Oligomerization catalyst

The oligo(cyclooctene) Co-OAc salen catalyst was then recycled without intermediate regeneration with acetic acid. As seen in Table 43.3, the initial rate of the oligomeric catalyst that was recycled after short reaction time (105 min) did not decrease significantly. However, the initial rate decreased substantially when the oligomeric catalyst was recycled after 12 h of the HKR reaction (run 2b). 

This result supports the idea that counterion addition to the epoxide also occurred with this material. The longer the exposure time of the catalyst to the HKR reagents (mainly epichlorohydrin), the greater is the extent of deactivation. Nevertheless, this oligomeric catalyst is certainly more effective than the monomeric catalyst as it can be used for shorter times to achieve comparable conversion and ee, thus reducing the influence of the reaction time on catalyst deactivation. 

Examples of w-allylnickel-X compounds (X = anionic ligand) other than 77-allylnickel halides which have been used in combination with (alkyl)aluminum halides as olefin oligomerization catalysts are 7r-allyl-nickel acetylacetonate (11) (Section III), 7r-allylnickel aziridide (4, 56), and bis(7r-allyl)nickel (6) (59). In addition to ir-allylnickel halides, organo-nickel halides such as tritylnickel chloride (60, 61) and pentafluoro-phenylbis(triphenylphosphine)nickel bromide (62), or hydridonickel halides, e.g., trans-hydridobis(triisopropylphosphine)nickel chloride (12) (Section III), give active catalysts after activation with aluminum halides... 

It is also important to note [13a] that for the generic catalyst, termination has a much lower barrier than insertion. Thus (HN=C(H)-C(H)=NH)NiC3H7+ is not going to be an efficient olefin polymerization catalyst. Rather, 2a, will at best be able to produce small oligomers of ethylene. This is in line with the experimental observation [16] that only bis-imines with bulky substituents are able to function as polymerization catalysts whereas less encumbered systems works as oligomerization catalysts. 

Metal-catalyzed oligomerizations are important reactions concerning both their chemistry and practical applications.12 36 37 Since the catalysts [(Ti(IV), Zr(TV), Ni(I), and Ni(II) complexes] used in coordination polymerization can also be active in oligomerization, catalyst systems and mechanisms will be discussed in more detail in Section 13.2.4. A short discussion of oligomerization with emphasis on the most important processes of practical significance, however, is appropriate here. 

Ziegler, Gellert, Holzkamp, Wilke, Duck and Kroll (72) have shown that the hydride transfer reaction of alkylaluminum occurs much more easily with trialkylaluminum than with the more electrophilic diethylaluminum chloride. Catalysts must be more anionic in order to produce oligomers which involve large amounts of hydride transfer than with polymerization catalysts where hydride chain transfer must be minimized. Thus the oligomerization catalyst employed by Bestian and Clauss was more anionic, or less cationic, than the usual polymerization catalyst where anionic chain transfer is minimized. 

Rh(diphos)(fj-BPh4) has been found to be a methylacetylene oligomerization catalyst which produces a variety of linear and branched dimers, as well as linear and cyclic trimers. When the reaction was carried out in the presence of carbon dioxide, a small quantity of 4,6-dimethyl-2-pyrone was also produced (115). 

Cai, EX. Studies of a new alkene oligomerization catalyst derived from nickel sulfate. Catal. Today, 1999, 51, 153-160. 

---