Photochemical Unmasking of Polyyne Rotaxanes

Article abstract of DOI:10.1021/jacs.0c05308

Bulky photolabile masked alkyne equivalents (MAEs) are needed for the synthesis of polyyne polyrotaxanes, as insulated molecular wires and as stabilized forms of the linear polymeric allotrope of carbon, carbyne. We have synthesized a novel MAE based on phenanthrene and compared it with an indane-based MAE. Photochemical unmasking of model compounds was studied at different wavelengths (250 and 350 nm), and key products were identified by NMR spectroscopy and X-ray crystallography. UV irradiation at 250 nm leads to unmasking of both MAEs. Irradiation of the phenanthrene system at 350 nm results in quantitative dimerization via [2 + 2] cycloaddition to form a [3]-ladderane; irradiation of this ladderane at 250 nm generates a dihydrotriphenylene, which can be oxidized easily to a triphenylene. Irradiation of the indane-based MAE at 350 nm in the presence of traces of oxygen forms an endoperoxide and a bisepoxide. Both MAEs have been incorporated into rotaxanes via copper-mediated active metal template Glaser or Cadiot-Chodkiewicz coupling. The identity of the rotaxanes was confirmed by NMR spectroscopy and mass spectrometry. The phenanthrene rotaxane decomposes during attempted photochemical unmasking, whereas photolysis of the indane rotaxane results in unmasking of the polyyne thread to form a rotaxane with a chain of 16 sp-hybridized carbon atoms. This approach opens avenues toward the synthesis of encapsulated carbon allotropes.

Full text of DOI:10.1021/jacs.0c05308

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Article
Photochemical Unmasking of Polyyne Rotaxanes
Steffen Woltering, Przemyslaw Gawel, Kirsten E. Christensen, Amber L. Thompson, and Harry L. Anderson
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Photochemical Unmasking of Polyyne Rotaxanes
Steffen L. Woltering, Przemyslaw Gawel, Kirsten E. Christensen, Amber L. Thompson,
and Harry L. Anderson*
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Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford OX1 3TA, UK.
ABSTRACT: Bulky photo-labile masked alkyne equivalents (MAEs) are needed for the synthesis of polyyne polyrotaxanes, as
insulated molecular wires and as stabilized forms of the linear polymeric allotrope of carbon, carbyne. We have synthesized a novel
MAE based on phenanthrene and compared it with an indane-based MAE. Photochemical unmasking of model compounds was
studied at different wavelengths (250 nm and 350 nm), and key products were identified by NMR spectroscopy and X-ray crystal-
lography. UV irradiation at 250 nm leads to unmasking of both MAEs. Irradiation of the phenanthrene system at 350 nm results in
quantitative dimerization via [2+2] cycloaddition to form a [3]-ladderane; irradiation of this ladderane at 250 nm generates a dihy-
drotriphenylene, which can be oxidized easily to a triphenylene. Irradiation of the indane-based MAE at 350 nm in the presence of
traces of oxygen forms an endoperoxide and a bisepoxide. Both MAEs have been incorporated into rotaxanes via copper-mediated
active metal template Glaser or Cadiot-Chodkiewicz coupling. The identity of the rotaxanes was confirmed by NMR spectroscopy
and mass spectrometry. The phenanthrene rotaxane decomposes during attempted photochemical unmasking, whereas photolysis of
the indane rotaxane results in unmasking of the polyyne thread to form a rotaxane with a chain of 16 sp-hybridized carbon atoms.
This approach opens avenues towards the synthesis of encapsulated carbon allotropes.
rocycle to be threaded round a polyyne, but it does not provide
access to polyyne polyrotaxanes with many threaded macro-
INTRODUCTION
The predicted electronic and mechanical properties of car-
byne, the linear sp-hybridized allotrope of carbon,1,2 have been
debated for many years,3,4 providing a motivation for synthe-
sizing this elusive carbon polymer. However, if carbyne could
be prepared as an undiluted carbon allotrope, it is expected to
be unstable and explosive, because acetylenic chains easily
react with each other to form cross-linked materials,5-7 in a
process that is highly exothermic.8 End-capped oligomeric
analogues of carbyne, known as polyynes, have been prepared
with chains of up to 44 sp-hybridized carbon atoms, stabilized
by bulky terminal groups.9,10 This strategy is unlikely to be
effective for stabilizing longer polyynes, because the influence
of the end-groups diminishes as the chain gets longer.
