Asymmetric synthesis of a chiral diarsine ligand via a cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylarsine

Article abstract of DOI:10.1016/j.tetasy.2014.06.005

The organopalladium complex containing ortho-metallated (S)-[1-(dimethylamino)ethyl]naphthalene as a chiral auxiliary has been successfully employed to promote the asymmetric cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylarsine. In the intramolecular cycloaddition reaction, a pair of separable diastereomeric palladium complexes was obtained in the ratio of 6:1. The chiral naphthylamine auxiliary could be removed chemoselectively from the template by treatment with HCl and subsequently NaI to generate the neutral diiodo complex [(As-As)PdI ₂]. Treatment of the diiodo complex with KCN gave the enantiomerically pure As-As bidentate ligand in quantitative yield. In contrast to the reported similar P-P and As-P analogues, both arsenic donors in the diiodo complex could be readily eliminated to produce a structurally novel dimetallic complex.

Full text of DOI:10.1016/j.tetasy.2014.06.005

Tetrahedron: Asymmetry 25 (2014) 1100–1103
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Asymmetric synthesis of a chiral diarsine ligand via a cycloaddition
reaction between 3,4-dimethyl-1-phenylarsole and
diphenylvinylarsine
b
a
b,
c
Weiwei Yao a, Mengtao Ma b, , Weifan Wang , Jianming Cheng , Li Xu , Sumod A. Pullarkat ,
⇑
⇑
Pak-Hing Leung c
a College of Pharmacy, Nanjing University of Chinese Medicine, Nanjing 210023, People’s Republic of China
b College of Science, Nanjing Forestry University, Nanjing 210037, People’s Republic of China
c Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 April 2014
Accepted 6 June 2014
The organopalladium complex containing ortho-metallated (S)-[1-(dimethylamino)ethyl]naphthalene as
a chiral auxiliary has been successfully employed to promote the asymmetric cycloaddition reaction
between 3,4-dimethyl-1-phenylarsole and diphenylvinylarsine. In the intramolecular cycloaddition reac-
tion, a pair of separable diastereomeric palladium complexes was obtained in the ratio of 6:1. The chiral
naphthylamine auxiliary could be removed chemoselectively from the template by treatment with HCl
and subsequently NaI to generate the neutral diiodo complex [(As–As)PdI2]. Treatment of the diiodo com-
plex with KCN gave the enantiomerically pure As–As bidentate ligand in quantitative yield. In contrast to
the reported similar P–P and As–P analogues, both arsenic donors in the diiodo complex could be readily
eliminated to produce a structurally novel dimetallic complex.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
chiral diarsine ligands.12–16 In studies concerning the preparation
and stereochemistry of chiral arsine ligands, several types of
Since the organoarsenic compound ‘Salvarsan’1–3 was used as a
modern chemotherapeutic agent at the beginning of the 1910s,
which led to the rapid expansion of arsenic chemistry, arsine
ligands have been found to be more active and selective than the
corresponding phosphines in many transition-metal-catalysed
As–N, As–S, As–P and As–As bidentate ligands have been resolved
via metal complexation techniques,17,18 although the resolution
process has the limitation of a 50% yield for the separation of one
enantiomer and could become relatively inefficient when the tar-
get ligand has more than one stereogenic centre. In view of our
interest in the synthesis and resolution of chiral arsines, we have
recently reported on the asymmetric synthesis of a series of As–P
bidentate ligands using the well-known Diels–Alder reaction
which could be used to produce five stereogenic centres in one
step.19–25 We wondered if the above synthetic method could be
extended to the preparation of chiral As–As bidentate ligands.
