The photochemical characteristics of aromatic enediyne compounds substituted with electron donating and electron withdrawing groups

Article abstract of DOI:10.1039/b305816f

trans- and cis-1-(4-Dimethylaminophenyl)-6-(4-nitrophenyl)hex-3-ene-1,5-diynes (trans- and cis-DANE) were synthesized and their photochemical properties were studied. The absorption spectra of trans-DANE red-shifted compared with the parent compound bisph

Full text of DOI:10.1039/b305816f

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The photochemical characteristics of aromatic enediyne
compounds substituted with electron donating and electron
withdrawing groups
Yoshihiro Miki, Atsuya Momotake and Tatsuo Arai*
University of Tsukuba, tennodai 1-1-1, Tsukuba, Ibaraki 305-8571, Japan.
E-mail: arai@chem.tsukuba.ac.jp; Fax: 81-298-53-4521; Tel: 81-298-53-4315
Received 27th May 2003, Accepted 24th June 2003
First published as an Advance Article on the web 9th July 2003
trans- and cis-1-(4-Dimethylaminophenyl)-6-(4-nitrophenyl)hex-3-ene-1,5-diynes (trans- and cis-DANE) were
synthesized and their photochemical properties were studied. The absorption spectra of trans-DANE red-shifted
compared with the parent compound bisphenylethynylethene (BEE) due to intramolecular charge transfer. The
fluorescence spectra, Stokes shift, fluorescence lifetime, fluorescence quantum yield, and quantum yield of trans-to-
cis photoisomerization of trans-DANE showed strong dependence upon the solvent polarity in the less-polar region.
No fluorescence emission from trans-DANE was observed in medium-polar and polar solvents. The quantum yield
of cis-to-trans isomerization was almost solvent independent. The donor–acceptor substituents shifted the
equilibrium between the trans perpendicular triplet state and the trans planar triplet state to the trans triplet state, and
resulted in an increase in the triplet lifetime. Comparison of the photochemical properties of trans-DANE with trans-
4-dimethylamino-4Ј-nitrostilbene (DANS) suggests that trans-DANE is a possible fluorescent probe in the non-polar
region.
Introduction
Enediyne compounds have been greatly investigated in the
areas of biological activity and physical properties. In the area
of biological activity, they have been investigated as antitumor
antibiotics. For example, the drugs esperamicin, calichemicin,
and dynemicin, each of which possesses an enediyne unit, have
been shown to produce a highly reactive aromatic 1,4-biradical
intermediate by heating (Scheme 1).1–4 Furthermore, the inter-
mediate has been shown to cause the cutting of DNA chains.
Since enediyne compounds have an olefin part, they can
undergo isomerization.5 Moreover, enediyne compounds are
expected to have applications as non-linear optical materials
Scheme 2 trans-cis Photoisomerization of BEE.
when donor–acceptor substituents are introduced, because the
conjugation may be spread over the whole molecule. Thus,
As an example of the photochemistry of a double bond,
stilbene is known to undergo cis–trans photoisomerization. As
an example of the structural characteristic of the photoexcited
state of the triple bond, Hamaguchi and co-workers proposed,
from CARS spectra, that the excited singlet state of di-
phenylacetylene takes a trans-bent structure with a centre of
symmetry (Scheme 3).7 Thus, double and triple bonds show very
interesting photochemical behaviour, and because aromatic
enediyne compounds have the photochemical characteristics of
enediyne compounds are investigated in many fields, but
reports of photoreactions are few. We decided to synthesize
enediyne compounds and to investigate the character of the
exited state and the photochemical reactivity.
Scheme
enediyne.
1
Generation of a 1,4-biradical intermediate from an
Previously we reported the photochemical reactivity of
aromatic enediyne compounds (bisphenylethynylethene; BEE,
Scheme 2).6 cis–trans Photoisomerization took place as the
main feature. For example, on irradiation with 328 nm light,
BEE gave a trans-to-cis isomer ratio at the photostationary
state (pss) of ([t]/[c])pss = 44/56 in benzene with a quantum yield
of cis-to-trans and trans-to-cis isomerization of 0.27 and 0.17,
respectively. In addition we have observed fluorescence
emission both from cis and trans isomers of BEE peaking at
360–380 nm even at room temperature with high quantum
yields of 0.31 and 0.42, respectively. Furthermore, the excited
triplet state was observed. Thus, aromatic enediyne compounds
show an interesting photochemical behaviour.
