Photocatalytic cascade carbon-carbon bond-forming radical reaction in aqueous media

Article abstract of DOI:10.1055/s-0033-1339182

The ruthenium photocatalyst Ru(bpy)₃Cl₂· 6H₂O has the potential to catalyze the cascade radical reaction with perfluoroalkyl radicals in aqueous media via two photoredox cycles. Georg Thieme Verlag Stuttgart. New York.

Full text of DOI:10.1055/s-0033-1339182

1578▌
LETTER
letter
Photocatalytic Cascade Carbon–Carbon Bond-Forming Radical Reaction in
Aqueous Media
Photocatalytic Cascade Carbon–Carbon Bond-Forming Radical Reaction
Eito Yoshioka, Shigeru Kohtani,* Eri Tanaka, Hideto Miyabe*
School of Pharmacy, Hyogo University of Health Sciences, Minatojima, Kobe 650-8530, Japan
Fax +81(78)3042794; E-mail: miyabe@huhs.ac.jp; E-mail: kohtani@huhs.ac.jp
Received: 08.05.2013; Accepted after revision: 13.05.2013
O
i-C3F7
O
Abstract: The ruthenium photocatalyst Ru(bpy)3Cl2·6H2O has the
potential to catalyze the cascade radical reaction with perfluoroal-
kyl radicals in aqueous media via two photoredox cycles.
Ru(bpy)3Cl2⋅6H2O (5 mol%)
i-C3F7I (5.0 equiv), (i-Pr)2NEt
NOBn
NOBn
I
LED lamp (1000 lm), argon
Key word: radical reaction, photochemistry, catalysis, fluorine,
heterocycles
1
2a
I
O
O
The use of water as the ultimate ‘green’ solvent to perform
organic reactions has been attributed to the development
of green chemistry.1 Particularly, C–C bond formation in
water has become the most exciting research area, as ex-
emplified by the recent dramatic progress.2,3 In principle,
the reactions of strictly neutral species are not affected by
water. Therefore, the employment of uncharged radical
species facilitates the construction of C–C bonds in aque-
ous media.4 Oshima and Shinokubo’s group demonstrated
that the initiator Et3B has the potential to induce radical
reactions in water.5 More recently, aqueous-medium rad-
ical reactions were performed by employing indium or
zinc as reported by Naito’s group and others.6
NOBn
NOBn
I
+
+
i-C3F7
i-C3F7
3a
4a
Scheme 1 Ruthenium-catalyzed cascade reaction
biphasic solution of 1 and i-C3F7I in H2O–MeCN (9:1,
v/v) was stirred with LED light irradiation under an argon
atmosphere. No reaction was observed in the absence of
catalyst (Table 1, entry 1). Next, the ruthenium catalyst
was employed with amines as the reductive quenchers of
ruthenium species having oxidant property. The genera-
tion of i-C3F7 radical was promoted by Ru(bpy)3Cl2·6H2O
(5 mol%) and (i-Pr)2NEt15,16 to give the products 2a, 3a,
and 4a in 77% combined yield and a 64:24:12 product ra-
tio after being stirred for three hours (Table 1, entry 2). In
this reaction, the aqueous phase changed into dark red col-
or; thus, this color change indicates the red crystalline ru-
thenium salt was mainly solved in the aqueous phase. The
replacement of H2O–MeCN with MeCN led to a slight de-
crease in yield (Table 1, entry 3). The reaction in DMF
took a long time and half of the substrate remained after
being stirred for 15 hours (Table 1, entry 4). A similar
trend was observed in the protic solvent MeOH (Table 1,
entry 5). In contrast, the reaction proceeded only in H2O
without any problem (Table 1, entry 6). In the absence of
solvent, the reaction on a 1 mmol scale was carried out in
i-C3F7I (0.7 mL, Table 1, entry 7). However,
Ru(bpy)3Cl2·6H2O is not soluble in i-C3F7I, and the reac-
tion did not proceed effectively to give the complex mix-
ture after being stirred for 15 hours. Evidently,
perfluorohaloalkanes themselves cannot act as solvents;
thus, the use of H2O as solvent led to an enhancement in
chemical efficiency.
