BODIPY-hexaphyrin hybrids

Article abstract of DOI:10.1002/chem.200902407

Expand your horizon: BODIPY-appended [28]-and [26]hexaphyrins (see figure) were prepared and the efficient intramolecular excitation energy transfer from the BODIPY part to the hexaphyrin part was revealed in both hybrids. Importantly this is the first ex

Full text of DOI:10.1002/chem.200902407

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DOI: 10.1002/chem.200902407
BODIPY–Hexaphyrin Hybrids**
Ji-Young Shin,[a] Takayuki Tanaka,[b] Atsuhiro Osuka,*[b] Qing Miao,[a] and
David Dolphin*[a]
Currently, expanded porphyrins have emerged as a prom-
ising class of molecules in light of their interesting structur-
al, electronic, and coordination properties.[1] Among these,
meso-aryl-substituted [26]hexaphyrins have been most ex-
tensively studied, since they exhibit intriguing attributes,
such as rectangular molecular shapes, strong aromaticity,
large two-photon absorption cross sections, and versatile
metal-coordinating abilities, which allow for multiple metal
complexes with interesting properties.[1g,2,3] As a further step
in the use of the hexaphyrin moiety for the creation of func-
tional molecules, hybrid systems are conceivable, in which
the hexaphyrin segment is linked with other electronically
active groups. However, synthetic efforts along such this
path has been rather limited to date.[4] In this paper, we
report the synthesis and characterization of boron dipyrro-
methene (BODIPY)-hexaphyrin hybrids. BODIPY dyes
have advantageous properties,[5] such as high molar-extinc-
tion coefficients, high fluorescence quantum yields, and rea-
sonably long excited singlet-state lifetimes and thermal/pho-
tochemical stability. As a result they have been extensively
used as biological labels,[5c] fluorescent sensors,[5d] synthetic
light harvesting antenna,[5e] and as a drug delivery re-
agent.[5f]
under unique reaction conditions,[2] this was followed with
our improved synthesis that provided 1 in much better
yields along with a set of other meso-pentafluorophenyl-sub-
stituted expanded porphyrins under modified Rothemund–
Lindsey conditions.[3] Hexaphyrins 1 and 2 are both in stable
oxidation states, which can be interconverted through a two-
electron reduction with NaBH4 and a two-electron oxidation
with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
(Scheme 1). Although 1 is a purple-colored aromatic macro-
Scheme 1. Interconversions between meso-pentafluorophenyl [26]-hexa-
phyrin 1 and meso-pentafluorophenyl [28]hexaphyrin 2.
cycle showing a strong diatropic ring current along its rec-
tangular molecular shape, the moderate diatropic ring cur-
rent of [28]hexaphyrin 2 has long been puzzling, since it
should be antiaromatic in its planar, rectangular conforma-
tion. Recently, we have shown that there is a fast dynamic
equilibrium among twisted Mçbius aromatic conformers and
antiaromatic rectangular conformers for 2 in solution at
room temperature, and that these two different types of con-
formers are energetically close but the formers are slightly
In 1999, Cavaleiroꢀs group reported the first synthesis of
meso-hexakis(pentafluorophenyl) substituted [26] and
[28]hexaphyrins, 1 and 2, in an overall yield of about 1%,
[a] Dr. J.-Y. Shin, Q. Miao, Prof. D. Dolphin
Department of Chemistry
University of British Columbia
2036 Main Mall, Vancouver, BC V6T 1Z1 (Canada)
Fax : (+1)604-822-9678
1
more stable.[6] Thus, the H NMR spectrum at room temper-
ature represents a snapshot of the averaged such fast confor-
mational dynamics. In accordance with this interpretation,
lowering the temperature to 173 K showed 2 predominantly
takes a twisted conformation having a diatropic ring current,
which has been assigned as a Mçbius aromatic species.[6a,7]
The formation of hexaphyrin hybrids is complicated by
the fact that meso-aryl [26]- and [28]hexaphyrins are only
stable when the meso-aryl substituents are 2,6-disubstituted.
In fact, meso-hexaphenyl-substituted [26]hexaphyrin-
E-mail: david.dolphin@ubc.ca
[b] T. Tanaka, Prof. A. Osuka
Department of Chemistry
Kyoto University
Oiwake-cho, Kitashirakawa, Sakyo-Ku, Kyoto, 606-852 (Japan)
Fax : (+81)075-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
[**] BODIPY=boron dipyrromethene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902407.
