Versatile bottom-up approach to stapled π-conjugated helical scaffolds: Synthesis and chiroptical properties of cyclic o-phenylene ethynylene oligomers

Article abstract of DOI:10.1002/anie.201206259

Carbon nanocoils (CNCs) are carbon allotropes with tubular diameters as small as 20 nm, and present interesting technological applications.[1] They have been used to generate magnetic fields through chiral electric currents, thus emulating the behavior of a solenoid,[2] and have shown pseudoelastic properties allowing elongations of up to 42% without permanent deformation.[3] Although in a broad sense [n]- helicenes and o-phenylenes can be considered CNCs, they feature extremely rigid backbones in the axial direction (helix propagation)[4] and densely packed π systems. This fact hinders the expected chiral electron transport (ET) because of competitive ET through theπ-stacked aromatic rings, thus limiting their use as a nanocoil.[5] Considering the structural features of these CNCs, we were interested in the conformationally flexible open-chain o-phenylene ethynylene oligomers (o-PEOs), which were mainly developed by Tew and coworkers[ 6] and can adopt helical arrangements by supramolecular interactions.[7] Nevertheless, such arrangements are solvent dependent and may not be suitable for applications demanding a permanent shape.[8] Within this context, the covalent stapling[9] of conformationally dynamic o-PEOs would lead to the corresponding less-tight helical compounds. These loops would have a helical conductive backbone, but the potential ET through theπ-stacked aromatic rings could be avoided by tuning the space defined by the staple (spacer group). This is essential to emulate the behavior of a macroscopic solenoid. Moreover, if the spacer group is conformationally flexible, the structure could be either compressed or stretched mechanically without significant energy variation. The stretching process of a very flexible chain could lead to a long-range pseudoelastic behavior. Additionally, the introduction of chiral spacers could lead to flexible, optically active structures adopting a helical shape, in which chiral currents could be potentially induced.[10] Moore and co-workers have reported a family of chiral m-PEOs using diastereoselective complexation with chiral guests[11, 12] or by introducing a chiral moiety as substituent within the oligomer chain.[13] The introduction of chirality into o-PEOs has been considerably less studied.[6] To the best of our knowledge, o-PEOs and m- PEOs with cyclic structures closed by a chiral unit have not been reported up to date.

Full text of DOI:10.1002/anie.201206259

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Communications
DOI: 10.1002/anie.201206259
Structure Elucidation
Versatile Bottom-up Approach to Stapled p-Conjugated Helical
Scaffolds: Synthesis and Chiroptical Properties of Cyclic o-Phenylene
Ethynylene Oligomers**
Noelia Fuentes, Ana Martin-Lasanta, Luis Alvarez de Cienfuegos, Rafael Robles,
Duane Choquesillo-Lazarte, Juan M. Garcꢀa-Ruiz, Lara Martꢀnez-Fernꢁndez, Inꢂs Corral,
Marꢀa Ribagorda, Antonio J. Mota, Diego J. Cꢁrdenas, M. Carmen CarreÇo, and
Juan M. Cuerva*
In memory of Christian Claessens
Carbon nanocoils (CNCs) are carbon allotropes with tubular
diameters as small as 20 nm, and present interesting techno-
logical applications.[1] They have been used to generate
magnetic fields through chiral electric currents, thus emulat-
ing the behavior of a solenoid,[2] and have shown pseudo-
elastic properties allowing elongations of up to 42% without
permanent deformation.[3] Although in a broad sense [n]-
helicenes and o-phenylenes can be considered CNCs, they
feature extremely rigid backbones in the axial direction (helix
propagation)[4] and densely packed p systems. This fact
hinders the expected chiral electron transport (ET) because
of competitive ET through the p-stacked aromatic rings, thus
limiting their use as a nanocoil.[5] Considering the structural
features of these CNCs, we were interested in the conforma-
tionally flexible open-chain o-phenylene ethynylene oligo-
mers (o-PEOs), which were mainly developed by Tew and co-
workers[6] and can adopt helical arrangements by supra-
molecular interactions.[7] Nevertheless, such arrangements are
solvent dependent and may not be suitable for applications
demanding a permanent shape.[8] Within this context, the
covalent stapling[9] of conformationally dynamic o-PEOs
would lead to the corresponding less-tight helical compounds.
These loops would have a helical conductive backbone, but
the potential ET through the p-stacked aromatic rings could
be avoided by tuning the space defined by the staple (spacer
group). This is essential to emulate the behavior of a macro-
scopic solenoid. Moreover, if the spacer group is conforma-
tionally flexible, the structure could be either compressed or
stretched mechanically without significant energy variation.
