Benzimidazoline-dimethoxypyrene. An effective promoter system for photoinduced electron transfer promoted reductive transformations of organic compounds

Article abstract of DOI:10.3987/COM-08-S(F)94

2-(p-Methoxyphenyl)-1,3-dimethylbenzimidazoline (ADMBI) and 2-(o-hydroxyphenyl)-1,3-dimethylbenzimidazoline (HPDMBI) are used as reducing reagents in 1,8-dimethoxypyrene (1,8-DMP) sensitized, photoinduced electron transfer (PET) reactions. This system was effectively used for PET induced, reductive transformations of various organic substrates, including α,β-epoxy ketones, the olefin tethered 2-bromomethyl-l-tetralone, and o-allyloxy-iodobenzene, as well as for the deprotection reactions of dodecyl-2-benzoylbenzoate and N-sulfonylindole. The results of studies show that 1,8-DMP is a more effective sensitizer than the previously used 9-methylcarbazole for deprotection of N-methyl-4-picolinium ester.

Full text of DOI:10.3987/COM-08-S(F)94

HETEROCYCLES, Vol. 77, No. 2, 2009
1147
HETEROCYCLES, Vol. 77, No. 2, 2009, pp. 1147 - 1161. © The Japan Institute of Heterocyclic Chemistry
Received, 4th August, 2008, Accepted, 18th September, 2008, Published online, 22nd September, 2008
DOI: 10.3987/COM-08-S(F)94
BENZIMIDAZOLINE-DIMETHOXYPYRENE.
AN
EFFECTIVE
PROMOTER SYSTEM FOR PHOTOINDUCED ELECTRON TRANSFER
PROMOTED REDUCTIVE TRANSFORMATIONS OF ORGANIC
COMPOUNDS
Eietsu Hasegawa,* Harumi Hirose, Kosuke Sasaki, Shinya Takizawa,
Takayuki Seida, and Naoki Chiba
Department of Chemistry, Faculty of Science, Niigata University, Ikarashi-2 8050,
Niigata 950-2181, Japan
Abstract – 2-(p-Methoxyphenyl)-1,3-dimethylbenzimidazoline (ADMBI) and
2
-(o-hydroxyphenyl)-1,3-dimethylbenzimidazoline (HPDMBI) are used as
reducing reagents in 1,8-dimethoxypyrene (1,8-DMP) sensitized, photoinduced
electron transfer (PET) reactions. This system was effectively used for PET
induced, reductive transformations of various organic substrates, including
α,β-epoxy ketones, the olefin tethered 2-bromomethyl-1-tetralone, and
o-allyloxy-iodobenzene, as well as for the deprotection reactions of
dodecyl-2-benzoylbenzoate and N-sulfonylindole. The results of studies show that
1
9
,8-DMP is a more effective sensitizer than the previously used
-methylcarbazole for deprotection of N-methyl-4-picolinium ester.
INTRODUCTION
About two decades ago, 1,3-dimethyl-2-phenylbenzimidazoline (DMPBI) was shown to be a
1
hydride-transfer reagent for reductive transformations of organic compounds. Subsequently, it was
demonstrated that DMPBI promotes radical reactions, in which it acts as an effective hydrogen
2
atom-donor. These earlier observations suggested that DMPBI and its derivatives behave in a manner
similar to biologically relevant dihydropyridines (i.e. NADH). This has been confirmed by the results of
3
further later investigations in this area.
In recent years, we have investigated photoinduced electron transfer (PET) reactions of carbonyl
4
-7
compounds and halides. In this effort, a substance possessing both electron-donating and hydrogen
th
This paper is dedicated to Professor Emeritus Keiichiro Fukumoto on the occasion of his 75 birthday.

