Template effects on the asymmetric cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylphosphine and their arsenic elimination reaction

Article abstract of DOI:10.1016/j.jorganchem.2009.01.044

The organopalladium complex containing ortho-metalated (S)-[1-(dimethylamino)ethyl]naphthalene as the chiral template was employed to promote the asymmetric cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylphosphine. It has been proven that the chiral template incorporating napthylamine is more efficient than the benzylamine based analogue as evidenced by the drastic improvement in stereoselectivity and reaction rate. However, when no chiral template was employed, trans-[PdI₂(3,4-dimethyl-1-phenylarsole)₂] reacted with trans-[PdI₂(diphenylvinylphosphine)₂] producing a structurally novel diiodo complex, as a result of an interesting selective cleavage of one As-C bond in the norbornene skeleton and subsequent rearrangements within the skeletal framework. The molecular structure of the diiodo product has been confirmed by X-ray crystallography. Structural analysis showed that in addition to the normal As-P five-membered ring, there is one new five-membered ring containing As-O bond being formed during the course of the reaction along with another seven-membered ring incorporating a hydroxy group.

Full text of DOI:10.1016/j.jorganchem.2009.01.044

Journal of Organometallic Chemistry 694 (2009) 1929–1933
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Template effects on the asymmetric cycloaddition reaction between
3,4-dimethyl-1-phenylarsole and diphenylvinylphosphine and their
arsenic elimination reaction
*
Mengtao Ma, Sumod A. Pullarkat, Ke Chen, Yongxin Li, Pak-Hing Leung
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:
The organopalladium complex containing ortho-metalated (S)-[1-(dimethylamino)ethyl]naphthalene as
the chiral template was employed to promote the asymmetric cycloaddition reaction between 3,4-
dimethyl-1-phenylarsole and diphenylvinylphosphine. It has been proven that the chiral template incor-
porating napthylamine is more efficient than the benzylamine based analogue as evidenced by the drastic
improvement in stereoselectivity and reaction rate. However, when no chiral template was employed,
trans-[PdI2(3,4-dimethyl-1-phenylarsole)2] reacted with trans-[PdI2(diphenylvinylphosphine)2] produc-
ing a structurally novel diiodo complex, as a result of an interesting selective cleavage of one As–C bond
in the norbornene skeleton and subsequent rearrangements within the skeletal framework. The molec-
ular structure of the diiodo product has been confirmed by X-ray crystallography. Structural analysis
showed that in addition to the normal As–P five-membered ring, there is one new five-membered ring
containing As–O bond being formed during the course of the reaction along with another seven-mem-
bered ring incorporating a hydroxy group.
Received 29 October 2008
Received in revised form 16 January 2009
Accepted 20 January 2009
Available online 4 February 2009
Keywords:
Asymmetric Diels–Alder reaction
As–P ligand
Template effects
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
arene and arsinidene complex which in one case has been trapped
through consecutive insertion reactions [8].
Quite recently, we have reported that the asymmetric
cycloaddition reaction between diphenylvinylphosphine and
3,4-dimethyl-1-phenylarsole (DMPA) can be promoted by the
organopalladium complex containing ortho-metalated (S)-[1-
(dimethylamino)ethyl]phenylene as the chiral auxiliary, the
arsenic donor in the dichloro and dibromo palladium complexes
containing the diphenylphosphino-substituted asymmetrical
heterobidentate arsanorbornene ligand could be eliminated
stereospecifically under mild reaction conditions to generate
the corresponding 1-(diphenylphosphino)-3,4-dimethyl-2,4-cyclo-
hexadiene, which remained as a bidentate ligand at the PdCl2 unit
In order to further understand the intricacies of the stereoselec-
tivity and stability factors influencing the As–C bonds in such sys-
tems, we undertook a synthesis of the cycloadduct where the
reaction was allowed to proceed under the assistance and influ-
ence of the corresponding naphthylamine auxiliary as well as in
the absence of any chiral auxiliary. It was observed that the
stereoselectivity of the Diels–Alder reaction between DMPA and
diphenylvinylphosphine has been vastly improved using the orga-
nopalladium complex containing ortho-metalated (S)-[1-(dimeth-
ylamino)ethyl]naphthalene as the chiral template in comparison
with the earlier reported cycloaddition [1]. More interestingly,
the trans-[PdI2(DMPA)2] complex reacted with trans-[PdI2(DPVP)2]
(DPVP = diphenylvinylphosphine) as a result of which a different
mode of arsenic elimination reaction coupled with a novel skeletal
rearrangement was found to occur.