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ACHTUNGTRENNUNG
(1.1.1.1.1.1) has been shown to be particularly unstable.[8]
We found a rare exception, however, in that 5,20-diphenyl-
10,15,25-30-tetrakis(pentafluorophenyl)-substituted
[26]hexaACHTUNGTRENNUNGphyrin is surprisingly stable, taking a rectangular
conformation having two small meso-phenyl substituents at
the short side.[9]
With these results in hand, a mixture of the formyl-
BODIPY compound 3 (Scheme 2) and tripyrrane 4 was
treated with BF3·OEt2 at low temperature under anaerobic
Figure 1. 19F NMR spectral comparison between a) 8 in CDCl3, b) 6 in
[D8]THF at 298 K, and c) 6 in [D2]1,1,2,2-tetrachloroethane at 373 K.
373 K at which temperature it is considered that [28]hexa-
phyrin 6 undergoes rapid conformational interconversion
that is faster than 19F NMR time-scale. As a result, the two
BODIPY units of 6 are considered to be linked at the
longer side of hexaphyrin with an averaged C2h symmetry
(Scheme 2).
1
The H NMR spectra of 6 and 8 are consistent with the
1
above structural assignments. The H NMR spectrum of 8 in
CDCl3 exhibits two doublets for the outer pyrrolic b-protons
at d=9.43 and 9.25 ppm, two doublets for the phenylene
protons at d=8.53 and 8.08 ppm, two doublets for the
BODIPY pyrrolic protons at d=7.16 and 6.50 ppm, a singlet
for the BODIPY methyl protons at d=2.79 ppm and a sin-
glet for the inner pyrrolic b-protons at d=À2.52 ppm, re-
spectively (Figure 2). These spectral data are fully consistent
with the rectangular shape placing two BODIPY groups at
the short side. The 1H NMR spectrum (Figure 3) of
Scheme 2. Formation of the Hexaphyrin–BODIPY hybrids 6–8:
a) 1. CH2Cl2, 2.5m BF3·OEt2 (CH2Cl2), 1.5 h, 08C, Ar; 2. DDQ;
b) CH2Cl2, DDQ.
conditions and the resulting mixture was oxidized with
DDQ. After a standard work-up, chromatographic separa-
tion on a silica gel column and recrystallization, [28]hexa-
phyrin 6 was obtained as a dark shiny solid in a 32% yield.
Similarly, [28]hexaphyrin 7 was prepared from the reaction
of 3 and 5 in a 43% yield. Finally, the further oxidization of
6 with DDQ quantitatively gave [26]hexaphyrin 8.
[28]hexaACHTNUGRTNEUNGphyrin 6 in CD2Cl2 presents broad singlets for the
outer and inner pyrrolic NH protons at d=8.36 and
5.11 ppm, four signals for the outer b-protons at d=7.74,
7.72, 7.68, and 7.51 ppm, signals for the phenylene protons
at d=7.58 ppm, two doublets for the BODIPY pyrrolic pro-
tons at d= 6.70 and 6.32 ppm, and a singlet for the
BODIPY methyl protons at d=2.61 ppm, respectively. The
inner b-protons of hexaphyrin are observed as two highly
shielded signals at d=2.98 ppm (Figure 3a) and split as two
High-resolution electrospray ionization time-of-flight (HR
ESI-TOF) mass spectroscopy revealed the parent ion peaks
À
of 6 and 8 at m/z 1717.3539 (calcd for C88H43B2F24N10 [M
H]À: 1717.3505) and m/z 1715.3372 (calcd for C88H41B2F24N10
[M H] : 1715.3348, respectively. The 19F NMR spectra of 6
À
À
and 8 show different patterns (Figure 1). The 19F NMR spec-
trum of 8 displays single sets of signals for the fluorine
atoms in the pentafluorophenyl groups and a quartet signal
for the fluorine atoms in the BODIPY part, indicating its
D2h symmetry. This evidence strongly suggests that 8 takes a
rectangular conformation where the two BODIPY units are
attached on the short sides. On the other hand, the
19F NMR spectrum of 6 shows two sets of signals in a 1:1
ratio for the fluorine atoms of the pentafluorophenyl groups
(Figure 1b). This spectral feature did not change even at
Figure 2. 1H NMR spectrum of 8 in CDCl3 at 298 K: * indicates solvent
or impurity peaks.
