Claisen rearrangement - chiral guanidinium salts - H-bonding catalysts
Significance
<P>Catalysis of the Claisen rearrangement by simple guanidinium
salts such as
N,
N′-diphenylguanidinium
salt
1 is reported. A broad variety of
substituted allyl vinyl ethers (see selected examples) were suitable
substrates for this interesting transformation. Moreover, highly
enantioselective Claisen rearrangements of ester-substituted
allyl vinyl ethers
2 have been achieved using
C
2-symmetric guanidinium salt
3 as the catalyst. Other types of substrates
failed to give satisfying results in the asymmetric variant. The
weakly coordinating counterion BArF
- was found
to be crucial for catalytic activity of the respective guanidinium
salt.</P>
Comment
<P>(Thio)ureas constitute a privileged class of H-bonding catalysts
(see Review below). However, in catalytic Claisen rearrangements
only modest rate accelerations have been realized by means of this
catalyst class (see, for example: M. Hiersemann, T. Strassner and
co-workers
J. Org. Chem. 2007,
72, 4001). Although guanidinium BArF
species exhibit similar p
K
a
values as
N,
N′-diarylthioureas,
they possess superior catalytic activity in this transformation.
This work complements the metal-catalyzed asymmetric Claisen rearrangement
developed by the Hiersemann group (
Angew.
Chem. Int. Ed. 2001,
40,
4700).</P>
Review
<P>M. S. Taylor, E. N. Jacobsen
Angew. Chem.
Int. Ed. 2006,
45,
1520-1543.</P>
