C-C-Coupling Reactions


Dimerisation

The dimerisation and oligomerisation of short chain alkenes (ethene, propene, butene) in the presence of homogeneous Nickel catalysts is a large scale industrial process for the production of linear alpha olefins (LAOs). Important applications are the Shell Higher Olefin Process (SHOP) and the Dimersol process by IFP. The obtained LAOs are important intermediates for e.g. alcohols, plasticizers and detergents. The SHOP and Dimersol catalysts operate according to a metal hydride mechanism which results in a statistical distribution of products.

Left: Metal hydride mechanism for alkene oligomerisation.
Right: proposed metallacycle mechanism for selective trimerisation and tetramerisation of ethene.

The rising demand for 1-hexenes and 1-octenes as co-monomers for the production of LLDPE stimulated the development of novel processes such as the selective trimerization and tetramerization of ethene by Sasol. For these chromium based catalysts a metallacycle mechanism has been proposed as depicted the Scheme above (right).

Methathesis

Metathesis is a catalytic reaction of alkenes, which results in the rearrangement of alkylidene groups. All major steps in the catalytic cycle are equilibrium steps, resulting in an overall equilibrium conversion determined by thermodynamics according to Scheme 1.

Reaction scheme of the Metathesis.

The development of highly active homogeneous metathesis catalysts that do not require co-catalyst activation has been a major breakthrough for organic synthesis in the past decades. The Nobel Prize for chemistry in 2005, rewarding three main investigators in the field, reflects this importance accordingly. Ruthenium-carbene complexes, known as Grubbs-catalysts, are tolerant toward a variety of functional groups in the alkene molecules as well as kinetically stable against water and air. Commercially available catalysts are shown in Scheme 2.

Selection of commercially available Grubbs catalysts.

Catalyst separation and recycling is a major issue when using homogeneous catalysts. Grubbs-type catalysts have been used in small scale synthesis of fine chemicals and pharmaceuticals, thus the total recovery of the metal from the product is important in order to match quality regulations. Furthermore, recycling of the expensive catalyst would benefit the process economics. Several approaches have been made including biphasic catalysis in aqueous, fluorous and ionic liquid media. The development of modified G2 catalysts by Grubbs and Hoyveda (denoted GH1 5 and GH2 6) as well as ionic liquid tagged metathesis catalysts by Clavier and Dixneuf 7 and 8 allowed the elegant recycling of the ruthenium catalyst for several times. The catalysts are shown in Scheme 4 together with a newly developed metathesis catalyst 9 based on GH2 with a polar substituent from Zannan Company of China.

GH1 and 2, ionic liquid tagged and Zannan catalysts.

The ionic liquid tagged catalyst 7 showed exceptionally good activity in the ring closing metathesis of N,N-diallyltosylamide, a standard test reaction for metathesis. Compared to G1 and GH1 catalysts the ionic liquid tagged catalyst could be recycled in the ionic liquid [BMIM][PF6] ten times without significant loss of activity.

Due to the high activity of the Grubbs-type catalysts, most biphasic metathesis reactions are mass transport controlled. In the case of highly viscous ionic liquid systems, the reactions at low temperatures can become severely limited by mass transport. In that case, only a minor amount of the expensive catalyst and ionic liquid would be utilised for the reaction, making such processes economically not viable.
The immobilization of Grubbs-type catalysts via SILP technology is a promising was to overcome mass transport limitations and additionally utilize continuous fixed bed reactors with simplified product removal.

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