1148
HETEROCYCLES, Vol. 77, No. 2, 2009
atom-donating abilities was required to serve as an effective reducing reagent. An evaluation of its
properties suggested that DMPBI would meet these requirements. About ten years ago, we first applied
8
DMPBI in PET promoted transformations of aromatic epoxy ketones to hydroxy ketones. We found that
the use of DMPBI led to processes that took place in much higher product yields than those that
4
,9
employed previously reported PET conditions. In the reaction investigated, DMPBI serves as a
two-electron- and one-proton-donor, therefore, addition of an appropriate proton-donor (ROH) is required
to complete the reaction (Type 1 in Scheme 1). A second break-through occurred when
2
-(o-hydroxyphenyl)-1,3-dimethylbenzimidazoline (HPDMBI) was employed in the above types of PET
reactions. In this case, no additional proton-donor is required because HPDMBI acts as a two-electron-
and two-proton-donor in which its phenolic group serves as a second proton source (Type 2 in Scheme
10
1
). Since the time of these early studies, we developed a variety of PET promoted reactions, in which
several benzimidazolines (DMBI), including DMPBI and HPDMBI, were used to achieve reductive
1
1-16
transformation of carbonyl compounds and halides.
Type 1
Me
Me
N
Me
N
e-
-H+
-e-
N
-
DMPBI
-
H+
N
H
N
N
(
ROH)
OR
Me
Me
Me
DMPBI
Type 2
Me
N
Me OH
N
Me
N
O
e-
-H+
-e-
-
HPDMBHI
-H+
OH
N
H
N
N
Me
Me
Me
HPDMBHI
Scheme 1
For promoting PET reactions, photosensitization methods have been employed frequently owing to
17
several advantageous features that do not exist with direct irradiation induced PET. By using
photosensitization, the photosensitizer is selectively excited by the light that is not absorbed by substrates.
As a result, undesired photoreactions emanating from the excited states of the substrates do not take place.
Dimethylamino- or methoxy-substituted arenes have been used as sensitizers for PET promoted reductive
1
8-20
transformations of a variety of organic compounds.
Recently, we demonstrated that both
1
,6-bis-dimethylaminopyrene (BDMAP) and 1,6-dimethoxypyrene (1,6-DMP) are effective sensitizers in
1
6
cooperation with benzimidazolines (DMBIs) for PET reduction reactions of carbonyl compounds. In the

HETEROCYCLES, Vol. 77, No. 2, 2009
1149
photosensitization cycle followed in these processes (Scheme 2), the sensitizer first absorbs light to
generate its singlet excited state, from which a single electron is transferred to a carbonyl substrate to
generate the carbonyl radical anion (ketyl radical). The simultaneously formed radical cation of sensitizer
is subsequently reduced by DMBI to regenerate its ground state. Finally, the formed radical cation of
DMBI reacts with ketyl radicals to give reduction products.
O
sensitizer
MeO
DMBI
X
Y
O
1sensitizer*
Me
N
sensitizer
products
N
H
h!
Me
X
MeO
1,8-DMP
DMBI
sensitizer
BDMAP (X = Me N)
,6-DMP (X = MeO)
ADMBI (Y = p-MeO)
HPDMBI (Y = o-HO)
2
1
DMBI
Scheme 2
RESULTS AND DISCUSSION
Below, we describe results arising from an investigation of PET sensitization reactions of organic
2
1
substrates in which 1,8-dimethoxypyrene (1,8-DMP) is used for the first time as an electron donor
sensitizer together with 2-(p-methoxyphenyl)-1,3-dimethylbenzimidazoline (ADMBI) and
-(o-hydroxyphenyl)-1,3-dimethylbenzimidazoline (HPDMBI). In earlier efforts, we have demonstrated
that the conversion of epoxy-ketones to the corresponding hydroxy-ketones is a useful process to evaluate
2
4
,5,7,8,10,12,13,16
new electron transfer conditions.
Thus, the PET sensitization method that relies on the use of
1
,8-DMP and DMBIs was first probed employing the ring opening reaction of epoxy-ketone 1a (Table 1).
As can be seen by viewing the data in Table 1, 1,8-DMP was found to be similarly effective to 1,6-DMP
and BDMAP for reductive ring opening PET reactions of 1a with ADMBI and HPDMBI in DMF (entries
1
6
1
and 2). Notably, addition of Mg(ClO ) resulted in a higher conversion of 1a and an increased yield of
4 2
2
a (compare entry 3 to entry 2). Solvent property is one of the key factors in governing PET reactions in
1
7
solution. As data in Table 1 show, reaction of 1a takes place irregardless of the polarity of the solvent
2
2
(
entries 4~7). It should be noted that the relatively less polar solvent benzotrifluoride (BTF) can also be
used in this process. Although BTF is suggested to be a more environmentally benign solvent than
2
3
24
benzene and methylene chloride, it has not been previously used for PET reactions. Another notable
observation is that the pale yellow solution of 1,8-DMP in BTF and methylene chloride became brown on
irradiation while a significant color change is not observed when 1a and HPDMBI are present.

