Helquats: A facile, modular, scalable route to novel helical dications

Article abstract of DOI:10.1002/chem.200801904

The synthesis and properties of helical extended diquat (helquat), and derivatives that bear resemblance to diquat and azoniahelicene, was reported. Triyne with elongated tethers connecting the heterocyclic moiety with the pendant alkyne functionalities u

Full text of DOI:10.1002/chem.200801904

DOI: 10.1002/chem.200801904
Helquats: A Facile, Modular, Scalable Route to Novel Helical Dications
[
a]
[a]
[a]
[b]
[a]
ˇ
Louis Adriaenssens, Lukꢀ sˇ Severa, Tereza Sꢀlovꢀ, Ivana Cꢁsa rˇ ovꢀ, Radek Pohl,
[
a]
[c]
[c]
[a]
ˇ
David Saman, Silvia V. Rocha, Nathaniel S. Finney, Lubomꢁr Pospꢁ sˇ il,
[
d]
[a]
Petr Slavꢁ cˇ ek, and Filip Tepl y´ *
Dedicated to Professor Rudolf Zahradnꢀk on the occasion of his 80th birthday
Electroactive paraquat 1 and diquat 2 (Figure 1) are
The comparatively unexplored realm of helically chiral N-
[4,5]
widely used herbicides produced in ton quantities every
heterohelicenia
on which to simultaneously expand the fields of cationic N-
(e.g., 3, ref. [4b,c]) represents a platform
[1]
year. Due to their powerful electron-accepting character
they are exceptional electron-transfer quenching agents. Re-
search into these dicationic species spearheaded the devel-
opment of supramolecular science and led to the evolution
[6]
[7]
heteroaromatics, helicenes, and heterohelicenes. The pos-
sibility of combining the remarkably diverse application
AHCTUNGTRENNUNG
[2]
of molecular devices. Extensive studies into their proper-
[3]
ties in biological and photocatalytic systems, and their mo-
lecular recognition capabilities further testify to the unique
position of these privileged dications.
extended diquat 4 (helquat 4) and derivatives thereof that
bear resemblance to both diquat 2, and azoniahelicene 3. In
the synthesis of the parent helquat 4, we capitalize on the
fact that a facile pyridine-type nitrogen quaternization pro-
tocol (5!6, Scheme 1) developed for monocation synthesis
Figure 1.
Scheme 1. a) TfOCH
Rh(PPh Cl] (5 mol%), MeCN, MW, 30 min, 908C, 99%; c) [Cp*Ru-
(cod)Cl] (10 mol%), H
fluoromethanesulfonyl.
2
CH
2
CꢀCH, CCl
4 2 2
, CH Cl , 408C, 23 h, 83%; b)
[
A
H
U
G
R
N
N
3 3
)
ˇ
ˇ
[
a] Dr. L. Adriaenssens, L. Severa, T. Sꢀlovꢀ, Dr. R. Pohl, Dr. D. Saman,
Dr. L. Pospꢂ sˇ il, Dr. F. Tepl y´
A
H
U
G
R
N
U
G
2
O/DMSO 99:1, air, 30 min, 1008C, 94%. Tf=tri-
Institute of Organic Chemistry and Biochemistry
Academy of Sciences of the Czech Republic
Flemingovo n. 2, 166 10 Prague 6 (Czech Republic)
Fax : (+420)220-183-578
[8]
by Vaquero, Cuadro et al. can be beneficially combined
with a reliable entry to helical scaffolds based on [2+2+2]-
E-mail: teply@uochb.cas.cz
[9]
triyne cycloisomerization pioneered by Star y´ and Starꢀ.
[
[
b] I. Cꢂsa rˇ ovꢀ
Department of Inorganic Chemistry, Charles University
Hlavova 2030, 128 40 Prague (Czech Republic)
Thus, triyne 6 is prepared in high yield from the known pyri-
dine precursor 5 (ref. [10]) and the key [2+2+2] cycloiso-
merization 6!4 is effected in minutes with Wilkinsonꢁs cat-
[
c] Dr. S. V. Rocha, Dr. N. S. Finney
Institute of Organic Chemistry, University of Zurich
Winterthurerstrasse 190, 8057 Zꢃrich (Switzerland)
alyst [Rh ACHTUNGRTNEUNG( PPh ) Cl] in acetonitrile under microwave condi-
3 3
tions. The practicality of the protocol is underscored by the
fact that a simple ethyl acetate wash of the crude product
removes the catalyst affording helquat 4 as the triflate salt
in good purity on milligram to gram scales. X-ray crystal
structure analysis unambiguously confirms the identity of 6
d] Dr. P. Slavꢂ cˇ ek
Department of Physical Chemistry, Institute of Chemical Technology
Technickꢀ 5, 166 28 Prague (Czech Republic)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801904.
