Aryl-substituted dimethylbenzimidazolines as effective reductants of photoinduced electron transfer reactions

Article abstract of DOI:10.1016/j.tet.2015.06.071

Photoinduced electron transfer (PET) reactions promoted by 2-aryl substituted 1,3-dimethylbenzimidazolines (Ar-DMBIH) were investigated. Excited states of Ar-DMBIH, formed by irradiation using light above 360 nm, initiate PET reductions of various organic substrates, including transformations of epoxy ketones to aldols, free radical rearrangements such as the Dowd-Beckwith ring-expansion and 5-exo hexenyl cyclization, deprotection of N-sulfonyl-indols, and allylation of acyl formates. In these processes, Ar-DMBIH possessing 1-naphthyl, 2-naphthyl, 1-pyrenyl and 9-anthryl substituents formally act as two electron and one proton donors while the hydroxynaphthyl substituted derivative serves as a two electron and two proton donor. On the basis of the results of absorption spectroscopy studies, cyclic voltammetry and DFT calculation, a mechanistic sequence for these reduction reactions is proposed that involves initial photoexcitation of the aryl chromophore of the Ar-DMBIH followed by single electron transfer (SET) to the organic substrate to generate the radical cation of benzimidazoline and the radical anion of the substrate.

Full text of DOI:10.1016/j.tet.2015.06.071

Tetrahedron 71 (2015) 5494e5505
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Aryl-substituted dimethylbenzimidazolines as effective reductants
of photoinduced electron transfer reactions
,
a
a
a
a
Eietsu Hasegawa a , Taku Ohta , Shiori Tsuji , Kazuma Mori , Ken Uchida ,
*
Tomoaki Miura a, Tadaaki Ikoma a c, Eiji Tayama a, Hajime Iwamoto a,
,b,
Shin-ya Takizawa d, Shigeru Murata d
a Department of Chemistry, Faculty of Science, Niigata University, 8050 Ikarashi-2, Nishi-ku, Niigata 950-2181, Japan
b Center for Instrumental Analysis, Niigata University, 8050 Ikarashi-2, Nishi-ku, Niigata 950-2181, Japan
c Core Research for Evolutionary Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
d Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 May 2015
Received in revised form 16 June 2015
Accepted 17 June 2015
Available online 24 June 2015
Photoinduced electron transfer (PET) reactions promoted by 2-aryl substituted 1,3-
dimethylbenzimidazolines (Ar-DMBIH) were investigated. Excited states of Ar-DMBIH, formed by irra-
diation using light above 360 nm, initiate PET reductions of various organic substrates, including
transformations of epoxy ketones to aldols, free radical rearrangements such as the Dowd-Beckwith ring-
expansion and 5-exo hexenyl cyclization, deprotection of N-sulfonyl-indols, and allylation of acyl for-
mates. In these processes, Ar-DMBIH possessing 1-naphthyl, 2-naphthyl, 1-pyrenyl and 9-anthryl sub-
stituents formally act as two electron and one proton donors while the hydroxynaphthyl substituted
derivative serves as a two electron and two proton donor. On the basis of the results of absorption
spectroscopy studies, cyclic voltammetry and DFT calculation, a mechanistic sequence for these reduction
reactions is proposed that involves initial photoexcitation of the aryl chromophore of the Ar-DMBIH
followed by single electron transfer (SET) to the organic substrate to generate the radical cation of
benzimidazoline and the radical anion of the substrate.
Keywords:
Photoinduced electron transfer
2-Aryl-1,3-dimethylbenzimidazoline
Epoxy ketone ring-opening
Radical rearrangement
Desulfonylation
Allylation
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
received great attention in the area of synthetic organic chemistry.2
More recently, metal-free protocols using organo-photocatalysts
The single electron transfer (SET) reduction and oxidation (re-
dox) abilities of electronic excited states of organic substances ex-
ceed those of their ground states. As a result, photoinduced electron
transfer (PET) is a useful method to generate radical ions of organic
molecules.1 A simple way to initiate these processes is by direct
photoexcitation of the organic substrate in the presence of an
electron donor or acceptor co-reactant. Another approach involves
the use of photosensitizers (photocatalysts), which can be designed
to absorb longer wavelengths of light and whose electronic excited
states can serve as electron donors or acceptors. The latter protocol
has advantageous features, which include the avoidance of un-
desired reactions of the excited state of the substrate and secondary
photoreactions of the initially formed product. In the past several
years, PET reactions catalyzed by visible light absorbing transition
metal complexes, such as those of ruthenium and iridium, have
have gained interest in light of their potential application to
green and sustainable synthetic transformations.3
Because amines possess relatively strong electron donating
abilities compared to those of other organic substances, they have
been employed often in the SET promoted reactions,4 and espe-
cially those initiated by PET.5 2-Aryl-1,3-dimethylbenzimidazolines
(Ar-DMBIH) are a family of N-heterocyclic aromatic amines that
promote various reduction reactions by serving as hydride, hy-
drogen atom or electron donors (Scheme 1).6 Owing to their multi-
donor character, Ar-DMBIH have been employed for reduction re-
actions of various organic substances, exemplified by their partic-
ipation as hydride donors to carbocations as well as cationic salts
(Scheme 1, path 1).7 Moreover, certain Ar-DMBIH derivatives react
to produce hydrogen gas upon treatment with BrønstedeLowry
acids (path 1).8 Ar-DMBIH also behave as radical chain promoters
through a pathway involving hydrogen atom donation followed by
ET reactions (path 2 and path 4).9 In addition, PET reactions of
various organic substances using Ar-DMBIH have been described by
us and others (path 3epath 5epath 4, or path 3epath 6).10,11 Finally,
* Corresponding author. E-mail address: ehase@chem.sc.niigata-u.ac.jp
(E. Hasegawa).
http://dx.doi.org/10.1016/j.tet.2015.06.071
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.

E. Hasegawa et al. / Tetrahedron 71 (2015) 5494e5505
5495
the utility of Ar-DMBIH as electron donors has been expanded by
their application in artificial photosynthesis systems12 as well as
organic semiconductor devices13 (path 3).
has an absorption maximum around 300 nm,16 the longer wave-
length absorption bands at 313 nm for 1a and 305 nm for 1g are
a consequence of the presence of the benzimidazoline chromo-
phore. In the absorption spectra of 1b, 1c and 1f, the maxima
present in the 307e314 nm range are attributed to both the
naphthalene and benzimidazoline chromophores.16 On the other
hand, characteristic absorption bands associated with the pyrene17
and anthracene rings are present in the spectra of 1d and 1e, re-
spectively.16 Under the >360 nm irradiation conditions used in the
PET reactions explored (see below), the pyrene moiety of 1d, an-
thracene moiety of 1e, and benzimidazoline moieties of 1a and 1g
absorb the incident light. On the other hand, competitive absorp-
tion by naphthalene and benzimidazoline chromophores is possi-
ble for 1b, 1c and 1f.
Based on the results of previous studies, we concluded that re-
ductive ring opening reactions of epoxy ketones 2 (Fig. 2) and
radical rearrangement reactions of halo-alkyl substituted cyclic
ketones 4 and 8 would serve as model processes to evaluate the Ar-
DMBIH based PET protocol described above.10aef,hej,l In PET pro-
moted reductive ring opening reactions of epoxy ketones 2 that
form the corresponding hydroxyl ketones 3, two hydrogen atoms
are formally transferred to the substrates (Scheme 2).10a,c,d,f,h,i Be-
cause 1ae1e are expected to serve as a two electron and one proton
donors, PET reactions of epoxy ketones promoted by these sub-
stances require the participation of another proton donor.