Scheme 3 Structure of excited singlet state of diphenylacetylene.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 6 5 5 – 2 6 6 0
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Table 1 Absorption maxima, fluorescence maxima and Stokes shift of
DANEs
stilbene and diphenylacetylene, their investigation is considered
to be significant.
In this paper, we wish to report the photoisomerization and
fluorescence properties of aromatic enediyne compounds
substituted with an electron donating and an electron with-
drawing group (1-(4-dimethylaminophenyl)-6-(4-nitrophenyl)-
hex-3-ene-1,5-diyne; DANE, Scheme 4). The intramolecular
charge transfer interaction may affect the reactivity of an
excited state.
λabs/nm
λfl/nm
trans
∆ν/cmϪ1
trans
Solvents
cis
trans
Cyclohexane
Benzene
324
418
333
423
335
427
329
415
315
410
325
421
327
422
320
411
469
499
581
3000
6600
Dichloromethane
Acetonitrile
Fig. 2 shows the fluorescence and fluorescence excitation
spectra of trans-DANE in cyclohexane and benzene at room
temperature. The fluorescence maxima appeared at 469 nm and
499 nm with vibrational progression in cyclohexane, while the
fluorescence spectrum was broadened and red shifted in
benzene. No fluorescence emission from trans-DANE was
observed in dichloromethane and acetonitrile. The fluorescence
quantum yields and the fluorescence lifetimes were determined
to be 0.33 and 0.55 ns in cyclohexane, 0.25 and 2.4 ns in
benzene, respectively. As listed in Table 1, the Stokes shifts
between the absorption maximum and fluorescence maximum
for trans-DANE were calculated to be 3000 and 6600 cmϪ1 in
cyclohexane and in benzene, respectively.
Scheme 4 Structures of DANEs.
The photoisomerization and fluorescence properties of
stilbenes substituted with a donor and an acceptor group at the
para position of the phenyl ring have already been studied.8–13
A
large solvent effect on the fluorescence spectra observed in a
stilbene with dimethylamino and nitro substituents at the para
position (DANS) indicated the important role of the twisted
intramolecular charge transfer state for the deactivation from
the excited state. The enediyne compound is closely related to
the stilbene, and tetra-substituted analogues at the olefinic C᎐C
᎐
double bond of enediyne have already been studied and
reported previously.14,15 However, disubstituted enediyne com-
pounds at the C᎐C double bond with a donor and an acceptor
᎐
group at the adjacent phenyl group have not been studied yet.
We wish to explore the excited state behaviour of donor–
acceptor substituted enediyne compounds closely related to the
stilbene from a basic interest.
Results and discussion
Fig. 2 Fluorescence (FL) and fluorescence excitation (FLE) spectra of
trans-DANE in cyclohexane (blue) and in benzene (green).
UV Absorption and fluorescence spectra
cis- and trans-DANEs were synthesized from N,N-dimethyl-
aniline and 4-bromonitrobenzene (Scheme 5). Fig. 1 shows a
typical example of the absorption spectra of cis- and trans-
DANEs in cyclohexane at room temperature. The band shapes
and absorption maxima of both cis- and trans-isomers exhib-
ited only weak solvent dependence (Table 1).
trans-DANE exhibited fluorescence spectra in non-polar and
slightly polar solvents, while cis-DANE did not give any fluor-
escence in any solvent. The fact that no fluorescence was
observed in the spectrum of cis-DANE is in contrast to the
observation of fluorescence emission for the cis isomer of BEE
in benzene.6 The present finding indicates that the introduction
of electron donating and electron accepting substituents at the
para position remarkably changes the excited singlet state
behaviour of the cis-isomer of aromatic enediyne compounds.