1072
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 1072 – 1076

COMMUNICATION
[11]
and 4 (Figure 2). The reaction 6!4 is, to our knowledge,
the first example of metal-catalyzed [2+2+2] cycloaddition
[12]
of cationic N-heteroaromatics reported to date.
Figure 3. Absorption (solid line) and fluorescence (dotted line) spectra of
helquat 4 in water.
Figure 2. Molecular structures of 6 and 4 from single-crystal X-ray struc-
ture determination. Triflate counterions were omitted for clarity.
The key transformation 6!4 is also catalyzed by a known
ruthenium catalyst [Cp*Ru ACHUTNGRENNUG( cod)Cl] (Cp*=pentamethylcy-
[13]
clopentadienyl, cod=1,5-cyclooctadiene) and can be con-
ducted under unique aqueous aerobic conditions
(
Scheme 1), representing a rare metal-catalyzed example of
[14]
air-tolerant [2+2+2] cycloadditions. Although this reac-
tion also proceeds at room temperature, it is slower with
conversion reaching 25% after 17 h. We note that this mild
air- and water-tolerant process leads to the water-soluble
Scheme 2. Reversible Weiz-type electrochemical manifold of 4; panel A:
[
15]
blue fluorophore 4. The fluorescence spectrum obtained
after excitation at 330 nm, shows a maximum at 400 nm and
extends into the visible region, accounting for the blue fluo-
rescence (Figure 3). The quantum yield was determined to
be 0.14 with fluorescence lifetime of 3 ns (see Supporting In-
formation for details).
cyclic voltammetry of 4 (0.4 mm) and (nBu
panel B upper: UV/Vis spectrum of 4 (3 mm) and (nBu
acetonitrile (c), after the one-electron reduction (7, g) and in a
fully reduced form (8, a); Panel B lower: Ab initio calculated spectra
of 4, 7, and 8 in acetonitrile. Counterions are omitted.
4
N)PF
6
(0.1m) in acetonitrile;
N)PF (0.1m) in
4
6
Analysis by cyclic voltammetry showed the dication 4 to
be reduced in two separate revers
reduction potentials of ꢁ742 and ꢁ982 mV (vs. standard hy-
drogen electrode, scan rate 0.5 Vs, see the corresponding
A
H
U
G
R
N
U
G
terestingly, calculations show that if the reduced species 8
[17]
resided predominantly in its triplet state, the UV/Vis ab-
sorption spectrum would exhibit no maximum in the region
around 690 nm. The presence of this band in the experimen-
tally determined UV/Vis spectrum strongly suggests that
species 8 exists mostly in the singlet form (see Supporting
Information for details). The corresponding triplet state of 8
cyclic voltammogram, Scheme 2, panel A). In a UV/Vis
spectroelectrochemical experiment, absorption bands of the
three electrochemical states in the manifold 4, 7, and 8 were
determined. The results obtained experimentally (Scheme 2,
panel B upper) are in excellent agreement with the theoreti-
cal UV/Vis absorption bands of the three species 4, 7, and 8
obtained by ab initio calculations (TD-DFT with BMK func-
tional and 6-31+G* basis set, Scheme 2, panel B lower). In-
ꢁ1
is calculated to be 0.5 eV (48 kJmol ) higher in energy as
compared to the singlet ground state of 8. Thus, the two
electrochemical experiments supported by ab initio calcula-
tions suggest that helquat 4 parallels electrochemical proper-
Chem. Eur. J. 2009, 15, 1072 – 1076
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1073

F. Tepl y´ et al.
ties of diquat 2 (refs. [3b,16]) and thus belongs to an electro-
chemically privileged family of Weiz-type structures.
Table 1. Rhodium-catalyzed synthesis of helquats.
[
18]
[a]
[b]
Entry
Substrate
Product
Yield [%]
To demonstrate the flexibility of the described synthetic
protocol, a variety of helquats (Table 1) was synthesized.
