Phosphinites derived from the 7-phosphanorbornene skeleton: First results in asymmetric catalysis

Article abstract of DOI:10.1021/ol035067g

(Matrix presented) The bisphosphinite 4 incorporating two 7-phosphanorbornene subunits is an efficient ligand for the Rh-catalyzed enantioselective hydrogenation of functional alkenes in terms of rate and enantioselectivity. This type of structures is rea

Full text of DOI:10.1021/ol035067g

ORGANIC
LETTERS
2003
Vol. 5, No. 17
3093-3094
Phosphinites Derived from the
7-Phosphanorbornene Skeleton:
First Results in Asymmetric Catalysis
Magali Clochard, Eric Mattmann, Franc¸ois Mercier,* Louis Ricard, and
Franc¸ois Mathey*
Laboratoire “He´te´roe´le´ments et Coordination” UMR CNRS 7653, DCPH,
Ecole Polytechnique, 91128 Palaiseau Cedex, France
francois.mathey@polytechnique.fr
Received June 12, 2003
ABSTRACT
The bisphosphinite 4 incorporating two 7-phosphanorbornene subunits is an efficient ligand for the Rh-catalyzed enantioselective hydrogenation
of functional alkenes in terms of rate and enantioselectivity. This type of structures is readily accessible by [4 + 2] cycloaddition of phospholes
with dienophilic alkenes and can be easily fine-tuned.
During recent years, the Zhang group has developed the
synthesis of the 7-phosphanorbornane structure and its
applications in enantioselective catalysis.1 From a synthetic
standpoint, the related 7-phosphanorbornene structure is very
readily accessible through the [4 + 2] cycloaddition of a
phosphole dienic system with a dienophilic alkene. We have
recently demonstrated that it is possible to enhance the dienic
reactivity of phospholes with an appropriate choice of the
P-substituents.2 The 1-alkoxyphospholes appear to be espe-
cially reactive. Thus, we decided to perform a preliminary
investigation of the potential of the readily available 7-alkoxy-
7-phosphanorbornenes in asymmetric catalysis. Our first
experiments were carried out with the monophosphinite 1
(enantiomeric purity 98%, (S)-configuration at phosphorus)2c
(Scheme 1). These experiments demonstrate that the rhodium
complex of 1 is an efficient hydrogenation catalyst in terms
of rate but not satisfactory in terms of enantioselectivity.
Scheme 1. Asymmetric Hydrogenation of Functional Alkenes
with Rh/Monophosphinite (1) Catalysta
a Conditions: rt, MeOH, H2 3 atm, 1 % [RhL2(cod)]PF6 as a
catalyst.
(1) (a) Zhu, G.; Chen, Z.; Jiang, Q.; Xiao, D.; Cao, P.; Zhang, X. J. Am.
Chem. Soc. 1997, 119, 3836. (b) Chen, Z.; Jiang, Q.; Zhu, G.; Xiao, D.;
Cao, P.; Guo, C.; Zhang, X. J. Org. Chem. 1997, 62, 4521. (c) Zhang, X.;
Jiang, Q.; Jiang, Y.; Xiao, D.; Cao, P. Angew. Chem., Int. Ed. 1998, 37,
1100. (d) Zhang, X.; Jiang, Q.; Xiao, D.; Zhang, Z.; Chao, P. Angew. Chem.,
Int. Ed. 1999, 38, 516. (e) Zhang, Z.; Zhu, G.; Jiang, Q.; Xiao, D.; Zhang,
X. J. Org. Chem. 1999, 64, 1774.
(2) (a) Mattmann, E.; Simonutti, D.; Ricard, L.; Mercier, F.; Mathey, F.
J. Org. Chem. 2001, 66, 755. (b) Mattmann, E.; Mathey, F.; Sevin, A.;
Frison, G. J. Org. Chem. 2002, 67, 1208. (c) Mattmann, E.; Mercier, F.;
Ricard, L.; Mathey, F. J. Org. Chem. 2002, 67, 5422.
Thus we decided to devise a simple synthesis of a chelating
enantiopure bis(7-phosphanorbornene) structure. The starting
1-cyano-3,4-dimethylphosphole3 was first converted into the
(3) Holand, S.; Mathey, F. Organometallics 1988, 7, 1796
10.1021/ol035067g CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/29/2003

enantiopure bis(phosphole) 2 by reaction with the lithium
derivative of (R,R)-1,2-diphenyl-1,2-ethanediol (99% ee)4 and
was stored as its borane complex 3. It readily reacts with
N-phenylmaleimide5 to give the expected bis(7-phosphan-
orbornene) 4 as a single enantiomer (Scheme 2).
can be seen, the alkoxy P-substituents are anti with respect
to the phosphanorbornene CdC double bonds. The P-Pt-P
bite angle is 88°. The two Pt-P bonds are slightly different
at 2.183(3) and 2.222(3) Å, and the geometry of platinum is
almost perfectly square-planar.
In the noncomplexed 4, the two 31P nuclei appear
equivalent (δ 31P +100.2 in C6D6) whereas the frozen
geometry of 5 induces a differentiation of the two nuclei (δ
Scheme 2. Synthesis of Bisphosphinite 4
2
1
31P +93.0 and +103.6 in CH2Cl2, JP-P ca. 25 Hz, JPt-P
3800 and 3933 Hz).
The bisphosphinite 4 was tested on the same rhodium-
catalyzed hydrogenation reactions of functional alkenes as
the monophosphinite 1 (Scheme 3).
Scheme 3. Asymmetric Hydrogenation of Functional Alkenes
with Rh/Bisphosphinite (4) Catalysta
a Conditions: rt, H2 3 atm, 1 % [Rh(4)(cod)]PF6 as a catalyst.
The stereochemistry of 4 was established by X-ray crystal
structure analysis of its cis-PtCl2 complex 5 (Figure 1). As
Although slightly less efficient than 1 in terms of rates, a
drastic enhancement of the enantioselectivity is observed with
4. These preliminary experiments illustrate the potential of
the readily available 7-phosphanorbornenes in enantioselec-
tive catalysis. A fine-tuning of the structure can easily be
achieved by a modification of the phosphole substitution
scheme and the bridge connecting the two phosphorus atoms.
Dienophiles other than N-phenylmaleimide can also be used.
These various possibilities are currently under investigation.
Acknowledgment. We thank CNRS and Ecole Polytech-
nique for support of this research.
Supporting Information Available: Experimental details
of reactions; 1H, 13C, 31P NMR and mass spectra of isolated
products; X-ray data for complex 5. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL035067G
Figure 1. Crystal structure of complex 5. Phenyls have been
omitted for clarity.
(4) Wang, Z.-M.; Sharpless, K. B. J. Org. Chem. 1994, 59, 8302.
(5) Mathey, F.; Mercier, F. Tetrahedron Lett. 1981, 22, 312.
3094
Org. Lett., Vol. 5, No. 17, 2003