1150
HETEROCYCLES, Vol. 77, No. 2, 2009
a
Table 1. 1,8- sensitized PET reaction of 1a with DMBI in various solvents.
O
O
OH
Ph
h! / 1,8-DMP / DMBI / additive
Me
solvent / N2
Me
Ph
O
1
a
2a
additive
equiv vs 1a)
irrad time
conv of
1a (%)
yield of
2a (%)
78
entry
DMBI
solvent
b
b,c
(
(min)
60
1
2
3
4
5
6
7
ADMBI
HPDMBI
HPDMBI
HPDMBI
HPDMBI
HPDMBI
HPDMBI
DMF
DMF
DMF
MeCN
THF
AcOH (7.0)
-
73
45
83
25
58
35
75
60
70
Mg(ClO ) (1.2)
30
89
4
2
-
60
72
-
-
-
60
83
CH Cl
60
80
2
2
BTF
60
83
a
1
a (0.20 mmol), 1,8-DMP (0.05 equiv vs 1a), DMBI (1.2 equiv vs 1a), solvent (2.0 mL),
1
λ > 340 nm. bDetermined by H NMR. cBased on the conversion of 1a.
The expected reaction pathway followed in these reactions is outlined in Scheme 2. Irradiation
selectively generates the excited state of 1,8-DMP (1,8-DMP*) whose reducing ability is similar to those
2
5
of 1,6-DMP and BDMAP. Single electron transfer (SET) from 1,8-DMP* to 1a occurs to produce the
•+
•–
radical cation of 1,8-DMP (1,8-DMP ) and the radical anion of 1a (1a ). Mg(ClO ) could suppress the
4
2
back electron transfer from 1a•– to 1,8-DMP•+ through an ion-pair exchange effect, and therefore the
2
6
•+
quantum efficiency of the reaction increases. 1,8-DMP returns to its ground state by SET from DMBI.
If SET with 1a is absent, 1,8-DMP* undergoes energy wasting decay and, in some cases decomposition
occurs as is observed in BTF and methylene chloride solutions lacking 1a.
We next examined reactions of 1a in which the DMBIs are replaced by other electron donors, including
1
9,27
ascorbic acid (AA)
and 3,5-bis(ethoxycarbonyl)-2,6-dimethylpyridine (Hantzch dihydropyridine,
1
0,28
HDHP)
which have been previously used for PET reactions. As the results presented in Table 2
demonstrate, by using AA, Et N and HDHP reaction does not take place and 1a is recovered almost
3
quantitatively (entries 1-3). On the other hand, Me NPh promotes reaction of 1a to some extent (entry 4)
2
and, as already described, both HPDMBI and ADMBI are effective. These observations can be
•+
understood by considering the thermodynamic/kinetic feasibility of SET from these donors to 1,8-DMP .
Looking at the calculated free energy changes (ΔG) given in Table 2, it is seen that endoergonic SET is
expected for AA, Et N and HDHP whose oxidation potentials are greater than that of 1,8-DMP. In
3
contrast SET between 1,8-DMP•+ and Me NPh, HPDMBI or ADMBI, whose oxidation potentials are
2
lower than that of 1,8-DMP, should be exoergonic.