In recent years, photocatalysts have been shown to induce
the synthetically useful redox transformation by MacMil-
lan, Yoon, Stephenson, and other groups.7–10 Although the
viability of these catalysts in water was reported,11 less is
known about the aqueous-medium C–C bond forma-
tion.11a–d Our laboratory is interested in the construction of
multi C–C bonds in water. The reported redox potentials
in the excited state [Ru(bpy)3]2+* and the ground state
[Ru(bpy)3]2+ are summarized in Supplementary Materi-
al.12 Because both the reducing and oxidizing abilities of
[Ru(bpy)3]2+* in H2O (–0.86 V and +0.84 V vs. SCE, re-
spectively) are slightly greater than those in MeCN, we
expected the possibility of ruthenium catalysis in aqueous
media. We now report the results of experiments to probe
the utility of Ru(bpy)3Cl2·6H2O in the aqueous-medium
cascade reaction.
As a model reaction which has been well studied by our
group,13 we started to look at the addition–cyclization of
substrate 1 with perfluoroalkyl radicals in aqueous media
(Scheme 1),14 since the ruthenium and iridium complexes
are efficient catalysts to cleave the C–X bond of activated
halogen compounds.10,11b The utility of the most widely
employed catalyst, Ru(bpy)3Cl2·6H2O, was checked. The
This reaction involves two photoredox cycles and an
iodine-atom-transfer cycle (Scheme 2). The redox cycles
are initiated by visible-light irradiation to produce the
metal-to-ligand charge transfer (MLCT) excited singlet
SYNLETT 2013, 24, 1578–1582
Advanced online publication: 14.06.2013
DOI: 10.1055/s-0033-1339182; Art ID: ST-2013-U0430-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

LETTER
Photocatalytic Cascade Carbon–Carbon Bond-Forming Radical Reaction
1579
Table 1 Reaction of 1 with i-C3F7I
Entry
Ru catalyst
Solvent
Time (h)
Ratio of 2a/3a/4aa
Yield (%)b
1
2
3
4
5
6
7
none
H2O–MeCN (9:1, v/v)
3
3
–
n.r.c
77
Ru(bpy)3Cl2·6H2O
Ru(bpy)3Cl2·6H2O
Ru(bpy)3Cl2·6H2O
Ru(bpy)3Cl2·6H2O
Ru(bpy)3Cl2·6H2O
Ru(bpy)3Cl2·6H2O
H2O–MeCN (9:1, v/v)
64:24:12
66:34:trace
66:34:trace
66:34:trace
62:23:15
–
MeCN
DMF
MeOH
H2O
3
58
15
15
3
39 (43)
50 (31)
75
none
15
complex mixture
a Determined by 1H NMR analysis. Compound 2a was obtained in a cis/trans ratio of ca. 9:1–7:3.
b Combined yield. The yield in parentheses is for the recovered substrate 1.
c n.r. = no reaction.
because the primary perfluoroalkyl radicals exhibit lower
electrophilicities than the secondary perfluoroalkyl radi-
cals.18 When the primary perfluoroalkyl iodides were em-
ployed, the potential of [Ru(bpy)3]2+* (–0.86 V in H2O) is
not sufficient to reduce n-C3F7I (reduction potential: –1.00
V, Table 2, entry 1).17 Therefore, the photoredox cycle 1
is assumed to be the main pathway and the photoredox cy-
cle 2 would be ignored. The reaction with bulky c-C6F11I
gave the products in 48% combined yield (Table 2, entry
3). Under the analogous conditions, CCl3Br worked well
to give a good yield of the brominated products 2e and 3e
(X = Br) via the less effective bromine-atom transfer pro-
cess (Table 2, entry 4). It is important to note that the bro-
minated products was not obtained effectively when
initiator Et3B was employed in the absence of a Lewis ac-
(i-Pr)2NEt
•
i-C3F7
+
[Ru(bpy)3]3+
•
(i-Pr)2NEt
i-C3F7I
photoredox cycle 2
i-C3F7
O
LED
1
[Ru(bpy)3]2+
[Ru(bpy)3]2+
*
NOBn
•
photoredox cycle 1
[Ru(bpy)3]+
•
(i-Pr)2NEt
i-C3F7
A
iodine-atom-
transfer cycle
+
•
i-C3F7I
(i-Pr)2NHEt
i-C3F7
O
2a
NOBn
i-C3F7I
•
B
Scheme 2 Reaction pathway
Table 2 Reactions with other Radical Precursors in H2O
R
O
state.7d,12 The following rapid intersystem crossing popu-
lates the ruthenium complex in the MLCT excited triplet
