Synthesis and basic properties of tetrathieno[2,3-a:3′,2′-c: 2″,3″-f:3a€?,2a€?-h]naphthalene: A new π-conjugated system obtained by photoinduced electrocyclization- dehydrogenation reactions of tetra(3-thienyl)ethene

Article abstract of DOI:10.1016/j.tetlet.2013.05.090

A method for the synthesis of tetrathieno[2,3-a:3′,2′-c: 2″,3″-f:3a€?,2a€?-h]naphthalene (3), utilizing photoinduced electrocyclization-dehydrogenation reactions of tetra(3-thienyl)ethene (1), was developed. Photoirradiation of a toluene or CHCl₃ solution of 1, containing a small amount of I₂, leads to modestly efficient production of 3. In contrast to the UV-vis absorption property of the typical p-type organic transistor material pentacene, that of 3 does not experience a time-dependent change under aerated conditions, indicating that 3 has high stability against molecular oxygen. The results of X-ray crystallographic analysis demonstrate that 3 possesses a columnar crystalline structure in which molecules are aligned in a face-to-face manner with a high degree of the π-π overlap between adjacent molecules. This phenomenon should result in efficient charge-carrier transport properties of the crystalline form of this substance.

Full text of DOI:10.1016/j.tetlet.2013.05.090

Tetrahedron Letters 54 (2013) 4049–4053
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and basic properties of
tetrathieno [2,3-a:30,20-c:200,300-f:3000,2000-h]naphthalene: a new
-conjugated system obtained by photoinduced electrocyclization–
p
dehydrogenation reactions of tetra(3-thienyl)ethene
Atsushi Yamamoto a, Eisuke Ohta a,b, Nao Kishigami a, Norihiro Tsukahara c, Yusuke Tomiyori a,
Hiroyasu Sato d, Yasunori Matsui a, Yusuke Kano a, Kazuhiko Mizuno a,b, Hiroshi Ikeda a,b,
⇑
a Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
b The Research Institute for Molecular Electronic Devices (RIMED), Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
c Department of Applied Chemistry, Faculty of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
d Rigaku, 3-9-12 Matsubara-cho, Akishima, Tokyo 196-8666, Japan
a r t i c l e i n f o
a b s t r a c t
A method for the synthesis of tetrathieno[2,3-a:30,20-c:200,300-f:3000,2000-h]naphthalene (3), utilizing photoin-
duced electrocyclization–dehydrogenation reactions of tetra(3-thienyl)ethene (1), was developed. Phot-
oirradiation of a toluene or CHCl3 solution of 1, containing a small amount of I2, leads to modestly
efficient production of 3. In contrast to the UV–vis absorption property of the typical p-type organic tran-
sistor material pentacene, that of 3 does not experience a time-dependent change under aerated condi-
tions, indicating that 3 has high stability against molecular oxygen. The results of X-ray crystallographic
analysis demonstrate that 3 possesses a columnar crystalline structure in which molecules are aligned in
a face-to-face manner with a high degree of the p–p overlap between adjacent molecules. This phenom-
enon should result in efficient charge-carrier transport properties of the crystalline form of this
substance.
Article history:
Received 22 April 2013
Revised 16 May 2013
Accepted 20 May 2013
Available online 24 May 2013
Keywords:
Thiophene-fused aromatics
6p-Electrocyclization
Photoreaction
Single electron transfer reaction
Ó 2013 Elsevier Ltd. All rights reserved.
Introduction
Among many of the developed approaches, a 6p-electrocycliza-
tion process followed by oxidative dehydrogenation of the photo-
Materials used in organic field-effect transistors (OFETs) need to
be readily prepared and durable. Pentacene, a common material
employed in OFETs,1,2 suffers from problems associated with its
relatively short lifetime, especially when molecular oxygen is pres-
ent in its environment.3 One strategy developed to circumvent the
problem associated with the instability of fused aromatic hydro-
carbons in air, involves the use of annulated thiophenes in OFETs.
