods for oxidation of primary alcohols to either aldehydes or carboxylic acids is the one, commonly known as the Anelli oxidation. This reaction is carried out in a two-phase (CH2Cl2/aq.buffer) system utilizing TEMPO/NaBr as a catalyst and NaOCl as the terminal oxidant The new system described here is an extension of the Anelli oxidation, but surprisingly, does not require the use of any organic solvents and replaces the KBr co-catalyst with the more benign, Na2B40y (Borax). The use of the new cocatalyst reduces the volume of the buffer solution and eliminates completely the need of a reaction solvent. The new system was successfully applied in the industrial synthesis of the 3,3-Dimethylbutanal, which is a key intermediate in the preparation of the new artificial sweetener Neotame. 

Dihydroxylations. By using molecular oxygen to sustain the oxidation, high atom-efficiency is attained. A new cocatalyst duet is Al-methylmorpholine and the flavin analogue l,3-dimethyl-5-ethyl-5,10-dihydrobenzopteridine-2,4-dione. ... 

New Cocatalysts in Toluene Chlorination. Intensive research into the development of a catalyst system suitable for industrial use has therefore been started. These investigations have resulted in the discovery of the benzothiazepine (143]-[146] cocatalysts, whose properties are greatly superior to those of current state-of-the-art products. Their / ara-selectivily in comparison with FeClj-sulfur (o / = 1.11) is much increased (up to o p=0.55) by using minimum quantities of cocatalyst (e.g., 30 - 50 ppm) in the presence of FeCl3 as the main catalyst and at industrially favorable temperatures of 40 - 50 °C. 

Nanoparticulate Mixed Oxides as New Cocatalysts 4.1 Motivation for Applying Mixed-Oxides as Cocatalysts... 

In a recent development, a new process of preparing borane-terminated isotactic polypropylene (z -PPs) via an in situ chain-transfer reaction was achieved by a styrene/hydrogen consecutive chain-transfer reagent, which avoids the use of a B—H containing chain-transfer agent.74 This has resulted in the utilization of milder polymerization conditions due to the use of the alkylaluminoxane cocatalyst (MAO) (50) (Fig. 33), which cannot normally be used in the presence of a B—H chain-transferring... 

Very recently, Hu et al. claimed to have discovered a convenient procedure for the aerobic oxidation of primary and secondary alcohols utilizing a TEMPO based catalyst system free of any transition metal co-catalyst (21). These authors employed a mixture of TEMPO (1 mol%), sodium nitrite (4-8 mol%) and bromine (4 mol%) as an active catalyst system. The oxidation took place at temperatures between 80-100 °C and at air pressure of 4 bars. However, this process was only successful with activated alcohols. With benzyl alcohol, quantitative conversion to benzaldehyde was achieved after a 1-2 hour reaction. With non-activated aliphatic alcohols (such as 1-octanol) or cyclic alcohols (cyclohexanol), the air pressure needed to be raised to 9 bar and a 4-5 hour of reaction was necessary to reach complete conversion. Unfortunately, this new oxidation procedure also depends on the use of dichloromethane as a solvent. In addition, the elemental bromine used as a cocatalyst is rather difficult to handle on a technical scale because of its high vapor pressure, toxicity and severe corrosion problems. Other disadvantages of this system are the rather low substrate concentration in the solvent and the observed formation of bromination by-products. 

Along with enabling technological advances in alkene polymerizations, the application of new ancillary ligands or cocatalysts has become significant in the field of alkene oligomerizations to obtain a-olefms. This research exploits a new phase in the development of known catalyst systems containing Gr, Ni, Pd, Ti metal centers, as well as Fe and Co as newcomers. 

In new studies heteropoly acids as cocatalysts were found to be very effective in combination with oxygen in the oxidation of ethylene.1311 Addition of phosphomo-lybdic acid to a chloride ion-free Pd(II)-Cu(II) catalyst system results in a great increase in catalytic activity and selectivity.1312 Aerobic oxidation of terminal alkenes to methy ketones can be performed with Pd(OAc)21313 or soluble palladium complexes. Modified cyclodextrins accelerates reaction rates and enhance selectivities in two-phase systems under mild conditions.1315 1316... 

By application of the dimethylsilyl-bridged bisfluorenyl neodymium chloride complex [Me2Si(Ci3H8)2]NdCl (cocatalysts Buli, DIBAH, BuMgCl, BuMgOct) a complete new class of polymers is accessible by the copolymerization of ethylene and BD [307]. By reaction of 1 eq. BD and 2 eq. of ethylene 1,2-cyclohexane units are formed along the polymer chain. More than half of the inserted BD units (53-58 mol %) are involved in the ring closure reaction. The residual non-cyclicized BD units exhibit a nearly identical ratio of trans-1,4- and 1,2-units. 

Several research approaches are pursued in the quest for more efficient and active photocatalysts for water splitting (i) to find new single-phase materials, (ii) to tune the band-gap energy of TJV-active photocatalysts (band-gap engineering), and (iii) to modify the surface of photocatalysts by deposition of cocatalysts to reduce the activation energy for gas evolution. Obviously, the previous strategies must be combined with the control of the s)mthesis of materials to customize the crystallinity, electronic structure, and morphology of materials at nanometric scale, as these properties have a major impact on photoactivity. 

The s)mthesis of a new transition metal precursor chemically fixed to main-group organometallic cocatalysts is an important subject—pointing the way toward heterometallic compounds with unusual properties and... 

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