To test these proposals, photoreactions of chalcone epoxide 2a
with 1 under the various conditions were explored (Table 2).
AreDMBIH 1be1e were found to serve as photoreductants for re-
actions of epoxy ketone 2a in THF containing AcOH that are induced
by irradiation with light of wavelengths >360 nm (Xe lamp). The
yields of these reactions, which are ca. 90% based on the conversion
of 2a, are much higher than those in which the phenyl-substituted
benzimidazoline 1a was employed as the promoter (compare en-
tries 2, 11, 12 and 14 to entry 1). Detailed studies using 1b as the
photoreductant unveiled several characteristic features of the ep-
oxide ring opening reaction. For example, PhCO2H as well as PhOH
serves as proton donors for the reductive ring opening process
(entries 3 and 4, also see entries 13 and 15). Moreover, reactions of
2a in CH2Cl2 and DMF proceeded smoothly to generate 3a in good
to excellent yields (entries 5 and 8). Extending the irradiation time
does not cause decomposition of 3a (compare entry 6 to entry 5).
Furthermore, decomposition products such as chalcone, aceto-
phenone and benzaldehyde and not 3a are produced when the
photoreactions are conducted in solvents that do not contain pro-
ton donors (entry 7). In addition, reactions of 2a promoted by 1b
using PhCO2H (entry 9) or PhOH (entry 10) under solvent free
Scheme 1. Oxidation pathways of 2-aryl-1,3-dimethylbenzimidazoline (Ar-DMBIH).
In investigations of PET promoted reduction reactions using Ar-
DMBIH,10 we have employed approaches that involve both direct
excitation of substrates10aee,g,j,k as well as photosensitization using
electron donating pyrenes10d,f,h,i or transition metal complexes.10h,l
As a consequence of the fact that visible light promoted organic
transformations are highly attractive,2,3 we focused our attention
on the development of Ar-DMBIH, which possess chromophores
that absorb at long wavelengths so that desired PET reactions could
be promoted without using expensive photocatalysts such transi-
tion metal complexes. The concept utilized to design Ar-DMBIH of
this type is outlined in Fig. 1. In previous investigations, we dem-
onstrated that phenyl-substituted DMBIH, 1a, behaves as a two
electron and one proton donor in reduction reactions of organic
substrates while its phenol counterpart, 1g, serves as a two electron
and two proton donor. Based on this observation and the well-
known effects of extended conjugation on the wavelength maxi-
mum for light absorption, we anticipated that Ar-DMBIH 1be1f
(Fig. 1), in which various polycyclic aryl moieties are connected to
the methylene carbon of the benzimidazoline skeleton, would be
suitable long wavelength absorbing PET promoters. In related
studies, it has been shown that Hantzsch esters possessing aryl-
substiuents also are potential SET photoreductants14 although the
use of these substances to promote reductive transformations has
not been explored. In the study described below, we prepared the
Ar-DMBIH 1be1f and demonstrated their utility in promoting PET
transformations of the wide variety of organic substrates to form
products listed in Fig. 2.
Fig. 1. 2-Aryl substituted 1,3-dimethylbenzimidazolines (Ar-DMBIH) (1).
2. Results and discussion
conditions formed 3a with low to moderate efficiencies. Finally,
good yields of 3a are associated with reductive ring opening re-
actions of 2a in THF, CH2Cl2 or PhCH3 solutions not containing
proton donating additives, which are promoted by 1f and 1g that
contain respective naphthol and phenol proton donor moieties
(entries 16e22). It should be noted that these two electron and two
proton-donors 1f and 1g have some advantage compared to the
Analysis of the absorption spectra of Ar-DMBIH 1ae1g displayed
in Fig. 3 and Fig. 4, shows that these substances at 0.1 M concen-
trations absorb light at wavelengths >360 nm, which are attained
by irradiation using Glass Filter L-39.15 Absorption spectroscopic
data for 1 are accumulated in Table 1. Because 1,2-diaminobenzene

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E. Hasegawa et al. / Tetrahedron 71 (2015) 5494e5505
Fig. 2. Substrates 2, 4, 8, 10, 12, 14 and products 3, 5, 6, 7, 9, 11, 13, 15 of photoreduction reactions promoted by Ar-DMBIH (1).
Fig. 3. Absorption spectra of 1 in MeCN at 4.0ꢁ10ꢀ5 M (left for 1ae1e, right for 1f and 1g).
two electron and one proton donors such as 1ae1e since it is
necessary for latter donors to find a suitable proton donor and to
optimize their quantities to improve the yields of 3.10a,d,f,18
as photoreductants although consumption of the starting epoxide
was observed to occur (entries 1 and 12). Furthermore, both 1b and
1c are equally effective in promoting reactions of 2b in CH2Cl2 (4 h
irradiation) (entries 3 and 10) and prolonged irradiation (12 h) of
a mixture of 1b and 2b did not lead to higher conversions (compare
entry 3 to entry 4). Reactions in highly polar solvents such as DMF
and MeCN were found to be less efficient than those in solvents
with medium polarity such as THF, CH2Cl2, PhCH3 and PhCF3 (en-
tries 2, 4e8).18 On the contrary, the hydroxy-aryl substituted pho-
toreductants 1f and 1g promote reaction to give 3b in excellent
yields upon irradiation for only 1 h and with 1f being more effective
Photoreactions promoted by Ar-DMBIH were also explored us-
ing benzalacetone epoxide 2b, which is a weaker electron acceptor
(Ered1/2, no peak less negative than ꢀ2.0 V vs SCE) than its phenyl
ketone counterpart 2a (Ered ¼ꢀ1.68 V). As can be seen by viewing
1/2
the results given in Table 3, reactions of 2b promoted by 1b and 1d
(AcOH in THF, 12 h irradiation) produced hydroxyl ketone 3b in
modestly good yields (entries 2 and 11). On the other hand, only
trace quantities of 3b were generated when 1a and 1e are employed

E. Hasegawa et al. / Tetrahedron 71 (2015) 5494e5505
5497
Fig. 4. Absorption spectra of 1 in THF at 0.1 M (left for 1ae1e, right for 1f and 1g) and transmittance spectrum of the glass filter L-39 (dotted line).
Table 2
Table 1
Absorption spectroscopic data of 1ae1g
Photoreactions of chalcone epoxide 2a promoted by 1ae1ga
1
lmax (log ε)a (nm)
lendb (nm)
1a
1b
1c
1d
1e
1f
263 (3.74), 313 (3.88)
271 (4.03), 281 (4.04), 291 (3.92), 314 (3.87)
264 (4.01), 273 (3.99), 312 (3.87)
267 (4.44), 276 (4.63), 313 (4.26), 326 (4.55), 342 (4.66)
318 (3.86), 348 (3.84), 366 (3.99), 386 (3.94)
268 (3.90), 278 (3.91), 290 (3.88), 307 (3.94)
273 (3.83), 305 (3.83)
368
427
420
487
544
447
399
Entry 1 Solvent ROH (equiv) Irrad time (h) Conv of 2a (%) Yield of 3a (%)b
1
2
3
4
5
6
7
8
1a THF
1b THF
1b THF
1b THF
1b CH2Cl2 AcOH (2.2)
1b CH2Cl2 AcOH (2.2)
AcOH (2.2)
AcOH (2.2)
PhCO2H (2.2) 1
1
1
86c
100
99c
100
100
100
100
100
90c
97c
93
55
88
96
93
95
92
0e
86
67
28
88
46f
40c
61
56f
54
75
67
82f
72
57
41
1g
a
Measured in MeCN (4.0ꢁ10ꢀ5 M, Fig. 3). lmax: absorption maximum (log ε: log of
PhOH (3.3)
1
1
4
4
1
molar attenuation coefficient).
b
Measured in THF (0.1 M, Fig. 4). lend: end-absorption (absorbance ca. 1.0).
