Olefin molecule, activation

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

Complex 4a (see Fig. 1) differs from these catalytically active complexes only in the substitution of the complexed olefin molecules and hydrogen atom by a 7r-allyl group. The ligands in these square-planar molecules can adopt two different arrangements around the central nickel atom The olefin can either be trans (31a) or cis (31b) to the phosphine molecule. Because precedent exists for both these arrangements [e.g., 12 (84) and 30 (82)]. it is difficult to decide which of the two structures (31a or 31b) represents the catalytically active species. It is of course possible that the differences observed in the catalytic properties of systems having different ligands L and Y (Section IV) is due (at least in part) to differences in the population of 31a and 31b. 

According to what may be called the methyl separation mechanism, the polymerization reaction involves the rupture of a carbon-carbon bond of the olefin to give a methyl and an olefin radical which then add to the double bond of another olefin molecule (Kline and Drake, 60). The molecule of isobutylene, for example, behaves as though it were activated in the following manner CHa-+C(CH3)=CH2. The positive and negative signs indicate the relative electronegativities (Kharasch, 61) and are not intended to indicate ionization. The addi-... 

It will be noted from the above examples that the tertiary butyl carbonium ions required for the reaction are constantly being replenished to establish a chain reaction. It is assumed that the reaction is initiated by olefin molecules accepting protons from the catalyst to form carbonium ions which react with isobutane to produce the necessary active tertiary butyl carbonium ions. 

To clarify the course of the first addition of an a-olefin molecule to the active vanadium center, the reaction of pentene-1 with the soluble V(acac)3/A1(C2H5)2C1 catalyst was studied 85). After pentene-1 was reacted with the catalyst in toluene at —78 °C, the reaction mixture was taken out by means of a syringe after different periods of time, and hydrolyzed with cold water. 

Scheme 5.9 shows a possible mechanism for this epoxidation reaction. First, H202 reacts with a base site on the HT surface to form a HOO species, which attacks a nitrile to generate peroxycarboximidic acid as an active intermediate oxidant. The oxygen of the peroxycarboximidic acid is transferred to an olefin molecule. Interestingly, the resulting amide can be further employed for the epoxidation of olefins in the presence of a HT catalyst [44b]. 

A consensus on or explanation for the influence of the oxidation state of titanium on olefin polymerisation activity has not been reached. The absence of any insertion of the coordinating ethylene into the Ti-C bond in Ti(II) species is noteworthy instead, two ethylene molecules, which coordinate at two coordination sites at Ti(II) species, undergo an oxidative addition, and thus the respective metallacycle, titanacyclopentane, is formed [305], Such a reaction for dimethyltitaniumcomplexed by l,2-bis(dimethylphosphione)ethane [Dmpe] is as follows [305] ... 

One should assume here, in accordance with general opinion, that the productive reaction complex in methylaluminoxane-activated group 4 metallocene catalysts is the [Cp 2Mt(R).olefin]+[Alx(R) i. 1OxX2] complex, which is generated by displacement of an [A1x(R)a. 1OxX2] ion from its [Cp 2Mt(R)]+ counterion by the coordinating olefin molecule [30]. 

Similarly to the case of heterogeneous Ziegler Natta olefin polymerisation catalysts, the coordination of the olefin molecule at the cationic metallocene species, due to n bond formation (Figure 2.1), leads to lowering of the energy of the resultant n complex, e.g. the [Cp 2Mt(R)-olefin]1 [Alx(R)x OxX2] complex, which results in activation of the catalyst Mt-C bond and olefin C=C bond for the insertion reaction [136]. 

The main elements of chirality possibly present in the intermediates and transition states that can be hypothesised within this framework are as follows [1]. Firstly, a prochiral a-olefin molecule, e.g. propylene, coordinating via its two faces at the catalyst active site gives rise to non-superpo sable re and si diastereoisomeric complexes (Figure 3.24) [362, 363]. According to the considered mechanisms, an isotactic polymer is generated by a long series of... 

Figure 3.26 Schematic drawing of a lateral cut of a violet TiCl3 layer. In the more hindered inward (z) position, the chlorine atom is replaced by the alkyl group from the activator, and within the polymerisation by the growing polymer chain. In the less hindered outward (o) position, the a-olefin molecule is coordinated. The chirality, A and A, of two titanium atoms in indicated. - 77 O and o Cl - vacant positions. Reproduced (by permission from Elsevier Science) from Ref. 1. Copyright 1989 Pergamon Press...

