Modular synthesis, orthogonal post-functionalization, absorption, and chiroptical properties of cationic [6]helicenes

Article abstract of DOI:10.1002/anie.201208926

Pick and choose: Novel cationic diaza-, azaoxo-, and dioxo[6]helicenes are readily prepared and functionalized selectively by orthogonal aromatic electrophilic and vicarious nucleophilic substitutions (see scheme). Reductions, cross-coupling, or condensat

Full text of DOI:10.1002/anie.201208926

Angewandte
Chemie
DOI: 10.1002/anie.201208926
Helicenes
Modular Synthesis, Orthogonal Post-Functionalization, Absorption,
and Chiroptical Properties of Cationic [6]Helicenes**
Franck Torricelli, Johann Bosson, Cꢀline Besnard, Mahshid Chekini, Thomas Bꢁrgi, and
Jꢀrꢂme Lacour*
Helicenes,[1] which are ortho-condensed polyaromatic com-
pounds, are omnipresent in chemistry, biochemistry, and
physics as a result of their many different properties and
applications.[2] Of importance, these properties can be
addressed by introducing substituents at the periphery of
the helical cores or by changing the nature of the atoms
within. From a synthetic standpoint, such modifications are
not always trivial. Introduction of heteroatoms or post-
functionalization of helicenes with exocyclic substituents
can be difficult to achieve selectively.[3] For this reason,
changes are usually made on building blocks prior to the
completion of the helical skeletons. Herein, in an effort to
overcome this limitation, we report a new class of cationic
diaza-, azaoxo-, and dioxo[6]helicenes. The compounds 1–3
(Figure 1) are prepared in one step from a single common
tion properties are easily tuneable up to the near-infrared
region thanks to these late-stage transformations.
Previously, quinacridinium derivatives of type 4 have been
reported (Figure 1). They are configurationally stable [4]heli-
cenes (DG° of racemization is ca. 42 kcalmolꢀ1) and very
stable carbocations (pKR+ ꢁ 19).[4] Quite a few applications of
these compounds have been developed.[5] None of them
mention, however, a peripheral functionalization of the
aromatic core. In fact, attempts to introduce substituents to
4 lead to mixtures of products because of the lack of chemical
differentiation among the three benzo rings.[6] It was thus
decided to prepare other cationic heterohelicenes, which
could be highly tuneable after their formation, such as the
[6]helicenes 1–3.
Access to these compounds was achieved using a modular
route made of stepwise additions of the aromatic subunits and
controlled sequential ring closures, as summarized in
Scheme 1. The first step was the addition of (2,6-dimethoxy-
phen-1-yl)lithium to 2-methoxy-1-naphthaldehyde. The
Figure 1. The cationic diaza 1, azaoxo 2, and dioxo 3 [6]helicenes, and
the [4]helicene quinacridinium derivatives of type 4. Only the P enan-
tiomers are shown. R=alkyl chains.
advanced synthetic intermediate. Selective functionalization
of 1 is readily achieved by series of orthogonal regioselective
aromatic electrophilic and vicarious nucleophilic substitu-
tions. Reduction, cross-coupling, or condensation reactions
also introduce diversity. As a consequence, UV/Vis absorp-
[*] F. Torricelli, Dr. J. Bosson, Prof. J. Lacour
Scheme 1. Reagents and reaction conditions: a) (2,6-dimethoxyphen-1-
yl)lithium, Et2O, 16 h, ꢀ78!258C, 68%; b) DDQ, CH2Cl2, 20 h, 258C,
98%; c) BBr3, CH2Cl2, 4 h, ꢀ78!258C, 94%; d) neat, 2 h, 2258C,
99%; e) (2-methoxynaphthalen-1-yl)lithium, CeCl3, Et2O,14 h, ꢀ78!
258C; f) HBF4, 97% over 2 steps. DDQ=2,3-dichloro-5,6-dicyano-1,4-
benzoquinone.
