Migratory insertion, ethylen

The osmium hydroxycarbene complex 18 is formed in an acid-assisted migratory-insertion reaction of OsClEt(CO)2(PPh3)2. This alkyl compound results from reaction of the ethylene adduct 19 with one equivalent of acid 46). 

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... 

We have delineated viable coordinated ligand reactions and their attendant intermediates for the stoichiometric conversion of CO ligands selectively to the C2 organics ethane, ethylene, methyl (or ethyl) acetate, and acetaldehyde. We now outline results from three lines of research (1) T -Alkoxymethyl iron complexes CpFe(C0)2CH20R (2) are available by reducing coordinated CO on CpFe(C0)3+ (1) [Cp = r -CsHs]. Compounds 2 then form t -alkoxyacetyl complexes via migratory-insertion (i,e. CO... 

The first step is coordination of the ethylene through its n orbital. The ethylene is trans to Cl with the C=C bond in the Cl-Ru-H plane. Facile migratory insertion (AE = 7.6 kcal.mol 1) of the coordinated ethylene in the Ru-H bond leads to an alkyl intermediate 6.2 kcal.mol 1 less stable than the n ethylene complex. The alkyl intermediate has a strong P C-H agostic interaction as illustrated by the unusually long agostic C-H bond (1.221 A) which helps to stabilize the unsaturation in the formally 14-electron alkyl intermediate. 

Based on DFT calculations Brandt et al. proposed a catalytic cycle via Ir(III) and Ir(V) intermediates, in which an additional dihydrogen molecule coordinated to an Ir-dihydride undergoes oxidative addition during migratory insertion [31]. However, since an extremely truncated model for the ligand and substrate (ethylene) was used which neglected the severe steric interactions present in the actual catalysts it... 

Evidence for the migratory insertion of ethylene [46] and vinylsilane [47] into the Ru-Si bond yielding vinylsilane and two bis(silyl)ethene regioisomers [E-l,2-bis(silyl)ethene and l,l-bis(silyl)ethene],respectively,has proved that in the reaction referred to as the metathesis of vinylsilanes and their cometathesis with olefins, instead of the C=C bond cleavage formally characterizing alkene metathesis (Eq. 24a), a new type of olefin conversion that is a silylative coupling of olefins with vinylsilanes occurs (Eq. 24b). 

A mechanistic scheme of this new type of silyl olefin conversion involves the migratory insertion of the olefin into the Ru-Si bond and vinylsilane into the M-H bond followed by /3-hydrogen and /3-silicon elimination to give l,2-bis(silyl)ethenes, l,l-bis(silyl)ethenes and ethylene (Scheme 1) [46,47]. 

Figure 6.3 Proposed catalytic cycle for ethylene polymerization by the Ziegler catalyst. From 6.5 to 6.6 n — 2 ethylene molecules undergo insertion. Note the alteration of coordination sites between the polymer chain and coordinated ethylene, that is, migratory insertion, is assumed.

Alkyl iridium compounds are also accessible via insertion (see Migratory Insertion) of alkenes into Ir H bonds. Analogously, alkenyl iridium compounds may be formed via insertion of alkynes into Ir-H bonds. These types of reactions have been studied to shed tight on the mechanism of alkene and alkyne hydrogenation processes. For example, HIr(CO)(PPh3)2 (65) will react with ethylene and higher olefins to produce the alkyl iridium compounds (equation 17). 

The mechanisms of alkene insertions are known for some square-planar Pt(II) complexes , but it is difficult to distinguish between pathways in which a four-coordinated alkene/hydride species collapses by migratory insertion to a three-coordinated (perhaps solvated) species, and pathways in which a five-coordinated alkene/hydride collapses to a four-coordinated product. Both pathways occur, depending on the ligands. The rates of insertion of ethylene into the neutral and cationic hydrides trans-PtHX(PEt3)2 and trans-[PtHL(PEt3)2] + (X = Cl , NOj", CN L = acetone, CO, PEtj, AsPha, P(OMe)3 and P(OPh)3) leads to Scheme 1 °. 

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