Finally, several binaphthol11 ligands were screened with
N-phenyl-N-hydroxy-acrylamide 1 using the optimized con-
ditions (Figure 1). However, none proved to be better than
geometry of the acrylamide is unkown,16 and the positive
role of the N-phenyl substituent is not clear at the moment.
Further experiments and modeling will be performed to gain
more information about the factors involved in the stereo-
chemical outcome of these reactions.
Finally, we have shown that the hydroxamic acid 3 is a
useful building block (Scheme 3). It is easily converted by
Scheme 3. Conversion of 3 into Aldehyde 14 or Alcohol 15
treatment with an excess of LiAlH4 at 0 °C into aldehyde
14 or alcohol 15 depending on the reaction conditions.
In conclusion, we have demonstrated that enantioselective
Diels-Alder reactions of N-hydroxy-N-phenyl acrylamide
can be efficiently promoted by using 3 equiv of Me3Al and
1.1 equiv of binaphthol. The reaction presumably proceeds
via a complex containing three aluminum atoms. This system
requires the use of a stoichiometric amount of the chiral
ligand, but binaphthol is easily recovered at the end of the
reaction by filtration. The development of a catalytic version
of this reaction is currently under investigation and will be
reported in due course.
Figure 1. Screening of binaphthol-type ligands.
the nonsubstituted binaphthol 2. Substituted binaphthols 9
and 13 gave enantioselectivities close to those observed with
binaphthol 2. TADDOL derivatives12 and bis-sulfonamides13
were also investigated, but the enantioselectivity remained
very low (<20% ee).
In the course of the complex preparation, gas evolution
was observed during the addition of the first 2 equiv of the
Me3Al solution to the chiral ligand but not during the addition
of the third one. Gas evolution was again observed during
the addition of the hydroxamic acid solution to the chiral
catalyst. Changing the order of addition of the reagents had
no influence on the enantioselectivity of the reaction. We
believe that a trimetallic complex is formed with one of the
metal centers covalently linked to the hydroxamic acid
moiety. This is in close analogy to a recent work of
Yamamoto where dialuminum complexes of binaphthol 10
were used to catalyze Diels-Alder reactions.14,15 The
Acknowledgment. This work was supported by the Swiss
National Science Foundation (Grant 21-55′386.98). O.C. is
grateful to the Roche Research Foundation for a fellowship.
We thank Prof. Dieter Seebach (ETH Zu¨rich) and Prof. Paul
Mu¨ller (University of Geneva) for providing us with samples
of TADDOL and modified binaphthol derivatives.
Supporting Information Available: Experimental pro-
cedures and full characterization for compounds 1, 3, 4-7,
14, and 15 as well as a detailed procedure for the determini-
nation of the enantioselectivities. This material is available
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