Ethylene insertion/elimination

Based on our observation in these two systems, it would appear that Cp Cr -alkyls, if rendered electrophilic and/or sufficiently coordinatively unsaturated, will both bind and insert a-olefins. However, the more heavily substituted alkyl ligands thus formed (i.e. CrBl-CH2-CH(R)-P vs. Crni-CH2-CH2-P resulting from ethylene insertion) seem to be very susceptible to facile 3-hydrogen elimination. Rapid chain transfer and very low molecular weights are the results of this tendency. Whether the latter is an innate property of all chromium alkyls or reflects the particular chemical nature of the Cp Cr-moiety remains to be established. To this end, investigation of chromium alkyls with a variety of other ancillary ligands are needed. 

Scheme 13.2 Ethylene insertion followed by elimination of 2-ethylimidazolium...

The mechanism of the unprecedented chromium-catalysed selective tetramerization of ethylene to oct-1-ene has been investigated. The unusually high oct-1-ene selectivity of this reaction apparently results from the unique extended metallacyclic mechanism in operation. Both oct-1-ene and higher alk-l-enes were formed by further ethylene insertion into a metallacycloheptane intermediate, whereas hex-1-ene was formed by elimination from this species as in other trimerization reactions. Further mechanistic support was obtained by deuterium labelling studies, analysis of the molar distribution of alk-l-ene products, and identification of secondary co-oligomerization reaction products. A bimetallic disproportionation mechanism was proposed to account for the available data.120... 

As far as the fate of 6.6 is concerned, there is a third possibility. The polymer chain may remain attached to the metal atom that is, the metal-alkyl bond remains intact. The product of the overall reaction in this case is 6.6. Polymerization of this type is often called living polymerization. Note that in this case there is no polymer chain termination step, and the catalytic cycle is not completed. In other words, although a single Ti4+ ion may be responsible for the polymerization of thousands of ethylene molecules, in the strictest sense of the term it is not a true catalytic reaction. In the event 6.6 is converted to 6.7 either by reaction with H2 or by /8-elimination, 6.7 further reacts with ethylene. Insertion of ethylene into the Ti-H bond regenerates 6.2 and completes the catalytic cycle. 

It is largely accepted that the active species in ethylene oligomerization is a nickel hydride species like 8. The mechanism for the hydride formation is supported by the reactions depicted in eqs. (7)-(9). Complex 9 eliminates butadiene at low temperatures and becomes active at 40 °C. Insertion of ethylene and elimination of styrene from structure 4 at 70 °C causes the complex to become active, while the more strongly bound cyclopentadienyl ligand in structure 10 needs 130 °C [49]. The elimination products of these reactions could be detected by GLC. 

Carbonyl groups are also utilized in catalytic C-C bond cleaving reactions. Under catalytic conditions, 8-quinolyl phenyl ketone 85 reacts with ethylene to give 8-quinolyl ethyl ketone 86 and styrene in quantitative yield [105]. Styrene is formed by cleavage of the phenyl-carbonyl bond, followed by ethylene insertion into the resultant phenyl-rhodium bond, and (3-hydride elimination. The accompanying formation of a rhodium-hydride complex is followed by incorporation of ethylene to furnish the ethyl ketone 86. 

A cationic nickel-hydride is the proposed catalytically active species. Insertion of norbornene results in the nickel-norbomyl cationic intermediate, where R is H originating from the nickel-hydride. Under the conditions employed, ethylene insertion is competitive with the first norbornene insertion, followed by norbornene insertion to yield the nickel-norbornyl intermediate where R is ethyl. The second norbornene insertion is stereoselective about 96% of the time a meso enchainment results. Insertion of ethylene, followed by -hydrogen ehmination forms dimers 4 and 5 (as well as 4% of the isomeric rac dimers). Insertion of the third norbornene is non-stereoselective or random the trimeric product of m so or rac insertion is isolated after ethylene insertion and /l-hydrogen elimination (trimers 6 and 7, where R=H, along with the trimers where R=ethyl). Extrapolation of the random insertion of norbornene observed in the trimers to the high polymer predicts that the nickel-based poly(norbornene) is best described as atactic which should result in a more soluble polymer than a more regular poly (norbornene). 