HETEROCYCLES, Vol. 77, No. 2, 2009
1151
a
Table 2. 1,8-DMP sensitized PET reaction of 1a with various donors.
O
O
OH
Ph
h! / 1,8-DMP / donor
Me
Me
Ph
DMF / N2
O
1a
2a
b
Eox
V vs SCE)
1.08
conv of
1a (%)
yield of
2a (%)
ΔG
1
/2
entry
donor
AA
c
c,d
(
kcal/mol)
(
1
2e
3
5.8
3.5
- f
- f
Et N
0.98
3
HDHP
0.93
0.78
0.30g
0.28g
2.3
- f
4e
5
6e
Me NPh
HPDMBI
ADMBI
-1.2
-12.2
-12.7
34
45
73
32
70
78
2
a
1
a (0.20 mmol), 1,8-DMP (0.05 equiv vs 1a), donor (1.2 equiv vs 1a), DMF (2.0 mL),
b
c
1
d
ox
o
λ > 340 nm, 60 min. ΔG = 23.06 [E
the conversion of 1a. AcOH (7.0 equiv vs 1a) was added. 1a was recovered. Reference 16.
(donor) - E (1,8-DMP)]. Determined by H NMR. Based on
/2
1
e
f
g
On the basis of above observations, photosensitization system consisting of 1,8-DMP and DMBIs is
expected to be ideally suited for PET reduction reactions of a variety of substances. To probe the viability
of this proposal, we examined two representative dehalogenation reactions of the olefin tethered
1
4
12
2
-bromomethyl-1-tetralone 3 and o-allyloxy iodobenzene 5 which produce the corresponding products
2
9
4
and 6 through an 5-exo radical cyclization pathway (Scheme 3 and Table 3).
O
O
Br
I
h! / 1,8-DMP / ADMBI
h! / 1,8-DMP / ADMBI
solvent / N2
solvent / N2
O
O
3
4
5
6
H
SET
SET
H
O
O
-I–
-
Br–
3
5
O
O
Scheme 3
PET spirocyclization reaction of 3 to give 4, sensitized by 1,8-DMP and ADMBI proceeds both in DMF
and BTF in yields that are comparable to those promoted by using 1,6-DMP and BDMAP (entries 1 and
16
2
1
). In the case of BTF, precipitation of imidazolium bromide interfered with light absorption by
,8-DMP. Addition of water to the BTF solution solved this problem to some extent, and the conversion
of 3 and the yield of 4 increased slightly (entry 3). The 1,8-DMP and ADMBI based photosensitization

1152
HETEROCYCLES, Vol. 77, No. 2, 2009
method was also effective in inducing the dehalogenation/cyclization reaction of olefin tethered aromatic
iodide 5 in DMF and BTF although the yields of 6 were relatively low (entries 4 and 5). Notably, when
ADMBI is replaced by the hydrogen atom donor 1,4-cyclohexadiene (CHD, 53 equiv), only a trace
amount of 6 is detected.
a
Table 3. 1,8-DMP sensitized PET reactions of 3 and 5 with ADMBI.
irrad time
conv of b
yield of
product (%)
entry
halide
solvent
product
b,c
(
h)
5
halide (%)
1
2
3
4
5
3
3
3
5
5
DMF
BTF
BTFd
DMF
BTF
95
76
83
85
73
4
4
4
6
6
62
71
81
36
28
5
5
8
8
a
3
(0.50 mmol); 5 (0.20 mmol), 1,8-DMP (0.05 equiv vs halide), ADMBI (1.2 equiv vs halide), solvent
b 1 c
(
5.0 mL for 3, 4.0 mL for 5), λ > 360 nm for 3; λ > 340 nm for 5. Determined by H NMR. Based on
d
2
the conversion of halide. H O (1 mL) was added as a co-solvent.
In some cases, above the DMBIs act as effective reducing reagents for PET reactions carried out in the
absence of 1,8-DMP. This technique serves as a practical alternative particularly for reactions of
substrates which absorb Pyrex™-glass filtered light of which ordinary photoreaction vessels are made.
For example, irradiation (λ > 280 nm) of 1,3-diphenyl-2-propene-1-one oxide 1b (chalcone epoxide, 0.40
mmol) with HPDMBI (1.2 equiv) in THF or BTF (4.0 mL) for 1 h leads to production of the desired
hydroxy-ketone 2b (81% yield at 84% conv of 1b in THF; 91% yield at 81% conv of 1b in BTF).
Similarly, when 3 (0.50 mmol) was irradiated with ADMBI (1.2 equiv) in MeCN or BTF (5.0 mL) for 4 h,
4
was obtained in moderate to good yields (60% in MeCN; 74% at 98% conv of 3 in BTF). Also,
irradiation of 5 (0.20 mmol) with ADMBI (1.2 equiv) in BTF (4.0 mL) for 4 h gave 6 in a 35% yield at
3% conversion of 5.
Photodeprotection reaction is recognized as a useful technique in photolithography as well as chemical
7
30
biology. As described above, BTF was found to be a tolerable solvent for PET reactions. Then, we
decided to apply 1,8-DMP sensitized PET method in various solvents including BTF to three
representative deprotection reactions.
Nitrobenzyl and phenacyl groups have been used previously for photoremovable protecting groups for
3
1
alcohols and amines, especially in peptide synthesis. About ten years ago, Porter, et al. reported that
benzoylesters of alcohols are removed efficiently by using photochemical methods. For example,
irradiation (λ > 280 nm) of dodecyl 2-benzoyl benzoate 7 with cyclohexyl amine (10 equiv) in 1:1
3
2
benzene and MeCN leads to generation of dodecanol 8 and the lactone 9 (95% each). When 1,8-DMP