The bilateral coverage of each coordination site in such a catalytic active site by a CH and a CH3 group (both in a /1-position to the bridgehead C atom) appears to render them indifferent to the enantiofacial orientation of an incoming a-olefin molecule, and thus atactic poly(a-olefin) is formed [30]. 

If only the thermal energy of the bombarding olefin molecules was available to overcome the activation energy barrier for reaction then there should be a marked dependence of the conversion on the jet temperature. The results (Fig. 15) show clearly that this is not so and in fact the points for different temperatures all fall on the same curve. Therefore the thermal energy of the alighting molecules is unimportant. Either the reactions must occur at 77°K after the molecules have lost their thermal energy, or if they do occur immediately after bombardment... 

Mechanism of stereoregulation on the basis of the data on polyolefin stereoregularity. The structure of a polymer chain is the recording of events proceeding in the insertion of olefin molecules into an active metal-carbon bond. To understand the stereochemistry of the propagation reaction, the data on the stereoregular structure of polymer chains are important. Recently, for this purpose, C-NMR spectroscopy has been extensively used... 

Addition of two X- to two olefin molecules, as in Eq. (40) [267]. This addition reaction prefers activated olefins. 

Quantum-chemical calculations showed that Co and Ru in cluster have different affinity to CO2, CO and O2 molecules. Ru is characterized by lower affinity to CO than Co, but more high affinity to oxygen. So it may be suggested that during the CO2 dissociation on the Co-Ru-clusters the preferable formation of new bonds Co-COads and Ru-Oads takes place. In complex Ru-Oads metal has positive charge and could activate the olefin molecule - typical donor of electrons. 

To clarify the mechanism of propylene adsorption on Ru-Co clusters the quantum-chemical calculation of interaction between it and Ru-Co, Ru-Ru, and Co-Co clusters were carried out. During the calculation it was assumed that carbon atoms of C-C bond are situated parallel to metal-metal bond. The distance at which the cluster and absorbable molecule begin to interact is characterized by the nature of active center. Full optimization of C3H6 molecule geometry confirms that propylene adsorbs associatively on Co-Co cluster and forms Jt-type complex. In other cases the dissociate adsorption of propylene is occurred. The presence of Ru atom provides significant electron density transfer from olefin molecule orbitals to d-orbitals of ruthenium in bimetallic Ru-Co- or monometallic Ru-Ru-clasters (independently on either the tertiary carbon atom is located on ruthenium or cobalt atom.). At the same time the olefin C-C bond loosens substantially down to their break. 

Doyle has put forward arguments against the intermediacy of such complexes in catalytic cyclopropanation . Firstly, metal coordination activates the alkene to nucleophilic attack. Hence, an electrophilic metal carbene would add only reluctantly or not at all. Secondly, the stable PdCl2 complexes of dienes 8 and 428 do not react with ethyl diazoacetate, even if Rh fOAc) or PdCljfPhCbOj is added. The diazoester is decomposed only when it is added to a mixture of the Pd complex and excess diene. These results exclude the metal-carbene-olefin intermediate, but they leave open the possibility of metal carbene interaction with an uncomplexed olefin molecule. The preferred formation of exo-cyclopropanes in the PdCyPhCN) -catalyzed reactions between 8 and N2CHCOOEt or N2CPh2, with exo. endo ratios virtually identical to those observed upon cyclopropanation of monoolefin 429, also rule out coordination of a palladium carbene to the exocyclic double bond of 8 prior to cyclopropanation of the endocyclic double bond. 

Fig.6 shows PO yield over 8 wt % Au/TiOj/SiO, as a function of time. The catalytic activity of Au/TiOj/SiOj catalyst is not stable. Water is continuously formed during the oxidation of propylene and the oxygenated intermediates may block the active sites and depress the adsorption of propylene on the surface of the catalyst. MCM materials have hydrophobic character and Ti-MCM preferentially adsorbs less polar olefin molecules. This decreases the competition from water and probably avoid the accumulation of the oxygenated intermediates to lead to more stable catalytic activity. 

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