Palladium-catalyzed asymmetric allylic alkylations of cycloalkenyl acetates with planar chiral phosphino-ferrocene carboxylic acids

Article abstract of DOI:10.1016/S0022-328X(01)01143-3

New phosphino-ferrocene carboxylic acids with only planar chirality (Sp)-3 and (Sp)-4 have been synthesized conveniently from (S,Sp)-5 and (S,Sp)-7. They were used as ligands in palladium-catalyzed allylic alkylation of cycloalkenyl acetates. With ligand (Sp)-4, high yield and good ee were given for reactions of a series of cycloalkenyl acetates and nucleophiles.

Full text of DOI:10.1016/S0022-328X(01)01143-3

Journal of Organometallic Chemistry 637–639 (2001) 845–849
www.elsevier.com/locate/jorganchem
Note
Palladium-catalyzed asymmetric allylic alkylations of cycloalkenyl
acetates with planar chiral phosphino-ferrocene carboxylic acids
Shu-Li You, Yu-Mei Luo, We-Ping Deng, Xue-Long Hou *, Li-Xin Dai
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu,
Shanghai 200032, People’s Republic of China
Received 17 May 2001; received in revised form 15 June 2001; accepted 26 June 2001
Abstract
New phosphino-ferrocene carboxylic acids with only planar chirality (Sp)-3 and (Sp)-4 have been synthesized conveniently from
(S,Sp)-5 and (S,Sp)-7. They were used as ligands in palladium-catalyzed allylic alkylation of cycloalkenyl acetates. With ligand
(Sp)-4, high yield and good ee were given for reactions of a series of cycloalkenyl acetates and nucleophiles. © 2001 Elsevier
Science B.V. All rights reserved.
Keywords: Alkylation; Asymmetric catalysis; Ferrocene; Planar chirality
1. Introduction
alyzed allylic substitution reactions [5,6]. During this
process, planar chiral phosphino-ferrocene carboxylic
acids (Sp)-3 and (Sp)-4 were important intermediates
for synthesizing planar chiral ligands [7,8]. However, we
found these two compounds were also useful ligands in
palladium-catalyzed allylic alkylation reaction. In this
paper, we report the synthesis of ligands with only
planar chirality (Sp)-3 and (Sp)-4, and their applica-
tions in palladium-catalyzed asymmetric allylic alkyla-
tion of cycloalkenyl acetates.
Palladium-catalyzed allylic substitution reactions
have been one of the most intense topics in asymmetric
synthesis during the past two decades [1]. Various lig-
ands have been synthesized and proved to be powerful
in these reactions, especially with 1,3-diphenylprop-2-
enyl acetate [2]. On the other hand, substitutions with
cyclic allylic substrate are more difficult to achieve high
enantioselectivity [3]. Among a few successful systems,
chiral phosphinocarboxylic acids, such as 1 and 2 (Fig.
1), have been found to be effective for these types of
substrates [4].
In our group, many ferrocene ligands have been
synthesized and applied into asymmetric palladium-cat-
2. Results and discussion
The synthesis of ligands (Sp)-3 and (Sp)-4 was shown
in Scheme 1 [6e,7a]. (S,Sp)-5 and (S,Sp)-7 could be
synthesized from commercially available ferrocene and
enantiopure aminoalcohol. Following Meyer’s method
[6e,7a,9], the oxazoline was converted conveniently to
Fig. 1. Ligands 1 and 2.
* Corresponding author. Tel.: +86-21-6416-3300; fax: +86-21-
6416-6128.
E-mail address: xlhou@pub.sioc.ac.cn (X.-L. Hou).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01143-3

846
S.-L. You et al. / Journal of Organometallic Chemistry 637–639 (2001) 845–849
Scheme 1. Condition and reagent. (a) (1) TFA, Na2SO4, H2O; (2) Ac2O, Pyr. (b) (1) NaOH, MeOH, THF; (2) H3O+
.
