Benzimidazolium Naphthoxide Betaine Is a Visible Light Promoted Organic Photoredox Catalyst

Article abstract of DOI:10.1021/acs.joc.8b00282

Benzimidazolium naphthoxide (^-ONap-BI⁺) was first synthesized and utilized as an unprecedented betaine photoredox catalyst. Photoexcited state of ^-ONap-BI⁺ generated by visible light irradiation catalyzes the reductive deiodination as well as desulfonylation reactions in which 1,3-dimethyl-2-phenylbenzimidazoline (Ph-BIH) cooperates with as an electron and hydrogen atom donor. Significant solvent effects on the reaction progress were discovered, and specific solvation toward imidazolium and naphthoxide moieties of ^-ONap-BI⁺ was proposed.

Full text of DOI:10.1021/acs.joc.8b00282

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Benzimidazolium naphthoxide betaine is a visible
light promoted organic photo-redox catalyst
Eietsu Hasegawa, Norihiro Izumiya, Tomoaki Miura, Tadaaki
Ikoma, Hajime Iwamoto, Shin-ya Takizawa, and Shigeru Murata
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00282 • Publication Date (Web): 14 Mar 2018
Downloaded from http://pubs.acs.org on March 15, 2018
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Benzimidazolium naphthoxide betaine is a visible light promoted organic
photo-redox catalyst
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Eietsu Hasegawa,*† Norihiro Izumiya,† Tomoaki Miura,† Tadaaki Ikoma,†‡ Hajime Iwamoto,†
Shin-ya Takizawa§ and Shigeru Murata§
†Department of Chemistry, Faculty of Science, Niigata University, 8050 Ikarashi-2, Nishi-ku,
Niigata 950-2181, Japan.
‡Center for Coordination of Research Facilities, Niigata University, 8050 Ikarashi-2, Nishi-ku,
Niigata 950-2181, Japan.
§Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo,
3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan.
ABSTRACT: Benzimidazolium naphthoxide (–ONap-BI+) was first synthesized and utilized as an
–
unprecedented betaine photo-redox catalyst. Photoexcited state of ONap-BI+ generated by visible
light irradiation catalyzes the reductive deiodination as well as desulfonylation reactions in which
1,3-dimethyl-2-phenylbenzimidazoline (Ph-BIH) cooperates with as an electron and hydrogen atom
donor. Significant solvent effects on the reaction progress were discovered and specific solvation
toward imidazolium and naphthoxide moieties of –ONap-BI+ was proposed.
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INTRODUCTION
Photoinduced electron transfer (PET) is a fundamental photochemical process in which radical ions
are formed via single electron transfer (SET) between a photoexcited material and a ground state
substrate. Because of the effective generation of radical ions from neutral organic molecules and the
production of free radical species after the release of electro- or nucleofugal groups from such
intermediates under the mild reaction conditions, PET reactions have been frequently applied to
organic synthesis.1 Thus, extensive investigations of PET reactions of organic compounds
conducted during the past several decades have led to the development of a variety of synthetically
useful organic transformations. In addition, these studies have uncovered a plethora of preparatively
useful photo-redox catalyzed reactions using visible light.2 Complexes of transition metals, such as
ruthenium and iridium, are commonly utilized to promote these transformations. However, more
economical as well as sustainable photocatalysts, which do not rely on the use of expensive
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precious metals, are in great demand. Among the strategies employed to overcome this limitation
are those that utilize organic photo-redox catalysts.3 Although a variety of organic electron-acceptor
photocatalysts, such as cyano arenes, aromatic ketones, quinones, cationic salts such as pyrylium,
thiapyrylium, quinolinium, acridinium, have been developed in previous investigations,1,3 their
electron-donor counterparts have been less well developed.4 Moreover, the fact that they are
potentially useful for promoting atom transfer radical polymerizations heightens the need for further
development of visible light absorbing organic electron donors.5
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In a previous effort in the area of organic PET reactions, we discovered that
2-aryl-1,3-dimethylbenzimidazolines (Ar-BIHs) act as both an effective electron as well as
hydrogen atom donors to promote reductive transformations of various organic substances.1c,4f,g,6
Our findings have stimulated other studies exploring applications of PET reactions of BIHs.7
Recently, we developed a new protocol in which hydroxynaphthyl substituted BIH (HONap-BIH)
participates as a visible light absorbing strong electron donor that acts as an effective
photo-reductant.6b In this effort, we found that addition of bases significantly enhances the
efficiencies of the reduction reactions of various organic substances due to the generation of
naphthoxide structure (Figure 1).6c,d Moreover, HONap-BIH is used in stoichiometrically larger
amounts compared to the substrate and it is transformed to the oxidized product,
1,3-dimethyl-2-(2-naphthox-1-yl)benzimidazolium (–ONap-BI+, 1). Because 1 contains a
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naphthoxide chromophore, which absorbs visible light, we expected that it would also act as a
visible light absorbing strong electron donor.
