1,3-Dimethyl-2-phenylbenzimidazoline (DMPBI)-acetic acid: An effective reagent system for photoinduced reductive transformation of α,β-epoxy ketones to β-hydroxy ketones

Article abstract of DOI:10.1055/s-2001-15055

A combination of 1,3-dimethyl-2-phenylbenzimidazoline (DMPBI) and acetic acid has been utilized for photoinduced reductive transformation of α,β-epoxy ketones to β-hydroxy ketones. Study on photoreactions using several proton donors revealed that acetic acid is superior to other proton donors such as HCl, p-TsOH, MeOH, and water. 1,3-Dimethyl-2-phenylbenzimidazolium was produced in the reaction with acetic acid while N-benzoyl-N,N′-dimethyl-o-phenylenediamine was formed in aqueous solvents. When THF solutions containing aryl carbonyl possessing α,β-epoxy ketones and DMPBI and acetic acid were irradiated (λ > 280 nm), β-hydroxy ketones were isolated in good to excellent yields. Photosensitized conditions (λ > 340 nm) were employed for the reactions of alkyl carbonyl possessing α,β-epoxy ketones.

Full text of DOI:10.1055/s-2001-15055

1248
PAPER
1,3-Dimethyl-2-phenylbenzimidazoline (DMPBI)–Acetic Acid: An Effective
Reagent System for Photoinduced Reductive Transformation of , -Epoxy
Ketones to -Hydroxy Ketones
1
,3-Dimethyl-
2
i
e
benzimidaz
t
oline
(
s
DMPBI)–
u
A
cetic
A
cid:
An
Ef
H
fective
R
eagent
S
a
ystem segawa,* Naoki Chiba, Aiko Nakajima, Kumiko Suzuki, Akira Yoneoka, Kazuki Iwaya
Department of Chemistry, Faculty of Science, Niigata University, Ikarashi-2 8050, Niigata 950–2181, Japan
Fax +81(25)2626116; E-mail: ehase@sc.niigata-u.ac.jp
Received 29 January 2001
ence of electron-donating compounds would become a
Abstract: A combination of 1,3-dimethyl-2-phenylbenzimidazo-
line (DMPBI) and acetic acid has been utilized for photoinduced re-
ductive transformation of , -epoxy ketones to -hydroxy ketones.
Study on photoreactions using several proton donors revealed that
acetic acid is superior to other proton donors such as HCl, p-TsOH,
MeOH, and water. 1,3-Dimethyl-2-phenylbenzimidazolium was
produced in the reaction with acetic acid while N-benzoyl-N,N -
dimethyl-o-phenylenediamine was formed in aqueous solvents.
When THF solutions containing aryl carbonyl possessing , -ep-
oxy ketones and DMPBI and acetic acid were irradiated ( > 280
nm), -hydroxy ketones were isolated in good to excellent yields.
Photosensitized conditions ( > 340 nm) were employed for the re-
actions of alkyl carbonyl possessing , -epoxy ketones.
new way to achieve this synthetically important process.3
About ten years ago, we first examined this concept by
conducting photoreactions of several aromatic epoxy ke-
tones in the presence of triethylamine and other electron
donors.4 About the same time, Cossy and co-workers re-
ported photoreactions of several aliphatic as well as aro-
matic epoxy ketones with triethylamine.5 In these
experiments, while the yields of hydroxy ketones were
modest, several problems were also encountered: other
compounds such as -diketones were the major products
in some cases4a and the reactions could not be completed
Key words: photoinduced electron transfer, 1,3-dimethyl-2-phe-
nylbenzimidazoline, acetic acid , -epoxy ketones, reductive ring
opening -hydroxy ketones
since prolonged irradiation caused decomposition of
-
hydroxy ketones.5 Our subsequent mechanistic study
demonstrated that selectivity of the reaction pathway ei-
ther to produce -diketones or -hydroxy ketones depends
on the nature of the radical cations of amines used.6 Now,
we report herein a method to achieve photoinduced trans-
formation of , -epoxy ketones to -hydroxy ketones by
using a combination of 1,3-dimethyl-2-phenylbenzimida-
zoline (DMPBI)7 and acetic acid, which is superior to the
previously reported DMPBI-based method.8
During several decades of the last century, photoreactions
of three-membered ring compounds gained much atten-
tion due to their interesting reactivities; among these com-
pounds are epoxides.1 In particular, photoreaction of , -
epoxy ketones were extensively investigated, which dem-
onstrated that several types of photochemical processes
are open to these compounds.2 Selectivity of the bond
cleavage of epoxy rings such as C -O, C -O, C -C usual-
ly depends on the substitution and the nature of the reac-
tive excited states. However, if a single electron is
injected to carbonyl substituents to generate their radical
anions (ketyl radicals), these intermediates would under-
go selective C -O bond cleavage to give distonic radical
anions followed by formal two-proton and single-electron
transfer (SET) to become -hydroxy ketones (Scheme 1).
