Photoinduced electron-transfer systems consisting of electron-donating pyrenes or anthracenes and benzimidazolines for reductive transformation of carbonyl compounds

Article abstract of DOI:10.1016/j.tet.2006.03.061

Photoinduced electron-transfer reactions of several ketone substrates were studied to evaluate the utilities of 1,6-bis(dimethylamino)pyrene (BDMAP), 1,6-dimethoxypyrene (DMP), 9,10-bis(dimethylamino)anthracene (BDMAA), and 9,10-dimethoxyanthracene (DMA) as electron-donating sensitizers cooperating with 2-aryl-1,3-dimethylbenzimidazolines. BDMAP and DMP generally led higher conversion of ketones and better yield of reduction products compared to BDMAA and DMA.

Full text of DOI:10.1016/j.tet.2006.03.061

Tetrahedron 62 (2006) 6581–6588
Photoinduced electron-transfer systems consisting of
electron-donating pyrenes or anthracenes and benzimidazolines
for reductive transformation of carbonyl compounds
a
a
a
a
Eietsu Hasegawa,a, Shinya Takizawa, Takayuki Seida, Akira Yamaguchi, Naoto Yamaguchi,
*
Naoki Chiba,a Tomoya Takahashi,a Hiroshi Ikedab and Kimio Akiyamac
aDepartment of Chemistry, Faculty of Science, Niigata University, Ikarashi, Niigata 950-2181, Japan
bDepartment of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
cInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
Received 26 December 2005; accepted 18 February 2006
Available online 2 May 2006
Abstract—Photoinduced electron-transfer reactions of several ketone substrates were studied to evaluate the utilities of 1,6-bis(dimethyl-
amino)pyrene (BDMAP), 1,6-dimethoxypyrene (DMP), 9,10-bis(dimethylamino)anthracene (BDMAA), and 9,10-dimethoxyanthracene
(DMA) as electron-donating sensitizers cooperating with 2-aryl-1,3-dimethylbenzimidazolines. BDMAP and DMP generally led higher
conversion of ketones and better yield of reduction products compared to BDMAA and DMA.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
anions (ketyl radicals) in PET reactions,5 we needed to find
effective PET conditions to generate such radical anions.
Especially, to promote reactions of substrates not efficiently
absorbing light filtered by PyrexÔ of which ordinary photo-
reaction vessels are made, electron-donating sensitizers ab-
sorbing light of longer wavelength were desired. Related
to this purpose, we have also found that some benzimidazo-
lines act as effective electron- and proton-donors to promote
PET reduction of various carbonyl compounds in which their
radical anions are generated.6 In this context, we became
interested in developing electron-donating sensitizers co-
operating with benzimidazolines for PET-promoted reduc-
tions of carbonyl compounds.
Electron transfer is a fundamental reaction process, which is
operating in reduction and oxidation (redox) reactions in
chemical and biological systems.1 Single electron transfer
(SET) of neutral organic molecules generates radical ions
that undergo various transformations.2 Whereas using redox
reagents as well as electrochemical procedures are tradi-
tional ways to generate these reactive species, photoinduced
electron-transfer (PET) is an alternative method.1 Without
a doubt, PET chemistry of organic molecules has been a
central topic in organic photochemistry over the past several
decades.3 Significant progress has been accomplished in
understanding PET reaction mechanisms and application
of PET reactions to organic synthesis. Reactivity of radical
cations generated by PET processes between electron-donat-
ing substrates and electron-accepting sensitizers has been
extensively investigated.3 On the other hand, reactivity of
radical anions has been less explored in PET reactions,4
which must be in part ascribed to the fact that practical elec-
tron-donating sensitizers are few as compared to electron-
accepting sensitizers such as aromatic nitriles, quinones,
and cationic salts.
For this objective, we chose dimethylamino- or methoxy-
substituted pyrenes or anthracenes,7 namely 1,6-bis-
(dimethylamino)pyrene (BDMAP), 1,6-dimethoxypyrene
electron-donating sensitizers
electron- and proton-donors
X
Me
N
X
Y
H
N
Me
X
X
In the course of our research program focused on reaction
mechanism and synthetic application of carbonyl radical
BDMAA (X = Me2N)
DMA (X = MeO)
BDMAP (X = Me2N)
DMP (X = MeO)
DMPBI (Y = H)
ADMBI (Y = 4-MeO)
DMTBI (Y = 4-Me)
DCDMBI (Y = 3,5-Cl2)
HPDMBI (Y = 2-HO)
* Corresponding author. Tel./fax: +81 25 262 6159; e-mail: ehase@chem.
sc.niigata-u.ac.jp
Chart 1.
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.03.061

6582
E. Hasegawa et al. / Tetrahedron 62 (2006) 6581–6588
O
O
O
Br
O
O
O
Me
Ph
ketone
R
O
substrates
Ph
CO2Me
CO2R
O
5c
7
5a (R = Me)
5b (R = Et)
1a (R = Ph)
1b (R = Me)
1c
3
OH
OH
O
O
OH OH
O
OH
Ph
products
OH
R
Ph Ph
CO2R
CO2Me
Me
Me
2a (R = Ph)
2b (R = Me)
2c
6a (R = Me)
6b (R = Et)
8
6c
4
Chart 2.
(DMP), 9,10-bis(dimethylamino)anthracene (BDMAA), and
9,10-dimethoxyanthracene (DMA),8 and five 2-aryl-1,3-
dimethylbenzimidazolines (DMBIs), which are shown in
Chart 1. Carbonyl substrates, epoxy ketones 1, a-bromo-
methyltetralone 3, carbon–carbon multiple bond tethered
ketones 5, and 3-methylbenzophenone 7, and the correspond-
ing PET reaction products 2, 4, 6, and 8 are represented in
Chart 2. In the reactions, unimolecular or bimolecular rear-
rangements of ketyl radicals of these substrates are expected
to proceed, i.e., Ca–O bond cleavage of the ketyl radicals of
1,10 Cb–Br bond cleavage of the ketyl radical of 3,6e intramo-
lecular ketyl radical addition to C–C multiple bonds for 5,11
and dimerization of ketyl radical of 7.6g We also conducted
PET reactions using above four sensitizers with N,N-
diethyl-N-trimethylsilylmethylamine (TMSA),12 and using
Ru(bpy)3Cl2, well-known PET sensitizer,13 with DMBI. In
this paper, we report the results obtained and discuss the
features and the utilities of these PET systems.