1b CH2Cl2
1b DMF
1b dd
d
AcOH (2.2)
PhCO2H (1.2) 1
PhOH (3.3)
AcOH (2.2)
AcOH (2.2)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
1b dd
1
1
1
1c THF
1d THF
52f
51
1d THF
1e THF
1e THF
PhCO2H (2.2) 1
AcOH (2.2)
PhCO2H (2.2) 1
1
63
56f
78
1f THF
1f THF
d
d
d
d
d
d
d
1
4
1
4
4
4
4
100
90
1f CH2Cl2
1f CH2Cl2
1f PhCH3
1f DMF
1g CH2Cl2
100f
100
100
61
Scheme 2. Transformation of epoxy ketones 2 to hydroxyl ketones 3 promoted by
1ae1e.
a
2a (0.40 mmol, 0.20 mmol for entries 9 and 10), 1 (1.2 equiv vs 2a), solvent
>360 nm.
(4.0 mL),
l
b
Isolated yield.
c
d
e
f
Determined by 1H NMR.
than 1g (entries 13 and 16). Also, prolonged irradiation in these
cases did not cause decomposition of product 3b (entries 14 and
17). Finally, it should be noted that the reaction promoted by 1f to
afford 3b occurred in moderate yield under the solvent free con-
ditions (entry 15) while 1b with PhCO2H did not promote reaction
of 2b although 1b was completely oxidized (entry 9).
As described above, 1be1e, in solvent systems containing ap-
propriate proton donors, and 1f and 1g in the absence of proton
donating additives are effective promoters of the reductive epoxide
ring opening reaction of 2a. Moreover, reactions of 2b promoted by
1f and 1g take place in higher yields than those induced by 1be1e.
Based on these findings, 1f and 1g were used to promote reduction
reactions of other aliphatic epoxy ketones including 2ce2e
(Table 4). In each process, the corresponding hydroxyl ketone
3ce3e was generated in excellent yield (entries 1, 2, 3 and 6). In the
reaction of 2d, DMF was found to be less tolerable solvent than
CH2Cl2 (entries 4 and 5).18
For details see Experimental section.
Chalcone (35%), acetophenone (9%), and benzaldehyde (4%) were observed.
Average of two independent experiments.
undergo rearrangement reactions, such as Dowd-Beckwith (DB)
ring-expansion19,20 and 5-exo hexenyl cyclization,21 to form the
corresponding products.
The use of Ar-DMBIH to promote ring expansion reactions of a-
halomethyl benzocyclic 1-alkanones 4 was investigated (Table 5).
The results show that while 1ae1e all promote reaction of 4a in
CH2Cl2, the ratio of ring expansion (5a) to reduction (6a) products
differs depending on the photoreductant used (entries 1, 2, 6, 9 and
13). Moreover, irradiation of a mixture of 1b and 4a in different
solvents produced mixtures containing varying ratios of 5a and 6a
(entries 2e4). In the reaction of 4a promoted by 1c in CH2Cl2,
a white solid appeared within 30 min during irradiation (entry 6),
a phenomenon, which was not observed in the reaction promoted
by 1b (entry 2).22 The reaction promoted by 1d took place smoothly
(entry 9) and, in this case, irradiation with shorter wavelength light
was similarly effective (entry 11) although some solid precipitate
formed after 1 h irradiation. Unexpectedly, the progress of the
PET promoted reduction reactions of a-halomethyl benzocyclic
1-alkanones such as 4 and 8 are known to take place via pathways
involving the intermediacy of primary alkyl radicals formed by
carbon-halogen bond cleavage.10b,e,hej,l These radical intermediates

5498
E. Hasegawa et al. / Tetrahedron 71 (2015) 5494e5505
Table 3
Table 5
Photoreactions of benzalacetone epoxide 2b promoted by 1ae1ga
Photoreactions of haloalkyl substituted cyclic keto esters 4 promoted by 1ae1ea
Entry 1 Solvent ROH (equiv) Irrad time (h) Conv of 2b (%) Yield of 3b (%)b
Entry
4
1
Solvent Irrad time (h) Conv of 4 (%)b Yields (%)
Ratio
1
2
3
4
5
6
7
8
1a THF
1b THF
1b CH2Cl2 AcOH (6.6)
1b CH2Cl2 AcOH (6.6)
1b PhCH3 AcOH (6.6)
1b PhCF3 AcOH (6.6)
1b DMF
1b MeCN AcOH (6.6)
1b dd
PhCO2H (1.2)
1c CH2Cl2 AcOH (6.6)
AcOH (6.6)
AcOH (6.6)
12
12
4
12
12
12
12
12
6
40
79
68
0
5c
6b
5:6
55
48
52
60
30
13c
9c
1
2
3
4
5
6
7
8
4a 1a CH2Cl2
4a 1b CH2Cl2
4a 1b THF
2
2
2
2
2
2
33
44
41
54
80
50e
50
68
91
100
90
95
20
73
34
17
12
67
49
9
16
9
9
50:50
57:43
36:64
69:31
55:45
57:43e
60:40
73:27
79:21
76:24
81:19
53:47
68
12
17
12
27
91c
60c
59
4a 1b DMSO
27
33
AcOH (6.6)
4a 1b dd
32
4a 1c CH2Cl2
4a 1d CH2Cl2 0.5
21e 16e
9
de
75
0
27
42
67
65
61
42
13
18
16
18
20
14
38
10
11
12
13
14
15
16
17
4
52
50
Trace
94
90
34
80
88
4a 1d CH2Cl2
4a 1d CH2Cl2
4a 1d CH2Cl2
4a 1d CH2Cl2
4a 1d dd
4a 1e CH2Cl2
4b 1d CH2Cl2
4c 1d CH2Cl2
4d 1b CH2Cl2
4d 1d CH2Cl2
4e 1b CH2Cl2
4e 1d CH2Cl2
1
2
6
2
2
2
2
2
2
2
2
2
1d THF
1e THF
AcOH (6.6)
AcOH (6.6)
d
d
d
d
d
12
12
1
4
4
76c
6
9
10
11f
12
13
14
15
16
17
18
19
1f CH2Cl2
1f CH2Cl2
1f dd
1g CH2Cl2
1g CH2Cl2
100
100
66
91
100
Trace <100:>0
83:17
17b Trace <100:>0
1
4
49b,g 10
a
4
5
22
14
Trace <100:>0
4
18
11
2b (0.40 mmol, 0.05 mmol for entry 9 and 0.20 mmol for entry 15), 1 (1.2 equiv
56:44
55:45
56:44
vs 2b), solvent (4.0 mL),
l >360 nm.
b
Isolated yield.
c
Determined by using 1H NMR.
d
e
For details see Experimental section.
a
4 (0.40 mmol, 0.20 mmol for entries 5 and 12), 1 (1.2 equiv vs 4), solvent
Negligible 1H NMR peaks other than those for 1b, oxidized 1b and 2b.
(4.0 mL),
l >360 nm.
b
Determined by 1H NMR.
Isolated yield.
For details see Experimental section.
Average of two independent experiments.