Owing to their relatively low reactivity with molecular oxygen, thi-
ophene-fused aromatic compounds are widely used as organic
optoelectronic materials. For example, 2,7-diphenyl[1]benzothie-
no[3,2-b][1]benzothiophene (DPh-BTBT, Chart 1)4,5 and dinaph-
tho[2,3-b:20,30-f]thieno[3,2-b]thiophene (DNTT),6–8 both prepared
by Takimiya, have attracted great attention as organic transistor
materials as a consequence of (1) high charge carrier mobilities
of them comparable to that of pentacene and (2) stabilities against
molecular oxygen of them much higher than that of pentacene.
product is a unique strategy for the synthesis of thiophene-fused
p
-conjugated systems. Applications of this process are exemplified
by the synthesis of tetrathieno[3,2-a:20,30-c:300,200-f:2000,3000-h]naph-
thalene devised by Harrit.9 Recently, Osuga employed this ap-
proach to prepare new substances that have potential
applications as organic electroluminescence materials.10
Inspired by Harrit’s early effort leading to the preparation of the
uniquely p-conjugated tetrathienonaphthalene, and ensuing stud-
ies of derivatives of this substance by Yamaguchi and co-work-
ers,11 we developed a plan for the synthesis of the regioisomeric
substance,
tetrathieno[2,3-a:30,20-c:200,300-f:3000,2000-h]naphthalene
⇑
Corresponding author. Tel./fax: +81 72 254 9289.
E-mail address: ikeda@chem.osakafu-u.ac.jp (H. Ikeda).
Chart 1. Structures of DPh-BTBT and DNTT.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.05.090

4050
A. Yamamoto et al. / Tetrahedron Letters 54 (2013) 4049–4053
Table 1
Photoinduced electrocyclization–dehydrogenation reactions of 1a
Solventb
kEX (nm)
Time (h)
Conv. (%)
Yields (%)
2
3
TOL
313c
350d
313c
350d
24
18
24
18
100
94
100
94
20
Trace
19
1
20
7
CHCl3
Scheme 1. Syntheses of 2 and 3 using a photoinduced electrocyclization–dehy-
drogenation sequence starting with 1.
Trace
23
a
b
c
Compound 1, 2.8 mmol; I2, 0.2 lmol (0.07 equiv).
6 mL.
High-pressure mercury lamp (500 W) with Pyrex cut filter.
Rayonet lamps (70 W).
(3, Scheme 1) from tetra(3-thienyl)ethene (1). We also envisaged
that 3 would have ideal optoelectronic properties and stabilities
against molecular oxygen. Owing to these predicted properties, 3
along with its ring-opened counterpart, 5,6-di(3-thienyl)dithie-
no[2,3-a:30,20-c]benzene (2), should be novel OFET materials. The
results of studies guided by these expectations show that 3 has a
small HOMO–LUMO gap, a low reactivity with molecular oxygen,
and a columnar-ordered crystalline structure that possesses a large
d
degree of intermolecular p–p overlap, and consequently that 3 is a
potential charge-carrier transport material.
Results and discussion
Preparation of 1
Tetrathienylethene 1, employed as a key synthetic intermediate
in the route for preparation of polycyclic thiophenes 2 and 3, was
generated using a sequence that is initiated by conversion of com-
mercially available 3-bromothiophene (4) to di(3-thienyl)metha-
nol (5) promoted by treatment with n-BuLi and methyl formate
(Scheme 2). Reaction of alcohol 5 under pyridinium chlorochro-
mate oxidation conditions12 yields di(3-thienyl) ketone (6), which
is then subjected to McMurry coupling13 using TiCl4 and Zn to pro-
duce 1 as colorless needles.
Scheme 3. Plausible reaction pathway for generation of 2 and 3 by photoinduced
electrocyclization–dehydrogenation of and potential reaction pathway for
formation of 9 and 10 by photoinduced dyotropic rearrangement from 7 and 8,
1
respectively.
the basis of their characteristic spectroscopic data and demon-
strated unambiguously using X-ray crystallographic analysis.
The photochemical transformation from 1 to 3 via 2 most likely
takes place through a stepwise pathway involving 7 and 8 as key
intermediates (Scheme 3). However, no direct evidence for the
existence of 7 and 8 in this pathway was obtained by monitoring
the reaction of 1 using UV–vis absorption and NMR spectroscopy.
