Photoinduced electron transfer reactions of 3,3-dialkylated 4,5-diphenyl-3h-pyrazoles: A new route to the formation of the solvent adducts

Article abstract of DOI:10.1002/jccs.200400061

3,3-Dialkyl-4,5-diphenyl-3H-pyrazoles undergo readily photoinduced electron transfer (PET) reaction with 2,4,6-triphenylpyrylium tetrafluoroborate (TPP⁺) in acetonitrile to produce cyclopropenes and 2H-pyrroles. During prolonged irradiation, the new ring-closure products derived from 2H-pyrroles as the secondary photoproducts are also produced. However, the corresponding ester analog exhibits different behavior to obtain the cyclopropene as the primary photoproduct and a [2+2] dimer of the cyclopropene as the secondary photoproduct. A rationale for the different behavior is offered.

Full text of DOI:10.1002/jccs.200400061

Journal of the Chinese Chemical Society, 2004, 51, 393-398
393
Photoinduced Electron Transfer Reactions of 3,3-Dialkylated
,5-Diphenyl-3H-pyrazoles: A New Route to the Formation of the Solvent
Adducts
4
Yao-Pin Yen* (
), Tseng-Min Huang (
), Yu-Ping Tseng (
),
Hsuan-Yu Lin (
) and Ching-Cheng Lai (
Department of Applied Chemistry, Providence University, Sha-Lu, Taichung, Taiwan 433, R.O.C.
)
3
,3-Dialkyl-4,5-diphenyl-3H-pyrazoles undergo readily photoinduced electron transfer (PET) reaction
+
with 2,4,6-triphenylpyrylium tetrafluoroborate (TPP ) in acetonitrile to produce cyclopropenes and 2H-
pyrroles. During prolonged irradiation, the new ring-closure products derived from 2H-pyrroles as the sec-
ondary photoproducts are also produced. However, the corresponding ester analog exhibits different behavior
to obtain the cyclopropene as the primary photoproduct and a [2+2] dimer of the cyclopropene as the second-
ary photoproduct. A rationale for the different behavior is offered.
Keywords: Photoinduced electron transfer; 3H-pyrazoles; Cyclopropene.
INTRODUCTION
RESULTS AND DISCUSSION
Photoinduced electron transfer (PET) reactions of cy-
Fluorescence Quenching of Singlet Excited Sensitizer by
3H-pyrazoles 1a-c
clic azoalkanes have been the subject of considerable interest
1
from a mechanistic viewpoint in recently years. However,
In order to examine the interaction between singlet ex-
cited sensitizer and 1a-c, fluorescence quenching experi-
ments were performed. The fluorescence of the sensitizer,
little is known about the photoinduced electron transfer reac-
tions of 3H-pyrazoles which have an unsaturated carbon-car-
bon double bond with an azo chromophore in its skeleton.
