Olefin complexes mechanisms

At 22 °C the overall second-order coefficient for reductions by Fe(n) and Cu(I) are, respectively, 4x10 and 5xl0 l.mole .sec Products in the presence of olefins demonstrate unequivocally the intermediacy of carbenoid transients, and a complex mechanism is put forward, viz. 

A key question remains how is the olefin formed in the overall process Molecular tantalum complexes are known to undergo facile a- and transfer processes, leading to tantalumalkylidene and tantalum tt-olefin complexes, respectively (mechanism 9, Scheme 29) [98]. Moreover, olefin polymerization with tantalum complexes belongs to the rare case in which the Green-Rooney mechanism seems to operate (Eq. 10, Scheme 29) [102]. Finally, intramolecular H-transfer between perhydrocarbyl ligands has been exemplified (Eq. 11, Scheme 29) [103,104]. 

Nickel(O) reacts with the olefin to form a nickel(0)-olefin complex, which can also coordinate the alkyl aluminum compound via a multicenter bond between the nickel, the aluminum and the a carbon atom of the trialkylaluminum. In a concerted reaction the aluminum and the hydride are transferred to the olefin. In this mechanistic hypothesis the nickel thus mostly serves as a template to bring the olefin and the aluminum compound into close proximity. No free Al-H or Ni-H species is ever formed in the course of the reaction. The adduct of an amine-stabihzed dimethylaluminum hydride and (cyclododecatriene)nickel, whose structure was determined by X-ray crystallography, was considered to serve as a model for this type of mechanism since it shows the hydride bridging the aluminum and alkene-coordinated nickel center [31]. 

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]. 

The calculated thermodynamic parameters in Table X are for 150 °C, a value close to typical operating temperatures, and without pressure corrections, there is no H2 and both alkane and alkene could be at unit concentration. The bottom part of the table is the same as Table VI. The free-energy profile of the transfer reaction mechanism is shown in Fig. 4. In the first step of the mechanism (IVR), sacrificial alkene (in this case ethylene) was bound to the (L)Ir(H)2 complex 4 to form an olefin complex 7. As mentioned above, the forward reaction of this step has no... 

Since molecular mechanics cannot be used to calculate the energy of transition states, suitable models were adopted. These models are extremely similar to the Jt-olefin complex with an orientation of the growing chain rather similar to that adopted when a a-agostic interaction is present. They were often called pre-insertion intermediates because the insertion transition state could be reached from these intermediates with a minimal displacement of the reacting atoms. 

Other possible mechanisms, corresponding to those discussed for the alkyne, have been considered. The oxidative addition pathway is excluded because all vinyl intermediates (see below) are found at high energy with respect to the n olefin complex. 

Examples of catalytic formation of C-C bonds from sp C-H bonds are even more scarce than from sp C-H bonds and, in general, are limited to C-H bonds adjacent to heteroatoms. A remarkable iridium-catalyzed example was reported by the group of Lin [116] the intermolecular oxidative coupling of methyl ethers with TBE to form olefin complexes in the presence of (P Pr3)2lrH5 (29). In their proposed mechanism, the reactive 14e species 38 undergoes oxidative addition of the methyl C-H bond in methyl ethers followed by olefin insertion to generate the intermediate 39. p-hydride elimination affords 35, which can isomerize to products 36 and 37 (Scheme 10). The reaction proceeds under mild condition (50°C) but suffers from poor selectivity as well as low yield (TON of 12 after 24 h). 

In aqueous hydrochloric acid solutions, mthenium(II) chloride catalyzed the hydrogenation of water-soluble olefins such as maleic and fumaric acids [6]. After learning so much of so many catalytic hydrogenation reactions, the kinetics of these simple Ru(II)-catalyzed systems still seem quite fascinating since they display many features which later became established as standard steps in the mechanisms of hydrogenation. The catalyst itself does not react with hydrogen, however, the mthenium(II)-olefin complex... 

The ozonolysis of olefins has been studied for over 60 years, and is stiU the subject of current research. The principle interest lies in the complex mechanism of dipolar additions, chain scissions, and recombinations that are the source of the complexity... 

Aluminium tri-n-alkyls are dimeric in solution, although - especially for higher alkyls - the fraction of monomer can become significant at higher temperatures [ 19] in the gas phase they are usually monomeric. Kinetic evidence indicates that olefin insertion involves a monomeric aluminium trialkyl this suggests a Cossee-type insertion mechanism. Kinetic data do not indicate the presence of an intermediate olefin 7c-complex [23]. However, if the olefin complexation energy at the 7c-complex stage is low, this would be expected. 