Polyynes have previously been stabilized by rotaxane for-
mation,11,12 helical wrapping13 and co-crystallization with met-
al complexes.14 Very long polyynes, consisting of more than
6,000 carbon atoms, have been created and studied inside car-
bon nanotubes.15 However, this type of encapsulation largely
screens the properties of the guest; for example, it is difficult
to measure the charge transport, optical or mechanical proper-
ties of a polyyne inside a carbon nanotube, because all these
aspects are dominated by the host nanotube. The most versa-
tile strategy for creating stable forms of carbyne is probably to
synthesize polyrotaxanes in which the carbon chain is threaded
through many macrocycles to form an ‘insulated molecular
wire’.16 Threading also offers a strategy to stabilize the cyclic
analogues of carbyne, cyclo[n]carbons,1 by catenane for-
mation, which would not be possible using encapsulation in-
side carbon nanotubes. Here we present an approach to the
synthesis of polyyne rotaxanes using photo-labile masking
groups. This chemistry opens an avenue towards carbyne pol-
yrotaxanes and cyclocarbon catenanes.
cycles, unless temporary alkyne-masking groups can be used
to prevent unthreading. Various masked alkyne equivalents
(MAEs) have been developed, particularly for the synthesis of
cyclo[n]carbons. MAEs are molecular subunits that can be
transformed into alkynes by activation with light,21-23 heat24,25
or chemical reagents26-28 (Scheme 1). Here, we explore the use
of MAEs in the synthesis of polyyne rotaxanes. Our choice of
MAEs is governed by the following factors: (i) the MAE must
be chemically stable under the conditions of rotaxane for-
mation; (ii) it should be bulky enough to act as a temporary
stopper and prevent unthreading of the rotaxane; (iii) unmask-
ing of the MAE must be possible in interlocked systems, and
(iv) the MAE should be easy to synthesize. Photo-labile MAEs
are appealing because they are expected to be stable under the
conditions of rotaxane formation. Here, we investigate a new
photo-labile MAE, based on the susceptibility of phenanthrene
to undergo reversible photochemical [2+2]-cycloadditions to
the double bond of the central ring (9,10-position). We com-
pare this new MAE with an indane-based MAE developed by
Tobe and coworkers.21 The photochemistry of both MAEs was
investigated and some unexpected products were identified.
Polyyne rotaxanes containing both types of MAEs were syn-
thesized, but only the rotaxane with indane-based MAEs un-
derwent clean photochemical unmasking.
Polyyne rotaxanes have been prepared previously using ac-
tive metal templates.12,17-20 This strategy enables a single mac-
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Cl
Cl
Cl
TIPS
TIPS
Scheme 1. Examples of Masked Alkyne Equivalents.
a)
Cl
Cl
R
R
R
1
2
3
4
5
6
7
8
TIPS
hν
H
H
R
R
R
Pd(PPh3)4, CuI
then
NaOH
(Ref. 21)
BuNH2, THF, 89%
1
2
–
hν
hν
TIPS
(Ref. 25)
(Ref. 22)
–
heat
b)
c)
TIPS
eight
steps
O
Cl
Cl
TIPS
–
R
R
Pd(PPh3)4, CuI
9
BuNH2, THF, 100%
3
4
Pb(OAc)4
heat
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I2
N
N
NH2
N
O
O
R
– 2CO
N
R
R
R
Ph2P
OC
PPh2
CO
(Ref. 27)
(Ref. 24)
Co
Co
OC
CO
(Ref. 26)
N
N
N
Ph
R
R
R
R
RESULTS & DISCUSSION
Synthesis and testing of MAEs
The dichlorocyclobutene 1 is readily prepared from phenan-
throline by photochemical addition of trichloroethylene fol-
lowed by elimination of hydrogen chloride, as reported by
Hacker and McOmie (Figure 1).29 Sonogashira coupling was
used to prepare the TIPS-protected model compound 2. The
crystal structure of 2 confirms the structure and gives an indi-
cation of its steric bulk (Figure 1c).30 We compared this phe-
nanthrene-based MAE with an indane-based MAE developed
by Tobe and coworkers.21 A drawback of the indane group is
its longer synthesis: indane dichloride 3 is prepared in eight
steps from cyclopentanone (Figure 1b).21d The TIPS-protected
model compound 4 was prepared for comparison with 2.