Department of Organic Chemistry, University of Geneva
Quai Ernest Ansermet 30, 1211 Geneva 4 (Switzerland)
E-mail: jerome.lacour@unige.ch
Homepage: http://www.unige.ch/sciences/chiorg/lacour/
Dr. C. Besnard
Laboratory of Crystallography, University of Geneva
Quai Ernest Ansermet 24, 1211 Geneva 4 (Switzerland)
M. Chekini, Prof. T. Bꢀrgi
Department of Physical Chemistry, University of Geneva
Quai Ernest Ansermet 30, 1211 Geneva 4 (Switzerland)
resulting alcohol 5 (68%) was oxidized to the ketone 6
(98%, DDQ, CH2Cl2). Then, treatment with BBr3 (1 equiv)
regioselectively generated the phenol 7, which underwent
a clean O-ring closure upon heating at 2258C (neat, combined
yield 93% for the two steps). Reaction of the resulting
tetracyclic ketone 8 with (2-methoxynaphthalen-1-yl)lithium
in the presence of CeCl3[7,8] then afforded, after work-up with
HBF4, [9][BF4] in excellent yield (97%). The whole sequence
is readily performed on multigram scale.
[**] We thank the University of Geneva, the Swiss National Science
Foundation and the European Marie Curie Foundation for financial
support. We also acknowledge the contributions of the Sciences
Mass Spectrometry (SMS) platform at the Faculty of Sciences,
University of Geneva.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208926.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
Helicenes of type 1, 2, and 3 were then prepared using
[9][BF4] as single common precursor. The diaza derivatives
1a–d (Scheme 2) were synthesized by treatment of [9][BF4]
with the corresponding anhydrous aliphatic amines (25 equiv,
Scheme 3. Reagents and reaction conditions: a) HNO3, 15 min, 258C;
b) NCS (3 equiv), CHCl3, AcOH, 15 h, 258C; c) NBS (3 equiv), CHCl3,
AcOH, 15 h, 258C; d) NIS (3 equiv), CHCl3, AcOH, 15 h, 258C; e) H2,
PtO2 (10 wt%), EtOH, 1 h, 258C; f) [Pd(PPh3)4] (10%), PhB(OH)2
(5 equiv), K2CO3 (5 equiv), 1,4-dioxane, 15 h, 808C; g) [Pd(PPh3)4]
Scheme 2. Reagents and reaction conditions: a) Anhydrous RNH2
(25 equiv), NMP, MW 1708C, 10 min; b) PPh3, DIAD, CH3COSH, THF,
258C; c) n-PrNH2, 498C, 30 min, then heated neat at 2008C, 5 min;
d) Pyr·HCl, 2248C, 2 min then ion exchange metathesis with HBF4.
DIAD=diisopropyl azodicarboxylate, MW=microwave, NMP=
N-methyl-2-pyrrolidone, THF=tetrahydrofuran.
ꢂ
(20%), CuI (30%), PhC CH (10 equiv), Et3N, 3 h, 908C. NBS=
N-bromosuccinimide, NCS=N-chlorosuccinimide, NIS=
N-iodosuccinimide.
NMP) and rapid heating under microwave irradiation (1708C,
10 min).[9] The resulting blue salts, 1a–d, were isolated in
moderate yields (43–47%). In the case of 1d, facile trans-
formation of the diol side chains into thioacetyl groups using
Mitsunobu-type conditions was achieved to give 1e (90%).
To obtain the purple azaoxa salt 2a, 9 was heated in neat
anhydrous n-propylamine at reflux (498C, 30 min). Then,
after complete evaporation of the amine in vacuo at 208C,
application of a burst of heat (2008C, 5 min) afforded the
mixed azaoxo derivative [2a][BF4] in 40% yield. Finally, rapid
treatment of [9][BF4] in molten Pyr·HCl (2248C, 2 min) and
ion-exchange metathesis (HBF4, 258C) afforded the red dioxo
helicene 3 as its BF4 salt in excellent yield (95%).