Herein we report on the asymmetric synthesis of a chiral diarsine
ligand via the cycloaddition reaction between 3,4-dimethyl-1-
phenylarsole and diphenylvinylarsine using the organopalladium
complex as a chiral promoter.
organic reactions, due to their poorer
r
-donor ability.4–9
Subsequently much attention has been devoted to the study of
the enantiomerically pure arsine compounds, for example the
chiral R-ethylmethylpropylarsine can be obtained with 60% enan-
tiomeric purity by reductive methylation of ethylpropylarsinic acid
and trapping by a palladium complex.10 The phosphine ligand
(2,20-bis(diphenylphosphino)-1,10-binaphthyl) (BINAP) is widely
used in asymmetric synthesis. The chiral arsine analogue of BINAP,
2,20-bis(diphenylarsino)-1,10-binaphthyl (BINAs) was found to be
an effective ligand for an alkenyl iodide-using an intramolecular
asymmetric Heck reaction.11
In contrast to the many chiral phosphine-based ligands that
have been synthesized, relatively few arsine ligands have been pre-
pared and applied in organic reactions or medicine etc, particularly
2. Results and discussion
2.1. Synthesis of a chiral diarsine ligand
Since the Pd–Cl bond in (+)-1 is thermodynamically stable and
kinetically inert, the chloro ligand in (+)-1 was replaced with
⇑
Corresponding authors. Tel.: +86 025 85427621.
E-mail addresses: mengtao@njfu.edu.cn (M. Ma), xuliqby@njfu.edu.cn (L. Xu).
http://dx.doi.org/10.1016/j.tetasy.2014.06.005
0957-4166/Ó 2014 Elsevier Ltd. All rights reserved.

W. Yao et al. / Tetrahedron: Asymmetry 25 (2014) 1100–1103
1101
Me
Me
Me
Me
Me
Me
Ph
As
Cl
Me
N
Me
N
AgClO4
Ph2As
+
Pd
Pd
Ph
As
As
-
ClO4
Ph Ph
(_)-2
1
(+)-
Me
Me
Ph
As
Ph
As
I
I
Ph2As
KCN
15 min in CDCl3/H2O
conc. HCl, rt, 5 min
NaI, rt, 3 min
Me
Me
Pd
As
(_)-4
Ph Ph
_
3
( )-
Scheme 1.
perchlorate by using AgClO4 in dichloromethane in quantitative
yield.27 The asymmetric cycloaddition reaction between the result-
ing intermediate perchlorate complex and diphenylvinylarsine was
completed in 24 h at 40 °C and resulted in the formation of two
diastereomers in a ratio of 6:1 (Scheme 1). The major isomer
(ꢀ)-2 was isolated by column chromatography in 75% yield,
[
a
]
D = ꢀ91.8 (c 0.49, CH2Cl2), and was then recrystallized from
chloroform–diethyl ether in the form of pale yellow needles. The
Diels–Alder reaction was further proven by 1H NMR spectrum in
which the chemical shift of the proton of the double bond in DMPA
of (+)-1 was changed from 6.69 and 6.97 ppm20 to 2.95 and
3.80 ppm.
The chiral amine auxiliary on complex (ꢀ)-2 can be removed
from the template complex chemoselectively using concentrated
HCl for 5 min at room temperature. The resultant dichloro complex
is very unstable in solution and therefore it is very difficult to pur-
ify by column chromatography or fractional crystallization. Hence
it was further converted into the corresponding diiodo complex
(ꢀ)-3 by treatment with NaI for 3 min at room temperature. The
diiodo complex (ꢀ)-3 was readily isolated by column chromatog-
raphy in 70% yield, [
a]
D = ꢀ78.6 (c 0.14, CH2Cl2).
Due to the configurational instability of the uncoordinated
As–As bidentate ligand, the liberation process must be carried
out under strictly inert atmospheric conditions. A mixture of (ꢀ)-
3 and excess KCN in CDCl3 and H2O was shaken vigorously for
15 min at room temperature to liberate the optically active biden-
tate ligand (ꢀ)-4 as an air-sensitive solid in quantitative yield,
[
a]D = ꢀ81.7° (c 6.16, CDCl3).
Figure 1. Molecular structure of complex 5.
2.2. Arsenic elimination reaction
During the recrystallization of diiodo complex (ꢀ)-3, deep red
crystals were obtained (Scheme 2). From the X-ray structural anal-
ysis of this complex, however, it was evident that this was not the
original diiodo complex (ꢀ)-3; on the contrary an unexpected com-
plex 5 was obtained (Fig. 1). Selected bond lengths and angles are
listed in Table 2.
