Insertion-elimination mechanism

Operation of the insertion-elimination mechanism has been demonstrated in the reaction of rhodium hydride complex, RhHL4 (L=PPh3), with two isomeric allyl phenyl carbonates [56]. Unbranched 2-butenyl phenyl carbonate was found to give branched allylic phenyl ether exclusively, whereas the decarboxylation of the branched l-methyl-2-propenyl phenyl carbonate afforded unbranched 2-butenyl phenyl ether. These results can be accounted for by assuming a precata-lytic and catalytic insertion-elimination process as shown in Scheme 7. 

Mainly two alternative mechanisms are discussed for the linear dimerization and oligomerization of monoolefins catalyzed by transition metal systems an insertion-elimination mechanism via a metal hydride (alkyl) species " and metallacycle pathways " . 

MS [30]. Theses results prove the proposed the insertion-elimination mechanism (see equations 2 and 3). 

Why is the metallacycle mechanism able to give selective trimerization while the insertion/elimination mechanism found for A1-, Zr-, or Ni-catalyzed ethylene... 

In ethylene oligomerization, oxidative addition plays no role. In the insertion-elimination mechanism, the metal oxidation state is constant throughout the catalytic cycle. In the metallacycle mechanism, the oxidative step is an oxidative coupling reaction (see below). 

Extrusion is the reverse reaction of insertion (CN +1, VE +2, ON unchanged). The reaction plays a very important role in ethylene oligomerization according to the insertion/elimination mechanism as the so-called P-H-elimination. Scheme 6.16.6 illustrates this elementary step for the extrusion of a 1-hexene product from a Ni-hexyl complex. The extrusion step is followed by 1-hexene dissociation from the complex (see above) to finally liberate the 1-hexene product. Below, in Scheme 6.16.8, the extrusion step is shown as part of a more complex reaction sequence for the liberation of a 1-alkene product from a chromium metallacyclic intermediate. 

Ethylene oligomerization catalyzed by AI-, Zr-, and Ni-complexes follows a so-called insertion/elimination mechanism that results in the production of 1-alkene mixtures of different chain lengths. The mechanistic reason for this product distribution is the fact that each metal-alkyl complex shows the same probability of chain growth independent of the chain length of the attached alkyl group. 

Oxidative addition of molecular hydrogen was considered to be involved in the alkyne hydrogenations catalyzed by [Pd(Ar-bian)(dmf)] complexes (4 in Scheme 4.4) [41, 42]. Although the mechanism was not completely addressed, 4 was considered to be the pre-catalyst, the real catalyst most likely being the [Pd(Ar-bian)(alkyne)] complex 18 in Scheme 4.11. Alkyne complex 18 was then invoked to undergo oxidative addition of H2 followed by insertion/elimination or pairwise transfer of hydrogen atoms, giving rise to the alkene-complex 19. 

Reactions of alkynyliodonium salts 119 with nucleophiles proceed via an addition-elimination mechanism involving alkylidenecarbenes 120 as key intermediates. Depending on the structure of the alkynyliodonium salt, specific reaction conditions, and the nucleophile employed, this process can lead to a substituted alkyne 121 due to the carbene rearrangement, or to a cyclic product 122 via intramolecular 1,5-carbene insertion (Scheme 50). Both of these reaction pathways have been widely utilized as a synthetic tool for the formation of new C-C bonds. In addition, the transition metal mediated cross-coupling reactions of alkynyliodonium salts are increasingly used in organic synthesis. 

Experimental data are also consistent with a dissociative oxygen insertion pathway initiated by the dissociation of the anion PhCOO-, followed by coordination of 02, intramolecular hydrogen migration, and recoordination of PhCOO-. This mechanism has been ruled out on the basis of density functional calculations that placed the free energy of the intermediate for O2 insertion at about 120 kJ/mol above the energy of the corresponding intermediate in the reductive elimination mechanism. 

In other words, a SiO/Me exchange is the result of compounding two improbable insertions, so that simple addition/elimination mechanisms predict SiO/H or Me/H to be the more common exchanges (other than the degenerate H/H exchange). Since this prediction is contrary to fact, some other mechanism must be responsible for SiO/Me exchanges. 

There are two possible mechanisms to this observed elimination, simple reductive elimination and a migratory insertion/decomplexation sequence [22], Normally, a simple reductive elimination mechanism is observed [140], but occasionally a migratory... 

Hydrogen. The reaction of O ( D2) with H2 takes place on the ground state potential surface of water, HiOf Ai). On the basis of trajectory calculations, (Whitlock et al., 1982) it has been suggested that, as is true for the hydrocarbons, parallel mechanisms involving insertion/elimination and direct abstraction govern the course of this reaction. The observation using laser induced fluorescence spectroscopy (Luntz et al., 1979 Smith and Butler, 1980) of a highly excited, non-Boltzmann rotational distribution and a nearly statistical vibrational distribution for v" = 1 and v" = 0 is consistent with the insertion/elimination... 

In addition to isolation and characterization of the ruthenacycle complexes 18 or 32, the detailed reaction mechanism of the [2 + 2 + 2] cyclotrimerization of acetylene was analyzed by means of density functional calculations with the Becke s three-parameter hybrid density functional method (B3LYP) [25, 33]. As shown in Scheme 4.12, the acetylene cyclotrimerization is expected to proceed with formal insertion/reductive elimination mechanism. The acetylene insertion starts with the formal [2 + 2] cycloaddition of the ruthenacycle 35 and acetylene via 36 with almost no activation barrier, leading to the bicydic intermediate 37. The subsequent ring-... 