Therefore, irradiation of , -epoxy ketones in the pres-
Irradiation of trans-1,3-diphenyl-2,3-epoxypropane-1-
one (chalcone epoxide) (trans-1a) with DMPBI (1.2–1.5
equivalents) in either aqueous THF or wet benzene pro-
duced
1,3-diphenyl-3-hydroxypropane-1-one
(2a)8
(Scheme 2). It was already found that the yield of 2a was
sensitive to the amount of water added: an appropriate
amount of water (approx. 14 equivalents) was necessarily
added to obtain 2a in a yield above 80% (Figure 1).8a
Chalcone was a major product in dry solvents while sub-
stantial amounts of benzaldehyde as well as acetophe-
none, retro-aldol-fragmentation products, were formed in
O
O
-
•
O
O-
O
OH
R3
R4
R3
R4
R3 H+, e-, H+
R3
R4
e-
β
α
•
R1
R1
R1
R1
R4
R2
R2
O
O
R2
R2
Scheme 1
Synthesis 2001, No. 8, 18 06 2001. Article Identifier:
1437-210X,E;2001,0,08,1248,1252,ftx,en;C00601SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

PAPER
1,3-Dimethyl-2-phenylbenzimidazoline (DMPBI)–Acetic Acid: An Effective Reagent System
1249
Me
O
O
OH
R2
R3
hν / DMPBI-MeCO2H
N
Ph
H
R2
R3
R1
R1
THF / N2
N
O
Me
1
2
DMPBI
Scheme 2
the presence of large excess of water. On the basis of hand, addition of MeOH (31 equivalents) did not signifi-
Scheme 1, one may anticipate that an efficient protonation cantly alter the product distribution from the reactions in
to the anionic intermediate must be essential for the better dry solvents. Consequently, acetic acid is the best proton
yielding of 2a. This assumption led us to use acetic acid donor for the photoinduced transformation of trans-1a to
as a proton donor instead of water. Then, we found that the 2a promoted by DMPBI.
yield of 2a kept above 80% within a wider range of the
amount of acetic acid used (1.3–70 equivalents) compared
to that of water (Figure).
On the basis of the above observations, we applied the re-
agent system of DMPBI and acetic acid to the photoreac-
tions of other epoxy ketones 1 (Scheme 2). When THF
solutions containing 1 and DMPBI (1.2 equivalents) and
acetic acid (6.6 equivalents) were irradiated by Pyrex-fil-
tered light ( > 280 nm) for 1 hour, most of hydroxy ke-
tones 2 were isolated in good to excellent yields (Table 1).
In the reactions of all chalcone epoxides 1a g examined
(entries 1–8), the yields of hydroxy ketones 2a–g were
greater than those (average 84%) in the reactions using
DMPBI and water.8b Both trans- and cis-epoxy ketones
reacted similarly to produce comparable yields of hy-
droxy ketones (compare entries 1 and 2, and entries 9 and
10). In the reaction of dypnone epoxide 1h, the isolated
yields of 2h were modest while substantial amounts of ac-
etophenone (7–14%) were produced (entries 9 and 10).
