Regioselective photoamination of 2-styrylthiophenes with ammonia

Article abstract of DOI:10.1002/jccs.200300015

Photoinduced electron transfer from the singlet excited state of 2-styrylthiophenes (1a-1f) to p-DCB (p-dicyanobenzene) has been proposed from the negative value of ΔG_ET (the free energy of the electron transfer) calculated by the Rehm-Weller e

Full text of DOI:10.1002/jccs.200300015

Journal of the Chinese Chemical Society, 2003, 50, 109-114
109
Regioselective Photoamination of 2-Styrylthiophenes with Ammonia
Jinn-Hsuan Ho (
Department of Chemistry, National Taiwan University, Taipei, Taiwan 116, R.O.C.
) and Tong-Ing Ho* (
)
Photoinduced electron transfer from the singlet excited state of 2-styrylthiophenes (1a-1f) to p-DCB
(p-dicyanobenzene) has been proposed from the negative value of DGET (the free energy of the electron trans-
fer) calculated by the Rehm-Weller equation. This leads to the formation of radical cations of 1a-1f and a radi-
cal anion of p-DCB. The p-nitro derivative 1g can not result in the radical cation due to the positive DGET
value which results from the unfavorable singlet excitation energy (E0,0) and oxidation potential. Regio-
selective amination by ammonia to the radical cations of 1a-1e has occurred to afford 1-amino-1-aryl-2-
(thien-2-yl)ethanes (2a-2e) in reasonable yields (50-90%). The regioselectivity arises from the attack of am-
monia at the localized positive charge on C-1 of the ethene bond of the radical cations of 1a-1e.
Keywords: Photosensitization; Electron transfer; Nucleophilic addition; Regioselectivity.
INTRODUCTION
Photoaddition of nucleophiles to the electron rich sub-
strates,1,2 such as styrenes,3 stilbenes,4 and arenes,5 via photo-
induced electron transfer is a potentially useful synthetic pro-
cedure. And photoamination reactions can provide conve-
nient ways for the synthesis of primary and secondary
amines5 directly. It is also a useful tool for the synthesis of
isoquinolines6 and aporphines.7
charge in one of the intermediates. Thus addition of ammonia
is limited to the other position of the radical cation intermedi-
ate. Therefore photoamination of p-methoxystilbene can give
only one regioselective product, 1-amino-1-phenyl-2-(4¢-
methoxyphenyl)ethane (Scheme I).
For the photoamination of the olefins with ammonia,
the influences of the arene groups have been mentioned, and
the effects of the heterocycle groups are seldom noticed.8 We
have studied the photophysics of 2-styrylthiophenes, and it is
found that thiophene moiety can act as electron donor.9 In this
work, we would like to report on the photoaddition of 2-
styrylthiophenes 1a-1e with ammonia and that the 2-thienyl
group is a more powerful group for the control of the regio-
selectivity than the para-methoxyphenyl group.
Regioselectivity for the substituent depended photo-
amination of stilbenes with ammonia in the presence of
p-dicyanobenzene (p-DCB) has been investigated.4 The oxy-
gen atom of the para methoxy group in stilbenes has played
an important role in localizing the appropriate positive
Scheme I
OCH3
Irradiation of
p-methoxystilbene
with p-DCB
NH2
OCH3
p-DCB
OCH3
OCH3
Dedicated to Professor Fa-Ching Chen on the occasion of his ninetieth birthday.

110 J. Chin. Chem. Soc., Vol. 50, No. 1, 2003
Ho and Ho
RESULTS AND DISCUSSIONS
1 and 0.005 M p-DCB was irradiated at 300 nm in a Rayonet
photochemical reactor. After irradiation, the reactions show
very low conversions (Table 1, Entry 1-5). In order to im-
prove the yield we have tried to change the reaction solvent,
the amount of p-DCB and the irradiation time.
