Chiral squares

In comparison to the hybrid squares [Pd(/ )-(+)-BINAP)(58)2] [OTfls (5.88) and [Pt (7 )-(+)-BINAP)(58)2][OTf]g (5.89), the all metal squares were formed in significantly greater diastereomeric excesses and were conformationally much more rigid. This was confirmed by VT H-NMR studies carried on these chiral squares with only metal corners. In brief the C2h-symmetrical connector ligands are locked into a chiral conformation in the square assemblies and must be restricted in rotation due to the presence of two - of just one - metal comers. As expected the use of the opposite chiral metallo-corners based on the (5)-(—)-BINAP ligand allowed the preparation of the opposite enantiomers of chiral molecular squares. The CD spectra of these chiral species confirmed the expected enantiomeric relationship. 

Brissy D, Skander M, Retailleau P, Marinetti A (2007) N-hetetocyclic carbenes in the synthesis of axially chiral square-planar platinum complexes. Organometallics 26 5782-5785... 

Figure Bl.5.8 Random distribution of (a) non-chiral adsorbates that gives rise to a surfaee having effeetive oo m-synnnetry (b) ehiral moleeules that gives rise to effeetive oo-synnnetry. (e) SH intensity versus the angle of an analyser for a raeemie (squares) and a non-raeemie (open eireles) monolayer of ehiral moleeules. The pump beam was p-polarized the SH polarization angles of 0° and 90° eorrespond to s- and p-polarization, respeetively. (From [70].)...

The absolute configuration of products obtained in the highly stereoselective cycloaddition reactions with inverse electron-demand catalyzed by the t-Bu-BOX-Cu(II) complex can also be accounted for by a square-planar geometry at the cop-per(II) center. A square-planar intermediate is supported by the X-ray structure of the hydrolyzed enone bound to the chiral BOX-copper(II) catalyst, shown as 29b in Scheme 4.24. 

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. 

Racemic amino acids have been resolved via stereoselective coordination to the square planar chiral matrix complex (178).584 The bisamidobispyridyl ligand (179) forms a square planar Ni11 complex with considerable tetrahedral twist due to repulsion of the ortho protons of the pyridyl rings.585... 

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