Fluorescence spectral change in mixed solvents
Fig. 3 shows the fluorescence spectral change with changing
ratio of dichloromethane : cyclohexane at room temperature.
The fluorescence maximum shifted to a longer wavelength with
increasing solvent polarity. The fluorescence intensity slightly
increased with an increasing ratio up to 4% dichloromethane,
and then drastically decreased with a further increase in the
ratio of dichloromethane. The fluorescence emission had
almost disappeared when the ratio of dichloromethane was
more than 60%. Inset in Fig. 3 shows the change of the Stokes
shift with increasing ratio of dichloromethane in cyclohexane.
The solvent polarity in the less polar region strongly affected
their Stokes shift. The fluorescence quantum yields with
Fig. 1 UV absorption spectra of trans-DANE (dot–dash line) and
cis-DANE (solid line).
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Scheme 5 Reagents and conditions: (a) NaI, NaOCl, MeOH; (b) trimethylsilylacetylene, CuI, Pd(PPh3)2Cl2, benzene, Et3N; (c) K2CO3, MeOH,
ether; (d) 5, CuI, PPh3, Pd(dba)2, benzene, n-BuNH2; (e) 3, CuI, PPh3, Pd(dba)2, benzene, n-BuNH2.
Table 2 Fluorescence maxima, Stokes shift and fluorescence quantum
yield of trans-DANE in
a mixed solvent (dichloromethane–
cyclohexane)
CH2Cl2 Content (vol %)
λfl/nm
∆ν/cmϪ1
Φfl
0
1
2
4
8
12
16
20
40
60
469, 499
479, 500
501
514
541
576
584
601
647
3000
3500
4400
4900
5900
7000
7300
7800
8900
9700
0.33
0.37
0.40
0.42
0.31
0.13
0.085
0.046
0.004
0.0004
682
Therefore, the intramolecular charge transfer state (ICT) is
supposed to be involved in the radiative deactivation from the
singlet excited state.
cis–trans Photoisomerization
Fig. 3 Fluorescence spectra of trans-DANE in mixed solvents of
dichloromethane–cyclohexane. Dichloromethane content (%): 0%
(dark blue), 1% (blue), 4% (light blue), 8% (green), 12% (yellowish
green), 16% (yellow), 20% (orange), 40% (red) and 60% (dark red). Inset
shows the change of the Stokes shift values with increasing ratio of
dichloromethane in cyclohexane
Fig. 4 shows the change in the absorption spectra during the
photoisomerization of trans-DANE in cyclohexane at room
temperature. On photoirradiation, the absorption maximum of
trans-DANE shifted to a longer wavelength and the absorbance
intensity decreased, indicating the occurrence of isomerization
to the cis isomer. The photostationary state ([t]/[c])pss was 10/90
in cyclohexane. The ([t]/[c])pss value is related to the ratio of the
extinction coefficient at the excitation wavelength of cis- (εc)
and trans-DANE (εt) and that of the isomerization quantum
yield from trans-to-cis (Φt
(Φc t). The value of εc/εt at 365 nm is 1.3/3.3. Therefore, the
ratio of Φt c/Φc t is estimated to be 0.78/0.22.
