The stereoselective synthesis of E-1,2-bis(3-indolizinyl)ethylenes by the reactions of 3-thioformylindolizines with n-tributylphosphine

Article abstract of DOI:10.1002/jhet.5570450509

(Chemical Equation Presented) 3-Thioformylindolizines undergo novel reductive coupling reaction in the presence of tributylphosphine to give E-1,2-bis(3-indolizinyl)ethylenes in high yield. These reactions proceed via (3-indolizinyl)methylene carbene intermediate and provide a new, stereoselective synthesis of the bis(3-indolizinyl)ethylene derivatives highly.

Full text of DOI:10.1002/jhet.5570450509

Sep-Oct 2008
1311
The Stereoselective Synthesis of E-1,2-Bis(3-indolizinyl)ethylenes
by the Reactions of 3-Thioformylindolizines with n-Tributylphosphine
Weiwei Liu,a,c* Shujiang Tu,b Ting He,c Yueqiang Zhao,a and Hongwen Huc*
a School of Chemical Engineering, Huaihai Institute of Technology, Lianyungang, 222005, P. R. China
b School of Chemistry and Chemical Engineering, Xuzhou Normal University, Xuzhou, 221116, P. R. China
cDepartment of Chemistry, Nanjing University, Nanjing, 210093, P. R. China
Received August 31, 2007
R1
R3
R4
H
C
R2
R4
R1
n-Bu3P
N
R2
S
N
C
THF
N
CH
R3
R1
H
R2
R4
R3
1
2
3-Thioformylindolizines undergo novel reductive coupling reaction in the presence of tributylphosphine
to give E-1,2-bis(3-indolizinyl)ethylenes in high yield. These reactions proceed via (3-
indolizinyl)methylene carbene intermediate and provide a new, stereoselective synthesis of the bis(3-
indolizinyl)ethylene derivatives highly.
J. Heterocyclic Chem., 45, 1311 (2008).
and ꢀ- electron excessive indolizinyl. Indolizine derivatives
themselves constitute an important and interesting class of
heterocycles with special electronic structure and increasing
applications in medicinal chemistry [12] and in optical and
electronic materials [13]. Therefore, their synthesis and
structural elaboration have also received much recent
attention [14]. Since 3-thioformylindolizines are relatively
easily accessible [11], we envision that not only they may
serve as good model compounds to investigate the
reactivity of thioaldehydes, but also the reactions of
thioformylindolizines may provide new synthetic methods
for the structural modification of indolizines. We repot here
the reductive coupling reactions of 3-thioformylindolizines
in the presence of n-tributylphosphine, leading to the highly
stereoselective synthesis of the novel stilbene heterocyclic
analogue E-1,2-bis(3-indolizinyl)ethylenes in good yield.
INTRODUCTION
Stilbene derivatives have a wide range of biological
activity [1]. Their unique photoelectronic properties have
also drawn much recent research interest [2]. Replacing
the two phenyls in stilbenes by heterocycles leads to their
heterocyclic analogs. In comparison with stilbenes, these
heterostilbenes have many different and special
properties. As examples, many heterostilbenes have
significantly red shifted absorption and emission spectra
[3], increased molecular polarizability [4], more sensitive
response to external electric and magnetic field change
[5], etc. As a result, heterostilbenes have gained wide
applications in diversified areas such as nonlinear optics
[6], solvatochromism [7], etc. In response to the
increasing applications of heterostilbenes, their synthesis
has emerged as an active new research area [8].
The chemical reactions of thiocarbonyl compounds have
been of much research interest because of their application
in the synthesis of heterocycles and natural products, and in
C-C bond formation [9]. It is anticipated that the smaller
steric hindrance at the thiocarbonyl group in thioaldehyle as
compared to the thioketones would enable the former to
undergo more diversified reactions. However, thio-
aldehydes are usually unstable and are prone to oligomerize
[10], and only when either electronic stabilization or steric
protection to the thioformyl functionality exists can
thioaldehydes be stable at ambient temperature. Therefore,
the chemistry of thiocarbonyl compounds have been mainly
investigated with thioketones as substrates [9,10], while the
thioaldehyde chemistry is much less explored. Belonging to
the small group of known stable thioaldehydes, 3-
thioformylindolizines [11] have their stability inherently
originating from the extensive electron delocalization
between the thioformyl and the strongly electron donating
RESULTS AND DISCUSSION
Reactions of 3-thioformylindolizines 1a-1g with two
phosplines (triphenylphosphine, n-tributylphosphine) were
tested. It is found that, refluxing 1a with 3~4 equivalent
amount of triphenylphosphine in dry THF under nitrogen
atmosphere for 16 h resulted in no appreciable reaction as
evidenced by TLC monitoring of the reaction course.