Photoamination of 2-styrylthiophenes 1a-1e can give
the regioselective products 1-amino-1-aryl-2-(thien-2-yl)-
ethanes 2a-2e (Scheme II), and their reaction conditions and
results are listed in Table 1. Initially the irradiaton condition
in the literature is employed,4 i.e. NH3-saturated mix-solvent
(CH3CN:C6H6:H2O = 8:1:1) containing 0.005 M compounds
It is known that solvent is very influential in a photo-
chemical reaction (Table 1). Compared to the acetonitrile sol-
vent (entry 6), extra water can increase the concentration of
dissolved ammonia gas in the media, but the activity of am-
monia will be retarded because of the interaction of water and
ammonia. The amount of ammonia molecules in NH3-satu-
rated acetonitrile is enough to react with the 0.005 M reac-
tants and the nucleophility of ammonia in acetonitrile is
stronger than that in the mix solvent.
Scheme II
NH2
HN
O
(i)
(ii)
S
S
S
Photoamination reactions can not be carried out in the
absence of p-DCB. The ratios of the aminated products are in-
creasing with the enhancement of the concentration of p-
DCB (entry 6-8). This is because more p-DCB molecules in-
crease the collision frequency of the reactants. To avoid inter-
fering with the absorbance of the 2-styrylthiophenes, 5 equiv-
R
R
R
1a-1g
2a-2e
3a-3e
a: R = OCH3, b: R = CH3, c: R = CH(CH3)2, d: R = H
e: R = F, f: R = N(CH3)2, g: R = NO2
reaction conditions: (i) hv 300nm, 2.5eq p-DCB, NH3-saturated CH3CN,
(ii) benzoylchloride, pyridine, CCl4, ice bath.
Table 1. The Reaction Conditions, Conversions and Yields of the Photoamination of 2-
Styrylthiophenes 1a-1e
Entry
Reactanta
Solventb
(p-DCB/R)
Irrad.time Conversions Yieldse
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
1a
1b
1c
1d
1e
1c
1c
1c
1c
1c
1c
1a
1a
1b
1c
1d
1d
1e
1e
1f
mix
mix
mix
mix
1
1
1
1
1
1
3
5
5
5
5
5
5
5
5
5
5
5
5
5
5
10
10
10
10
10
2
2
2
0.5
1
3
3
5
5
5
3
5
3
5
5
5
10
10
13
14
50
72
62
83
80
14c
43c
67c
14c
22c
92c
91
76
70
47
91
80
71
56
---
---
mix
7
---
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
---
---
---
---
---
54
88
100
100
68
100
79
100
0d
1g
0d
a [1] = 0.005 M.
b The solvents are saturated with ammonia and the mix-solvent consisted of
CH3CN:C6H6:H2O = 8:1:1.
c The ratio of the aminated product estimating by the crude 1H NMR spectrum.
d Only cis/trans-photoisomerization.
e The yields of the product were based on the consumption of the starting material.

Photoamination of Styrylthiophenes with Ammonia
J. Chin. Chem. Soc., Vol. 50, No. 1, 2003 111
alents p-DCB are used and less than 5% incident light is ab-
sorbed by it. (for example, e (300 nm) for 1d = 15164
M-1cm-1; e (300 nm) for p-DCB = 120 M-1cm-1).
monia molecules can not add on the compound 1f. Another
explanation as suggested by one of the referees, could be due
to the competition of intramolecular charge transfer for 1f,
since N,N-dimethylamino group is known to be a good elec-
tron donor. For the compound 1g, the DGET value is positive
so that a photoamination reaction can not occur.
Irradiation time is also one of the factors affecting the
reaction yields (entry 8-11). The reaction is clear and only
one product is produced within 3-hr of irradiation by estimat-
ing the crude H-NMR spectrum. If the irradiation time is in-
creased to 5 hours, some unidentified side products start to
appear in the crude H-NMR spectrum. The separated yields
of the aminated products also became lower at the longer irra-
diation time. It means that the products are not stable and are
transformed slowly under this irradiation condition.