Table 3 summarizes the quantum yields of cis-to-trans and
trans-to-cis isomerization as well as the photostationary state in
various solvents. The quantum yield of trans-to-cis isomeriz-
ation was 0.13 in cyclohexane and decreased with increasing
solvent polarity. The values were almost constant in benzene,
CH2Cl2, and acetonitrile. The quantum yields of cis-to-trans
isomerization had almost the same value among the solvents
changing ratio of dichloromethane–cyclohexane are summar-
ized in Table 2. These results indicate that a polar solvent may
stabilize the intramolecular charge transfer state to give fluor-
escence emission at longer wavelengths and to accelerate the
deactivation from the excited singlet state. The Stokes shifts
increased with increasing solvent polarity. In the solution con-
taining 60% of CH2Cl2 in cyclohexane, the Stokes shift became
as large as 9700 cmϪ1. These results indicate that electronic
structure and/or molecular structure greatly depend on the
solvent polarity. The solvent effect on the fluorescence spectra
observed for DANS has been explained by the mechanism: the
intramolecular charge transfer state (ICT) or/and twisted
intramolecular charge transfer state (TICT) has been proposed
for the deactivation from the excited singlet state of DANS.8–9
c) and cis-to-trans direction
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Table 3 Quantum yields of photoisomerization and the trans/cis ratio
ization took place in the excited triplet state after undergoing
intersystem crossing. Similarly, permanent bleaching was
observed in the transient spectra starting from the trans-DANE
due to the photochemical trans-to-cis isomerization. The triplet
lifetime was 200 ns under air and 70 ns under oxygen, respect-
ively. Thus, the quenching rate constant was estimated to be
at the photostationary state
Solvent
Φt-to-c
Φc-to-t
([t]/[c])pss
Cyclohexane
Benzene
Dichloromethane
Acetonitrile
0.13
0.037
0.044
0.063
0.037
10/90
46/54
39/61
25/75
0.049
0.038
0.034
2 × 109 MϪ1 sϪ1
.
The triplet lifetime for BEE was determined to be 370 ns in
benzene, where the planar triplet state and perpendicular triplet
state were expected to be equilibrated with a ratio of 86 : 14.6
The considerably long lifetime observed for DANE indicates
that the equilibrium is shifted to the planar conformation in
DANE compared to BEE. One can roughly estimate the ratio
of the planar triplet state and perpendicular triplet state ([3p*]/
[3t*]) from the observed triplet lifetime and the decay rate
constant from 3p* (kdp = 2 × 107 sϪ1) and 3t* (kdt = 2 × 104 sϪ1).16
Thus, ([3p*]/[3t*]) was estimated to be 1/40, indicating that most
of the triplet state exists as the planar conformation. The ([3p*]/
[3t*]) value is 1/6 in BEE. Therefore introduction of the donor
and acceptor groups at the para position in BEE resulted in a
considerable change of the equilibrium towards the planar
conformation to increase the triplet lifetime.
Comparison with push–pull stilbenes
Fig. 4 Change of absorption spectrum of trans-DANE on irradiation
with 365 nm light in cyclohexane under argon: before (dash-dot line)
and after irradiation (solid line).
The basic photochemical properties of trans-DANE were com-
pared with trans-4-dimethylamino-4Ј-nitrostilbene (DANS),
which is a typical photoresponsive chromophore with a
conjugated linkage of electron-accepting and electron-donating
groups.8–11 Two points are noteworthy.
i) Although the distance between electron-donating and
accepting groups on trans-DANE is longer than that of DANS
due to the two conjugated triple bonds, absorption maxima of
DANE and DANS in various solvents appeared mostly in the
same region; 410–422 nm for trans-DANE and 410–440 nm for
DANS. The absorption spectra of trans-DANE showed less
solvent dependence than that of DANS. If it exhibits large
hyperpolarizabilities without a red-shift in more polar solvents,
then trans-DANE will be potentially useful for non-linear
optical applications.
examined in these experiments, ca. 0.04–0.05. Therefore, the
low quantum yield of cis-to-trans isomerization is one of the
reasons for the considerably high cis isomer composition of
the photostationary state in cyclohexane. The Φt
value
c
decreased with increasing solvent polarity to decrease the cis
isomer composition at the photostationary state. The above
results strongly indicate that the excited state behaviour of
DANE was affected by solvent polarity and the change is
remarkable between cyclohexane and benzene. This excited
state property is due to the donor–acceptor substituent groups
on the enediyne.
ii) The fluorescence quantum yield of DANS decreases with
increasing solvent polarity but is still measurable even in polar
solvents such as acetone or DMF, while that of trans-DANE
was falling off drastically to be 0.0004 in dichloromethane–
cyclohexane = 60/40. Thus, the fluorescence change of trans-
DANE is more sensitive than that of DANS in the non-polar
region, suggesting trans-DANE is a possible fluorescent probe
in the non-polar region.