+
TPP , was efficiently quenched by 1a-c in acetonitrile, and
2
Kitamura has synthesized some 3H-pyrazoles which have
the quenching rate constants, k
Stern-Volmer constants, k , and the lifetime of the singlet
excited sensitizer, t . The lifetime of TPP was 2.7 ns. The
quenching rate constants of 1a-c were calculated to be 7.3 ´
q
, were calculated from the
an electron-withdrawing substituent at C
5
and reported their
q s
t
+
+
3
PET reactions with TPP . In addition to the primary photo-
product, cyclopropene, a new minor solvent adduct as a sec-
ondary photoproduct through cyclopropene derivative was
also found in the reaction. It occurred to us that the relatively
complex photochemical pathway of the solvent adduct could
be simplified if the 3H-pyrazole has a phenyl group instead
s
9
10
10
-1 -1
10 , 2.1 ´ 10 and 1.6 ´ 10 M s , respectively. To explain
these results, the Rehm-Weller correlation was examined be-
tween 1a-c and the excited singlet sensitizer. For oxidation
ox
potential, E , during measurement of the 1a-c, irreversible
ox
of an electron-withdrawing group at the C
tionally the solvent adduct could be diminished if the
H-pyrazole has two electron-withdrawing groups at both C
and C positions. In order to test the hypothesis of possibly
5
position. Addi-
waves were obtained and the E values of 1a, 1b, and 1c
were determined to be 2.32, 1.89, and 1.65 V vs SCE, respec-
red
3
4
tively. On the other hand, the reduction potential, E , of the
red
5
excited sensitizer, E + Eo,o (where Eo,o indicates the singlet
simplifying the change of the secondary photoproduct to the
primary photoproduct or possibly changing the reaction
pathway, we synthesized 3,3-dialkyl 4,5-diphenyl-3H-pyra-
zoles and dimethyl-3,3-cyclopentyl-3H-pyrazole-4,5-dicar-
boxylate and investigated their PET reactions. In this paper
we would like to describe results of their PET reactions with
excitation energy obtained from the fluorescence o,o band),
+
4
of TPP is 2.53 V vs SCE. The DG values of 1a-c derived
5
ox
red
2
0
from Rehm-Weller’s equation, E - E - Eo,o – e / ea (a
2
-1
6
0
Coulomb term, e /ea = 0.7 Kcal mol in acetonitril ), were
-
1
-5.2, -15.1 and -20.7 Kcal mol , respectively. These results
suggest that these reactions can proceed via an electron trans-
fer.
+
TPP .
*
Corresponding author. E-mail: ypyen@pu.edu.tw

3
94 J. Chin. Chem. Soc., Vol. 51, No. 2, 2004
Yen et al.
PET Decomposition of 3H-pyrazoles 1a-c
Table 1. Product Yields of Photoinduced Electron Transfer
+
Reactions of 3H-Pyrazoles 1a-c in Acetonitrile
During some PET decompositions of 1a-b by TPP in
acetonitrile, cyclopropenes 2a-b and products 3a-b were pro-
duced as the primary photoproducts. If irradiation is pro-
longed, some additional secondary photoproducts 4a-b also
appeared. However, in contrast, decomposition of 1c by
a
Compound Decomposition (%)
Yield
2
3
4
5
1
a
20
27
38
36
41
71
58
14
16
13
13
-
-
1
-
1
-
-
-
-
-
-
-
3
2
7
+
TPP , only gave the corresponding cyclopropene 2c. In pro-
1b
18
2
6
longed irradiation, a dimer 5 instead of 3c was produced. The
product distributions are summarized in Table 1. The struc-
tures of 2a-c, 3a-b and 4a-b were determined by their spec-
1
c
12
2
1
-
a
1
13
Conversion yields based on the reactant consumed and obtained
by GC analysis.
tral data ( H and C NMR, MS, IR) and their NOE measure-
ments (see the supporting information). Products 3a-b were
adducts of the nitrogen-eliminated intermediate from the cor-
responding 1a-b to acetonitrile. Products 4a-b were the sub-
sequent cyclization and dehydrogenation product from the
corresponding 3a-b. The structure of 5 was characterized by
its spectral data and by its X-Ray crystal structure (Fig. 1).
The course of PET decomposition of these compounds
is shown separately in Fig. 2.
The curves show the general trend that while the
amount of product 2 increased with irradiation time, the
amount of products 3-5 either remained constant (4a-b, 5) or
increased a little with time (3a-b). The product yield of 2 pla-
teaus after 4 or 5 h of irradiation, and the yields of 3a-b in-
creased with an induction time. This suggested that product 2
likely underwent secondary PET decomposition but at vary-
ing efficiencies. Indeed, the irradiation of cyclopropenes
+
2
a-c in the presence of TPP under the same conditions
showed the rapid decomposition of 2a-c and the production
of 3a-b and 5, respectively.
Fluorescence Quenching of Single Excited Sensitizer by
Cyclopropenes 2a-c
Fig. 1. ORTEP plots of dimer 5.