DMP
BDMAP
i
i
0
0.2 0.4 0.6 0.8
0.4 0.6 0.8
1
1.2
+E
+E
BDMAA
DMA
i
i
0.7
0.9
1.1
+E
1.3
-0.2
0
0.2 0.4 0.6
+E
Figure 1. Cyclic voltammograms of sensitizers (E in V vs SCE).
2. Results and discussion
2.1. Fundamental properties of sensitizers and benz-
imidazolines, and basic concept of PET systems
Expected PET reaction pathways are presented in Scheme 1.
Selective photoexcitation of a sensitizer produces its singlet
excited state (1sensitizer*). The excited sensitizer donates
single electron to a ketone within its lifetime to produce
a radical cation of the sensitizer (sensitizerꢀ+) and a radical
anion of the ketone. Feasibility of this initial SET step is
Selected photophysical and electrochemical data of the
sensitizers are summarized in Table 1. These pyrenes and
anthracenes can absorb the light of longer wavelength than
both ketone substrates and DMBIs studied. As shown in
Figure 1, cyclic voltammograms of BDMAP and DMP
demonstrate reversible redox processes (differences of redox
peak potentials are 56 mV and 66 mV, respectively), while
those of BDMAA and DMA do not.
ox
evaluated using the equation DG ¼ E ꢁ Eex ꢁ Ered þ C,
1=2
in which E and Eex are a half-wave oxidation p1o=2tential
ox
1=2
of the ground state and an excitation energy of a sensitizer,
red
1=2
respectively, E is a half-wave reduction potential of ketone
substrate studied,14 and C is the coulomb term that depends
Table 1. Photophysical and electrochemical data of sensitizersa
Sensitizer lmax (log 3) (nm)
lend (nm) lFmax (nm) t (ns) Eex (kcal/mol)b
E
in V vs SCE Eox* in V vs SCE
ox
1=2
BDMAP
DMP
BDMAA
DMA
373 (4.35) 450
335 (4.38), 351 (4.57), 376 (4.16), 397(4.24) 420
450
402, 423
—
436
5.3c
64 (2.8)
72 (3.1)
+0.43d
+0.85d
+0.24
+0.98
ꢁ2.4
7.0e
ꢁ2.2
f
f
397 (3.76)
361 (3.79), 381 (3.95), 402 (3.87)
490
430
—
ca. 60 (2.6)f,g
70 (3.0)
ca. ꢁ2.4
14.3h
ꢁ2.0
a
Measured in MeCN. lmax, Absorption maximum; lend, end absorption; lFmax, fluorescence maximum; t, lifetime; Eex, excitation energy; E1o=x2, Half-wave
oxidation potential of ground state and E ox*, oxidation potential of the excited state that is obtained by the equation Eox ꢁ Eex
.
1=2
b
c
d
e
f
Values in parentheses are reported in eV.
In 5% aqueous THF (Ref. 7d).
Standard potential.
The lifetime in DMF was 6.5 ns (this work).
BDMAA was non-fluorescent.
Estimated from the absorption spectrum.
In heptane (Ref. 9b).
g
h

E. Hasegawa et al. / Tetrahedron 62 (2006) 6581–6588
Table 2. PET reactions of 1b with DMBIa
6583
O
O
O
O
OH
Ph
hν / sens / DMBI / additive
1sensitizer*
Me
Me
DMF
Ph
O
1b
2b
products
sensitizer
hν
Exp
Sensitizer
DMBI
Additive
Conv. of
Yield of
1bb (%)
2bb,c (%)
DMBI
sensitizer
1
2
3
4
5
6
7
8
BDMAP
DMP
BDMAA
DMA
BDMAP
DMP
BDMAA
DMA
ADMBI
ADMBI
ADMBI
ADMBI
HPDMBI
HPDMBI
HPDMBI
HPDMBI
AcOH
AcOH
AcOH
AcOH
—
—
—
—
64
67d
57
54
41
30
15
30
80
84
—
64
93
86
92
90
DMBI
e
Scheme 1.
on the polarity of solvent used; the value for MeCN was
reported to be ꢁ0.06 eV.15 Formed sensitizerꢀ+, except for
BDMAAꢀ+, is exoergonically reduced by DMBIs to its
a
Compound 1b (0.40 mmol), sensitizer (0.05 equiv vs 1b), DMBI
(1.2 equiv vs 1b), AcOH (5.0 equiv vs 1b), DMF (4 mL), l>340 nm for
1 h.
ox
1=2
ox
1=2
ground state because E of DMBIs (E in V vs SCE:
+0.33, +0.28, +0.29, +0.40, and +0.30 for DMPBI, ADMBI,
DMTBI, DCDMBI, and HPDMBI, respectively) are lower
than that of each sensitizer. Then, a radical cation of
DMBI (DMBIꢀ+) and a radical anion of the ketone react
with each other. For an effective sensitization (Scheme 1),
a radical ion pair generated from a sensitizer and a ketone
should smoothly dissociate. Therefore, a polar solvent is rec-
ommended to be used because such dissociation proceeds
more efficiently in polar solvents than in nonpolar sol-
vents.16,17 Also noted that, in these PET reactions, HPDMBI
is expected to act as a two-electron- and two-proton-donor,6f,g
while other DMBIs are expected to perform as two-electron-
and one-proton-donors that require addition of appropriate
proton-donors (ROH) for reduction of ketones (Scheme 2).6
b
c
d
e
Determined on the basis of isolated compounds.
Based on the conversion of 1b.
Determined with 1H NMR.