End-capped triyne 9 was transformed readily to hexasubsti-
tuted benzene derivative 10 showing that internal alkyne
arms are well tolerated. Triyne 11, with elongated tethers
connecting the heterocyclic moiety with the pendant alkyne
functionalities, also smoothly underwent cycloisomerization
giving helquat 12 featuring two seven-membered rings. Sig-
nificantly, introduction of diversity in the region anticipated
[
c]
1
93
91
[19]
to be crucial to the tuning of the helix inversion barrier is
also facile as exemplified by racemic syntheses of methyl-
substituted helquats 14, and 16, and benzo-homologues, 18
and 20. Structures of two representative examples of helical
2
3
4
[20]
dications, 18 and 20, are depicted in Figure 4. Similarly to
9
9
4
3
[
d]
[
e]
94
76
[f]
5
[
g]
6
69
Figure 4. Molecular structures of 18 and 20 from single-crystal X-ray
structure determination. Triflate counterions were omitted for clarity.
[
(
a] Reaction conditions: triyne (0.01–0.07 mmol), [Rh
5 mol%), MeCN, MW, 30 min, 908C. [b] Yield of isolated product.
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(PPh
3 3
) Cl]
[
[
[
c] Reaction temperature 1108C. [d] EtOH, 0.5 g scale (0.9 mmol).
e] Reaction performed in EtOH, 908C, 30 min, then 1108C, 30 min.
f] [Rh
A
H
U
G
R
N
U
G
3 3
) Cl] (10 mol%), EtOH, MW, 30 min, 1108C. [g] [Rh-
the synthesis of parent compound 4, all helquats in Table 1
are isolated by a simple ethyl acetate wash of the crude
product affording the helquats in good purity and high
yields. Importantly, the seven helquats reported in this Com-
munication can be accessed uniformly in only three steps
from commercially available starting materials. Each three-
step sequence entails a Sonogashira coupling, bisquaterniza-
tion, and finally [2+2+2] cycloisomerization (see Support-
ing Information for details).
3 3
AHCTUNGTRENNG(U PPh )
ic entry into a novel family of dications which we suggest to
call helquats (helical extended diquats). They feature an
electroactive-extended diquat-type motif in a helical ar-
rangement. The evidence for the reversible electrochemical
Weiz-type manifold in 4 and also regular columnar stacks in
crystal structures of 4, 18, and 20 suggest the potential of
helquats as electroactive functional elements of future nano-
In conclusion, capitalizing on a [2+2+2]-cycloisomeriza-
tion strategy, we have developed a robust three-step synthet-
[21]
science, for example, molecular electronics. The conceptu-
1074
www.chemeurj.org
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 1072 – 1076

Novel Helical Dications
COMMUNICATION
al simplicity, practicality, atom- and step-economy of the re-
ported synthetic strategy will open this class of compounds
for multidisciplinary research efforts ranging from organoca-
talysis to material applications to bio-applications. Potential
uses of helical dications, structural tailoring to tune the sin-
glet-triplet energy gap in fully reduced species such as 8,
and synthetic entry into non-racemic series are currently
being pursued.
Pecka, J. Jind rˇ ich, G. Calleja, P. R. Craig, J. Michl, J. Org. Chem.
2
005, 70, 405–412; L. Boubekeur-Lecaque, B. J. Coe, K. Clays, S.
Foerier, T. Verbiest, I. Asselberghs, J. Am. Chem. Soc. 2008, 130,
286–3287; F. M. Raymo, M. D. Bartberger, K. N. Houk, J. F. Stod-
3
dart, J. Am. Chem. Soc. 2001, 123, 9264–9267 and references there-
in.
[
7] For reviews on helicenes and heterohelicenes, see A. Rajca, M.
Miyasaka, in Functional Organic Materials (Eds.: T. J. J. Mꢃller,
U. H. F. Bunz), Wiley-VCH, Weinheim, 2007, pp. 543–577; A.
Urbano, Angew. Chem. 2003, 115, 4116–4119; Angew. Chem. Int.
Ed. 2003, 42, 3986–3989; C. Schmuck, Angew. Chem. 2003, 115,
2
552–2556; Angew. Chem. Int. Ed. 2003, 42, 2448–2452; H. Hopf,
Classics in Hydrocarbon Chemistry: Syntheses, Concepts, Perspec-
tives, Wiley-VCH, Weinheim, 2000, pp. 323–330; T. J. Katz, Angew.
