Dimerization catalyzed

Yet another possibility is illustrated by the propene (or ethylene) dimerization catalyzed by 7r-l,l,3,3-tetraphenylallylnickel bromide (26) activated with ethylaluminum dichloride the isolation of considerable amounts of 1,1,3,3-tetraphenylpropene (27) from the reaction mixture suggests that a hydrogen atom has been transferred from the substrate olefin to the sterically hindered 1,1,3,3-tetraphenylallyl system under formation of 3 [Eq. (7)] (81). The subsequent formation of the HNiY species from 3 can then take place by insertion of a second propene molecule and /3-hydrogen elimination, as discussed above. 

The catalysis afforded by the La3 + system for the transesterifications of paraoxon in ethanol and methanol is quite spectacular relative to the background reactions that are assumed to be promoted by the lyoxide. The reaction rate constant of ethoxide with paraoxon in ethanol at 5.1 x 10-3 dm3 mol-1 s-133 is roughly a factor of two lower than the rate constant of methoxide with paraoxon in methanol (1.1 x 10 2dm3mol 1 s-1).17a However a solution 2mmoldm-3 in total [La3 + ], which contains 1 mmol dm-3 of Lal+, has a maximum rate constant of 7 x 10-4s-1 for decomposition of 1 in ethanol at pH of 7.3, and accelerates the rate of ethanolysis of paraoxon by a factor of 4.4 x 10n-fold relative to the ethoxide reaction at the same pH.34 By way of comparison, the acceleration afforded by a 1 mmol dm-3 solution of the La + dimer catalyzing the methanolysis of 1 at the maximal pH of 8.3 (kobs = 0.0175 s 1) is 109-fold greater than its background methoxide reaction. On this simple basis La2+ in ethanol appears to be catalytically superior to La2+ in methanol, but this stems almost exclusively from the pH values... 

Another example is butene dimerization catalyzed by nickel complexes in acidic chloroaluminates 14). This reaction has been performed on a continuous basis on the pilot scale by IFF (Difasol process). Relative to the industrial process involving homogeneous catalysis (Dimersol process), the overall yield in dimers is increased. Similarly, selective hydrogenation of diene can be performed in ionic liquids, because the solubility of dienes is higher than that of monoene, which is higher than that of paraffins. In the case of the Difasol process, a reduction of the volume of the reaction section by a factor of up to 40 can be achieved. This new Difasol technology enables lower dimer (e.g., octenes) production costs 14). 

When oxygen is removed from a reaction solution of tetrakis-(dimethylamino)ethylene (TMAE), the chemiluminescence decays slowly enough to permit rate studies. The decay rate constant is pseudo-first-order and depends upon TMAE and 1-octanol concentrations. The kinetics of decay fit the mechanism proposed earlier for the steady-state reaction. The elementary rate constant for the dimerization of TMAE with TMAE2+ is obtained. This dimerization catalyzes the decomposition of the autoxidation intermediate. 

In the alkyne dimerization catalyzed by palladium systems, all proposed mechanisms account for an alkynyl/alkyne intermediate with cis addition of the alkynyl C-Pd bond to the alkyne in a Markovnikov fashion, in which the palladium is placed at the less-substituted carbon, both to minimize steric hindrance and to provide the most stable C-Pd bond (Scheme 2a). The reverse regioselectivity in the palladium-catalyzed dimerization of aryl acetylenes has been attributed to an agostic interaction between the transition metal and ortho protons of the aromatic ring in the substrate (Scheme 2b) [7, 8],... 

In the alternative kinetic model, the monomer-catalyzed steps (ki) were replaced by dimer-catalyzed steps (k A and km) allowing different rates for the homochiral and heterochiral catalytic processes. 

Carbodiimides undergo cyclooligomerization reactions. In this regard they are similar to isocyanates, the mono imides of carbon dioxide. For example, aliphatic carbodiimides undergo rapid dimerization catalyzed by tetrafluoroboric acid at room temperature to give salts of the cyclodimers 183. Neutralization with dilute sodium hydroxide, or better filtration through basic AI2O3, afford l,3-dialkyl-2,4-bisalkylimino-l,3-diazetidines 184. ... 

Scheme 3. Reaction pathways for ethylene dimerization catalyzed by titanium.

Figure 4. Ethylene dimerization catalyzed by titanium selectivity vs. conversion.

Tyrosyl-tRNA-synthetase from Bacillus stearothermophilus is an enzyme (MW = 2 x 47 500 Da) that as a dimer catalyzes the aminoacylation of tRNATyr with tyrosine in the following two steps. 

Xanthine oxidase (275 kDa 2 dimer) catalyzes the two-electron oxidation of xanthine to uric acid (Equation 6.7). 

The ethylrhodium(III) formed in this reaction results from insertion of ethylene into an intermediate cationic Rh(III) hydride. This is an initial step in ethylene dimerization catalyzed by Rh(I). Protonation of other / -CpRh(alkene)2 complexes can be studied by H NMR (see Table 8) . 

One characteristic property of GCS is the preservation of constant catalytic activity over time. This allows catalytic reactions to be performed in both flow and bateh reactors. It can be seen in Figure 28 that the rate of ethylene dimerization catalyzed by GCS in a flow system remains constant for several tens of hours, whereas their homogeneous analogues are quiekly deactivated. 

Fig. 25 Potential energy surfaces for the most feasible two-state reaction pathways for ethylene dimerization catalyzed by Cr(II)OH (If), via either a Cr-carbene mechanism or a metallacycle mechanism determined at the M06 level of theory. Also shown are the crossing points optimized at CASSCF level. The triplet metallacycle reaction pathway is depicted in blue, and the triplet Cr-carbene reaction pathway is shown in dark red. The quintet parts are in black. Energies are in kcal moP and relative to il. Bond lengths are in angstroms. Angles are in degrees...

Figure 5 A general mechanism for autocatalysis, in which template dimers catalyze the coupling of reactants to form trimer complexes, which can then dissociate into template dimers and free template monomers.

TABLE2.5 [i-Naphthol Dimerization Catalyzed by Fe(lll) Complexes Using Macroligands... 

Besides acrylonitrile and acrylates other functional olefins such as vinyl or allyl compounds can also be dimerized. An important example for vinyl compounds is styrene, which can be dimerized to 1,3-diphenyH-butene. Owing to its high tendency to poljmerize spontaneously, the reaction conditions must be chosen carefuUy. Thus the dimerization catalyzed by PdCl2 at 100 °C yielded only 33% of a dimer fraction with more than 60% of a dark polymeric residue [49]. Using Ni( 1/ -03115)2 as the catalyst, styrene is converted to l,3-diphenyl-/mn5-l-butene [58, 59]. With [PdCl r] -C,U,)] dimers and trimers are obtained (Equation 43) [60]. 

M. Sold, A. Lledos, M. Duran and J. Bertran, Proton transfer in the water dimer catalyzed by doubly charged cations (Zn2+, Be2+, and Mg2+), Theor. Chim. Acta, 81 (1992) 303. 

Takai A, Habermeyer B, Fukuzumi S (2011) Facile formation of a meso-meso linked porph5oin dimer catalyzed by a manganese(lV)-oxo porph3oin. Chem Commun 47 6804— 6806... 

Scheme 72 Zeise s dimer catalyzed hydration/annulation of Y-hydroxy-a,(3 -acetylenic esters...

Scheme 82 Zeise s dimer catalyzed intramolecular hydroalkoxylation of 4- and 5-alkynols...

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