Detected with 1H NMR but not isolated.
observed without irradiation in the cases of BDMAP and
BDMAA. Therefore, we concluded that 1a is too reactive
to evaluate these sensitizers. Then, we conducted PET reac-
tions of 1b under two different sensitization conditions,
using sensitizer–ADMBI–AcOH6d and sensitizer–HPDMBI
(Table 2).6f In most of the cases, 2b was obtained in good
yields based on the conversion of 1b. However, in the case
of BDMAA with ADMBI and AcOH, 2b could not be
isolated although 1b was consumed (exp 3). Because the
decomposition of 2b was suspected during irradiation,
2b was subjected to the same reaction condition using
BDMAA. However, 2b was quantitatively recovered. Al-
though this unique behavior of BDMAA could not be ratio-
nalized at the moment, combination of ADMBI and AcOH
seems not to be tolerated with BDMAA (also see exp 3 in
Table 6). In the reactions using HPDMBI, the conversion
of 1b in the case of BDMAA was lower than those of other
sensitizers (compare exp 7 to exps 5, 6, and 8). This must be
in part ascribed to endoergonic electron transfer between
BDMAAꢀ+ and DMBIs (Scheme 1).
Me
N
O
-2e-, -2H+
HPDMBI
(Y = 2-HO)
N
Me
Me
N
Me
N
Y
H
N
Me
DMBI
-2e-, -H+
OR
ROH
-H+ (ROH)
N
Me
Y
Scheme 2.
2.2. Ca–O bond cleavage of a,b-epoxy ketones 1
We then studied substituent effects of the phenyl group of
DMBI on PET reaction of 1c with BDMAP under the condi-
tions same as experiment 1 in Table 2. Plots of the conver-
The first examples of PET reactions of a,b-epoxy ketones
with amines were independently reported by Cossy10a
and us.10b At that moment, triethylamine was used as an
electron- and proton-donor, and yields of the expected b-
hydroxy ketones were moderate. Furthermore, it was found
that b-diketones were major products instead of the desired
b-hydroxy ketones in the reactions of 1,3-diaryl-2,3-epoxy-
1-propanones (chalcone epoxides) with triethylamine.10b
A breakthrough to solve this problem was a discovery of
DMBIs, which act as effective electron- and proton-donors
to give b-hydroxy ketones.6
ox
sion of 1c versus E of DMBIs are presented in Figure 2.
1=2
The conversion of 1c decreased as Eo1=x2 increases, being con-
sistent with the expected SET between BDMAPꢀ+ and DMBI
(Scheme 1).
A set of Ru(bpy)3Cl2 and amines is a well-known sensitiza-
tion system for reductive transformation of organic com-
pounds,13d–h and Ru(bpy)3Cl2 is considered to have some
advantages compared with organic sensitizers, for example,
photoexcitation using longer wavelength of light is possible,
and separation of Ru(bpy)3Cl2 is more easily performed.
Then, we examined PET reaction of 1b using Ru(bpy)3Cl2
with ADMBI and AcOH or with HPDMBI (Table 3). In this
sensitization system, Ru(I) is an expected reducing agent.
Based on the standard potential of Ru(I) (E¼ꢁ1.30w
ꢁ1.33 V vs SCE),13a–c,18 reducing ability of Ru(I) must be
First, we conducted PET reactions of epoxy ketones 1 to
compare DMP, BDMAA, and DMA with previously exam-
ined BDMAP.6 Whereas reactions of 1a with ADMBI in
DMF–AcOH afforded 2a (86% for BDMAP, 85% for
DMP, 68% for BDMAA, 62% for DMA based on the con-
versions of 1a), some conversions from 1a to 2a were also

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E. Hasegawa et al. / Tetrahedron 62 (2006) 6581–6588
Table 4. PET reactions of 3 with ADMBI or TMSAa
Br
100
90
80
70
60
50
40
30
20
O
O
hν / sens / amine
DMF
3
4
Exp
Sensitizer
Amine
Conv. of 3b (%)
Yield of 4b,c (%)
1
2
3
4
5
6
7
8
BDMAP
DMP
BDMAA
DMA
BDMAP
DMP
BDMAA
DMA
ADMBI
ADMBI
ADMBI
ADMBI
TMSA
TMSA
TMSA
TMSA
83
70
42
56
68
53
19
27
57
54
53
59
67
59
69
46
a
Compound 3 (0.50 mmol), sensitizer (0.05 equiv vs 3), ADMBI
(1.2 equiv vs 3), TMSA (5.0 equiv vs 3), DMF (5 mL), l>360 nm for 5 h.
0.25
0.3
0.35
0.4
0.45
ox
Determined with 1H NMR.
Based on the conversion of 3.
b
c
+E 1/2 in V vs SCE
Figure 2. Plots of conversion of 1c versus oxidation potentials of DMBIs in
PET reaction of 1c using BDMAP and DMBIs with AcOH in DMF.
be ascribed to an efficient and irreversible fragmentation
of TMSAꢀ+.4a,19
Table 3. PET reactions of 1b with Ru(bpy)3Cl2 and DMBIa
O
2.4. Intramolecular ketone–olefin or –acetylene coupling
of C–C multiple bond tethered ketones 5
O
OH
Ph
h
ν
/ Ru(bpy) Cl / DMBI / additive
3 2
Me
Me
solvent
Ph
O
1b
2b
Cossy and co-workers reported that PET-promoted intra-
molecular coupling reactions of ketone carbonyls with C–C
multiple bonds.11 However, the light of shorter wavelength
(254 nm) was usually used, and highly toxic hexamethyl-
phosphorictriamide (HMPA) was in some cases required
to obtain cyclization products in better yields. Therefore,
we became interested in testing applicability of our PET
methods to these synthetically relevant transformations.
Then, we conducted reactions of 5a with HPDMBI using
four sensitizers. The results presented in Table 5 clearly in-
dicate that both the conversion of 5a and the yield of 6a were
essentially same regardless of the sensitizer (exps 1–4).
Similar cyclization reactions of 5b and 5c were achieved by
BDMAP with HPDMBI to produce 6b and 6c, respectively.
Notably, addition of Mg(ClO4)2 significantly increased the
Exp
DMBI
Solv
Additive
Conv. of
Yield of
1bb (%)
2bb,c (%)
1
2
3
4
5
ADMBI
DMF
DMF
MeCN
THF
AcOH
—
—
—
—
71
45
41
25
36
41
70
75
71
72
HPDMBI
HPDMBI
HPDMBI
HPDMBI
MeOH
a
Compound 1b (0.40 mmol), Ru(bpy)3Cl2 (0.01 equiv vs 1b), DMBI
(1.2 equiv vs 1b), AcOH (5.0 equiv vs 1b), solvent (4 mL), l>390 nm
for 3 h.