state [Ru(bpy)3]2+*. In cycle 1, (i-Pr)2NEt is oxidized by
[Ru(bpy)3]2+* to form [Ru(bpy)3]+, which reduces i-C3F7I
(reduction potential: –0.66 V)17 to provide the i-C3F7 rad-
ical. In cycle 2, the excited state [Ru(bpy)3]2+* serves as a
reducing agent for i-C3F7I. Consequently, the secondary
i-C3F7 radical can be generated via both photoredox cy-
cles. The i-C3F7 radical predominantly adds to the polari-
ty-mismatched electron-deficient alkene moiety of 1 to
yield the intermediate radical A.13 In the iodine-atom-
transfer cycle, 2a is obtained via intermediate B. These
catalytic cycles efficiently proceeded in aqueous media,
although the reaction rate becomes slower in organic sol-
vent (Table 1). These results might be explained in terms
of the slight difference between the redox potential of
[Ru(bpy)3]2+* in pure MeCN and in H2O.12
Ru(bpy)3Cl2⋅6H2O (5 mol%)
RX (5.0 equiv)
NOBn
+
+
4d
3b–e
(i-Pr)2NEt (1.1 equiv)
1
H2O, argon
LED lamp (1000 lm)
X
2b–e
b R = n-C3F7, X = I d R = c-C6F11, X = I
c R = n-C4F9, X = I e R = CCl3, X = Br
Entry RX
Potential
Time
Ratio of
Yield
(%)c
(V vs. SCE)a (h)
2/3/4b
1
2
3
4
5
n-C3F7I
–1.00
3
3
84:16:trace
84:16:trace
57:24:19
88
n-C4F9I
c-C6F11I
CCl3Br
i-C3H7I
77
3
48 (27)
15
15
83:17: trace 78
n.r.d
–
Next, different radical precursors were employed in H2O
(Table 2). Reactions with primary perfluoroalkyl radical
sources, n-C3F7I and n-C4F9I, were also facile (Table 2,
entries 1 and 2). The formation of the polarity-matched
perfluoroalkylation products 3b and 3c slightly decreased,
a Reduction potential in MeCN.
b Determined by 1H NMR analysis. Compounds 2b–e were obtained
in a cis/trans ratio of ca. 9:1–6:4.
c Combined yield. The yield in parentheses is for the recovered sub-
strate.
d n.r. = no reaction.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1578–1582

1580
E. Yoshioka et al.
LETTER
id.19 However, the ruthenium catalyst could not cleave the
C–I bond of i-PrI (Table 2, entry 5).
R
O
O
Ru(bpy)3Cl2⋅6H2O (5 mol%)
RI (5.0 equiv), (i-Pr)2NEt
NOBn
NOBn
I
To understand the generality of this method, we next ex-
plored the reaction of substrates 5 and 6 having a carbon–
carbon triple bond (Scheme 3). The reaction of alkyne 5
with i-C3F7I proceeded with high regioselectivity. The
i-C3F7 radical exclusively added to the electron-deficient
alkene moiety of 5 to give the polarity-mismatched perflu-
oroalkylation products (Z)-6 and (E)-6 in 79% combined
yield and 88:12 ratio. The high regioselectivity was main-
tained in the reaction of alkyne 7. The reaction of sub-
strate 9 having a carbon–carbon double bond and an
oxime ether group gave the iodinated adduct 10, although
the desired cyclic product was not formed. Interestingly,
the i-C3F7 radical did not added to the oxime ether group
but selectively reacted with the carbon–carbon double
bond, probably due to the electrophilic property of per-
fluoroalkyl radicals.
H2O, argon
LED lamp (1000 lm)
11
12a R = i-C3F7
70%, cis/trans = 81:19
12b R = n-C3F7
74%, cis/trans = 66:34
O
R
O
Ru(bpy)3Cl2⋅6H2O (5 mol%)
RI (5.0 equiv), (i-Pr)2NEt
NOBn
NOBn
H2O, argon
LED lamp (1000 lm)
13
C
R
O
R
O
NOBn
NOBn
+
i-C3F7
O
O
Ru(bpy)3Cl2⋅6H2O (5 mol%)
i-C3F7I (5.0 equiv), (i-Pr)2NEt
NOBn
15b
14b
NOBn
R = n-C3F7
59%, 14b/15b = 58:42
H2O-CH3CN (9:1, v/v), argon
LED lamp (1000 lm)
H
I
Scheme 4 Reaction of substrates 11 and 13
5
6 79%, Z/E = 88:12
because the oxidation potential of the tert-butyl radical
predicted by a quantum-chemical calculation is –0.07 V
[Me3C+/Me3C•],20 which is sufficiently negative com-
pared to the reduction potentials of [Ru(bpy)3]3+ and
i-C3F7
O
O
Ru(bpy)3Cl2⋅6H2O (5 mol%)
i-C3F7I (5.0 equiv), (i-Pr)2NEt
NOBn
I
NOBn
[Ru(bpy)3]2+*
.
