Insertion ethylene

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 most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

Additionally, and interestingly, the DFT calculations suggest that the Zr-N bonds that lie on the same plane as the polymerization sites expand and contract according to the reaction coordinate of the ethylene insertion (2.23-2.34 A), while the Zr-O bond length remains virtually unchanged (Fig. 13). From studying these results, we... 

Ethylene polymerization by Ni(II) a-diimine catalysts is over 1000 times more active than that of the analogous Pd(II) catalysts [25]. The respective barriers for migratory insertion have been determined by II NMR analysis at low temperature and reflect the polymerization rate difference. While the barrier to ethylene insertion... 

NMR studies clearly indicated a rapid and reversible ethylene insertion-deinsertion process, as illustrated in Eq. (18). 

Figure 4.77 The optimized structure of the transition state II for the ethylene-insertion reaction II III (4.106), with forward activation energy A > = 6.90 kcalmol-1 relative to the metal-ethylene complex II.

Figure 4. Structures resulting from ethylene insertion and chain termination due to the generic catalyst (HN=C(H)-C(H)=NH)PdC3H7+. Ethylene complex (3a) insertion transition state (TS[ 3a-4a]) termination transition state (TS[3a-5a) new olefin product from termination process (5a).

Though only an alkyl complex is possible in the ethylene insertion, there are several possible products of the C02 insertion, such as p -OCOH, p2-02CH, and p -COOH species, as shown in Figure 4. Thus, the following issues were investigated for the C02 insertion reactions (1) Which species is more easily formed, the metal-formate (M-OCOH) or the metal-carboxylic acid (M-COOH) (2) What are the most important interactions in the C02 insertion (3) How different is the C02 insertion from the C2H4 insertion ... 

The polymerization of ethylene was also qualitahvely inveshgated by pulse injec-hons of ethylene into helium flowing over thorium (67) and uranium (86) metallocene hydrocarbyl complexes supported on 7-AI2O3.950 at 25 °C, both revealing similar achvihes [171, 173]. Supported thorium half-sandwich complexes 65 exhibited higher achvity than surface species, resulhng from coordinatively more saturated tris(cyclopentadienyl) and metallocene U/Th-alkyl/hydride complexes, that is, 77, 79, 82, 90 and 91 [171]. C CP MAS NMR spectra revealed no clear evidence of ethylene insertion into [Th-CHs] or [AL5-CH3] moiehes of material... 

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

Scheme 9.11 depicts trans addition of the external nucleophile water resulting in a transoid P-hydroxyethylpalladium intermediate (66).498 Studies with stereoiso-meric DHC=CHD molecules support this view.509,510 Results with D20, however, indicates no deuterium incorporation into the product molecule. This is consistent with an intramolecular cis addition of HO-, namely, ethylene insertion into the Pd-O bond [70 to 71, Eq. (9.101)] ... 

Figure 3. a) Predicted correlation between the ethylene-insertion barrier AEa and (51V) for V(=0 X)Me3 complexes (Adapted from ref. 12b). b) The same for V(=Y)Me3 complexes. (Figure 3a is adapted with permission from reference 12b. Copyright 1998 Wiley-VCH.)... 

Table II Predicted 51V Chemical Shifts and Ethylene Insertion Barriers for V(=Y)Me3...

Beyond this work, the chemistry of neutral r 2-ethylene complexes remains largely unexplored. However, it has been reported that in the presence of B(C6F5)3 at 60 °C, 158 converts to the platinacyclic complex 176 (Scheme 13).67 This conversion, which also occurs in the absence of the borane at 80 °C, results from ethylene insertion into the Pt—Fh linkage, followed by rapid intramolecular orthometallation. A comparable reaction has also been noted with propene, though in this instance the intermediate r 2-complexes are never observed (see also Section III.C.l). 

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