For the photoamination products of 2-styrylthiophenes,
1-amino-1-aryl-2-(thien-2-yl)ethanes (2a-2e), are easily rec-
ognized by their NMR spectrum. For example, for the aminated
product 2d, the 1H NMR peaks for the CH2 protons of the eth-
ane group are at 3.07 ppm (dd, J = 14.5, 8.0 Hz, 1H) and 3.19
ppm (dd, J = 14.5, 5.1 Hz, 1H), and for the CH proton of the
ethane group is at 4.19 ppm (dd, J = 8.0, 5.1 Hz, 1H). The 13C
NMR peaks for the CH and CH2 carbons of the ethane group
are at 57.6 and 40.3 ppm, respectively. To determine the
regioselectivity, the position of the amino group has been de-
termined by mass spectrum. For the aliphatic amines, the
most intense peak will arise from the a-cleavage, and the
mass weight of the base peaks for the compounds 2a-2e cor-
respond to the 4-substituted-benzylideneamines which are
formed via the a-cleavage of compounds 2a-2e. N-(1-Phen-
yl-2-(thien-2-yl)ethyl)benzamides 3a-3e, the benzoylization
of the compounds 2a-2e, also display the corresponding
fragmental peaks in the mass spectra to explain the regio-
selectivity. Finally, the structures and the regioselectivity of
the aminated products are confirmed by the X-ray spectrum
of the compound 3d (supporting data).
Finally, the best irradiation condition is chosen: a NH3-
saturated acetonitrile containing 0.005 M compounds 1 and
0.025 M p-DCB is irradiated at 300 nm in a Rayonet photo-
chemical reactor. The very high reaction conversions and
yields can be achieved at this condition (entry 12-19). But ir-
radiations of 2-(4-dimethylaminostyryl)thiophene 1f and
2-(4-nitrostyryl)thiophene 1g undergo only cis/trans-iso-
merization in this condition.
In order to confirm the photoinduced electron transfer
between 2-styrylthiophenes and p-DCB, one can use the
Rehm-Weller equation10 to estimate the free energy of reac-
tion. (Table 2) The E0,0 energy of the compounds 1a-1g has
been calculated from the maximum wavelength of absorption
and emission, and the oxidation potentials Eox can also be
measured in acetonitrile. The free energies of the electron
transfer DGET are listed in Table 2. The negative DGET values
for the compounds 1a-1e are in accord with the results of the
photoamination reactions. The DGET value is also negative
for compound 1f, but there is no photoamination for it. This
may be due to the fact that positive charge is localized on the
nitrogen atom of the N,N-dimethylamino group, so that am-
A plausible reaction mechanism for the regioselective
photoamination of 2-styrylthiophenes is proposed. Irradiated
compounds 1 at 300 nm can produce the singlet excited state
(1*). At the same time, the single electron transfer (SET) oc-
curs from 1* to p-DCB to form the cation radical of the com-
pound 1 (1+•) and the anion radical of p-DCB (p-DCB-•). Then
Table 2. The Spectral Dataa and DGET Values for the Photoinduced Electron Transfer
between 2-Styrylthiophenes 1a-1g and p-DCB by Rehm-Weller Equation.
Rehm-Weller Eq: DGET = Eox - Eredb - E0,0 - 0.06
c
Reactant
Abs.(nm)
Em.(nm)
E
0,0(eV)
Eox
DGET
1a
1b
1c
1d
1e
1f
332
325
326
322
321
338
372
388
380
381
377
374
424
588
3.47
3.54
3.53
3.57
3.59
3.04
2.59
1.08
1.20
1.23
1.29
1.43
0.64
1.63
-0.81
-0.75
-0.72
-0.70
-0.58
-0.82
0.62
1g
a The absorption and emission spectra were measured in acetonitrile.
b The reduction potential of p-DCB is -1.64 V.
c The oxidation potentials vs Ag/AgCl in acetonitrile.

