ALKENE OLIGOMERISATION

1-Alkenes, or linear a-olefms as they are called in industry, are desirable starting materials for a variety of products. Polymers and detergents are the largest end-uses. We mention a few applications  

The ethene-based processes give only even-numbered alkenes (including the one making alcohols) while the other two processes produce both odd and even numbered alkenes. In the last two decades there has been a trend to close down wax cracking alkene plants, and the products are now increasingly derived from ethene. Note that ethene is also a cracker product. 

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Co-addition and Co-oligomerisation.—Nickel and palladium catalysts are again much in evidence. The same 7r-tetramethylcyclobutadienenickel dichloride plus alkylaluminium halide mixed catalysts which have been investigated in relation to alkene oligomerisation (see above) catalyse coaddition of ethylene and propene. Kinetic results for this reaction are reported. Product analysis and distribution permit postulation of mechanisms of addition of butadiene and but-2-yne to give 4,5-dimethyl-1-... 

Keim W, Behr A, Roper M (1982) Alkene and alkyne oligomerisation, cooligomerisation and telomeiization reactions. In Comprehensive organometallic chemistry, vol 8. Pergamon Press, Oxford... 

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],... 

The higher alkene feed (C10-14) for the production of detergent alcohols is either a product from the wax-cracker (terminal and internal alkenes) or the byproduct of the ethene oligomerisation process (internal alkenes). In the near future a feed from high-temperature Fischer-Tropsch may be added to this. The desired aldehyde (or alcohol) product is the linear one and the cobalt catalyst must therefore perform several functions ... 

Alcohol dehydration. Other routes to linear higher alcohols are available such as the reduction of fatty acids and an aluminium based oligomerisation of ethene followed by oxidation. The higher alcohols are used as such or they are dehydrated to make 1-alkenes. 

Ethene oligomerisation. In view of the above limitations there is a demand for a process that selectively makes linear 1-alkenes. Three processes are available, two based on aluminium alkyl compounds or catalysts and one on nickel catalysts. The aluminium processes use aluminium in a stoichiometric fashion and they produce a narrow molecular weight distribution (a Poisson distribution, vide infra). 

Keywords Fluorinated alkenes nucleophilic attack fluoride ion oligomerisation rearrangements displacement carbanions. 

A particularly useful reaction of this type involves the direct formation of hexakis(trifluoromethyl)cyclopentadiene (71) (Scheme 31), or the corresponding cyclopentadienide (72), from the diene (38) by a fluoride ion induced reaction with pentafluoropropene [67-69]. Recent work [54] has shown that very active sources of fluoride ion can be generated by direct reaction of amines, especially TDAE (43), with perfluorinated alkenes or perfluorinated aromatic compounds and these essentially solventless systems promote both oligomerisations (see above) and polyfluoroalkylations. The absence of solvent makes recovery of product very easy, e.g. in high-yielding formation of (73), (74) or (75) (Scheme 32). 

Oligomerisations of fluorinated alkenes, via carbanion formation by addition of fluoride ion to the double-bond, was discussed in Sect. 5.3 (Scheme 51). 

However, ylides may be generated by reactions of amines with various perfluorinated -alkenes or -cycloalkenes [110] and attack of these ylides also promotes oligomerisation [62, 111, 112] (Scheme 52). 

Various oligomers of fluorinated alkenes and cycloalkenes have been prepared by fluoride ion induced oligomerisation of various monomers (Sect. 5.3), and the chemistry of these systems provides some unique reactions. The oligomers of special interest here may be described as of types (95) or (96) (Scheme 59), i. e. systems with either four (95) or three (96) perfluoro-alkyl or -cycloalkyl groups attached to the double bond, whereas systems with two perfluoroalkyl groups attached, i. e. (97) and (98), have a chemistry more similar to fluorinated alkenes that may be derived from other sources. 

Finally perfluorinated alkenes, prepared according to various routes (decarboxylation of perfluorinated acids or anhydrides, action of organolithium, magnesium reagents or metallic zinc-copper couple to perfluoroalkyl iodides, oligomerisation of TFE and of HFP, addition of perfluoroalkyl iodides to per-fluoroallyl chloride [304], addition of KF to perhalogenated esters or alkanes... 

Lewis acid-free catalysts for acyclic diene metathesis obviate the formation of carbocations, which in turn completely eliminates competing reactions, mostly involving cationic oligomerisation via 1-alkene bonds. Thus, metathesis polycondensation occurs quantitatively to yield high molecular weight poly(l-alke-nylenejs with vinyl end groups and ethylene as a byproduct. 

Despite its fame as the first initiator used in cationic polymerisation, sulphuric acid has not frequently been used for fundamental studies in this field. Ite low vapour pressure hinders proper purification procedures even in high vacuum, and its limited solubflity in the solvents currently used for these investigations is another serious drawback. While alkenes are only oligomerised by this acid, aromatic monomers can give high polymers under suitable conditions. Our present discussion will be limited to the only system investigated in depth, i.e. the polymerisation of stjrene. 

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