All three As–C bonds in the As–As bidentate arsanorbornene
skeleton undergone cleavage. This is in contrast to the analogous
As–P dichloro and dibromo complexes in which case only the
O
Me
Me
Ph
As
Ph2As
Pd
AsPh2
Pd
bridged head arsenic atom collapsed, while the
P was still
O2
retained.19–21 The coordination about each Pd atom is a distorted
octahedron. Analysis of the X-ray data showed that the two diph-
enylarsine ligands occupy the axial positions, whereas the terminal
iodo ligand, the two bridging iodo ligands and another palladium
metal can be considered to occupy the equatorial positions. The
two neighbouring side by side diphenylarsine groups were linked
via an oxygen bridge. This further proves that AsAC bonds are
I
I
I
I
I
5
Pd
I
As
Ph2As
AsPh2
Ph Ph
O
_
3
( )-
Scheme 2.

1102
W. Yao et al. / Tetrahedron: Asymmetry 25 (2014) 1100–1103
generally much weaker than their P–C counterparts. The PdAAs
bond length [2.379(1) Å] in 5 is in the range of those bond
distances observed in similar complexes.19–25 The As–O bond
[1.785(2) Å] is nearly identical to a reported corresponding bond
[1.786(4) Å].21 The lengths of three PdAI bonds [2.665(1),
2.743(1) and 2.954(1) Å] in 5 are longer than the reported Pd–I
bonds of the similar diiodide palladium complexes [2.620(1)–
2.660(1) Å] [19,21]. The AsAPdAAs bond angle [173.8(1)°] is
almost linear.
4.3. Removal of the chiral auxiliary: preparation of diiodo
complex (ꢀ)-3
Complex (ꢀ)-2 (0.29 g, 0.32 mmol) was dissolved in dichloro-
methane (50 mL) and treated with excess concentrated hydrochlo-
ric acid (2 mL) at room temperature for 5 min. The mixture was
then washed with water (3 ꢁ 50 mL). Next, sodium iodide (0.1 g)
in water (50 mL) was added and stirred vigorously for 3 min. The
organic layer was washed with water (3 ꢁ 50 mL) and dried
(MgSO4). The solvent was removed and the complex (ꢀ)-3 was iso-
lated by column chromatography on a silica column with dichloro-
3. Conclusion
methane (0.19 g, 70%). [
a
]
D = ꢀ78.6 (c 0.14, CH2Cl2). Mp: 85–86 °C.
Anal. Calcd for C26H26As2I2Pd: C, 36.8; H, 3.1. Found: C, 36.9; H, 3.2.
In conclusion, we have successfully synthesized a new enantio-
merically pure As–As bidentate compound in moderate selectivity
via the asymmetric cycloaddition reaction between 3,4-dimethyl-
1-phenylarsole and diphenylvinylarsine. It was found that the
arsenic elimination reaction was observed in the resultant diiodo
complex [(As–As)PdI2] in which three AsAC bonds collapsed to
produce a new dimetal complex.
1H NMR (CDCl3, d): 1.48 (s, 3H, @CCH3), 1.57 (s, 3H, @CCH3), 2.05
3
2
2
(dd, JHH = 9.5, JHH = 13.6 Hz, 1H, CHCH2), 2.55 (d, JHH = 13.0 Hz,
3
3
1H, CHCH2), 2.96 (dt, JHH = 2.1, JHH = 9.4 Hz, 1H, CHCH2), 3.07 (d,
3JHH = 2.1 Hz, 1H, AsCH), 3.52 (d, JHH = 2.6 Hz, 1H, AsCH), 7.37–
3
8.02 (m, 15H, aromatics). 13C NMR (CDCl3, d): 14.3, 15.7, 30.5,
31.4, 52.2, 56.3, 128.6, 129.1, 129.8, 129.9, 130.0, 130.9, 131.0,
131.1, 131.3, 131.6, 133.3, 133.7, 134.3, 135.8.