+
The fluorescence of TPP was quenched by cyclopro-
1
2
8
4
0
12
8
12
1
2
3
4
a
a
a
a
1b
2b
3b
4b
1c
2c
5
8
4
0
4
0
0
0
2
4
6
8
10
2
4
6
8
0
2
4
6
8
Time(hr)
Time(hr)
Time(hr)
Fig. 2. Progress of PET decomposition of 3H-pyrazoles 1a, 1b, 1c (!) was monitored by GLC and appearance of products
a, 2b, 2c (•), 3a, 3b (i), 4a, 4b (#) and 5 (—); left (1a), middle (1b) and right (1c).
2

3
,3-Dialkylated 4,5-Diphenyl-3H-pyrazoles
J. Chin. Chem. Soc., Vol. 51, No. 2, 2004 395
Table 2. Quenching Rate Constants, Oxidation Potentials and
Table 3. Product Yields of Photoinduced Electron Transfer
Free Energy Changes of Cyclopropenes 2a-c
Reactions of Cyclopropenes 2a-c in Acetonitrile
-
1
-1
ox
a
b
-1
a
k (M s )
E
(ev)
DG (Kcal mol )
Compound
Decomposition (%)
Yield
q
1
0
0
0
2
2
2
a
a
b
c
2.0 ´ 10
2.1 ´ 10
1.3 ´ 10
1.44
1.42
2.14
-25.6
-25.7
-9.4
3
4
5
1
2
a
14
54
38
50
41
-
-
31
-
-
-
1
3
1
VS SCE. Measured by cyclic voltametry in dry acetonitrile.
Derived from Rehm-Weller’s equation, DG = E - E - Eo,o
2b
15
30
30
-
b
ox
red
-
28
-
-
2
e /ea.
o
2c
44
a
Conversion yields based on the reactant consumed and obtained
by GC analysis.
penes 2a-c. The quenching rate constants, oxidation poten-
tials and the free energy changes, DG, between sensitizer and
cyclopropenes are summarized in Table 2. These results sug-
gested to involve a single electron transfer from the 1a-c to an
+
+
gest that 2a-c can undergo PET reactions with TPP .
excited sensitizer TPP resulting in the formation of an initial
+
·
+·
radical cation 6 and the neutral radical TPP·. 6 immedi-
PET Decomposition of Cyclopropenes 2a-c
ately loses a nitrogen to form the radical cation 7 and redis-
2
PET decompositions of cyclopropenes 2a-b gave ini-
tially two similar products 3a-b, in 54% and 50% yield, re-
spectively. After prolonged irradiation of 2a-b, two new ad-
ditional products 4a-b were also isolated (31 and 28%, re-
spectively) (Table 3). Their structures were characterized by
their spectral data. The nature of the secondary reactions was
determined in the following additional experiments. Irradia-
tion of a solution of an isolated sample of 3a was found to
give 4a in 75% yield. Similarly, irradiation of 3b gave 4b in
tributes to a more stable configuration 8. At this stage there
are two possible pathways for the interpretation of intermedi-
ate 8 leading to cyclopropene 2. The first pathway is the back
electron transfer (BET) from the TPP· to 8 to form a diradical
or vinylcarbene intermediate and then it undergoes 1,3-cy-
clization to produce cyclopropene 2 (path A). Alternatively,
8 can undergo intramolecular cyclization to form the cyclo-
propene radical cation and subsequently follows the BET
from TPP· to yield cyclopropene 2 (path B). Besides the
5
1% yield. Thus, the complete sequences of reaction during
cyclopropene, the formation of the primary photoproducts
3a-b is proposed to proceed via the following reaction path-
way: addition of acetonitrile with 8a-b results in the forma-
tion of intermediates 9a-b. The back electron transfer from
TPP· to 9a-b would produce 3a-b (path C).
irradiation of 1a-b can be summarized in the following
manner:
+
hn / TPP
1
a-b
2
a-b + 3a-b + 4a-b
l >400 nm
The presence of such primary photoproducts 3a-b
+
hn / TPP
3
a-b + 4a-b
3
could be due to the phenyl group (R ) with high resonance
l >400 nm
stability that would stabilize the intermediate 8 and favor ad-
dition with acetonitrile. Hence, it appears that the presence of
phenyl groups indeed accomplished the stated goal to change
the complex pathway to a simple pathway.