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BODIPY–Hexaphyrin Hybrids
COMMUNICATION
Figure 3. 1H NMR spectra of 6 in a) [D6]DMSO and b) CD2Cl2 at 298 K:
* indicates solvent and its side peaks.
slightly shielded broad signals in [D6]DMSO, which support
rapid interconversions among the twisted Mçbius aromatic
and antiaromatic-rectangular conformations. Solvent plays a
key in the conformational dynamics. As shown in Figure 3a,
the NH peaks are significantly downfield shifted owing to
hydrogen bonding between DMSO and the amino hydro-
gen’s. As expected, the 1H NMR spectra of 6 in [D8]THF
shows that the conformational interconversions slow down
at reduced temperature (Figure S8 in the Supporting Infor-
mation).
As shown in Figure 4, the optical spectra of [28]hexaphyr-
in 6 and the [26]hexaphyrin 8 show bands at l=515 and
614 nm and at l=514 and 576 nm, corresponding to the ab-
sorption band of BODIPY and the Soret-like band of hexa-
phyrin, respectively, which are red-shifted from the original
bands of BODIPY (l=510 nm), [28]hexaphyrin 2 (l=
590 nm), and [26]hexaphyrin 1 (l=567 nm). The observed
larger red-shift for the Soret-like band of 6 may not necessa-
rily indicate a larger electronic interaction between the
[28]hexaphyrin and BODIPY segments, but rather the non-
negligible influence of the attached BODIPY groups upon
the conformational dynamics of the [28]hexaphyrin moiety.
In line accordance with this conjecture, variable-tempera-
ture optical spectra of 6 in the range of 173–298 K showed
typical spectral changes for a [28]hexaphyrin (Figure S23 in
the Supporting Information). Thus, the Soret-like absorption
band of the [28]hexaphyrin segment was sharpened and red-
shifted by 14 nm upon lowering the temperature from 298 K
to 173 K. Similar temperature-dependent spectral changes
were also observed for 7 (Figure S24 in the Supporting In-
formation). On the other hand, the temperature-dependent
red shift was only modest for 8, in line with the conforma-
tional rigidity of the [26]hexaphyrin segment.[6b]
Figure 4. Optical spectra of a) 1 (c) and 8 (c) and b) 2 (c) and 6
(c) with a spectrum of reference BODIPY in CH2Cl2 (normalized at
absorption maxima); absorption (a) and emission (b).
MTHF). Upon photoexcitation at the Soret-like band of the
hexaphyrin segment, the fluorescence of 6 was observed to
be broad, showing peaks at l=1055 and 1235 nm at 298 K
but became sharper, showing peaks l=1014, 1164, and
1198 nm upon lowering the temperature to 77 K. This tem-
perature dependence is analogous to that of 2, indicating
the restricted conformational dynamics that favor the twist-
ed Mçbius conformation at low temperature. On the other
hand, the fluorescence of 8 is relatively sharp with a peak at
l=1029 nm even at 298 K and becomes even sharper with
peaks at l=1016, 1165, and 1203 nm at 77 K. Intramolecular
singlet excitation energy transfer from the BODIPY seg-
ments to the hexaphyrin segment was examined by compar-
ing the fluorescence intensities upon excitation at the peak
of the Soret-like band of the hexaphyrin segment versus at
the peak of the BODIPY absorption. Figure 5 shows the
fluorescence spectra of 6 and 8 taken upon excitation at in-
dicated wavelengths with adjusted absorbance so that the
optical densities of the two excitation positions are identical.
