Alkynes as Linchpins for the Additive Annulation of Biphenyls: Convergent Construction of Functionalized Fused Helicenes

Article abstract of DOI:10.1002/anie.201606330

A new approach to fused helicenes is reported, where varied substituents are readily incorporated in the extended aromatic frame. From the alkynyl precursor, the final helical compounds are obtained under mild conditions in a two-step process, in which th

Full text of DOI:10.1002/anie.201606330

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201606330
German Edition: DOI: 10.1002/ange.201606330
Fused Helicenes
Alkynes as Linchpins for the Additive Annulation of Biphenyls:
Convergent Construction of Functionalized Fused Helicenes
Rana K. Mohamed, Sayantan Mondal, Joseph V. Guerrera, Teresa M. Eaton,
Thomas E. Albrecht-Schmitt, Michael Shatruk, and Igor V. Alabugin*
Abstract: A new approach to fused helicenes is reported,
where varied substituents are readily incorporated in the
extended aromatic frame. From the alkynyl precursor, the
final helical compounds are obtained under mild conditions in
biphenyl units. This work reports a simple method for such
transformation for the model system and illustrates the utility
of this approach for the preparation of distorted helicene
aromatics.
À
a two-step process, in which the final C C bond is formed via
Curvature introduces a new design element for controlling
the organization of aromatics into 3D crystal architecture.[4]
By using a nonplanar core, the aromatic rings may adopt
unusual intermolecular interaction patterns that are typically
unavailable to flat molecules. In helicenes, distortion from
a planar topology is induced due to ortho-annelation of the
polycyclic aromatic framework. Owing to unique spectral,
optical, and structural properties,[5] helicenes are investigated
for application in nanoscience,[6] asymmetric catalysis,[7]
chemical biology, supramolecular, and polymer chemistry.[8]
Synthetic methods that allow facile control over size,
rigidity, thermal stability, and functionality are crucial for
advancing the utility of helicenes.[9,10] Recent reports,[11,12]
have leveraged the propensity of alkynyl precursors to form
carbon rich conjugated structures.[13] Despite significant
progress in the area, the list of general methods that can
produce a large library of helicenes with a broad selection of
precisely placed functional groups is still limited.[14]
An attractive metal-free approach to additive annulation
of biphenyls would be to couple iodo-cyclization of alkynes
with directed Mallory photocyclization of the o-terphenyl
intermediate.[15,16] The advantage of this approach is that the
dearomatized dihydrophenanthrene intermediates derived
from the iodo-substituted substrates can undergo irreversible
rearomatization via simple elimination of HX under mild
conditions. The use of such “preoxidized” substrates with an
electronegative substituent, X, on one of the reacting carbons
at the ring closure point obviates the need for an external
oxidant for the aromatization step. Most of the effort has
focused on X = Cl,[17] Br,[18] OMe[19] and more recently
X = F[20] while surprisingly little is known about iodinated o-
terphenyls.[21]
a photochemical cyclization/ dehydroiodination sequence. The
distortion of the p-system from planarity leads to unusual
packing in the solid state. Computational analysis reveals that
substituent incorporation perturbs geometries and electronic
structures of these nonplanar aromatics.
T
he armchair edge is a common structural feature of carbon-
rich nanostructures (nanotubes, graphene ribbons etc). Scott
described a creative Diels–Alder strategy for the growth of
such nanostructures from the “biphenyl-like” bay regions.[1]
Although this approach is hampered by the weak reactivity of
acetylene, more reactive analogues such as nitroethylene can
be used, albeit still at relatively harsh conditions.[1,2] Intrigued
by the recent report of selective edge halogenation proce-
dures for nanographenes[3] that may allow alkyne introduc-
tion at these positions, we became interested in testing the
possibility of using alkynes as a two-carbon unit for connect-
ing two armchair-edged carbon nanostructures (Scheme 1).
The basic step of such process is the annulation of two
Undeterred by this scarcity, we decided to explore the
possibility of coupling iodocyclizations[22] to light-induced
cyclodehydroiodination (CDHI). Aryl iodides can be con-
veniently prepared from alkynes in iodocyclization processes
that give the bonus of annealing an additional cycle with the
properly positioned iodo group. In this scenario, the intro-
duction of iodine is transient (it can be considered a traceless
directing group if the two cyclization steps are taken
together). In the overall cascade, the alkyne moiety serves
as a staple that “stiches” the two biphenyl systems together. A
similar approach was used by Liu and co-workers[23] but the
last step was a Pd-catalyzed Mizoroki–Heck coupling. In this
work, we report an equally efficient photochemical proce-
Scheme 1. Proposed use of alkynes for annealing two biphenyl systems
and its possible implications for chemistry of carbon nanostructures.
[*] Dr. R. K. Mohamed, Dr. S. Mondal, J. V. Guerrera, T. M. Eaton,
Prof. T. E. Albrecht-Schmitt, Prof. M. Shatruk, Prof. I. V. Alabugin
Department of Chemistry and Biochemistry, Florida State University
Tallahassee, FL 32310 (USA)
E-mail: alabugin@chem.fsu.edu
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201606330.