In order to shed light on the properties of the excited singlet
state of DANE, either ICT or TICT depending on the solvent
polarity, we should prepare model compounds and examine
their fluorescence properties. Thus, much further work is
required on this point. However, the above results that the
solvent effect on the fluorescence properties of trans-DANE is
different from that of DANS indicates that ICT rather than
TICT plays an important role for the deactivation from the
excited single state of trans-DANE.
Transient absorption spectra
On 308 nm laser excitation, the T-T absorption spectra for
trans-DANE were observed in benzene under argon at room
temperature (Fig. 5). The decay observed at 500–800 nm was
assigned to the T-T absorption spectra and the rising compon-
ent at 420 nm was assigned to the production of cis-DANE.
The triplet lifetime was determined to be 2 µs at 600 nm. The
rising time constant at 420 nm was also 2 µs, which was in
accord with the decaying time constant. The concomitant
increase in the absorbance of trans-DANE with the decay of
the T-T absorption spectra indicates that the cis–trans isomer-
Conclusion
In summary, absorption spectra of trans-DANE, having an
donor–acceptor substituent at the para position of BEE, red-
shifted compared with the parent compound BEE due to
intramolecular charge transfer. The absorption spectra of
trans-DANE showed weak solvent dependence. The fluor-
escence spectra, Stokes shift, fluorescence lifetime, fluorescence
quantum yield, and quantum yield of trans-to-cis photoisomer-
ization were affected by a small change of solvent polarity in
the less-polar region. No fluorescence emission was observed
in medium-polar and polar solvents. The quantum yield of
Fig. 5 Transient absorption spectra of trans-DANE in benzene,
observed on laser flash photolysis with a focused 308 nm laser beam.
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cis-to-trans isomerization was almost solvent independent. The
triplet state of trans-DANE was observed on direct excitation
with 308 nm laser light in benzene. The donor–acceptor substi-
tuent shifted the equilibrium between the trans perpendicular
triplet state and the trans planar triplet state to the trans triplet
state, and resulted in an increase of the triplet lifetime.
extracted with ether. The organic layer was dried over MgSO4
and concentrated in vacuo to give 2.58 g (89%) of 5 as yellow
crystals. δH (400 MHz; CDCl3; Me4Si) 8.18 (2H, d, J 8.8 Hz,
ArH), 7.62 (2H, d, J 9.2 Hz, ArH), 3.34 (1H, s, H).
cis-1-Chloro-4-nitrophenylbut-1-ene-3-yne (6). A mixture of 5
(2.05 g, 13.9 mmol), cis-dichloroethylene (5.40 g, 55.7 mmol),
Pd(dba)2 (335 mg, 0.58 mmol), PPh3 (460 mg, 1.75 mmol), and
CuI (334 mg, 1.75 mmol) in degassed n-BuNH2 (63 ml) and
benzene (108 ml) was stirred at room temperature overnight.
After the reaction was complete, the reaction mixture was
poured into water and extracted with ether. The organic layer
was washed with saturated aqueous NH4Cl, then dried over
MgSO4 and concentrated in vacuo. The residue was purified by
Experimental
Materials
Preparation of compounds 1–5 are similar to those reported in
other papers.17–19
N,N-Dimethyl-4-iodoaniline (1). Aqueous sodium hypo-
chlorite (61.0 g, 82.0 mmol) was added dropwise over 2 h to the
solution of N,N-dimethylaniline (6.91 g, 57.0 mmol) and
sodium iodide (9.57 g, 63.0 mmol) in methanol (140 ml) at 10
ЊC. The mixture was stirred at 10–15 ЊC for 4 h, then allowed to
warm up to room temperature for 12 h. After the reaction was
complete, the reaction mixture was poured into water and
extracted with CH2Cl2. The organic layer was washed with
saturated aqueous NH4Cl, then dried over MgSO4 and concen-
trated in vacuo. The residue was purified by silica-gel column
chromatography (eluent: hexane) to give 7.24 g (51%) of 1 as
yellow crystals. δH (400 MHz; CDCl3; Me4Si) 7.46 (2H, d, J 8.4
Hz, ArH), 6.49 (2H, d, J 8.8 Hz, ArH), 2.92 (6H, s, CH3).