Determined with 1H NMR.
b
c
Based on the conversion of 1b.
weaker than those of the excited states of above electron-
donating pyrenes and anthracenes (see Eox* in Table 1). As
anticipated, although the reactions of 1b proceeded, yields
of 2b were relatively low as compared to those reported in
Table 2.
Table 5. PET reactions of 5 with HPDMBIa
OH
O
hν / sens / HPDMBI / additive
DMF
(R = Me, Et)
2.3. Cb–Br bond cleavage of a-bromomethyltetralone 3
CO2R
CO2R
6
5
Next, we conducted PET reactions of 2-bromomethyl-2-
(3-butenyl)-1-tetralone 3 with ADMBI or TMSA (Table 4).
Although yields of the expected spiro-cyclization product 4
were good in all cases, the conversions of 3 in the reactions
with BDMAP or DMP were greater than those with
BDMAA or DMA. This tendency was more significant in
the reactions with TMSA. SET steps between ADMBI and
sensitizersꢀ+, except for BDMAAꢀ+, are exoergonic as de-
scribed above. On the other hand, calculated free energy
Exp
5
Sensitizer
Additive
Conv. of
Yield of
5b (%)
6b,c (%)
1
2
3
4
5
6
7
5a
5a
5a
5a
5b
5b
5c
BDMAP
DMP
BDMAA
DMA
BDMAP
BDMAP
BDMAP
—
—
—
—
71
72
75
78
68
93
58
59
62
55
60
71
88
86
—
Mg(ClO4)2
—
ox
changes for SET between TMSA (E ¼+0.49 V vs SCE)
1=2
a
and sensitizersꢀ+ suggest that SET with DMPꢀ+ is exoergonic
(DG¼ꢁ8.3 kcal/mol) while SET with BDMAPꢀ+ is slightly
endoergonic (DG¼+1.4 kcal/mol). However, the conver-
sions of 3 were not significantly different from each other
(greater than 50% in both cases). This phenomena would
Compound 5 (0.40 mmol), sensitizer (0.05 equiv vs 5a; 0.1 equiv vs 5b
and 5c), HPDMBI (1.2 equiv vs 5), Mg(ClO4)2 (1.2 equiv vs 5b), DMF
(4 mL for exps 1–4; 2 mL for exps 5–7), l>340 nm for 22 h for exps
1–5 and 7; for 16 h for exp 6.
Determined with 1H NMR.
b
c
Based on the conversion of 5.

E. Hasegawa et al. / Tetrahedron 62 (2006) 6581–6588
6585
conversion of 5b and the yield of 6b for the shorter reaction
time (compare exp 6 to exp 5). This phenomenon would be ra-
tionalized by the assumption that back-electron transfer from
5ꢀꢁ to BDMAPꢀ+ is suppressed through ion-pair exchange
with Mg(ClO4)2, and therefore the reaction is accelerated.20
sensitizers than BDMAA and DMA. As a result, we recom-
mend both BDMAP and DMP as visible light absorbing and
electron-donating sensitizers for PET reactions. Another no-
table point is that DMP, unprecedented PET sensitizer, acted
reasonably well in above examples. We will further explore
the PET reactions of DMP and other alkoxy substituted
pyrenes. The results and discussion described in this paper
will hopefully provide useful information for any individual
who is interested in PET chemistry to perform reductive
transformation of organic compounds.
2.5. Pinacol coupling of 3-methylbenzophenone 7
We recently found that 3-methylbenzophenone 7 was a suit-
able substrate to probe the mechanism of PET reactions with
DMBIs.6g Thus, we decided to examine the PET reaction of
7 using the sensitizers with ADMBI and AcOH, or with
HPDMBI (Table 6). Whereas benzpinacol 8 was obtained al-
most quantitatively in the case of DMP with ADMBI and
AcOH (exp 2), BDMAA did not work at all (exp 3). Both
BDMAP and DMA were similarly effective (exps 1 and 4).
In the cases of HPDMBI, both the conversion of 7 and the
yield of 8 were greater in the reactions with BDMAP and
DMP than in those with BDMAA and DMA. It should also
be noted that selective formation of 8 rather than benzhydrol
is similar to the product selectivity observed in the PET reac-
tions of 7 with DMPBI and AcOH or with HPDMBI without
sensitizers.6g In the latter case, specific interactions of radi-
cal ion pairs are involved. Therefore, observed product selec-
tivity in this work is consistent with the proposed reaction
pathways in which reaction between independently gener-
ated 7ꢀꢁ and DMBI+ꢀ proceeds (Scheme 1).