Chem. 2000, 112, 1997–1999; Angew. Chem. Int. Ed. 2000, 39, 1921–
Acknowledgements
Support of this work by the IOCB under Grant No. Z4 055 0506 and the
Ministry of Education of the Czech Republic (Projects: COST D36
OC140 and 6046137307) is gratefully acknowledged. We thank Dr. I.
Star y´ , Dr. D. Schrçder, and Dr. Martin B eˇ lohradsk y´ for stimulating dis-
1923; S. K. Collins, M. P. Vachon, Org. Biomol. Chem. 2006, 4,
2518–2524; A. Rajca, S. Rajca, M. Pink, M. Miyasaka, Synlett 2007,
1799–1822; for recent reports, see J. Ichikawa, M. Yokota, T. Kudo,
S. Umezaki, Angew. Chem. 2008, 120, 4948–4951; Angew. Chem.
Int. Ed. 2008, 47, 4870–4873; H. Sugiura, R. Amemiya, M. Yamagu-
chi, Chem. Asian J. 2008, 3, 244–260; K. Shiraishi, A. Rajca, M.
Pink, S. Rajca J. Am. Chem. Soc. 2005, 127, 9312 ꢁ9313; Y. Kita-
hara, K. Tanaka, Chem. Commun. 2002, 932–933; S. K. Collins, A.
Grandbois, M. P. Vachon, J. Cꢅtꢆ, Angew. Chem. 2006, 118, 2989–
ˇ
cussions and advice, Dr. I. Cervenꢀ for her experimental help in the ini-
tial phase of this project, the group of Dr. J. Cva cˇ ka for mass spectra, Dr.
P. Fiedler for IR spectra, and Dr. J. Fiedler for UV/Vis spectra.
Keywords: air-tolerant cycloisomerization
electrochemistry · helical structures · heteroaromatic cations
·
diquat
·
2
992; Angew. Chem. Int. Ed. 2006, 45, 2923–2926; D. C. Harrowven,
I. L. Guy, L. Nanson; Angew. Chem. 2006, 118, 2300–2303; Angew.
Chem. Int. Ed. 2006, 45, 2242–2245; Angew. Chem. 2006, 118, 2300–
2
303; Angew. Chem. Int. Ed. 2006, 45, 2242–2245; C. Wachsmann,
E. Weber, M. Czugler, W. Seichter, Eur. J. Org. Chem. 2003, 2863–
876; T. Verbiest, S. Sioncke, A. Persoons, L. Vyklick y´ , T. J. Katz,
Angew. Chem. 2002, 114, 4038–4040; Angew. Chem. Int. Ed. 2002,
1, 3882–3884; T. B. Norsten, A. Peters, R. McDonald, M. T. Wang,
[
1] a) R. C. Brian, R. F. Homer, J. Stubbs, R. L. Jones, Nature 1958, 181,
46–447; b) The Pesticide Manual (Ed.: C. D. S. Tomlin), 14th ed.,
British Crop Protection Council, 2006; c) http://www.pan-uk.org/
pestnews/actives/paraquat.htm.
2
4
4
N. R. Branda, J. Am. Chem. Soc. 2001, 123, 7447–7448; Y. Xu, Y. X.
Zhang, H. Sugiyama, T. Umano, H. Osuga, K. Tanaka, J. Am.
Chem. Soc. 2004, 126, 6566–6567; S. Honzawa, H. Okubo, S. Anzai,
M. Yamaguchi, K. Tsumoto, I. Kumagai, Bioorg. Med. Chem. 2002,
[
2] a) B. L. Allwood, F. H. Kohnke, A. M. Z. Slawin, J. F. Stoddart, D. J.
Williams, J. Chem. Soc. Chem. Commun. 1985, 311–314; b) J. W.
