Pyridine-phosphinite ligands

Based on crystal stmctures of Ir complexes with bicyclic pyridine-phosphinite ligands of this type, the stmcture shown in Figure 6 was postulated for the catalyst-substrate... 

The stereocentres in the terpenoid side chain of the tocopherols, fat soluble antioxidants which are the main components of vitamin E, have been installed using this chemistiy in an impressive demonstration of the power of this methodology. " Various iridium catalysts were screened for the hydrogenation of y-tocotrienyl acetate 19 with pyridine-phosphinite ligand 20 giving almost exclusively the natural (J )-isomer of y-tocopherol acetate 21 (Scheme 14.7). The presence of the existing stereocentre in the molecule exerts almost no influence over the reaction, as demonstrated by hydrogenation with an achiral iridium catalyst. 

Although phosphinite- and NHC-based iridium catalysts show very similar enantiose-lectivities in the hydrogenation of various olefins, replacement of the phosphinite group by an A-heterocyclic carbene (NHC) unit results in particularly effective catalysts which are much better suited for the hydrogenation of acid-sensitive substrates beeause of the lower acidity of iridium hydride intermediates produced. The new NHC-pyridine ligands are also likely to prove useful for other applications in asymmetric catalysis." ... 

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