HETEROCYCLES, Vol. 77, No. 2, 2009
1153
a
Table 4. 1,8-DMP sensitized PET reaction of 7 with DMBI.
O
O
CO C H
O
2
12 25
h! / 1,8-DMP / DMBI / additive
Ph
Ph
C H OH
+
1
2
25
solvent / N2
7
8
9
b,c
additive
equiv vs 7)
conv of 7
yields (%)
entry
DMBI
solvent
b
(
(%)
8
~100
95
9
1
2
3
HPDMBI
ADMBI
ADMBI
DMF
DMF
BTF
-
52
~100
99
79
84
-d
AcOH (7.0)
AcOH (7.0)
93
a
7
(0.20 mmol), 1,8-DMP (0.05 equiv vs 7), DMBI (1.2 equiv vs 7), solvent (2.0 mL), λ > 360 nm, 6 h.
1 c d
b
Determined by H NMR. Based on the conversion of 7. Small amount of 9 (~3%) was detected.
sensitized method using DMBI was applied to the deprotection reaction of 7, the desired dodecanol 8 is
obtained in excellent yields (> 90%) (Table 4). While 9 is also obtained in reasonable yields in DMF,
only small amount of this substance is detected when BTF is used as solvent. This finding is difficult to
•–
explain. A plausible mechanism for the deprotection reaction is shown in Scheme 4. Ketyl radical (7 )
accepts two protons and a single electron from DMBI•+ and AcOH to produce an intermediate
diarylcarbinol which then undergoes lactonization to produce 8 and 9.
OH CO C H
OH CO C H
OH CO C H
2 12 25
2
12 25
2
12 25
H+
H+
SET
Ph
Ph
Ph
8
+
9
7
H
Scheme 4
3
3
Desulfonylation reactions of N-sulfonamides generally take place under relatively vigorous conditions.
Recently, Padwa, et al. reported a PET based mild desulfonylation method using Et N and nBu SnH. This
3
3
method was applied to the desulfonylation reaction of N-sulfonylindole 10, which produces the
34
deprotected product 11 (96%). Accordingly, sulfonyl substitution at the benzene-ring occurs in the
absence of nBu SnH. We believed that this methodology, using an environmentally undesired organotin
3
reagent and high energy short wavelength light (254 nm), could be advantageously modified by
substituting the PET conditions we have developed (Table 5). Indeed, desulfonylation of the
N-sulfonylindole 10 takes place smoothly and in excellent yield by employing PET reaction with
1
,8-DMP and HPDMBI in MeCN (entry 1). Although a longer irradiation time is required when reaction
is carried out in BTF, the yield of 11 remains high. The expected pathway followed in this process is
•–
red
shown in Scheme 5. Radical anion 10 , formed by SET between 1,8-DMP* and 10 (E
= -0.95 V vs
/2
1

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HETEROCYCLES, Vol. 77, No. 2, 2009
a
Table 5. 1,8-DMP sensitized PET reaction of 10 with HPDMBI.
h! / 1,8-DMP / HPDMBI
CO Me
CO Me
2
2
N
N
solvent / N2
SO Ph
H
2
1
0
11
b,c
entry
solvent
MeCN
BTF
irrad time (min)
conv of 10 (%)
yield of 11 (%)
1
2
30
100
97
93
84
120
a
1
0 (0.40 mmol), 1,8-DMP (0.05 equiv vs 10), HPDMBI (1.2 equiv vs 10), solvent (4.0 mL), λ > 360 nm.
b
c
Isolated yield. Based on the conversion of 10.
SCE), undergoes desulfonylation to give indole anion and sulfonyl radical which accept a respective
•+
proton and hydrogen atom from HPDMBI to produce 11 and PhSO H.
2
+
H / H
1
0
CO Me
+
PhSO2
11
+
PhSO H
2
2
N
Scheme 5
The final process we have explored is the deprotection of N-methylpicolinium carboxylate 12 that was
35
first reported by Falvey and co-workers. In Falvey’s work, a stoichiometric amount of 9-methylcarbazol
(
9-MC) was used as a sensitizer and an excess amount of CHD was employed as a hydrogen atom and
proton donor. We reexamined this process using both 1,8-DMP and 9-MC with CHD in MeCN (Table 6).
Using stoichiometric amounts of these sensitizers, no significant differences in conversion and product
a
Table 6. 1,8-DMP sensitized PET reaction of 12 with CHD.
Me
Ph
N
Ph
h! / sensitizer / CHD
O
OH
+
Me
N Me
Ph
Ph
MeCN / N2
ClO4
O
O
1
2
13
14
b,c
sensitizer
equiv vs 12)
irrad
wavelength
conv of 12
yields (%)
entry
b
(
(%)
13
77
74
6
14
80
79
11
1
2
3
4
9-MC (1.0)
1,8-DMP (1.0)
9-MC (0.05)
λ > 320 nm
λ > 320 nm
λ > 340 nm
λ > 360 nm
100
100
18
100
b
1,8-DMP (0.05)
89
~100
c
a
1
1
2 (0.02 mmol), CHD (1.0 ml), MeCN (4.0 mL), 3 h. Determined by H NMR. Based on the
conversion of 12.