rings to react with nucleophiles,[11] [1a][BF4] was treated with
an excess of NaCN (3 equiv) in an open flask (Scheme 4).[12]
Full conversion was achieved but the resulting salt, [17][BF4],
bearing two nitrile groups at positions 5 and 13, was found to
With these derivatives in hand, in particular salt [1a][BF4],
orthogonal routes to the selective functionalization of the
helical core were developed. Electrophilic aromatic substitu-
tion reactions were first studied. A higher reactivity of the
median phenyl ring over the flanking naphthyl subunits was
predicted on account of the two donor nitrogen atoms
attached to this ring (instead of one for the naphthyl
groups). This assumption was rapidly confirmed. Treatment
of [1a][BF4] under nitration or halogenation reaction con-
ditions (Scheme 3) afforded the corresponding dinitro 10,
dichloro 11, and dibromo 12 derivatives in good to excellent
yields (77, 97, and 96% respectively). A perfect regioselec-
tivity was observed, wherein only positions 8 and 10 of the
helical core, ortho/para to the two N atoms, reacted under
these reaction conditions. Interestingly, with NIS, a milder
monoiodination was obtained (13, 90%). Moreover, addi-
tional derivatizations were feasible with 10 and 12. For
example, [10][BF4] was readily reduced (H2, cat. PtO2) to the
corresponding diamino derivative 14 (56%), and [12][BF4]
was reacted under Suzuki–Miyaura and Sonogashira condi-
tions to yield the corresponding diphenyl 15 (69%) and
diphenylethynyl 16 (62%) derivatives.[10]
Scheme 4. a) NaCN (3 equiv), DMF, 60 h, 258C; b) n-PrNH2, 16 h,
258C; c) TMSN3 (3 equiv), Bu2SnO (20%), PhCl, 16 h, 608C.
DMF=N,N-dimethylformamide, TMS=trimethylsilyl.
be sensitive to purification conditions (SiO2, 17%). A similar
reactivity was obtained in the treatment of [12][BF4] with
n-PrNH2 (neat, 258C) to afford the trisubstituted [18][BF4] in
good yield (74%). These vicarious nucleophilic substitutions
afford, therefore, another means by which to modify the
helical core under orthogonal (nucleophilic oxidative rather
than electrophilic) conditions.[13] Salt [17][BF4] can be further
transformed by reaction with trimethylsilyl azide to obtain the
bis(tetrazole) analogue [19][BF4] (60%).
Crystals of [12][BF4] were grown by slow diffusion of Et2O
into a CH2Cl2 solution. Structural analysis by X-ray diffrac-
tion revealed, as expected, a helical conformation of the
ortho-condensed framework (Figure 2).[14] The compound 12
presents a larger helical pitch (3.31 ꢀ) and angle (64.58)
between the two mean planes defined by the edge rings in
Alternatively, taking into consideration the cationic
character of the compounds 1 and the propensity of naphthyl
2
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
Table 1: Electronic properties: maximum absorption band (lmax) and
extinction coefficients (e). CH2Cl2 solutions (2ꢁ10ꢀ5 molLꢀ1).
Entry
Substituents
Compound
lmax [nm]
e [mꢀ1 cmꢀ1
]
1
2
3
4
5
6
7
8
9
8,10-NO2
8,10-Cl2
8,10-Br2
8-I
10
11
12
13
16
15
14
19
17
18
567
620
620
620
637
643
700
620
690
600
18300
13000
13000
13000
15400
14600
5100
17350
17700
30400
ꢂ
8,10-C CPh
8,10-Ph
8,10-NH2
5,13-tetrazole
5,13-CN
Figure 2. ORTEP view of the crystal structure of [12][BF4]·CH2Cl2;
M enantiomer shown. Thermal ellipsoids are drawn at 50% probability.
Hydrogen atoms and solvent molecule are omitted for clarity.[27]
10
5-HN-nPr 8,10-Br2
comparison to those of the classical carbo[6]helicene[2b,15]
(3.22 ꢀ and 58.58 respectively).[16]
two cyano EWGs, and is in sharp contrast to the EDG-
substituted 18 (lmax = 600 nm).[19]
As briefly mentioned, the novel salts [1a][BF4], [2a][BF4],
and [3][BF4] are colorful dyes and their electronic absorption
spectra were recorded and are presented in Figure 3 (CH2Cl2,
2 ꢁ 10ꢀ5 molLꢀ1).[17] All derivatives are characterized by
moderately strong absorptions in the visible region
(13050 < e < 16800mꢀ1 cmꢀ1). The dioxo [3][BF4] displays
a maximum absorption band (lmax) at 574 nm. No shift of
this band is observed when one oxygen atom is replaced by
a nitrogen atom.[18] The presence of two nitrogen atoms
The enantiomeric resolution of 1a was also performed
using the hexacoordinated phosphorus bis(tetrachlorobenze-
nediolato)-mono([1,1’]binaphthalenyl-2,2’-diolato)phospha-
te(V) anion (BINPHAT; 20, D or L enantiomer; see the
Supporting Information) as a resolving agent.[20] The racemic
salt [1a][BF4] and [Me2NH2][L-20] (1.2 equiv) were mixed in
a 1:1 mixture of CH2Cl2 and acetone. After concentration,
NMR spectroscopic analysis (CDCl3, 400 MHz) was per-
formed and revealed an efficient enantiodifferentiation of
1a by anion 20.[21,22] The crude mixture was dissolved in pure
acetone and, upon cooling to 08C, the formation of a precip-
itate was noticed. The blue solid was collected by filtration.
causes, however, a moderate red shift of 45 nm (lmax
=
619 nm).
