Bisoxazoline ligand

Part of the success of the BOX and PyBOX motifs lies in their convenient one step synthesis from malononitrile and dipicolinic acid, which are commercially available at low expense.

In general, for methylene bridged BOX ligands the stereochemical outcome is consistent with a twisted square planar intermediate that was proposed based on related crystal structures.

[8] On the other hand, two-point binding on a Lewis acid bearing the meridially tridentate PyBOX ligand would result in a square pyramidal complex.

[9] Metal complexes incorporating bis(oxazoline) ligands are effective for a wide range of asymmetric catalytic transformations and have been the subject of numerous literature reviews.

[18] Oxazoline ligands were first used for asymmetric catalysis in 1984 when Brunner et al. showed a single example, along with a number of Schiff bases, as being effective for enantioselective carbenoid cyclopropanation.

[19] Schiff bases were prominent ligands at the time, having been used by Ryōji Noyori during the discovery of asymmetric catalysis in 1968[20] (for which he and William S. Knowles would later be awarded the Nobel Prize in Chemistry).

[26] In the same year Andreas Pfaltz et al. reported the use of C2‑symmetric semicorrin ligands for enantioselective carbenoid cyclopropanations, achieving impressive results with ee's of between 92-97%.

Box stereochemical model
Box stereochemical model
PyBox stereochemical model
PyBox stereochemical model
Evans 1997: [ 14 ] BOX assisted Aldol reaction
Aggarwal 1998: [ 15 ] BOX assisted Diels-Alder reaction resulting in verbenone synthesis. The final conversion with diphenylphosphoryl azide involves a modified Curtius rearrangement
Nishiyama 1989: [ 16 ] Enantioselective hydrosilylation
The development of bis(oxazoline) ligands
C 2 -symmetric bis(oxazoline) ligands with axial chirality