CLUSTER
Rh-Catalyzed Aqueous Pauson–Khand-Type Cycloaddition
1555
Table 3 Microwave-Assisted Asymmetric Aqueous Pauson–
ligands. Fine-tuning the chiral ligands to further increase
both product and optical yield is currently under way.
Khand-Type Cyclizationa
Ph
Ph
O
Rh-L*
Acknowledgment
4-chlorobenzyl formate
MW
We thank the University Grants Committee Areas of Excellence
Scheme in Hong Kong (Project No. AoE/P-10/01) and The Hong
Kong Polytechnic University Areas of Strategic Development Fund
for financial support. FYK is grateful to the Research Grants
Council of Hong Kong (Project No. CERG:PolyU5006/05P) for
partial support of this investigation.
H
O
O
Entry
Ligand L*
Yield (%)b ee (%)
1
2
(S)-BINAP (1)
23
89
89
(S)-Tol-BINAP (2)
(S)-Xylyl-BINAP (3)
(S)-BisBenzodioxanPhos (4)
(S)-P-Phos (5)
30
References and Notes
3
32
87
(1) (a) Gedye, R.; Smith, F.; Westaway, K.; Ali, H.; Baldisera,
L.; Laberge, L.; Rousell, J. Tetrahedron Lett. 1986, 27, 279.
(b) Giguere, R. J.; Bray, T. L.; Ducan, S. M.; Majetich, G.
Tetrahedron Lett. 1986, 27, 4945.
4
42
93
5
<5
60
(2) For recent reviews, see: (a) Kappe, C. O. Angew. Chem. Int.
Ed. 2004, 43, 6250. (b) Kappe, C. O.; Dallinger, D. Nature
Rev. Drug Discov. 2006, 5, 51.
(3) For a review, see: Nuchter, M.; Ondruschka, B.; Bonrath,
W.; Gum, A. Green Chem. 2004, 6, 128.
6
(S)-Xylyl-P-Phos (6)
(S)-MeO-BIPHEP (7)
(S)-8
72
84
7
55
91
8
27
98
(4) (a) Fuji, K.; Morimoto, T.; Tsutsumi, K.; Kakiuchi, K.
Angew. Chem. Int. Ed. 2003, 42, 2409. (b) Fuji, K.;
Morimoto, T.; Tsutsumi, K.; Kakiuchi, K. Tetrahedron Lett.
2004, 45, 9163. For book chapters and reviews, see: (c) Li,
C. J.; Chan, T. H. Organic Reactions in Aqueous Media;
John Wiley and Sons: New York, 1997. (d) Aqueous-Phase
Organometallic Catalysis; Cornils, B.; Herrmann, W. A.,
Eds.; Wiley-VCH: Weinheim, 1998. (e) Lindström, U. M.
Chem. Rev. 2002, 102, 2751. (f) Tundo, P.; Anastas, P. T.
Green Chemistry: Challenging Perspectives; Oxford
University Press: Oxford, 2000. (g) Nelson, W. M. Green
Solvents for Chemistry, Perspective and Practice; Oxford
University Press: Oxford, 2003. (h) Clean Solvents,
Alternative Media for Chemical Reactions and Processing;
Abraham, M. A.; Moens, L., Eds.; American Chemical
Society Symposium Series 819, ACS: Washington DC,
2002.
(5) For recent reviews on catalytic Pauson–Khand-type
reactions, see: (a) Gibson, S. E.; Stevenazzi, A. Angew.
Chem. Int. Ed. 2003, 42, 1800. (b) Shibata, T. Adv. Synth.
Catal. 2006, 348, 2328. (c) Morimoto, T.; Kakiuchi, K.
Angew. Chem. Int. Ed. 2004, 43, 5580.
(6) (a) Kwong, F. Y.; Li, Y.-M.; Lam, W. H.; Qiu, L.; Lee, H.
W.; Chan, K. S.; Yeung, C.-H.; Chan, A. S. C. Chem. Eur. J.
2005, 11, 3872. (b) Kwong, F. Y.; Lee, H. W.; Qiu, L.; Lam,
W. H.; Li, Y. M.; Kwong, H. L.; Chan, A. S. C. Adv. Synth.
Catal. 2005, 347, 1750. (c) Kwong, F. Y.; Lee, H. W.; Lam,
W. H.; Qiu, L.; Chan, A. S. C. Tetrahedron: Asymmetry
2006, 17, 1238. (d) Lee, H. W.; Chan, A. S. C.; Kwong, F.
Y. Chem. Commun. 2007, 2633.
9
(R,R)-Walphos (9)
(R,S)-Taniaphos (10)
no rxn
no rxn
n.d.
n.d.
10
OMe
O
N
MeO
MeO
PAr2
PAr2
O
O
PAr2
PAr2
PAr2
PAr2
N
O
OMe
Ar = Ph, (S)-BINAP (1)
Ar = Tol, (S)-Tol-BINAP (2)
Ar = 3,5-xylyl, (S)-xylyl-BINAP (3)
(S)-BisbenzodioxanPhos (4)
Ar = Ph, (S)-P-Phos (5)
Ar = 3,5-xylyl, (S)-xylyl-P-Phos (6)
Ph2P
PPh2
PPh2
MeO
MeO
PAr2
PAr2
NMe2
Me
Ph2P
Fe
Fe
H
H
Ar = Ph, (S)-MeO-BIPHEP (7)
Ar = 3,5-Di-t-Bu-4-MeO-C6H2 [(S)-8]
Walphos (9)
Taniaphos (10)
a Reaction conditions: [Rh(COD)Cl]2 (5 mol%), L* (11 mol%), enyne
(0.3 mmol), 4-chlorobenzyl formate (1.5 mmol), and H2O (1.0 mL)
were placed in MW vials under nitrogen and the reactions were con-
ducted at 130 °C for 50 min under microwave conditions.
b Isolated yield.
the highest yield of the cyclopentenone product (entry 6).
The modified MeO-BIPHEP (8) furnished the product in
excellent enantioselectivity of 98%, which is the highest
ee achieved so far for the O-tethered aromatic enyne (en-
try 8). Chiral ferrocenyl ligands such as Walphos (9) and
Taniaphos (10) were found to be inferior in these aqueous
cascade processes (entries 9 and 10).
(7) For a review on axially chiral diphosphine ligands, see:
Tang, W.; Zhang, X. Chem. Rev. 2003, 103, 3029.
(8) Pai, C.-C.; Li, Y.-M.; Zhou, Z.-Y.; Chan, A. S. C.
Tetrahedron Lett. 2002, 43, 2789.
(9) Genêt, J.-P. Acc. Chem. Res. 2003, 36, 908.
(10) Wu, J.; Chan, A. S. C. Acc. Chem. Res. 2006, 39, 711.
(11) Representative Procedures
In summary, we have succeeded in showing the first aque-
ous dual catalysis in microwave-irradiated medium.11
Various oxygen-, nitrogen-, and carbon-tethered enynes
were successfully transformed to their corresponding cy-
clopentenones. Excellent product enantioselectivities (up
to 98%) were obtained with axially chiral diphosphine
[Rh(COD)Cl]2 (7.4 mg, 15.0 mmol) and dppp (13.6 mg, 33.0
mmol) were charged into the reaction vial on bench-top at
room temperature. The reaction vial was then transferred to
the dry box for being evacuated and backfilled with nitrogen
(3 cycles). Then, 4-chlorobenzyl formate (255 mg, 1.5
mmol, 5 equiv with respected to enyne) was added under
Synlett 2008, No. 10, 1553–1556 © Thieme Stuttgart · New York