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dure, which offers the practical advantage of avoiding the use
of a transition metal catalyst.
The starting point for our approach was the synthesis of
a diverse library of a-iodo-b-substituted naphthalene building
blocks N1–10. For this purpose, we took advantage of our
Bu3SnH-mediated radical cyclization/fragmentation cascades
of ortho-enynes,[24] followed by in situ quantitative iododes-
tannylation, with I2 in DCM, which provided N1–10, as shown
in Scheme 2.
Scheme 3. ICl cyclization does not have to be selective when coupled
with convergent photochemical CDHI.
Table 1: The library of fused helicenes H1–H10.
Helicene
Yield Helicene
Yield
35%
Scheme 2. The synthetic route for preparing the alkynyl precursors of
helicenes A1–A10.
H1
82% H6
The second set of required structural units (2-ethynylbi-
phenyls) was assembled via Suzuki cross coupling of o-
bromophenyl TMS alkynes with different boronic acids, B1–
B3 followed by desilylation with K2CO3 in 1:1 THF:MeOH.
Subsequent Sonogashira cross coupling of the a-iodo, b-
substituted naphthalenes[23] and the ethynyl biphenyl deriv-
atives C yielded a library of the key bis(biaryl) acetylene
precursors A1–A10.[25] To expand the general applicability of
the protocol for the synthesis of 5,5 fused carbohelicenes such
as helicene H10, we ran the Sonogashira cross coupling with
1-iodo-2,2’-binaphthalene and 2-ethynyl-1,1’-biphenyl and
obtained the internal alkyne precursor A10 in good yield of
86%.
The first core ring of the helicene frame is assembled by
treating each bis(biaryl)acetylene A1–A10 with 1.1 equiv of
ICl in DCM at À788C for 2–3 hours. The iodonium-induced
carbocyclization could afford either the substituted iodochry-
sene, iodophenanthrene, or a mixture of both. The conven-
ience of this convergent synthetic strategy is that the
subsequent photochemical CDHI step should convert either
structural isomer into the same final helical products, H1–
H10, as shown in Scheme 3.
H2
H3
89% H7
91% H8
93%
81%
H4
H5
91% H9
90%
80%
90% H10
The methodology can be readily extended to the higher
homologues such as [5,5]helicene H10. ICl-induced carbo-
cyclization onto the alkyne provided the iodinated compound
in almost quantitative yield. Photochemical closure was
successful in affording the [5,5] helicene H10 in 80% yield.
In general, we found that the addition of propylene oxide
(excess) was helpful to the reaction, likely assisting as an acid
scavenger of in situ generated HI. One notable benefit of this
directed photochemical CDHI process is that the lack of
external oxidant prevents further cyclization of the newly
formed helicenes.[27]
The helical structures of H2 and H7 were unambiguously
determined by X-ray crystallographic analysis (Figure 1),
which confirmed an extended doubly twisted p-system for
both structures. The compounds crystallize in the space
groups P21/c and Pbca, respectively. For both structures,
significant bond length alternation originates from deviation
Interestingly, we observed selective carbocyclization into
the iodophenanthrene derivatives. The observed regioselec-
tivity is likely due to less steric congestion upon coordination
of I+ to the alkyne away from the naphthyl moiety.
Photochemical closure of iodophenanthrene derivatives
A’1–10, driven by the release of HI, readily afforded the
functionalized fused helicenes shown in Table 1. This process
allows access to structurally diverse substrates containing
functional groups such as Ph (H1), tolyl derivatives (H2–H4),
methoxy and dimethoxy aryls (H8–H9), as well as hetero-
cyclic thiophene and pyridine units placed at the termini of
the helicene scaffold (H5–H6). However, photocyclization of
the aryl iodide produced from pyridyl acetylene A6 was
inefficient, yielding helicene H6 in 35% yield.[26]
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Figure 1. Crystal structures of a) [4,5]-fused helicenes H2 and b) H7,
with the bond lengths shown in ꢀ.
from planarity and from differences in the aromatic character,
particularly in the central naphthalene subunit. The bond
lengths, the dihedral angles between the planes of the
terminal rings, A–A’ and B–B’, and the distances of the
inner and outer helical pitch between rings A and A’ and rings
B and B’ are shown in Figure 1.
The presence of different substituents in the helical
skeleton leads to substantial differences in the crystal pack-
ings of H2 and H7. In both crystal structures, the molecules
form p-p stacks along the b axis or the a axis of the lattice,
respectively. In the crystal structure of H2 which contains
columns of molecules with the same handedness, the stacks
are held together by effective p–p interactions between rings
A (dring-ring = 3.40 and 3.52 ꢀ), which result in the CH3
substituents of one stack facing the CH3 substituents of the
neighboring stack (Figure 2a). Such arrangement leads to
alternating p-p-interdigitated and CH3-lined regions. The
CH3 groups are engaged in s–p interactions with the inner
ring of the neighboring molecule, with the H-centroid
distance of 2.59 ꢀ (Figure 2b). The entire crystal packing
thus exhibits extensive intermolecular p–p and s–p contacts
that minimize the free space between the stacks (Figure 2c).