1
Since H NMR analysis of the crude reaction mixture of
cis-1h demonstrated that only trace amounts of acetophe-
none existed, 2h probably partially decomposed to give
Figure
acetophenone during column separation. It should be also
Therefore, it might be easier to optimize the reaction con-
ditions determining the appropriate amount of a proton
donor for a certain substrate by use of acetic acid rather
than water. In these reactions, DMPBI is oxidized through
sequential liberation of two electrons and one proton to
become an imidazolium ion, which could be easily re-
turned to DMPBI by treatment with metal hydride such as
NaBH4 if it is isolated. However, in the reaction in aque-
ous solvents, N-benzoyl- N,N -dimethyl-o-phenylenedi-
amine (3) was formed.8 Since 3 was probably produced
from the reaction of imidazolium and hydroxide ions, ace-
tic acid might be advantageous to suppress decomposition
of imidazolium because of generation of a less nucleo-
philic acetate anion. As expected, no 3 was formed but
1,3-dimethyl-2-phenylbenzimidazolium acetate (4) was
produced in nearly quantitative yield in the reaction with
acetic acid although its isolation could not be completed.
Since formation of 2a appears to be sensitive to the nature
of the proton donor added, photoreactions of trans-1a
with DMPBI in other proton donors were briefly conduct-
ed. In the cases of more acidic proton donors such as HCl
(1.3 equivalents) and p-TsOH (1.1 equivalents), the reac-
tions were significantly retarded, which might be due to
the deactivation of DMPBI by protonation. On the other
noted that the yield of 2k was significantly improved from
that (37%) in the reaction of 1k using DMPBI and wa-
ter.8b
Our next interest was to develop the reaction conditions
utilizing longer wavelengths, preferably visible light.
Then, 1,6-bisdimethylaminopyrene (BDMAP)9 was cho-
sen as a sensitizer since this yellow compound absorbs
light above 340 nm, max = 373 nm (log = 4.35) and its
end-absorption is close to 450 nm. Irradiation ( > 360
nm) of BDMAP (0.05 equiv) with trans-1a and DMPBI
(1.2 equivalents) and acetic acid (2.2 equivalents) for 1
hour completely consumed trans-1a and produced 2a in
89% yield, while the conversion of trans-1a (52%) and
the yield of 2a (25%) were significantly low in the ab-
sence of BDMAP. A plausible mechanism is as follows.
Under the above conditions, BDMAP first absorbs the in-
cident light to become in the fluorescent excited state,
which donates a single electron to trans-1a. The resulting
radical cation of BDMAP accepts a single electron from
DMPBI to give the radical cation of DMPBI, which sub-
sequently reacts with the radical anion of trans-1a. Since
the oxidation potential of the singlet excited state of BD-
MAP is estimated to be –2.4 V,4a,9 SET from the excited
Synthesis 2001, No. 8, 1248–1252 ISSN 0039-7881 © Thieme Stuttgart · New York

1250
E. Hasegawa et al.
PAPER
Table 1 Photoreaction of Aryl Carbonyl Possessing Epoxy Ketones 1 with DMPBI–HOAc in THF
Entry
1
R1
R2
R3
Conv.
(%)
Yield of 2a
Bu3SnHd
(%)
h
1
2
trans-1a
cis-1a
1b
Ph
Ph
H
Ph
H
100
100
100
100
100
100
100
100
100
100
100
100
100
100
96
94
93
98
90
95
88
92
66b
58 b
69
72
72
70c
93
78
94
83
88
89
Ph
H
3
-Naph
Ph
Ph
4
1c
p-MeOPh
H
5
1d
Ph
H
-Naph
85
85
84
87
87
75
84
71
90
20
6
1e
Ph
H
p-MePh
p-MeOPh
p-ClPh
Ph
7
1f
Ph
H
8
1g
Ph
H
9
trans-1h
cis-1h
1i
Ph
Me
Ph
H
10
11
12
13
14
Ph
Me
Ph
H
84
81
75
92
89
75
53
61
1j
Ph
H
i-Pr
1k
Ph
H
COPh
Ph
1l
PhCH=CH
H
a Isolated yields.
b Determined by 1H NMR.
c The mixture of trans and cis (84:16) determined by 1H NMR.
d Cited from ref. 3k.