Pyrex-filtered irradiation (
c
d
e
f
Table 4
Photoreactions of aliphatic epoxy ketones 2ce2e promoted by 1f and 1ga
l
>280 nm).
g
Naphthol 7 (3%) was included.
greater than that in the solution phase reaction (compare entry 12
to entry 9). Although the reaction mechanism in the solvent free
condition is unclear, it is possible that the larger amounts of 6a
produced in reactions taking place under solvent-free conditions
are a consequence of close contact between I and 1, which favors
hydrogen atom transfer.
Entry
2
1
Solvent
Irrad time (h)
Conv of 2 (%)
Yield of 3 (%)b
1
2
3
4
5
6
2c
2d
2d
2d
2d
2e
1f
1f
1g
1f
1g
1f
CH2Cl2
CH2Cl2
CH2Cl2
DMF
DMF
CH2Cl2
2
1
1
1
1
1
100
100
100
82c
92
94
95
39c
19c
94
52c
100
a
2 (0.40 mmol for CH2Cl2, 0.10 mmol for DMF), 1f (1.2 equiv vs 2), solvent (4.0 mL
for CH2Cl2, 1.0 mL for DMF),
l >360 nm.
b
Isolated yield.
c
Determined by 1H NMR.
reaction of 4a was significantly slowed when 1e is used as the
photoreductant although 1e can absorb the incident light (entry 13
and Fig. 4).
The mechanistic pathway for conversion of 4a to 5a and 6a in-
volves the intermediacy of radical I (Scheme 3), which is initially
formed via PET process with 1 and undergoes competitive hydro-
gen atom abstraction to produce 6a or a ring-expansion process to
form II, which serves as the precursor of 5a. Consequently, the use
of lower concentrations of the hydrogen atom donors such as 1
should enhance formation of 5a. As expected, the ratio of 5a to 6a
was observed to increase as the progress of photoreaction of 4a
promoted by 1d increases and 1d is consumed (see entries 7e10).
In addition, reaction of 4a in THF, which also is a hydrogen atom
donor, takes place to give increased amounts of 6a (compare entry
3 to entry 2). Solvent free reactions of 4a using 1b and 1d as pro-
moters were found to proceed (entries 5 and 12). Although the
conversion of 4a in the reaction with 1b is significantly higher than
that attending the solution phase reaction, the ratios of 6a are
equivalent each other (compare entry 5 to entry 2). On the other
hand, the ratio of 6a in the solvent free reaction of 4a with 1d is
Scheme 3. Mechanism involving competitive hydrogen atom abstraction and ring-
expansion in PET reactions of 4a promoted by 1.
Reactions of other
a-halomethyl benzocyclic 1-alkanones
4be4e promoted by 1d were also examined. While reaction of
the indanone derivative 4b occurred smoothly to give 5b and 6b
along with a small amount of naphthol 7 (entry 14), its larger ring
counterpart 4c was less reactive and the yields of expected prod-
ucts were low (entry 15). Notably, the carbonechlorine bond of 4d
is cleaved in photoreduction reactions promoted by 1b and 1d al-
though the progress of these processes was slow (entries 16 and
17). On the other hand, the corresponding iodide 4e reacts rapidly
to produce the expected products 5e and 6e (entries 18 and 19).

E. Hasegawa et al. / Tetrahedron 71 (2015) 5494e5505
5499
Interestingly, over equal irradiation time periods the conversion of
4a is lower than that of 4e using 1b as the promoter (compare entry
2 to 18). In contrast the conversion of bromide 4a is significantly
higher than that of 4e when 1d was employed as the photo-
reductant (compare entry 9 to 19). At this moment, it seems diffi-
cult to rationalize these characteristic effects of 1b and 1d on the
reaction progress of 4a and 4e.
In these photoreduction reactions, Ar-DMBIH should be oxi-
dized to form the corresponding imidazolium ion. The fact that this
oxidation process takes place is confirmed by the observation that
imidazolium 1dþ formed in the reaction of 4a promoted by 1d
(Scheme 4). Accordingly, following 6 h irradiation of a mixture of 1d
and 4a (entry 10 in Table 5), the pale yellow solid formed was
isolated and identified to be the imidazolium salt 1dDBrL by using
1H NMR spectroscopy. Treatment of 1dDBrL with NaBH4 in MeOH
afforded 1d (94%).
(>90% based on the conversion of 8) in the above reactions are
greater than those (40e80%) associated with previously reported
PET reactions with Ar-DMBIH.10e,h,i,l
AreDMBIH were applied to promote the 5-exo radical cycliza-
tion reaction of 2-allyloxy iodobenzene 10 (Table 7). The results
show that irradiation of solutions containing 1b or 1c and 10 led to
formation of the expected cyclization product 11 with yields that
are dependent on the solvent. The reactions in DMSO proceeded in
only modest yield (entries 2 and 4) and at a significantly higher rate
than that in CH2Cl2 (entries 1 and 3), which is the reverse of the
observation made for the reaction of 8 (see Table 6).
Table 7
Photoreaction of 2-allyloxy iodobenzene 10 promoted by 1b and 1ca
Entry
1
Solvent
Conv of 10 (%)b
Yield of 11 (%)b
1
2
3
4
1b
1b
1c
1c
CH2Cl2
DMSO
CH2Cl2
DMSO
4
52
14
63
Trace
30
9
43
a
10 (0.20 mmol), 1 (1.2 equiv vs 10), solvent (2.0 mL), l>360 nm, 5 h.
b
Determined by 1H NMR.
Interesting observations made in the study described above
include (1) reaction of 8 with 1b or 1c proceeds more smoothly
than that using 1d while 1d is the most effective photoreductant for
DB ring-expansion of 4a and (2) DMSO is more suitable than CH2Cl2
as a solvent for reaction of 10 but not for that of 8 while such
a significant difference in the conversion of 4a between CH2Cl2 and
DMSO was not observed.
Scheme 4. Isolation and hydride reduction of 1dþBrꢀ.
We next determined the feasibility of PET reactions of the
butenyl side chain containing
a-bromomethyl tetralone 8 pro-
moted by 1b, 1c and 1d (Table 6). These processes are expected to
generate radical intermediate that undergoes 5-exo cyclization.
Reactions of 8 in CH2Cl2 and DMSO promoted by using 1b produced
the expected spirocyclic product 9 (>90%, based on the conversion
of 8) although the reaction in DMSO is relatively slow (entries 1 and
2). When 1c was used for reaction of 8 in CH2Cl2, the conversion
was slightly lower than that using 1b although the process took
place in a similarly high yield (compare entry 3 to entry 1). In
a manner similar to that of 4a (see entry 6 of Table 5), a white solid
appeared during irradiation of 1c with 8. However, this occurrence
did not significantly interfere with the reaction. The conversion of 8
in the reaction promoted by 1c in DMSO is also lower than that
using 1b (compare entry 4 to entry 2), in which no solid pre-
cipitation was observed. In the reaction promoted by 1d, the con-
version of 8 was significantly low although the mass balance
remains high (entry 5). It should be mentioned that the yields of 9
The applicability of this PET protocol using Ar-DMBIH to NeS
bond cleavage reactions was determined by investigating desulfo-
nylation reactions of N-sulfonylated indoles 12 (Table 8).10i,23
Photoreaction of 12a promoted by 1b in the presence of AcOH
took place smoothly to form the deprotected indole 13 (entry 1). On
the other hand, by the use of lower quantities of 1f and without
addition of AcOH, the process was highly efficient (entry 2). This
protocol was applied to photo-detosylation of 12b as well as
demesylation of 12c, both of which took place to give 13 in excel-
lent yields (entries 3 and 4). The results of studies with 12d show
that an electron withdrawing substituent such as CO2Me at the C2
position of the indole ring is not necessary for the occurrence of the
photo-desulfonylation process. However, a greater quantity of 1f
and longer irradiation times are required when this C2-substituent
is absent (entry 5). In these reactions, naphthol moiety of 1f
Table 8
Table 6
Photo-desulfonylation reactions of N-sulfonylated indoles 12 promoted by 1b and
1fa
Photoreaction of bromoalkyl and alkene substituted cyclic ketone 8 promoted by
1be1da
Entry 12
1
AcOH (equiv) Irrad time (h) Conv of 12 (%) Yield of 13 (%)b
Entry
1
Solvent
Conv of 8 (%)b
Yield of 9 (%)b
1
2
3
4
5
12a 1b 2.2
2
0.5
1
0.5
2
100
100
100
100
100
91
95
98
1
2
3
4
5
1b
1b
1c
1c
1d
CH2Cl2
DMSO
CH2Cl2
DMSO
CH2Cl2
97c
35
85
29
25
91c
33
80
26
22
12a 1f
12b 1f
12c 1f
12d 1f
d
d
d
d
100
99c
a
12 (0.40 mmol), 1 (1.8 equiv of 1b and 1.2 equiv of 1f vs 12ae12c, 1.5 equiv of 1f
a
8 (0.20 mmol), 1 (1.2 equiv vs 8), solvent (2.0 mL),
Determined by 1H NMR.