HETEROCYCLES, Vol. 77, No. 2, 2009
1155
yields are detected (entries 1 and 2). However, when the quantities of the sensitizers are decreased to
catalytic levels and irradiation takes place with longer wavelength light, significant differences appear
between the 9-MC and 1,8-DMP promoted reactions (entries 3 and 4).
The large differences observed between PET reaction of 12 sensitized by 1,8-DMP versus 9-MC appears
to be due to differences in their UV-VIS absorption profiles. Namely, end-absorption of 9-MC is around
3
60 nm while that of 1,8-DMP extends to around 400 nm in MeCN while oxidizaing abilities of their
25,36
excited states are comparable.
Another notable observation is that the results of cyclic
voltammometric studies show that 9-MC displays irreversible redox behavior while 1,8-DMP behaves
reversibly. This finding suggests that the radical cation formed from 9-MC is less stable than that derived
from 1,8-DMP. A plausible mechanistic route for PET deprotection of 12 is outlined in Scheme 6.
3
5
red
Accordingly, the radical 15, generated from SET between 1,8-DMP* and 12 (E
= -0.95 V vs SCE),
/2
1
undergoes fragmentation to give diphenylacetate and the picolinium radical, which then abstract a
respective proton and hydrogen atom to form 13 and 14, repectively.
Me
Ph
N
Ph
+
H / H
O
O
+
H C
2
N Me
13
+
14
Ph
Ph
O
O
15
Scheme 6
•+
The interesting issues in this process are, 1) what process intervenes to return 1,8-DMP to its neutral
form to participate in the catalytic cycle, and 2) what species donates a proton to the acetate. Unlike
3
7
DMBIs, CHD does not have a sufficiently strong electron donating ability (+1.74 V vs SCE) to transfer
•+
single electron to 1,8-DMP . A more plausible candidate for this role is the CHD radical that is
38
generated by hydrogen atom transfer between CHD and the picolinium radical (Scheme 7). SET
•+
between the CHD radical and 1,8-DMP generates the CHD cation that should be acidic enough to
protonate acetate anion and produce benzene.
H C
N Me
2
SET
Ph
+
1,8-DMP
+
H
H
1
,8-DMP
CHD
1
4
O
13
Ph
O
benzene
Scheme 7

