Achim Kless

THE FIRST CHIRAL EARLY-LATE HETEROBIMETALLIC COMPLEX : A TITANIUM(IV)-PALLADIUM(II) COMPLEX BASED ON SALENOPHOS

Abstract The synthesis of the chiral polyfunctional diphosphine salenophos 1 is described. The new ligand bearing hard and soft coordination sites chelates titanium(IV) as well as palladium(II) in a defined manner provided a certain order during the introduction of the two metals was kept. In this way the first chiral early-late heterobimetallic complex could be synthesized. It was fully characterized by NMR spectroscopy and X-ray structural analysis. Remarkable features of the heterobimetallic complex are the trans coordination of the phosphine groups on palladium and the distance between Ti and Pd which is more than 4.13 A.

Hydroxyalkylphosphines in asymmetric hydrogenations

Abstract The synthesis of chiral bisphosphines bearing a hydroxyl group in a rigid backbone is described. These compounds which are analogues of the prominent ligand DIOP formed definite rhodium complexes. IR spectra provided evidence that the hydroxyl groups do not participate in the complexation to the metal or to one of the acetal oxygen atoms. The complexes have proven successful in the asymmetric hydrogenation of prochiral functionalized olefins in different solvents. In methanol only small effects could be detected in comparison to parent complexes which do not carry a hydroxyl group. However, when hydrogenations were performed in methylene chloride or toluene significant differences in the enantioselectivities (by up to 17 %ee) were observed, especially in the reaction with itaconic acid or its dimethyl ester, respectively. In the latter cases the effect is contingent on the spatial orientation of the hydroxyl group in the catalyst.

Synthesis and catalytic properties of an acyclic analogue of hydroxy norphos

Abstract The synthesis of (S)-1,2-bis(diphenylphosphino)butane-4-ol 1 an acyclic analogue of hydroxy norphos is described, starting from L-ascorbic acid. Problems which arose during the cleavage of OH-protective groups in the presence of phosphino groups are discussed. The hydrogenation properties of the catalyst derived from the new ligand are compared with those obtained with hydroxy norphos.

Chiral phosphinephosphites having axial and central chirality in asymmetric hydroformylations

Abstract Chiral phosphinephosphites were prepared by the reaction of enantiomerically pure cis - or trans -3-diphenylphosphinotetrahydrofuran-4-ol with atropisomeric chlorophosphites. These ligands were tested in the rhodium catalyzed hydroformylation of allyl acetate. Selectivities up to 44% ee were observed in dependence on the configuration of the applied phosphinephosphites and the bulk of the aromatic groups bound to the phosphorus. The results clearly show that both, central and axial chirality are responsible for the stereochemical outcome of this reaction.

A new chiral multidentate ligand for asymmetric catalysis

Abstract The synthesis of enantiomerically pure ( R,R )-N,N′-bis(3-diphenylphosphino-2-hydroxybenzylidene)-1,2-diphenyl-1,2-ethanediamine ( 1 ) is described. This multidentate salentype ligand offers several possibilities for the construction of chiral mono-nuclear or bi-nuclear metal complexes, which may be useful in bifunctional asymmetric catalysis.

Zur Komplexierung von enantiomerenreinen cis- und trans-3-Diphenylphosphino-4-hydroxy-tetrahydrofuranen mit [Rh(COD)2]BF4

The complexation behaviour of cis- und trans-3-diphenylphosphino-4-hydroxyl-tetrahydrofurans with [Rh(COD)2]BF4 is studied with the help of NMR and IR spectroscopy. In dependence of the spatial arrangement of hydroxyl and phosphino group the formation of different intra- and intermolecular bridged O-P complexes has been observed.

Synthesis of an optically active hydroxy diphosphine, a new ligand for asymmetric catalysis

Abstract The synthesis of (4R,5R)-4,5-bis(diphenylphosphinomethyl)-2-hydroxymethyl-2-phenyl-1,3-dioxolane 1 which is related to DIOP is described. The application of this new ligand in asymmetric catalysis is illustrated in the stereodifferentiating hydrogenation.

Studies on the formation of uniform η3-coordinated triphos-Mo(0)-complexes

Abstract A comparative study on the reaction of (CO)3Mo(CH3CN)3 with a series of homologous chiral and prochiral trisphosphines is presented. Only one of the tripodal ligands formed the desired unique complex (CO)3Mo(η3-triphos) (3b) being the first example of a Mo(0)-complex containing a seven- as well as an eight-membered chelate ring. Spectroscopic properties of this complex and two different η2-coordinated species are discussed. The molecular structure of the η3-coordinated complex 3b has been determined by X-ray crystallography.