Average of two independent experiments.
l>360 nm, 2 h.
vs 12d), CH2Cl2 (4.0 mL),
l
>360 nm.
b
c
b
Isolated yield.
c
Determined by 1H NMR.

5500
E. Hasegawa et al. / Tetrahedron 71 (2015) 5494e5505
presumably acts as an internal proton donor, similar to the role it
plays in the reactions of epoxy ketones 2.
In the final chemical phase of this effort, Ar-DMBIH were applied
to promote carbonecarbon bond forming reactions between a-keto
esters 14 and allyl bromide. PET processes of this type were un-
covered earlier in a study using other DMBIH.10k The results show
that irradiation of 1a in the presence of 14a and allyl bromide
(Table 9) led to formation of the expected allylation product 15a in
modest yield (entry 1). Visible light irradiation of 1d with 14a also
promotes the reaction although a longer reaction time is required
(entry 2). Non-conjugated ester substrates such as 14b also un-
derwent the allylation reaction to give 15b although in rather low
yield (entry 3).
Table 9
Photo-allylation reactions of a
-keto esters 14 promoted by 1b and 1da
Entry 14
1
l
(nm) Irrad time (h) Conv of 14 (%) Yield of 15 (%)b
1
2
3
14a 1b >360
14a 1d >390
14b 1b >360
4
8
12
92
80
88
63
38
36
a
14 (0.40 mmol), 1 (1.2 equiv vs 14), allyl bromide (3.0 equiv), CH2Cl2 (4.0 mL).
Isolated yield.
b
Discussion on the mechanistic pathways followed in the pho-
toinduced reduction reactions described above is described below.
DFT calculations were performed in order to gain insight into the
structural and electronic properties of the Ar-DMBIH. As described
above, photoexcited Ar-DMBIH, and in particular 1b, 1d, and 1f,
effectively promote reduction reactions of various substrates.
Therefore, calculations were first performed on these Ar-DMBIH
(Figs. 5 and 6, data for other 1 are reported in Supplementary
Fig. 6. Molecular orbital coefficients of 1b, 1d, and 1f obtained by using DFT calcula-
tions (B3LYP/6-31G(d)).
data). As seen by viewing the results displayed in Fig. 5, the ben-
zimidazoline and aryl rings are nearly perpendicular to each other
in energy minimized structures of 1b, 1d, and 1f and, in the case of
1f, hydrogen bonding exists between naphthol hydroxyl group and
ring nitrogen.
Inspection of the calculated frontier orbital coefficients of 1b,1d,
and 1f, depicted in Fig. 6, suggests that the highest energy occupied
molecular orbitals (HOMO) of 1 are located on the benzimidazoline
moiety, an observation that is fully consistent with the fact that
these substances have nearly the same oxidation potentials re-
gardless of the aryl substituent at C2 position of the benzimidazo-
line (Table 10). On the other hand, the coefficients of the lowest
energy unoccupied molecular orbitals (LUMO) are more highly
populated on the aryl rings. Therefore, electronic interactions be-
tween the frontier orbitals could be less efficient because of the
minimal overlap that exists between orbitals on the
Fig. 5. Energy minimized structures of 1b, 1d, and 1f obtained from DFT calculation
(B3LYP/6-31G(d)).

E. Hasegawa et al. / Tetrahedron 71 (2015) 5494e5505
5501
Table 10
displayed in Fig. 7 show that a broad band with a maximum at
430 nm grows until 500 ns, after, which it decays exponentially
with no change of spectral shape. The 430 nm band was assigned to
the triplet state of pyrene rather than the pyrene radical anion.26,27
Generation of the triplet pyrene from 1d-Meꢂþ/ꢂꢀ is energetically
possible because the triplet energy of pyrene (2.1 eV)28 is lower
than that of 1d-Meꢂþ/ꢂꢀ (2.41 eV). The lifetime of triplet pyrene was
Oxidation potential data of 1ae1ga
1
1a
1b
1c
1d
1e
1f
1g
Eox (V vs SCE) þ0.34 þ0.36 þ0.35 þ0.35 þ0.35 þ0.34 þ0.33
1/2
a
Measured in MeCN.
benzimidazoline and aryl moieties (Fig. 5). On the other hand, the
HOMO-1 of these compounds have some electronic distributions in
the aryl rings. If aryl moieties are selectively irradiated, electronic
excitation from the HOMO-1 to the LUMO could occur.
On the basis of the results described above, a plausible sequence
for electronic transitions in the Ar-DMBIH, exemplified by pyrene
substituted DMBIH 1d, can be proposed (Scheme 5). Because the
absorption band of 1d in the long wavelength region (Fig. 3) is the
same as that for pyrene, a light irradiation of 1d initially produces
estimated to be 12 ms from fitting the decay profile at 430 nm (see
the Supplementary data). A plausible route to produce triplet pyr-
ene involves charge recombination of the initially formed, zwit-
terionic biradical 1dꢂþ/ꢂꢀ like, charge-separated state. Therefore,
the proposed mechanism involving the formation of 1dꢂþ/ꢂꢀ shown
in Scheme 5 is plausible for electronic processes that take place
earlier than 100 ns after excitation.29
a pyrene localized excited state 1d , corresponding to an electronic
*
transition from HOMO-1 to the LUMO of 1d. Intramolecular SET
from ground state benzimidazoline moiety to the excited pyrene
gives the charge separated, zwitterionic biradical 1dꢂþ/ꢂꢀ, corre-
sponding to relaxation of an electron in the doubly occupied HOMO
into the singly occupied HOMO-1. The pyrene radical anion then
donates an electron to the substrate producing its radical anions
along with benzimidazoline radical cation (1dꢂþ). Alternatively, 1d
*
directly donates an electron to the substrate to give the pyrene
radical cation of 1d and the radical anion of the substrate. Then,
intramolecular SET from benzimidazoline moiety to pyrene radical
cation moiety produces the same 1dꢂþ formed via the former route.