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Figure 1. Evolution of benzimidazolium naphthoxide photocatalyst from benzimidazoline electron
donor reagent
In this article, we describe the results of an investigation probing the ability of betaine 1 to serve
as a visible light absorbing electron-donor photocatalyst (Figure 1). In the designed reaction
sequence, photo-excited 1 initiates reaction and 1,3-dimethyl-2-phenylbenzimidazoline (2) serves as
a reductant by donating an electron and a hydrogen atom.6 Two types of substances, including
o-allyloxyiodobenzene (3) and sulfonamides 5, 7 and 9 which undergo respective reductive
deiodination and desulfonylation reactions,6d were selected as substrates to probe the viability of the
photocatalytic system of 1 with 2 (Figure 2).
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Figure 2. PET promoted reduction reactions of o-allyloxyiodobenzene (3) and sulfonamides 5, 7
and 9.
RESULTS AND DISCUSSION
Betaine 1 was prepared via the sequential steps presented in Scheme 1 (see details in Experimental
Section). HONap-BIH was first treated with phenacyl bromide to give the imidazolium bromide,
HONap-BI+Br–. Then, deprotonation of this salt by KOH produced 1.
Scheme 1. Preparation of betaine 1
Because the absorption spectrum of 1 (Figure 3) contains a maximum at 415 nm (ε
= 8268 M-1
cm-1) that arises from the presence of a naphthoxide moiety (see Figure S1), 1 should absorb visible
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light. Thus, the photocatalytic reduction reaction of 3 (0.1 M) promoted by 1 in the presence of 2
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was first examined by using a 500 W Xe light source, a cut-off glass filter (λ > 390 nm), and a 6 h
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irradiation time. As seen in Table 1, a catalytic quantity of 1 did promote the deiodination reaction
of 3 to produce the expected cyclization product 4 in good yields in DMSO as well as DMF (entries
1, 2, 4, 5, 9, 10). While only 1 mol% of 1 could lead to the completion of the reaction (entry 10), no
reaction proceeded in the absence of 1 (entry 11). Irradiation time did not significantly influence the
yield of 4 (compare entries 2 and 5 to 3 and 7, respectively), which suggests that prolonged
irradiation does not cause the decomposition and further reaction of 4. On the other hand, no
reaction proceeded in the absence of 2 (entry 6) or without irradiation (entry 8).
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Figure 3. Absorption spectra of 1 and 2 in DMSO (4.0 x 10-5 M), and the transmittance spectrum of
the glass filter used for photoreaction.
Table 1. Photo-reductive cyclization of 3 promoted by the catalytic quantity of 1 with 2a
Entry
1
1 (mol%)
Solvent
DMSO
Irrad tme (h) Conv of 3 (%)b
Yield of 4 (%)b,c
20
6
100
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DMSO
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
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1
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100
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3
47 (66)
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6d
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44 (86)
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8e
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0
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1
-
a3 (0.10 mmol), 1 (1~20 mol% vs 3), 2 (1.2 equiv vs 3), solvent (1.0 mL); 500 W Xe-lamp (
λ > 390
nm). bDetermined by 1H NMR, cThe yield in the parenthesis is based on the conversion of 3. dIn the
absence of 2. eStirred in the dark.
Owing to the characteristic betaine structure of 1, we were interested in studying solvent effects
on the reaction. We anticipated that the Lewis base and acid character of solvents, estimated by
using respective donor numbers (DN) and acceptor numbers (AN),8 would influence the nature of
the imidazolium and naphthoxide moieties in 1 and have possible effects on the reaction progress.