H2O, argon
LED lamp (1000 lm)
This consideration indicates that the cationic intermediate
D should be formed in the absence of an amine additive.
Therefore, the reaction was checked without (i-Pr)2NEt
(Scheme 5). When 20 mol% of Ru(bpy)3Cl2·6H2O and ten
equivalents of i-C3F7I were employed in 1 M NaHCO3–
H2O, the reaction proceeded slowly in the absence of
(i-Pr)2NEt. This reaction is a ‘real catalytic radical reac-
7
8 80%, Z/E = 76:24
i-C3F7
I
BnO
BnO
N
O
N
O
Ru(bpy)3Cl2⋅6H2O (5 mol%)
i-C3F7I (5.0 equiv), (i-Pr)2NEt
H2O, argon
LED lamp (1000 lm)
9 was recovered in 37% yield
i-C3F7
O
i-C3F7
O
Ru(bpy)3Cl2⋅6H2O
(20 mol%)
i-C3F7I (10 equiv)
10 35%
9
NOBn
NOBn
13
+
Scheme 3 Reaction of substrates 5, 7, and 9
1 M NaHCO3–H2O
argon, 15 h
LED lamp (1000 lm)
To gain further insight into the present photocatalysis, the
substrates of choice were 11 and 13, which showed low
reactivities toward the Et3B-initiated reaction (Scheme 4).
The use of the photocatalytic method improved the chem-
ical efficiency. The cyclic products 12a and 12b were ob-
tained in good yields. The photocatalysis of 13 proceeded
effectively. In marked contrast, the formation of cyclic
products was not observed in the Et3B-initiated reaction of
13, probably due to the less effective iodine-atom transfer
from primary n-C3F7I to stable tertiary alkyl radical C.
The formation of the products 14b and 14b having a C–C
double bond would result from the oxidation of inter-
52% (14a/15a = 69:31)
14a
15a
•
i-C3F7
i-C3F7I
radical C
cation D
[Ru(bpy)]3+
R
O
photoredox cycle 2
NOBn
LED
[Ru(bpy)]2+
[Ru(bpy)]2+
*
photoredox cycle 1
+
[Ru(bpy)]+
•
radical C
cation D
i-C3F7
cation D
i-C3F7I
mediate radical C with [Ru(bpy)]3+ and/or [Ru(bpy)]2+*
,
Scheme 5 Reaction of 13 in the absence of (i-Pr)2NEt
Synlett 2013, 24, 1578–1582
© Georg Thieme Verlag Stuttgart · New York

LETTER
Photocatalytic Cascade Carbon–Carbon Bond-Forming Radical Reaction
1581
tion’ without depending on the radical-chain processes
such as an iodine-atom transfer. Two acceptable photore-
dox cycles are shown in Scheme 5. This reaction would
proceed via both the photoredox cycles, considering the
redox potentials of i-C3F7I and ruthenium species playing
the role of electron donor and acceptor.
References and Notes
(1) Anastas, P. T.; Warner, J. C. In Green Chemistry Theory and
Practice; Oxford University Press: New York, 1998.
(2) Li, C.-J.; Chan, T.-H. In Comprehensive Organic Reactions
in Aqueous Media; John Wiley and Sons: Hoboken, 2007.
(3) (a) Kobayashi, S. Pure Appl. Chem. 2007, 79, 235. (b) Li,
C.-J. Chem. Rev. 2005, 105, 3095.
A favorable experimental feature of photocatalysis is that
the reactions proceed in aqueous media under mild reac-
tion conditions, providing a C–C bond-forming method.
We are now initiating a program aimed at studying the
real catalytic radical processes in great detail, which are
not depending on the radical-chain processes but on the
redox processes.
(4) Perchyonok, V. T. In Radical Reactions in Aqueous Media;
RSC Green Chemistry No. 6, RSC Publishing: Cambridge,
2010.
(5) Yorimitsu, H.; Shinokubo, H.; Oshima, K. Synlett 2002, 674.