Intermolecular versions of this SET sequence have been proposed
previously to describe reactions in which certain electron-donating
substituent possessing pyrenes are used as photosensitizers along
Fig. 7. Transient absorption spectra for 1d-Me in CH2Cl2 (4ꢁ10ꢀ5 M) detected after
nanosecond laser flash at 355 nm. Numbers in the figures indicates delay time after
flash.
with DMBIH.10h,i On the basis of the redox potentials of 1d (Eox
3. Conclusion
1/
¼þ0.35 V), pyrene (Eox ¼þ1.16 V and Ered ¼ꢀ2.09 V for pyrene;
2
1/2
1/2
Eex¼þ3.34 eV for singlet excitation energy, Eox
¼ꢀ2.18 V,
*
In the study described above, we demonstrated that 2-aryl-1,3-
dimethylbenzimidazolines (Ar-DMBIH), in which aryl chromo-
phores and benzimidazoline moiety are connected by a methylene
carbon, act as effective reductants under the photoirradiation
Ered
*
¼þ1.25 V for the redox potentials of the excited singlet),24 and
the substrates, (e.g., Ered ¼ꢀ1.68 V for 2a, e1.64 for 4a,10lꢀ1.82 V
1/2
for 8,10lꢀ1.78 V for 12a), both of the above pathways should be
conditions (l>360 nm). Although the electron donating properties
energetically feasible.
Scheme 5. Plausible pathways of 1d promoting the reduction reactions.
In an attempt to obtain the information about the nature of
of all Ar-DMBIH, as estimated by their oxidation potentials, are
essentially same, the electron donating activities of their photo-
excited states are different and vary according to the nature of the
aryl substituents at C2 of the DMBIH moiety. In addition, Ar-DMBIH
behave differently depending on the substrates and reactions that
they promote, and in some cases solvents have a profound effect on
the efficiencies of the photoreactions. Both the naphthyl or pyrenyl
substituted DMBIH are effective in promoting carbon-halogen bond
species formed by irradiation of 1d, a laser-flash photolysis study
was performed. However, 1d was found to be labile under the ex-
perimental conditions.25 Thus, 1,2,3-trimethyl-2-pyrenyl benzimi-
dazoline (1d-Me, Eox ¼þ0.32 V) was employed in these
1/2
experiments, in which transient absorption measurements were
made in the10L7e10ꢀ4 s time regime following pulsing at 355 nm.
Inspection of the time dependent transient absorption spectra

5502
E. Hasegawa et al. / Tetrahedron 71 (2015) 5494e5505
cleavage, which initiates Dowd-Beckwith ring-expansion and 5-exo
hexenyl cyclization processes and allylation reactions of acyl for-
mates. On the other hand, hydroxynaphthyl substituted DMBIH,
which acts as a formal two-electron and two-proton-donor, is ef-
fective in promoting transformations of epoxy ketones to aldols and
deproptection reactions of N-sulphonyl indoles. Notably, we dem-
onstrated that the imidazolium salt product can be recovered after
the reaction and transformed to the starting Ar-DMBIH by hydride
reduction. Unfortunately, the species generated by the photoexci-
tation of Ar-DMBIH and that responsible for the initial single
electron reduction of the substrates could not be identified by using
time resolved laser spectroscopy. However, it is believed that in the
case of the pyene substituted DMBIH, photoexcitation of aryl
chromophore followed by intramolecular SET generates a zwitter-
ionic biradical that could serve as the electron donor in the PET
process. Since some of Ar-DMBIH designed and prepared in this
study have absorption spectra that extend into the visible region,
their application in visible light promoted transformations are
possible. We are pursuing this proposal in our current
investigations.
was stirred for 6 h in an ice-water bath and then molecular sieves
were removed by filtration. The residue obtained after concentra-
tion of the filtrate in vacuo was subjected to column chromatog-
raphy (benzene with 1% triethylamine) to give 1b (1.85 g, 6.8 mmol,
67%). In a similar fashion, 1c (1.61 g, 5.9 mmol, 71%) from DMPDA
(1.12 g, 8.2 mmol) and 1e (1.30 g, 4.0 mmol, 50%) from DMPDA
(1.09 g, 8.0 mmol) were prepared. 1d (1.74 g, 5.0 mmol, 61%) from
DMPDA (1.12 g, 8.2 mmol) was rinsed with MeOH after column
separation. While 1b, 1c, and 1e were crystallized from dime-
thoxyethane and EtOH, 1d was crystallized from CH2Cl2 and MeOH
before using for the photoreactions. 1f (595 mg, 2.1 mmol, 49%)
from DMPDA (584 mg, 4.3 mmol) was obtained by rinsing the solid
with EtOH after column chromatography (EtOAc/benzene¼1/6
with 1% triethylamine), and then used for the photoreaction.
4.2.1. 1,3-Dimethyl-2-(1-naphthyl)benzimidazoline (1b). Pale yel-
low solid; mp 123.5e124.0 ꢃC; 1H NMR (400 MHz, CDCl3)
d 8.67 (br
s, 1H), 7.89 (t, J¼8.6 Hz, 2H), 7.59 (br s, 1H), 7.49e7.39 (m, 3H),
6.79e6.75 (m, 2H), 6.51e6.47 (m, 2H), 5.41 (br s, 1H), 2.55 (s, 6H);
13C NMR (100 MHz, CDCl3)
d 142.1, 134.4, 133.3, 132.2, 130.3, 129.0,
128.5, 125.7, 124.7, 119.2, 105.8, 33.3; HRMS (ESI) m/z calcd for
4. Experimental section
4.1. General
C
19H17N2 [MꢀH]þ 273.1386, found 273.1378.
4.2.2. 1,3-Dimethyl-2-(2-naphthyl)benzimidazoline (1c). Pale yel-
low solid; mp 124.5e126.0 ꢃC; 1H NMR (400 MHz, CDCl3)
NMR spectra were recorded using CDCl3 or DMSO-d6 solutions
with tetramethylsilane (Me4Si) as an internal standard at 400 MHz
for 1H NMR and 100 MHz for 13C NMR. High-resolution mass
spectra (HRMS) were recorded on a double focusing mass spec-
trometer by using electrospray ionization (ESI). Uncorrected melt-
ing points are reported. Oxidation and reduction potentials in
MeCN were measured using cyclic voltammetry and the previously
described procedure.10h Calibration of the potential values were
performed using the formal potentials of ferrocene/ferrocenium
couple, Eꢃ¼0.069e0.072 V and 0.434e0.446 V versus Ag/AgNO3
and SCE, respectively. Photoreactions were conducted using solu-
tions in Pyrex test tubes (1.4 cm diameter) immersed in a water
bath at room temperature and irradiated using a 500 W Xe lamp.
Column chromatography was performed using silica gel. Pre-
parative thin-layer chromatography (TLC) was performed on
20ꢁ20 cm plates coated with silica gel. Anhydrous solvents for
photoreactions and preparations of some substrates were obtained
as follows. Tetrahydrofuran (THF) was distilled over sodium-
benzophenone under N2. CH2Cl2 and PhCH3 were purified in
a same manner by the treatment with H2SO4, water, 5% NaOH,
water, and CaCl2 and then distilled over CaH2. MeCN was distilled
over P2O5 and subsequently distilled with K2CO3. Anhydrous N,N-
dimethylformamide (DMF), dimethyl sulfoxide (DMSO), PhCF3, and
MeOH were purchased and used without distillation. Other re-
agents and solvents were purchased and used without purification.