To evaluate this issue, reaction of 3 was conducted for shorter irradiation time (1 h) in various
solvents including N,N-dimethylpyrimidineurea (DMPU). As can be seen by viewing the data in
Table 2, the progress of the reaction of 3 does not simply correlate with solvent polarities as
estimated by using ET(30) values.9 In fact, the reaction takes place in nonpolar solvents such as
toluene (entry 11). Although the absorption spectra of 1 in MeCN and CH2Cl2 are not significantly
different from those in DMSO and DMF (See Figure S2 and Table S1), the reaction does not occur
in the former solvents (compare entries 9 and 10 with entries 3 and 4). On the other hand, addition
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of only 5 molar excesses (vs 3) of DMSO, DMPU and NEt3 (DN = 31.7)8b was found to promote
reactions of 3 in CH2Cl2 (DN = 1) producing 4 in respective 24%, 41% and 53% yields and 26%,
51% and 57% conversion, and however this yield enhancement was not observed for the reaction in
the modest donor solvent MeCN (DN = 14.1) (see Table S4). Protic solvents such as MeOH, which
has a significantly high AN, are expected to hydrogen bond with the naphthoxide moiety of 1 and
cause a decrease in its electron donating ability.10 This effect and the fact that a significant blue shift
of the absorption spectrum of 1 takes place in MeOH are likely reasons for the lack of
photocatalytic reactivity when MeOH was used as solvent (entry 6, also see Figure S2 and Table
S1).
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Table 2. Photo-reductive cyclization of 3 promoted by 1 and 2 in various solventsa
b
entry
1
solvent
DMPU
Pyridine
DMSO
DMF
ET
DN c
33
ANc
-
conv of 3 (%)c yield of 4 (%)d,e
42.1
40.2
45.1
43.2
37.4
55.4
38.1
42.2
45.6
40.7
33.9
50
48
71
51
33
0
41 (82)
29 (60)
47 (66)
44 (86)
22 (67)
0
2
33.1
29.8
26.6
20.0
19
14.2
19.3
16.0
8.0
3f
4g
5
THF
6
MeOH
AcOEt
Acetone
MeCN
CH2Cl2
PhCH3
41.5
9.3
7
17.1
17
44
41
> 0
> 5
37
12 (27)
31 (76)
trace
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12.5
18.9
20
9
14.1
1
10
5
11
0.1
3.3
21 (57)
a3 (0.10 mmol), 1 (10 mol% vs 3), 2 (1.2 equiv vs 3), solvent (1.0 mL); 500 W Xe-lamp (
λ
> 390
b
nm), 1 h. Empirical parameter of solvent polarity (ET(30), kcal/mol).9 cDonor number (DN,
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kcal/mol) and acceptor number (AN) of solvents.8 dDetermined by H NMR. eThe yield in the
3
4
f
g
parenthesis is based on the conversion of 3. Same as entry 3 of Table 1. Same as entry 7 of Table
5
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1.
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Next, DFT calculations and electrochemical measurements were performed to gain more insight
into the structure and redox properties of 1 (Figure 4).11 The results reveal that the imidazolium and
naphthoxide rings in the energy minimized structure of 1 are not in parallel planes, thus preventing
π-conjugation of these moieties (Figure 4a). Moreover, the oxy anion of the naphthoxide moiety is
located close to the imidazolium nitrogen. In addition, although frontier orbitals are distributed over
the entire molecule, the HOMO coefficients are greater in the naphthoxide moiety and the LUMO
coefficients are relatively larger in the imidazolium moiety (Figure 4b). Thus, an electrostatic
interaction exists between the negatively charged naphthoxide and the positively charged
imidazolium groups. This interaction causes stabilization of 1, as reflected by the observation that
the oxidation potential of 1 (Eox1/2 = +0.64 V vs SCE in MeCN) is much greater than expected for a
simple naphthoxide derivative (Eox = +0.1 V for 2-naphthoxy anion12 and +0.03 V for
1/2
1-methy-2-naphthoxy anion in MeCN) (see Figures S3, S4 and Table S2). The electron donating
ability of 1 should be enhanced in solvents that have high DN (entries 1~4 in Table 2) or low AN
values (entries 5, 7, 8 and 11 in Table 2) because they should strongly coordinate to the imidazolium
moiety and not tightly interact with naphthoxy anion.10 Moreover, stronger coordination of DMSO
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to 1 than MeCN, which should more enhance the imidazolium as well as naphthoxide character of 1
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1
in former solvent, is confirmed by the comparison of H NMR of 1 in these solvents (Figure 5). A
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peak of HA appears in upper field and that of HB appears in more downfield in DMSO-d6 compared
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to CD3CN (see details in Experimental Section and SI).
