Cyclometallated Pd Complexes as Templates

Ort/ o-cyclometallated palladium complexes discussed in Section 2.2.2 have found another very important application as chiral templates that promote Diels-Alder and other reactions. This chemistry has been developed by Leung and co-workers and has allowed the synthesis of an impressing number of mono- and diphosphines bearing P-stereogenic atoms with peculiar scaffolds 

DMPP itself is not a reactive diene in Diels-Alder reactions,but it is activated by coordination to transition metal ions. Complex 198 contains a labile perchlorato ligand that is easily displaced by the dienophile, which possesses a coordinating atom (O, S, As or P) in the group E. The cycloaddition reaction occurs intramolecularly in a highly organised environment, which leads to the coordinated exo cycloadduct 199 exclusively. A standard decoordination step affords the desired enantiopure ligands 200. Only the exo-syn isomers are formed, which bear the lone pair at the phosphorus atom and the dienophile functionalities at the same side of the molecule.  

In contrast, complex 201 contains an inert Pd-Cl bond, which is not cleaved by the dienophile. Therefore the cycloaddition reaction takes place inter-molecularly, yielding the thermodynamically more stable endo-syn products 203 after decoordination. As can be expected, the stereoselectivity is low (less than 50% de), although the two diastereomers are often separable and the stereoselectivity can be improved by optimising the structure of the cyclo-metallated amine.  

These Diels-Alder cycloadditions have provided many funetionalised phosphines as single products, which are listed in Table 2.15. 

A disadvantage is that reactions are slow, taking days or even weeks to reach completion although intermolecular cycloadditions are usually faster than the intramolecular ones. Another problem is that the bridgehead phosphorus atoms are not configurationally stable in the free phosphines. 

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