Substrates 2ae2e,10c 4a,19f 4b,10j 4c,19f 4d,10l 4e,10l 8,10h 10,10c and
12a,23 12b,30 12c,31 12d,32 which are known compounds, were
prepared by using reported procedures. Compounds 14a and 14b
are commercially available.
d
7.92e7.82 (m, 5H), 7.55e7.51 (m, 2H), 6.77e6.72 (m, 2H),
6.49e6.45 (m, 2H), 5.06 (s, 1H), 2.59 (s, 6H); 13C NMR (100 MHz,
CDCl3) 142.1, 136.4, 134.2, 128.9, 128.6, 128.1, 127.9, 126.5, 126.3,
d
125.6, 119.4, 105.8, 94.3, 33.2; HRMS (ESI) m/z calcd for C19H17N2
[MꢀH]þ 273.1386, found 273.1383.
4.2.3. 1,3-Dimethyl-2-(1-pyrenyl)benzimidazoline
(1d). Yellow
8.86 (br s,
solid; mp 161.0e161.5 ꢃC; 1H NMR (400 MHz, CDCl3)
d
1H), 8.22e8.20 (m, 3H), 8.14e8.00 (m, 4H), 6.84e6.79 (m, 2H),
6.56e6.52 (m, 2H), 5.84 (br s, 1H), 2.60 (s, 6H); 13C NMR (100 MHz,
CDCl3)
d 142.1, 132.1, 130.8, 130.6, 128.3, 128.0, 127.3, 126.0, 125.4,
125.3, 124.7, 124.5, 119.4, 105.9, 33.4; HRMS (ESI) m/z calcd for
C
25H19N2 [MꢀH]þ 347.1543, found 347.1543.
4.2.4. 2-(9-Anthryl)-1,3-dimethylbenzimidazoline
(1e). Orange
9.13 (d,
solid; mp 177.0e178.0 ꢃC; 1H NMR (400 MHz, CDCl3)
d
J¼9.2 Hz, 1H), 8.56e8.54 (m, 2H), 8.07e8.00 (m, 2H), 7.55e7.45 (m,
3H), 7.39e7.35 (m, 1H), 6.85e6.81 (m, 2H), 6.65 (s, 1H), 6.57e6.53
(m, 2H), 2.53 (s, 6H); 13C NMR (100 MHz, CDCl3)
d 142.4, 132.1,
130.8, 130.0, 129.7, 128.7, 127.2, 126.7, 126.6, 125.2, 125.0, 124.6,
121.5, 119.6, 106.3, 87.8, 33.5; HRMS (ESI) m/z calcd for C23H19N2
[MꢀH]þ 323.1543, found 323.1533.
4.2.5. 1,3-Dimethyl-2-(2-hydroxy-1-naphthyl)benzimidazoline
(1f). Pale yellow solid; mp 123.5e124.0 ꢃC; 1H NMR (400 MHz,
CDCl3)
7.51e7.45 (m, 1H), 7.35 (t, J¼7.4 Hz, 1H), 7.20 (d, J¼9.2 Hz, 1H),
6.90e6.84 (m, 2H), 6.68e6.63 (m, 2H), 5.84 (s, 1H), 2.68 (s, 6H); 13
NMR (100 MHz, CDCl3) 157.6, 142.2, 134.5, 131.6, 129.2, 128.5,
d
10.43 (br s, 1H), 8.02 (d, J¼8.8 Hz, 1H), 7.85e7.80 (m, 2H),
C
d
127.0, 122.8, 121.0, 120.1, 109.0, 108.6, 87.8, 33.9; HRMS (ESI) m/z
4.2. Preparations of 2-aryl-1,3-dimethylbenziimidazoline
(DMBIH) (1)
calcd for C19H17N2O [MꢀH]þ 289.1335, found 289.1325.
4.3. Preparation of 1,2,3-trimethyl-2-
pyrenylbenziimidazoline (1d-Me)
DMBIH 1a,7a 1b,8b, and 1g,8c are known compounds. 1c, 1d, 1e,
and 1f were prepared by using modified literature procedur-
es.6a,8b,c,10g A typical procedure for preparation of 1b is described
below. To a CH2Cl2 (10 mL) containing N,N0-dimethyl-o-phenyl-
enediamine (DMPDA) (1.38 g, 10.1 mmol) with molecular sieves 4A
(ca. 10g) under N2 seated in ice-water bath was slowly added 1-
naphthoaldehyde (1.42 mL, 10.5 mmol) in CH2Cl2 (20 mL). After
addition of acetic acid (0.23 mL, 4.0 mmol), the resulting mixture
To a benzene solution (8 mL) containing N,N0-dimethyl-o-phe-
nylenediamine (400 mg, 2.9 mmol) with molecular sieves 4A (ca.
6.5g) was slowly added 1-acetylpyrene33 (2.13 g, 8.7 mmol) in
benzene (13 mL) under N2. After addition of trifluoroacetic acid
(0.23 mL, 3.0 mmol), the resulting mixture was stirred at reflux for
24 h and filtered. To the residue obtained after concentration of the

E. Hasegawa et al. / Tetrahedron 71 (2015) 5494e5505
5503
filtrate in vacuo was added water. Extraction with CH2Cl2 gave an
extract that was washed with water, sat NaHCO3, brine, and dried
over anhydrous MgSO4. Solid obtained by concentration in vacuo
was rinsed with Et2O to give 1d-Me (633 mg, 1.8 mmol, 60%), which
was then crystallized from CH2Cl2 and EtOH.
were determined by using 1H NMR and appropriate internal ref-
erences such as triphenylmethane and 1,3,5-trimethoxybenzene.
Known products 5ae5c,19f 6ae6c,19f 7,10j 9,10h and 11,10c were
characterized by comparing their 1H NMR data with those reported
earlier.
4.3.1. 1,2,3-Trimethyl-2-(1-pyrenyl)benzimidazoline
(1d-Me).-
8.95
4.4.4. Solvent free photoreaction of 1 with 2 or 4. A CH2Cl2 solution
of 2 or 4 (0.05e0.20 mmol) and 1 (0.06e0.24 mmol) with an ap-
propriate quantity of PhCO2H or PhOH for 2 was prepared in
a round-bottomed flask and then concentrated in vacuo. The thin-
layered residue was flashed with N2 and irradiated (see
Supplementary data). The same work-up, separation and product
identification procedures as those used for the photoreactions in
solutions were performed.
Yellow solid; mp 249.5e250.0 ꢃC; 1H NMR (400 MHz, CDCl3)
d
(d, J¼9.6 Hz, 1H), 8.15e8.19 (m, 4H), 8.06e8.11 (m, 2H), 7.99 (t,
J¼7.6 Hz, 1H), 7.73 (d, J¼9.6 Hz, 1H), 6.72e6.76 (m, 2H), 6.33e6.37
(m, 2H), 2.50 (s, 6H),1.93 (s, 3H); 13C NMR (100 MHz, CDCl3)
d 140.2,
133.9, 132.1, 131.2, 131.1, 130.5, 128.1, 127.2, 126.9, 126.8, 126.0, 125.9,
125.6,125.5,125.1,123.4,118.2,103.6, 91.3, 28.3,16.3; HRMS (ESI) m/
z calcd for C26H22N2 [M]þ 362.1778, found 362.1774.