via phosphorus and the
ination process in such cases was found to be influenced by the
halo ligand in the [PdX2(As–P)] complex.
g
2-C–C double bond [1]. The arsenic elim-
Compared to the stable phosphole complexes in which a few
phosphorus elimination reactions have been reported [2–4],
arsenic elimination appeared to be a common phenomenon in
the cycloaddition reactions involving arsoles [5–7]. For example,
2,3,4,5-tetramethyl-1-phenylarsole and 1,2,3,4,5-pentamethylar-
sole metal complexes readily undergo Diels–Alder reaction with
acetylenedicarboxylic acid dimethyl ester. The 7-arsanorbornadi-
enes formed as intermediates are unstable and decompose into
2. Results and discussion
2.1. Asymmetric cycloaddition reaction between DMPA and
diphenylvinylphosphine promoted by the Pd
naphthylamine template
Treatment of complex (+)-1 with silver perchlorate yielded the
intermediate perchlorate complex in essentially quantitative yield.
This highly reactive species was not isolated and upon subsequent
* Corresponding author. Tel.: +65 63168905; fax: +65 63166984.
E-mail address: pakhing@ntu.edu.sg (P.-H. Leung).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.01.044

1930
M. Ma et al. / Journal of Organometallic Chemistry 694 (2009) 1929–1933
removal of AgCl and excess silver perchlorate, the dichloromethane
solution of the complex was treated directly with a stoichiometric
amount of diphenylvinylphosphine (Scheme 1). This cycloaddition
reaction was monitored by 31P{1H} NMR spectroscopy and was
found to be completed within 40 min at room temperature. Prior
to purification, the 31P{1H} NMR spectrum of the crude Diels–Alder
reaction in CDCl3 exhibited only two singlets at d 50.2 and 48.6 in
the ratio 19:1, thus indicating that only two stereochemically dis-
tinct cycloadducts were formed with high stereoselectivity being
exercised. Compared to the same reaction conducted using benzyl-
amine auxiliary as chiral template, the selectivity improved
roughly six fold (from 3.2:1 to 19:1) and the reaction time reduced
significantly from 3 h to 40 min [1]. The reason is that the
naphthylamine auxiliary is superior to the benzylamine analogue
in certain asymmetric synthesis scenarios due to the unique ste-
reochemistry associated with the rigid ortho-metalated naphthyl-
amine ring [9]. For example, there is an internal steric repulsion
between the methyl substituent on the stereogenic carbon and
its neighboring naphthylene proton in complex (+)-1. The crystal-
lographic determinations and rotating overhauser effect (ROESY)
NMR investigations confirmed that the organometallic ring is
locked into the static k conformation, both in the solid state and
in solution [10]. Thus the prochiral NMe groups are fixed into the
non-equivalent axial and equatorial positions. These NMe groups
control the stereochemistry of the neighboring coordination sites.
On the other hand, the stereochemistry of the five-membered
organometallic ring in the corresponding benzylamine complex
cannot be well defined, as the puckered benzylamine ring under-
goes rapid transformation between the two non-equivalent d and
k conformations in solution.
Fig. 1. Molecular structure of complex (À)-2.
Table 1
Selected bond lengths (Å) and angles (°) for complex (À)-2.