With these two ligands, the enantioselectivity of the
products was determined only by the planar chirality
[10]. As part of our program for studying the role of
planar chirality [6,7,11], the results here were very
interesting. It indicated also that the ferrocene modified
planar chiral pocket could show better asymmetric
induction in some reaction than the phenyl derived
chiral pocket ligands [7,8], for that a moderate enan-
tioselectivity could be caused by the planar chirality of
phosphino-ferrocene carboxylic acid moiety.
esters (S,Sp)-6 and (S,Sp)-8. In the presence of NaOH,
(S,Sp)-6 was hydrolyzed to (Sp)-3 in high yield after
being acidified with HCl. Following the same method,
(Sp)-4 was also synthesized from (S,Sp)-8 in high yield.
Both of them were orange solids and were very stable in
air.
To examine the catalytic efficiency of (Sp)-3 and
(Sp)-4 in palladium-catalyzed allylic alkylation reac-
tions of cycloalkenyl acetates, the most used condition,
reaction of cyclohexenyl acetate with the nucleophile
derived from dimethylmalonate was chosen (Scheme 2)
and the results were summarized in Table 1.
From the results listed in Table 1, we found the
reaction ran very slowly with ligand (Sp)-3 at ambient
temperature. However, the reaction could be completed
in 1 day by raising the temperature to 50 °C. The
product 10 was given in 96% yield and 30.0% ee.
Ligand (Sp)-4 was shown to be more effective in this
reaction. Even at ambient temperature, the reaction
was completed in 1 h with 50.1% ee and 98% yield. On
considering both reactivity and enantioselectivity, lig-
and (Sp)-4 is better.
3. Conclusions
New planar chiral ferrocene phosphino-carboxylic
acids (Sp)-3 and (Sp)-4 have been synthesized and used
as ligands in palladium-catalyzed allylic alkylation with
cycloalkenyl acetates. With 1,1%-disubstituted ligand
(Sp)-4, high yield and moderate to good ee could be
achieved for a series of cycloalkenyl acetates and
nucleophiles.
With ligand (Sp)-4, different cycloalkenyl acetates
and nucleophiles were investigated (Scheme 3). The
results are summarized in Table 2.
4. Experimental
4.1. General methods
All reactions proceeded smoothly and the products
were given in high yield. In previous works [3,4], the ee
of the products were determined mainly by using opti-
All reactions were performed under an atmosphere of
either Ar or N2 using oven-dried glassware. Solvents
were treated prior to use according to the standard
method. The commercially available reagents were used
as received without further purification. Melting points
1
cal rotation, H-NMR chiral shifting reagents or GC.
In addition to these methods, in this work, the ee’s were
also determined by separating the products using chiral
HPLC.
1
were uncorrected. H-NMR spectra were recorded in a
In order to investigate how the steric demanding of
the nucleophiles affects the enantioselectivity, more
steric demanding nucleophiles like sodium diethyl-
malonate and sodium di-tert-butylmalonate were used
in the reaction of cyclohexenyl acetate. We found that
the ee of the products decreased a little. The result was
also unsatisfactory for cyclopentenyl acetate, only
31.8% ee was given. However, the highest ee of 65.5
was obtained for the seven-member ring substrate, cy-
cloheptenyl acetate.
Bruker AMX-400 (400 MHz) or AMX-300 (300 MHz)
spectrometer in CDCl3 at room temperature (r.t.). 31P-
NMR spectra were recorded in a Bruker AMX-400
(162 MHz) spectrometer and the chemical shifts were
Scheme 2.

S.-L. You et al. / Journal of Organometallic Chemistry 637–639 (2001) 845–849
847
Table 1
Palladium-catalyzed allylic alkylation of 9 with different ligands a
Entry
Ligand
Temperature
Time
Yield (%) b
ee (%) c
1
2
3
(Sp)-3
(Sp)-3
(Sp)-4
r.t.
50
r.t.
2 days
1 day
1 h
B10%
96
98
–
30.0 (R)
50.1 (R)
a Reactions were performed with molar ratio: [Pd(h3-C3H5)Cl]2–ligand–9–NaCH(CO2Me)2=2:8:100:200.
b Isolated yield based on 9.
c Determined by HPLC (chiracel OD column) and the absolute configuration of product was assigned through comparison of the sign of specific
rotations with the literature data [3e].
referenced to external 85% H3PO4. Chemical shifts were
given in parts per million relative to Me4Si as an
internal standard. Optical rotations were measured us-
ing a Perkin–Elmer 241 MC polarimeter with a ther-
mally jacketed 10 cm cell at 25 °C (concentration c
given as g 100 ml−1). IR spectra were measured in
cm−1, using a Shimadzu IR-440 infrared spectrophoto-
meter. Mass spectra and high-resolution mass spectra
were taken using HP5989A and Finnigan MAT mass
spectrometers, respectively. Elemental analyses were
performed in a Foss-Heraeus Vario EL instrument. The
ee values were determined by chiral HPLC on a Chiral-
cel AS column.