1
The mother liquor was concentrated and H NMR analysis
(CDCl3) indicated the presence of essentially one diastereo-
meric salt this time. Both fractions of the BINPHAT salts
were converted into hexafluorophosphate salts by treatment
with an excess of HPF6 and KPF6 in a mixture of CH2Cl2 and
water. The salts (+)-[1a][PF6] (e.r. 98:2) and (ꢀ)-[1a][PF6]
(e.r. 96:4) resulted from the mother liquor and solid fractions
in 45% and 35% yields, respectively.[23] Electronic circular
dichroism (ECD) spectra (see the Supporting Information)
displayed symmetrical curves in the l = 240 to 700 nm region.
The absolute configuration of the helicene 1a was then
established by vibrational circular dichroism (VCD).[24] IR
absorption and VCD spectra were measured for solutions
(CD2Cl2) of both (+)- and (ꢀ)-[1a][PF6] and compared to the
Boltzmann averaged spectrum calculated for (P)-1a
(Figure 4) taking into account the most stable conformers
found. Overall, a good agreement between the experimental
and theoretical spectra is observed, thus allowing the assign-
ment of P and M configurations for the carbenium ion in salts
(+)- and (ꢀ)-[1a][PF6], respectively.
Figure 3. UV/Vis spectra of a) [1a][BF4] (blue), b) [2a][BF4] (purple),
and c) [3][BF4] (red) recorded in CH2Cl2 (CH2Cl2, 2ꢁ10ꢀ5 molLꢀ1).
Of importance to the current study is a rather strong
modulation of the visible absorption properties offered by the
substituents introduced at the periphery of cation 1a. Results
are summarized in Table 1 and the corresponding spectra are
displayed in the Supporting Information. When substituents
are introduced on the upper phenyl ring (entries 1–7), the
maximum absorption band varies from lmax = 567 to 700 nm.
Electron-withdrawing (EWG) and electron-donating (EDG)
groups afford blue and red shifts, respectively. Interestingly,
the opposite trend is observed when substitution occurs on
the naphthyl rings (entries 8–10, 600 < lmax < 690 nm), with
the lowest energy transition being that made by 17, bearing
Finally, the determination of the racemization barrier was
attempted. DMSO solutions of (ꢀ)-[1a][PF6] were heated at
1808C and monitored in ECD at a single wavelength (l =
620 nm).[25] No evidence could be found for a racemization
after 5 hours of continuous heating. Performing the experi-
ment at higher temperature (ꢃ 1908C) in dibutyl sulfoxide
failed as 1a degraded in situ. As no racemization was
observed after 5 hours at 1808C, it is fair to say that 1a
presents a barrier of racemization of at least 37 kcal.molꢀ1
which is within the range for that of [6]helicene.[1,26]
,
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
d) O. Crespo, B. Eguillor, M. A. Esteruelas, I. Fernꢂndez, J.
Garcꢄa-Raboso, M. Gꢅmez-Gallego, M. Martꢄn-Ortiz, M.
Olivꢂn, M. A. Sierra, Chem. Commun. 2012, 48, 5328 – 5330;
e) E. Anger, M. Srebro, N. Vanthuyne, L. Toupet, S. Rigaut, C.
Roussel, J. Autschbach, J. Crassous, R. Rꢆau, J. Am. Chem. Soc.
2012, 134, 15628 – 15631; f) C. M. ꢇlvarez, H. Barbero, L. A.
Garcꢄa-Escudero, J. M. Martꢄn-Alvarez, C. Martꢄnez-Pꢆrez, D.
Miguel, Inorg. Chem. 2012, 51, 8103 – 8111; g) K. Fuchibe, H.
Jyono, M. Fujiwara, T. Kudo, M. Yokota, J. Ichikawa, Chem. Eur.
J. 2011, 17, 12175 – 12185; h) E. Anger, M. Rudolph, C. Shen, N.
Vanthuyne, L. Toupet, C. Roussel, J. Autschbach, J. Crassous, R.
