Synthesis of new phosphino-oxazoline ligands for asymmetric catalysis

Borabox ligands proved to be efficient ligands for controlling the enantioselectivity of various
metal-catalyzed reactions. Therefore modification of an existing borabox backbone was
implemented and new borabox ligands modified on C(5) position of the oxazoline ring were
prepared and tested in the copper-catalyzed asymmetric cyclopropanation. In this study high
stereocontrol of the reaction was observed. However the presence of sterically demanding
groups at position C(5) did not improve the results compared to the C(5) non-substituted
analogs.
The synthesis of analogous boron-bridged phosphino-oxazolines was attempted via several
synthetic approaches in order to prepare new zwitterionic N,P-ligands. The simple stepwise
substitution by subsequent addition of lithiated oxazoline and phosphine was not possible. It
either led to borabox ligands or to undesired dimeric species, which were inert towards
reaction with other nucleophiles.
We decided to tune the electronic properties of the boron compound by variation of the
substituents in order to avoid multiple substitution or undesired dimer formation. Therefore
aminochloroborates were examined due to their lower reactivity compared to chloroboranes
or chloroborates. A derivative with a phosphine-aminoborate backbone was prepared but
unfortunately the decreased reactivity of the nitrogen-substituted boron center did not allow
another nucleophilic addition of the oxazoline moiety. In order to avoid dimer formation the
reactivity of potassium diaryldifluoroborates was investigated. These tetrasubstituted boron
compounds reacted with lithium oxazolines and provided products of nucleophilic
substitution at the boron center. The resulting oxazoline-substituted fluoroborates could be
isolated as zwitterions after protonation of the oxazoline nitrogen atom. However, the second
intended substitution with the phosphine moiety was not possible. In addition, quantum
chemistry calculations were carried out to support the experimental studies.
The synthesis of new NeoPHOX ligands derived from inexpensive chiral aminoacids L-serine
and L-threonine was developed. These chiral ligands were tested in the iridium-catalyzed
asymmetric hydrogenation and palladium-catalyzed allylic substitution. In both reactions the
enantioselectivities achieved were excellent for most of the substrates tested. In the iridium
catalyzed hydrogenation it was found that presence of an acid-stable protecting group of
tertiary alcohol (R2) is necessary in order to achieve full conversions. The enantioselectivities
obtained in the catalytic asymmetric hydrogenation and allylic substitution with the L-serine
and L-threonine derived ligands were almost identical to those reported for tert-butyl-
substituted NeoPHOX ligands, which are derived from very expensive amino acid tert-
leucine.
The use of Ir catalysts for the diastereoselective hydrogenation of Diels-Alder products was
investigated. The best results were obtained with a pyridine-phosphinite complex that
afforded the saturated cyclohexane derivatives with diastereoselectivities of up to 98:2 and
full conversion. The reaction is strongly catalyst-controlled, so it is possible to obtain each of
the two diastereomeric products with high selectivity using either (R)- or (S)-catalyst.