However, refluxing 1a with the more nucleophilic
n-tributylphosphine under similar conditions led to smooth
reaction, which was completed within 10 h. Chromato-
graphic separation of the reaction mixture afforded an
orange colored product which, according to spectroscopic
(1H NMR, IR, MS) and elemental analytical data, turned
out to be the E-1,2-bis(2-phenyl-3-indolizinyl)ethylene 2a
(Scheme 1 and Table 1). Different conditions for this
reaction were examined. Reaction of 1a with n-Bu3P in
refluxing DMF resulted in lower yield caused by the

1312
W. Liu, S. Tu, T. He, Y. Zhao, and H. Hu
Vol 45
occurrence of side reactions. Reaction in THF at lower
temperature, on the other hand, led to a lengthening of
reaction time. Increasing the amount of n-Bu3P shortened
the reaction time, however, the separation process was
found to be hampered by the necessity of removing of the
excess amount of n-Bu3P after the reaction. The optimal
results were found to be achieved by refluxing the thio-
formylindolizine with the n-Bu3P in a 1:3.5 mole ratio in
THF under nitrogen atmosphere until the disappearance of
the thioformylindolizine as indicated by TLC monitoring.
Under these conditions, reactions of 1b-1g were similarly
conducted to give the corresponding E-1,2-biindolizin-3-
yl ethylenes 2b-2g [15]. The results are in Table 1.
ꢀ
Figure 2. Packing diagram of 2d.
These reactions displayed significant difference with
the reactions of thioketones with phosphines. Ogata et al.
reported that in the reaction of thiobenzophenone with
tributylphosphine [17], 1,1,2,2-tetraphenyl ethane was the
main product (65 %), and the olefinic product (tetraphenyl
ethene) was a minor product (28 %), in the reaction of
thio-fluorenone with tributylphosphine [18], bifluorenyl-
idene (63 %) was the main product, and bifluorenyl was a
minor product.
Scheme 1
R1
R3
R4
R2
H
C
R4
R1
n-Bu3P
N
R2
S
N
C
H
THF
N
CH
R3
R1
R2
R4
R3
1
2
In our reaction of the thioaldehydes 1 with the
phosphine, however, we have not found any hydrogenated
product of 2. A plausible mechanism for this reaction is
proposed in Scheme 2. Similar to that in the reactions of
thioketones with phosphine [18], desulfurization by the
trivalent phospine Bu3P leads to a (3-indolizinyl)methyl-
ene intermediate I.
Table 1
Reactions of 3-thioformylindolizenes (1) with n-tributylphossphine.