The stretching process of a very flexible chain could lead to
a long-range pseudoelastic behavior. Additionally, the intro-
duction of chiral spacers could lead to flexible, optically active
structures adopting a helical shape, in which chiral currents
could be potentially induced.[10] Moore and co-workers have
reported a family of chiral m-PEOs using diastereoselective
complexation with chiral guests[11,12] or by introducing a chiral
moiety as substituent within the oligomer chain.[13] The
introduction of chirality into o-PEOs has been considerably
less studied.[6] To the best of our knowledge, o-PEOs and m-
PEOs with cyclic structures closed by a chiral unit have not
been reported up to date.
Recently, we described the synthesis of simple organic
compounds which exert elementary electronic functions,[14] by
using a bottom-up approach. Herein, we disclose a similar
approach to synthesize the smallest members of a new CNC
family, which would be able to retain their striking properties.
Surprisingly, the stapling of simple foldamers has been
scarcely reported in literature.[15] We show that o-PEOs
functionalized with simple oxygenated functions such as
allylic ethers and benzylic alcohols can be conveniently
stapled using simple stapling reactions such as esterification
with diacid derivatives or alkene metathesis reactions.
Notably, remarkable chiroptical responses could be achieved
with the introduction of a very simple and available optically
[*] Dr. N. Fuentes,[+] A. Martin-Lasanta,[+] Dr. L. Alvarez de Cienfuegos,
Prof. Dr. R. Robles, Dr. J. M. Cuerva
L. Martꢀnez-Fernꢁndez, Dr. I. Corral
Dpto. de Quꢀmica, M-13, Facultad de Ciencias, UAM
Cantoblanco, 28049 Madrid (Spain)
Dpto de Quꢀmica Orgꢁnica, Facultad de Ciencias
Universidad de Granada (UGR)
18071 Granada (Spain)
[+] These authors contributed equally to this work.
[**] This work was supported by the Junta de Andalucꢀa (P09-FQM-
04571), MINECO (CTQ2011-22455, CTQ2011-24783), and Comu-
nidad de Madrid and European Social Fund (SOLGEMAC-S2009/
ENE-1617). We thank CSIRC-UGR and “Centro de Computaciꢂn
Cientꢀfica-UAM” for computation time. The project “Factorꢀa de
Cristalizaciꢂn, CONSOLIDER INGENIO-2010” and the beamline
I19 at Diamond (UK) provided X-ray structural facilities for this
work. DChL, NF, LMF, and AML thank CSD2006-00015, UGR, and
MEC for funding.
E-mail: jmcuerva@ugr.es
Dr. D. Choquesillo-Lazarte, Prof. Dr. J. M. Garcꢀa-Ruiz
Laboratorio de Estudios Cristalogrꢁficos IACT, CSIC- UGR
18071 Granada (Spain)
Dr. A. J. Mota
Departamento de Quꢀmica Inorgꢁnica, (UGR)
18071 Granada (Spain)
Dr. M. Ribagorda, Prof. Dr. D. J. Cꢁrdenas, Prof. Dr. M. C. CarreÇo
Dpto de Quꢀmica Orgꢁnica, M-01, Facultad de Ciencias, Universi-
dad Autꢂnoma de Madrid, Cantoblanco, 28049 Madrid (Spain)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206259.
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active staple such as a tartrate diester. A significant transfer of
chirality from the stereogenic centers to the helical frame-
work in these new cyclic structures was observed.
stereogenic carbon atoms of the tartrate is the same in all
cases, the presence in this region of positive bands in the
The starting o-PEOs A–H were prepared using Sonoga-
shira couplings.[16] We chose open-chain o-PEOs derivatives
A and D, having a para substitution pattern at the terminal
aryl ring, the meta-substituted analogues B and E, and the
meta,para-substituted derivative C (Scheme 1). These substi-
tution patterns allowed evaluation of the influence of the
different geometries on the properties of the final stapled
products. Compounds F, G, and H, having two meta,meta-
disubstituted terminal aromatic rings were also obtained so as
to place two linkers in the closed targets. Benzyl alcohols (A–
C, F) or allyl ethers (D, E, G, and H) situated on the terminal
aryl groups of the o-PEOs systems (Scheme 1) facilitated the
attachment to the spacer. Thus, compounds bearing the free
hydroxy groups were esterified with different diacids, pre-
viously activated as acid chlorides, as spacers.[17] The results,
shown in Scheme 1, evidenced that open-chain difunctional-
ized compounds A–C were able to efficiently accommodate
diesters derived from malonate (1, 5, and 7), succinate (2),
and glutarate (4). Compounds 3 and 6, having an l-tartrate
moiety, were obtained as a 80:20 mixture of diastereomers, as
determined by HPLC. This evidenced the presence of two
chiral elements in the structures: the stereogenic centers
present in the l-tartrate unit and a new chiral helical
1
fragment. All the structures were fully characterized by H
and 13C NMR spectroscopy, MS, and, in most cases, by X-ray
diffraction.[16]
The X-ray analysis confirmed our hypothesis of the partial
disconnection between the superposed benzene rings
(Figure 1). The distance between the aromatic superposed
rings, ranging from 4.5 to 5.6 ꢀ, was longer than the typical p–
p stacking distance (3.5–3.8 ꢀ). The helical arrangement of
these stapled structures was also seen in the X-ray structure.