112 J. Chin. Chem. Soc., Vol. 50, No. 1, 2003
Ho and Ho
EXPERIMENTAL SECTION
General Methods
Scheme III
1H and 13C NMR spectra were recorded on CDCl3 solu-
tions on Bruker AM-200 WB FT-NMR or Bruker AM-300
WB FT-NMR spectrophotometers. Chemical shifts are re-
ported in parts per million (ppm) relative to TMS as internal
standards. Mass spectra were determined with a Finnigan
MAT TSCE-46C spectrometer and high resolution mass
spectra with a JEOL JMS-HX110 spectrometer. A Rayonet
photochemical reactor (300 nm) was used as the irradiation
source. UV-Vis absorption spectra were determined with an
Hitachi U-2000 spectrometer and fluorescence spectra with
an Hitachi F-3000 spectrometer. The redox potentials were
determined with an electrochemical analyzer BAS-100,
which was equipped with a AgCl/Ag reference electrode, a
glassy carbon working electrode and a platinum auxiliary
electrode.
S
m/e = 112
S
S
NH2
NH2
R
R
m/e(intensity%)
146(100)
120(100)
148(100)
106(100)
R = OCH3
R = CH3
R = CH(CH3)2
R = H
NH2
NH
R = F
124(100)
R
m/e(intensity%)
R = OCH3
R = CH3
R = CH(CH3)2
R = H
240(47)
224(84)
252(95)
210(34)
228(47)
N
O
HN
O
R = F
S
R
R
1+• undergoes nucleophilic addition of ammonia, deprotona-
tion, back single electron transfer (BSET) from p-DCNB-•
and protonation to give the final aminated products 2 (Scheme
IV).
The General Procedure for the Preparation of
2-(4¢-substitutedstyryl)thiophenes
All reactants, 2-styrylthiophenes 1a-1g were synthe-
sized by Wittig reaction. The mixture of 4-substituted-benzyl
chloride (26 mmol) and triphenyl phosphite (31 mmol) was
refluxed under nitrogen atmosphere for 4 hours. After reflux-
ing, the reaction was cooled down to room temperature and to
which was added 20 mL dry DMF as solvent. Then sodium
methoxide (52 mmol) was added quickly into the reaction at 0
°C. After stirring for 10 min, the 2-thiophenecarboxylalde-
hyde (26 mmol) was added slowly. While the addition was
completing, the reaction mixture was kept stirring for several
hours at room temperature. Following, the reaction mixture
was poured into a lot of ice water, and a white solid appeared.
Purification of 2-styrylthiophenes was by recrystallization
from ethanol.
Scheme IV
SET
NH3
hv
1
1*
1
1-NH3
2
p-DCB p-DCB
-H+/BSET(p-DCB)/+H+
1
S
S
R
R
R = OCH3
S
S
OCH3
R
The positive charge of 1+• can be produced on either site
The General Procedure for the Photoamination of
2-Styrylthiophenes with Ammonia in the Presence of
p-Dicyanobenzene
of the ethene part. For the intermediate 1a+•, it has been
known that the oxygen atom can localize the cation produced
on the benzylic position near the p-methoxyphenyl group.
However, the sulfur atom on the thiophene group can also sta-
bilize the positive charge on the other side of the ethene, and
it even shows a stronger ability of localization of the positive
charge. So photoamination of 1a gives only the product 2a,
and photoaminations of the other reactants 1b-1e give the
regioselective products 2b-2e. The 2-thienyl group is stron-
ger than the p-methoxyphenyl group in terms of electron do-
nating ability9 to stabilize the positive charge thus leading to
regiospecific amine addition products.