Figure 4. The results from DFT calculations of 1 (B3LYP/6-311+G(d,p)). a) Optimized structure
depicted by a ball-and-bond type model from different angles. b) Frontier orbital distributions.
7.59–6.71ppm
7.65–6.71ppm
HB
CH3
N
7.67–7.62 ppm
7.64–7.59 ppm
N
O
HA
CH3
HB
6.64 ppm
6.72 ppm
8.00–7.95 ppm
7.82–7.76 ppm
1
Figure 5. Comparison of H NMR chemical shifts of representative protons of 1 (HA and HB) in
DMSO-d6 (above) and CD3CN (below).
In order to gain information about excited species derived from 1, laser flash photolysis
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experiments using 1 were performed in the absence and presence of 3 in DMSO (Figure 6). Figure
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6a contains transient absorption spectra of 1 at 510 nm and 700 nm. The long lifetime of 17 ꢀs and
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significant quenching by molecular oxygen indicate that the observed transient absorption is due to
the excited triplet state of 1 (see Figure S8). In Figure 6b are shown transient absorption spectra of 1
with 3. In addition to the triplet peaks, a new peak around 400 nm is observed, which is assigned to
a reaction product of excited 1 and ground state of 3. However, the intensity and lifetime of the
triplet signal for 1 observed at 510 nm are unaffected by addition of 3 (see Figure S9), which
indicates that the triplet state does not react with 3. Therefore, a short-lived species derived from 1
upon irradiation that could not be detected by nanosecond laser experiment is tentatively considered
to undergo SET to 3.
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4x10-3
3
a)
b)
1 ꢀs
5 ꢀs
10 ꢀs
20 ꢀs
50 ꢀs
2x10-3
1
2
1
0
0
-1
-2
1 ꢀs
5 ꢀs
10 ꢀs
20 ꢀs
40 ꢀs
400
500
600
700
800
400
500
600
700
800
Wavelength / nm
Wavelength / nm
Figure 6. a) Transient absorption spectra of Ar-saturated DMSO solution of 1 (0.15 mM). b)
Transient absorption spectra of Ar-saturated DMSO solution of 1 (0.15 mM) and 3 (1.5 mM).
On the basis of the results described above, it is reasonable to propose the reaction pathways
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shown in Scheme 2 for the reaction of 3 photocatalyzed by 1 and 2. In the process, visible light
irradiation generates the photo-excited state of 1, which should have short lifetime (see above) and
a high oxidizing ability as reflected in an oxidation potential (Eox*) of -2.08 V, estimated by using
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the end-absorption wavelength (456 nm with
ε
= 500 M-1 cm–1) to determine excited state energy
(Eex = 2.72 eV) and oxidation potential (Eox1/2 = +0.64 V) of 1. Consequently, SET from the excited
state of 1 to 3 (Ered = –1.51 V, see Figure S5 and Table S3) should be energetically favorable.13
1/2
Subsequent SET to the formed radical cation of 1 (imidazolium naphthoxyl radical 11) from 2
(Eox1/2 = +0.34 V)6b regenerates 1 and forms the radical cation of 2 (2•+). Deiodination of the formed
radical anion of 3 produces aryl radical 12, which subsequently undergoes rapid 5-exo cyclization
to give the alkyl radical 13. Hydrogen atom transfer between 2•+ and 13 produces the observed
product 4 and imidazolium ion 14. When 2 was replaced by Hantzsch ester (Eox1/2 = +0.93 V)4g and
the mixture of NEt3 (Eox = +0.93 V)4g and HCOOH in DMF, no formation of 4 was observed
1/2
because SET processes between 11 and these amines would be endoergonic (see Table S4).
Scheme 2. Plausible reaction pathways of the reductive deiodination of 3 catalyzed by the
photoexcited 1 and 2
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Next, in order to further evaluate the PET catalyst system of 1 with 2, desulfonylation reactions
of sulfonamides 5, 7 and 9 (0.1 M) were chosen as another probe reactions (Table 3). The
nitrogen-sulfur bond cleavage of their radical anions, in which either homolytic or heterolytic
cleavages may occur depending on the substituents, are expected (Scheme 3).4a,4g,14 Desulfonylation
reactions of N-sulfonylindols 5 (-1.67 V, -1.74 V, and -1.92 V for 5a, 5b, and 5c respectively)
proceed smoothly to give the corresponding free indoles 6 in excellent yields (entries 1, 8 and 9).