4. Experimental
4.1. General
NMR spectra were recorded in CDCl3 with Me4Si as an
1
internal standard at 200 and 270 MHz for H NMR, and
50 MHz for 13C NMR. Uncorrected melting points are re-
ported. Absorption and fluorescence spectra were measured
in MeCN. The fluorescence lifetime was determined by
time-correlated single photon counting technique based on
picoseconds laser pulse excitation. Oxidation and reduction
potentials in MeCN were measured with cyclic voltammetry
using platinum electrodes as working and counter elec-
trodes, Ag/0.01 M AgNO3 as a reference electrode, and
0.1 M Et4NClO4 as a supporting electrolyte at the scan
rate of 100 mV/s. Sample solutions were purged with N2
before measurements. Ferrocene was used as a reference.21
Standard Potentials of ferrocene/ferrocenium couple were
measured to be +0.066 V and +0.439 V versus Ag/AgNO3
and SCE, respectively. Then, peak potentials of sensitizers,
DMBIs, and ketone substrates were converted to those
against SCE. Half-wave potentials were obtained from these
peak potentials by subtracting or adding 0.029 V.22 Photo-
reactions were conducted in a Pyrex test tube (1.5 cm dia-
meter) immersed in a water bath at room temperature with
either a 500 W Xe lamp or 500 W Xe–Hg lamp as a light
source. Column chromatography was performed with
silica-gel (Wakogel C-200). Preparative TLC was performed
on 20 cmꢂ20 cm plates coated with silica-gel (Wakogel
B-5F). Anhydrous DMF was purchased and used for the
photoreactions. MeCN was distilled over P2O5 and subse-
quently with K2CO3. THF was distilled over sodium–benzo-
phenone under N2. Other reagents and solvents were
purchased and used without further purification. DMPBI,23a
ADMBI,6g HPDMBI,6g DMTBI,23b and DCDMBI23b were
prepared by the previously reported methods.6g BDMAP,9a
DMP,9a BDMAA,9c DMA9a,24 were prepared by the slight
modifications of the literature procedures (see below). Sub-
strates (1a,10b 1b,25 1c,25 5a,11 5b,11 5c,11 and 76g) and
photoproducts (2a,10b 2b,25 2c,25 6a,11 6b,11 6c,11 and 86g)
are known compounds. Preparation of 3 and spectral data of
3 and 4 are described below because their characterizations
have not been completed.6e
Table 6. PET reactions of 7 with DMBIa
O
OH OH
hν / sens / DMBI / additive
Ar
Ar
Ar
Ph
DMF
(Ar = 3-MeC6H4)
Ph Ph
8
7
Exp
Sensitizer
DMBI
Additive
Conv. of
Yield of
7b (%)
8b,c (%)
1
2
BDMAP
DMP
BDMAA
DMA
BDMAP
DMP
BDMAA
DMA
ADMBI
ADMBI
ADMBI
ADMBI
HPDMBI
HPDMBI
HPDMBI
HPDMBI
AcOH
AcOH
AcOH
AcOH
—
—
—
—
78
100
6
64
34
57
17
22
86
100
0
79
100
92
3
4d
5
6
7
53
77
8d
a
Compound 7 (0.20 mmol), sensitizer (0.05 equiv vs 7), DMBI (1.2 equiv
vs 7), AcOH (6.6 equiv vs 7), DMF (2 mL), l>360 nm for 4 h.
Determined with 1H NMR.
Based on the conversion of 7.
Benzhydrol was also obtained: 13% for exp 4 and 6% for exp 8.
b
c
d
3. Conclusion
We have studied PET reactions of several ketone substrates
using electron-donating pyrenes or anthracenes as sensitizers
with benzimidazolines as electron- and proton-donors (re-
ducing reagents). Differences in effectiveness of these sen-
sitizers were observed depending on the substrates and
conditions. Among properties to be required for an effective
sensitizer is the stability of its radical cation. Based on the cy-
clic voltammetry of sensitizers (Fig. 1), radical cations of the
pyrenes must be more stable than those of the anthracenes in
the sensitization cycle in Scheme 1, which is compatible with
some of the results described above. Therefore, we would
like to conclude that BDMAP and DMP are more reliable
4.2. Preparations of sensitizers
4.2.1. Preparation of BDMAP.9a 1,6-Diaminopyrene was
prepared by the reaction of pyrene with HNO3, followed
by the reduction of 1,6-dinitropyrene with Na2S$9H2O.
Then, an aqueous MeOH solution (H2O+MeOH¼
21 mL+86 mL) of 1,6-diaminopyrene (1.9 g, 8.3 mmol),

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E. Hasegawa et al. / Tetrahedron 62 (2006) 6581–6588
CaCO3 (1.8 g, 18 mmol), and MeI (20.7 mL, 0.33 mol) was
heated at 55 ꢃC for 29 h. After addition of MeI (20.0 mL,
0.32 mol), the mixture was heated for another 36 h. Precip-
itated yellow solid obtained after cooling was heated with
NaOEt (19.7 g, 0.29 mol) at 90 ꢃC for 48 h. After concentra-
tion and addition of H2O, extraction with C6H6 was per-
formed. The extract was treated with H2O, satd aqueous
NaCl, and dried over anhydrous MgSO4, and then concen-
trated. Silica-gel column chromatography (CH2Cl2) gave
yellow solid that was recrystallized twice from EtOH to
give BDMAP (709 mg, 2.5 mmol, 30%) as yellow plates:
mp 163.2–164.5 ꢃC (lit.9a 164–165 ꢃC).
tetralone (1.66 g, 8.31 mmol) that was obtained through the
sequence of NaH promoted reaction of 4-bromo-1-butene
with ethyl 1-tetralone-2-carboxylate28 and NaOH promoted
hydrolytic decarboxylation, and stirred for 1 h under N2.
Then, CH2Br2 (2.9 mL, 41.6 mmol) was added and the re-
sulting mixture was heated at 75 ꢃC for 22 h. After addition
of H2O, extraction with Et2O was performed. The extract
was treated with satd aqueous Na2S2O3, satd aqueous
NaHCO3, satd aqueous NaCl, and dried over anhydrous
MgSO4, and then concentrated. Silica-gel column chromato-
graphy (C6H6/n-C6H14¼1/1) and subsequent distillation
gave 3 (924 mg, 3.15 mmol, 38%) as colorless oil: bp
105–110 ꢃC (0.45 mmHg); IR (Neat) 1681 cmꢁ1; 1H NMR
(270 MHz) d 1.73–2.37 (m, 6H), 2.93–3.15 (m, 2H), 3.66
(d, J¼10.4 Hz, 1H), 3.76 (d, J¼10.4 Hz, 1H), 4.91–5.04
(m, 2H), 5.66–5.81 (m, 1H), 7.22–7.34 (m, 2H), 7.45–7.51
(m, 1H), 8.02–8.05 (m, 1H) ppm; 13C NMR (50 MHz)
d 24.9, 27.9, 31.1, 32.5, 39.1, 48.5, 115.1, 126.8, 128.1,
128.8, 131.3, 133.6, 137.6, 142.9, 198.4 ppm. HRMS
(ESI) calcd for C15H17O79BrNa+: 315.0361, found:
315.0355; C15H17O81BrNa+: 317.0341, found: 317.0335.