The two established pathways for transition metal-catalyzed alkene isomerization are the jr-allyl metal hydride and the metal hydride addition-elimination mechanisms. The metal hydride addition-elimination mechanism is the more common pathway for transition metal-catalyzed isomerization. In this mechanism, free alkene coordinates to a metal hydride species. Subsequent insertion into the metal-hydride bond yields a metal alkyl. Formation of a secondary metal alkyl followed by y3-elimination yields isomerized alkene and regenerates the metal hydride. The jr-allylhydride mechanism is the less commonly found pathway for alkene isomerization. Oxidative addition of an activated allylic C-H bond to the metal yields a jr-allyl metal hydride. Transfer of the coordinated hydride to the opposite end of the allyl group yields isomerized alkene. 

The other type of process of C-0 bond activation that is different from the direct oxidative addition of the C-0 bond to M(0) complexes to form r 3-allyltransition metal complexes is insertion-elimination type or SN2 type as shown in Eqs. 4 and 5. Although the two processes are conceptually different, it is sometimes difficult to distinguish the two mechanisms. When the insertion-elimination process... 

The net H2 elimination process is predicted to be nearly thermoneutral. The highest-energy transition state on this path has an energy requirement of 5-9 kcal/mol, depending on the level of theory employed. This may be contrasted with recent DFT results on Co and Fe mediated H2 elimination from C2H6. These latter calculations find a lower C-H insertion transition state, perhaps due to deeper wells corresponding to the initial M+-ethane complexes. The calculations also predict that the 1,2-elimination is favored over the 1,1-elimination mechanism. The net CH4 elimination is predicted to be rather endothermic at all levels of theory, with a high C-C insertion barrier. This is consistent with the lack of experimental observation of CH4. 

The mechanism of trimerisation of butadiene by a mixed cobalt(ii) chloride-aluminium triethyl catalyst has been inferred from the natures of the three products characterised. The determination of the enthalpy of dimerisation of aluminium triethyl provides a useful piece of thermochemical data for quantitative discussion of the role and energetics of aluminium triethyl in this type of reaction. Polymerisation of isoprene in the presence of Fe(acac)3-aluminium triethyl-pyridine derivatives mixtures has a negative apparent activation enthalpy, which can be attributed to the instability of the catalytic complex at elevated temperatures. Bis-cyclo-octatetraeneiron(o) is an effective oligomerisation catalyst. The composition of products accessible only by hydrogen migration indicates an oxidative addition-reductive elimination mechanism rather than insertion. 

Certain low-valent early transition metal complexes catalyze the dimerization of ethylene and propylene selectively to 1-butene and 2,3-dimethyl-l-butene. The regioselec-tivity of this dimerization of propene signals a different mechanism than the insertion and elimination mechanism presented in the previous section. The formation of 1-butene occurs selectively because of the absence of a persistent metal hydride complex that isomerizes this olefin to the more stable 2-butene. 

While in alkane metathesis mechanism (Scheme 20, b), the n-decane undergoes o-bond metathesis to generate methane and the W-bis-decyl species which, upon P-H elimination, produces the W-H with a coordinated olefin. Further, the a-hydrogen transfer from the alkyl to alkylidyne forms the hydrido W-bis-carbene [55, 76]. This upon [2-1-2] cycloaddition and cycloreversion gives an internal olefin and hydrido W-bis-carbene. Successive insertion/elimination steps (by chain walking) [77] give the terminal alkene, which reacts to a new W-alkylidene. The CH activation of the pendant W-hydride with -decane followed by p-H elimination provides 1-decene. A second metathesis between 1-decene and newly formed W-alkylidene followed by hydrogenolysis produces the alkane. 

The mechanism of Ni-catalyzed ethylene oligomerization involves both nickel hydride and nickel alkyl species. The mechanism is known in the literature as the metal-hydride mechanism, Cossee-Arlman mechanism, or ethylene insertion - -hydride elimination mechanism and results in a Schulz-Flory distribution of the oligomerization products. The mechanism is depicted in Figure 6.16.4. Note that two other coordination sites at the nickel are occupied by one bidentate ligand or two monodentate ligands (see Section 2.4 for details) that have been omitted in Figure 6.16.4 for clarity. 

A digital functional approach has been employed to investigate important steps in the Heck reaction catalyzed by a bis(carbene)Pd complex and one in which the Pd is coordinated by a bidentate carbene-phosphine ligand. The crucial steps of olefin insertion into the palladium-aryl bond and / -hydride elimination were investigated. For the bis(carbene)Pd catalyst, a mechanism was proposed, which proceeds via halide abstraction, to give a cationic species, prior to olefin coordination and insertion. The total insertion/elimination process was found to be exothermic (—8.9 kcal moP ). For the carbene-phosphine ligated system, the vacant site for olefin coordination was provided by phosphine dissociation. The energetics for the total insertion/elimination process was very similar to that of the bis-carbene system. 

The proposed mechanism for acetylene polymerization is shown in Scheme II. It assumes a typical Ziegler-Natta insertion-type mechanism with a cis approach of the monomer leading to ds-polymer. TWo possible modes of termination are assumed j8-hydride elimination and terminal cyclization (Scheme III). The first route produces an acetylenic tail, and the second gives a ben-zylic end. 

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