rification. Photoreactions were conducted in a Pyrex tube (1.5 cm
diameter) immersed in a water bath with a 500-W Xe-lamp as a light
source. Separation of photoreaction mixtures was performed by col-
umn chromatography with Wakogel C-200 silica gel and prepara-
tive TLC on 20 cm 20 cm plates coated with Wakogel B-5F silica
gel. Identification of photoproducts was mainly accomplished by
their IR and NMR spectra. NMR spectra were measured in CDCl3
with TMS as internal standard at 90 MHz, 200 MHz for 1H NMR;
22.49 MHz and 49 MHz for 13C NMR. Mps are uncorrected.
state of BDMAP to aliphatic carbonyl compounds, whose
reduction potentials are around –2.2 V,10 would be also
feasible. As shown in Table 2, the photosensitized condi-
tion ( > 340 nm) using BDMAP (0.05 equivalent), DMP-
BI (2.5 equivalents) and acetic acid was employed for the
reactions of alkyl carbonyl possessing monocyclic and bi-
cyclic epoxy ketones 1m–p which could not be efficiently
activated under the direct irradiation condition using Pyr-
ex-filtered light. Although the irradiation conditions have
not yet been completely optimized, the isolated yields of
hydroxy ketones 2m–p were found to be similar to those
in the reactions using DMPBI and water.8b
1,3-Dimethyl-2-phenylbenzimidazoline (DMPBI)7,13
To a MeOH solution (80 mL) of 1,3-dimethyl-2-phenylbenzimida-
zolium iodide (2.1 g, 6 mmol) was slowly added NaBH4 (567.5 mg,
15 mmol) under N2 in an ice-water bath. The resulting mixture was
stirred under N2 at ambient temperature for 1 h followed by concen-
tration, addition of H2O (30 mL), and extraction with Et2O (100
mL). The organic layer was treated with sat. Na2S2O3, sat. NaCl,
and anhyd MgSO4. The crude product mixture obtained by concen-
tration was subjected to silica gel column chromatography with
benzene containing 1% Et3N. The colorless solid obtained was
washed with EtOH and subsequently recrystallized from EtOH to
give colorless needles, 800.9 mg (60%, 3.57 mmol); mp 95–96°C
(Lit.12 mp 96°C). It should be noted that DMPBI sometimes decom-
poses in CDCl3, which depends on the condition of CDCl3. There-
fore, CDCl3 is recommended to be passed through a short alumina-
column before the NMR experiment of DMPBI.
Finally, it should be interesting to compare this reagent
system with other reducing agents which could convert
, -epoxy ketones to -hydroxy ketones.3 For example,
the yields of -hydroxy ketones reported in this study are
as good as those obtained in typical free-radical reactions
which are quite effective but require the use of toxic tribu-
tyltin hydride (see Tables 1, 2).3k
Reaction substrates were prepared according to the literature proce-
dures: 1a–o,3k,4a,11 1p.3f All photoproducts are known: 2a–o,3k,4a,11
2p.3f 1,6-Bis(dimethyl-amino)pyrene (BDMAP) was prepared by
the reported procedure.12 THF was distilled from Na benzophe-
none under N2. Benzene was treated with H2SO4, 5% NaOH, CaCl2,
and then distilled with CaH2. H2O for the reaction was obtained
through an ion-exchange column. HOAc, anhyd DMF, and other
solvents and reagents were purchased and used without further pu-
Photoreactions of trans-1a with DMPBI; Typical Procedure
A solution of trans-1a (89.7 mg, 0.40 mmol) and DMPBI (107.7
mg, 0.48 mmol) in THF (8 mL) in the presence of HOAc (0.15 mL,
2.64 mmol) was purged with dry N2 for 5 min followed by irradia-
tion for 1 h. To the photolysate was added H2O (30 mL) followed
Synthesis 2001, No. 8, 1248–1252 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
1,3-Dimethyl-2-phenylbenzimidazoline (DMPBI)–Acetic Acid: An Effective Reagent System
1251
Table 2 Photosensitized Reaction of Alkyl Carbonyl Possessing Epoxy Ketones 1 with DMPBI–HOAc in DMF
Entry
1
R1
R2
R3
Irrad. Time Conv
Yield of 2b
Bu3SnHc
(h)
10
12
(%)
70
(%)
h
1
2
1m
1n
Me
Me
H
H
Ph
65 (93)
34 (52)
24
40
91
n-Pent
65
80
O
3
4
1o
1p
12
12
79
73
56 (71)
31 (42)
44
23
O
O
O
a HOAc: 6.6 equiv for entry 1; 13.2 equiv for entries 2–4.