A 100-mL acetonitrile solution containing 0.005 M
2-styrylthiophenes and 0.025 M p-DCB in a quartz tube was
deaerated by bubbling ammonia gas for a while. The bubbles
of ammonia were absorbed by the solution and they were not
seen at the beginning. The bubbles were seen gradually and
stopped bubbling when the solution was saturated with am-
monia. Then the sample was irradiated with a Rayonet Photo-
chemical Reactor at 300 nm. After irradiation, the solution
was acidified with 0.1 M hydrochloric acid solution and di-
luted with water. The aqueous layer was extracted with ethyl

Photoamination of Styrylthiophenes with Ammonia
J. Chin. Chem. Soc., Vol. 50, No. 1, 2003 113
acetate several times to remove the unreacted reactants and
p-DCB. Then it was basified by 0.1 M sodium hydroxide so-
lution and extracted with ethyl acetate. The ethyl acetate
layer was dried by anhydrous MgSO4 and removed in vac-
uum. The residual mixture was separated by chromatography
on silica gel (elute: a mix solution of n-hexane and ethyl ace-
tate). The aminated products 2a-2e were gotten. The yields
are listed in Table 1, and the spectral data is listed below.
m/z 222 (M++1, 22), 124 (C7H7NF+, 100).
The General Procedure for the Benzolylation of
1-Amino-1-phenyl-2-thienylethanes
A 10 mL tetrachloromethane containing approx. 30-50
mg isolated 1-amino-1-phenyl-2-thienylethanes (2a-2e) was
kept in an ice bath with stirring. The appropriate amount of
benzoyl chloride was added by drops and continued the reac-
tion for 30 minutes. Then pyridine was added dropwise to
trap the hydrochloride molecules. The reaction solution was
basified and diluted by water then extracted with ethyl ace-
tate several times. The EtOAc layer was dried by anhydrous
MgSO4, and the solvent was removed in vacuum. The resid-
ual mixture was separated by PTLC chromatography on sil-
ica gel (elute: a mixed solution of n-hexane and ethyl ace-
tate).
1-Amino-1-(4¢-methoxyphenyl)-2-thienylethane (2a)
1H NMR (300 MHz, CDCl3, TMS) d 2.19 (br), 3.11 (dd,
J = 14.5, 8.0 Hz, 1H), 3.18 (dd, J = 14.5, 5.8 Hz, 1H), 3.77 (s,
3H), 4.16 (t, J = 6.8 Hz, 1H), 6.76 (d, J = 3.6 Hz, 1H), 6.85 (d,
J = 8.7 Hz, 2H), 6.88 (dd, J = 5.1, 3.6 Hz, 1H), 7.12 (d, J = 5.1
Hz, 1H), 7.27 (d, J = 8.7 Hz, 2H).
1-Amino-1-(4¢-methylhenyl)-2-thienylethane (2b)
1H NMR (300 MHz, CDCl3, TMS) d 2.34 (s, 3H), 3.06
(dd, J = 14.5, 8.6 Hz, 1H), 3.18 (dd, J = 14.5, 5.0 Hz, 1H),
4.16 (dd, J = 8.6, 5.0 Hz, 1H), 6.81 (d, J = 3.5 Hz, 1H), 6.92
(dd, J = 5.1, 3.5 Hz, 1H), 7.11-7.15 (m, 3H), 7.25 (d, J = 8.1
Hz, 2H). MS (EI, 70 eV) m/z 218 (M++1, 11), 120 (C8H11N+,
100).