This is probably due to the occurrence of efficient dehydrogenation
reactions from 7 to 8. The only moderate yields of 2 and 3 in these
processes might be a consequence of the formation of side prod-
ucts 9 and 10, which could arise by secondary photoinduced dyo-
tropic rearrangement14 of 7 and 8, respectively. However, no direct
evidence for the existence of 9 and 10 was also obtained in the
experiment.
Syntheses of 2 and 3
Studies were carried out to uncover the best conditions for the
syntheses of the polycyclic thiophenes 2 and 3 using tandem
photoinduced electrocyclization–dehydrogenation reactions of 1
(Table 1). Irradiation of a toluene (TOL) solution of 1 (4.7 ꢀ
10ꢁ2 M) and a small amount of I2 using a high-pressure mercury
lamp (313 nm, 500 W) for 24 h results in formations of 2 and 3
in 20% and 1%, respectively. In contrast, when a TOL solution of 1
and a small amount of I2 are irradiated with Rayonet lamps
(350 nm, 70 W) for 18 h, 3 is generated predominantly (20%) along
with only a trace quantity of 2. Photoreactions of 1 in CHCl3 con-
taining I2 generate 2 and 3 in 19% and 7%, respective yields when
a high-pressure mercury lamp is used as the light source, and 3
is produced nearly exclusively (23% and trace of 2) when Rayonet
lamps are employed. The structures of 2 and 3 were elucidated on
The possibility of promoting the electrocyclization–dehydroge-
nation reactions of 1 to form 2 and 3, by utilizing single electron-
transfer (SET) conditions and the oxidants, was explored (Table 2).
Table 2
SET-initiated electrocyclization–dehydrogenation reactions of 1 in CH2Cl2
Oxidant
T (°C)
Time (h)
Conv. (%)
Yields (%)
2
3
ꢁ
ðp-BrC6H4Þ3NÅþSbCl6
a
25
6
100
48
6
(n-Bu4N)2[Ce(NO3)6]b
25
0
1
1
100
100
27
41
0
3
c
FeCl3
Compound 1, 0.1 mmol; ðp-BrC6H4Þ NÅþSbCl6ꢁ, 0.1 mmol (1.0 equiv); CH2Cl2,
a
3
20 mL.
b
Compound 1, 0.2 mmol; (n-Bu4N)2[Ce(NO3)6], 0.3 mmol (1.5 equiv); CH2Cl2,
50 mL.
c
Compound 1, 0.3 mmol; anhydrous FeCl3, 1.5 mmol (5.0 equiv); CH2Cl2, 30 mL.
Scheme 2. Preparation of 1.

A. Yamamoto et al. / Tetrahedron Letters 54 (2013) 4049–4053
4051
Indeed, treatment of a CH2Cl2 solution of 1 with tris(p-bromophe-
nyl)aminium hexachloroantimonate ½ðp-BrC6H4Þ NÅþSbCl6ꢁꢂ pro-
3
motes electrocyclization–dehydrogenation reactions to give
and 3 in 48% and 6% yields, respectively. The use of FeCl3
2
to
18–21
promote these reactions leads to nearly the same yields of these
products, while (n-Bu4N)2[Ce(NO3)6]22,23 is less effective. Radical
cations 7Åþ and 8Åþ must serve as intermediates in the mechanistic
routes for these SET reactions. In addition, the low yields of 3 might
be a consequence of either the high oxidation potential of 1 com-
pared to 2 or some unexpected decompositions of 7Åþ and 8Åþ
.
Figure 2. Cyclic voltammograms of 1 (red, EAP = +1.11 and +1.42 V), 2 (green,
Photophysical and electrochemical studies of 2 and 3
EAP = +1.41 V), and 3 (blue, EAP = +1.35 V) in CH2Cl2 containing 0.1 M n-Bu4NþClO4ꢁ
.
Scan rate, 0.1 V sꢁ1; reference electrode, SCE; working and counter electrodes, Pt.
