Palladium complexes chiral squares

In 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. 

Formation of the palladium(II) complexes can be achieved using standard protocols and resulting in square planar complexes with mer (pincer) chelate structure. The six-membered, very flexible metallacycles featuring alkyl linker chains display chiral puckering that would make the use of chiral analogues difficult in asymmetric catalytic applications. 

FIGURE 9-7 Chiral Isomers of Square-Planar Complexes (meso-stilbenediamine)(iSo-butylenedi-amine)platinum(ll) and palladium(ll). (From W. H. Mills and T. H. H. Quibell, J. Chem. Soc.,... 

FIGURE 9.8 Chiral Isomers of Square-Planar Complexes. (Meso-stilbenediamine)( so-butylenediamine) platinum(ll) and palladium(ll). 

Miscellaneous Oxidation Reactions. The kinetic resolution of secondary alcohols is achieved by a palladium-catalyzed enan-tioselective oxidation using Pd[(—)-sparteine]Br2/(—(-sparteine or Pd(CH3CN)2(Br)2/chiral diamine sparteine mimic under an oxygen atmosphere. The chiral amine-dibromide complexes are observed to oxidize secondary alcohols more rapidly than the dichloride complexes (eq 7). This has been attributed to a greater counterion distortion from ideal square planar geometry, which could lower the energy barrier to 8-hydride elimination. 

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