N-(2-thiophen-2-yl-1-(4¢-methoxyphenyl)ethyl)benzamide
(3a)
1H NMR (300 MHz, CDCl3, TMS) d 3.43 (d, J = 6.5 Hz,
2H), 3.78 (s, 3H), 5.41 (td, J = 8.1, 6.5 Hz, 1H), 6.41 (d, J =
8.1 Hz, 1H), 6.73 (dd, J = 4.2, 0.8 Hz, 1H), 6.85 (d, J = 8.6
Hz, 2H), 6.87 (m, 1H), 7.10 (dd, J = 5.1, 1.2 Hz, 1H), 7.21 (d,
J = 8.8 Hz, 2H), 7.39-7.48 (m, 3H), 7.71 (dd, J = 7.8, 1.4 Hz,
2H). MS (EI, 70 eV) m/z 338 (M++1, 11), 240 (C15H14NO2 ,
+
1-Amino-1-(4¢-isopropylhenyl)-2-thienylethane (2c)
1H NMR (300 MHz, CDCl3, TMS) d 1.25 (d, J = 6.8 Hz,
6H), 2.87 (heptet, J = 6.8 Hz, 1H), 3.05 (dd, J = 14.5, 9.1 Hz,
1H), 3.18 (dd, J = 14.5, 4.7 Hz, 1H), 4.16 (dd, J = 9.1, 4.7 Hz,
1H), 6.82 (d, J = 3.2 Hz, 1H), 6.92 (dd, J = 4.9, 3.2 Hz, 1H),
7.14 (d, J = 4.9 Hz, 1H), 7.20 (d, J = 8.1 Hz, 2H), 7.30 (d, J =
8.1 Hz, 2H). 13C NMR (75 MHz, CDCl3) d 147.9, 142.5,
141.7, 126.8, 126.5, 126.3, 125.8, 123.9, 57.4, 40.4, 33.7,
24.0. MS (EI, 70 eV) m/z 148 (C10H14N+, 100).
47), 105 (C7H5O+, 100).
N-(2-thiophen-2-yl-1-(4¢-methylphenyl)ethyl)benzamide
(3b)
1H NMR (300 MHz, CDCl3, TMS) d 2.32 (s, 3H), 3.43
(d, J = 6.5 Hz, 2H), 5.44 (td, J = 6.5, 7.6 Hz, 1H), 6.50 (d, J =
7.6 Hz, 1H), 6.74 (dd, J = 3.5, 1.0 Hz, 1H), 6.87 (dd, J = 5.1,
3.5 Hz, 1H), 7.10 (dd, J = 5.1, 1.0 Hz, 1H), 7.12 (d, J = 8.3
Hz, 2H), 7.19 (d, J = 8.3 Hz, 2H), 7.38-7.47 (m, 3H), 7.71
(dd, J = 8.2, 1.8 Hz, 2H). MS (EI, 70 eV) m/z 322 (M++1, 1),
224 (C15H14NO+, 84), 105 (C7H5O+, 100).
1-Amino-1-phenyl-2-thienylethane (2d)
1H NMR (300 MHz, CDCl3, TMS) d 3.07 (dd, J = 14.5,
8.0 Hz, 1H), 3.19 (dd, J = 14.5, 5.1 Hz, 1H), 4.19 (dd, J = 8.0,
5.1 Hz, 1H), 6.80 (d, J = 3.3 Hz, 1H), 6.91 (dd, J = 5.1, 3.3
Hz, 1H), 7.14 (dd, J = 5.1, 1.0 Hz, 1H), 7.2-7.4 (m, 5H);
13C-NMR (CDCl3) d 145.0, 141.3, 128.4, 127.2, 126.8, 126.4,
125.9, 123.9, 57.6, 40.3; MS (EI, 70 eV) m/z 204 (M++1, 62),
106 (C7H8N+, 100).
N-(2-thiophen-2-yl-1-(4¢-isopropylphenyl)ethyl)benzamide
(3c)
1H NMR (300 MHz, CDCl3, TMS) d 1.22 (d, J = 6.8 Hz,
6H), 2.88 (heptet, J = 6.8 Hz, 1H), 3.44 (d, J = 6.6 Hz, 2H),
5.45 (td, J = 7.6, 6.6 Hz, 1H), 6.47 (d, J = 7.6 Hz, 1H), 6.75
(d, J = 3.4 Hz, 1H), 6.88 (dd, J = 5.0, 3.4 Hz, 1H), 7.10 (d, J =
5.0 Hz, 1H), 7.17 (d, J = 8.4 Hz, 2H), 7.23 (d, J = 8.4 Hz, 2H),
7.34-7.47 (m, 3H), 7.71 (dd, J = 8.1, 1.6 Hz, 2H). MS (EI, 20
eV) m/z 252 (C17H18NO+, 95), 105 (C7H5O+, 100).