The weak electron accepting substrate 5c requires longer irradiation times for completion.
Interestingly, irradiation of the solution not containing 2 resulted in some consumption of 5a with
the formation of 6a, which may be a consequence of the stoichiometric reaction with 1 present
(entry 2). As expected, these processes do not occur in the absence of 1 or without irradiation
(entries 3 and 4). The observations suggest that the desulfonylation reactions follow a catalytic
pathway that is similar to that presented in Scheme 2. Notably, the reactions proceed not only in
toluene but also in CH2Cl2, although again not in MeCN (entries 5–7). In addition, while
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photoreaction of N-sulfonyl benzamide 7 (-1.58 V) in the presence of 2 but in the absence of 1
forms 8 in 16% at 17% conversion of 7 (not shown in Table 3), the photocatalytic reaction using
both 1 and 2 leads to complete consumption of 7 and generation of 8 in an excellent 96% yield
(entry 10).15 In contrast, diphenyl tosylamine 9 (-2.07 V), which is the weakest electron acceptor, is
less reactive than 7 (entry 11). Moreover, DMSO was found to be superior to DMF as a solvent for
reaction of 9 that produces 10, even though 9 was not completely consumed using a long irradiation
time (entry 12). DMSO is also a suitable solvent for the desulfonylation of 5a promoted by the
commercially available household white LED (7.3 W) irradiation for 24 h giving 6a in 97% yield
while same reaction did not lead to the completion in DMF (see Table S5).
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Table 3. Photo-desulfonylation of N
-sulfonamide 5, 7, 9 catalyzed by 1 and 2a
entry
substrate solvent
Irrad time (h) conv (%)b
product
6a
yield (%)b,c
1
DMF
1
100
6
98
5a
5a
5a
5a
5a
5a
5a
5b
5c
7
2d
3e
4f
DMF
1
4 (67)
0
6a
DMF
1
0
6a
DMF
1
0
0
6a
5
MeCN
CH2Cl2
PhCH3
DMF
1
6
4 (67)
97
6a
6
1
100
100
100
100
100
9
6a
7
1
98
6a
8
1
93
6a
9
DMF
2
89
6b
8
10
11
12
DMF
0.5
6
96
DMF
trace
9
10
DMSO
6
42
33 (79)
9
10
aSubstrate (0.10 mmol), 1 (10 mol% vs substrate), 2 (1.2 equiv vs substrate), solvent (1.0 mL); 500
b
1
c
W Xe-lamp (
λ
> 390 nm). Determined by using H NMR. Yields in the parenthesis are based on
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the conversion of 5. dIn the absence of 2. eIn the absence of 1. fStirred in the dark.
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Scheme 3. Desulfonylation of sulfoneamide radical anions
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While photo-redox reactions of β-ketosulfones were previously performed by using a ruthenium
complex as a photocatalyst, an organic donor photocatalyst such as Eosin Y was recently reported to
be ineffective for alkylcarbonyl substrate.16 Thus, our photocatalyzed reaction condition was briefly
applied for desulfonylation reactions (λ > 390 nm) of β
-ketosulfones 15a (Ered = –1.48 V) and
1/2
15b (Ered = –1.89 V) (Scheme 4). Reactions of 15 (0.1 mmol) using 1 (10 mol%) and 2 (1.2
1/2
equiv) in DMSO (1 mL) produced the respective ketones 16a and 16b in good to excellent yields .
Scheme 4. Photo-desulfonylation of β-ketosulfones 15
CONCLUSION
As demonstrated in the investigation described above, betaine 1 serves as an effective electron
donating, organic photocatalyst for PET reactions. In the effort, solvent effects on the efficiencies of
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the reactions were observed and attributed to specific solvation, such as donor and acceptor
interactions with the benzimidazolium and naphthoxide moieties of 1. Therefore, the findings show
that a proper choice of solvent is crucial for the success of these photocatalytic reductions. While
various betaines have been developed and their properties have been characterized,17 these
zwitterionic substances to the best of our knowledge have not been previously used as photo-redox
catalysts.18 Determination of the substrate scope as well as the detailed mechanism of these
photo-catalytic reactions are under current investigation.