4.4. Photoreactions of 2-aryl-1,3-dimethylbenziimidazoline 1
with various substrates
4.5. Recovery and subsequent transformation of an imida-
zolium salt (1dD) (entry 10 of Table 5)
4.4.1. Photoreactions of 1 with epoxy ketone 2, sulfonyl indole 12, or
acyl formate 14. Independent solutions of 2, 12 or 14 (0.40 mmol)
and 1 (0.48 mmol) in a solvent (4 mL) with or without AcOH,
PhCO2H, or PhOH for 2 and 12, allyl bromide for 14, were purged
with N2 for 5 min prior to irradiation. The solutions were irradi-
A solution of 4a (124.5 mg, 0.40 mmol) and 1d (167.2 mg,
0.48 mmol) in CH2Cl2 (4 mL) was purged with N2 for 5 min prior to
irradiation. This solution was irradiated for 6 h. The precipitate
formed by addition of Et2O to the photolysate was separated by
filtration. To a solution of the precipitate in MeOH (8 mL) was added
NaBH4 (62.3 mg, 1.65 mmol) under N2 in an ice-bath. The resulting
mixture was stirred for 1 h, diluted with water and extracted with
EtOAc. The extract was washed with sat Na2S2O3, brine and dried
over MgSO4 and concentrated in vacuo giving a residue that was
analyzed by using 1H NMR and identified as 1d (157.7 mg,
0.45 mmol, 94%).
ated (l>360 nm, except for entry 2 of Table 9 using the light above
390 nm) for appropriate time periods When an acid was present,
water was added to the photolysate, and the mixture was
extracted Et2O. The extract was washed with water, sat NaHCO3
for AcOH and PhCO2H or 20% NaOH for PhOH, brine, and dried
over anhydrous MgSO4. When allyl bromide or no additive was
present, the precipitate formed by addition of Et2O to the photo-
lysate was separated by filtration. The residue obtained by the
concentration of the extract or the filtrate was subjected to col-
umn chromatography (CH2Cl2, and then CH2Cl2/EtOAc¼5/1 for 2
and 14, CH2Cl2 for 12) to give starting materials and products 3, 13,
or 15. When isolation of the products by chromatography could
not be achieved, yields were determined by using 1H NMR with
appropriate internal references such as triphenylmethane and
1,3,5-trimethoxybenzene. Known products 3ae3e,10c 13a,23 and
15a10k were characterized by comparing their 1H NMR data with
those reported earlier. 13d is commercially available. Spectral data
of unknown 15b is given below.
4.5.1. 1,3-Dimethyl-2-(1-pyrenyl)benzimidazoliumbromide(1dþBrꢀ).
Pale yellow solid; mp 258.0e260.5 ꢃC; 1H NMR (400 MHz, DMSO-d6)
d
8.66 (d, J¼8.0 Hz, 1H), 8.55e8.50 (m, 4H), 8.44e8.40 (m, 2H),
8.27e8.21 (m, 3H), 7.91 (d, J¼9.2 Hz, 1H), 7.88e7.82 (m, 2H), 3.87 (s,
6H); 13C NMR (100 MHz, DMSO-d6)
150.0, 134.2, 132.5, 130.9, 130.7,
d
130.6, 130.4, 130.2, 129.0, 127.6, 127.5, 127.4, 127.2, 127.0, 125.4, 123.8,
123.32, 123.25, 114.2, 113.9, 33.1; HRMS (ESI) m/z calcd for C25H19N2
[MꢀBr]þ 347.1543, found 347.1536.
4.6. Density functional theory (DFT) calculations
Calculations were carried out using the Gaussian 03 program
package.34 Geometry optimizations were performed on ground
states at the B3LYP/6-31G(d) level. In addition, frequency calcula-
tions were performed on all optimized structures to confirm that
no imaginary frequencies were obtained. The molecular orbitals
(MOs) were visualized with Gauss View 3.09 software.
4.4.2. Ethyl 2-hydroxy-2-phenethyl-4-pentenate (15b). Pale yellow
oil; 1H NMR (400 MHz, CDCl3)
d 7.30e7.24 (m, 2H), 7.20e7.14 (m,
3H), 5.84e5.73 (m, 1H), 5.13 (s, 1H), 5.11e5.08 (m, 1H), 4.26e4.13
(m, 2H), 3.34 (br s, 1H), 2.86e2.78 (m, 1H), 2.52e2.39 (m, 3H),
2.12e1.95 (m, 2H), 1.30 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d
175.9, 141.6, 132.2, 128.4, 128.3, 125.9, 119.0, 76.8, 61.9, 44.1, 40.5,
30.3, 14.3; HRMS (ESI) m/z calcd for C15H20O3 [MþNa]þ 271.1305,
4.7. Laser flash photolysis
found 271.1303.
Transient absorption measurements were performed using an
in-house-built spectrometer equipped with a nanosecond YAG la-
4.4.3. Photoreaction of 1 with haloalkyl keto ester 4, alkene tethered
bromoalkyl ketone 8, or allyloxy iodobenzene 10. A solution of
a substrate 4, 8 or 10 (0.40 or 0.20 mmol) and 1 (0.48 mmol or
0.24 mmol) in a solvent (4 mL or 2 mL) was purged with N2 for
5 min prior to irradiation. Each solution was irradiated. When
DMSO was used as a solvent, the photolysate was diluted with
water and extracted with Et2O. Each extract was washed with
water, sat NaHCO3, brine, and dried over anhydrous MgSO4. When
THF or CH2Cl2 was used, the precipitate formed by addition of Et2O
to the photolysate was separated by filtration. The residue obtained
by the concentration of the extract or filtrate was subjected to
column chromatography (n-hexane/EtOAc¼3/1) to give recovered
4 and products 5 and 6. Since separation of 4 and 6 by chroma-
tography could not be achieved, yields were determined by using
1H NMR. The conversion of 8 and 10 as well as the yields of 9 and 11
ser that irradiates the third harmonic of
l
¼355 nm as a pump light
and a continuous wave Xe lamp probe light, of which the spectrum
is appropriately shaped with high and low cut-filters. The probe
light passing through the sample solution was dispersed by
a monochromator and then detected with a photomultiplier tube.
The output signal from the photomultiplier tube was recorded with
a digital oscilloscope. Dead time of this measurement system is
100 ns.
Acknowledgements
This study was partially supported by grants provided to E.H.
from the Uchida Energy Science Promotion Foundation and the
Sasaki Environment Technology. T.I. also appreciate the financial

5504
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supports of a CREST grant from the Japan Science and Technology
Agency (JST), a Grant-in-Aid for Scientific Research (No. 2610088)
from the Japanese Ministry of Education, Culture, Sports, Science
and Technology (MEXT), a grant from Idemitsu Kosan Co., Ltd., and
a grant from the Network Joint Research Center for Materials and
Devices.
Supplementary data
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Supplementary data (These data include a experimental set-up
of solvent free photoreaction, cyclic voltammograms of 1ae1g and
1d-Me, selected substrates, absorption spectra of selected sub-
strates, DFT calculation data of 1, decay profile of transient ab-
sorption spectrum for 1d-Me, 1H NMR and 13C NMR of 1be1f, 1d-
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interaction of 1 at the high concentration. We measured the spectrum of the
mixture of 1a and naphthalene expecting their CT interaction, and also mea-
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no clear CT absorption bands were observed in both cases.
16. National Institute of Standard and Technology (NIST) Chemistry Web Book,
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18. A reviewer questioned why 1.2 equiv of 1f and 1g are enough to be effective
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proton donors are used for the reaction with 1aee in solutions. Transformation
of 2 to 3 promoted by 1 is considered to proceed within PET generated radical
ion pairs, not after dissociation to become free radical ions. Therefore, less polar
solvents are more suitable for the reaction than highly polar ones (see Tables
2e4). In the cases of 1f and 1g, two electron and two proton transfers to 2,
which are required for the formation of 3, would smoothly take place within
their radical ion pairs.
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29. Concentrations of the components in the preparative photoreactions (ca.
10ꢀ1 M) are much greater than those used in the laser photolysis experiments