Synthesis of Long Oxahelicenes by Polycyclization in a Flow Reactor

Article abstract of DOI:10.1002/anie.201700341

A series of oxahelicenes composed of ortho/meta-annulated benzene/pyridine and 2H-pyran rings were synthesized on the basis of the cobalt(I)-mediated (or rhodium(I)- or nickel(0)-mediated) double, triple, or quadruple [2+2+2] cycloisomerization of branched aromatic hexa-, nona-, or dodecaynes, thus allowing the construction of 6, 9, or 12 rings in a single operation. The use of a flow reactor was found to be beneficial for the multicyclization reactions. The stereogenic centers present in some of the oligoynes steered the helical folding in such a way that the final oxa[9]-, [13]-, [17]- and [19]helicenes were obtained in both enantiomerically and diastereomerically pure form. Specifically, the oxa[19]helicenes beat the current record in the length of a helicene backbone. Single-molecule conductivity was studied by the mechanically controllable break-junction method with a pyridooxa[9]helicene.

Full text of DOI:10.1002/anie.201700341

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700341
German Edition: DOI: 10.1002/ange.201700341
Helical Structures
Synthesis of Long Oxahelicenes by Polycyclization in a Flow Reactor
+
+
+
ˇ
Jind rˇ ich Nejedl y´ , Michal Sꢀmal , Ji rˇ ꢁ Rybꢀ cˇ ek , Miroslava Tobrmanovꢀ, Florence Szydlo,
Christophe Coudret, Maria Neumeier, Jaroslav Vacek,* Jana Vacek Chocholou sˇ ovꢀ,
ˇ
Milo sˇ Bud eˇ sˇ ꢁnsk y´ , David Saman, Lucie Bednꢀrovꢀ, Ladislav Sieger, Irena G. Starꢀ,* and
Ivo Star y´ *
Dedicated to Professor Thomas J. Katz
Abstract: A series of oxahelicenes composed of ortho/meta-
annulated benzene/pyridine and 2H-pyran rings were synthe-
sized on the basis of the cobalt(I)-mediated (or rhodium(I)- or
nickel(0)-mediated) double, triple, or quadruple [2+2+2]
cycloisomerization of branched aromatic hexa-, nona-, or
dodecaynes, thus allowing the construction of 6, 9, or 12 rings
in a single operation. The use of a flow reactor was found to be
beneficial for the multicyclization reactions. The stereogenic
centers present in some of the oligoynes steered the helical
folding in such a way that the final oxa[9]-, [13]-, [17]- and
The current record in helicene length is held by Mori
et al., who recently accomplished the synthesis of the
[16]helicene and its derivative 1 (Figure 1) by the use of
a sextuple photocyclodehydrogenation to fold a cleverly
designed single-strand arylene–vinylene precursor into
[
5]
a helix. Even more carbocycles are present in [17]heliphene
2 (angular [9]phenylene) reported by Vollhardt and co-
workers, whose landmark synthesis relied on the triple
cobalt(I)-mediated [2+2+2] cycloisomerization of an aro-
[6]
matic nonayne. The low yields of such multiple cyclizations
(10 and 2–3.5%, respectively) manifest, however, both the
limits of the current synthetic methodologies and the
[
19]helicenes were obtained in both enantiomerically and
diastereomerically pure form. Specifically, the oxa-
19]helicenes beat the current record in the length of a helicene
[
7]
[
structural complexity of the highest helicenes. Not surpris-
ingly, the lower homologues, such as the [15]- and [13]thia-
backbone. Single-molecule conductivity was studied by the
mechanically controllable break-junction method with
a pyridooxa[9]helicene.
[
8]
helicene reported by Yamada et al., the [14]- and [12]hel-
[
9]
icene synthesized by Martin and Baes, and the [13]helicene
[10]
described by Martin et al., are also quite rare. Intriguingly,
Werz and co-workers recently demonstrated the synthesis of
[
1]
L
ong helically chiral (hetero)[n]helicenes (n ꢀ 11) are both
challenging synthetic targets and potentially useful materials
[
11]
a truncated [12]helicene analogue. In contrast, successful
and diverse approaches to [11]helicene and its analogues were
[2]
[3]
for molecular (electro)mechanics, (opto)electronics, and
[4]
chiroptics. Although numerous methods for the preparation
of helicenes, heterohelicenes, and their analogues have been
developed, only the multiple photocyclodehydrogenation of
diaryl olefins and the [2+2+2] cycloisomerization of alkynes
have so far passed the tough test for synthetic methods that
are suitable for the preparation of molecular screws contain-
[1]
ing more than eleven all-ortho condensed rings.
