Olefin dimerisation

Olefin dimerisation with Ni-NHC complexes became a topic of interest following reports of Ni(II) phosphine complexes being employed in imidazolium-based ionic liquid solvents [23, 24]. It had previonsly been established that aIkyl-Ni(II) complexes containing NHC ligands can rapidly decompose via imidazolium formation (Scheme 4.1) [5], and it was thus of interest to explore the effect that an excess of the imidazolinm cation would have on this reaction. 

Catalyst systems of the type [NiL X + AlEt Xj (where L = PR and X = halide) afford highly active catalysts for olefm dimerisation. However, when complex 11 (Scheme 13.8) is treated with AlEt Cl in the presence of 1-butene, in toluene at 20°C the only products observed were decomposition products, 12,13,14 no butene dimers were obtained [22], At low temperatures (-15°C) and using the complex with 1,3-diiso-propylimidazolin-2-ylidene as the NHC ligand, small amounts of butene dimers were observed. It is apparent from these results that Ni-NHC complexes are capable of olefin dimerisation, however, decomposition of the catalyst via reductive elimination predominates. 

Similarly olefine dimerisation is a 2 + 2 cycloaddition. However this nomenclature is sometimes ambiguous. 

Scheme 8.1 Reaction pathways in olefin dimerisation, excluding higher oligomerisation...

In the foUowiag cases, only those reactions ia which there is no chain growth, or at most dimerisation, are considered (see Olefin polymers). Alkyl titanium haUdes can be prepared from alkyl aluminum derivatives. The ring stmcture imparts regiospecificity to the ensuing carbometalation (216) ... 

G. Lefebvre and Y. Chauvin Dimerisation and codimerisation of olefinic compounds by coordination catalysis, pp. 108-203. 

The final method was an electrochemical reductive dimcrisation working extremely efficiently and capable of dimerising many electron-deficient olefins. 

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],... 

Scheme 4 Use of 5c/MAO or 5j/MAO to mediate the head-to-head dimerisation of a-olefins...

Die Photosensibilisation hat sich als besonders niitzlich bei der C4-Cyclo-dimerisation von Olefinen mit isolierten Doppelbindungen erwiesen. Mit Ausnahme der durch Quecksilber photosensibilisierten Dimerisation des Athylens zu. Cyclobutan in der Gasphase (61) konnten jedoch bisher nur cyclische Olefine photosensibilisiert dimerisiert werden. 

Like a,/Lunsaturated carbonyl compounds, vinyl silanes and particularly vinylsiloxanes do not dimerise readily and can thus be used in excess to drive selective CM reactions. Pietraszuk et al. [56] have exploited this to couple vinyl-triethoxysilane with a variety of olefin partners in good yield and with high stereoselectivity (Scheme 8) [57]. Given the importance of vinylsiloxanes as nucleophilic components in Pd-catalysed coupling reactions [58], their facile and stereoselective modification by CM is a significant development. 

Since our initial reports [ 139,140] on the immobilisation and CM reactions of olefins on solid supports significant progress has been made. As is the case with most reactions on solid supports, resin-bound metathesis reactions (particularly CM reactions and variations thereof) are more selective as dimerisation pathways are suppressed (although intraresin metathesis reactions are by no means excluded [ 141,142]) and the reaction can be driven to completion by using an excess of the other olefin substrates (the dimers of which can be removed by filtration). In many cases the required products can be cleaved from the resin after the reaction, and as such are available in relatively pure form for... 

The most recent phenomenon observed to occur within the confines of 24 is the stereoselective photodimerisation of olefins [68]. Without the presence of the cage, the dimerisation of 28 results in a mixture of the syn- and anti- isomers (29 and 30 respectively, Scheme 8). With the cage present, two molecules of 28 are encapsulated before the photodimerisation occurs. Within the cavity the syn isomer is the only one capable of forming for steric reasons. This type of selectivity has been observed for several related compounds. Recently, photodimerisation has also been observed to be controlled by supramolecular templation in the solid state [69]. 

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... 

Note Thiazol-2-ylidenes, although stable at ambient temperatures, readily dimerise reversibly to the respective electron-rich olefin. 

Note The tendency to dimerise and form electron-rich olefins follows the sequence isothiazol-3-ylidene > thiazol-2-ylidene > imidazol-2-ylidene. 