My last comment concerns the reaction of palladium olefin complexes with carbon monoxide discovered by Tsuji. I agree that this is most likely to proceed by an insertion rather than an ionic mechanism. Chloride attack on coordinated olefin is rare however. Chloride ion is an inhibitor, for example in the palladous chloride catalyzed hydration of ethylene (0). I, therefore, wondered whether carbon monoxide was affecting the ease with which chloride attacks olefin. One can postulate that carbon monoxide participates in this insertion either as a gas phase reactant or by first forming a carbonyl olefin complex. Such complexes of the noble metals were unknown, but examining the reaction between carbon monoxide and the halogen bridged olefin complexes of platinum revealed that they are formed very readily... 

The nature of the bonding in Zeise s anion and other r -olefin complexes is illustrated in Fig. 18.2. Without the push-pull mechanism, the 7r electrons of the olefin would have little or no tendency to allow themselves... 

Conjugated dienes can be dimerized or trimerized at their 1,4 positions (formally, 4 + 4 and 4 + 4 + 4 cycloadditions) by treatment with certain complexes or other transition-metal compounds.1088 Thus butadiene gives 1,5-cyclooctadiene and 1,5,9-cyclodo-decatriene.1089 The relative amount of each product can be controlled by use of the proper catalyst. For example, Ni P(OC6H4-o-Ph)3 gives predominant dimerization, while Ni(cyclooctadiene)2 gives mostly trimerization. The products arise, not by direct 1,4 to 1,4 attack, but by stepwise mechanisms involving metal-olefin complexes.1090... 

When an appropriate chiral phosphine ligand and proper reaction conditions are chosen, high enantioselectivity is achieved. If a diphosphine ligand of C2 symmetry is used, two diastereomers of the enamide coordination complex can be produced because the olefin can interact with either the re face or the si face. This interaction leads to enantiomeric phenylalanine products via diastereomeric Rh(III) complexes. The initial substrate-Rh complex formation is reversible, but interconversion of the diastereomeric olefin complexes may occur by an intramolecular mechanism involving an olefin-dissociated, oxygen-coordinated species (18h). Under ordinary conditions, this step has higher activation enthalpies than the subsequent oxidative addition of H2, which is the first... 

The mechanism of the polymerization of this monomer has been studied in far greater detail than any other. It is clear from the outset that a much more complex mechanism is involved than is the case for olefins. A large proportion of the initiator is used to form polymer whose molecular weight is only a few hundreds and the overall molecular weight distribution is so broad as to be rivalled only by those found in polyethylene produced by the high pressure process (19, 39). The initiator disappears almost instantaneously on mixing the reactants (19, 38). Under these conditions, an almost monodisperse polymer would be expected if chain transfer or termination processes are absent. 

Figure 1 outlines the key intermediates of a catalytic cycle where the rate-determining step is the formation of an n-alkyl derivative of the trans-bisphosphine via a coordinated olefin complex. This presumed catalytic cycle appears to satisfy proposals by Cavalieri d Oro et al. (9), C. V. Pittman et al. (10), and J. Hjortkjaer (II). Although no single mechanism of hydroformylation was established, the cycle is shown here to illustrate the key nature of the equilibrium between the trisphosphine (D) and the trans-bisphosphine (E). 

In the hydrosilylation of alkenes catalyzed by Group IV metallocene complexes, Cp2MCl2/2BuLi(M = Zr, Hf, Ti)37,38 (vide supra), the olefin-first mechanism including cr-bond metathesis of r/2-alkene-MCp2 and HSiR3 is proposed by Kesti and Waymouth (Scheme 9)37. After the formation of /J-silylalkyl—M—H species 56 via cr-bond metathesis... 

In this dissociative pathway (which is assumed to be the major one today) first a phosphine is displaced from the metal center to form an active 14-electron-intermediate 42. After alkene coordination cis to the alkylidene fragment the 16-electron-olefine-complex 43 undergoes [2 + 2]-cycloaddition to give a metallacylobutane 44. Compound 44 breaks down in a symmetric fashion to form carbene complex 45. The ethylene is replaced in the conversion to complex 46. In the next steps (they are not further discribed above), another intramolecular [2 + 2]-cycloaddition joins up the eight-membered ring 11 regenerating the catalyst 42. Each step of the reaction is thermodynamically controlled making the whole RCM reversible. With additional excess of phosphine added to the reaction mixture an associative mechanism is achieved, in which both phosphines remain bound. 

No precise information about the olefin polymerisation mechanism has been obtained from kinetic measurements in systems with heterogeneous catalysts analysis of kinetic data has not yet afforded consistent indications either concerning monomer adsorption on the catalyst surface or concerning the existence of two steps, i.e. monomer coordination and insertion of the coordinated monomer, in the polymerisation [scheme (2) in chapter 2], Note that, under suitable conditions, each step can be, in principle, the polymerisation rate determining step [241]. Furthermore, no % complexes have been directly identified in the polymerisation process. Indirect indications, however, may favour particular steps [242]. Actually, no general olefin polymerisation mechanism that may be operating in the presence of Ziegler-Natta catalysts exists, but rather the reaction pathway depends on the type of catalyst, the kind of monomer and the polymerisation conditions. 

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