Cocatalysts sensitivity

Other apphcations of sodium bromide iaclude use ia the photographic iadustry both to make light-sensitive silver bromide [7785-23-1] emulsions and to lower the solubiUty of silver bromides during the developing process use as a wood (qv) preservative in conjunction with hydrogen peroxide (14) as a cocatalyst along with cobalt acetate [917-69-1] for the partial oxidation of alkyl side chains on polystyrene polymers (15) and as a sedative, hypnotic, and anticonvulsant. The FDA has, however, indicated that sodium bromide is ineffective as an over-the-counter sleeping aid for which it has been utilized (16). 

Tertiary stibines have been widely employed as ligands in a variety of transition metal complexes (99), and they appear to have numerous uses in synthetic organic chemistry (66), eg, for the olefination of carbonyl compounds (100). They have also been used for the formation of semiconductors by the metal—organic chemical vapor deposition process (101), as catalysts or cocatalysts for a number of polymerization reactions (102), as ingredients of light-sensitive substances (103), and for many other industrial purposes. 

Independent of the ligand system, two different activation methods have been used in performing the propylene polymerization experiments. In both cases, the catalytic activities and molecular weights of the polymers are a sensitive function of the aluminum content provided by the activators. This dependence suggested an additional reversible chain transfer to aluminum when activating with MAO. As lower contents of A1 are provided in the polymerization system in the case of in situ activation with TIBA/borate, the only mechanism occurring is the chain back-skip. Furthermore, the differences in the polymer microstructures prepared with MAO and borate as cocatalysts are reflected. They sustain the proposed reversible chain transfer. 

A major limitation of such Group IVB metallocene catalysts is that they are air- and moisture-sensitive and not tolerant to heteroatom-containing monomers. In the case of heteroatom-containing monomers the unbonded electron pairs on the heteroatom, such as oxygen, preferentially coordinate to the Lewis acid metal center in place of the carbon-carbon double bond. Some so-called middle and late transition metal organometallics are more tolerant to the presence of such heteroatoms and can be used as effective cocatalysts. These include some palladium, iron, cobalt, and nickel initiators. 

Finally, it should be noted that cationic polymerizations are very sensitive to impurities. These can act as cocatalysts, accelerating the polymerization, or as inhibitors (e.g., tertiary amines) they can also give rise to chain transfer or chain termination and so cause a lowering of the degree of polymerization. Since these effects can be caused by very small amounts of impurities (10 mol% or less), careful purification and drying of all materials and equipment is imperative. 

Rudler and coworkers reported that in the case of moderately acid-sensitive epoxides the use of biphasic reaction conditions (H2O/CH2CI2) proved to be sufficient in order to obtain the epoxides with good selectivity because under biphasic conditions the contact of epoxides with water is minimized. Because of the lability of pyridines under the reaction conditions employed, alternative and more stable cocatalysts such as pyrazole (12 mol%, biphasic conditions), bipyridine (6 mol%, biphasic conditions H20/CH2Cl2) and bipyridine-A,Af -dioxide (1.2mol%) were employed together with MTO (Scheme Pyrazole is stable against oxidation and with this additive... 

Tris(pentafluorophenyl)borane, known as "FAB" (structure below), is the most common arylborane used as cocatalyst for single site catalysts. FAB is a strongly Lewis acidic, air-sensitive solid (T 126-131 °C) that is only slightly soluble in hydrocarbon solvents. The structure of FAB is given below. 

DCC can be used to prepare 5-alkyl and 5-aryl thiocarboxylates (1) from carboxylic acids and thiols according to equation (5). This method has been successfully applied to the synAesis of thiol esters with sensitive substituents, e.g. 5-methyl thioacrylate, a natural product. In particular, N-protected amino acid and peptide 5-phenyl esters, which are useful building blocks in peptide synthesis, are obtained in excellent yields without racemization. N-Hydroxyphthalimide and DMAP have been used as cocatalysts to facilitate the reaction. The preparation of the Wittig reagent (5) by this route is shown in equation (6). 