1254 (s), 1165 (s), 696 (m). Anal. Found: C, 66.57; H,
6.23; N, 2.32. Calc. for C31H34FeNO3P: C, 67.00; H,
6.13; N, 2.52%.
4.3. (Sp)-2-(Diphenylphosphino)-1-ferrocene carboxylic
acid (Sp)-3
Under Ar, to a solution of (S,Sp)-6 (555 mg, 1
mmol) in 30 ml THF and 10 ml MeOH, 4 ml 2.5 N
NaOH (10 mmol) was added. The mixture was refluxed
for 2 h, cooled to r.t. and the solvent was removed.
With an ice bath the mixture was acidified with 1 N
HCl to pH 1. The resulting solution was extracted with
CH2Cl2, the organic layer was dried and the solvent
was removed in vacuo. The crude product was purified
by column chromatography (EtOAc–petroleum
ether=1:2) to afford (Sp)-3 (392 mg, 95%) as an
orange solid: m.p. 158–160°C; [h]2D0= −296 (c 0.31,
4.2. (S,Sp)-2-(Diphenylphosphino)-[1-(N-acetyl)-2-tert-
butyl-2-amino ethoxycarboxyl]-1-ferrocene (S,Sp)-6
A solution of (S,Sp)-5 (1.98 g, 4 mmol) in THF (40
ml) was treated with powdered Na2SO4 (29 g, 60
mmol), water (4 ml, 220 mmol) and trifluoroacetic acid
(1.74 ml, 22 mmol), The suspension was stirred for 3
days, and anhydrous Na2SO4 (8.4 g, 60 mmol) was
added. Filtration and concentration at B30 °C af-
forded the unstable ammonium salt, which was dis-
solved in CH2Cl2 (70 ml), cooled in an ice bath, and
treated sequentially with Ac2O (14 ml, 15 mmol) and Py
(22 ml, 27 mmol). The reaction mixture was allowed to
warm to ambient temperature over 7 h. The solution
was washed with cold 3 N HCl (3×100 ml) and
saturated NaHCO3 (100 ml). The organic layer was
dried and the solvent was removed in vacuo. The crude
product was purified by column chromatography
(EtOAc–petroleum ether=1:3) to afford 6 (1.40 g,
63%) as a yellow solid: m.p. 188–189 °C (dec); [h]2D0=
1
CHCl3). H-NMR (400 MHz, CDCl3) l 7.49–7.51 (m,
2H), 7.37–7.39 (m, 3H), 7.17–7.26 (m, 5H), 5.09 (m,
1H), 4.94 (t, J=2.5 Hz, 1H), 4.24 (s, 5H), 3.79 (m, 1H);
31P-NMR (161.92 MHz, CDCl3) l −17.46. EIMS; m/z
(relative intensity): 414 [M+, 100], 368 (86), 303 (39),
Scheme 3.
Table 2
Palladium-catalyzed allylic alkylation of cycloalkenyl acetates with
(Sp)-4 a
Entry
Substrate
R
Yield (%) b
ee (%) c
1
−231 (c 0.33, CHCl3). H-NMR (400 MHz, CDCl3) l
1
2
3
4
5
n=6
n=6
n=6
n=5
n=7
Me
Et
98
95
91
95
94
50.1
40.3
48.4
31.8
65.5
7.52 (m, 2H), 7.39–7.41 (m, 3H), 7.22–7.24 (m, 3H),
7.14–7.15 (m, 2H), 5.92 (d, J=9.4 Hz, 1H), 5.08 (m,
1H), 4.64 (dd, J=6.8 Hz, 11.5 Hz, 1H), 4.48 (t, J=2.6
Hz, 1H), 4.17 (s, 5H), 4.09 (m, 1H), 3.95 (dd, J=4.0
Hz, 11.5 Hz, 1H), 3.80 (m, 1H), 1.74 (s, 3H), 1.01 (s,
9H). EIMS; m/z (relative intensity): 555 [M+, 100], 490
(14), 414 (73), 369 (51), 213 (26). IR (KBr, cm−1) 3332
(m), 3057 (w), 2966 (m), 1702 (s), 1652 (s), 1434 (m),
tBu
Me
Me
a Reactions were performed with molar ratio: [Pd(h3-C3H5)Cl]2–
(Sp)-4–cycloalkenyl acetates–NaCH(CO2R)2=2:8:100:200.
b Isolated yield based on cycloalkenyl acetates.
c Determined by HPLC.