(6) (a) Ueda, M.; Miyabe, H.; Nishimura, A.; Miyata, O.;
Takemoto, Y.; Naito, T. Org. Lett. 2003, 5, 3835. (b) Huang,
T.; Keh, C. C. K.; Li, C. J. Chem. Commun. 2002, 2440.
(7) For some reviews, see: (a) Shi, L.; Xia, W. Chem. Soc. Rev.
2012, 41, 7687. (b) Ischay, M. A.; Yoon, T. P. Eur. J. Org.
Chem. 2012, 3359. (c) Tucker, J. W.; Stephenson, C. R. J. J.
Org. Chem. 2012, 77, 1617. (d) Narayanam, J. M. R.;
Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102.
(8) (a) Nicewicz, D. A.; MacMillan, D. W. C. Science 2008,
322, 77. (b) Ischay, M. A.; Anzovino, M. E.; Du, J.; Yoon,
T. P. J. Am. Chem. Soc. 2008, 130, 12886. (c) Narayanam, J.
M. R.; Tucker, J. W.; Stephenson, C. R. J. J. Am. Chem. Soc.
2009, 131, 8756. (d) Larraufie, M.-H.; Pellet, R.;
Typical Procedure for the Reaction with i-C3F7I
To a suspension of substrate 1, 5, 7, 9, 11, or 13 (1.00 mmol) in
H2O–MeCN (9:1, v/v, 10 mL) or H2O (10 mL) was added
(i-Pr)2NEt (192 μL, 1.10 mmol), then the suspension was degassed
using three pump-thaw cycles under argon atmosphere at 0 °C.
Ru(bpy)3Cl2·6H2O (32 mg, 0.050 mmol) and i-C3F7I (705 μL, 5.00
mmol) were added to the suspending solution at r.t. and irradiated
with white LED lamp (1000 lm). After being stirred for 3 h, the re-
action mixture was concentrated under reduced pressure. Purifica-
tion of the residue by flash silica gel column chromatography
(EtOAc–hexane = 1:10–1:4) afforded the products.
Fensterbank, L.; Goddard, J.-P.; Lacôte, E.; Malacria, M.;
Ollivier, C. Angew. Chem. Int. Ed. 2011, 50, 4463. (e) Lin,
S.; Ischay, M. A.; Fry, C. G.; Yoon, T. P. J. Am. Chem. Soc.
2011, 133, 19350. (f) McNally, A.; Prier, C. K.; MacMillan,
D. W. C. Science 2011, 334, 1114. (g) Maity, S.; Zhu, M.;
Shinabery, R. S.; Zheng, N. Angew. Chem. Int. Ed. 2012, 51,
6119. (h) Miyake, Y.; Nakajima, K.; Nishibayashi, Y. J. Am.
Chem. Soc. 2012, 134, 19350.
The Analytical Data of Typical Compounds 14a and 15a
Product 14a
1
A colorless oil. IR (KBr): 2962, 2931, 1723, 1646, 1456 cm–1. H
NMR (400 MHz, CDCl3): δ = 7.45–7.35 (5 H, m), 5.06 (1 H, d, J =
11.0 Hz), 4.94 (1 H, d, J = 11.0 Hz), 3.89 (1 H, d, J = 12.1 Hz), 3.82
(1 H, d, J = 12.1 Hz), 2.78 (1 H, t, J = 16.0 Hz), 2.56 (1 H, t, J = 16.0
Hz), 1.73 (3 H, s), 1.52 (3 H, s), 1.40 (3 H, s). 13C NMR (100 MHz,
CDCl3): δ = 171.4, 135.0, 130.2, 129.4, 128.8, 128.5, 122.7, 120.8
(qd, J = 290, 28 Hz), 120.6 (qd, J = 290, 28 Hz), 91.1 (dsep, J = 209,
33 Hz), 76.7, 49.8 (d, J = 3 Hz), 43.8 (d, J = 4 Hz), 33.5 (d, J = 18
Hz), 26.9, 21.2, 20.7. 19F NMR (376 MHz, CDCl3): δ = –77.6 (3 F,
m), –77.9 (3 F, m), –189.5 (1 F, m). HRMS (ESI+): m/z calcd for
C19H21F7NO2Na [M + Na+]: 450.1274; found: 450.1304.
(9) For photocatalysis of halogen compounds, see: (a) Tucker, J.
