J. Sun et al. / Journal of Organometallic Chemistry 694 (2009) 3219–3221
3221
LCQ Electrospray Mass Spectrometer. Normal mass spectra were
taken using HP 5989A mass spectrometer. Optical rotations were
recorded on a Perkin Elemer 241 Polarimeter. Ee values were
determined by a Perkin–Elmer 200 HPLC on a chiral Chiralcel OJ-
H or AD-H column. The diastereomeric excesses were determined
by HPLC and 1H NMR.
L
V(II)
B
ArCHO
ZnCl2
Zn
L
Cl
V
5.2. General Procedure for pinacol coupling reaction
TMSCl
O
O
VO(acac)
+
V(IV)
L
L
2
Cl
A
Ar
Ar
H H
To a mixture of 8 (0.10 mmol) and VO(acac)2 (0.10 mmol) was
added anhydrous THF (2 mL) under argon. The deep blue mixture
was stirred for 10 min at room temperature, and then Zn powder
(2 mmol) was added. After stirred for another 10 min, the mixture
was cooled to 0 °C and the freshly distilled Me3SiCl (1.5 mmol) was
added. After 30 min, aldehyde (1 mmol) was added and the mix-
ture was stirred at 0 °C for 36 h. The suspension was filtered
through a glass funnel, and the filtrate was treated with 2 M HCl
(5 mL) and stirred for 1 h. Then the mixture was extracted with
EtOAc (3 Â 10 mL) and the organic layer was washed with a satu-
rated NaHCO3 solution (2 Â 10 mL). After dried over anhydrous so-
dium sulfate and removal of solvent under reduced pressure, the
crude products were purified by flash chromatography on silica
gel (petroleum ether/EtOAc = 5:1) to remove the impurities. The
ratio of dl to meso of the diols was measured by 1H NMR analysis.
The ee values of the dl diols were determined by chiral HPLC using
a Daicel Chiralcel OJ-H or AD-H column.
L = salan ligands
C
2Me3SiCl
OH
OSiMe3
Ar
H+
Ar
Ar
Ar
OH
OSiMe3
D
E
Fig. 2. Proposed mechanism for vanadium-catalyzed asymmetric pinacol coupling
reaction.
3. Mechanism study
The proposed mechanism for the 8-VO(acac)2 catalyzed enantio-
selective pinacol coupling reaction was shown in Fig. 2. Reaction of
chiral oxovanadium (IV) complex with Me3SiCl affords the ligand
exchange dichloro intermediate A. Following reduction with Zn
generates a low valent V(II) species B. Subsequently addition of
aldehyde results in the formation of V–O bond between the metal
atom and the oxygen atoms of the aldehydes. This diketyl radical
intermediate C is thought as the plausible transition state structure.
The electrons transfer from the vanadium atom to two carbonyl
substrates results in the formation of the new C–C bond and the oxi-
dation state of vanadium is changed from +2 to +4. Cleavage of V–O
bonds with Me3SiCl affords the diol silyl ether D and the intermedi-
ate A is regenerated. The whole catalytic circulation is finished.
Acknowledgements
We gratefully acknowledge China Pharmaceutical University,
the National Natural Science Foundation of China (20472028,
20332050 and 20672053), the 863 High Technology Program, and
Science Foundation of Jiangsu Province for their financial support.
References
[1] J.K. Whitesell, Chem. Rev. 89 (1989) 1581.
[2] A. Gansäuer, D. Bauer, J. Org. Chem. 63 (1998) 2070.
[3] A. Gansäuer, H. Bluhm, Chem. Rev. 100 (2000) 2771.
[4] A. Chatterjee, N.N. Joshi, Tetrahedron 62 (2006) 12137.
[5] T. Tsuritani, S. Ito, H. Shinokubo, K. Oshima, J. Org. Chem. 65 (2000) 5066.
[6] J.L. Oller-Lopez, A.G. Campana, J.M. Cuerva, J.E. Oltra, Synthesis (2005) 2619.
[7] B. Hatano, A. Ogawa, T. Hirao, J. Org. Chem. 63 (1998) 9421.
[8] T. Hirao, M. Asahara, Y. Muguruma, A. Ogawa, J. Org. Chem. 63 (1998) 2812.
[9] T. Hirao, B. Hatano, Y. Imamoto, A. Ogawa, J. Org. Chem. 64 (1999) 7665.