Steed, J. L. Atwood, Supramolecular Chemistry, Wiley, Chichester,
2
000; c) V. Balzani, A. Credi, M. Venturi, Molecular Devices and
1
0, 3213–3218; L. Owens, C. Thilgen, F. Diederich, C. B. Knobler,
Machines, Wiley-VCH, Weinheim, 2003; d) W. Sliwa, B. Bachowska,
T. Girek, Curr. Org. Chem. 2007, 11, 497–513.
Helv. Chim. Acta 1993, 76, 2757–2774; E. Murguly, R. McDonald,
N. R. Branda, Org. Lett. 2000, 2, 3169–3172; I. Sato, R. Yamashima,
K. Kadowaki, J. Yamamoto, T. Shibata, K. Soai, Angew. Chem.
[
3] a) L. Michaelis, Biochem. Z. 1932, 250, 564–567; b) P. M. S. Monk,
The Viologens, Wiley, Chichester, 1998; c) C. K. Ryu, R.-Y. Wang,
R. H. Schmehl, S. Ferrere, M. Ludwikow, J. W. Merkert, C. E. L.
Headford, C. M. Elliott, J. Am. Chem. Soc. 1992, 114, 430–438.
4] a) K. Sato, S. Arai, in Cyclophane Chemistry for the 21st Century
2
001, 113, 1130–1132; Angew. Chem. Int. Ed. 2001, 40, 1096–1098;
M. T. Reetz, S. Sostmann, Tetrahedron 2001, 57, 2515–2520; A. Ter-
fort, H. Gçrls, H. Brunner, Synthesis 1997, 79–86; L. Rulꢂ sˇ ek, O.
Exner, L. Cwiklik, P. Jungwirth, I. Star y´ , L. Pospꢂ sˇ il, Z. Havlas, J.
Phys. Chem. C 2007, 111, 14948–14955; M. C. CarreÇo, M. Gon-
zꢀlez-Lꢇpez, A. Urbano, Chem. Commun. 2005, 611–613; M. Ging-
ras, F. Dubois, Tetrahedron Lett. 1999, 40, 1309–1312; D. PeÇa, A.
Cobas, D. Pꢆrez, E. Guitiꢀn, L. Castedo, Org. Lett. 2003, 5, 1863–
[
(Ed.: H. Takemura), Research Signpost, Trivandrum, 2002, pp. 186–
1
1
97; b) S. Arai, T. Yafune, M. Ohkubo, M. Hida, Tetrahedron Lett.
989, 30, 7217–7218; c) K. Sato, S. Arai, T. Yamagishi, T. Tanase,
Acta Crystallogr. Sect. C 2003, 59, o162–o164.
[
[
5] a) C. Herse, D. Bas, F. C. Krebs, T. Bꢃrgi, J. Weber, T. Wesolowski,
B. W. Laursen, J. Lacour, Angew. Chem. 2003, 115, 3270–3274;
Angew. Chem. Int. Ed. 2003, 42, 3162–3166; b) B. Laleu, P. Mobian,
C. Herse, B. W. Laursen, G. Hopfgartner, G. Bernardinelli, J.
Lacour, Angew. Chem. 2005, 117, 1913–1917; Angew. Chem. Int.
Ed. 2005, 44, 1879–1883.
6] For reviews, see W. Sliwa, Curr. Org. Chem. 2003, 7, 995–1048; S.
Arai, M. Hida, Adv. Heterocycl. Chem. 1992, 55, 261–358; C. K.
Bradsher, Compr. Heterocycl. Chem. 1984, 2, 525–579; H. Ihmels,
Sci. Synth. 2004, 15, 907–946; for recent accounts, see T. M. Swager,
S.-W. Zhang, J. Am. Chem. Soc. 2003, 125, 3420–3421; J. A. Murphy,
S. Zhou, D. W. Thomson, F. Schoenebeck, M. Mahesh, S. R. Park, T.
Tuttle, L. E. A. Berlouis, Angew. Chem. 2007, 119, 5270–5275;
Angew. Chem. Int. Ed. 2007, 46, 5178–5183; C. Nicolas, J. Lacour,
Org. Lett. 2006, 8, 4343–4346; A. D. C. Parenty, K. M. Guthrie, Y.
Song, L. V. Smith, E. Burkholder, L. Cronin, Chem. Commun. 2006,
1
7
866; T. Iwasaki, Y. Kohinata, H. Nishide, Org. Lett. 2005, 7, 755–
58; T. Verbiest, S. Van Elshocht, M. Kauranen, L. Hellemans, J.
Snauwaert, C. Nuckolls, T. J. Katz, A. Persoons, Science 1998, 282,
913–915; K. Nakano, Y. Hidehira, K. Takahashi, T. Hiyama, K.
Nozaki, Angew. Chem. 2005, 117, 7298–7300; Angew. Chem. Int.
Ed. 2005, 44, 7136–7138; J. E. Field, T. J. Hill, D. Venkataraman, J.