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EXPERIMENTAL SECTION
General methods. NMR spectra were recorded on CDCl3 with tetramethylsilane (Me4Si) as an
internal standard, DMSO-d6, and CD3CN solutions at 400 MHz for 1H NMR and 100 MHz for 13C
NMR. Proton-decoupled carbon data of 13C NMR are reported. High resolution mass spectra
(HRMS) were recorded on a doublefocusing mass spectrometer by using Atmospheric Pressure
Chemical Ionization (APCI). Oxidation and reduction potentials in MeCN were measured using
cyclic voltammetry and a previously described procedure.4f Calibration of the potential values were
performed using the formal potentials of ferrocene/ferrocenium couple, which are 0.067 V and
0.442 V versus Ag/AgNO3 and SCE, respectively. Half-wave potentials (E1/2) reported in the
manuscript were obtained from the peak potentials by subtracting or adding 0.029 V.19 Light
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sources for photoreactions were a 500 W Xe lamp with glass filter L-42 (λ > 390 nm) and a 7.3 W
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7
8
white LED. Column chromatography was performed with silica gel. Anhydrous solvents for
photoreactions were obtained as follows. 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. EtOAc was washed with 5% Na2CO3, water, NaCl and K2CO3 and
subsequently distilled. Pyridine was washed with KOH, water and subsequently distilled with CaH2.
Acetone was purified only by distillation. Anhydrous DMF, DMSO, and MeOH were purchased
and used without distillation. Other reagents and solvents were used without further purification.
Benzimidazoline 220 is known compound and HONap-BIH6b was prepared according to the
previously reported procedure. Substrates 3,21 5a,22 5b,23 5c,24 7,15 9,25 15a,26 and 15b16a which are
known compounds, were prepared by using reported procedures. Preparation procedure and
spectroscopic data of the betaine 1 are described below.
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Preparation of 1,3-dimethyl-2-(2-naphthox-1-yl)benzimidazolium (1) .
Synthesis of 1,3-dimethyl-2-(2-hydroxy-1-naphthyl)benzimidazolium bromide. A mixture of
1,3-dimethyl-2-(2-hydroxy-1-naphthyl)benzimidazoline (HONap-BIH, 901 mg, 3.10 mmol) and
2-bromoacetophenone (926 mg, 4.65 mmol) in THF (30 mL) was stirred at room temperature for 3
h. Precipitates formed by the addition of Et2O (ca 20 mL) were collected by filtration. The solid was
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crystallized from MeOH-Et2O to give 1,3-dimethyl-2-(2-hydroxy-1-naphthyl)benzimidazolium
bromide (1.062 g, 2.87 mmol, 93%). Colorless solid; mp 283.0–284.0 °C; 1H NMR (400 MHz,
6
7
8
9
DMSO-d6)
δ
11.47 (bs, 1H), 8.28 (d, J = 8.8 Hz, 1H), 8.18–8.12 (m, 2H), 8.06 (d, J = 8.0, 1H),
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7.82–7.77 (m, 2H), 7.57–7.44 (m, 3H), 7.32 (d, J = 8.0, 1H), 3.82 (s, 6H); C NMR (100 MHz,
DMSO-d6)
δ 156.9, 148.1, 135.7, 132.1, 131.8, 129.1, 129.0, 127.6, 126.7, 124.4, 122.5, 118.2,
113.7, 99.0, 32.5; HRMS (ESI) m/z calcd for C19H17N2O [M-Br]+ 289.1335, found 289.1336.
Synthesis
of
1,3-dimethyl-2-(2-naphthox-1-yl)benzimidazolium
(1
).
To
1,3-dimethyl-2-(2-hydroxy-1-naphthyl)benzimidazolium bromide (793 mg, 2.00 mmol) was added
a MeOH (25 mL) containing KOH (259 mg, 4.62 mmol). The resulting mixture was stirred at room
temperature for 1 h and then extracted with CH2Cl2 (20 mL x8) after addition of water (30 mL). The
extract was washed with brine and dried over anhydrous MgSO4. The residue obtained by
concentration was subjected to column chromatography using MeCN (to remove impurity) and
subsequently MeOH. The obtained solid was crystallized from CH2Cl2 and Et2O to give
1,3-dimethyl-2-(2-naphthox-1-yl)benzimidazolium (1) (477 mg, 1.66 mmol, 83%). Yellow solid;
mp 218.0–220.0 °C; 1H NMR (400 MHz, DMSO-d6)
δ 8.00–7.95 (m, 2H), 7.67–7.62 (m, 2H), 7.59
(d, J = 9.2 Hz, 1H), 7.55 (d, J = 7.8 Hz, 1H), 7.18–7.13 (m, 1H), 6.91–6.87 (m, 1H), 6.71 (d, J = 8.0
1
Hz, 1H), 6.64 (d, J = 9.2 Hz, 1H), 3.75 (s, 6H); H NMR (400 MHz, CD3CN); 7.82-7.76 (m, 2H),
7.65-7.56 (m, 4H), 7.21-7.16 (m, 1H), 6.96-6.91 (m, 1H), 6.76-6.71 (m, 2H), 3.76 (s, 6H); 13C
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NMR (100 MHz, DMSO-d6)
δ
171.7, 155.0, 135.2, 133.5, 131.9, 128.4, 128.0, 127.0, 125.3, 122.9,
4
5
6
7
8
119.1, 118.0, 112.6, 94.4, 32.3; HRMS (ESI) m/z calcd for C19H17N2O [M+H]+ 289.1335, found
289.1339.