The results of a photophysical investigation showed that 2 and
3 in CH2Cl2 have respective UV–vis absorption maxima at kAB = 338
and 364 nm, and that they exhibit very weak fluorescence (UFL
<0.01) with emission maxima at kFL = 366 and 390 nm, respectively
(Table 3). The UV–vis absorption spectra of 2 and 3 remain un-
changed when aerated CH2Cl2 solutions containing these sub-
stances are allowed to stand in the dark for 48 h at room
temperature (Fig. 1a and b). This result shows that 2 and 3 have
higher stabilities against molecular oxygen than pentacene whose
Table 3
UV–vis absorption and fluorescence maxima (kAB and kFL), fluorescence quantum
yield (UFL), anodic peak potentials (EAP), energy levels of the HOMO (EH), and
differences in energy level between the HOMO and LUMO (EH–L) for pentacene, DPh-
BTBT, DNTT, and 1–3
kAB
kFL
UFL
EAP
EH (eV)
EH–L
(nm)
(nm)
(V versus
SCE)
(eV)
c
c
Pentacene 578a,b
DPh-BTBT 336g,h
DNTT
1
2
3
582a,b
—
—
—
—
+0.51a,b,d
+1.32d,h,i
+1.03d,i,k
+1.11a,m
—
2.21e,f
c
c
—
—
—
ꢁ5.41h,j 3.89h,j
ꢁ5.19j,k 3.38j,k
c
c
400a,k
324a
338a
364a
l
l
ꢁ5.24n
ꢁ5.66n
ꢁ5.44n
3.94n
4.59n
4.01n
366a
390a
0.006a,o +1.41a,m
0.008a,o +1.35a,m
Figure 3. Energy levels of the HOMOs and LUMOs of 1–3 calculated using B3LYP/6-
31G(d,p).
a
b
c
In CH2Cl2.
See Ref. 24.
Not available.
UV–vis absorption spectrum in aerated CH2Cl2 drastically changes
upon standing for 24 h at room temperature.6
d
Obtained by assuming the following empirical relationship, EAP (in V versus
SCE) = EAP (in V versus Fc/Fc+) +0.41 V.
e
Observations made in electrochemical studies show that the
cyclic voltammogram of 1 contains irreversible anodic peaks at
the potential EAP = +1.11 and +1.42 V versus SCE, and that of 2
contains one irreversible peak at +1.41 V versus SCE (Fig. 2 and
Table 3). The poor electrochemical reversibilities indicate that
radical cations 1Åþ and 2Åþ, arising by SET oxidation of 1 and 2, as
expected, undergo rapid chemical reactions. The voltage similarity
(+1.42 vs +1.41 V) between the second anodic peak of 1 and the
first peak of 2 is likely caused by the generation of 2 from 1Åþ. In
addition, the anodic peak of cyclization product 3 appears at a
lower potential (EAP = +1.35 V) than that of 2.
See Ref. 3.
Calculated using B3LYP/6-31+G(d).
In THF.
See Ref. 4.
In benzonitrile.
Calculated using B3LYP/6-31G(d).
See Ref. 6.
No attempt.
f
g
h
i
j
k
l
m
The first EAP versus SCE. For the detail, see Fig. 2.
Calculated using B3LYP/6-31G(d,p).
Determined using an integrating sphere system.
n
o
The order of the electrochemically determined EAP values of 1–3
agrees well with the energy level of the HOMOs (EH) for these sub-
stances obtained by using density functional theory (DFT) calcula-
tions (Table 3 and Fig. 3). In spite of the less planar structure of 1,
the EH of 1 is significantly higher than those of 2 and 3. This note-
worthy difference is a unique feature of tetrathienylethene frame-
works and is responsible for the ease with which 1 undergoes
photo- and electrochemical electrocyclization–dehydrogenation
reactions.17,23,25–27 The results of DFT calculations also demon-
strate that the difference in energy level between the HOMO and
LUMO (EH–L = ELꢁEH) for 3 is nearly equal to that of DPh-BTBT
(Chart 1 and Table 3),5 suggesting that 3 has a high potential for
use as an OFET material.
Figure 1. Time course of the UV–vis absorption spectra of (a) 2 (1 ꢀ 10ꢁ5 M) and (b)
3 (1 ꢀ 10ꢁ5 M) in aerated CH2Cl2 for 48 h.