The other components are C5+ olefins and dienes and in particular cycto-pentadiene which easily dimerises to a Cio compound di-cyclo-pentadiene). As well as a strong odour, these materials readily polymerise to form gum in the gasoline and the raw pyrolysis gasoline is usually hydro-treated prior to use. In some cases, these C5 dienes are extracted and used to form low melting resins. One approach to upgrading the pyrolysis gasoline stream is shown in Figure 5.5 . 

In this approach, pyrolysis gasoline first enters a C5/C6+ splitter which passes the C5 fraction to a di-cyc/o-pentadiene unit which dimerises the cyc/o-pentadiene in the C5 stream and the dimer is extracted. Excess C5 is returned to the system via an isoprene extraction unit. The mixture is then hydrogenated and olefins are saturated to paraffins. 

Within the scope of this review we shall only consider those compounds possessing one or more alkenyl functions susceptible to activation by electrc hilic attack. Included in this family is a vast array of monomers varying in basicity from ethylene, which is so resistant to protonation that the ethyl carbenium ion has hitherto eluded observations even under the most drastic conditions (see below), and which in fact is equally resistant to cationic polymerisation, to N-vinylcarbazole, whose susceptibility to this type of activation is so pronounced that it can be polymerised by almost any acidic initiator, however weak. We shall also deal with olefins which, because of steric hindrance, can only dimerise (e.g., 1,1-diphenylethylene) or cannot go beyond the stage of protonated or esterified monomeric species (e.g., 1,1-diphenylpropene). The interest of such model compounds is obvious they allow clean and detailed studies to be conducted on the kinetics and mechanism of the initiation steps and on the properties of the resulting products which simulate the active species in cationic polymerisation. The achievements and shortcomings of the latter studies will be discussed below. 

In cyclohexane the reaction was also first order in olefin and second order in acid, but the predominantly dimeric state of the acid in this solvent suggested that the one molecule of the aggregate was responsible for the attack on the olefin. Of course the transition state might again involve the concerted participation of two acid molecules arising from the opening of the dimer. In the second study, Latremouille and Eastham also obtained third order kinetics for the reaction of trifluoroacetic acid with isobutene in 1,2-dichloroethane. Given the low value of the dimerisation constant in this solvent it seems obvious that here too the Ad 3 mechanism should hold, a point which was in fact implicitly touched on by the authors in the discussion of their results. 

These basic differences are strong indication of a change in mechanism between the two processes. Our interpretation is that 1,1-diphenylethylene dimerises via a pseudocationic mechanism where the active species are the trichloroacetate ester molecules (absence of spectrum) and the rate-determining step is the dimerisation itself (second order in olefin). On the other hand, 1,1-di-p-methoxyphenylethylene dimerises under the action of a carbenium ion intermediate (typical spectrum), and the rate-determining step is the protonation reaction (first order in olefin and higher activation energy). 

The mechanism of the dimerisation of this olefin with triflic acid has been the object of several recent investigations Since this is one of the best-understood... 

Recent work on the dimerisation of 1,1-diphenylethylene by aluminium chloride produced conclusive evidence that direct initiation does not lead to the total ctmsump-tion of the catalyst. This excellent piece of research diowed that about 2.5 aluminium atoms are needed to give rise to one carbenium ion. Similar indications were reported by Kennedy and Squires for the low temperature polymerisation of isobutene by aluminium chloride. They underlined the peculiar feature of limited yields obtained in flash polymerisations with small amounts of catalyst. The low conversions could be increased by further or continuous additions of the Lewis acid. Equal catalyst increments produced equal yield increments It was also shown that introductions of small amounts of moisture or hydrogen chloride in the quiescent system did not reactivate the polymerisation. This work was carried out in pentane and different purification procedures for this solvent resulted in the same proportionality between polymer yield and catalyst concentration. Experiments were also performed in which other monomers (styrene, a-methylstyrene, cyclopentadiene) were added to the quiescent isobutene mixture. The polymerisation of these olefins was initiated but limited yields were again obtained. Althou the full implications of these observations must await more precise data, we agree with the authors interpretation that allylic cations formed in the isobutene polymerisation, while incapable of activating that monomer, are initiators for the polymerisation of the more basic monomers added to the quiescent mixture. The low temperature polymerisation of isobutene by aluminium chloride was also studied... 

Yamamoto et al. pursued their studies on the photoinduced dimerisation and polymerisation of olefins in the presence of electron acceptors. A mechanism was discussed involving the excitation of the EDA complex to give the olefin cation-radical which can then undergo various reaction pathways with itself and with the monomer. 

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