848
S.-L. You et al. / Journal of Organometallic Chemistry 637–639 (2001) 845–849
121 (19), 57 (42), 44 (59). IR (KBr, cm−1) 3400 (w),
3054 (m), 2923 (m), 1705 (s), 1437 (s), 1156 (s), 1069
(w). HRMS Found: 414.0469. Calc. for C23H19O2PFe:
414.0466.
product. The enantiomeric purities were determined
by HPLC analysis.
4.6.1. Propanedioic acid, 2-cyclohexen-1-yl-dimethyl
ester
4.4. (S,Sp)-1-Diphenylphosphino-1%-[N-acetyl-2-iso-
propyl-2-aminoethoxycarbonyl]-2%-(trimethylsilyl)-
ferrocene (S,Sp)-8
1H-NMR (300 MHz, CDCl3) l 5.74–5.82 (m, 1H),
5.52 (dd, J=2.3, 10.2 Hz, 1H), 3.75 (s, 6H), 3.29 (d,
J=9.2 Hz, 1H), 2.86–2.95 (m, 1H), 1.96–2.06 (m,
2H), 1.26–1.81 (m, 4H). Chiralcel As, flow rate: 0.7
ml min−1, n-hexane–i-PrOH=100:4, 226 nm, 10.0
min (S), 11.2 min (R).
By a similar procedure as for (S,Sp)-6, (S,Sp)-8
was obtained in 54% yield (10 mmol scale) from
(S,Sp)-7 as a yellow solid: m.p. 85–86 °C; [h]2D0=
−132.0° (c 0.32, CHCl3). 1H-NMR l 7.24–7.40 (m,
10H), 6.05 (d, J=8.9 Hz, 1H), 4.85 (m, 1H), 4.45 (s,
1H), 4.35 (m, 1H), 4.07–4.28 (m, 7H), 1.99 (m, 1H),
1.87 (s, 3H), 1.03 (d, J=6.8 Hz, 3H), 1.01 (d, J=6.8
Hz, 3H), 0.27 (s, 9H); 31P-NMR (161.92 MHz,
CDCl3) l −17.29. MS; m/z (relative intensity): 613
[M+, 27], 486 (100), 442 (29), 321 (28), 170 (25), 86
(18). IR (KBr, cm−1) 3285, 2960, 1714, 1649, 1550,
1434, 1247, 1157, 835, 696. Anal. Found: C, 64.02;
H, 6.46; N, 2.19. Calc. for C33H40NO3PSiFe: C,
64.60; H, 6.57; N, 2.28%.
4.6.2. Propanedioic acid, 2-cyclohexen-1-yl-diethyl
ester
1H-NMR (300 MHz, CDCl3) l 5.73–5.80 (m, 1H),
5.53–5.57 (m, 1H), 4.16–4.24 (m, 4H), 3.24 (d, J=
9.4 Hz, 1H), 2.90 (m, 1H), 1.97–2.02 (m, 2H), 1.71–
1.77 (m, 2H), 1.57–1.63 (m, 1H), 1.37–1.41 (m, 1H),
1.27 (t, J=7.1 Hz, 6H). Chiralcel As, flow rate: 0.7
ml min−1, n-hexane–i-PrOH=95:5, 226 nm, tR 7.99,
9.44 min.
4.6.3. Propanedioic acid,
2-cyclohexen-1-yl-di-tert-butyl ester
4.5. (Sp)-1-Diphenylphosphino-2%-(trimethylsilyl)-
1%-ferrocene carboxylic acid (Sp)-4
1H-NMR (300 MHz, CDCl3) l 5.71–5.76 (m, 1H),
5.58–5.62 (m, 1H), 3.03 (d, J=9.2 Hz, 1H), 2.79 (m,
1H), 1.96–2.02 (m, 2H), 1.47 (s, 18H), 1.40–1.82 (m,
4H). Chiralcel As, flow rate: 0.7 ml min−1, n-hex-
ane–i-PrOH=100:0.1, 226 nm, tR 16.8, 19.0 min.