The unit cell of the crystal obtained for 3 (Figure 1b)
consisted of two different helical diastereoisomers (M,R,R
and P,R,R), although 3 had been synthesized as a 80:20
mixture of both diastereomers.
Comparison of UV/vis spectra provided evidence of the
change from conformationally flexible o-PEOs to the close-
shaped systems.[16] The wide absorption band appearing in the
open structures at about lmax = 270 nm, resulting from the
aromatic chromophores, was red shifted in all cases to l =
300 nm after the stapling reaction.[18] The CD spectra of
compounds 3, 6, 11, 15, and 17, having the (À)-O,O’-di-
pivaloyl-l-tartrate moiety, are depicted in Figure 2. Taking
into account the presence of both M- and P-helical diaste-
reomers in these derivatives, the chiroptical responses are
noteworthy. In the region of l < 270 nm, the CD absorptions
of the tartrate moiety[19] overlap with bands corresponding to
the aromatic framework. As the R,R configuration of the
Scheme 1. Synthesized CNCs 1–17 from o-PEOs A–H. [a] Yield based
on recovered starting material.[16] [b] Chiralpack IC column using
n-hexane/iPrOH (90:10) as the eluent with a 0.5 mLminÀ1 flow rate.
[c] Chiralpack IB column using n-hexane/iPrOH (90:10) as the eluent
with a 0.5 mLminÀ1 flow rate. [d] Used 2 equiv of malonyl chloride.
[e] Used 2 equiv of (À)-O,O’-di-pivaloyl-l-tartaric acid.
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Figure 1. Representative X-ray structures: a) 10 (side view left and top
view right). b) P,R,R-3 and M,R,R-3.[27]
monotartrates 6 and 15, and negative ones in 3 (Figure 2a)
indicated a significant contribution of the aromatic chiral
helical framework to these absorptions. The CD bands in the
region l > 270 nm were also of opposite sign for 3 when
compared with those of 6 and 15. The p-substituted mono-
stapled tartrate 3 gave a chiroptical response in the region of
about l = 295 nm, which was much more intense (De = 47.35)
and pseudoenantiomeric to that of the meta-substituted
analogues 6 (De = À5.07) and 15 (De = À12.21). Thus the
chiral l-tartrate unit was able to induce a helix of different
configuration depending on the relative m or p positions of
the benzyl alcohols in the initial o-PEOs precursors. The
intensity of these bands significantly depends on the relative
substitution of the terminal aromatic rings. Pure (M,R,R)-3[20]
and (P,R,R)-3 diastereomers could be obtained after separa-
tion of the 80:20 mixture initially obtained by HPLC using
a chiral stationary phase.[16] As can be seen in Figure 2b
pseudoenantiomeric CD bands were observed for each
diastereomer. The absorptions of the 80:20 mixture of 3
were very similar to those of the (M,R,R)-3 diastereomer. The
theoretical simulation of the ECD spectra[16] of the two
possible (M,R,R) and (P,R,R) helical structures of 3 as well as
that of the 80:20 and 20:80 mixtures of both diastereomers
(Figure 2c), allowed the identification of the absolute con-
figuration of the major species contributing to the 80:20
mixture of 3 experimentally obtained. Theoretical ECD
spectra were calculated[21] in the frame of time-dependent
density functional theory (TDDFT), by combining the PBE0
functional,[22] and the 6-31G* basis set[23] as implemented in
Gaussian09 program suite.[24] As can be seen, the ECD curve
for the 80:20 mixture of (M,R,R)-3 and (P,R,R)-3 featured
a negative band in the l = 330–400 nm spectral region, a very
intense positive absorption at about l = 300 nm, and another
negative band at about l = 270 nm. The qualitative agreement
with the experimental ECD spectrum obtained for the 80:20
mixture of 3 and pure (M,R,R)-3 (Figure 2b) is very good and
allowed assignment of the (M,R,R)-3 configuration to the
major diastereomer. A pseudoenantiomeric ECD spectrum
was calculated for the reverse diastereomer ratio (Figure 2c),
Figure 2. CD spectra (1ꢃ10À5 m in CH2Cl2). a) Mono and doubly
stapled derivatives (À)-3, (+)-6, (+)-11, (+)-15, and (À)-17. b) Pure
diastereomers (M,R,R)-3 (solid line) and (P,R,R)-3 (dotted line), 80:20
mixture of (M,R,R)-3, and (P,R,R)-3 (dash-dot line). c) TD-PBE0/6-
31G* spectra[25] of (M,R,R)-3/(P,R,R)-3 (80:20; solid line) and (P,R,R)-
3/(M,R,R)-3 (80:20; dotted line).