[
+]
ˇ
[+]
[+]
[
*] J. Nejedl y´ , Dr. M. Sꢀmal, Dr. J. Rybꢀ cˇ ek, M. Tobrmanovꢀ,
Dr. F. Szydlo, Dr. C. Coudret, M. Neumeier, Dr. J. Vacek,
ˇ
Dr. J. Vacek Chocholou sˇ ovꢀ, M. Bud eˇ sˇ ꢁnsk y´ , Dr. D. Saman,
Dr. L. Bednꢀrovꢀ, Dr. L. Sieger, Dr. I. G. Starꢀ, Dr. I. Stary´
Institute of Organic Chemistry and Biochemistry
Czech Academy of Sciences
Flemingovo nꢀm. 2, 16610 Prague 6 (Czech Republic)
E-mail: jaroslav.vacek@uochb.cas.cz
stara@uochb.cas.cz
stary@uochb.cas.cz
Dr. L. Sieger
Department of Physics, CTU in Prague
Faculty of Electrical Engineering
Technickꢀ 2, 16627 Prague 6 (Czech Republic)
+
[
] These authors contributed equally.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
Figure 1. The longest helicenes and their analogues synthesized to
date. TIPS=triisopropylsilyl, Tol=p-tolyl.
1002/anie.201700341
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[
9]
[12]
reported by Martin and Baes, Wynberg et al., Rajca and
[13]
[14]
co-workers, Tanaka and co-workers, Werz and co-work-
ers, and us. Although the asymmetric synthesis of long
[
11]
[15]
[16]
(
hetero)[n]helicenes (n ꢀ 11) has been attempted rarely,
recent results are truly encouraging: We demonstrated the
diastereoselective cobalt(I)-mediated double [2+2+2] cyclo-
isomerization of a centrally chiral hexayne to generate the
[
15]
ortho/meta-fused [11]helicene analogue with d.r. 90:10.
Analogously, Tanaka and co-workers used the rhodium(I)-
catalyzed enantioselective double [2+2+2] cycloisomeriza-
tion of an achiral hexayne to obtain the all-ortho-fused
[
14]
oxa[11]helicene with 91% ee.
Herein, we report the synthesis of a series of oxahelicenes
in both racemic and enantiomerically pure form by the
multiple [2+2+2] cycloisomerization of branched aromatic
oligoynes. This approach produced the longest helicene
analogues prepared so far, such as (À)-(M,R,R)-3
(
Figure 1). We also initiated studies on the single-molecule
conductivity of helicenes by employing the mechanically
controllable break-junction method.
On the basis of the principles of the stepwise assembly of
oligo(m-phenyleneethynylene)s, studied extensively by
[17]
Moore and co-workers as foldamers,
we attempted the
preparation of the oxahelicene precursors 7 and 9–14 (achiral
or enantiomerically pure; Table 1). The synthesis of (À)-
(R,R)-9 exemplifies the approach to the key arylene–ethyny-
lene short monodispersed oligomers (Scheme 1; for the
preparation of 7 and 9–14, see the Supporting Information).
The Sonogashira monocoupling of diiodide 4 with alkyne (À)-
Scheme 1. Synthetic route to (À)-(R,R)-7 and (À)-(R,R)-9, oligoyne
precursors of the corresponding long oxahelicenes: a) (À)-(R)-5
(
R)-5 (1.5 equiv) led to the desymmetrized iodide (À)-(R)-6
(
1.5 equiv), [Pd(PPh ) Cl ] (5 mol%), CuI (10 mol%), iPr NH, benzene
3 2 2 2
as the main product (56%), accompanied by the symmetrical
hexayne (À)-(R,R)-7 (25%) resulting from double coupling.
After their separation by liquid chromatography, the iodide
(
3:1), room temperature, 2 h, 56% for (À)-(R)-6, 25% for (À)-(R,R)-7;
b) 8 (1.1 equiv), [Pd(PPh ) Cl ] (5 mol%), CuI (10 mol%), iPr NH,
3
2
2
2
toluene (2:1), room temperature, 16 h, 36%. Tol=p-tolyl.
(
À)-(R)-6 was subjected to a final coupling reaction with
diyne 8 to provide the target nonayne (À)-(R,R)-9 in
satisfactory yield.
[13]helicene (À)-(M,R,R)-18 (entry 4). The successful triple
[2+2+2] cycloisomerization of the latter oligoyne prompted
us to attempt the quadruple cyclization of dodecaynes to close
12 rings in a single operation. Indeed, the enantiomerically
pure pyridine-derived dodecayne (À)-(R,R)-9 furnished the
diastereomerically pure pyridooxa[17]helicene compound
(À)-(M,R,R)-19 (entry 5). Importantly, the achiral naphthyl-
derived dodecayne 13 underwent the desired multiple cycli-
zation to give the oxa[19]helicene rac-20 (entry 6). Finally, the
enantiomerically pure naphthyl-derived dodecayne (À)-
(R,R)-14 was successfully converted into the diastereomeri-
cally pure oxa[19]helicene (À)-(M,R,R)-3 (entry 7).