Entry
R1
R2
R3
R4
Time / h
Yield/%
2a
2b
2c
2d
2e
2f
H
CH3
H
CH3
H
CH3
H
Ph
Ph
Ph
CH3
t-Bu
t-Bu
CH3
H
H
H
H
H
H
H
H
H
CH3
H
H
10
8
84
84
86
79
90
93
91
10
14
34
18
12
Addition of this carbene to the C=S bond of another
3-thioformylindolizine gave the thiirane II. Thiiranes are
known to undergo facile desulfurization in the presence of
phosphines [18]. Desulfurization of II either spon-
taneously under thermal conditions or promoted by the
phosphine resulted in the formation of the ethylene
products 2. The ground state of arylcarbenes is proposed
to be the triplet state [19]. In the reactions of thioketone
with phosphines [18], hydrogen abstraction of the triplet
carbene [diphenylmethylene and (9-fluorenyl)methylene]
from tributylphosphine and addition to the C=S bond of
the second thioketone run parallelly, with the hydrogen
abstraction being predominant, giving the ethanes as main
products and the ethylene as minor products. In the case
of the reactions of 1 with n-Bu3P, much smaller steric
hindrance toward the carbene addition to the thioformyl
as compared with the addition to the more sterically
hindered thiocarbonyl in thioketones may contribute to
the prevailing addition to C=S bond at the expense of
hydrogen abstraction. Moreover, the hydrogen abstraction
reactivity of the indolizinylcarbene may be further
decreased by the effective mesomeric stabilization as
H
CH3
2g
An X-ray crystallographic analysis of 2d was carried
out, and the ORTEP drawing and cell packing diagram
are given in Figure 1 and 2 respectively. Figure 1 clearly
shows the E-configuration of 2d. It is also seen that, in
2d, the two indolizines are coplanar with the central C=C
bond, constructing a large delocalized ꢀ-system. This
enables favorable intermolecular interactions in the
crystal packing and resulted in ꢀ-ꢀ-stacking as indicated
by the interlayer distance of 3.7 Å [16]. The E-
configuration of the other E-1,2-biindolizin-3-ylethylenes
can be deduced from the chemical shift of the olefinic
protons in the 1H NMR spectra.
shown in
I (Scheme 2). In the triplet diradical
intermediate formed during the addition of the triplet
carbine to the C=S bond, serious steric hindrance between
the two indolizinyls in the cis- conformation renders the
trans-conformation to be predominant, which after
Figure 1. Molecular structure of 2d.

Sep-Oct 2008
The Stereoselective Synthesis of E-1,2-Bis(3-indolizinyl)ethylenes
1313
Scheme 2
n-Bu3P
Ph
S
Ph
Ph
N
N
..
..
N
+
-n-Bu3PS
CH
CH
CH
-
Carbene
H
I
1
Ph
S
N
CH
Ph
N
Ph
Ph
N
CH2
N
n-Bu3P
-n-Bu3PS
H
HC
CH
C
C
S
H
N
N
Ph
Ph
Ph
N
II
2
H2C
Ph
CH2
N
similarly conducted to give the corresponding E -1,2-bis(3-
indolizinyl)-ethylenes 2b-2g.
intersystem crossing and bond formation affords the
thiirane with trans-configuration.
(E)-1,2-Bis(2-phenylindolizin-3-yl)ethene (2a). Orange
crystals; m.p. 198-200 °C (from acetone). ir (KBr): ꢀmax 3054,
In summary, 3-thioformylindolizines undergo novel
desulfurazative coupling reactions in the presence of
n-tributylphosphine to give E-1,2-bis(3-indilizinynyl)
ethylenes stereoselectively in satisfactory yield. These
reactions provided a new efficient and stereospecific
synthesis of E-1,2-bis(3-indilizinynyl)ethylenes, and
displayed the different reaction pattern of these indolizine
thioaldehydes in reactions with phosphine from the
reaction of thioketones with phosphines.
-1
1
3025, 1625, 1597, 962, 760, 725, 700 cm . H nmr (300MHz,
DMSO-d6): ꢀ = 8.42 (d, 2H, J = 7.0 Hz, H-5, H-5'), 7.49 (d, 2H,
J = 8.7 Hz, H-8, H-8'), 7.36-7.29 (m, 10H, 2ꢀPh), 7.03 (s, 2H,
trans, -CH =CH-), 6.82 (m, 2H, H-7, H-7'), 6.70 (m, 2H, H-6,
H-6'), 6.58 (s, 2H, H-1, H-1'). MS m/z (%): 410 (M+, 84), 230
(100), 205 (17), 193 (5). Anal. Calcd. for C30H22N2 (%): C,
87.80; H, 5.37; N, 6.83. Found (%): C, 87.75; H, 5.44; N, 6.89.
(E)-1,2-Bis(1-methyl-2-phenylindolizin-3-yl)ethene (2b).
Orange crystals; m.p. 226-228 °C (from petroleum ether). ir
(KBr): ꢀmax 3066, 3023, 2916, 1753, 1600, 951, 733, 700 cm-1.