Figure 1. (a,b) Synthesis of MAEs models 2 and 4; (c) two or-
thogonal views of the structure of compound 2 in the crystal
(thermal ellipsoids plotted at the 40% level).
Irradiation of compound 2 as a solution in CDCl3 (0.2 M
concentration) with UV light (wavelength 250 nm) for 21 h
leads to formation of phenanthrene in 80% yield, as illustrated
1
by the H NMR spectra in Figure 2. TIPS-triyne 5 is also
formed in this reaction (Figure 3), although in poor yield (20%
by NMR analysis) together with a complex mixture of prod-
ucts. A reference experiment showed that TIPS-triyne 5 is
stable under the conditions of this photolysis, implying that the
side products come from competing side reactions of 2, rather
than from decomposition of triyne 5.
1
Figure 2. H NMR spectra (CDCl3, 298 K, 400 MHz) of a solu-
tion of 2 (red peaks) before and after irradiation with UV light
(250 nm, no purification). Phenanthrene (blue peaks) is the major
product (80%) after irradiation for 21 h. Triyne 5 could be detect-
ed by 1H and 13C NMR (yield 20% by 1H NMR).
We also tested irradiation of 2 with UV light at longer
wavelengths. Surprisingly, irradiation at 350 nm gave no phe-
nanthrene; instead the [2+2] dimer, [3]-ladderane 6, is formed
The unexpected outcome of the irradiation of 2 at 350 nm
led to us to question whether [3]-ladderane 6 might be an in-
termediate in the cycloreversion observed on irradiation of 2 at
250 nm. This can be ruled out as dimer 6 does not yield the
unmasked triyne 5 when irradiated with 250 nm UV light;
instead it generates a different product 7 (50% yield) with only
one phenanthrene unit expelled (Figure 3a). The formation of
different products on irradiation at 250 and 350 nm must re-
flect the reactivity of different excited states. Compound 7 is
also formed as a minor product (~10%) when 2 is irradiated
with 250 nm UV light (peak at 3.77 ppm in Figure 2).
1
quantitatively as a single diastereomer (as seen from the H
NMR spectra in Figure 4 and X-ray crystal structure in Figure
3b).30 The geometry of 6 is similar to those of previously re-
ported ladderanes,31 but steric congestion results in an unusu-
ally long bond length for the C-C bonds shared between two
cyclobutane units (bond length 1.604(2) Å; see Supporting
Information, Section S6 for a comparison of bond lengths
from CSD and Section S7 for computational studies).
2
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R
R
a)
TIPS
1
TIPS
UV light
2
R
R
80%
(250 nm)
3
H
H
+
4
5
6
TIPS
TIPS
8
, 96%
O2
2
5
, 20%
7
8
UV light
(350 nm)
R
R
9
R
UV light
(250 nm)
R
R
R
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H
H
+
50%
Figure 4. 1H NMR spectra (C6D6, 298 K, 400 MHz) of a solution
of 2 (red peaks) before and after irradiation with UV light (350
nm, no purification). A [3]-ladderane 6 (green peaks) is formed
quantitatively after irradiation for 14 h. (The outcome of photoly-
sis in CDCl3 is the same.)