Reau, J. Am. Chem. Soc. 2011, 133, 3800 – 3803.
[3] a) S. Shi, T. J. Katz, B. V. Yang, L. Liu, J. Org. Chem. 1995, 60,
1285 – 1297; b) Y. Dai, T. J. Katz, J. Org. Chem. 1997, 62, 1274 –
1285; c) H. Okubo, D. Nakano, S. Anzai, M. Yamaguchi, J. Org.
Chem. 2001, 66, 557 – 563; d) S. Maiorana, A. Papagni, E.
Licandro, R. Annunziata, P. Paravidino, D. Perdicchia, C.
Giannini, M. Bencini, K. Clays, A. Persoons, Tetrahedron 2003,
Figure 4. Experimental VCD spectra (CD2Cl2, 298 K) of (ꢀ)-[1a][PF6]
(blue) and (+)-[1a][PF6] (red). Calculated spectrum of (P)-1a (green).
´
59, 6481 – 6488; e) K. Paruch, L. Vyklicky, D. Z. Wang, T. J. Katz,
C. Incarvito, L. Zakharov, A. L. Rheingold, J. Org. Chem. 2003,
68, 8539 – 8544; f) D. Z. Wang, T. J. Katz, J. Golen, A. L.
Rheingold, J. Org. Chem. 2004, 69, 7769 – 7771; g) K. Nakano,
Y. Hidehira, K. Takahashi, T. Hiyama, K. Nozaki, Angew. Chem.
2005, 117, 7298 – 7300; Angew. Chem. Int. Ed. 2005, 44, 7136 –
7138; h) J.-D. Chen, H.-Y. Lu, C.-F. Chen, Chem. Eur. J. 2010, 16,
11843 – 11846.
Herein, we have thus presented novel cationic diaza-,
azaoxo-, and dioxo[6]helicenes. These three classes of deriv-
atives were prepared in one step from a common advanced
intermediate. Straightforward, yet orthogonal, aromatic elec-
trophilic and vicarious nucleophilic substitution reactions
afforded a series of mono-, di-, and trisubstituted diazaheli-
cenes which were additionally derivatized through cross-
coupling, reduction, or condensation processes. The impor-
tance of this regioselective late-stage functionalization of the
helical core was further evidenced in the strong modulation of
the visible absorption properties of the cationic dyes. The
diazahelicene can be resolved into single enantiomers, which
demonstrates a high barrier to racemization.
[4] a) B. W. Laursen, F. C. Krebs, Angew. Chem. 2000, 112, 3574 –
3576; Angew. Chem. Int. Ed. 2000, 39, 3432 – 3434; b) B. W.
Laursen, F. C. Krebs, Chem. Eur. J. 2001, 7, 1773 – 1783; c) 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.
[5] a) 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; b) B. Laleu,
M. S. Machado, J. Lacour, Chem. Commun. 2006, 2786 – 2788;
c) P. Mobian, C. Nicolas, E. Francotte, T. Bꢃrgi, J. Lacour, J. Am.
Chem. Soc. 2008, 130, 6507 – 6514; d) D. Conreaux, N. Mehanna,
C. Herse, J. Lacour, J. Org. Chem. 2011, 76, 2716 – 2722; e) J.
Guin, C. Besnard, P. Pattison, J. Lacour, Chem. Sci. 2011, 2, 425 –
428; f) J. Elm, J. Lykkebo, T. J. Sørensen, B. W. Laursen, K. V.
Mikkelsen, J. Phys. Chem. A 2012, 116, 8744 – 8752; g) O. Kel, P.
Sherin, N. Mehanna, B. Laleu, J. Lacour, E. Vauthey, Photochem.
Photobiol. Sci. 2012, 11, 623 – 631; h) A. Ueda, H. Wasa, S.
Suzuki, K. Okada, K. Sato, T. Takui, Y. Morita, Angew. Chem.
2012, 124, 6795 – 6799; Angew. Chem. Int. Ed. 2012, 51, 6691 –
6695.
[6] For instance, treatments of 4 with electrophilic reagents like N-
chloro- and N-bromosuccinimide (1 equiv, AcOH/CHCl3) afford
unreacted starting material along with complex mixtures of
regioisomeric mono- and polyhalogenated derivatives.
[7] Without a stoichiometric amount of CeCl3, the yield drops to
64%.