+
hn / TPP
l >400 nm
4
a-b
In contrast, PET decomposition of 2c only yielded a dimer
product, 5, in 44% yield, and the complete sequence of irradi-
ation of 1c can also be summarized as follows:
Since the products 3a-b are also produced in the PET
reactions of 2a-b, the formation of 3a-b is proposed to un-
dergo the following pathways. The cyclopropenes 2a-b react
+
hn / TPP
+
+·
+·
1
c
2c + 5
with TPP to form the radical cation 2 . The C-C bond of 2
is then cleaved to form 8a-b. The addition of acetonitrile with
a-b results in the formation of the intermediate 9a-b. The
l >400 nm
+
hn / TPP
l >400 nm
5
8
back electron transfer from TPP· to 9a-b would produce the
solvent-adducts, 3a-b. Since all the wavelengths of the UV
absorption spectra of 3a-b are shorter than 400 nm, the for-
mation of the final products 4a-b can be explained by the PET
reaction of 3a-b through cyclization and dehydrogenation.
To explain the above PET results, we propose the mechanism
displayed in Scheme I. The formation of the primary products
2
a-c likely followed the reaction pathway proposed by
2
+
Kitamura. The PET reaction of 1a-c with TPP has been sug-

3
96 J. Chin. Chem. Soc., Vol. 51, No. 2, 2004
Yen et al.
Scheme I
R3
+
R3
R3
R2
R1
R3
R2
R1
R3
R2
TPP
R3
N
N
-TPP
N
N
R1
N
N
6
1
1
1
a; R1 = R2 = CH , R = Ph
3 3
b; R1 R2 = (CH ) , R = Ph
2 4
3
-
N2
c; R1 R2 = (CH2)4,
R3
R3 = CO Me
R3
2
R2
R1
R2
R1
R3
R3
or
cycl
(
Path A)
BET
R3
R3
R3
R3
R2
R2
R1
R2
R3
(Path B)
cycl
R3
R3
BET
PET
R1
R1
R2
R1
2
8
7
R3
N
(Path C)
TPP
3
*
R3
R3
R2
R3
R2
R1
R1
N
R3
9
BET
2c
R2
R1
R3
N
R3
R3
R3
3
-2H
5
R2
R1
N
4
+
·
leading to the formation of the radical ion 2 and TPP·. Re-
On the other hand, the PET reaction of 2c will undergo
3
*
a different pathway to form the [2 + 2] dimer 5 as the second-
ary photoproduct. It may be due to 2c having two electron-
combination of this radical ion pair partially gives [2] ,
7
which reacts with 2c to give the dimer 5.
3
withdrawing substituents (R ) that would destabilize the for-
mation of 8c, making this pathway not competitive against
the dimerization of 2c. Hence, it appears that the presence of
electron-withdrawing groups indeed accomplished the stated
goal for blocking the formation of the solvent-adduct. A fac-
ile process can be proposed for the formation of 5 from 2c.
EXPERIMENTAL SECTION
General Information
Melting points were determined on a Yamaco micro
melting points apparatus and are uncorrected. IR spectra
+
The excited TPP reacts with 2c at a diffusion-controlled rate

3
,3-Dialkylated 4,5-Diphenyl-3H-pyrazoles
J. Chin. Chem. Soc., Vol. 51, No. 2, 2004 397
1
0
1
were obtained on a Perkin Elmer 2000 spectrophotometer in
3
(mp 43.5-44 °C ); H NMR (250 MHz, CDCl ) d: 7.61-7.65
1
KBr. H NMR spectra were determined on a Bruker AC-250
(m, 4H), 7.41-7.47 (m, 4H), 7.31-7.34 (m, 2H). 1.49 (s, 6H);
IR (neat): 3060, 3025, 2943, 2857, 1807, 1596, 1491, 1445,
spectrometer in CDCl
chromatography (GC) analyses were performed on an HP
890 chromatograph. Mass spectra were recorded using an
3
with TMS as an internal standard. Gas
-
1
+
1271, 754, 689 cm ; MS m/z (%): 220 (M , 28), 205 (100),
5
189 (13), 178 (15), 166 (12), 128 (14), 115 (13), 77 (18).