and is in agreement with the ECD spectra experimentally
recorded for the pure diastereoisomer (P,R,R)-3.
Pseudoenantiomeric chiroptical responses were also
experimentally observed in the CD spectra of the doubly
stapled meta-substituted derivatives 11 [De (298 nm) =
À105.14], having two tartrate moieties, and 17 [De
(292 nm) = 35.02], having one tartrate and one 2-butene
linker (Figure 2a). As we were dealing with mixtures of
(P,R,R) and (M,R,R) diastereomers, the positive broad bands
at about l = 360 nm for 6 and 15 suggested that the major
diastereomer in this case the P-configured helical moiety.[20]
In a similar way, the configuration of the major diastereomers
present in 11 and 17 were assigned as (P,R,R) and (M,R,R),
respectively. Thus, a preferred handed P helicity was induced
when one (À)-O,O’-di-pivaloyl-l-tartaric acid was introduced
as a chiral staple in the m-substituted o-PEOs B and H, to fix
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the major helical structure in 6 and 15. The same P helicity
was induced when two l-tartrate units acted as staples to close
the meta,meta-disusbtituted tetraol o-PEO F, thus leading to
11. The opposite M helicity was induced when the p-
substituted o-PEO A was transformed into the monostapled
derivative 3 and when the meta,meta-disubstituted F was
transformed sequentially, by the l-tartrate reaction and olefin
metathesis, into 17. This remarkable chirality induction is due
to an intramolecular central-to-helix chirality transfer and the
configuration of the major diastereomer formed is structure
dependent.
Theoretical calculations also allowed prediction of pseu-
doelastic behavior for these systems. An inspection of the
potential energy profile of compound 1, chosen as a model,
when submitted to mechanical stress, was calculated. DFT
calculations (M06/6-31G*)[26] revealed two different regimes
for the compression and the lengthening (Figure 3). An
almost quadratic behavior was observed at small elongations
(Æ 0.2 ꢀ) with an estimated Hook force constant of 20.2
kcalꢀÀ2 molÀ1.[4] In the lengthening regime, except for small
elongations, the system dependence was practically linear and
elongations up to 2 ꢀ (55% longer) were accessible with low-
energy requirements (< 3 kcalmolÀ1). This pseudoelastic
behavior is quite similar to that of CNCs.[3]
Keywords: circular dichroism · cyclization ·
density functional theory · helical structures · oligomers
.
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Figure 3. Potential energy profile of compound 1 toward elongation.
In conclusion, we have developed a versatile synthesis of
flexible helical p-conjugated cyclic nanocoils from easily
prepared o-PEOs, which retain some of their interesting
properties. Our approach allowed the preparation of homo-
chiral systems by introduction of l-tartrate esters. This simple
chiral unit was able to induce either P or M configurations in
the helical systems, as evidenced by the chiroptical responses
observed. These helices could work as nanosolenoids, nano-
springs, pressure sensors (when submitted to an external
source of pressure), or adaptative ligands for metals because
of their flexible backbone. The method described herein is
general enough to envision further synthetic applications en
route to larger chiral coils with different connecting groups.
Received: August 4, 2012
Revised: September 13, 2012
Published online: November 14, 2012
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1994. See also the theoretical calculations of the CD. This
assignment also fit with the known shape and sign of the CD of
related [n]-helicenes: F. Furche, R. Ahlrichs, C. Wachsmann, E.
13040 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 13036 –13040