4052
A. Yamamoto et al. / Tetrahedron Letters 54 (2013) 4049–4053
repeating overlap and long p–p spacing distance) in the crystalline
state (for the detail, see the Supplementary data). From these
points of view, development of 3 as an OFET material is an intrigu-
ing subject now being probed in our laboratory.
Acknowledgments
A.Y. gratefully acknowledges financial support (No. 25-304) by
the Sasakawa Scientific Research Grant from the Japan Science
Society. E.O. acknowledges financial support in the form of a
Grant-in-Aid for Young Scientist (B) (No. 24750044) from the Min-
istry of Education, Culture, Sports, Science and Technology (MEXT),
Japan. K.M. is grateful for financial support in the form of a Grant-
in-Aid for the Scientific Research (C) (No. 23550058) from the
MEXT of Japan. K.M. also thanks the Cooperation for Innovative
Technology and Advanced Research in Evolutional Area (Wakay-
ama) program. H.I. gratefully acknowledges financial support in
the form of a Grant-in-Aid for Scientific Research on Priority Areas
‘New Frontiers in Photochromism’ (Nos. 20044027 and 21021025
in the Area No. 471), Innovative Areas ‘p-Space’ (Nos. 21108520
and 23108718 in the Area No. 2007), and the Grant-in-Aid for Sci-
entific Research on Innovative Areas ‘Stimuli-responsive Chemical
Species’ (No. 24109009), the Scientific Research (B) (Nos.
20044027 and 23350023), and the Challenging Exploratory Re-
search (Nos. 21655016 and 24655037) from the MEXT of Japan.
Figure 4. ORTEP plots of structural data of (a) 1; (b) 2; and (c) 3 obtained by using
X-ray crystallographic analysis showing 30%-probability thermal ellipsoids for 1, 2
(T = 296 K) and 50%-probability ones for 3 (T = 93 K); (d) a columnar ordering of 3 in
the crystalline state.
X-ray crystallographic analyses of 1–3
Supplementary data
Inspection of the 1H NMR spectrum of 1 shows that its four thi-
enyl moieties are magnetically equivalent, suggesting that these
groups are rapidly rotating in solution at ambient temperature.
In the X-ray structure of 1,28 several disorders are observed in
the orientations of the thienyl moieties (Fig. 4a). Similarly, the
molecular structure of 2 in the crystalline state is comprised of a
mixture of disordered parallel and anti-parallel conformers
(Fig. 4b), in which the two thienyl moieties are oriented in the
same and opposite directions, respectively. The results of X-ray
crystallographic analysis show that 3 has a planar structure that
the molecules adopt a columnar order with a face-to-face manner,
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
05.090.
References and notes
1. Horowitz, G.; Fichou, D.; Peng, X.; Garnier, F. Synth. Met. 1991, 41, 1127–
1130.
2. Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur, A. Chem. Mater.
1996, 8, 2542–2544.
3. Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist, T. Chem. Mater.
2004, 16, 4980–4986.
4. Takimiya, K.; Ebata, H.; Sakamoto, K.; Izawa, T.; Otsubo, T.; Kunugi, Y. J. Am.
Chem. Soc. 2006, 128, 12604–12605.
5. Takimiya, K.; Kunugi, Y.; Otsubo, T. Chem. Lett. 2007, 36, 578–583.
6. Yamamoto, T.; Takimiya, K. J. Am. Chem. Soc. 2007, 129, 2224–2225.
7. Haas, S.; Takahashi, Y.; Takimiya, K.; Hasegawa, T. Appl. Phys. Lett. 2009, 95,
022111.
8. Sánchez-Carrera, R. S.; Atahan, S.; Schrier, J.; Aspuru-Guzik, A. J. Phys. Chem. C
2010, 114, 2334–2340.
a large degree of overlap and short p–p spacing distance of 3.45 Å
between heteroarene rings of adjacent molecules (Fig. 4c and d).
This structural feature should enable 3 to have ideal charge-carrier
transport properties. More generally, observations in this study
indicate that electrocyclization–dehydrogenation reactions, which
produce products that contain thiophene moieties annulated with-
in aromatic systems, represent a promising methodology to pro-
duce substances that meet the requirements for OFET materials.