7
, 50%
6π e
conrotatory
electrocyclic
ring closure
R
R
6
, 100% (R = TIPS)
hν
[2+2] cyclo-
reversion
[2+2] cycloreversion
hν
hν
R
R
R
R
R
R
The constitution of 7 was deduced from the fact that it is
formed from 6 concomitantly with one equivalent of phenan-
hν
Δ
1
R
H
R
R
R
R
R
threne, and from the integration of the H NMR signals. The
H
H
H
H
H
crystal structure revealed that 7 has a surprising stereochemis-
try with a central trans-dihydrotriphenylene.30 This suggests
that a photochemical domino reaction takes place after phe-
nanthrene is extruded by a [2+2]-cycloreversion to form in-
termediate A (Figure 3a). This cyclobutene appears to undergo
a disrotatory 4π electrocyclic ring opening (photochemically
allowed) to give a cyclohexadiene B, which then undergoes a
conrotatory 6π electrocyclic ring opening (thermally allowed)
to form a [10]annulene intermediate C, which recloses via a
disrotatory 6π electrocyclic ring closure to yield 7. Alterna-
tively, the central cyclobutane ring in A may undergo a photo-
chemical [2+2]-cycloreversion to generate C in one step. A
similar cycloreversion was observed in the formation of
[12]annulene from a ladderane.32
A
B
C
4π e disrotatory
electrocyclic
ring opening
6π e disrotatory
electrocyclic
ring opening
b)
6
Intermediates A–C have not been observed but these mech-
anisms appear to be the most likely explanation for the genera-
tion of 7 from 6. The crystal structure has four molecules of 7
in the asymmetric unit, all with essentially the same molecular
geometry: a representative molecule is shown in Figure 3c.
c)
When exposed to air, dihydrotriphenylene 7 is slowly oxi-
dized to triphenylene 8 in the course of weeks. This process
can be accelerated by stirring a CH2Cl2 solution under an oxy-
gen atmosphere for five days to achieve full oxidation. The
structure of 8 was confirmed by NMR, MS and X-ray crystal-
lography (Figure 3c).30
Indane derivative 4 is a known compound,21d but its photo-
chemistry has not been reported. We found that irradiation
with 250 nm UV light leads to rapid formation of indane and
triyne 5 (both in 70% yield; Figure 5 and 6). Competition ex-
periments show that 4 reacts about five times faster than 2
under identical photolysis conditions.
7
8
Figure 3. Reactivity of phenanthrene MAE model 2. a) Irradiation
of 2 at 250 nm or 350 nm, and irradiation of dimer 6 at 250 nm,
showing a possible mechanism for the formation of dihydrotri-
phenylene 7 (R = TIPS). b) Two orthogonal views of the structure
of 6 in the crystal. c) Structures of dihydrotriphenylene 7 and
triphenylene 8. Thermal ellipsoids plotted at the 40% level for (b)
and (c).
3
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in Figure 7); this set of peaks disappears to be replaced by
1
2
3
4
5
6
another set of peaks at 3.6 ppm and 3.2 ppm (purple peaks in
Figure 7). Formation of indane is also observed, indicating
that unmasking occurs to some extent as well. The two prod-
ucts, 9 and 10, were isolated and characterized (isolated yields
of 9 and 10 after 3.5 h of irradiation 19% and 28%, respective-
1
ly; 55% of 10 after 8 h of irradiation: H NMR spectra shown
in Figure 7b). The crystallographic structure determination of
epoxide 10 (Figure 6b)30 enabled us to deduce the structure of
peroxide 9. The reactions are summarized in Figure 6a. Pre-
sumably, traces of oxygen are converted to singlet oxygen and
undergo hetero-Diels-Alder reaction33 with the diene moiety of
4. The formation of bis-epoxides from unsaturated endoperox-
ides has been reported previously.34 As expected, products 9
and 10 are not formed when rigorously O2-free solutions of 4
are irradiated. Tobe et al. reported the formation of a cyclooc-
tatetraene and a semibullvalene (each in 7% yield, using a
low-pressure mercury lamp in hexane) when irradiating TMS-
and TBDMS-analogues of 4.21d We also observed formation of
these products, each in about 10% yield, when irradiating with
7
8
9
10
11
12
13
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21
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1
Figure 5. H NMR spectra (CDCl3, 298 K, 400 MHz) of a solu-
tion of 4 (red peaks) before and after irradiation with UV light
(250 nm, no purification). Indane (blue peaks) is formed in 70%
yield after irradiation for 1.3 h. The triyne 5 (formed in 70%
yield) could be detected by 13C NMR.
1
250 nm light, identified by their characteristic H NMR spec-
tra.