1H nmr (300 MHz, C6D6): ꢀ = 7.56-7.53 (m, 6H, H-5, H-5',
2ꢀPh), 7.41-7.31 (m, 6H, 2ꢀPh), 7.23 (d, 2H, J = 8.9 Hz, H-8,
H-8'), 6.87 (s, 2H, trans, -CH=CH-), 6.50 (m, 2H, H-7, H-7'),
6.28 (m, 2H, H-6, H-6'), 2.29(s, 6H, 2ꢀCH3). MS m/z (%): 438
(M+, 100), 244 (68); Anal. Calcd. for C32H26N2 (%): C, 87.67; H,
5.94; N, 6.39. Found (%): C, 87.53; H, 6.08; N, 6.42.
EXPERIMENTAL
Melting points were determined on a Yanaco melting point
apparatus and are uncorrected. IR Spectra were recorded on a
Nicolet FT-IR 5DX spectrometer with KBr pellets. 1H NMR
Spectra were recorded on a Bruker ACF-300 spectrometer with
TMS as internal reference. Mass spectra were obtained on a
ZAB-HS mass spectrometer at 70 eV. Elemental analytical were
performed on a Foss Heraeus CHN-O-Rapid analyzer.
General procedure for preparation of (E)-1,2-bis(3-
indolizinyl)ethene (2a-g).A solution of 2-phenyl-3-thioformyl-
indolizine (1a) (0.35 g, 1.5 mmol) in anhydrous THF (8 mL)
was stirred at r.t. and after the air was replace by N2 gas, adding
tributylphosphine (1.1 g, 5.4 mmol). The reaction was kept at
reflux temperature till 1a disappeared (monitored by TLC). The
solvent was removed in vacuo, the mixture was cooled and the
E-1,2-bis(2- phenyl-3-indolizinyl)ethylene (2a) was separated
on a column of silica gel G with petroleum ether as eluents for
quick elution. Under these conditions, reaction of 1b-1g were
(E)-1,2-Bis(7-methyl-2-phenylindolizin-3-yl) ethene (2c).
Orange crystals; m.p. 240-242 °C (from petroleum ether). ir
(KBr): ꢀmax 3046, 3020, 2904, 1638, 1598, 961, 754, 711 cm-1.
1H nmr (300MHz, C6D6): ꢁ = 7.68(d, 4H, J = 1.47Hz, 2ꢀPh),
7.65 (d, 2H, J = 1.15 Hz, H-5, H-5'), 7.25 (m, 6H, 2ꢀPh), 7.01
(s, 2H, H-8, H-8'), 6.84 (s, 2H, trans,-CH=CH-), 6.50 (s, 2H,
H-1, H-1'), 5.96 (m, 2H, H-6, H-6'), 1.9 (s, 6H, 2ꢀCH3). MS m/z
(%): 438 (M+, 79), 244 (50), 219 (24), 207 (100); Anal. Calcd.
for C32H26N2 (%): C, 87.67; H, 5.94; N, 6.39. Found (%): C,
87.62; H, 6.02; N, 6.34.
(E)-1,2-Bis(1,2-dimethylindolizin-3-yl)ethene (2d). Yellow
crystals; m.p. 180-182 °C (from petroleum ether). ir (KBr): ꢀmax
3058, 3032, 2961, 2907, 1755, 1674, 933, 721 cm-1. 1H nmr (300

1314
W. Liu, S. Tu, T. He, Y. Zhao, and H. Hu
Vol 45
Eur. J. Pharma. 2003, 478, 81.
MHz, C6D6): ꢀ = 7.83 (s, 2H, H-5, H-5'), 7.29 (s, 2H, H-8, H-8'),
7.03 (s, 2H, trans, -CH=CH-), 6.55 (s, 2H, H-7, H-7'), 6.29 (m,
2H, H-6, H-6'), 2.47 (s, 6H, 2ꢀCH3), 2.34 (s, 6H, 2ꢀCH3). MS
m/z (%): 314 (M+, 100), 299 (50), 182 (53), 157 (15); Anal.
Calcd. for C22H22N2 (%): C, 84.00; H, 7.01; N, 8.92. Found
(%):C, 84.01; H, 7.08; N, 9.06.