Attempts at the multiple [2+2+2] cycloisomerization of
the model oligoynes started with a comparison of the
efficiency of various metal complexes in the cyclization of
the readily accessible hexayne 10 (stoichiometric or super-
stoichiometric amounts of MLn were used to guarantee
complete conversion). Although [Ni(cod) ] (in a Schlenk
2
flask) and [Cp*Rh(CH =CH ) ] (in a microwave reactor)
2
2 2
exhibited slightly better performance than [CpCo(CO) ]/PPh
2
3
(
in a Schlenk flask under irradiation with visible light) or
[
Rh (CH =CH ) Cl ] (in a microwave reactor) in providing
2
2
2
4
2
oxa[9]helicene rac-15 (Table 1, entry 1), [CpCo(CO) ] (in
2
a flow reactor) outperformed [Ni(cod) ]/PPh (in a Schlenk
The preparative yields of the cyclized products were 18–
53%, thus indicating a loose correlation between the yield
and the length of the helix (the average yield per [2+2+2]
cycloisomerization reaction was calculated to be 63–83%).
Importantly, the use of the high-temperature high-pressure
flow reactor was a key to this success; in the case of multiple
cyclization reactions of the longest oligoynes, the yield
surpassed that observed upon microwave or conventional
heating. This positive effect of the flow reactor might be
attributed to efficient heat transfer and therefore the short
reaction time (8–16 min), which resulted in reduced inter-
mediate/product deterioration. The asymmetric synthesis of
2
3
flask) in promoting the cyclization of enantiomerically pure
(
À)-(R,R)-7 into oxa[9]helicene (À)-(M,R,R)-16 as a single
diastereomer (entry 2).
Accordingly, subsequent multiple [2+2+2] cycloisomeri-
zation reactions were preformed in the presence of [CpCo-
(
CO) ] in a flow reactor. The other metal complexes
2
mentioned above were ineffective in cyclizations of the
longer oligoynes. Thus, the achiral naphthyl-derived hexayne
1
1 was converted into [11]helicene rac-17 (entry 3), and the
enantiomerically pure pyridine-derived nonayne (À)-(R,R)-
delivered the diastereomerically pure pyridooxa-
1
2
2
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Table 1: Multiple [2+2+2] cycloisomerization of oligoynes 7 and 9–14 into the corresponding long oxahelicenes 3 and 15–20.
[a] Reaction conditions A: THF, room temperature, 1–16 h; B: decane, 1408C, 30 min, irradiation with a halogen lamp; C: THF, 1408C, 15 min,
microwave reactor; D: THF, 2508C, 8–16 min, flow reactor. [b] In the case of racemates, only the M enantiomer is shown for clarity. [c] The number of
ortho/meta condensed rings is given in square brackets. [d] Yield of the isolated product. cod=1,5-cyclooctadiene, Cp*=1,2,3,4,5-pentamethyl-
cyclopentadienyl.
oxa[n]helicenes (À)-(M,R,R)-3, 16, 18, and 19 (n = 9, 13, 17,
availability of (R)- and (S)-but-3-yn-2-ol as the key chiral
and 19), which is unprecedented in the family of long
building block and the powerful stereocontrol induced by 1,3-
[
18,19]
[
n]helicenes (n > 11), benefited from both the commercial
allylic-type strain
in the folding of the enantiomerically
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pure oligoyne precursors into the diastereomerically pure
helices.
Despite our efforts to obtain material suitable for X-ray
crystallographic analysis, the well-soluble helical products 3
and 15–20 bearing up to eight tolyl groups provided crystals of
insufficient quality. Therefore, we inferred their helical
study single-molecule conductivity by using the mechanically
[22]
controllable break-junction method (MCBJ). Helicenes are
unique multichannel systems combining a chiral p-conjugated
pathway with an achiral p–p stacked pathway in a single
[
23]
molecule
and may express, among other phenomena,
[
24]
symmetry breaking in charge/spin transport.
In a pilot
structure from the electronic circular dichroism (ECD) and
study, we measured by the MCBJ method the single-molecule
conductance of pyridooxa[9]helicene (À)-(M,R,R)-16 repeat-
edly sandwiched between gold nanoelectrodes. The 1D
conductance histogram revealed the presence of a conduc-
tance peak corresponding to the single-molecule junction
1
H NMR spectra, which confirmed the C symmetry of the
2
cyclized products. For example, the folding of the centrally
chiral dodecayne (À)-(R,R)-9 into the corresponding oxa-
[
17]helicene (À)-(M,R,R)-19 led to a dramatic change in the
ECD spectrum (see the Supporting Information). The helical
product gave rise to new strong bands in the ECD spectrum,
as a helically chiral chromophore was formed. The negative
dichroism associated with the longest-wavelength band at
(Figure 3; for a 2D histogram, see the Supporting Informa-
À4
tion). Its maximum was found at 8.8 ꢁ 10 G/G in relation to
0
À3
the calculated value of 4.5 ꢁ 10 G/G (ATK, QuantumWise).
