Ethylene insertion into

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)] ... 

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). 

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... 

FIGURE 55. Calculated reaction profile at the MP4//MP2 level of ethylene insertion into the Pt—H bond of PtH(PH3)2X. The calculated energies (kcalmoU1) refer to X = SnCl3 the numbers in parentheses refer to X = Cl. Reprinted with permission from Reference 172. Copyright 1998 American Chemical Society... 

Figure 3.18 Theoretical study of ethylene insertion into the Zr-C bond in the silylene-biscyclopentadienyl(methyl)zirconium cation [H2Si(Cp)2Zr—Me]+. E — energy...

The results of the MD simulations clearly demonstrate that the insertion starting from the higher energy isomers of the ethylene-chelate complexes in which the chelating bond has been broken have much smaller activation barriers, that are comparable to those observed in ethylene homopolymerization. This, however, does not explain the differences in the copolymerization activity of Pd and Ni-diimine complexes, as the barriers for the ethylene insertion into Ni-alkyl bond are smaller (14.2 kcal/mol) than those for Pd-alkyl bond (16.8 kcal/mol). Thus, it may be concluded that the ethylene insertion following the insertion of the polar monomer is not a crucial factor for the diimine catalyst copolymerization activity. It is the initial poisoning of the catalyst by formation of the... 

The reaction is stereoselective, 1,4-hexadiene being mainly obtained in the E configuration the Z-isomer is present only to a small extent and its formation can be further decreased by donor ligands such as Bu3P=0 and (Me2N)3P=0. The reaction is also regioselective the positional isomer CH2=CHCH(CH3)CH=CH2, derived from ethylene insertion into the more substituted carbon-rhodium bond of the allylic system, is present to less than 1 % when ethanol is in large excess. 

Surprisingly, competition of 3a/5a for ethylene under various reaction conditions gave no indication at all of C—C coupling via ethylene insertion into a zirconium-to-carbon o--bond (27). At low temperatures, i.e., under conditions when the (5-tram-/5-cw-butadiene)zirconocene equilibration (3a 5 5a) is sufficiently slow (22, 23), it is only the (s-trans-jf -diene)metallocene isomer (3a) that reacts with ethylene. With increasing temperature, increasing amounts of (s-crs-butadiene)ZrCp2 (5a) are also consumed, but only at a limited rate that allows restoration of the 3 5... 

The CO insertion reaction into the metal hydride bond is in fact a member of the class of ligand insertion reactions to which much theoretical work has been devoted (28,29-35). Some years ago we analyzed the ethylene insertion into the rhodium hydride bond of a Rh(III) hexacoordinated complex (. We later focused our attention on the CO insertion reaction into the Mn-H bond ofHMn(CO)5 (37-39) and very recently we have undertaken the study of the CO2 insertion reaction into the Cr-H bond of HCr(CO)5 (C. Bo and A. Dedieu, Inorg. Chem., in press). We will concentrate here on the CO insertion reaction and compare it to the two other insertion reactions. The study of the reaction (1) was carried out at both the SCF and... 

However, this concept cannot be transferred to the polymerization of 1-alkenes. Recently, critical remarks on the carbene mechanism of ethylene insertion into the Co—CHj bond were published Zambelli et al. performed an elegant study on the discrimination of the carbene mechanism for the stereospecific polymerization of propylene. According to the carbene mechanism the insertion of the first C3H6 molecule into the Mt—bond have to result in the formation of chain with an isobutyl end group i) enriched by in the methyl and methylene groups (scheme (17 a) >) ii) enriched by in the methyl group with threo- or erithro-configuration (see structures (12a) and (12b)). 

A recent detailed theoretical study of the platinum-catalyzed hydrosilylation of ethylene [15] led to a conclusion that this process proceeds through the Chalk-Harrod mechanism. The rate-determining step in this mechanism is the isomerization of the Pt(silyl)(alkyl) complex formed by ethylene insertion into the Pt-H bond, and the activation barrier of this step is 23 kcal moP for R = Me and -26 kcal mol for R = Cl). In the modified Chalk-Harrod mechanism, however, the rate-determining step is ethylene insertion into the Pt-SiRa bond and its barrier is 44 kcal moP for R = Me and 60 kcal moP for R = Cl. 

The product in this case consists of the corresponding acyl or aroyl derivatives. The CO insertion appears to be more rapid than ethylene insertion into the same bond, explaining why the activity of the nickel(II) precursor in the copolymerization reaction increases when carbon monoxide is absent in the initial stages of the process. 

Ethylene insertion into an Mg-C bond is possible under very mild conditions by an alkyl chain transfer via chaingrowing polymerization catalyzed by a lanthanocene. This new reaction is an efficient method for the synthesis of P-Mg-P1 compounds (P = C4-C2oo alkyl chain) (Scheme 257).91S,91Sa... 

Theoretical calculations at DFT level for ethylene insertion into Ti-Me bonds of cationic alkylamidinato complexes [TiMe(R1NGRNR1)2]+ (R = H, Ph R1 = H, SiMe3) have been performed,320 as have calculations for a bis(/5-diketonato)titanium model system (Scheme 137) in the presence of ethylene. Special attention is paid to the possible occurrence of agostic alkyl complexes and to the mechanism of ethylene uptake, chain propagation, and termination.321... 

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. 

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

Addition of ethylene to an allylic anion is not involved since 1- and 2-butene would give rise to a common intermediate and identical products would result. Preferential metalation of the terminal vinylic position of 1-butene also seems unlikely since products derived from vinylic lithium intermediates are not obtained with isobutylene. To explain the product structure, ethylene insertion into the allylic carbon-lithium bond may possibly involve both a four-centered and six-centered transition state (Reactions 11a and lib) with the latter predominating in butene-2. The exclusive formation of 2-methylpentene-l and its homologs from the telomerization with isobutylene is easily understood since this product would result from both reaction pathways. 

Ethylene inserts into the Pt—H bond of trans-PtCl(C2H4XPEt3)2-... 

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