[10] X. Xu, T. Hirao, J. Org. Chem. 70 (2005) 8594.
[11] S.I. Fukuzawa, Y. Yahara, A. Kamiyama, M. Hara, S. Kikuchi, Org. Lett. 7 (2005)
5809.
[12] M. Tamiya, K. Ohmori, M. Kitamura, H. Kato, T. Arai, M. Oorui, K. Suzuki, Chem.
Eur. J. 13 (2007) 9791.
4. Conclusion
In summary, we have developed a highly enantioselctive pina-
col coupling reaction of aromatic aldehydes catalyzed by a chiral
salan–vanadium complex in the presence of Zn and Me3SiCl. The
1,2-diols were obtained in high diasteroselectivities (up to 90/10)
and moderate to high enantioselectivities (up to 82% ee). The
mechanism of the reaction has also been discussed. To our knowl-
edge, this approach provides the first example of asymmetric pina-
col coupling catalyzed by a chiral vanadium complex.
[13] T. Mukaiyama, N. Yoshimura, K. Igarashi, A. Kagayama, Tetrahedron 57 (2001)
2499.
[14] M. Bandini, P.G. Cozzi, S. Morganti, A. Umani-Ronchi, Tetrahedron Lett. 40
(1999) 1997.
[15] U. Groth, M. Jeske, Angew. Chem. Int. Ed. 39 (2000) 574.
[16] R. Halterman, C. Zhu, Z. Chen, M.S. Dunlap, M.A. Khan, K.M. Nicholas,
Organometallics 19 (2000) 3824.
5. Experimental
[17] D. Enders, E.C. Ullrich, Tetrahedron: Asymmetry 11 (2000) 3861.
[18] Y. Hashimoto, U. Mizuno, H. Matsuoka, T. Miyahara, H. Takakura, M.
Yoshimoto, K. Oshima, K. Utimoto, S. Matsubara, J. Am. Chem. Soc. 123
(2001) 1503.
[19] A. Bensari, J. Renaud, O. Riant, Org. Lett. 3 (2001) 3863.
[20] A. Chatterjee, T.H. Bennur, N.N. Joshi, J. Org. Chem. 68 (2003) 5668.
[21] Y. Li, Q. Tian, J. Zhao, Y. Feng, M. Li, T. You, Tetrahedron: Asymmetry 15 (2004)
1707.
[22] J. Wen, J. Zhao, X. Wang, J. Dong, T. You, J. Mol. Catal. A 245 (2006) 242.
[23] J. Wen, J. Zhao, X. Dong, T. You, J. Mol. Catal. A 245 (2006) 278.
[24] J. Wen, Q. Tan, T. You, J. Mol. Catal. A 258 (2006) 159.
[25] N. Takenaka, G.Y. Xia, H. Yamamoto, J. Am. Chem. Soc. 126 (2004) 13198.
[26] H. Yang, H. Wang, C. Zhu, J. Org. Chem. 72 (2007) 10029.
[27] J. Sun, C. Zhu, Z. Dai, M. Yang, Y. Pan, H. Hu, J. Org. Chem. 69 (2004) 8500.
[28] Y. Huang, F. Yang, C. Zhu, J. Am. Chem. Soc. 127 (2005) 16386.
[29] J. Sun, Z. Dai, M. Yang, X. Pan, C. Zhu, Synthesis (2008) 2100.
[30] H. Egami, T. Katsuki, J. Am. Chem. Soc. 129 (2007) 8940.
[31] H. Egami, T. Katsuki, Angew. Chem. Int. Ed. 47 (2008) 5171.
5.1. General
All reactions were carried out under an argon atmosphere using
standard Schlenk techniques. Unless otherwise indicated, all mate-
rials were obtained from commercial sources and were used with-
out further purification. THF and ethyl ether were distilled from
sodium/benzophenone. Dichloroethane was distilled from CaH2.
Acetonitrile was distilled from P2O5 before use. Liquid aldehydes
and trimethylchlorosilane were freshly distilled. Elemental analy-
ses were carried out on a Perkin–Elmer 240 C elemental analyzer.
NMR data were recorded on a Bruker AMX-300 spectrometer with
chemical shifts referenced to SiMe4 as internal standard. Infrared
spectra were measured in cmÀ1, using a 5DX-FT-2 spectrometer.
Electrospray ionization mass spectrums were recorded on Finnigan