Pd1–C1
Pd1–As1
As1–C21
C21–C28
C22–C24
C26–C27
C1–Pd1–N1
N1–Pd1–P1
N1–Pd1–As1
C26–As1–C21
C24–C22–C21
C24–C26–C27
C27–C28–C21
2.024(14)
2.443(2)
1.984(13)
1.57(2)
Pd1–N1
Pd1–P1
As1–C26
C21–C22
2.095(13)
2.264(4)
1.968(14)
1.579(19)
1.548(19)
1.540(17)
95.7(4)
173.7(4)
83.2(1)
105.3(12)
113.0(13)
105.0(10)
1.277(18)
1.590(18)
82.3(5)
174.0(3)
99.4(3)
C24–C26
C27–C28
The major diastereomer (À)-2 was obtained as pale yellow nee-
dles from dichloromethane–diethyl ether in 73% isolated yield,
C1–Pd1–P1
C1–Pd1–As1
P1–Pd1–As1
C28–C21–C22
C22–C24–C26
C28–C27–C26
[
a
]D = À33.0° (c 0.6, CH2Cl2). The molecular structure and absolute
confirmation of complex (À)-2 have been resolved by X-ray crys-
tallography (Fig. 1). Selected bond lengths and angles are listed
in Table 1. It is structurally the same as the one obtained by utiliz-
ing the analogous benzyl complex, the arsenic is trans to the aro-
matic carbon and the phosphorus is trans to nitrogen as expected
from the hard soft preference exerted by the template in such sys-
tems. Four new chiral centers have been generated with R absolute
stereochemistry at As(1) and R, R and S stereochemistry at C(21),
C(26) and C(27). The naphthylamine auxiliary could be removed
chemoselectively from (À)-2 by treatment with concentrated
77.5(6)
112.0(12)
110.0(10)
109.1(10)
hydrochloric acid to generate the dichloro complex (À)-3 at room
temperature. The same phenomenon as that in the case of our ear-
lier report was observed with the two As–C bridgehead bonds in
Me
Me
Me
Me
Me
Me
Ph
As
Me
Me
N
N
Cl
AgClO4
+
Pd
Pd
As
Ph2PCH=CH2
P
Ph
-
Ph2
ClO4
Me
1
(+)-
(-)-2
Me
Me
Ph
As
Me
Cl
Cl
Cl
Cl
air
-PhAsO(OH)2
conc. HCl
Pd
Pd
P
P
Ph2
Ph2
(-)-3
4
(+)-
Scheme 1.

M. Ma et al. / Journal of Organometallic Chemistry 694 (2009) 1929–1933
1931
(À)-3 easily collapsing to give the new
(+)-4 [1].
g
2-P palladium complex
2.2. Cycloaddition reaction between DMPA and
diphenylvinylphosphine without template
DMPA was coordinated to the [PdCl2(NCMe)2], resulting in the
cis and trans isomeric mixture of the dichloro complex 5 with the
two DMPA ligands coordinated to the Pd center (Scheme 2). In fact,
the mixture of trans and cis dichloro complexes of 5 need not be
separated because when the complex 5 was subsequently con-
verted to the analogous diiodo complex 6 in high yield (97%) by
treatment with sodium iodide at room temperature, only the trans
isomer was obtained. This can be confirmed by the single crystal X-
ray analysis of complex 6 in which the arsenic is trans to each other
(Fig. 2). Selected bond lengths and angles are listed in Table 2. The
geometry at palladium is a distorted square planar with bond
angles at metal center ranging between 87.7(1)–92.3(1) and
180.0(1)°.
The diiodo complex 6 was then allowed to react with trans-
[PdI2(DPVP)2] at room temperature. The Diels–Alder reaction was
monitored by the 31P{1H} NMR spectrum. Analysis of the crude
reaction mixture indicated the formation of several products
amongst which complex 7 was subsequently isolated by recrystal-
lization and other products cannot be isolated. The molecular
structure of complex 7 was confirmed by X-ray crystallography
and indicated the formation of a ligand system was the result of
a very unusual rearrangement in norbornene skeletal system
formed as an intermediate (Fig. 3). Selected bond lengths and an-
gles are listed in Table 3. Upon further detailed analysis of the
31P{1H} NMR spectral data of the reaction it was seen that when
the two trans diiodo complexes, 6 and [PdI2(DPVP)2], were mixed
together, a new peak at d 13.0 appeared immediately which can
be attributed to the trans or cis intermediate [PdI2(DMPA)(DPVP)]
[11]. After 1 day, the Diels–Alder reaction product was found to
be formed as observed from the 31P{1H} signal at d 37.7 which is
identical to the value obtained for the enantiomerically pure
Fig. 2. Molecular structure of complex 6.
Table 2
Selected bond lengths (Å) and angles (°) for complex 6.