4.2.2. Preparations of DMP and DMA.26 Pyrene (4.3 g,
21 mmol) and K2Cr2O7 (6.2 g, 21 mmol) in 8 M H2SO4
(44 mL) were heated at 90 ꢃC for 1 h and subsequently at
120 ꢃC for 3 h. The mixture was poured into H2O and the
precipitated red-brown solid was filtered. The solid was
subjected to silica-gel column separation (AcOH) to give
a mixture of 1,6-pyrenedione and 1,8-pyrenedione (1.2 g,
5.2 mmol, 25%).27 A part of the mixture (250 mg,
1.08 mmol), Na2S2O4 (1.13 g, 6.48 mmol), and n-Bu4NBr
(35 mg, 0.11 mmol) in aqueous THF (H2O+THF¼4.7 mL+
2.7 mL) were stirred at room temperature for 1 h. KOH
(1.39 g, 24.8 mmol) in H2O (3.6 mL) was added, and
Me2SO4 (2.1 mL, 22.7 mmol) was slowly added in an ice-
water bath. After the resulting mixture was stirred at room
temperature for 22 h, addition of H2O was followed by
extraction with CH2Cl2. The extract was treated with H2O
and dried over anhydrous MgSO4, and then concentrated.
Silica-gel column chromatography (CH2Cl2) gave a mixture
of 1,6-dimethoxypyrene and 1,8-dimethoxypyrene. The
mixture was subjected to fractional recrystallization three
times from C6H5Cl, and then DMP (52.7 mg, 0.20 mmol,
19%) was obtained as yellow needles: mp 250–252 ꢃC
(lit.9a 244–245 ꢃC). DMA was prepared from anthraquinone
(624 mg, 3.00 mmol) in the similar manner to DMP (see
above). Successive recrystallization of the crude product
from EtOH gave DMA (466 mg, 1.95 mmol, 65%) as pale
yellow plates: mp 204–207 ꢃC (lit.24 198–199 ꢃC).
4.4. General procedure of PET reactions
A solution of a ketone substrate (0.20–0.50 mmol) and DMBI
(0.24–0.60 mmol) or TMSA (2.50 mmol) with a sensitizer
(generally 1.0–4.0ꢂ10ꢁ2 mmol and 4.0ꢂ10ꢁ3 mmol for
Ru(bpy)3Cl2) in DMF (2–5 mL) or other solvents (MeCN,
THF, MeOH 4 mL) in the presence or absence of AcOH
(1.32–2.00 mmol) or Mg(ClO4)2 (0.48 mmol) was purged
with N2 for 5 min prior to irradiation. This solution was irradi-
ated with the light (l>340 and 360 nm for pyrenes and anthra-
cenes, and 390 nm for Ru(bpy)3Cl2) using a cut-off glass-filter
foranappropriateirradiationtime.Whileaphotolysatewithout
additive was concentrated, a photolysate containing AcOH or
Mg(ClO4)2 was subjected to the extraction with EtOAc or
C6H6. The extract was treated with satd aqueous NaHCO3,
satd aqueous NaCl, and dried over anhydrous MgSO4, and
then concentrated. For the reactions of 7, the resulting residue
was directly analyzed with 1H NMRusingtriphenylmethaneas
an internal standard. For other reactions, the residues were sub-
jected to silica-gel TLC or column separation for the reactions
of 1, 3 or 5 using appropriate mixed solvents (EtOAc/n-C6H14
for 1, C6H6/n-C6H14 for 3, EtOAc/C6H6 for 5) to give the start-
ing ketones and the products. Photoproducts were identified by
analyses with their NMR and IR spectra. Product 4(3/1mixture
of two diastereomers): colorless oil; IR (Neat) 1678 cmꢁ1; 1H
NMR (270 MHz)d 1.00(d, J¼6.5 Hz, 3H, majorisomer), 1.07
(d, J¼6.8 Hz, 3H, minor isomer), 1.12–2.28 (m, 9H), 2.91–
3.06 (m, 2H), 7.18–7.31 (m, 2H), 7.40–7.46 (m, 1H), 8.02–
8.05 (m, 1H) ppm; 13C NMR (50 MHz) d 19.8, 20.2, 26.3,
26.7, 34.2, 34.3, 34.4, 34.5, 35.4, 35.7, 36.5, 42.6, 44.6, 53.4,
126.4, 127.9, 128.5, 131.5, 132.8, 143.5, 202.2 ppm. Anal.
Calcd for C15H18O: C, 84.07;H, 8.47; found:C, 83.91;H, 8.63.
4.2.3. Preparation of BDMAA.9c 9,10-Diaminoanthracene
was obtained by the reaction of anthraquinone (6.0 g,
29 mmol) with formamide, followed by a treatment with
KOH. To the crude 9,10-diaminoanthracene were added
MeOH (8 mL), MeI (8.0 mL, 129 mmol), and subsequently
Na2CO3 (3.2 g, 30 mmol) in H2O (8 mL). After the resulting
mixture was stirred under N2 for 16 h at room temperature
and heated at 60 ꢃC for 24 h, addition of H2O was followed
by extraction with CHCl3. The extract was treated with H2O,
satd aqueous NaHCO3, and dried over anhydrous MgSO4,
and then concentrated. Silica-gel column chromatography
(CHCl3/n-C6H14¼2/1) gave yellow solids, which were re-
crystallized twice from EtOH to give BDMAA (552 mg,
2.09 mmol, 19% from anthraquinone) as yellow plates: mp
197–200 ꢃC.
4.3. Preparation of 2-bromomethyl-2-(3-butenyl)-1-
tetralone 3
Acknowledgements
E.H. and H.I. thank the Ministry of Education, Culture,
Sports, Science, and Technology of Japan for financial
support in the form of Grants-in-Aid for Scientific Research
C (No. 15550028) and for Scientific Research in Priority
To the mixture of NaH (ca. 66% in oil, 604 mg, 16.6 mmol)
and HMPA (2.9 mL, 16.6 mmol) in THF (23 mL) was
slowly added a THF solution (7 mL) of 2-(3-butenyl)-1-

E. Hasegawa et al. / Tetrahedron 62 (2006) 6581–6588
6587
1996, 61, 6799–6804; (f) Banerjee, A.; Falvey, D. E. J. Org.