[8] a) T. Imamoto, T. Kusumoto, Y. Tawarayama, Y. Sugiura, T.
Mita, Y. Hatanaka, M. Yokoyama, J. Org. Chem. 1984, 49, 3904 –
3912; b) T. Imamoto, N. Takiyama, K. Nakamura, T. Hatajima,
Y. Kamiya, J. Am. Chem. Soc. 1989, 111, 4392 – 4398; c) V.
Dimitrov, S. Bratovanov, S. Simova, K. Kostova, Tetrahedron
Lett. 1994, 35, 6713 – 6716; d) V. Dimitrov, M. Genov, S. Simova,
A. Linden, J. Organomet. Chem. 1996, 525, 213 – 224; e) V.
Dimitrov, K. Kostova, M. Genov, Tetrahedron Lett. 1996, 37,
6787 – 6790; f) S. Panev, V. Dimitrov, Tetrahedron: Asymmetry
2000, 11, 1517 – 1526; g) S. Panev, A. Linden, V. Dimitrov,
Tetrahedron: Asymmetry 2001, 12, 1313 – 1321.
Received: November 7, 2012
Published online: && &&, &&&&
Keywords: arenes · helical structures · synthetic methods · UV/
.
Vis spectroscopy · vicarious nucleophilic substitution
[1] a) Y. Shen, C.-F. Chen, Chem. Rev. 2012, 112, 1463 – 1535; b) F.
Dumitrascu, D. G. Dumitrescu, I. Aron, ARKIVOC 2010, (i), 1 –
´
32; c) I. Stary, I. G. Starꢂ in Strained Hydrocarbons (Ed.: H.
Dodziuk), Wiley-VCH, Weinheim, 2009, pp. 166 – 204; d) A.
Rajca, S. Rajca, M. Pink, M. Miyasaka, Synlett 2007, 1799 – 1822;
e) A. Rajca, M. Miyasaka in Functional Organic Materials:
Syntheses, Strategies and Applications (Eds.: T. J. J. Mꢃller,
U. H. F. Bunz), Wiley-VCH, Weinheim, 2007, pp. 543 – 577;
f) S. K. Collins, M. P. Vachon, Org. Biomol. Chem. 2006, 4,
2518 – 2524; g) A. Urbano, Angew. Chem. 2003, 115, 4116 – 4119;
Angew. Chem. Int. Ed. 2003, 42, 3986 – 3989; h) T. J. Katz,
Angew. Chem. 2000, 112, 1997 – 1999; Angew. Chem. Int. Ed.
2000, 39, 1921 – 1923; i) K. P. Meurer, F. Vçgtle, Top. Curr. Chem.
1985, 127, 1 – 76; j) W. H. Laarhoven, W. J. C. Prinsen, Top. Curr.
Chem. 1984, 125, 63 – 130.
[2] For recent articles involving [6]helicenes specifically: a) J.
ˇ
ˇ
´
ˇ ˇ
Zꢂdny, A. Jancarik, A. Andronova, M. Sꢂmal, J. Vacek Chocho-
ˇ
ˇ
ˇ
lousovꢂ, J. Vacek, R. Pohl, D. Saman, I. Cꢄsarovꢂ, I. G. Starꢂ, I.
´
Stary, Angew. Chem. 2012, 124, 5959 – 5963; Angew. Chem. Int.
Ed. 2012, 51, 5857 – 5861; b) Y. Nakai, T. Mori, Y. Inoue, J. Phys.
Chem. A 2012, 116, 7372 – 7385; c) T. Kogiso, K. Yamamoto, H.
Suemune, K. Usui, Org. Biomol. Chem. 2012, 10, 2934 – 2936;
[9] The reactions can be performed using regular heating (oil bath)
at 1408C for 4 h with similar yields.
4
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
[10] In fact, the reaction conditions for the cross-coupling experi-
ments are somewhat forcing (high mol% of Pd, 5 equiv of
[19] Comparison of the spectra of 18 and 12 indicates that the donor
aminopropyl group is responsible for a blue shift with the
bromine having little influence on the absorption properties
(comparison of 12 and 1a).
ꢂ
PhB(OH)2 or 10 equiv PhC CH). The major by-product is the
derivative of monofunctionalization with one bromine being
replaced by a hydrogen atom.
[20] a) J. Lacour, A. Londez, C. Goujon-Ginglinger, V. Buß, G.