1-Amino-1-(4¢-fluorophenyl)-2-thienylethane (2e)
1H NMR (300 MHz, CDCl3, TMS) d 3.03 (dd, J = 14.4,
8.0 Hz, 1H), 3.14 (dd, J = 14.4, 5.2 Hz, 1H), 4.18 (dd, J = 8.0,
5.2 Hz, 1H), 6.77 (dd, J = 3.4, 0.6 Hz, 1H), 6.90 (dd, J = 5.1,
3.4 Hz, 1H), 6.99 (dd, J = 8.7, 8.7 Hz, 2H), 7.13 (dd, J = 5.1,
1.2 Hz, 1H), 7.31 (dd, J = 8.4, 5.4 Hz, 2H). MS (EI, 20 eV)
N-(2-thiophen-2-yl-1-phenylethyl)benzamide (3e)
1H NMR (300 MHz, CDCl3, TMS) d 3.45 (d, J = 6.6 Hz,

114 J. Chin. Chem. Soc., Vol. 50, No. 1, 2003
Ho and Ho
2H), 5.48 (td, J = 7.5, 6.6 Hz, 1H), 6.53 (d, J = 7.5 Hz, 1H),
6.73 (dd, J = 3.5, 1.0 Hz, 1H), 6.87 (dd, J = 5.1, 3.5 Hz, 1H),
7.10 (dd, J = 5.1, 1.0 Hz, 1H), 7.27-7.48 (m, 8H), 7.72 (dd, J
= 7.9, 1.4 Hz, 2H); MS (EI, 70 eV) m/z 308 (M++1, 7), 210
(C14H12NO+, 34), 105 (C7H5O+, 100). HRMS Calcd. 307.1031,
found 307.1024.
ACKNOWLEDGEMENT
We are grateful to the National Science Council of the
Republic of China (Taiwan) for financial support. We thank
Prof. Yu Wang and Mr. Gene-Hsiang Lee for the X-ray crys-
tallographic measurement.
N-(2-thiophen-2-yl-1-(4¢-fluorophenyl)ethyl)benzamide
(3f)
Received August 23, 2002.
1H NMR (300 MHz, CDCl3, TMS) d 3.41 (d, J = 6.5 Hz,
2H), 5.44 (td, J = 7.3, 6.5 Hz, 1H), 6.50 (d, J = 7.3 Hz, 1H),
6.72 (d, J = 3.5 Hz, 1H), 6.89 (dd, J = 5.1, 3.5 Hz, 1H), 7.00
(td, J = 8.7, 2.1 Hz, 2H), 7.12 (dd, J = 5.1, 1.0 Hz, 1H), 7.25
(dd, J = 8.5, 5.4 Hz, 2H), 7.35-7.49 (m, 3H), 7.71 (dd, J = 8.2,
1.7 Hz, 2H); MS (EI, 20 eV) m/z 326 (M++1, 1), 228
(C13H11FNO+, 47), 105 (C7H5O+, 100).
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CONCLUSION
A series of 2-styrylthiophenes 1a-1g with various sub-
stituents were synthesized. Photochemistry of 1a-1g in the
presence of an electron-deficient sensitizer p-DCB was stud-
ied. The generation of cation radicals of 1a-1e and anion radi-
cals of p-DCB is supported by the negative values calculated
from the Rehm-Weller equation which is based on the oxida-
tion potentials of 1a-1e. Regioselective photoamination by
ammonia is observed to afford 1-amino-1-aryl-2-(thien-2-
yl)ethanes (2a-2e) in relatively good yields. Both p-N,N-
dimethylamino (1f) and p-nitro (1g) derivatives are photo-
chemically inactive. A 1g radical cation is not formed due to
the unfavorable oxidation potential and singlet excitation en-
ergy.
10. Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259-271.