Org. Chem. 2003, 68, 6071–6078; I. Pischel, S. Grimme, S. Kotila,
ˇ
M. Nieger, F. Vçgtle, Tetrahedron: Asymmetry 1996, 7, 109–116; S.
Vysko cˇ il, M. Smr cˇ ina, M. Lorenc, I. Ti sˇ lerovꢀ, R. D. Brooks, J. J.
Kulagowski, V. Langer, L. J. Farrugia, P. Ko cˇ ovsk y´ , J. Org. Chem.
2001, 66, 1359–1365; M. Miyasaka, A. Rajca, M. Pink, S. Rajca, J.
Am. Chem. Soc. 2005, 127, 13806–13807; R. Fasel, M. Parschau, K.-
H. Ernst, Nature 2006, 439, 449–452; Z. Y. Wang, E. K. Todd, X. S.
Meng, J. P. Gao, J. Am. Chem. Soc. 2005, 127, 11552–11553; R.
El Abed, B. Ben Hassine, J.-P. GenÞt, M. Gorsane, A. Marinetti,
Eur. J. Org. Chem. 2004, 1517–1522; S. Maiorana, A. Papagni, E.
Licandro, R. Annunziata, P. Paravidino, D. Perdicchia, C. Giannini,
M. Bencini, K. Clays, A. Persoons, Tetrahedron 2003, 59, 6481–
6488; C. Bazzini, S. Brovelli, T. Caronna, C. Gambarotti, M. Gian-
none, P. Macchi, F. Meinardi, A. Mele, W. Panzeri, F. Recupero, A.
Sironi, R. Tubino, Eur. J. Org. Chem. 2005, 1247–1257.
1
194–1196; M. Sinan, M. Panda, A. Ghosh, K. Dhara, P. E. Fan-
wick, D. J. Chattopadhyay, S. Goswami, J. Am. Chem. Soc. 2008,
30, 5185–5193; D. Monchaud, M. P. Teulade-Fichou, Org. Biomol.
1
Chem. 2008, 6, 627–636; M. F. BraÇa, M. Cacho, A. Gradillas, B. de
Pascual-Teresa, A. Ramos, Curr. Pharm. Des. 2001, 7, 1745–1780;
A. Fꢃrstner, A. Ernst, Tetrahedron 1995, 51, 773–786; M. Valꢀ sˇ ek, J.
Chem. Eur. J. 2009, 15, 1072 – 1076
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1075

F. Tepl y´ et al.
[
8] a) A. NfflÇez, A. M. Cuadro, J. Alvarez-Builla, J. J. Vaquero, Chem.
Commun. 2006, 2690–2692; b) A. NfflÇez, A. M. Cuadro, J. Alvarez-
Builla, J. J. Vaquero, Org. Lett. 2004, 6, 4125–4127; c) A. NfflÇez,
A. M. Cuadro, J. Alvarez-Builla, J. J. Vaquero, Org. Lett. 2007, 9,
mate cleavage catalyzed by [Cp*Ru AHCTUNTGERNNUN(G cod)Cl] in living cells, see C.
Streu, E. Meggers, Angew. Chem. 2006, 118, 5773–5776; Angew.
Chem. Int. Ed. 2006, 45, 5645–5648.
[15] For bisimidazolium water-soluble blue fluorophore of analogous
structure, see M. Ostermeier, C. Limberg, B. Ziemer, V. Karunakar-
an, Angew. Chem. 2007, 119, 5423–5426; Angew. Chem. Int. Ed.
2007, 46, 5329–5331.
[16] Reduction potentials obtained under the same conditions for diquat
dichloride are ꢁ242 and ꢁ735 mV vs. standard hydrogen electrode.
[17] For related investigation of singlet-triplet dichotomy in extended
paraquat, see a) J. Casado, S. Patchkovskii, M. Z. Zgierski, L. Her-
mosilla, C. Sieiro, M. M Oliva, J. T. L. Navarette, Angew. Chem.
2008, 120, 1465–1468; Angew. Chem. Int. Ed. 2008, 47, 1443–1446;
b) W. W. Porter, III, T. P. Vaid, A. L. Rheingold, J. Am. Chem. Soc.
2005, 127, 16559.
2
977–2980.
ˇ
[
9] I. G. Starꢀ, I. Star y´ , A. Kollꢀrovi cˇ , F. Tepl y´ , D. Saman, M. Tich y´ , J.
