Dimerisation and Oligomerisation

Summaries on dimerisation and oligomerisation reactions in ionic liquids have been published previously.[3,4] Predominantly complexes of either palladium or nickel have been employed in ionic liquids for dimerisation/oligomerisation reactions, but there are also examples where iron,[5] tungsten1 and rhodium17 have been used. Apart from metal catalysed dimerisation/oligomerisation reactions in ionic liquids, examples of electrochemical dimerisation have been reported, which include arylhalidcs17 and 3-(4-fluorophenyl) tluophcnc1 as substrates. 

In contrast to the diphenylamine derivatives, transformation of the nitrogen radical (XII) to a nitroxyl radical, N-0-, has not been observed. Due to the longer lifetime of the nitrogen-centred radical (XII) by resonance stabilisation, dimerisation and oligomerisation take place whilst maintaining the -NH- function. Indications have been found that the products of Reaction (4.37) may result from direct reaction of phenyl-a-naphthylamine with oxygen [27, 35]. 

The first class includes non-redox reactions like isomerisation, dimerisation or oligomerisation of unsaturated compounds, in which the role of the catalyst lies in governing the kinetic and the selectivity of thermodynamically feasible processes. Electrochemistry associated to transition metal catalysis has been first used for that purpose, as a convenient alternative to the usual methods to generate in situ low-valent species which are not easily prepared and/or handled [3]. These reactions are not, however, typical electrochemical syntheses since they are not faradaic they will not be discussed in this review. 

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],... 

Dimerisation of olefins is a major industrial process, and is carried out on a multi million ton scale annually.111 One of the most important methods is represented by the Shell Higher Olefin Process (SHOP), which can even be run under biphasic conditions. In the oligomerisation of ethylene, the catalyst is generated in situ in 1,4-butanediol from a nickel salt, Na[BH4] and a chelating ligand. The olefins formed in the reaction are immiscible with the polar solvent and are isolated by phase separation and subsequent distillation.[2]... 

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... 

The oligomerisation stage with an acidic catalyst is likely to involve the reactive terminal double bond of the side chain in cardanol and formation of an allylic carbocation (ref. 2). It has been studied by initiation with boron trifluoride etherate (ref. 243) and the effect of temperature, ratio of catalyst to cardanol and duration of reaction time on its dimerisation examined (ref.244). In the following scheme represents for example HOCeH4(CH2)7-... 

Palladium compounds are also useful catalysts for oligomerisation and co-oligomerisation reactions. Dimerisation or co-dimerisation of olefins catalysed by palladium(n) chloride or, better, PdCl2(C6H5CN)2 involves a palladium(ii)-hydride intermediate as the catalytically active species. A similar reaction to dimerisation is coupling, e.g. of benzene to give biphenyl, which is also catalysed by palladium(ii) chloride and proceeds via palladium-phenyl u-bonded species. The nature of intermediates in... 

The mechanism of trimerisation of butadiene by a mixed cobalt(ii) chloride-aluminium triethyl catalyst has been inferred from the natures of the three products characterised. The determination of the enthalpy of dimerisation of aluminium triethyl provides a useful piece of thermochemical data for quantitative discussion of the role and energetics of aluminium triethyl in this type of reaction. Polymerisation of isoprene in the presence of Fe(acac)3-aluminium triethyl-pyridine derivatives mixtures has a negative apparent activation enthalpy, which can be attributed to the instability of the catalytic complex at elevated temperatures. Bis-cyclo-octatetraeneiron(o) is an effective oligomerisation catalyst. The composition of products accessible only by hydrogen migration indicates an oxidative addition-reductive elimination mechanism rather than insertion. 

Investigation of the nature and activity of catalytic species in copper(i) chloride-promoted dimerisation of acetylene indicates that the mechanism either involves insertion of a C2H unit into a copper-chlorine bond, or the reaction of CgH with two copper atoms to give a 3-centre, 2-electron-bonded species. Cyclic oligomerisation of diphenylacetylene in the presence of AlPha proceeds through ir-complexes as intermediates benzyne intermediates are suggested, on the basis of observed products, in oligomerisation of but-2-yne and of oct-4-yne in the presence of n-butyl- or /7-tolyl-chromium compounds. 

Oligomerisation of 1-butene in sulphuric acid dimerisation of isobutylene and isoamylene... 

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