Note that extensive p–p contacts exist both inside the
columnar stacks and between them.
The crystal packing of H7 is markedly different. It is
dominated by intermolecular p–p contacts between the
curved p-surfaces within the same stack while interactions
between the stacks are provided by s-p- contacts (Figure 3a).
Thus, the interdigitation between the stacks is minimal, which
leads to the crystal packing with quite large open channels.
These openings are filled with strongly disordered hexane
molecules (Figure 3b) The crystal packing in this compound
can be considered reminiscent of metal–organic or covalent–
organic frameworks,[28] but in the present case the molecules
are held together via the combination of shape complemen-
tarity and weak supramolecular interactions to create the
open-pore structure.
Figure 2. Crystal packing of H2: a) a side view of stacks revealing
regions dominated by p–p and s–p (dashed lines) interactions; b) the
s–p interaction between the methyl groups and the inner rings of the
molecules; c) crystal packing viewed down the b axis.
inner ring of the neighboring molecule, in the structure of H7,
the cyano group positions its terminal N atom over the outer
A’ ring, with the N–centroid distance of 3.740 ꢀ. Thus, the
variation of the substituent offers a viable approach to
achieving specific crystal packing effects.
The photophysical properties of several H1–H10 in
CH2Cl2 are summarized in the Supporting Information (SI).
In short, the UV-visible absorption and fluorescence of
Interestingly, unlike the CH3-substituted derivative H2, in
which each methyl group shows preferred interaction with the
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prevent conjugation throughout the extended polyaromatic
system (Figure S5a in SI).
The calculated effect of peripheral incorporation of
heteroatoms (S, N) into the fused helicene, H5 suggests
large geometric and electronic changes—the presence of
a nitrogen lone pair planarizes the respective fjord region in
a favorable hydrogen bonding interaction (2.2 ꢀ) with the
À
proximal aromatic C H. Electronically, the HOMO–LUMO
gap is relatively attenuated (2.01 eV) with the HOMO
significantly localized to the sulfur containing fused phenan-
threne unit (see Figure S5b).
In conclusion, we describe a versatile method for the
synthesis of complex, fused functionalized helicenes in high
chemical yield. One of the key building blocks is provided by
the selective Sn-mediated cyclization of aromatic ortho-
enynes.[24] Sonogashira coupling followed by ICl-induced
cyclization provide an iodinated precursor for the final
photocyclization in yields consistently exceeding 80%. The
dehydrohalogenative photocyclization of iodinated o-ter-
phenyls is viable and offers an efficient synthetic route to
4,5- and 5,5-fused double helicenes with differently substi-
tuted aromatic peripheries. The crystal packing is strongly
influenced by the peripheral substituents, suggesting that
their presence can be leveraged to impact photophysical and
electrochemical properties of these materials.
Acknowledgements
Figure 3. Crystal packing of H7·0.5C6H14: a) a side view of stacks
revealing extensive p–p interactions within each stack and weaker s–p
contacts (dashed lines) among the stacks; b) crystal packing viewed
down the a axis.
I.A. is grateful to the National Science Foundation (CHE-495
1465142) for support of this research project. M.S. acknowl-
edges partial support from the NSF (CHE-1464955).
selected helicenes H1–H8 are in the range of 360–405 nm for
Keywords: alkynes · helicenes · photocyclodehydroiodination
labs and 370–460 nm for lem
.
All calculations were performed using the Gaussian09
program package as discussed in the SI. In the design of
materials for organic field effect transistors (OFETs), it is
important to maximize electron mobility, which depends on
many factors and can be enhanced if the reorganization
energy (i.e., the difference in the geometries of the neutral
and oxidized forms) is minimized.
A simple way to evaluate the effect of substituents at
reorganization energies is to examine the HOMO structure. If
a substituent is placed next to a node, then it will not have
a large effect on reorganization energy. In contrast, if
a substituent is not at the node and there is a lot of density
at the respective position, it may have a large effect on the
reorganization energies.[29]
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Received: June 29, 2016
Published online: && &&, &&&&
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Communications
Fused Helicenes
A new approach to fused helicenes is
reported, in which varied substituents are
readily incorporated in the extended aro-
matic frame. From an alkynyl precursor,
the final helical compounds are obtained
in a two-step process, in which the final
R. K. Mohamed, S. Mondal, J. V. Guerrera,
T. M. Eaton, T. E. Albrecht-Schmitt,
M. Shatruk,
I. V. Alabugin*
&&&& — &&&&
À
C C bond is photochemically forged by
Alkynes as Linchpins for the Additive
Annulation of Biphenyls: Convergent
Construction of Functionalized Fused
Helicenes
coupling cyclization and dehydroiodina-
tion. The distortion of the p-system from
planarity leads to unusual packing in the
solid state.
6
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