1
HP 5890 and 5890 MSD. Fluorescence spectra were mea-
sured on a Perkin-Elmer LB-50 spectrofluorimeter.
2b (25% yield). H NMR (250 MHz, CDCl
3
) d: 7.65-
7.75 (m, 4H), 7.41-7.51 (m, 4H), 7.30-7.40 (m, 2H), 1.89-
1
3
1
3
.99 (m, 4H), 1.74-1.83 (m, 4H); C NMR (CDCl ) d: 129.8,
Materials
128.5, 128.4, 128.3, 128.0, 126.2, 32.3, 30.4, 26.3, 25.4; IR
(neat): 3062, 3019, 2949, 2851, 2401, 1808, 1493, 1446,
3
,3-Dimethyl-4,5-diphenyl-3H-pyrazole (1a), 3,3-cy-
-
1
+
clopentyl-4,5-diphenyl-3H-pyrazole (1b) and dimethyl-3,3-
cyclopentyl-3H-pyrazole-4,5-dicarboxylate (1c) were pre-
1216, 759 cm ; MS m/z (%): 246.2 (M , 100), 231.2 (8),
217.1 (100), 178.1 (29), 166.1 (10), 139.1 (18), 111.1 (28);
8
pared according to published procedures. The UV absorp-
HRMS calcd for C19H18: 246.1409; Found 246.1406.
1
tion spectra of all these compounds exhibit a shoulder absorp-
tion band around 290-380 nm. (The extinction coefficients
2c (35% yield); H NMR (250 MHz, CDCl
3
) d: 3.86 (s,
) d: 160.3, 127.6,
52.6, 40.4, 34.5, 26.3; IR (neat): 2955, 2869, 1835, 1721,
1
3
6H), 1.63-1.78 (m, 8H); C NMR (CDCl
3
3
3
3
-1
-1
for 1a-c were 4.9 ´ 10 , 6.6 ´ 10 and 2.5 ´ 10 M cm ,
-
1
+
respectively.)
1436, 1252, 1062 cm ; MS m/z (%): 210 (M , 4), 167 (10),
50 (25), 119 (38), 118 (28), 91 (100), 67 (39), 59 (61);
1
Oxidation Potentials of the 3H-Pyrazoles
HRMS calcd for C11
H
14
O
4
: 210.0892; Found: 210.0900.
) d: 7.22-7.30
1
Oxidation potentials of 3H-pyrazoles 1a-c were mea-
sured by cyclic voltammetry (CHI 600A) at a platinum elec-
trode in argon-saturated dry acetonitrile with 0.10 M tetra-
ethylammonium perchlorate as the supporting electrolyte.
3a (9% yield); H NMR (250 MHz, CDCl
3
1
3
(m, 6H), 7.07-7.14 (m, 4H), 2.24 (s, 3H), 1.41 (s, 6H);
NMR (CDCl ) d: 170.0, 167.7, 136.8, 135.2, 133.8, 129.2,
C
3
128.3, 128.2, 127.4, 127.1, 78.4, 23.4, 18.2; IR (neat): 3059,
-
1
-1
The scan speed was 200 mVs , and the reference electrode
was the SCE. Obtained peaks were irreversible in all cases,
and the oxidation potentials were determined from the half-
peak potentials.
2974, 2928, 1444, 1379, 1292, 761, 701 cm ; MS m/z (%):
+
261 (M , 100), 246 (11), 205 (46), 178 (11), 83 (31); HRMS
calcd for C19
H
19N: 261.1518; Found: 261.1515.