TIPS
TIPS
a)
UV light (250 nm)
TIPS
TIPS
TIPS
TIPS
The crystal structure of 10 (Figure 6b) shows that the six-
membered cyclohexane-bisepoxide ring is remarkably flat
(root-mean-squared deviation for 6 carbon atoms: 0.004 Å);
this conformation has been reported previously in similar cy-
clohexane derivatives.35 Comparing the crystal structures of 2
and 10 reveals that the geometry of the enediyne unit is unaf-
fected by the decoration of the other side of the cyclobutene
ring; bond lengths and bonding angles are essentially identical.
–
5, 70%
4
5
UV light
(350 nm)
–
TIPS
TIPS
TIPS
TIPS
+
+
O
O
In summary, both the indane and the phenanthrene group
are viable MAEs for masking polyynes. Unmasking is quicker
and more efficient for the indane but the phenanthrene MAE is
more accessible. The alkyne masked with indane 4 gives tri-
yne 5 in 70% yield; the one masked with phenanthrene 2 gives
5 in 20% yield.
O
9
10
O
Rotaxane synthesis
With these encouraging results in hand, we synthesized in-
terlocked structures based on the phenanthrene and indane
MAEs. Bulky trityl stoppers were added to ensure that the
macrocycle cannot slip off after a rotaxane is formed.36 The
synthesis of a phenanthrene rotaxane 16 and the corresponding
reference thread 17 are shown in Schemes 2 and 3.
b)
Figure 6. a) Reactivity of indane MAE model 4. b) Two orthogo-
nal views of the structure of 10 in the crystal (thermal ellipsoids
plotted at the 40% level).
In contrast to the clean photochemistry with 250 nm light,
irradiation of indane 4 at 350 nm gives a complex mixture of
products (Figures 6 and 7). Initially, a new set of 1H resonanc-
es appears at 6.5 ppm and 4.6 ppm (green peaks
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Figure 7. a) 1H NMR spectra (CDCl3, 298 K, 400 MHz) of a solution of 4 (red peaks) before and after irradiation with UV light over 8 h.
(350 nm, no purification). In addition to indane (blue peaks), two new species are formed (green and purple). b) 1H NMR spectra (CDCl3,
298 K, 400 MHz) of the purified compounds; the green peaks are assigned to peroxide 9 and purple peaks belong to bisepoxide 10.
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Scheme 2. Synthesis of phenanthrene-masked rotaxane 16. Scheme 3. Synthesis of phenanthrene reference thread 17.
Page 6 of 12
1
2
3
4
Ar Ar
Ar
Cl
Cl
Ar
Ar
TMS
Ar
OH
1
11
5
6
14
TMS
(COCl)2, THF
then MeLi/TMS-C4-TMS, 0 °C
then K2CO3, MeOH, 58%
7
8
Pd(PPh3)4, CuI
BuNH2, THF, 38%
TMS
1) K2CO3, MeOH
2) CuCl, TMEDA, O2, CH2Cl2, 50%
9
Cl
Ar
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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Ar
H
tBu
tBu
13
Ar
12
tBu
tBu
A
B
Pd(PPh3)4, CuI
BuNH2, THF, 51%
Ar Ar
Ar
tBu
tBu
f
C
g
N
N
D
TMS
e
d
a
c
E
O
O
b
14
17
1) K2CO3, MeOH
2) I2, K2CO3, 14, CuI
60 °C, THF, 56%
The synthesis of indane rotaxane 27 proved more challeng-
ing than anticipated. Sonogashira couplings using TMS-
acetylene instead of TIPS-acetylene are less efficient on di-
chloride 3 (53% yield for TMS-acetylene, 100% for TIPS-
acetylene). Therefore, initial attempts focused on TIPS pro-
tected derivatives. We were not able to couple stopper 12 to
the indane 18 (Scheme 4) and TIPS indane stopper 21 could
only be obtained by Cadiot-Chodkiewicz coupling12,38,39 to 20,
the monodeprotected derivative of 4. Attempted deprotection
of 21 to give 22 was not successful; a range of fluoride sources
(TBAF, CsF, KF) led to rapid decomposition of 21 and analy-
sis of the degradation products suggested that the trityl unit is
cleaved from the molecule (Scheme S7). To overcome this
problem, SiEt3 (TES) was used as a protecting group, as it
does not require fluoride for deprotection. Surprisingly, the
Cadiot-Chodkiewicz coupling, which yielded the TIPS ana-
logue 21, did not work on 23. It is puzzling that changing the
silyl protecting group (for a smaller one) in a remote part of
the molecule has such a drastic effect on reactivity (Scheme
4).