Chem. 1997, 62, 6245–6251; (g) Pandey, G.; Hajira, S.; Ghorai,
M. K.; Kumar, K. R. J. Am. Chem. Soc. 1997, 119, 8777–8787;
(h) Pandey, G.; Rao, K. S. S. P.; Rao, K. V. N. J. Org. Chem.
2000, 65, 4309–4314.
Areas (Area No. 417), respectively. E.H. thanks financial
support from the Uchida Energy Science Promotion Founda-
tion. H.I. gratefully acknowledges financial support from the
Izumi Science and Technology Foundation and the Shorai
Foundation. We thank Dr. D. D. M. Wayner (National
Research Council of Canada) for his valuable comments
on electrochemistry. We also thank Professor K. Okada
(Osaka City University) for his donation of Ru(bpy)3Cl2.
Generous assistance provided by Professors T. Horaguchi
(Niigata University), M. Ueda (Tohoku University), and
S. Tero-Kubota (Tohoku University) is also acknowledged.
8. Although these pyrenes and anthracenes are previously known
(Ref. 9) and both BDMAP (Refs. 6,7d) and DMA (Ref. 7a,h)
have been used for some PET reactions, PET reactions of
DMP and BDMAA have not been reported. In addition, no sys-
tematic study on the utilities of these four compounds as sensi-
tizers for PET reactions has been conducted.
9. (a) Zweig, A.; Maurer, A. H.; Roberts, B. G. J. Org. Chem.
1967, 32, 1322–1329; (b) Dreeskamp, H.; Pabst, J. Chem.
Phys. Lett. 1979, 61, 262–265; (c) Chung, Y.; Duerr, B. F.;
McKelvey, T. A.; Najappan, P.; Czarnik, A. W. J. Org. Chem.
1989, 54, 1018–1032.
References and notes
10. (a) Cossy, J.; Aclinou, P.; Bellosta, V.; Furet, N.; Baranne-
Lafont, J.; Sparfel, D.; Souchaud, C. Tetrahedron Lett. 1991,
32, 1315–1316; (b) Hasegawa, E.; Ishiyama, K.; Horaguchi,
T.; Shimizu, T. J. Org. Chem. 1991, 56, 1631–1635; (c) Cossy,
J.; Bouzide, A.; Ibhi, S.; Aclinou, P. Tetrahedron 1991, 47,
7775–7782; (d) Kirschberg, T.; Mattay, J. J. Org. Chem.
1996, 61, 8885–8896; (e) Hasegawa, E.; Ishiyama, K.; Fujita,
T.; Kato, T.; Abe, T. J. Org. Chem. 1997, 62, 2396–2400; (f)
Zhang, J.; Jin, M.-Z.; Zhang, W.; Yang, L.; Liu, Z.-L. Tetra-
hedron Lett. 2002, 43, 9687–9689; (g) Recent review:
Hasegawa, E.; Kamata, M. CRC Handbook of Organic Photo-
chemistry and Photobiology, 2nd ed.; Horspool, W. M.,
Lenci, F., Eds.; CRC: Boca Raton, FL, 2003; Chapter 53,
pp 1–17.
11. Belotti, D.; Cossy, J.; Pete, J. P.; Portella, C. J. Org. Chem.
1986, 51, 4196–4200.
12. (a) Hasegawa, E.; Xu, W.; Mariano, P. S.; Yoon, U. C.; Kim,
J. U. J. Am. Chem. Soc. 1988, 110, 8099–8111; (b) Hasegawa,
E.; Brumfield, M. A.; Yoon, U. C.; Mariano, P. S. J. Org. Chem.
1988, 53, 5435–5442.
1. (a) Eberson, L. Electron Transfer Reactions in Organic Chem-
istry; Springer: Berlin, 1987; (b) Advances in Electron Transfer
Chemistry; Mariano, P. S., Ed.; JAI: Greenwich, CT, 1991–
1999; Vols. 1–6; (c) Electron Transfer in Chemistry;
Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vols. 1–5.
2. (a) Schmittel, M.; Burghart, A. Angew. Chem., Int. Ed. 1997,
36, 2550–2589; (b) Berger, D. J.; Tanko, J. M. The Chemistry
of Double–Bonded Functional Groups; Patai, S., Ed.; Wiley:
New York, NY, 1997; pp 1281–1354; (c) Schmittel, M.; Ghorai,
M. K. Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, 2001; Vol. 2, pp 5–54.
3. (a) Photoinduced Electron Transfer; Fox, M. A., Chanon, M.,
Eds.; Elsevier: Amsterdam, 1988; Parts A–D; (b) Kavarnos,
G. J. Fundamental of Photoinduced Electron Transfer; VCH:
New York, NY, 1993; (c) CRC Handbook of Organic Photo-
chemistry and Photobiology, 1st ed.; Horspool, W. M., Song,
P. S., Eds.; CRC: Boca Raton, FL, 1994, and 2nd ed.;
Horspool, W. M., Lenci, F., Eds.; CRC: Boca Raton, FL, 2003.
4. Representative reviews for PET-promoted reductions of car-
bonyl compounds with amine donors. (a) Yoon, U. C.; Marinao,
P. S.; Givens, R. S.; Atwater, B. W., III. Advances in Electron
Transfer Chemistry; Mariano, P. S., Ed.; JAI: Greenwich, CT,
1994; Vol. 4, pp 117–205; (b) Cossy, J.; Pete, J. P. Advances
in Electron Transfer Chemistry; Mariano, P. S., Ed.; JAI:
Greenwich, CT, 1996; Vol. 5, pp 141–195.
13. Representative examples for PET reactions using Ru(bpy)3Cl2
with amines. (a) DeLaive, P. J.; Lee, J. T.; Sprinschinik, H. W.;
Abruna, H.; Meyer, T. J.; Whitten, D. G. J. Am. Chem. Soc.
1977, 99, 7094–7097; (b) Ballardini, R.; Varani, G.; Indelli,
M. T.; Scandola, F.; Balzani, V. J. Am. Chem. Soc. 1978, 100,
7219–7223; (c) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.;
Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K.