1
3b (2% yield); H NMR (250 MHz, CDCl
3
) d: 7.15-
7
.33 (m, 6H), 7.03-7.11 (m, 4H), 2.30 (s, 3H), 2.20-2.29 (m,
1
3
PET Photolyses
3
2H), 2.11-2.20 (m, 2H), 1.77-1.91 (m, 4H); C NMR (CDCl )
Degassed acetonitrile solutions of 1a or 1b or 1c (10
d: 164.8, 137.7, 135.4, 134.0, 129.3, 128.7, 128.2, 127.4,
+
mM) and TPP (1.0 mM) were irradiated with a Hanovia me-
127.1, 89.8, 37.2, 34.4, 26.0, 18.2; IR (neat): 3062, 2919,
-
1
+
dium-pressure 450 W mercury lamp through a filter solution
2847, 1440, 1373, 1101, 763, 687 cm ; MS m/z (%): 287 (M ,
9
(
CuSO
4
+ NaNO
2
+ NH
4
OH) (l = 400-500 nm). The reaction
100), 272 (13), 259 (51), 246 (25), 216 (18), 202 (29); HRMS
was monitored by GC/MS, and the yield of the product was
determined by quantitative GLC and based on the number of
moles of reactant consumed.
calcd for C21
H
21N: 287.1674; Found: 287.1670.
1
4a (3% yield); H NMR (250 MHz, CDCl
3
) d: 8.79-8.86
(m, 2H), 8.55-8.61 (m, 1H), 8.18-8.24 (m, 1H), 7.65-7.79 (m,
1
3
4
3
H), 2.95 (s, 3H), 1.72 (s, 6H); C NMR (CDCl ) d: 167.3,
Isolation of the Decomposition Products of 1a-c
The primary photoproducts, cyclopropenes 2a-c and
solvent-adducts 3a-b as well as the secondary photoproducts,
157.7, 131.2, 131.1, 131.0, 127.5, 127.2, 126.9, 126.6, 126.2,
125.6, 123.8, 123.7, 123.6, 74.3, 25.2, 22.6; IR (neat): 3083,
-
1
2971, 2926, 2855, 1436, 1378, 759 cm ; MS m/z (%): 259
+
4
a-b and a new dimmer product 5 were isolated from the
(M , 100), 244 (50), 215 (10), 203 (56), 101 (9); HRMS calcd
TPP-sensitized decomposition of 1a-c. 1a or 1b or 1c (0.73
for C19
H
17N: 259.1362; Found: 259.1353.
+
1
mmol) and TPP (0.07 mmol) were dissolved in 300 mL of
4b (3% yield); H NMR (250 MHz, CDCl
3
) d: 8.76-
acetonitrile and irradiated with a Hanovia medium-pressure
8.88 (m, 2H), 8.51-8.63 (m, 1H), 8.01-8.22 (m, 1H), 7.65-
7.78 (m, 4H), 2.91-3.03 (s, 3H), 2.39-2.40 (m, 4H), 2.18-2.31
4 2 4
Hg lamp through a filter solution (CuSO + NaNO + NH OH)
1
3
for 4-6 h after nitrogen purging. After concentration of the so-
lution, products were separated by flash chromatography
3
(m, 2H), 1.85-1.96 (m, 2H); C NMR (CDCl ) d: 155.3,
131.6, 131.2, 127.6, 127.3, 127.0, 126.4, 126.3, 125.2, 124.0,
123.7, 123.6, 84.4, 37.3, 27.3, 22.0; IR (KBr): 3078, 2952,
(
2 3
Al O , n-hexane:ether = 20:1). 2a (17% yield). mp 34-36 °C

3
98 J. Chin. Chem. Soc., Vol. 51, No. 2, 2004
Yen et al.