4. Experimental
4.1. General
4.4. Liberation of diarsine ligand (ꢀ)-4
A solution of (ꢀ)-3 (0.10 g, 0.12 mmol) and potassium cyanide
(1.0 g) in CDCl3 (0.6 mL) and H2O (0.1 mL) was shaken vigorously
for 15 min. The organic layer was separated, washed with water
(3 ꢁ 0.5 mL) and dried (MgSO4). Upon removal of the solvent, the
free ligand (ꢀ)-4 was obtained as an air-sensitive solid in quantita-
Reactions involving air-sensitive compounds were performed
under an inert atmosphere of argon using standard Schlenk tech-
niques. Solvents were dried and freshly distilled according to stan-
dard procedures and degassed prior to use when necessary. NMR
spectra were recorded at 25 °C on Bruker Avance 400 and 500
spectrometers. Optical rotations were measured on the specified
solution in a 0.1 dm cell at 20 °C with a Perkin–Elmer 341 polarim-
eter and the units are deg cm2 gꢀ1. Elemental analysis was per-
formed by the Elemental Analysis Laboratory of the Division of
Chemistry and Biological Chemistry at Nanyang Technological
University. Melting points were measured on a Stanford Research
Systems OptiMelt MPA 100 instrument and are uncorrected.
Diphenylvinylarsine26 and (+)-120 were prepared following litera-
ture procedures. Caution! Perchlorate salts of metal complexes
are potentially explosive compounds and should be handled with
care.
tive yield. [
a
]
D = ꢀ81.7 (c 6.16, CDCl3). 1H NMR (CDCl3, d): 1.43 (s,
3
3
3H, CH3), 1.48 (d, 3H, CH3), 2.02 (dt, JHH = 1.8, JHH = 11.4 Hz, 1H,
3
2
3
Ph2AsCH), 2.42 (ddd, JHH = 2.3, JHH = 4.4, JHH = 13.3 Hz, 1H,
CHCH2), 2.65 (s, 1H, PhAsCH), 2.79 (dd, 1H, 2JHH = 4.7, 3JHH = 8.9 Hz,
CHCH2), 2.93 (s, 1H, PhAsCH), 7.23–7.60 (m, 15H, aromatics).
4.5. Preparation of complex 5
Complex (ꢀ)-3 (0.09 g, 0.11 mmol) was dissolved in dichloro-
methane–diethyl ether and allowed to stand at room temperature
for several days; deep red crystals of 5 were obtained (0.02 g, 22%).
Mp: 174–175 °C. Anal. Calcd for C48H40As4I4O2Pd2: C, 34.5; H, 2.4.
Found: C, 34.1; H, 2.6. 1H NMR (CD2Cl2, d): 7.34–7.79 (m, 5H,
aromatics).
4.2. Cycloaddition reaction: preparation of complex (ꢀ)-2
A solution of (+)-1 (0.63 g, 1.10 mmol) in dichloromethane
(30 mL) was stirred for 2 h in the presence of a solution of silver
perchlorate (0.40 g) in water (1 mL). The organic layer, after
removal of AgCl, was then washed with water (3 ꢁ 50 mL), dried
(MgSO4) and subsequently treated with diphenylvinylarsine
(0.28 g, 1.10 mmol) at 40 °C for 24 h. The solvent was removed
from the reaction mixture, and complex (ꢀ)-2 was isolated by col-
umn chromatography on a silica column with dichloromethane–
diethyl ether to give a pale yellow solid, which was recrystallized
from chloroform–diethyl ether in the form of needles (0.74 g,
Table 1
Crystallographic data for complex 5
5
Formula
C48H40As4I4O2Pd2
1668.88
P1
Formula weight
Space group
Crystal system
a (Å)
b (Å)
c (Å)
Triclinic
10.0982(3)
10.6280(3)
12.4365(4)
100.939(2)
100.910(2)
97.918(2)
1265.74(7)
1
223(2)
2.189
0.71073
5.780
780
a
(°)
75%). [
a
]
D = ꢀ91.8 (c 0.49, CH2Cl2). Mp: 152–153 °C. Anal. Calcd
b (°)
for C40H42As2ClNO4Pd: C, 53.8; H, 4.7; N, 1.6. Found: C, 53.6; H,
c
(°)
4.9; N, 1.6. 1H NMR (CDCl3, d): 1.41 (s, 3H, @CCH3), 1.60 (s, 3H,
V (Å3)
3
2
Z
@CCH3), 2.03 (d, JHH = 6.2 Hz, 3H, CHCH3), 2.08 (d, JHH = 14.0 Hz,
1H, CHCH2), 2.51 (d, JHH = 14.0 Hz, 1H, CHCH2), 2.80 (s, 3H,
NCH3), 2.90 (s, 3H, NCH3), 2.95 (s, 1H, AsCH), 3.21 (d, JHH = 9.2 Hz,
1H, CHCH2), 3.80 (s, 1H, AsCH), 4.44 (q, JHH = 6.2 Hz, 1H, CHCH3),
2
T (K)
Dcalcd (g cmꢀ3
k (Å)
)
3
3
l
(mmꢀ1
)
6.87–8.04 (m, 21H, aromatics). 13C NMR (CDCl3, d): 14.1, 14.9,
25.3, 32.0, 35.3, 52.3, 52.7, 53.2, 54.9, 75.5, 123.9, 125.2, 125.7,
126.4, 127.6, 128.7, 128.9, 129.2, 129.3, 129.7, 130.2, 130.5,
130.6, 131.0, 131.6, 131.9, 132.1, 132.8, 133.0, 133.2, 133.4,
133.6, 133.9, 136.1, 136.8, 136.9, 151.9 152.2.
