2- propene, dimerisation

When dicarbene complexes of the form 21 were tested for 1-bntene or propene dimerisation, npon activation with AfEt Cl or MAO in tolnene, rapid deactivation took place yielding Ni(0) [25]. It was shown that this decomposition did indeed involve carbene-hydride and carbene-aUcyl rednctive elimination. Some dimerisation was evident at -15°C (TON = 50), however decomposition of the intermediate Ni species seemed too rapid for effective catalysis. In contrast, when the complexes were... 

Note that the catalyst of Figure 10.6 contains the same titanium part as that of Figure 10.5, but that they differ in the aluminium Lewis acid and anions formed. The use of diethylaluminium chloride (DEAC, the common initiator for heterogeneous titanium catalysts) gave propene dimerisation only. This... 

Propane formation occurs through a three-step process with hydrogenation of acetone on the platinum sites, dehydration of isopropanol on the acid sites and hydrogenation of propene on the platinum sites. 2-Methylpentane being a primary product results probably from propene dimerisation on the acid sites followed by hydrogenation. 

Use of less sterically hindered examples of 5 in combination with MAO allows for active catalysts for the linear (head-to-head) dimerisation of a-olefins such as 1-butene, 1-hexene, 1-decene and Chevron Phillips C20-24 a-olefin mixture (Scheme 4) [47], The mechanism for dimerisation is thought to involve an initial 1,2-insertion into an iron-hydride bond followed by a 2,1-insertion of the second alkene and then chain transfer to give the dimers. Structurally related cobalt systems have also been shown to promote dimerisation albeit with lower activities [62], Oligomerisation of the a-olefms propene, 1-butene and 1-hexene has additionally been achieved with the CF3-containing iron and cobalt systems 5j and 6j yielding highly linear dimers [23],... 

Using the catalyst shown in Figure 10.5 propene leads to dimerisation only. The difference in behaviour between ethene and propene is explained by the hydrogen at a tertiary carbon that is formed using propene, which undergoes /3-hydride elimination. 

As might be expected from their inherent strain, many cyclopropenes undergo rearrangement, dimerisation or even polymerisation under relatively mild conditions. The conditions required for reaction are, however, very variable and some cyclo-propenes, such as 3,3-dimethylcyclopropene, are stable at relatively high temperature (150 °C in this case). Three main reactions are described below — the ene-reaction, [2+ 2]-dimerisation, and rearrangement to vinylcarbenes. 

The first example for biphasic oligomerisation of olefins in ionic liquids was published in 1990, reporting on the dimerisation of propene by nickel(II) catalysts in chloroaluminate ionic liquids of the general formula [cation]Clx-(AlCl3)y with either [C4Ciim]+, [C4py]+ or [(C4)4P]+ as cation.[10] It was found that in basic ionic liquids, y < 0.5, no catalysis took place. Excess chloride, which is present in such basic chloroaluminates, poisons the catalyst and it was shown that nickel compounds of the type NiCkCPRok... 

Several (bis)carbene nickel complexes were tested as catalysts for the dimerisation of propene or 1-butene (Scheme 8.3). IS While the activity of these complexes was very poor in the dimerisation of 1-butene when toluene was used as solvent, turnover frequencies as high as 7,000 mol mol h 1 were observed at ambient temperature with C4Ciim]Cl-AlCL-iV-methylpyrrole (0.45 0.55 0.10) as solvent. With propene as substrate, TOFs of 75,000 mol-moL -lf1 were achieved. Compared to NiCl2(PCy3)2, the activity of the carbene complexes is considerably higher, but selectivity towards the desired, highly branched propene dimer is low. 

A similar mechanism has been advanced for the dimerisation of hexafluoropropene [50] (Figure 4.17) and other fluorinated propenes [45, 51] to analogous dimers. 

Other biphasic C—C bonding reactions were carried out with fluorous solvents, for instance Suzuki- and Sonogashira-couplings [124] or ethene or propene oligomerizations [125, 126], Further new solvent systems use ionic liquids for the linear dimerisation of 1-butene to octenes [127] or the hydrovinylation of styrene with a combination ionic liquid/supercritical carbon dioxide [128] (cf. Section 7.4). 

Catalysts derived from CpTaCli, are also able to dimerise ethylene to 1-butene. (C5Me5)2MMc2 (M = Th.O) on AI2O3 are considerably better propene hydrogenation catalysts than CP3M-R derivatives they are converted to hydrides. Ethylene is polymerised. ... 

Several multicomponent metal oxide catalysts, developed for this process, have achieved excellent product selectivity with a high conversion of propene Mo-Bi-Fe-Co-M-K-O (M = V or W) used for the first step can attain >90% acrolein yields [6,7] while for the second step Mo-V-Cu-based oxides can lead to >97% acrylic acid yields [8,9], giving, in theory, an overall acrylic acid yield from propene of 87%. In addition to the compositional differences in fhe catalysis for the two-step process, there is also a difference in the optimal reaction temperatures 320-330°C for the first step and 210-255°C for the second step. One has to keep in mind that propene and oxygen can form an explosive mixture and therefore, certain limitations in the feed composition (propene oxygen (air) steam) exist. In addition, the acrylic acid easily dimerises at temperatures above 90 C, meaning that the reactor effluent should be quickly quenched after the second catalyst bed to temperatures below this critical value. 

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