silica-gel column chromatography (eluent: hexane–CHCl3
=
4 : 1) to give 633 mg (22%) of 6 as yellow crystals. δH (400 MHz;
CDCl3; Me4Si) 8.21 (2H, d, J 8.8 Hz, ArH), 7.64 (2H, d, J 8.8
Hz, ArH), 6.57 (1H, d, J 8.0 Hz, olefinic proton), 6.12 (1H, d,
J 7.2 Hz, olefinic proton).
trans-1-Chloro-4-(4-nitrophenyl)but-1-ene-3-yne
(7).
A
mixture of 5 (500 mg, 3.40 mmol), trans-dichloroethylene
(1.32 g, 13.6 mmol), Pd(dba)2 (64.9 mg, 0.11 mmol), PPh3
(89.2 mg, 0.34 mmol), and CuI (64.8 mg, 0.34 mmol) in
degassed n-BuNH2 (12 ml) and benzene (18 ml) was stirred at
room temperature overnight. After the reaction was complete,
the reaction mixture was poured into water and extracted with
ether. The organic layer was washed with saturated aqueous
NH4Cl, then dried over MgSO4 and concentrated in vacuo. The
residue was purified by silica-gel column chromatography
(eluent: hexane–CHCl3 = 4 : 1) to give 134 mg (19%) of 7 as
yellow crystals. δH (400 MHz; CDCl3; Me4Si) 8.20 (2H, d, J 8.8
Hz, ArH), 7.58 (2H, d, J 8.8 Hz, ArH), 6.74 (1H, d, J 13.4 Hz,
olefinic proton), 6.18 (1H, d, J 13.8 Hz, olefinic proton).
1-(4-Dimethylaminophenyl)-2-trimethylsilylethyne (2).
A
mixture of 1 (7.24 g, 29.3 mmol), trimethylsilylacetylene (2.16 g,
22.0 mmol), [PdCl2(PPh3)2] (890 mg, 1.27 mmol), and CuI (450
mg, 2.37 mmol) in triethylamine (150 ml) was stirred at 40 ЊC
for 12 h. After the reaction was complete, the reaction mixture
was poured into water and extracted with ether. The organic
layer was washed with saturated aqueous NH4Cl followed by
saturated aqueous NaCl, then dried over MgSO4 and concen-
trated in vacuo. The residue was purified by silica-gel column
chromatography (eluent: hexane–CHCl3 = 9 : 1) to give 3.24 g
(69%) of 2 as yellow crystals. δH (400 MHz; CDCl3; Me4Si) 7.32
(2H, d, J 8.8 Hz, ArH), 6.57 (2H, d, J 8.8 Hz, ArH), 2.95 (6H, s,
CH3), 0.21 (9H, s, CH3).
cis-1-(4-Dimethylaminophenyl)-6-(4-nitrophenyl)hex-3-ene-
1,5-diyne (cis-DANE). A mixture of 3 (1.27 g, 1.36 mmol),
6 (633 mg, 3.06 mmol), Pd(dba)2 (1.66 g, 2.89 mmol), PPh3
(2.66 g, 10.2 mmol), and CuI (1.08 g, 5.69 mmol) in degassed n-
BuNH2 (40 ml) and benzene (110 ml) was stirred at room tem-
perature overnight. After the reaction was complete, the reac-
tion mixture was poured into water and extracted with ether.
The organic layer was washed with saturated aqueous NH4Cl,
then dried over MgSO4 and concentrated in vacuo. The residue
was purified by silica-gel column chromatography (eluent:
hexane–CHCl3 = 4 : 1) to give 650 mg (67%) of cis-DANE as
orange crystals. δH (400 MHz; CDCl3; Me4Si) 8.21 (2H, d, J 8.8
Hz, ArH), 7.64 (2H, d, J 8.8 Hz, ArH), 7.37 (2H, d, J 8.8 Hz,
ArH), 6.65 (2H, d, J 8.8 Hz, ArH), 6.22 (1H, d, J 10.4 Hz,
olefinic proton), 5.98 (1H, d, J 10.8 Hz, olefinic proton), 3.01
(6H, s, CH3); Elemental analysis. Anal. Calcd. for C20H16N2O2:
C, 75.93; H, 5.10; N, 8.86; Found: C, 76.39; H, 5.44; N, 8.58%.