Pd1–I1
Pd1–As1
As1–C1
C1–C2
C2–C5
2.600(1)
2.394(1)
1.911(2)
1.341(3)
1.496(3)
1.500(3)
180.0(1)
92.3(1)
Pd1–I1A
Pd1–As1A
As1–C4
C2–C3
2.600(1)
2.394(1)
1.916(2)
1.497(3)
1.340(3)
C3–C4
C3–C6
I1–Pd1–I1A
I1–Pd1–As1
I1–Pd1–As1A
C1–As1–C4
C1–C2–C3
C3–C4–As1
I1A–Pd1–As1
I1A–Pd1–As1A
As1–Pd1–As1A
C2–C1–As1
87.7(1)
92.3(1)
180.0(1)
110.5(1)
115.9(2)
87.7(1)
88.0(1)
115.4(2)
110.1(2)
C4–C3–C2
tr ans-PdI2(DPVP)2
2 DMPA
NaI
PdCl2(DMPA)2
PdCl2(NCMe)2
PdI2(DMPA)2
5
tr ans and cis-
tr ans-6
CH3
CH2
Me
Ph
Me
Ph
O
Ph
Pd
I
As
P
As
P
I
Pd
OH
7
I
I
Ph
Ph
Ph
O2 (air)
H2O
Ph
Me
OH
Me
As
OH
OH
Ph
Me
OH
CH2
Ph
M e
Ph
CH2 OOH
Me
OH
Ph
Pd
O
I
I
I
As
P
As
I
As
O2
Me
OH
H2O
Pd
Pd
Pd
OH
I
I
I
I
P
P
P
Schenck Ene Reaction
Reduction
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Scheme 2.

1932
M. Ma et al. / Journal of Organometallic Chemistry 694 (2009) 1929–1933
The diiodo complex 7 is very different from the
g
2-P palla-
dium complexes formed as a result of As–C bridghead bond col-
lapse, only one As–C bond broke in this case, and meanwhile
one new five-member ring containing As–O and the seven-mem-
ber ring containing hydroxyl group were generated during the
course of the reaction. To our knowledge this is the first instance
of such unique bridgehead collapse coupled with rearrangement
observed in cyclic arsenic systems. We are currently in the pro-
cess of fine-tuning the conditions in order to improve the yield
of the reaction which will allow us to do a more thorough inves-
tigation of the mechanism involved as well as pave the way for
potential application of the ligand system in various catalytic
scenarios.
In conclusion, we have found that the template has a vital effect
on the reaction between DMPA and diphenylvinylphosphine. Com-
pared to the benzyl analogue, the naphthylamine is a better chiral
auxiliary in exerting stereospecific control over the formation of
the hetero bidentate As–P ligand. However, in the absence of any
template, a new arsenic elimination coupled with ring rearrange-
ment was observed. Finally, it is confirmed that it is a common fea-
ture that As–C bonds cleaved in such arsanorbornene (As–P) units
but interestingly there was a selective cleavage of only one As–C
bridgehead bond in the latter scenario where no chiral template
was employed. We are currently investigating the reactions
employing other substrates in order to further understand the pro-
cesses involved in both the template promoted and non-template
promoted reactions involving cyclic arsines.
Fig. 3. Molecular structure of complex 7.
3. Experimental
Table 3
Selected bond lengths (Å) and angles (°) for complex 7.
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. The 1H
and 31P{1H} NMR spectra were recorded at 25 °C on Bruker Avance
300 and 400 spectrometers. Optical rotations were measured on
the specified solution in a 0.1 dm cell at 20 °C with a Perkin–Elmer
341 polarimeter. Elemental analysis was performed by the Ele-
mental Analysis Laboratory of the Division of Chemistry and Bio-
logical Chemistry at Nanyang Technological University. Melting
points are uncorrected.
Pd1–P1
Pd1–I2
As1–O1
C7–C8
C8–C9
C10–C11
C10–C12
C12–C13
P1–Pd1–As1
As1–Pd1–I2
As1–Pd1–I1
O1–As1–C7
C9–C8–C7
C7–C8–C10
C8–C10–C12
O2–C12–C13
C13–C12–C10
C7–C14–C13
2.276(1)
2.620(1)
1.786(4)
1.490(7)
1.304(9)
1.516(8)
1.549(8)
1.525(7)
83.7(1)
Pd1–As1
Pd1–I1
As1–C7
C7–C14
C8–C10
C10–O1
C12–O2
C13–C14
P1–Pd1–I2
P1–Pd1–I1
I2–Pd1–I1
C8–C7–C14
C9–C8–C10
C8–C10–O1
O1–C10–C12
O2–C12–C10
C12–C13–C14
2.345(1)
2.649(1)
1.976(6)
1.537(7)
1.509(8)
1.531(7)
1.416(7)
1.553(7)
91.8(1)
173.4(1)
89.2(1)
89.6(2)
172.7(1)
95.4(1)
112.9(5)
126.8(6)
106.9(5)
106.5(4)
107.1(5)
116.3(4)
125.9(6)
107.3(5)
108.5(5)
113.0(5)
111.2(4)
111.7(4)
(+)-1 [17], diphenylvinylphosphine [18] and DMPA [19] were
prepared following the literature procedures.