Metal cyclopentadienyl complexes can also be used as cocatalysts, with the intent of creating chromocene-like structures on the surface of the catalyst, as shown in Scheme 46. Chromocene catalysts, which contain mono-attached chromium species incorporating one cyclopentadienyl ligand, are noted for their sensitivity to H2. It is believed that Cr/silica catalysts can be modified to make this species by the addition of metal cyclopentadienyls to the reactor, such as LiCp or MgCp2 [695],or by use of a combination of cyclopentadiene or indene with an aluminum alkyl cocatalyst [696]. When these modified catalysts are allowed to polymerize ethylene in the presence of a remarkable broadening of the polymer MW distribution is observed, mainly as a result of a shift of the low-MW part of the MW distribution. The chromocene surface species is known for its ability to incorporate H2 (thus lowering the polymer MW) and also to reject 1-hexene. Thus, these unusual cocatalysts have the potential to reverse the normal branch profile of polymers made with Cr/silica catalysts (i.e., to put more branches into the longer chains). 

FIGURE 213 MW distributions of polymers made with Cr/silica-titania catalyst in the presence of metal cyclopentadienyl cocatalysts. Enhanced sensitivity to H2 leads to an extreme shift to low-MW polymer. 

Sometimes the chromium species generated by reaction of the catalyst with the cocatalyst become highly sensitive to H2 as a MW regulator, much like the organochromium catalysts. For example, an attempt was made to minimize the MI (raise the MW) by choosing a combination of catalyst and reaction variables that are all known to raise the polymer MW. A low pore volume Cr/silica was activated at only 600 °C, and the catalyst was treated with fluoride to increase the activity. It was then reduced in CO at 340 °C, again to improve activity and to lower the MI potential of the catalyst. To further lower the MI (actually the HLMI in... 

This phenomenon is rather specific. That is, a Cr/silica catalyst is preferred, not a Cr/silica-titania or a Cr/silica-alumina. It must be activated at a high temperature or treated with fluoride, perhaps to reduce potential ligands. Then it must be reduced in CO. When contacted with some cocatalysts, especially aluminum alkyls, the catalyst then becomes highly sensitive to H2. As illustrated in Table 60, in this series of experiments there was a huge jump in MI only when all of these treatments were combined. Activation at 600 °C does not work unless the catalyst also contains fluoride. Activation at 800 °C is effective without fluoride, but the effect is more pronounced with fluoride. The data shown in Figure 214 illustrate the huge shift in the MW distribution resulting from this combination of catalyst and reaction variables. 

As mentioned above, the a-olefin products that are generated in situ consist of two superimposed distributions a Schulz-Flory distribution of even-numbered linear a-olefins and a spike of 1-hexene. The amount of 1-hexene generated compared to the Schulz-Flory distribution and the flatness or sharpness of the Schulz-Flory distribution can be sensitively dependent on the catalyst, the cocatalyst, and the reaction conditions [27,238,681,682, 698-700]. 

The reaction is sensitive to steric hinderance. Aromatic ketones are reduced to hydrocarbons. Unsaturated ketones are fully reduced and with no selectivity. Complexes of the type Ir(Chel)(CH2=CH2)2Cl, with Chel = 2,2 -bipyridine or phenantholine derivatives, behave as catalyst precursors for hydrogen transfer from isopropanol to ketones and Schiff bases. Potassium hydroxide is required as cocatalyst to convert the isopropanol coordinated to the Ir(I) ion, in the neutral isopropoxy derivative. Enolates that are present would act as inhibitors when coordinated to the cationic derivative. Ethylene complexes are better precursors than the corresponding cyclooctadiene derivatives, because they are activated more easily and more completely, and they show high catalytic activity. The most active complexes is the 3,4,7,8-Me4 phen derivative, which, at 83°C, gives turnovers of up to 2850 cycles/min. Reduction of 4-r-butylcyclohexanone affords 97% of the tra/u-alcohol. 

The Stille reaction is somewhat more sensitive to the steric hindrances than the Negishi reaction involving organozincs. According to this, 2,2-, 2,2 -di-, and 2,6,2 -trisubstituted biaryls have been obtained in good yields [80], however the fully ortho-substituted biaryls, if any, were prepared in poor yields. The yields of extremely encumbered tetra-ort/io-substituted biaryls can be somewhat improved by addition of copper(I) bromide or copper(l) iodide as cocatalysts (2-4 eq. to Pd) [85,86]. 

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