F
O
O
G
A
H
tBu
tBu
15
B
C
tBu
tBu
O
N N
D
E
tBu
tBu
f
O
g
d
e
c
F
b
O
O
a
G
H
16
Synthesis of rotaxane 16 starts with desymmetrization of di-
chloride 1 via a Sonogashira coupling to TMS-acetylene to
give chloride 13. In a subsequent Sonogashira coupling, diyne
stopper 12 was installed to give stoppered masked phenan-
threne 14 (Ar = 4-methylphenyl).
Stopper 12 was synthesized in one step from alcohol 11.36
We found that it is necessary to separate the trityl unit and the
MAE by two acetylene units to prevent a steric clash of the
stoppers in the rotaxane forming step. A design based on a
single acetylene spacer between stopper and MAE failed to
give interlocked products (SI, Scheme S5).
After TMS-deprotection of 14 with K2CO3/MeOH, the phe-
nanthrene rotaxane 16 was formed in 56% yield via copper-
mediated oxidative homocoupling using macrocycle 15.37
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Scheme 5. Synthesis of rotaxane 28.
Scheme 4. Unsuccessful approaches to the synthesis of
1
2
3
4
compound 22.
Ar
Ar
Cl
Ar
Cl
TIPS
Cl
Ar
Ar
Pd(PPh3)4, CuI
BuNH2, THF, 15%
Ar
+
3
5
Cl
+
6
7
8
9
18
12
Ar
Ar
H
Pd(PPh3)4, CuI
BuNH2, THF
25
Ar
TIPS
12
Pd(PPh3)4, CuI
Ar
Ar
Ar
TES
Ar
Ar
BuNH2, THF, 91%
10
11
12
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Ar
Ar
Ar
Ar
Ar
Ar
Ar
CuI, BuNH2
K2CO3, THF
Ar
Ar
Ar
20
TIPS
(iv)
+
Ar
Ar
62%
K2CO3
TES
NBS
TBAF
or CsF
Br
Ar
H
Br
MeOH
AgNO3
19
21
22
Ar
Ar
Ar
K2CO3
MeOH
24
22
26
TES
CuI, BuNH2
K2CO3, THF
TES
14, CuI THF, THF
47% over 3 steps
Ar
Ar
Br
+
A
tBu
tBu
Ar
B
C
tBu
tBu
O
23
19
24
N N
D
E
The successful route to 24 proceeds via a low-yielding So-
nogashira coupling (15%) to give 25 (Scheme 5). Subsequent-
ly a Sonogashira coupling gave 24 in 91% yield. In contrast to
phenanthrene rotaxane 16, we were unable to prepare indane
rotaxane 27 via a copper-mediated oxidative homocoupling;
only non-interlocked products were obtained. However, if
indane stopper 24 is deprotected to 22 and converted to the
corresponding alkynyl bromide 26, rotaxane 27 can be formed
in 47% yield (over three steps: TES-deprotection, bromination
of half the material and Cadiot-Chodkiewicz coupling).12,39
Reference thread 29 was synthesized via a copper-catalyzed
homocoupling (Scheme 6).
e
tBu
tBu
O
f
b
a
d
F
O
O
G
c
UV light (250 nm)
cyclohexane-d12, 6 h, 34%
27
H
A
B
C
N
N
D
1
tBu
tBu
E
The H NMR spectra of rotaxanes 16 and 27 are compared
with those of the free threads 17 and 29, and macrocycle 15, in
Figure 8. Compounds 16, 17, 27 and 29 exist as mixtures of
meso and racemic diastereomers (SI, Scheme S8), causing
splittings in the NMR resonances. The splittings in the signals
of the phenanthroline macrocycle are more pronounced in 16,
compared to 27; this can be attributed to the higher magnetic
anisotropy and greater lateral extension of the phenanthrene
group compared to the indane. In both rotaxanes, NOEs were
observed between the aryl protons of the stopper (f and g in
16, e and f in 27) and proton A of macrocycle 15, indicating
that the macrocycle can shuttle over the MAE units. The iden-
tity of rotaxanes was further confirmed by mass spectrometry;
in contrast to the hydrocarbon precursors and dumbbells, all
rotaxanes can be detected easily using ESI-MS (see SI for MS
spectra).