J. Am. Chem. Soc. 1979, 101, 4815–4824; (d) Pac, C.;
Miyauchi, Y.; Ishitani, O.; Ihama, M.; Yasuda, M.; Sakurai,
H. J. Org. Chem. 1984, 49, 26–34; (e) Ishitani, O.; Yanagida,
S.; Takamuku, S.; Pac, C. J. Org. Chem. 1987, 52, 2790–
2796; (f) Willner, I.; Tsfania, T.; Eichen, Y. J. Org. Chem.
1990, 55, 2656–2662; (g) Fukuzumi, S.; Mochizuki, S.;
Tanaka, T. J. Phys. Chem. 1990, 94, 722–726; (h) Okada, K.;
Okubo, K.; Morita, N.; Oda, M. Chem. Lett. 1993, 2021–2024.
5. Hasegawa, E. J. Photoscience 2003, 10, 61–69.
6. (a) Hasegawa, E.; Kato, T.; Kitazume, T.; Yanagi, K.;
Hasegawa, K.; Horaguchi, T. Tetrahedron Lett. 1996, 37,
7079–7082; (b) Hasegawa, E.; Tamura, Y.; Tosaka, E. Chem.
Commun. 1997, 1895–1896; (c) Hasegawa, E.; Yoneoka, A.;
Suzuki, K.; Kato, T.; Kitazume, T.; Yanagi, K. Tetrahedron
1999, 55, 12957–12968; (d) Hasegawa, E.; Chiba, N.;
Nakajima, A.; Suzuki, K.; Yoneoka, A.; Iwaya, K. Synthesis
2001, 1248–1252; (e) Hasegawa, E.; Takizawa, S.; Iwaya, K.;
Kurokawa, M.; Chiba, N.; Yamamichi, K. Chem. Commun.
2002, 1966–1967; (f) Hasegawa, E.; Chiba, N.; Takahashi, T.;
Takizawa, S.; Kitayama, T.; Suzuki, T. Chem. Lett. 2004, 33,
18–19; (g) Hasegawa, E.; Seida, T.; Chiba, T.; Takahashi, T.;
Ikeda, H. J. Org. Chem. 2005, 70, 9632–9635.
7. Representative examples for PET reactions using dimethyl-
amino- or methoxy-substituted arenes as sensitizers. (a) Lee,
J. A.; Israel, S. H. J. Org. Chem. 1983, 48, 4557–4563; (b)
Hamada, T.; Nishida, A.; Yonemitsu, O. J. Am. Chem. Soc.
1986, 108, 140–145; (c) Nishida, A.; Hamada, T.; Yonemitsu,
O. J. Org. Chem. 1988, 53, 3386–3387; (d) Okada, K.; Oka-
moto, K.; Oda, M. J. Am. Chem. Soc. 1988, 110, 8736–8738;
(e) Pandey, G.; Rao, K. S. S. P.; Rao, K. V. N. J. Org. Chem.
red
1=2
14. Reduction potentials (E in V vs SCE) of some representative
substrates are ꢁ1.59 V for 1a, ꢁ1.82 V for 3, and ꢁ1.78 V for
7, respectively. In the cases of 1b, 1c, and 5a, the reduction
peaks were not observed below ꢁ2.3 V.
15. (a) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259–271; (b)
Weller, A. Z. Phys. Chem. Neue Folge 1982, 133, 93–98.
16. In principle, a polar solvent accelerates initial PET processes
(Refs. 3,15,17), and is desirable for a PET reaction involving
sensitization. However, if a reaction requires a chemical pro-
cess within a radical ion pair, e.g., proton transfer from a radical
cation to a radical anion (Ref. 17), nonpolar solvents are in
some cases more favorable to lead better yields of products

6588
E. Hasegawa et al. / Tetrahedron 62 (2006) 6581–6588
as compared to polar solvents (Ref. 10e). Therefore, a choice of
a suitable solvent for each case is important for preparative
PET reactions.
21. (a) Gritzne, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461–466;
(b) Workentin, M. S.; Parker, V. D.; Morkin, T. L.; Wayner,
D. D. M. J. Phys. Chem. A 1998, 102, 6503–6512.
22. Mann, C. K.; Barnes, K. K. Electrochemical Reactions in
Nonaqueous Systems; Marcel Dekker: New York, NY, 1970.
23. (a) Chikashita, H.; Itoh, K. Bull. Chem. Soc. Jpn. 1986, 59,
1747–1752; (b) Lee, I.-S. H.; Jeoung, E. H.; Kreevoy, M.
J. Am. Chem. Soc. 1997, 119, 2722–2728.
24. Meek, J. S.; Monroe, P. A.; Bouboulis, C. J. J. Org. Chem.
1963, 28, 2572–2577.
25. Hasegawa, E.; Ishiyama, K.; Kato, T.; Horaguchi, T.; Shimizu,
T.; Tanaka, S.; Yamashita, Y. J. Org. Chem. 1992, 57, 5352–
5359.
17. Devadoss, C.; Fessenden, R. W. J. Phys. Chem. 1991, 95,
7253–7260.
18. Tokel-Takvoryan, N. E.; Hemingway, R. E.; Bard, A. J. J. Am.
Chem. Soc. 1973, 95, 6582–6589.
19. (a) Zhang, X.; Ye, S. R.; Hong, S.; Freccero, M.; Albini, A.;
Falvey, D. E.; Mariano, P. S. J. Am. Chem. Soc. 1994, 116,
4211–4220; (b) Su, Z.; Mariano, P. S.; Falvey, D. E.; Yoon,
U. C.; Oh, S. W. J. Am. Chem. Soc. 1998, 120, 10676–
10686.
20. Representative examples of inorganic salt effects on PET reac-
tions. (a) Simon, J. D.; Peters, K. S. J. Am. Chem. Soc. 1983,
105, 4875–4882; (b) Mizuno, K.; Ichinose, N.; Otsuji, Y.
Chem. Lett. 1985, 455–458; (c) Review: Loupy, A.; Tchoubar,
B.; Astruc, D. Chem. Rev. 1992, 92, 1141–1165.
26. Kraus, G. A.; Man, T. O. Synth. Commun. 1986, 16, 1037–1042.
27. Fatiadi, A. J. J. Chromatogr. 1965, 20, 319–324.
28. Bowman, W. R.; Westlake, P. J. Tetrahedron 1992, 48, 4027–
4038.