It is important to note that illumination at l>570 nm only
excites the hexaphyrin segment, but that at the peak of the
BODIPY absorption excites both the chromophores. By
comparing the absorbances of the reference molecules, the
ratios of the absorptions were estimated to BODIPY (87%)
and [28]hexaphyrin (13%) for
6 at l=509 nm, and
BODIPY (76%) and [26]hexaphyrin (24%) for 8 at l=
512 nm, respectively (Figure S27 and S28 in the Supporting
Information). On the basis of these data, the relative fluo-
rescence intensities for the hexaphyrin unit upon the
The fluorescence spectra of 6 and 8 (Figure 5) were mea-
sured at 298 K and 77 K in 2-methyltetrahydrofuran (2-
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D. Dolphin, A. Osuka et al.
quantitatively. Purification through a
short silica gel column was sufficient
to obtain pure 8 with violet color.
Compound 6: 1H NMR (600 MHz,
[D6]DMSO, 298 K): d=9.94 (bs, outer-
NH, 2H), 7.58 (d, J=8.0 Hz, 4H,
phenylene), 7.48 (d, 2H, J=8.0 Hz,
phenylene), 7.43 (bs, 2H, hexa-bCH),
7.29 (bs, 2H, hexa-bCH), 7.16 (bs, 2H,
hexa-bCH), 7.10 (bs, 2H, hexa-bCH),
6.85 (d, J=4.2 Hz, 4H, BODIPY-
bCH), 5.10 (d, J=4.2 Hz, 4H,
BODIPY-bCH), 6.11 (bs, 2H, inner
bCH of hexaphyrin), 5.60 (bs, 2H,
inner bCH of hexaphyrin), 2.90 (bs,
4H, inner bCH of hexaphyrin),
2.59 ppm (s, 12H, CH3); 19F NMR
(564.73 MHz, [D8]THF, 298 K): d=
À139.53 (d, J=19.0 Hz, 4F, C6F5-
ortho), À140.24 (d, J=19.0 Hz, 4F,
C6F5-ortho), À148.13 (q, J=31.1 Hz,
4F, BF2), À156.40 (t, J=20.7 Hz, C6F5-
para), À156.95 (t, J=20.7 Hz, 2F,
C6F5-para), À164.21 (m, 4F, C6F5-
meta), À164.38 ppm (m, 4F, C6F5-
meta); HR-ESI-MS: m/z: calcd for
Figure 5. Fluorescence spectra of 6 (top) and 8 (bottom) at room temperature (left) and 77 K (right) in 2-
methyltetrahydrofuran measured upon photoexcitation at the Soret region of hexaphyrin (c) and BODIPY
(a).
C88H43B2F24N10:1717.3505
[MÀH]À;
found: 1717.3539; UV/Vis (MTHF)
BODIPY-excitation versus the direct hexaphyrin-excitation
have been determined to be about 67% for 6 and 39% for
8 at 298 K, which can be regarded as the extent of the intra-
molecular energy transfer from the BODIPY to the hexa-
phyrin part.
lmax(e)=513 (144000), 620 (135000), 772 nm (14000mÀ1 m3 cmÀ1).
Compound 7: 1H NMR (600 MHz, [D6]DMSO, 298 K): d=9.10 (bs, 2H,
outer NH), 7.78 (d, J=8.4 Hz, 4H, Ar), 7.64 (t, J=8.0 Hz, 2H, Ar), 7.57
(d, J=8.4 Hz, 4H, Ar), 7.50 (d, J=7.9 Hz, 4H, Ar), 7.41 (d, J=4.4 Hz,
2H, hexa-bCH), 7.38 (m, 4H, Ar), 6.91 (d, J=4.4 Hz, 2H, hexa-bCH),
6.84 (d, J=4.1 Hz, 4H, BODIPY-bCH), 6.65 (d, J=4.4 Hz, 2H, hexa-
bCH), 6.57 (d, J=4.1 Hz, 4H, BODIPY-bCH), 6.49 (d, J=4.4 Hz, 2H,
hexa-bCH), 6.16 (bs, 2H, inner bCH of hexaphyrin), 5.58 (bs, 2H, inner
bCH of hexaphyrin), 2.59 ppm (s, 12H, CH3); HR-ESI-MS: m/z: calcd
for C88H56B2Cl8F4N10 :1634.2310 [M]+; found: 1634.2352; UV/Vis
(MTHF) lmax(e)=341 (52000), 512 (149000), 618 (84000), 780 (16000),
860 nm (15000mÀ1 dm3 cmÀ1).
