C2-symmetric chiral bisoxazoline (BOX) ligand-metal complexes
have emerged as an effective catalyst for carrying out a wide range
of enantioselective reactions.
[1]
The presence
of a C2-symmetric axis in the bisoxazoline ligands minimizes
the number of possible transition states in a particular reaction.
[2]
Of the various chiral
bisoxazoline ligands used, ligands (1-7) with a one carbon bridging between the
oxazoline rings are most frequently utilized. Several research groups
have utilized these bisoxazoline ligands in combination with a wide
range of mild Lewis acids as catalysts for carrying out different
enantioselective reactions.
[4-13]
Recently,
chiral BOX-metal(II) complexes covalently anchored to silica and
mesoporous MCM-41 have been used as a new heterogenous catalyst for
enantioselective Friedel-Crafts hydroxylation.
[14]
An intriguing feature
of the BOX-metal(II) complexes is that the metal(II)-derived chiral
Lewis acids shows pronounced counter ion
[15a-c]
as
well as solvent dependence in context of yield, enantioselectivity,
rate, and success of reaction. The selectivity observed is due to
the fact that the bisoxazoline ligands form six membered metal chelates
which are conformationally constrained and the chiral centers in
these ligands are located in close proximity to donor nitrogen,
thereby imposing a strong directing effect on the catalytic sites.
[16]
Preparation of the catalyst
A wide variety of chiral bisoxazoline ligands are commercially
available or can be synthesised.
[1a]
[3]
[14]
[17]