Bernardinelli, Org. Lett. 2000, 2, 4185 – 4188; b) L. Vial, J.
Lacour, Org. Lett. 2002, 4, 3939 – 3942; c) J. Vachon, G.
Bernardinelli, J. Lacour, Chem. Eur. J. 2010, 16, 2797 – 2805.
[21] J. Lacour, D. Moraleda, Chem. Commun. 2009, 7073 – 7089.
[22] Because of the presence of the anion 20, which acts as a NMR
chiral solvating agent, two sets of signals are observed in the
1H NMR spectrum: one for each the P and M enantiomers of 1a.
Integration of the respective signals indicates a 1:1 diastereo-
meric ratio. See the Supporting Information.
[23] The enantiomeric purity was determined by CSP-HPLC (CHIR-
ALPAK AD-H) on the neutral hydride adduct obtained by
simple treatment of [1a][PF6] with NaBH4 in EtOH. See the
Supporting Information.
[24] a) L. A. Nafie, T. A. Keiderling, P. J. Stephens, J. Am. Chem. Soc.
1976, 98, 2715 – 2723; b) G. Holzwarth, E. C. Hsu, H. S. Mosher,
T. R. Faulkner, A. Moscowit, J. Am. Chem. Soc. 1974, 96, 251 –
252; c) T. B. Freedman, X. L. Cao, R. K. Dukor, L. A. Nafie,
Chirality 2003, 15, 743 – 758.
[11] A. I. Meyers, K. A. Lutomski, Synthesis 1983, 105 – 107.
[12] Most probably, 1a undergoes successive nucleophilic attacks of
cyanide anions at the activated positions 5 and 13 followed each
time by hydride abstraction reactions under the oxidative
conditions in air.
[13] For a recent review on vicarious nucleophilic substitutions, see:
M. Ma˛kosza, Chem. Soc. Rev. 2010, 39, 2855 – 2868; for a selected
example of VNS functionalization on a dye, see: V. Leen, M.
Van der Auweraer, N. Boens, W. Dehaen, Org. Lett. 2011, 13,
1470 – 1473.
[14] Both monoclinic (racemate) and orthorhombic (conglomerate
enantiopure) structures were found in the crystallization batch;
the first kind being of higher quality and the only one reported.
One side chain possesses an anti conformation while the other
presents a gauche orientation. In the orthorhombic packing,
both side chains were gauche, that is with the terminal methyl
groups pointing back to the aromatic core. The pitch measured
was 3.31 and 3.17 ꢀ for the monoclinic and orthorhombic
structures, respectively.
[15] a) I. R. Mackay, J. M. Robertson, J. G. Sime, J. Chem. Soc. D
1969, 1470 – 1471; b) C. De Rango, G. Tsoucaris, J. P. Declerq, G.
Germain, J. P. Putzeys, Cryst. Struct. Commun. 1973, 2, 189 – 192.
[16] The angle between the two mean planes defined by the inner
naphthyl rings of 12 is of 52.68, which is significantly larger than
that of the related [4]helicene 4 (39.28, CCDC 205385).
[17] Compounds 1 present essentially identical UV-Vis properties in
CH2Cl2.
[25] This experiment was performed on an ECD apparatus having
a low to high temperature attachment which allows ECD
experiments from ꢀ80 to + 2308C.
[26] Considering first-order kinetics for the racemization, a barrier of
37 kcalmolꢀ1 corresponds to a half-life of 15 h at 1808C.
[27] CCDC 904858 ([12][BF4]·CH2Cl2) contains the supplementary
crystallographic 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.
[18] Differences between [2a][BF4] and [3][BF4] can however be
observed at higher energies in the spectra.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Helicenes
F. Torricelli, J. Bosson, C. Besnard,
M. Chekini, T. Bꢁrgi,
J. Lacour*
&&& — &&&
Modular Synthesis, Orthogonal Post-
Functionalization, Absorption, and
Chiroptical Properties of Cationic
[6]Helicenes
Pick and choose: Novel cationic diaza-,
azaoxo-, and dioxo[6]helicenes are readily
prepared and functionalized selectively by
orthogonal aromatic electrophilic and
vicarious nucleophilic substitutions (see
scheme). Reductions, cross-coupling, or
condensation reactions introduce addi-
tional diversity and allow tuning of the
absorption properties up to the near-
infrared region. The diaza salts can be
resolved into single enantiomers.
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!