| STUDY |
In progress of a different sort, DSM chemists have simplified the catalyst for certain Heck reactions by eliminating the ligands. Heck and similar reactions are aromatic substitutions leading to carbon-carbon bond formation. They are catalyzed usually by palladium with phosphine ligands. Most reactions work well with 3 mol% of palladium and 9 mol% of phosphine ligand, Hans de Vries says. But using that much of a very expensive metal in a commercial plant is not economically feasible, and the phosphine ligands contaminate the product. In addition, the reaction works well with aryl iodides but poorly with aryl bromides, which are cheaper. Understanding the catalytic cycle, the DSM chemists realized that eliminating the ligand would solve all the problems. Hans de Vries explains the thinking: With aryl iodides, the reaction is fast enough to keep all the palladium in the catalytic cycle. Aryl bromides react more slowly, and so not all the palladium is in the catalytic cycle. Some of it starts aggregating and eventually precipitates. The problem is keeping palladium in solution for catalysis, which is the role of the phosphine ligands. But the phosphine ligands also make the catalyst less reactive toward aryl bromides. Because aggregation is probably higher order in palladium, and catalysis is probably first order in palladium, the DSM chemists thought that lowering the palladium concentration might slow aggregation more than catalysis. And it works: At 0.1 to 0.01 mol% palladium, catalysis works just fine. "The solution is very counterintuitive because when a catalyst does not work, people tend to throw in more rather than cut back," Hans de Vries says. |
| UPDATE | 07.03 |
| AUTHOR | DSM Pharma Chemicals's de Vries Hans |
| LITERATURE REF. | This data is not available for free |
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