b Isolated yields (based on the conversion of 1).
c Cited from ref. 3k.
by extraction with Et2O (50 mL). The organic layer was treated with
H2O, sat. NaHCO3, sat. NaCl, and anhyd MgSO4. The crude product
mixture obtained by concentration of the extract was subjected to
column chromatography (CH2Cl2 followed by CH2Cl2–EtOAc, 5:1)
to give 2a (87.3 mg, 0.39 mmol). Reactions using various amounts
of acetic acid as well as reactions of cis-1a and 1b–l were performed
by essentially same procedure.
BDMAP Photosensitized Reaction of 1m with DMPBI; Typical
Procedure
A solution of 1m (64.9 mg, 0.40 mmol) and DMPBI (244.3 mg,
1.00 mmol), and BDMAP (5.8 mg, 0.02 mmol) in DMF (4 mL) in
the presence of HOAc (0.15 mL, 2.64 mmol) was purged with dry
N2 for 5 min followed by irradiation through Toshiba UV-37 fil-
tered light for 10 h. To the photolysate was added H2O (30 mL), fol-
lowed by extraction with EtOAc (50 mL). The extract was treated
with H2O, sat. NaHCO3, sat. NaCl, and anhyd MgSO4. The crude
product mixture obtained by concentration of the extract was sub-
jected to column chromatography (CH2Cl2 followed by CH2Cl2–
EtOAc, 5:1) to give 2m (42.5 mg, 0.26 mmol) and unreacted 1m
(19.4 mg, 0.12 mmol). Reaction conditions for 1n p were essen-
tially the same as above except for the amount of HOAc (0.30 mL,
5.28 mmol) and the irradiation time (12 h). Modified reaction con-
ditions were employed for trans-1a: DMF (8 mL), HOAc (0.05 mL,
0.88 mmol), Toshiba L-39 filtered light, 1 h.
Oxidized Products from DMPBI
In the reaction of trans-1a (89.7 mg, 0.40 mmol), DMPBI (107.7
mg, 0.48 mmol), and HOAc (0.03 mL, 0.52 mmol) in benzene (8
mL), an insoluble solid appeared during irradiation (1 h). This was
filtered and washed with benzene to give a colorless solid (78.9 mg,
0.28 mmol) which was identified as 1,3-dimethyl-2-phenylbenzim-
idazolium acetate (4) by comparing its IR and 1H NMR spectra with
those of the known iodide. 1H NMR analysis of the reaction mixture
obtained by concentration of the filtrate revealed that some amount
of the acetate existed with unreacted DMPBI: imidazolium acetate
(38.8 mg, 0.14 mmol) and DMPBI (10.5 mg, 0.05 mmol). Column
separation of this reaction mixture gave 2a (77.2 mg, 0.34 mmol).
Total yield of imidazolium acetate was 95% based on the consumed
DMPBI (about 100% based on the conversion of trans-1a).
Acknowledgement
This work was supported by a Grant-in-Aid for Scientific Research
(10640575) from the Ministry of Education, Science, Sports and
Culture of Japan. We thank Professor Masaki Kamata (Faculty of
Education and Human Science) for his helpful comments. We thank
Professor Emeritus Yoshiki Okamoto and Professor Takaaki Hora-
guchi (Faculty of Science) for their generous support.
Imidazolium Acetate 4
Mp 60.5–61°C.
–
IR (KBr): = 1572, 1404 (-CO2 )cm–1.
1H NMR: = 4.05 (s, 6H, 2 N-Me), 7.65–7.88 (m, 7H, 4H of ben-
zimidazolium ring and m,p-3H of 2-Ph), 7.91–7.99 (m, 2H, o-2H of
2-Ph).
References
For comparison, imidazolium iodide: 1H NMR: = 4.02 (s, 6H, 2
N-Me), 7.67–7.85 (m, 7H, 4H of benzimidazolium ring and m,p-3H
of 2-Ph), 8.02–8.09 (m, 2H, o-2H of 2-Ph).
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E. Hasegawa et al.
PAPER
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Synthesis 2001, No. 8, 1248–1252 ISSN 0039-7881 © Thieme Stuttgart · New York