-
1
+
2
6
866, 1600, 1445, 1377, 756, 726 cm ; MS m/z (%): 285 (M ,
Am. Chem. Soc. 1994, 116, 2576. (e) Ikeda, H.; Minegishi,
T.; Miyashi, T. J. Chem, Soc. Chem. Commun. 1994, 297. (f)
Zona, T. A.; Goodman, J. L. J. Am. Chem. Soc. 1995, 117,
5879. (g) Karatsu, T.; Itoh, H.; Kikunaga, T.; Ebashi, Y.;
Hotta, H.; Kitamura, A. J. Org. Chem. 1995, 60, 8270. (h)
Ikeda, H.; Minegishi, T.; Takahashi, Y.; Miyashi, T. Tetra-
hedron lett. 1996, 39, 4377.
6), 270 (19), 256 (21), 244 (100), 214 (35), 201 (42); HRMS
calcd for C21
H
19N: 285.1517; Found 285.1513.
1
5
(4% yield). mp 196-197 °C; H NMR (250 MHz,
CDCl
3
) d: 1.48-1.60 (m, 8H), 1.65-1.75 (m, 8H), 3.72 (s,
1
9
2H); IR (neat): 2950, 2848, 1737, 1460, 1358, 1276, 1102,
-
1
58 cm ; MS m/z (%): 421 (M+1, 23), 329 (15), 307 (36), 301
2
3
. Miyagawa, N.; Karatsu, T.; Kitamura, A. Chem. Lett. 1997,
(
(
19), 289 (25), 269 (17), 154 (100), 89 (62), 77 (65); HR-MS
1
005.
+
FAB) calcd for C22
21.1859.
H
29
O
8
(M+H ): 421.1863; Found:
. Akaba, R.; Niimura, Y.; Fukushima, T.; Kawai, Y.; Tajima,
T.; Kuragami, T.; Negishi, A.; Kamata, M.; Sakuragi, H.;
Tokumaru, K. J. Am. Chem. Soc. 1992, 114, 4460.
. Chanon, M.; Ebersen, L. In Photoinduced Electron Trans-
fer; Fox, M. A.; Chanon, M. Eds.; Elsevier: New York, 1988;
Part A, p. 470.
4
4
ACKNOWLEDGEMENT
5
. Rehm, D.; Weller, A. Ber. Bunsenges. Physik. Chem. 1969,
We thank the National Science Council of the Republic
7
3, 834.
of China for financial support (contract NSC-89-2113-M-
6
. Kavarnos, G. J. In Photoinduced Electron Transfer 1, Topics
in Current Chemistry 156; Springer-Verlag: New York,
1
26-001).
1
990, p 31.
7. Brown-Wensley, K. A.; Mattes, S. L.; Faird, S. J. Am. Chem.
Soc. 1978, 100, 4162.
Received August 20, 2003.
8. (a) Williams, W. M.; Dobier, W. R. J. Am. Chem. Soc. 1972,
8
5
4, 3955. (b) Leigh, W. J.; Arnold, D. R. Can. J. Chem. 1979,
7, 1186. (c) Yen, Y. P.; Chen, S. F.; Heng, Z. C.; Huang, J.
REFERENCES
C.; Kao, L. C.; Lai, C. C.; Liu, R. S. H. Heterocycles. 2001,
5, 1859.
. Eriksen, J.; Foots, C. S. J. Am. Chem. Soc. 1980, 102, 6083.
0. Baumstark, A. L.; McCloskey, C. J.; Witt, K. E. J. Org.
Chem. 1978, 43, 3609.
5
1
. (a) Karatsu, T.; Hotta, H.; Kitamura, A. J. Chem. Soc. Chem.
Commun. 1991, 1451. (b) Zona, T. A.; Goodman, J. L. J. Am.
Chem. Soc. 1993, 115, 4925. (c) Adam, W.; Sendelbach, J. J.
Org. Chem. 1993, 58, 5310 and 5316. (d) Adam, W.; Sahin,
C.; Sendelbach, J.; Walter, H.; Chen, G.-F.; Williams, F. J.
9
1