1156
HETEROCYCLES, Vol. 77, No. 2, 2009
CONCLUSION
Electron transfer is a fundamental process that produces charged radicals that serve as intermediates in
39,40
many chemical and biological reduction and oxidation (redox) reactions.
While the use of redox
reagents as well as electrochemical procedures are common ways to ion radical species, reactive species
17
in organic chemistry, PET represents a mild, practical alternative. Above, we have demonstrated that
DMBI and 1,8-DMP is a useful promoter system for promoting PET reductive transformations of a
variety of organic compounds. Among characteristic features of the PET method based on this
combination are that 1) two types of DMBIs can be chosen as reducing agents in PET promoted
reductions, 2) 1,8-DMP is an effective photosensitizer that absorbs incident light near the visible region,
thus avoiding unwanted photoreactions of substrates, 3) DMBIs are compatible with 1,8-DMP sensitized
PET reaction conditions while DMBIs can be also used in the absence of sensitizers, 4) 1,8-DMP is not
only effective with DMBIs but also with CHD, and 5) BTF, proposed as an environmentally benign
solvent, can be employed in these PET reaction conditions.
EXPERIMENTAL
General procedures.
NMR spectra were recorded in CDCl with Me Si as an internal standard at 200 MHz and 270 MHz for
3
4
1
13
H NMR, and 50 MHz and 68 MHz for C NMR. Uncorrected melting points are reported. Absorption
and fluorescence spectra were measured in MeCN. Oxidation and reduction potentials in MeCN were
1
6
measured with cyclic voltammetry according to the previously reported procedure. Photoreactions were
conducted in a Pyrex test tube (1.5 cm diameter) immersed in a water bath at room temperature with a
5
00 W Xe lamp as a light source. Column chromatography was performed with silica gel (Wakogel
C-200). Preparative TLC was performed on 20 cm x 20 cm plates coated with silica gel (Wakogel B-5F).
Anhydrous DMF was purchased and used for the photoreactions. MeCN was distilled over P O and
2
5
subsequently with K CO . THF was distilled over sodium–benzophenone under N . Other reagents and
2
3
2
solvents were purchased and used without further purification. ADMBI and HPDMBI were prepared by
1
5
16
the previously reported methods. 1,8-DMP was similarly prepared to 1,6-DMP by the slight
4
1
42
43
modification of the literature procedure (see below). 9-MC and HDHP were prepared according the
4
4
4
16
12
32
34
35
44
to literature reported procedures. Substrates (1a, 1b, 3, 5, 7, 10, and 12 ) and photoproducts (2a,
4
16
12
32
32
34
35
35
2
b, 4, 6, 8, 9, 11, 13, and 14 ) are known compounds.
Preparation and property of 1,8-DMP.
1
6
Procedure to obtain the mixture of 1,6-pyrenedione and 1,8-pyrenedione was previously reported.
A
typical procedure is given as follows. To the crude solid containing a mixture of 1,6-pyrenedione and

HETEROCYCLES, Vol. 77, No. 2, 2009
1157
1
2
,8-pyrenedione (13 mmol), and n-Bu NBr (419 mg, 1.3 mmol) in aqueous THF (THF + H O = 58 mL +
4 2
0 mL) was added Na S O (13.6 g, 78 mmol) in H O (40 mL). The resulting mixture was stirred at room
2
2
4
2
temperature for 1 h. Then, KOH (16.8 g, 299 mmol) in H O (40 mL) was added, and Me SO (25.8 mL,
2
2
4
2
73 mmol) was slowly added in an ice–water bath. After the resulting mixture was stirred at room
temperature for 22 h, addition of H O was followed by extraction with CH Cl . The extract was washed
2
2
2
with H O and dried over anhydrous MgSO , and then concentrated in vacuo. Silica-gel column
2
4
chromatography of the residue (CH Cl / nC H = 1 / 2) gave a mixture of 1,6-dimethoxypyrene and
2
2
6
14
1
,8-dimethoxypyrene (724 mg, 2.76 mmol, 21%). This mixture was subjected to fractional crystallization
three times from C H Cl, and then 1,6-DMP (239 mg, 0.91 mmol, 7%) was obtained as yellow needles:
6
5
41
mp 246.5–247.0 °C (lit., 244–245 °C). Then, 1,8-DMP was concentrated in the filtrate, from which
1
2
,8-DMP (142 mg, 0.54 mmol, 4%) was obtained as yellow needles: mp 208.3-210.0 °C (lit.,45
-1 1
01-202 °C). 1,6-DMP: IR (KBr) 2950, 1518, 1030 cm ; H-NMR (270 MHz) δ 4.17 (s, 6H), 7.54 (d, 2H,
J = 8.37 Hz), 7.94 (d, 2H, J = 9.18 Hz), 8.06 (d, 2H, J = 8.37 Hz), 8.26 (d, 2H, J = 9.18 Hz); 1,8-DMP: IR
-
1 1
(
KBr) 2950, 1588, 1020 cm ; H-NMR (270 MHz) δ 4.16 (s, 6H), 7.51 (d, 2H, J = 8.51 Hz), 7.77 (s, 2H),
1
3
8
1
.03 (d, 2H, J = 8.51 Hz), 8.41 (s, 2H); C-NMR (68 MHz) δ 56.1 (q), 108.0 (d), 120.1 (d), 120.8 (s),
24.5 (d), 124.7 (d), 125.6 (s), 125.9 (s), 152.9 (s).
Solubilities of 1,6-DMP and 1,8-DMP in several solvents were qualitatively checked by adding DMP (1
mmol) into a solvent (1.0 mL). Both 1,6-DMP and 1,8-DMP were insoluble in MeOH. While 1,6-DMP
was not completely dissolved in MeCN and BTF, 1,8-DMP was soluble in these solvents. Both 1,6-DMP
and 1,8-DMP were soluble in DMF, CH Cl , and THF.
2
2
General procedure of PET reactions.
A solution of a substrate (1, 3, 5, 7, 10: 0.20–0.50 mmol) and DMBI (0.24–0.60 mmol) with or without
-
2
1
,8-DMP (1.0–2.5 x 10 mmol) in a solvent (DMF, MeCN, CH Cl , THF, BTF 2.0–5.0 mL), in the
2 2
presence or absence of AcOH (1.40 mmol) or Mg(ClO ) (0.24 mmol), was purged with N for 5-10 min
4
2
2
prior to irradiation. In the case of 12, a solution of 12 (0.02 mmol), CHD (1.0 mL, 10.6 mmol) and 9-MC
or 1,8-DMP (0.02 mmol or 0.001 mmol) in MeCN (4.0 mL) was purged with N for 10 min before
2
irradiation. These solutions were irradiated with the light using cut-off glass-filters (λ > 320 nm, 340 nm,
and 360 nm for sensitization reactions) or not using a filter (λ > 280 nm through Pyrex) for an appropriate
irradiation time. Photolysates obtained from the reactions of 1a, 5, 7 in DMF and 5 in BTF were
subjected to the extraction with Et O for 1a and EtOAc for 5, 7. The extracts were washed with water and
2
satd. aqueous NaCl, and dried over anhydrous MgSO and then concentrated. Photolysates from other
4
reactions in MeCN, CH Cl , THF, and BTF were directly concentrated. The resulting residues were
2
2
1
analyzed with H-NMR using triphenylmethane for 1a, 3, 7, 1,4-dimethoxybenzene for 5, and