F (000)
R1 (obs data)a
0.0254
0.0616
wR2 (obs data)b
a
R1
wR2
=
R
=
||F0| ꢀ |Fc||/
R|F0|.
p
b
{
R
[w(F20 ꢀ F2c)2]/
R
[w(F20)2]}, wꢀ1
=
r
2(F0)2 + (aP)2 + bP.

W. Yao et al. / Tetrahedron: Asymmetry 25 (2014) 1100–1103
1103
Table 2
Research Foundation for the Returned Overseas Chinese Scholars,
State Education Ministry, China (Grant No. 20131792) and the Nat-
ural Science Foundation of Jiangsu Province, China (Grant No.
BK20130952) are gratefully acknowledged.
Selected bond lengths (Å) and angles (deg) for complex 5
Pd(1)–As(1)
Pd(1)–I(1)
Pd(1)–I(2)
As(1)–O(1)
As(1)–C(1)
As(2)–C(19)
I(2)–Pd(1)#1
As(1)–Pd(1)–As(2)
As(2)–Pd(1)–I(1)
As(2)–Pd(1)–I(2)#1
As(1)–Pd(1)–I(2)
I(1)–Pd(1)–I(2)
As(1)–Pd(1)–Pd(1)#1
I(1)–Pd(1)–Pd(1)#1
I(2)–Pd(1)–Pd(1)#1
O(1)–As(1)–C(1)
O(1)–As(1)–Pd(1)
C(1)–As(1)–Pd(1)
O(1)#1–As(2)–C(13)
O(1)#1–As(2)–Pd(1)
C(13)–As(2)–Pd(1)
2.379(1)
2.665(1)
2.954(1)
1.785(2)
1.934(4)
1.931(3)
2.7431(3)
173.8(1)
88.0(1)
89.9(1)
93.4(1)
110.8(1)
92.9(1)
166.0(1)
55.2(1)
Pd(1)–As(2)
Pd(1)–I(2)#1
Pd(1)–Pd(1)#1
As(1)–C(7)
As(2)–O(1)#1
As(2)–C(13)
O(1)–As(2)#1
As(1)–Pd(1)–I(1)
As(1)–Pd(1)–I(2)#1
I(1)–Pd(1)–I(2)#1
As(2)–Pd(1)–I(2)
I(2)#1–Pd(1)–I(2)
As(2)–Pd(1)–Pd(1)#1
I(2)#1–Pd(1)–Pd(1)#1
O(1)–As(1)–C(7)
C(7)–As(1)–C(1)
C(7)–As(1)–Pd(1)
O(1)#1–As(2)–C(19)
C(19)–As(2)–C(13)
C(19)–As(2)–Pd(1)
2.379(1)
2.743(1)
2.966(1)
1.931(3)
1.783(2)
1.938(3)
1.783(2)
87.8(1)
89.5(1)
131.8(1)
92.4(1)
117.4(1)
92.3(1)
62.2(1)
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Acknowledgements
Financial supports from the National Natural Science Founda-
tion of China (Grant Nos. 21372117 and 81173540), the Scientific