Ethylene dimerization, oxidative

Deactivation processes competing with fluorescence are mainly nonradiative deactivation to the S0 state (IC) and nonradiative transition to a triplet state (intersystem crossing, ISC). Photochemical products are often formed from this triplet state. Important photochemical reactions are the E—yZ isomerization of ethylene, the oxidation of pyrazoline to pyrazole, and the dimerization of cou-marins. 

Although the actual oxidation state of chromium in the active catalyst is unclear, the reaction has again been interpreted as an ethylene dimerization leading to a metallacycle, in this case followed by ethylene addition and p-hydrogen elimination (Figure 16). 

Nickel oxide and nickel complexes supported on silica, silica-alumina, different zeolites, and polymeric materials have been reported to be active for ethylene dimerization. Yashima et reported that ethylene dimerization can... 

The aqueous solution layer that forms at the metal interface can ultimately provide a medium for the dissolution of Pd ions or oxidized Pd clusters into the supported liquid layer where they can then act as homogeneous catalysts. As was discussed earlier, the acetoxylation of ethylene can be carried out over various Pda,OAcj, clusters where alkali metal acetates are typically used as promoters. DFT calculations were carried out on both the Pd2(OAc)2 and Pd3(OAc)e clusters in order to examine the paths that control the solution-phase chemistry. The Pd3(OAc)e cluster is the most stable structure but is known experimentally to react to form the Pd2(OAc)2 dimer and monomer complexes in the presence of alkali metal acetates. The reaction proceeds by the dissociative adsorption of acetic acid to form acetate ligands. Elthylene subsequently inserts into a Pd-acetate bond. The cation is then reduced by the reaction to form the neutral Pd°. The reaction is analogous to the Wacker reaction in which ethylene is oxidized over Pd + to form acetaldehyde. Pd° is subsequently reoxidized by oxygen to form pd2+[35,36,44]... 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

A -Piperideine-N-oxide was obtained along with a dimeric product by oxidation of N-hydroxypiperidines with mercuric acetate or potassium ferricyanide (107-109). 2l -Pyrroline-N-oxide is formed by oxidation of N-ethylpyrrolidine with hydrogen peroxide with simultaneous formation of ethylene (110). 

Some types of reactions involving gases that have been studied in IFs are hydrogenations [16, 25-37 ], oxidations [38, 39], and hydroformylations [25, 40 5]. In addition, some dimerizations and allcylations may involve the dissolution of condensable gases (e.g., ethylene, propylene, isobutene) in the IF solvent [46-50]. 

The phase behavior of several polybibenzoates with oxyalkylene spacers has been reported [11,14,15,20-27]. These spacers include the dimer of trimethylene glycol and different ethylene oxide oligomers. The most noticeable characteristic of these polybibenzoates with ether groups in the spacer is the considerable decrease of the rate of the mesophase-crystal transformation. Thus, Fig. 8 shows the DSC curves corresponding to a sample of poly[oxybis(trimethylene)p,p -bibenzoate], PDTMB, with a structure similar to that of P7MB but with the... 

An alternative route to PET is by the direct reaction of terephthalic acid and ethylene oxide. The product bis(2-hydroxyethyl)terephthalate reacts in a second step with TPA to form a dimer and ethylene glycol, which is released under reduced pressure at approximately 300°C. 

Keller and Bhasin were first to report in 1982 [1] on the catalytic one-step oxidative dimerization or coupling of methane (OCM) to C2 hydrocarbons, ethane and ethylene. Numerous investigations have followed this seminal work and a large number of catalysts have been found which give total selectivity to C2 hydrocarbons higher than 90% at low (<2%) methane conversion [2-6]. 

Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly(ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers only strained (and more electrophilic) monomers can react with the anion. 

At least for ethylene hydrogenation, catalysis appears to be simpler over oxides than over metals. Even if we were to assume that Eqs. (1) and (2) told the whole story, this would be true. In these terms over oxides the hydrocarbon surface species in the addition of deuterium to ethylene would be limited to C2H4 and C2H4D, whereas over metals a multiplicity of species of the form CzH D and CsHs-jD, would be expected. Adsorption (18) and IR studies (19) reveal that even with ethylene alone, metals are complex. When a metal surface is exposed to ethylene, selfhydrogenation and dimerization occur. These are surface reactions, not catalysis in other words, the extent of these reactions is determined by the amount of surface available as a reactant. The over-all result is that a metal surface exposed to an olefin forms a variety of carbonaceous species of variable stoichiometry. The presence of this variety of relatively inert species confounds attempts to use physical techniques such as IR to char-... 

Dimethyltitanium complex 25, bearing an ethylene and methyl ligands, catalyzed the dimerization of ethylene via a metallacyclopentane intermediate 26 (Eq. 1) [30]. During the dimerization, no insertion of ethylene into the Ti-Me bond was observed due to the perpendicular orientation between methyl and ethylene ligands. This inertness could be attributed to the low oxidation state of 25, i.e. Ti(II). 

Alkylation reactions reveal a mechanistic aspect of the cuprate reactions different from that of addition reactions. Theoretical analyses of reactions of alkyl halides (Mel and MeBr) [123, 124] and epoxides (ethylene oxide and cyclohexene oxide) [124] with lithium cuprate clusters (Me2CuLi dimer or Me2CuLi-LiCl, Scheme 10.11) resolved long-standing questions on the mechanism of the alkylation reaction. Density functional calculations showed that the rate-determining step of the... 

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