9. Fischer, E.; Larsen, J.; Christensen, J. B.; Fourmigué, M.; Madsen, H. G.; Harrit, N.
J. Org. Chem. 1996, 61, 6997–7005.
10. Nishide, Y.; Osuga, H.; Iwata, K.; Tanaka, K.; Sakamoto, H. Bull. Chem. Soc. Jpn.
2008, 81, 1322–1330.
11. Dou, C.; Saito, S.; Gao, L.; Matsumoto, N.; Karasawa, T.; Zhang, H.; Fukazawa, A.;
Yamaguchi, S. Org. Lett. 2013, 15, 80–83.
Conclusion
12. Lucas, P.; Mehdi, N. E.; Ho, H. A.; Bélanger, D.; Breau, L. Synthesis 2000, 9, 1253–
1258.
13. McMurry, J. E. Chem. Rev. 1989, 89, 1513–1524.
14. Note that a similar dyotropic rearrangement is known to take place in di- or
tetra(3-thienyl)ethene derivatives that have two methyl groups at 2-position
of the thienyl groups.15–17
15. Irie, M.; Lifka, T.; Uchida, K.; Kobatake, S.; Shindo, Y. Chem. Commun. 1999,
747–750.
16. Ikeda, H.; Kawabe, A.; Sakai, A.; Namai, H.; Mizuno, K. Res. Chem. Intermed.
2009, 35, 893–908.
The effort described above has resulted in the synthesis of 3 uti-
lizing a route that features key photoirradiation promoted electro-
cyclization–dehydrogenation reactions of 1 followed by an I2-
induced oxidative dehydrogenation process. In addition, although
an electrocyclization reaction of 1 takes place under SET condi-
tions, the process affords mainly the monocyclization product 2.
Although moderately yielding of new p-conjugated systems, these
synthetic processes only require filtration through a short column
with silica gel. Moreover, the current studies demonstrate that the
hexacyclic heteroarene 3 has high stability against molecular
oxygen comparable to DPh-BTBT and DNTT and that 3 has the
EH–L value nearly equal to those of them, especially DPh-BTBT.5 Fi-
nally, owing to the fact that its crystalline structure is columnar in
a face-to-face manner, 3 should have a high charge-carrier trans-
portability. Unfortunately, it was turned out that similar ability is
not expected for 2, probably due to a poor ordering (a lack of
17. Ikeda, H.; Sakai, A.; Kawabe, A.; Namai, H.; Mizuno, K. Tetrahedron Lett. 2008,
49, 4972–4976.
18. Tovar, J. D.; Rose, A.; Swager, T. M. J. Am. Chem. Soc. 2002, 124, 7762–
7769.
19. Navale, T. S.; Zhai, L.; Lindeman, S. V.; Rathore, R. Chem. Commun. 2009, 2857–
2859.
20. Pei, J.; Zhang, W.; Mao, J.; Zhou, X. Tetrahedron Lett. 2006, 47, 1551–1554.
21. Wang, J.; Xu, H.; Li, B.; Cao, X.; Zhang, H. Tetrahedron 2012, 68, 1192–1197.
22. Dehmlow, E. V.; Makrandi, J. K. J. Chem. Res. (S) 1986, 32–33.
23. Ikeda, H.; Minegishi, T.; Miyashi, T. J. Chem. Soc., Chem. Commun. 1994, 297–
298.

A. Yamamoto et al. / Tetrahedron Letters 54 (2013) 4049–4053
4053
24. Payne, M. M.; Delcamp, J. H.; Parkin, S. R.; Anthony, J. E. Org. Lett. 2004, 6, 1609–
27. Ikeda, H.; Sakai, A.; Namai, H.; Kawabe, A.; Mizuno, K. Tetrahedron Lett. 2007,
48, 8338–8342.
1612.
25. Suzuki, T.; Shiohara, H.; Monobe, M.; Sakimura, T.; Tanaka, S.; Yamashita, Y.;
Miyashi, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 455–458.
26. Suzuki, T.; Ohta, E.; Kawai, H.; Fujiwara, K.; Fukushima, T. Synlett 2007, 851–
869.
28. Crystallographic data (excluding structure factors) for the structures in this
paper have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication Nos. CCDC-932660 (1), -932661 (2), and -932662
(3).