O
O
tBu
tBu
F
O
O
tBu
tBu
28
G
H
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Photochemical unmasking of polyyne rotaxanes
Page 8 of 12
Scheme 6. Synthesis of dumbbell 30.
1
2
3
4
Unmasking of the rotaxanes 16 and 27 is significantly less
efficient than unmasking of the model compounds 2 and 4.
While 2 and 4 can be unmasked in a range of solvents (CDCl3,
THF-d8, benzene-d6), giving comparable results, the only suit-
1
5
able solvent that does not lead to extensive broadening of H
NMR signals during irradiation of the rotaxanes is cyclohex-
ane. The results from irradiation of rotaxanes 16 and 27 in
cyclohexane-d12 at 250 nm are shown in Figure 9.
6
7
8
During the irradiation with 250 nm light, both rotaxanes ex-
pel the masking groups (indane in the case of 27, phenan-
threne in the case of 16). As for the model compounds, indane
is generated faster than phenanthrene (unmasking of 27 oc-
curred over 4 h vs. 12 h for 16 under the same conditions).
Longer wavelengths led to undesired side reactions, even
when visible blue light was used (450 nm, tested on 17); long-
er wavelengths than 450 nm showed no effect.
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The signals of the macrocycle are a good probe for monitor-
ing the unmasking reaction. In the case of 27, unmasking is
indicated by sharpening of the signals of the macrocycle. The
unmasked product 28, and peaks for partially unmasked rotax-
anes, were also observed via ESI-MS over the course of the
reaction. After chromatography, octatyne rotaxane 28 was
isolated in 34% yield from rotaxane 27 (Scheme 5). Similar
oligoyne rotaxanes to 28, with 4–12 alkyne units, have been
prepared previously via active metal template synthesis.12
Figure 8. Comparison of the 1H NMR spectra (CDCl3, 298 K, 500 MHz) of phenanthrene thread 17, phenanthrene rotaxane 16, macrocy-
cle 15, indane rotaxane 27, and indane thread 29. Signal labeling according to Schemes 2 and 5. Signals from the macrocycle are high-
lighted in green.
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8
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16
17
18
19
20
21
22
23
24
25
26
Figure 9. Unmasking of rotaxanes 16 and 27. a) 1H NMR spectra (cyclohexane-d12, 298 K, 400 MHz) of a solution of 27 before and after
27
28
29
30
31
32
33
34
35
36
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irradiation (250 nm) for 4 h. Green peaks are assigned to the macrocycle; red peaks to indane. Green peaks sharpen over time, indicating a
higher symmetry. Unmasked octayne rotaxane 28 was isolated from the reaction mixture (34% yield). b) H NMR spectra (cyclohexane-
d12, 298 K, 400 MHz) of a solution of 16 before and after irradiation (250 nm) for 12 h. Green peaks are assigned to the macrocycle; red
peaks to phenanthrene. Phenanthrene is generated in the course of the reaction, but the macrocycle peaks broaden and lose intensity due to
decomposition. No unmasked rotaxane 28 was isolated from the photolysis of 16.
1
Sharpening of the macrocycle peaks does not occur on irra-
diation of phenanthrene rotaxane 16 (Figure 9b); all peaks
CONCLUSIONS
We have shown that MAEs can be successfully applied to
make long polyyne rotaxanes. The unmasking reaction of two
different MAEs was thoroughly studied and unexpected pho-
tochemical side products were identified by X-ray crystallog-
raphy. The indane-based MAE developed by Tobe and
coworkers can be unmasked yielding a polyyne rotaxane com-
prising eight -C≡C- units, whereas the phenanthrene MAE
proved to be unsuitable, due to competing side reactions dur-
ing photochemical unmasking. This is the first demonstration
that a polyyne can be released from a masked interlocked pre-
cursor. Our results indicate that encapsulated versions of car-
byne and cyclocarbons should be accessible, but to achieve
this objective, it will be necessary to use a smaller macrocycle
(or increase the bulk of the MAE), so that the MAE groups act
as temporary stoppers.
belonging to the rotaxane broaden significantly and lose inten-
sity, in spite of the release of phenanthrene. ESI-MS shows no
new peaks, only the peak for starting material 16 (intensity
decreases over time) was observed. No identifiable products
could be isolated from photolysis of 16.