Caution! Perchlorate salts of metal complexes are potentially
explosive compounds and should be handled with care.
3.1. Cycloaddition reaction: preparation of complex (À)-2
A solution of (+)-1 (0.59 g, 1.03 mmol) in dichloromethane
(50 mL) was stirred for 2 h in the presence of a solution of silver
perchlorate (0.36 g) in water (1 mL). The organic layer, after the re-
moval of AgCl, was then washed with water (3 Â 50 mL), dried
(MgSO4), and subsequently treated with diphenylvinylphosphine
(0.22 g, 1.03 mmol) for 40 min at room temperature. Removal of
the solvent gave (À)-2 as a yellow solid, which was then recrystal-
lized from dichloromethane–diethyl ether to give the complex as
version of the cycloadduct obtained as part of our previous studies
involving DMPA and diphenylvinyphosphine albeit in that case the
reaction was promoted by the chiral auxiliary [1]. Cyclic phosphine
oxides with contracted internal C–P–C angles have been found to
undergo oxygen insertion into a C–P bond [12,13]. The same phe-
nomenon may occur in the cycloadduct resulting in oxygen being
inserted into the As–C bond of the complex and thereafter, due
to the inherent instability of such compounds, it results in a hydro-
lysis [12]. Subsequently a Schenck ene reaction [14] caused double
bond migration to a methyl group [15]. Finally, the reduction of the
peroxide [16] gave the alcohol intermediate which eliminated a
molecular of water to yield the complex 7. Taking into consider-
ation the low yield and numerous unidentified side products ob-
tained along with the fact that the presence of phenylarsonic
acid in similar reactions have been reported earlier [1], the possi-
bility of the acid acting as a reductant in this reaction could not
be ruled out.
pale yellow needle crystals (0.66 g, 73%). [
a
]
D = À33.0° (c 0.6,
CH2Cl2). M.p.: 161–162 °C. Anal. Calc. for C40H42AsClNO4PPd Á 1/
3CH2Cl2: C, 55.3; H, 4.9; N, 1.6. Found: C, 55.7; H, 5.0; N, 1.7%.
31P{1H} NMR (CDCl3, d): 50.2. 1H NMR (CDCl3, d): 1.41 (s, 3H,
@CCH3), 1.61 (s, 3H, @CCH3), 1.92 (m, 1H, CHCH2), 2.03 (d,
3
2
3JHH = 6.1 Hz, 3H, CHCH3), 2.52 (dd, JPH = 24.5, JHH = 13.6 Hz, 1H,
CHCH2), 2.79 (s, 3H, NCH3), 2.84 (s, 1H, AsCH), 2.93 (d, 4JPH = 3.1 Hz,
3
3H, NCH3), 3.15 (t, JHH = 9.0 Hz, 1H, PCH), 3.81 (s, 1H, AsCH), 4.46
(qn, 3JHH = 4JPH = 5.9 Hz, 1H, CHCH3), 6.68–8.26 (m, 21H, aromatics).

M. Ma et al. / Journal of Organometallic Chemistry 694 (2009) 1929–1933
1933
Table 4
NMR (CD2Cl2, d): 1.64 (s, 3H, CCH3), 2.04 (s, 1H, OCH), 2.74 (m,
4
Crystallographic data for complexes (°)-2, 6 and 7.
1H, CHCH2), 3.02 (d, JHH = 3.4 Hz, 1H, AsCH), 3.26 (m, 1H, CHCH2),
2
3.92 (d, JPH = 2.6 Hz, 1H, PCH), 5.00 (s, 1H, @CH2), 5.13 (d,
(À)-2
C40H42AsClNO4PPd Á
1/3CH2Cl2
876.79
6
7
4JHH = 4.2 Hz, 1H, @CH2), 7.00–8.19 (m, 15H, aromatics). 13C NMR
(CD2Cl2, d) [20]: 20.1, 30.5, 38.6, 38.9, 73.1, 73.2, 86.6, 113.9,
125.2, 128.1, 128.2, 128.7, 129.8, 129.9, 130.3, 132.1, 132.4,
132.9, 134.1, 134.2, 135.0, 135.1, 136.6. EI-MS: m/z 837.3, [M]+.