0
The diffuse character of the peak was attributed to the
superposition of a variable gold electrode geometry and the
attachment configuration of the molecule. The conductance
value reflects the presence of a cross-conjugated part of the
molecule (a meta-substituted central benzene unit), which is
expected to diminish the probability of tunneling along the p-
conjugated pathway owing to the destructive interference
3
89 nm pointed to M helicity of the molecule, as already
[20]
observed in the CD spectra of nonracemic helicenes and
their congeners.
[
19b,21]
Moreover, the helical arrangement of
the longest [19]helicenes (À)-(M,R,R)-3 and rac-20 was
1
1
unequivocally confirmed by detailed analysis of their H– H
ROESY NMR spectra. As exemplified for rac-20 (Figure 2),
the spatial proximity (2.08 ꢀ) of pseudoaxial hydrogen atoms
of the innermost methylene units to the pseudoaxial a single
diastereomer of the outermost methylene units (17 bonds
apart) led to a through-space correlation between these atoms
and a clearly visible cross-peak in the ROESY spectrum.
The availability of the series of oxa[9]-, [13]-, and
[
25]
effect. A detailed study focused on the transport properties
of helicenes will be reported separately.
In summary, we have developed a synthetic route to the
longest oxahelicenes prepared to date, comprising up to 19
ortho/meta-fused benzene/2H-pyran rings in their helical
backbone. The use of a flow reactor in the key step, the
final multiple cobalt(I)-mediated alkyne [2+2+2] cycloiso-
merization to form up to 12 CÀC bonds and 12 rings in a single
[
17]helicenes (À)-(M,R,R)-16, 18 and 19 terminated with
pyridine units as suitable anchoring groups encouraged us to
operation, was found to be advantageous for the efficient
folding of the branched aromatic oligoynes into oxa[9]-, [11]-,
[
(
13]-, [17]- and [19]helicenes in reasonable preparative yields
18–53%). Furthermore, the introduction of stereogenic
Figure 3. Single-molecule conductance of (À)-(M,R,R)-16, as measured
by the MCBJ method (graph) and calculated by DFT (vertical mark).
The experimental 1D conductance histogram was constructed from
10544 individual opening conductance traces indicating the presence
of a molecule (70% of all curves; 2% bin size; measured in mesitylene
at room temperature under argon). A theoretical model (DFT/PBE/
SZP, DZP) of the Au(111)–single molecule–Au(111) junction used in
the conductance calculations (NEGF, Landauer formalism, ATK) is
shown in the inset.
1
1
Figure 2. Part of the H– H ROESY NMR spectrum of rac-20 showing
a through-space correlation between protons from adjacent backbone
loops. The molecular structure was optimized by the DFT (B3LYP/cc-
pVDZ) method with the GD3 dispersion correction in vacuo.
4
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Angewandte
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centers of known absolute configuration into the oligoyne
precursors resulted in complete stereoselectivity in the
cyclization step to provide the inherently chiral products in
enantio- and diastereomerically pure form. Finally, we have
shown that the single-molecule conductivity of such complex
p-electron systems equipped with anchoring pyrido groups
can be investigated by means of the mechanically controllable
break-junction method. Further studies in this direction are in
progress.
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[
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Manuscript received: January 11, 2017
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Helical Structures
Think big: Oxahelicenes comprising up to
9 fused rings (see example) were syn-
1
ˇ
J. Nejedl y´ , M. Sꢁmal, J. Rybꢁ cˇ ek,
thesized by multiple cobalt(I)-mediated
cycloisomerization reactions of oligo-
ynes, most efficiently in a flow reactor.
The stereogenic centers in enantiomeri-
cally pure substrates steered the diaste-
reoselective polycyclization of the oligo-
ynes. Single-molecule conductivity was
studied in a pyridooxa[9]helicene by the
break-junction method.
M. Tobrmanovꢁ, F. Szydlo, C. Coudret,
M. Neumeier, J. Vacek,*
J. Vacek Chocholou sˇ ovꢁ, M. Bud eˇ sˇ ꢂnsk y´ ,
ˇ
D. Saman, L. Bednꢁrovꢁ, L. Sieger,
I. G. Starꢁ,* I. Star y´ *
&&&& — &&&&
Synthesis of Long Oxahelicenes by
Polycyclization in a Flow Reactor
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!