[2] (a) Luo, J. D.; Haller, M.; Ma, H.; Liu, S.; Kim, T. D.; Tian,
Y. Q.; Chen, B. Q.; Jang, S. H.; Dalton, L. R. and Y.Alex, A. K. J. Phys.
Chem. B 2004, 108, 8523; (b) Chidichimo, G.; Salerno, G.; Veltri, L.;
Gabriele, B. and Nicoletta, F. Liquid Crystals 2004, 31, 733; (c) Diaz,
M. C.; Iescas, B. M.; Seoane, C. and Martin, N. J. Org. Chem. 2004, 69,
4492.
(E)-1,2-Bis(2-t-butylindolizin-3-yl)ethene (2e). Yellow
crystals; m.p. 160-162 °C (from petroleum ether). ir (KBr): ꢁmax
3116, 3038, 2961, 2862, 976, 737 cm-1. 1H nmr (300 MHz,
DMSO-d6): ꢀ = 8.54 (d, 2H, J = 7.0 Hz, H-5, H-5'), 7.44 (d, 2H,
J = 8.8 Hz, H-8, H-8'), 7.13 (s, 2H, trans, -CH=CH-), 6.73 (m,
2H, H-7, H-7'), 6.62 (m, 2H, H-6, H-6'), 6.46 (s, 2H, H-1, H-1'),
1.38 (s, 18H, 2ꢀt-Bu). MS m/z (%): 370 (M+, 100), 313 (53),
210 (72), 185 (19); Anal. Calcd. for C26H30N2 (%): C, 84.32; H,
8.11; N, 7.57. Found (%): C, 84.24; H, 8.07; N, 7.42.
(E)-1,2-Bis(1-methyl-l,2-t-butylindolizin-3-yl)ethene (2f).
Yellow crystals; m.p. 186-188 °C (from acetone). ir (KBr): ꢁmax
3067, 3030, 2956, 2865, 1616, 981, 735 cm-1. 1H nmr (300MHz,
Acetone-d6): ꢀ = 8.54 (d, 2H, J = 6.0 Hz, H-5, H-5'), 7.39 (d, 2H,
J = 9.0 Hz, H-8, H-8'), 7.06 (s, 2H, trans, -CH=CH-), 6.64 (m,
2H, H-7, H-7'), 6.49 (m, 2H, H-6, H-6'), 2.46 (s, 6H, 2ꢀCH3-),
1.47 (s, 18H, 2ꢀt-Bu). MS m/z (%): 398 (M+, 75), 341 (100),
224 (25), 199 (20); Anal. Calcd. for C28H34N2 (%): C, 84.42; H,
8.54; N, 7.04. Found (%): C, 84.57; H, 8.70; N,6.85.
(E)-1,2-Bis(2,7-dimethylindolizin-3-yl)ethene (2g). Orange
crystals; m.p. 199-201 °C (from petroleum ether). ir (KBr): ꢁmax
3098, 3046, 2954, 2907, 1638, 940, 753 cm-1. 1H nmr (300 MHz,
C6D6): ꢀ = 7.79 (s, 2H, H-5, H-5'), 7.03 (s, 4H, H-8, H-8', trans,-
CH=CH-), 6.48 (s, 2H, H-1, H-1'), 6.16 (d, 2H, J = 6.9 Hz, H-6,
H-6'), 2.60 (s, 6H, 2ꢀCH3), 2.13 (s, 6H, 2ꢀCH3). MS m/z (%):
314 (M+, 100), 299 (38), 182 (77), 157 (26); Anal. Calcd. for
C22H22N2 (%): C, 84.00; H, 7.01; N, 8.92. Found (%): C, 84.06;
H, 7.00; N, 8.85.
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Section E 2003, E59, o1039.
Acknowledgement. We are grateful to the National Natural
Science Foundation of China (No. 20672090), the Six Kinds of
Professional Elite Foundation of Jiangsu Province (No. 07-A-
024), the Key Laboratory of Marine Biotechnology Foundation
of Jiangsu Province(No. 2006HS014) and the Huaihai Institute
of Technology Disciplinary Construction Foundation (No.
XK200311) for financial support.
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