New fluorophilic phosphines for use in catalysis
07 November 2005
Advances in the synthesis of ligands for use in rhodium-based catalysts could lead to industrially viable applications, report UK researchers.
The formation of long chain linear aldehydes from alkenes is an important industrial process used in the production of soaps and plasticisers. Rhodium-based catalysts give higher selectivities and reactivities than the catalysts used in industry, but they decompose as the products are removed from the reaction medium by distillation. To try to overcome this problem of catalyst-product separation, fluorous biphase catalysis was established.
A fluorous biphase system consists of a fluorine-containing phase that includes a dissolved catalyst and a second organic or aqueous phase that contains the substrate. When the system is warmed the two phases can mix, allowing the reaction to proceed. The two phases separate when they cool, dividing the products from the catalyst.
To make the catalyst preferentially soluble in the fluorous phase perfluoroalkyl groups, or ponytails, can be attached to the phosphine ligands on the rhodium catalyst. However, this has often resulted in poor regioselectivity in the formation of aldehydes. Eric Hope and colleagues at the University of Leicester and the University of St Andrews, UK, successfully prepared new ligands containing these ponytails and used them in the catalytic reaction of octene. They established, for the first time, that catalyst activity, regioselectivity and the ability to separate the catalyst from the product vary according to the number and type of ponytails incorporated into the ligand rather than the electronic environment of the phosphine. This knowledge is a significant advancement towards a catalytic system that can be employed in industry.
Caroline A Moore
D J Adams, J A Bennett, D J Cole-Hamilton, E G Hope, J Hopewell, J Kight, P Pogorzelec and A M Stuart, Dalton Trans., 2005 (DOI: 10.1039 /b510766k)