A similar procedure as for (Sp)-3 gave (Sp)-4 in
83% yield (1 mmol scale) from (S,Sp)-8: [h]2D0=
1
+36.5° (c 0.19, CHCl3). H-NMR (400 MHz, CDCl3)
l 7.25–7.38 (m, 10H), 4.90 (dd, J=1.4, 2.3 Hz, 1H),
4.47 (m, 1H), 4.43 (s, 1H), 4.33 (t, J=2.5 Hz, 1H),
4.29 (m, 1H), 4.23 (s, 1H), 4.19 (s, 1H), 0.28 (s, 9H);
31P-NMR (161.92 MHz, CDCl3) l −17.52. EIMS;
m/z (relative intensity): 486 [M+, 100], 471 (13), 442
(26), 321 (13), 226 (19). IR (KBr, cm−1) 2500–3000
(w), 1376 (s), 1459 (s), 1247 (m), 836 (s). Anal.
Found: C, 63.98; H, 5.53. Calc. for C26H27O2SiPFe:
C, 64.20; H, 5.59%.
4.6.4. Propanedioic acid, 2-cyclopenten-1-yl-dimethyl
ester
1H-NMR (300 MHz, CDCl3) l 5.82–5.85 (m, 1H),
5.64–5.67 (m, 1H), 3.75 (s, 6H), 3.26–3.37 (m, 2H),
2.30–2.38 (m, 2H), 2.09–2.16 (m, 1H), 1.54–1.63 (m,
1H). Chiralcel As, flow rate: 0.7 ml min−1, n-hex-
ane–i-PrOH=90:10, 226 nm, tR 9.18, 9.99 min.
4.6.5. Propanedioic acid, 2-cyclohepten-1-yl-dimethyl
ester
4.6. General procedure for the palladium-catalyzed
allylic alkylation of cycloalkenyl acetate
1H-NMR (300 MHz, CDCl3) l 5.81–5.85 (m, 1H),
5.57–5.62 (m, 1H), 3.74 (s, 6H), 3.47 (d, J=8.5 Hz,
1H), 3.05 (m, 1H), 2.12–2.18 (m, 1H), 1.92–1.97 (m,
1H), 1.59–1.69 (m, 3H), 1.33–1.41 (m, 2H). Chiralcel
As, flow rate: 0.7 ml min−1, n-hexane–i-PrOH=
90:10, 226 nm, tR 8.63, 9.59 min.
[Pd(h3-C3H5)Cl]2 (3.7 mg, 0.01 mmol) and ligand
(0.04 mmol) were dissolved in dry THF (2 ml), and
then stirred for 30 min at r.t. under Ar. To this
solution were successively added cycloalkenyl acetate
(0.5 mmol), sodium dimethylmalonate (1.0 mmol, pre-
pared in situ). The reaction mixture was stirred at r.t.
and monitored by TLC. After completion, the reac-
tion mixture was diluted with Et2O (20 ml) and
washed twice with ice-cold saturated aqueous NH4Cl.
The organic phase was dried over anhydrous Na2SO4
and then concentrated under reduced pressure. The
residue was purified by column chromatography
(EtOAc–petroleum ether=1:10) to afford the pure
Acknowledgements
This research was financially supported by National
Natural Science Foundation of China, the Major Ba-
sic Research Development Program (Grant No.
G2000077506), National Outstanding Youth Fund,
Chinese Academy of Sciences, and Shanghai Commit-
tee of Science and Technology.

S.-L. You et al. / Journal of Organometallic Chemistry 637–639 (2001) 845–849
849
[6] (a) S.-L. You, Y.-G. Zhou, X.-L. Hou, L.-X. Dai, Chem.
Commun. (1998) 2765;
References
(b) L.-X. Dai, X.-L. Hou, W.-P. Deng, S.-L. You, Y.-G. Zhou,
Pure Appl. Chem. 71 (1999) 1401;
(c) W.-P. Deng, X.-L. Hou, L.-X. Dai, Y.-H. Yu, W. Xia, Chem.