Org. Chem. 1998, 63, 4046–4050; J. Mꢂ sˇ ek, F. Tepl y´ , I. G. Starꢀ, M.
Tich y´ , D. Saman, I. Cꢂsa rˇ ovꢀ, P. Vojtꢂ sˇ ek, I. Star y´ , Angew. Chem.
ˇ
2
008, 120, 3232–3235; Angew. Chem. Int. Ed. 2008, 47, 3188–3191;
ˇ
F. Tepl y´ , I. G. Starꢀ, I. Star y´ , A. Kollꢀrovi cˇ , D. Saman, L. Rulꢂ sˇ ek, P.
Fiedler, J. Am. Chem. Soc. 2002, 124, 9175–9180; I. G. Starꢀ, I.
ˇ
ˇ
Star y´ , A. Kollꢀrovi cˇ , F. Tepl y´ , S. Vysko cˇ il, D. Saman, Tetrahedron
Lett. 1999, 40, 1993–1996; S. Han, A. D. Bond, R. L. Disch, D.
Holmes, J. M. Schulman, S. J. Teat, K. P. C. Vollhardt, G. D. Whiten-
er, Angew. Chem. 2002, 114, 3357–3361; Angew. Chem. Int. Ed.
2
002, 41, 3223–3227; S. Han, D. R. Anderson, A. D. Bond, H. V.
[18] K. Deuchert, S. Hꢃnig, Angew. Chem. 1978, 90, 927–938; Angew.
Chem. Int. Ed. Engl. 1978, 17, 875–886; J. Volke, Collect. Czech.
Chem. Commun. 1968, 33, 3044–3048; C. L. Bird, A. T. Kuhn,
Chem. Soc. Rev. 1981, 10, 49–82.
Chu, R. L. Disch, D. Holmes, J. M. Schulman, S. J. Teat, K. P. C.
Vollhardt, G. D. Whitener, Angew. Chem. 2002, 114, 3361–3364;
Angew. Chem. Int. Ed. 2002, 41, 3227–3230.
[
10] K. T. Potts, C. P. Horwitz, A. Fessak, M. Keshavarz-K, K. E. Nash,
P. J. Toscano, J. Am. Chem. Soc. 1993, 115, 10444–10445.
11] CCDC 688168 (4) and 688169 (6) contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[19] Preliminary variable temperature NMR investigation with the
parent helquat 4 ditriflate suggests the helix inversion barrier in
ꢁ
1
[
6
[D ]acetone has the activation free energy of 64 kJ.mol at 408C.
[20] CCDC 688170 (18) and 688171 (20) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[21] Molecular wire prototype based on self-assembly of uncharged ex-
tended helicene into columnar stacks has been reported, see S. Xiao,
M. Myers, Q. Miao, S. Sanaur, K. Pang, M. L. Steigerwald, C. Nuck-
olls, Angew. Chem. 2005, 117, 7556–7560; Angew. Chem. Int. Ed.
2005, 44, 7390–7394; for theoretical account on conductivity of heli-
cenes, see G. Treboux, P. Lapstun, Z. H. Wu, K. Silverbrook, Chem.
Phys. Lett. 1999, 301, 493–497.
[
[
[
12] For related enyne ring-closing metathesis on N-heteroaromatic cat-
ions, see ref. [8a].
13] Y. Yamamoto, T. Arakawa, R. Ogawa, K. Itoh, J. Am. Chem. Soc.
003, 125, 12143–12160.
14] For rare [2+2+2] cycloadditions in presence of O
Y. Sakurai, S. Sakaguchi, Y. Ishii, Tetrahedron Lett. 1997, 38, 3923–
926; G. A. Ardizzoia, S. Brenna, G. LaMonica, A. Maspero, N.
Masciocchi, J. Organomet. Chem. 2002, 649, 173–180; for Ru-cata-
lyzed [2+2+2] cycloadditions in mixtures of H O/MeOH under N
2
2
, see T. Yokota,
3
2
2
atmosphere at 758C, see V. Cadierno, S. E. Garcꢂa-Garrido, J.
Gimeno, J. Am. Chem. Soc. 2006, 128, 15094–15095; for allylcarba-
Received: September 16, 2008
Published online: November 5, 2008
1076
www.chemeurj.org
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 1072 – 1076