Compound 8: 1H NMR (600 MHz, CDCl3, 298 K) d=9.51 (d, J=4.6 Hz,
4H, hexa-bCH), 9.25 (d, J=4.6 Hz, 4H, hexa-bCH), 8.53 (d, J=8.0 Hz,
4H, Ar), 8.09 (d, J=8.0 Hz, 4H, Ar), 7.16 (d, J=3.9 Hz, 4H, BODIPY-
bCH), 6.50 (d, 4H, J=3.9 Hz, BODIPY-bCH), 2.79 ppm (s, 12H, CH3);
19F NMR (564.73 MHz; CDCl3) d=À136.93 (d, J=18.1 Hz, 8F, C6F5-
ortho), À147.31 (q, J=31.9 Hz, 4F, BF2), À152.97 (t, J=20.7 Hz, 4F,
C6F5-para), À163.07 ppm (t, J=18.1 Hz, 8F, C6F5-meta); HR-ESI-MS:
m/z: calcd for C88H41B2F24N10 :1715.3348 [MÀH]À; found: 1715.3372;
UV/Vis (MTHF) lmax(e)=512 (135000), 573 (163000), 721 nm
(17000mÀ1 m3 cmÀ1).
In summary, the facile synthesis of BODIPY–hexaphyrin
hybrids 6, 7, and 8 was achieved from the formyl-substituted
BODIPY 3 and tripyrranes 4 and 5 in moderate yields. In
the hybrid 8, the two BODIPY parts are attached at the
short side of the rectangular [26]hexaphyrin segment, where-
as the [28]hexaphyrin segment in the hybrid 6 exhibits tem-
perature-dependent conformational dynamics similar to the
parent [28]hexaphyrin 2. In the hybrids, efficient singlet ex-
citation-energy transfer occurs from the BODIPY segments
to the [28]hexaphyrin or [26]hexaphyrin. It is to be noted
that the intramolecular-energy transfer in 6 and 7 is the first
example involving a Mçbius aromatic molecular part as the
energy acceptor. Detailed time-resolved photophysical prop-
erties of these compounds and advanced multi-hexaphyrin–
BODIPY arrays will be examined in the future.
Experimental Section
Acknowledgements
BODIPY–hexaphyrin hybrids 6–8: Compound 3 (114.5 mg, 353.3 mmol)
was dissolved in CH2Cl2 (12 mL) at 08C under Ar. To this solution,
BF3·OEt in CH2Cl2 (70 mL, 2.5m) was added and the resulting solution
Financial support for this work was provided by the Natural Sciences and
Engineering Research Council (NSERC) of Canada. This work was parti-
ally supported by Grants-in-Aid for Scientific Research (No. 19205006,
and 20108001 “pi-Space”) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. TT thanks the JSPS fellowship for
Young Scientists.
was stirred for 5 min.
A CH2Cl2 (8 mL) solution of 4 (179 mg,
321.3 mmol) was added and the solution was stirred for 2.5 h. DDQ
(149 mg) was then added and the resulting solution was stirred for
30 min. The reaction was quenched by the addition of water. The prod-
ucts were extracted with CH2Cl2 and separated by silica gel column chro-
matography using 1% MeOH/CH2Cl2 as eluent. A distinct blue fraction
was collected to give [28]hexaphyrin 6 in 32% yield. Similarly, 7 was pre-
pared from 5 in 43% yield under the same conditions. [28]Hexaphyrin 6
was oxidized with DDQ (1.5 equiv) at 08C to give [26]hexaphyrin 8
Keywords: aromaticity · BODIPY · expanded porphyrin ·
electron transfer · hexaphyrin
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BODIPY–Hexaphyrin Hybrids
COMMUNICATION
c) P. J. Emmerson, S. Archer, W. El-Hamouly, A. Mansour, H. Akil,
F. Medzilhradsky, Biochem. Pharmacol. 1997, 54, 1315–1322; d) G.
Ulrich, R. Ziessel, A. Harriman, Angew. Chem. 2008, 120, 1202–
1219; Angew. Chem. Int. Ed. 2008, 47, 1184–1201; e) R. W. Wagner,
J. S. Lindsey, Pure Appl. Chem. 1996, 68, 1373–1380; f) F. Camerel,
L. Bonardi, M. Schmutz, R. Ziessel, J. Am. Chem. Soc. 2006, 128,
4548–4549.