Commun. (2000) 285;
(d) S.-L. You, X.-L. Hou, L.-X. Dai, Tetrahedron: Asymmetry
11 (2000) 1495;
[1] Reviews: (a) G. Consiglio, R.M. Waymouth, Chem. Rev. 89
(1989) 257;
(b) C.G. Frost, J. Howarth, J.M.J. Williams, Tetrahedron:
Asymmetry 3 (1992) 1089;
(c) B.M. Trost, D.L. Van Vranken, Chem. Rev. 96 (1996) 395.
[2] Recent examples: (a) Y. Wang, H. Guo, K. Ding, Tetrahedron:
Asymmetry 11 (2000) 4153;
(e) W.-P. Deng, S.-L. You, X.-L. Hou, L.-X. Dai, Y.-H. Yu, W.
Xia, J. Sun, J. Am. Chem. Soc. 123 (2001) 6508.
[7] (a) S.-L. You, X.-L. Hou, L.-X. Dai, B.-X. Cao, J. Sun, Chem.
Commun. (2000) 1933;
(b) K. Yonehara, T. Hashizume, K. Mori, K. Ohe, S. Uemura,
J. Org. Chem. 64 (1999) 9374;
(c) N.-F.K. Kaiser, U. Bremberg, M. Larhed, C. Moberg, A.
Hallberg, J. Organomet. Chem. 603 (2000) 2;
(d) M.T. Reetz, S. Sostmann, J. Organomet. Chem. 603 (2000)
105.
(b) S.-L. You, X.-L. Hou, L.-X. Dai, X.-Z. Zhu, Org. Lett. 3
(2001) 149;
(c) S.-L. You, X.-L. Hou, L.-X. Dai, J. Organomet. Chem.
(2001), in press.
[3] (a) K. Ito, R. Kashiwagi, S. Hayashi, T. Uchida, T. Katsuki,
Synlett (2001) 284;
[8] J.M. Longmire, B. Wang, X. Zhang, Tetrahedron Lett. 41 (2000)
5435.
[9] (a) A.M. Warshawsky, A.I. Meyers, J. Am. Chem. Soc. 112
(1990) 8090;
(b) T.D. Nelson, A.I. Meyers, J. Org. Chem. 59 (1994) 2655.
[10] (a) T. Hayashi, M. Tajika, K. Tamao, M. Kumada, J. Am.
Chem. Soc. 98 (1976) 3718;
(b) S. Kudis, G. Helmchen, Angew. Chem. Int. Ed. Engl. 37
(1998) 3047;
(c) B.M. Trost, R. Radinov, J. Am. Chem. Soc. 119 (1997) 5962;
(d) D.A. Evans, K.R. Campos, J.S. Tedrow, F.E. Micheal, M.R.
Gagne`, J. Am. Chem. Soc. 122 (2000) 7905;
(e) P. Sennhenn, B. Gabler, G. Helmchen, Tetrahedron Lett. 35
(1994) 8595.
(b) T. Hayashi, M. Konoshi, M. Fukushima, T. Mise, M.
Kagotani, M. Tajika, M. Kumada, J. Am. Chem. Soc. 104
(1982) 180;
(c) C. Bolm, K. Muniz-Fernandez, A. Seger, G. Raabe, K.
Gunther, J. Org. Chem. 63 (1998) 7860.
[4] (a) Y. Okada, T. Minami, Y. Umezu, S. Nishikawa, R. Mori, Y.
Nakayama, Tetrahedron: Asymmetry 2 (1991) 667;
(b) G. Knu¨hl, P. Sennhenn, G. Helmchen, J. Chem. Soc. Chem.
Commun. (1995) 741.
[11] (a) X.-T. Zhou, Y.-R. Lin, L.-X. Dai, J. Org. Chem. 64 (1999)
1331;
[5] Reviews: (a) T. Hayashi, A. Togni (Eds.), Ferrocene, VCH,
Weiheim, Germany, 1995;
(b) C.J. Richards, A.J. Locke, Tetrahedron: Asymmetry 9 (1998)
2377.
(b) X.-D. Du, L.-X. Dai, X.-L. Hou, L.-J. Xia, M.-H. Tang,
Chin. J. Chem. 16 (1998) 90.