1158
HETEROCYCLES, Vol. 77, No. 2, 2009
1
,3,5-trimethoxybenze for 12 as internal standards. For reactions of 1b and 10, the residues were
subjected to silica-gel column or TLC separation using appropriate solvents (column: CH Cl for 1b,
2
2
TLC: EtOAc / n-C H = 1 / 3 for 10) to give the starting materials and the products. Photoproducts were
6
14
identified by analyses with their NMR and IR spectra.
ACKNOWLEDGEMENTS
We are grateful to Professor Patrick S. Mariano (University of New Mexico, USA) for his valuable
comments and editing. This work was partly supported by the grant from the Uchida Energy Science
Promotion Foundation.
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16
1. As previously reported, both BDMAP and 1,6-DMP are reasonably effective sensitizers. However
some problems are associated with these compounds. For example, BDMAP is reported to often
2
0
undergo decomposition during irradiation, and the solubility of 1,6-DMP was found to be low in
MeCN that is commonly used for PET reactions. On the other hand, 1,8-DMP is readily soluble in

1160
HETEROCYCLES, Vol. 77, No. 2, 2009
MeCN (see Experimental Section).
CH Cl , C H CF , THF, respectively.
3
0
2
2
2. Polarity of the solvents estimated by ET (kcal/mol): 45.7, 43.3, 41.0, 39.3, 37.4 for MeCN, DMF,
2
4
2
2
6
5
3
3. A. Ogawa and D. P. Curran, J. Org. Chem., 1997, 62, 450; J. J. Maul, P. J. Ostrowski, G. A.
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2
2
4. Recently, we reported a first example of ground-state ET reactions in BTF: E. Hasegawa, Y. Ogawa,
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2
008, 64, 7724.
º
5. On the basis of the standard potential (E = +0.83 V vs SCE, obtained from reversible cyclic
voltammogram) and the excitation energy (Eex = 73.0 kcal/mol, 3.16 eV) of 1,8-DMP, oxidation
ox
º
ox
potential of its excited state (E *) is estimated to be -2.33 V by using the equation of E – E . E *
ex
values for 1,6-DMP and BDMAP can be similarly caluculated to be -2.27 and -2.34 V based on the
1
6
reported data.
2
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3
5
(
Eex = 82 kcal/mol, 3.55 eV) of 9-MC, its excited state oxidation potential (Eox*) is calculated to
be -2.37 V.

HETEROCYCLES, Vol. 77, No. 2, 2009
1161
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3
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2
5
1
,8-DMP, on the basis of the reported data for some conjugated alkyl radicals such as PhCH2,
ox
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