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Photoreaction procedure. Solutions containing substrates 3, 5, 7, 9, or 15, and 1 and 2 in Pyrex
test tubes (1.4 cm diameter), immersed in a water bath at room temperature, were irradiated using a
Xe lamp or with a white LED without using a water bath. A typical procedure is described below.
An appropriate solvent (1.0 mL) containing 1 (2.9 mg, 10 mol%), 2 (26.9 mg, 0.12 mmol) and 3
(26.0 mg, 0.10 mmol) was purged with N2 for 10 min and then irradiated with a Xe lamp through
L-42 glass filter. The photolysate was diluted with water (30 mL) and extracted with Et2O (20
mL×3). The combined extracts were washed with water (30 mL×2), brine (30 mL), dried over
anhydrous MgSO4 and concentrated in vacuo to give a residue. The conversion of 3 and the yield of
1
4 were determined by using H NMR analysis of the residue with 1,4-dimethoxybenzene or
triphenylmethane as an internal reference in CDCl3. Photo-products 4,21 6a27 and 815 are known
compounds, and 6b, 10, 16a and 16b are commercial materials.
Density functional theory (DFT) calculations. Calculations were carried out using the Gaussian
09 program package.11 Geometry optimization was performed on a ground state structure at the
restricted B3LYP/6-311+G(d,p) level. In addition, frequency calculations were performed on the
optimized structure of 1 to confirm that no imaginary frequencies are obtained. The optimized
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structure and molecular orbitals (MOs) were visualized with GaussView 5.0.9 software.
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Laser flash photolysis. Transient absorption experiments with 1 in the presence and absence of 3
were carried out with an updated version of homemade laser flash photolysis setup described
elsewhere.6b The third harmonics of a nanosecond Nd:YAG Laser (wavelength of 355 nm, pulse
length of 6 ns, intensity of 2 mJ/0.20 cm2/pulse) and a 75 W Xe arc lamp equipped in an ellipsoidal
mirror lamp house were used as pump and probe light sources, respectively. The probe light
transmitted through the sample solution was dispersed by using a monochromator and detected with
a photomultiplier tube. The output current from the photomultiplier was amplified with a fast
current amplifier and recorded with a digital oscilloscope. The time resolution and dead time for
this setup were 20 ns and 80 ns, respectively. Wavelength resolution was 6.8 nm. The samples were
dissolved in dehydrated DMSO and placed in a 1 cm path-length quartz cuvette. The solutions were
deoxygenated by Ar bubbling before the measurement unless otherwise stated.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
1
Data of absorption spectra, cyclic voltammograms, and H NMR of some compounds. Additional
photoreactions, DFT calculations, and laser photolysis (PDF). 1H NMR and 13C NMR charts of 1.
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AUTHOR INFORMATION
Corresponding Author
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*E-mail for E.H.: ehase@chem.sc.niigata-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This study was partially supported by JSPS KAKENHI Grant Numbers JP16K05689 (Grant-in-Aid
for Scientific Research (C) for E.H.) and JP16KT0049 (Grant-in-Aid for Scientific Research (B) for
T.I.). We thank Mr. Kazuki Nemoto (Niigata University) for his contribution to preliminary
experiments.
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(13) An alternative possibility is that naphthoxide benzimidazoline (–ONap-BIH) is first generated
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by in-situ hydride transfer from 2 to 1 and then photoexcited ONap-BIH promotes reaction is
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–
unlikely. When 1 is treated with 2 in DMSO-d6, no sizable H-NMR peaks of ONap-BIH were
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