Diphosphine ligands, chelating, effect

Although chelating aryl diphosphine ligands are effective for hydrogenating dehydroamino acids with high stereoselectivity, minor changes in the composition of the substrate can drastically lower the enantiomeric excess. The best results are obtained for the following structures ... 

The Effect of Chelating Diphosphine Ligands on Homogeneous Catalytic Decarbonylation Reactions Using Cationic Rhodium Catalysts... 

Even though several chelating diphosphines have been considered as likely ligands for this reaction (using the chelate effect to stabilize the ligand under the harsh conditions of the process), their performance has not traditionally been analyzed in terms of bite angle. 

Phosphorus-based systems constitute an important family of ligands in homogeneous catalysis. Even though the use of basic phosphines, which increases the nucleophilicity of the rhodium center, was initially considered a very promising strategy, their instability under the harsh conditions of the process constituted a major drawback for their application. Attempts to circumvent the decomposition were undertaken by using bidentate diphosphines, which stabilize the active rhodium species by the chelate effect (vide supra). 

In an effort to develop new systems able to combine both stability and activity, Siiss-Fink and co-workers reported a family of trans-spanning, but flexible diphosphines.These ligands form stable complexes, due to the chelate effect, which resemble the ones obtained with PEts, (/r . r-[RhI(CO)(PEt3)2]), with the two phosphorus atoms in a /r< //i -disposition. These ligands, as benzoic acid diphenylphosphine derivatives, are supposed to increase the electron density on the rhodium center due to a weak Rh-O interaction. 

It has been found that among chelating chiral diphosphines, ligands with C2-symmetry are the most effective, and therefore, they have become the most popular ligands... 

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