Formula
C24H26As2I2Pd
C26H26AsI2O2PPd Á
1/2CH2Cl2
878.52
Formula weight
Space group
Crystal system
a (Å)
b (Å)
c (Å)
824.49
C2/c
ꢀ
C2
P1
Monoclinic
30.3585(16)
17.5355(7)
25.7191(8)
90
90.066(5)
90
13691.6(10)
12
173(2)
1.276
0.71073
1.294
5352
Monoclinic
17.0458(9)
7.5236(3)
20.0657(8)
90
95.381(3)
90
2562.0(2)
4
173(2)
2.138
0.71073
5.707
1552
1.135
Triclinic
9.9521(3)
11.7499(4)
14.3409(5)
108.790(2)
91.610(2)
111.672(2)
1454.62(8)
2
173(2)
2.006
0.71073
4.058
837
3.5. X-raycrystal structure determination of complexes (À)-2, 6 and 7
a
(°)
Crystal data for all complexes and a summary of the crystallo-
graphic analysis are given in Table 4. Diffraction data were col-
b (°)
c
(°)
lected on a Bruker X8 CCD diffractometer with Mo K
a radiation
V (Å3)
Z
(graphite monochromator). SADABS absorption corrections were ap-
plied. All non-hydrogen atoms were refined anisotropically, while
the hydrogen atoms were introduced at calculated positions and
refined riding on their carrier atoms. The absolute configuration
of the chiral complex was determined unambiguously by using
the Flack parameter.
T (K)
Dcalc (g cmÀ3
k (Å)
)
l
(mmÀ1
)
F(000)
GOF on F2
0.909
0.0864
0.2032
1.038
0.0459
0.1041
R1 (observed data)
wR2 (observed data)
0.0179
0.0428
4. Supplementary material
3.2. Synthesis of the dichloro complex 5
CCDC 694659, 694661 and 649745 contain the supplementary
crystallographic data for (À)-2, 6 and 7. These data can be obtained
free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
The complex [PdCl2(NCMe)2] (0.11 g, 0.43 mmol) and the ligand
DMPA (0.20 g, 0.86 mmol) in dichloromethane (60 mL) were stir-
red at room temperature overnight. The solvent was removed
and the residue was crystallized with chloroform–diethyl ether
to give the product (trans and cis mixture) as yellow crystals
(0.45 g, 82%), which was further recrystallized with dichlorometh-
ane–diethyl ether to produce pure trans isomer 5. M.p.: 177–
178 °C. Anal. Calc. for C24H26As2Cl2Pd: C, 44.9; H, 4.1. Found: C,
44.4; H, 4.4%. 1H NMR (CDCl3, d): 2.04 (s, 6H, CH3), 6.60 (s, 2H,
@CH), 7.33–7.62 (m, 5H, aromatics).
Acknowledgements
We thank Nanyang Technological University for support of this
research and for a scholarship to Ma M.T.
References
3.3. Synthesis of the trans-diiodo complex 6
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The solution of [PdCl2(DMPA)2] (trans and cis mixture) (0.45 g,
0.70 mmol) in dichloromethane (50 mL) was added to sodium
iodide (0.5 g) in acetone (50 mL) and was stirred vigorously for
10 min. The solvent was removed and the residue was extracted
with dichloromethane, Removal of solvent gave 6 as a solid, which
was then recrystallized from dichloromethane–diethyl ether to
give the product as red crystals (0.56 g, 97%). M.p.: 192–193 °C.
Anal. Calc. for C24H26As2I2Pd: C, 35.0; H, 3.2. Found: C, 34.8; H,
3.1%. 1H NMR (CDCl3, d): 2.11 (s, 6H, CH3), 7.03 (s, 2H, @CH),
7.33–7.72 (m, 5H, aromatics).
3.4. Synthesis of the diiodo complex 7
The complexes [PdI2(DMPA)2] (0.45 g, 0.55 mmol) and
[PdI2(DPVP)2] (0.43 g, 0.55 mmol) in dichloromethane (90 mL)
were stirred at 30 °C for 5 days. The solvent was removed and
the residue was recrystallized with dichloromethane–diethyl ether
from À78 °C to room temperature to produce brown crystals
(0.05 g, 5%). M.p.: 180–181 °C. 31P{1H} NMR (CD2Cl2, d): 79.0. 1H
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[20] Because there is extensive 31P–13C coupling present in compound 7, hence it is
difficult to definitely assign the 13C signals.