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Chiral Concave N-Heterocyclic Carbenes
Tim Reimers, Ulrich Lüning*
Otto-Diels-Institut für Organische Chemie, Olshausenstraße 40, 24098 Kiel, GER
treimers@oc.uni-kiel.de, luening@oc.uni-kiel.de
In the last two decades, N-heterocyclic carbenes (NHC) have gained large interest.
They are widely applied in organocatalysis and also as ligands in transition metal
catalyzed reactions, e.g. Grubbs’ Second Generation Catalyst. [1]
Due to their versatility,
a control of their reactivity and selectivity is desirable.
For example, the incorporation of an NHC in a concave bimacrocycle was realized
by Winkelmann (for precursor see structure A). The influence of the concave shape of
[2]
the catalyst in respective reactions has already been shown. In order to achieve
enantioselectivity, axial chirality was created by exchanging a phenyl bridgehead for a
naphthyl unit (structure B).
O
N
N
O
O O
Me
Me
O
O
* N N * * N N *
O O
O O
A B C
O O
Figure 1: Three concave NHC precursors: A is achiral, B and C are axially chiral,
but only C is configurationally stable.
If the backbone of the NHC precursor is not modified, a rotation of the N-heterocycle
along the CAr-N bond is observed. This leads to an equilibrium of the two
enantiomers at room temperature and separation is not possible.
This contribution shows an approach to avoid the rotation by adequate substitution
to reach configurational stability. The synthesis of a respective axially chiral bimacro-
[3]
cycle has been accomplished (structure C). The stabilization of the configuration has
been proven by NMR experiments and by chiral HPLC. The application of axially
chiral bimacrocyclic N-heterocyclic carbenes in enantioselective supramolecular
catalysis is the final goal.
[1] M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org. Lett. 1999, 1, 953-956.
[2] O. Winkelmann, U. Lüning, Supramol. Chem. 2009, 21, 223-229.
[3] T. Reimers, C. Näther, U. Lüning, Eur. J. Org. Chem. 2011, 1040-1046.
[2]