4-Dimethylaminophenylethyne (3). A mixture of 2 (2.28 g,
10.5 mmol) and K2CO3 (14.5 g, 105 mmol) in MeOH–ether
(180 ml–130 ml) was stirred at room temperature overnight.
After the reaction was complete, the reaction mixture was
poured into water and extracted with ether. The organic layer
was dried over MgSO4 and concentrated in vacuo to give 1.27 g
(83%) of 3 as a yellow liquid. δH (400 MHz; CDCl3; Me4Si) 7.36
(2H, d, J 9.2 Hz, ArH), 6.61 (2H, d, J 8.8 Hz, ArH), 2.97 (7H, s,
CH3 and H).
1-(4-Nitrophenyl)-2-trimethylsilylethyne (4). A mixture of 4-
bromonitrobenzene (5.00 g, 24.8 mmol), trimethylsilylacetylene
(2.91 g, 29.7 mmol), [PdCl2(PPh3)2] (344 mg, 0.49 mmol), and
CuI (93 mg, 0.49 mmol) in triethylamine–benzene (100 ml–100
ml) was stirred at 60 ЊC overnight. After the reaction was com-
plete, the reaction mixture was poured into water and extracted
with ether. The organic layer was washed with saturated aque-
ous NH4Cl followed by saturated aqueous NaCl, then dried
over MgSO4 and concentrated in vacuo. The residue was
purified by silica-gel column chromatography (eluent: hexane)
to give 4.31 g (80%) of 4 as yellow crystals. δH (400 MHz;
CDCl3; Me4Si) 8.15 (2H, d, J 8.8 Hz, ArH), 7.57 (2H, d, J 8.4
Hz, ArH), 0.25 (9H, s, CH3).
trans-1-(4-Dimethylaminophenyl)-6-(4-nitrophenyl)hex-3-ene-
1,5-diyne (trans-DANE). A mixture of 3 (468 mg, 3.23 mmol),
7 (333 mg, 1.61 mmol), Pd(dba)2 (614 g, 1.07 mmol), PPh3
(984 mg, 3.75 mmol), and CuI (400 mg, 2.10 mmol) in degassed
n-BuNH2 (14 ml) and benzene (40 ml) was stirred at room
temperature overnight. After the reaction was complete, the
reaction mixture was poured into water and extracted with
ether. The organic layer was washed with saturated aqueous
NH4Cl, then dried over MgSO4 and concentrated in vacuo. The
residue was purified by silica-gel column chromatography (elu-
ent: hexane–CHCl3 = 4 : 1) to give 50 mg (10%) of trans-DANE
as orange crystals. δH (400 MHz; CDCl3; Me4Si) 8.17 (2H, d,
J 8.8 Hz, ArH), 7.56 (2H, d, J 8.8 Hz, ArH), 7.33 (2H, d, J 8.8
Hz, ArH), 6.62 (2H, d, J 8.8 Hz, ArH), 6.38 (1H, d, J 16.0 Hz,
olefinic proton), 6.16 (1H, d, J 16.4 Hz, olefinic proton), 2.99
(6H, s, CH3); Elemental analysis. Anal. Calcd. for C20H16N2O2:
C, 75.93; H, 5.10; N, 8.86; Found: C, 75.09; H, 5.37; N, 8.66%.
4-Nitrophenylethyne (5). A mixture of 4 (4.31 g, 19.7 mmol),
K2CO3 (6.87 g, 49.7 mmol) in MeOH (100 ml) and ether (75 ml)
was stirred at room temperature for 2 h. After the reaction was
complete, the reaction mixture was poured into water and
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 6 5 5 – 2 6 6 0
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (417) and the 21 Century COE
Program from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of the Japanese Government
and by the Asahi Glass Foundation.
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