Angewandte
Chemie
Helical Structures Very Important Paper
One-Step Synthesis of [16]Helicene**
Kazuyuki Mori, Takashi Murase,* and Makoto Fujita*
Abstract: A single-strand arylene–vinylene precursor contain-
ing four phenylene and three naphthylene units linked together
with six vinylene spacers undergoes helical folding via sextuple
photocyclization to give a [16]helicene core in a single step.
The phenylene and naphthylene units are arranged in the
precursor such that unfavorable side reactions (anthracene or
benzoperylene formation) are avoided, and this is the key to
the success of the one-step synthesis of [16]helicene, which is
the longest [n]helicene that has been synthesized to date.
T
he [n]helicenes have attracted continuous interest in the
past few decades, not only because of their structural curiosity
and beauty but also for the unique properties caused by their
helically extended chiral p systems.[1,2] With increasing
n number, the p electrons start to form additional layers;
a double layer at n = 7 and a triple layer at n = 13. In the
triple-layered structures, the framework around the middle is
tightly compressed and it is hard to release the steric
distortion. This makes the synthesis of higher [n]helicenes
with n numbers of 13 and over more difficult. In fact, despite
numerous attempts to synthesize the higher helicenes,
[14]helicene, reported by Martin and Baes in 1975,[3a] is still
the longest homologue and provides one of only a few
examples of a triple-layered [n]helicene.[3] Herein, we report
the one-step synthesis of [16]helicene by the multiple photo-
cyclization of single-strand oligo(arylene–vinylene) precursor
1 (Scheme 1). Although the oxidative photocyclization of
stilbenes[4] is the most frequently employed reaction in
helicene synthesis, attempts to apply this fundamental reac-
tion to the synthesis of higher helicenes (n ꢀ 13) have failed.[5]
The key to the success of the [16]helicene synthesis in this
study is the rational design of precursor 1, in which the
phenylene and naphthylene units are arranged such that
unfavorable side reactions are avoided.
Scheme 1. One-step synthesis of [16]helicene 2 by multiple oxidative
photocyclizations of precursor 1.
easy access to the precursors by Wittig olefination and facile
E/Z-photoisomerization that allows the use of E/Z mixtures
as precursors. Martinꢀs [14]helicene was formed by the
photocyclization of a precursor olefin with a [3] + [6] + [3]
or [4] + [4] + [4] triad structure, where “[n]” and “ + ” denote
ortho-fused [n]helicene subunits and vinylene linkers, respec-
tively.[3a] Our synthetic strategy utilizes only the simplest
subunits [1] and [2] (namely, phenylene and naphthylene
units). Larger [n] subunits were not included in our precursor
design because of increasing synthetic difficulty, and because
the [6] + [6] precursor failed to furnish [13]helicene under any
conditions, despite favorable theoretical predictions.[5] We,
however, avoided the simplest [1] + [1] + [1] and [2] + [2]
sequences in the precursor to exclude the formation of an
unfavorable [5]helicene framework that easily cyclizes into
benzoperylene (Scheme 2a).[7] The [2] + [1] + [2] sequence
was also avoided because it has been reported that this
sequence predominantly gives planar dinaphthanthracene
rather than the expected [7]helicene (Scheme 2b).[6a] Based
on these considerations, we deduced the following guideline
for the design of the precursor olefins: [2] units must be
separated by two [1] units. Namely, only ··· + [2] + [1] + [1] +
[2] + [1] + [1] + [2] + ··· sequences can undergo multiple oxi-
dative photocyclizations without unfavorable benzoperylene
or anthracene formation.
The oxidative photocyclization of (Z)-stilbene frame-
works is still the most common and powerful method for
synthesizing a variety of helicene homologues[6] because of
[*] K. Mori, Prof. Dr. M. Fujita
Department of Applied Chemistry
School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: mfujita@appchem.t.u-tokyo.ac.jp
Dr. T. Murase
Department of Material and Biological Chemistry
Faculty of Science, Yamagata University
1-4-12 Kojirakawa-machi, Yamagata-shi, Yamagata 990-8560 (Japan)
E-mail: tmurase@sci.kj.yamagata-u.ac.jp
To confirm our synthetic guideline for the precursor
synthesis, the simplest [2] + [1] + [1] + [2] precursor 3 was
prepared and its photocyclization into [9]helicene 4 was
examined. Precursor 3 was easily prepared from (Z)-4,4’-
dibromostilbene[8] by bis-formylation (nBuLi, DMF; 84%)
and Wittig olefination with (2-naphthylmethyl)triphenylphos-
phonium bromide[9] (82%). A toluene solution of precursor 3
was irradiated with a high-pressure Hg lamp at 908C for 8 h in
[**] This research was supported by Grants-in-Aid for Specially Pro-
moted Research (24000009) and for Young Scientists (A)
(25708008).
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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