W.; Nguyen, J. D.; Narayanam, J. M. R.; Krabbe, S. W.;
Stephenson, C. R. J. Chem. Commun. 2010, 46, 4985.
(b) Shih, H.-W.; Wal, M. N. V.; Grange, R. L.; MacMillan,
D. W. C. J. Am. Chem. Soc. 2010, 132, 13600. (c) Nguyen,
J. D.; D’Amato, E. M.; Narayanam, J. M. R.; Stephenson, C.
R. J. Nat. Chem. 2012, 4, 854. (d) Andrews, R. S.; Becker, J.
J.; Gagné, M. R. Angew. Chem. Int. Ed. 2012, 51, 4140.
(e) Freeman, D. B.; Furst, L.; Condie, A. G.; Stephenson, C.
R. J. Org. Lett. 2012, 14, 94.
(10) For the photocatalysis using perfluoroalkyl iodides, see:
(a) Nagib, D. A.; Scott, M. E.; MacMillan, D. W. C. J. Am.
Chem. Soc. 2009, 131, 10875. (b) Pham, P. V.; Nagib, D. A.;
MacMillan, D. W. C. Angew. Chem. Int. Ed. 2011, 50, 6119.
(c) Nagib, D. A.; MacMillan, D. W. C. Nature (London)
2011, 480, 224.
(11) For the photocatalysis in aqueous media, see: (a) Nguyen, J.
D.; Tucker, J. W.; Konieczynska, M. D.; Stephenson, C. R.
J. J. Am. Chem. Soc. 2011, 133, 4160. (b) Rueping, M.; Zhu,
S.; Koenigs, R. M. Chem. Commun. 2011, 47, 8679.
(c) Tucker, J. W.; Zhang, Y.; Jamison, T. F.; Stephenson, C.
R. J. Angew. Chem. Int. Ed. 2012, 51, 4144. (d) Zen, J. M.;
Liou, S. L.; Kumar, A. S.; Hsia, M. S. Angew. Chem. Int. Ed.
2003, 42, 577.
Product 15a
1
A colorless oil. IR (KBr): 2932, 2869, 1725, 1688, 1456 cm–1. H
NMR (400 MHz, CDCl3): δ = 7.45–7.36 (5 H, m), 5.04 (1 H, d, J =
11.2 Hz), 4.96 (1 H, br s), 4.95 (1 H, d, J = 11.2 Hz), 4.69 (1 H, br
s), 3.33–3.25 (2 H, m), 3.08 (1 H, t, J = 7.8 Hz), 2.57 (1 H, dd, J =
28.9, 16.4 Hz), 2.26 (1 H, dd, J = 16.4, 10.1 Hz), 1.74 (3 H, s), 1.02
(3 H, s). 13C NMR (100 MHz, CDCl3): δ = 172.3, 140.5, 135.2,
129.4, 128.9, 128.6, 120.7 (qd, J = 288, 28 Hz), 120.6 (qd, J = 288,
29 Hz), 114.7, 91.9 (dsep, J = 209, 32 Hz), 76.9, 47.7, 43.8, 41.5 (d,
J = 5 Hz), 33.0 (d, J = 18 Hz), 23.1 (d, J = 3 Hz), 20.6. 19F NMR
(376 MHz, CDCl3): δ = –75.9 (3 F, br quin, J = 10 Hz), –78.1 (3 F,
m), –185.1 (1 F, m). HRMS (ESI+): m/z calcd for C19H20F7NO2Na
[M + Na+]: 450.1274; found: 450.1268.
(12) (a) Balzani, V.; Bolletta, F.; Gandolfi, M. T.; Maestri, M.
Top. Curr. Chem. 1978, 75, 1. (b) Campagna, S.; Puntoriero,
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J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.;
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4815. (e) Lin, C.-T.; Böttcher, W.; Chou, M.; Creutz, C.;
Sutin, N. J. Am. Chem. Soc. 1976, 98, 6536.
Acknowledgment
This work was supported in part by Grant-in-Aid for Scientific Re-
search (C) (H.M.) and for Young Scientists (B) (E.Y.) from the Mi-
nistry of Education, Culture, Sports, Science and Technology of
Japan.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
(13) Yoshioka, E.; Kohtani, S.; Sawai, K.; Kentefu; Tanaka, E.;
Miyabe, H. J. Org. Chem. 2012, 77, 8588.
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ungIifoop
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© Georg Thieme Verlag Stuttgart · New York
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LETTER
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Synlett 2013, 24, 1578–1582
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