[1] a) J. L. Sessler, A. Gebauer, E. Vogel, The Porphyrin Handbook,
Vol. 2 (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Academic
Press, San Diego, 1999, Chapter 8; b) B. Franck, A. Nonn, Angew.
Chem. 1995, 107, 1941–1957; Angew. Chem. Int. Ed. Engl. 1995, 34,
1795–1811; c) A. Jasat, D. Dolphin, Chem. Rev. 1997, 97, 2267–2340;
d) T. D. Lash, Angew. Chem. 2000, 112, 1833–1837; Angew. Chem.
Int. Ed. 2000, 39, 1763–1767; e) H. Furuta, H. Maeda, A. Osuka,
Chem. Commun. 2002, 1795–1804; f) J. L. Sessler, D. Seidel, Angew.
Chem. 2003, 115, 5292–5333; Angew. Chem. Int. Ed. 2003, 42, 5134–
5175; g) T. K. Chandrashekar, S. Venkatraman, Acc. Chem. Res. 2003,
36, 676–691; h) S. Shimizu, A. Osuka, Eur. J. Inorg. Chem. 2006,
1319–1335.
[2] M. G. P. M. S. Neves, R. M. Martins, A. C. Tomꢂ, A. J. D. Silvestre,
A. M. S. Silva, V. Fꢂlix, M. G. B. Drew, J. A. S. Cavaleiro, Chem.
Commun. 1999, 385–386.
[3] a) J.-Y. Shin, H. Furuta, K. Yoza, S. Igarashi, A. Osuka, J. Am. Chem.
Soc. 2001, 123, 7190–7191; b) M. Suzuki, S. Shimizu, J.-Y. Shin, A.
Osuka, Tetrahedron Lett. 2003, 44, 4597–4601; c) T. K. Ahn, J. H.
Kwon, D. Y. Kim, D. W. Cho, D. H. Jeong, S. K. Kim, M. Suzuki, S.
Shimizu, A. Osuka, D. Kim, J. Am. Chem. Soc. 2005, 127, 12856–
12861; d) S. Mori, S. Shimizu, J.-Y. Shin, A. Osuka, Inorg. Chem.
2007, 46, 4374–4376.
[6] a) J. Sankar, S. Mori, S. Saito, H. Rath, M. Suzuki, Y. Inokuma, H.
Shinokubo, K. S. Kim, Z. S. Yoon, J.-Y. Shin, J. M. Lim, Y. Matsuzaki,
O. Matsushita, A. Muranaka, N. Kobayashi, D. Kim, A. Osuka, J.
Am. Chem. Soc. 2008, 130, 13568–13579; b) K. S. Kim, Z. S. Yoon,
A. B. Ricks, J.-Y. Shin, S. Mori, J. Sankar, S. Saito, Y. M. Yung, M. R.
Wasielewski, A. Osuka, D. Kim, J. Phys. Chem. A 2009, 113, 4498–
4506.
´
´
[7] M. Ste˛pien, L. Latos-Graz˙ynski, N. Sprutta, P. Chwalisz, L. Szteren-
berg, Angew. Chem. 2007, 119, 8015–8019; Angew. Chem. Int. Ed.
2007, 46, 7869–7873.
[8] a) C. Brꢃckner, E. D. Sternberg, R. W. Boyle, D. Dolphin, Chem.
Commun. 1997, 1689–1690; b) M. Suzuki, A. Osuka, Org. Lett. 2003,
5, 3943–3946; c) Y.-S. Xie, K. Yamaguchi, M. Toganoh, H. Uno, M.
Suzuki, S. Mori, S. Saito, A. Osuka, H. Furuta, Angew. Chem. 2009,
121, 5604–5607; Angew. Chem. Int. Ed. 2009, 48, 5496–5499.
[9] M. Suzuki, A. Osuka, Chem. Eur. J. 2006, 12, 1754–1759.
[4] Y. Inokuma, A. Osuka, Org. Lett. 2004, 6, 3663–3666.
[5] a) A. Loudet, K. Burgess, Chem. Rev. 2007, 107, 4891–4932; b) R.
Raymond, G. Ulrich, A. Harriman, New J. Chem. 2007, 31, 496–501;
Received: August 31, 2009
Published online: November 2, 2009
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