Ligand-promoted reductive elimination of ketones is directly observed with alkyl-acyl rhodium complexes formed via oxidative addition of either Wilkinson s catalyst or [ethylene insertion into the metal hydride bond33-37. 

Relative Eneigies of Stationary Structures in Ethylene Insertion and jS-Elimination with [H2SiCp2MCH3] (Kiloealories per Mole) Calculated... 

Another catalytic cycle studied by Matsubara, Morokuma, and coworkers [77] is the hydroformylation of olefin by an Rh(I) complex. Hydroformylation of olefin by the rhodium complex [78-80] is one of the most well known homogeneous catalytic reactions. Despite extensive studies made for this industrially worthwhile reaction [81, 82], the mechanism is still a point of issue. The active catalyst is considered to be RhH(CO)(PPh3)2, 47, as presented in Fig. 25. The most probable reaction cycle undergoes CO addition and phosphine dissociation to generate an active intermediate 41. The intramolecular ethylene insertion, CO insertion, H2 oxidative addition, and aldehyde reductive elimination are followed as shown with the surrounding dashed line. Authors have optimized the structures of nearly all the relevant transition states as well as the intermediates to determine the full potential-... 

Mechanistic investigations with special nickel complex catalysts have shown that nickel hydrides with chelating P-0 groups are the catalytically active species. The metal hydride reacts with ethylene to give alkylnickel intermediates, which can grow further by ethylene insertion or eliminate the corresponding a-olefins. A simplified mechanism is shown in Scheme 3-8 [9],... 

Though these two products cannot be interconverted (i.e. no evidence of ethylene insertion or elimination), an over-pressure of CO does induce migratory insertion for 438 yielding Tp Rh C( = 0)Et (C6H5)(C0) (439), which reductively eliminates PhC( = 0)Et when treated with ZnBr2. 

The butyl product of the second ethylene insertion undergoes p-hydrogen elimination (see Chapter 10) to generate free butene and the starting rhodium hydride. Thus, the rhodium complex catalyzes the dimerization of ethylene to form butene. 

However, it is worth emphasizing that since the Si—C reductive elimination needs much greater activation energy than the oxidative addition of H—SiMea and ethylene insertion into the Rh—SiMes bond, the modified Chalk-Harrod mechanism is more favorable than the Chalk-Harrod mechanism in the Rh-catalyzed hydrosi-lylation of ethylene (43). 

There is an obvious problem with this route Why does the polymer chain not chain-terminate by p elimination The answer seems to be that the high-valent d metal has insufficient ability to back-donate in order to break the C—H bond recall that 3.7 failed to -eliminate for the same reason. A second difficulty is that ethylene insertion into an alkyl group is rather rare (see Section 3.3). 

Burger, B. J. Thompson, M. E. Cotter, W. D. Bercaw, J. E. Ethylene insertion and -hydrogen elimination for permethylscandocene alkyl complexes. A study of the chain propagation and termination steps in Ziegler-Natta polymerization of ethylene. J. Am. Chem. Soc. 1990,112, 1566-1577. 

C NMR spectroscopy showed that CPE was preferentially incorporated in the copolymers through the 1,2-insertion mode, without ring-opening metathesis. The 1,2-enchainment pattern is a result of the facile coordination of ethylene to the active metal center ((-hydride elimination of a CPE-ended chain, which is needed to form a 1,3-enchained CPE unit, is relatively slow compared to ethylene insertion. In the NMR spectrum of the copolymer containing 28 mol% CPE, short blocks of PCPE units were detected. 

To calculate the product distribution of ethylene oligomerization, the reaction orders for the insertion and the elimination steps with respect to ethylene are highly relevant. For most ethylene oligomerization processes, it is reasonable to assume that ethylene insertion is first order with respect to ethylene concentration while elimination is zero order with respect to ethylene. Under these preconditions, Eq. (6.16.1) describes the so-called coefHcient that gives the ratio of the rates of elimination over insertion (Onken and Behr 1996) ... 

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