As a reference, we synthesized the corresponding octayne
dumbbell 30 by irradiating indane thread 29 with UV light
(250 nm): the yield (31%) is similar to the unmasking of ro-
1
taxane 27 (Scheme 6). The H NMR spectra of indane rotax-
ane 27, unmasked rotaxane 28, macrocycle 15 and unmasked
thread 30 are compared in Figure 10. No strong NOEs are
observed between the stopper units and the macrocycle of 28,
suggesting that the macrocycle resides near the center of the
polyyne chain.
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Figure 10. a) 1H NMR spectra (CDCl3, 298 K, 500 MHz) of masked indane rotaxane 27, unmasked octayne rotaxane 28, free macrocycle
15 and unmasked thread 30. Signals numbered according to Scheme 5. The shifts of macrocycle 15 are hardly affected by the threaded
dumbbell in rotaxane 28.
358. (b) Tykwinski, R. R. Carbyne: The Molecular Approach. Chem.
Rec. 2015, 15, 1060–1074. (c) Casari, C. S.; Tommasini, M.; Tykwin-
ASSOCIATED CONTENT
Supporting Information
ski, R. R.; Milani, A. Carbon-atom wires: 1-D systems with tunable
properties, Nanoscale 2016, 8, 4414–4435. (d) Casari, C. S.; Milani,
A. Carbyne: From the Elusive Allotrope to Stable Carbon Atom
Wires. MRS Commun. 2018, 8, 207–219.
(3) (a) Kertesz, M.; Koller, J.; Azman, A. Ab initio Hartree–Fock
crystal orbital studies. II. Energy bands of an infinite carbon chain. J.
Chem. Phys. 1978, 68, 2779–2782. (b) Tongay, S.; Senger, R. T.;
Dag, S.; Ciraci, S. Ab-initio Electron Transport Calculations of Car-
bon Based String Structures. Phys. Rev. Lett. 2004, 93, 136404. (c)
Yang, S.; Miklos Kertesz, M. Bond Length Alternation and Energy
Band Gap of Polyyne. J. Phys. Chem. A 2006, 110, 9771–9774. (d)
Zanolli, Z.; Onida, G.; Charlier, J.-C. Quantum Spin Transport in
Carbon Chains. ACS Nano 2010, 4, 5174−5180.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacsXXXXXXX.
Details of synthetic protocols, copies of NMR and MS spec-
tra, crystallographic data (PDF).
Crystallographic data for compound 2 (2003495) (CIF)
Crystallographic data for compound 6 (2003496) (CIF)
Crystallographic data for compound 7 (2003497) (CIF)
Crystallographic data for compound 8 (2003498) (CIF)
Crystallographic data for compound 10 (2003499) (CIF)
AUTHOR INFORMATION
(4) Liu, M.; Artyukhov, V. I.; Lee, H.; Xu, F.; Yakobson, B. I.
Carbyne from First Principles: Chain of C Atoms, a Nanorod or a
Nanorope. ACS Nano 2013, 7, 10075−10082.
Corresponding Author
*harry.anderson@chem.ox.ac.uk
(5) Tarakeshwar, P.; Buseck, P. R.; Kroto, H. W. Pseudocarbynes:
Charge-Stabilized Carbon Chains. J. Phys. Chem. Lett. 2016, 7,
1675–1681.
(6) Hoye, T. R.; Baire, B.; Niu, D.; Willoughby, P. H.; Woods, B.
P. The Hexadehydro-Diels-Alder Reaction. Nature 2012, 490, 208–
211.
(7) (a) Schrettl, S.; Stefaniu, C.; Schwieger, C.; Pasche, G.; Oveisi,
E.; Fontana, Y.; Fontcuberta i Morral, A.; Reguera, J.; Petraglia, R.;
Corminboeuf, C.; Brezesinski, G.; Frauenrath, H. Functional carbon
nanosheets prepared from hexayne amphiphile monolayers at room
temperature. Nat. Chem. 2014, 6, 468–467. (b) Chernick, E. T.;
Tykwinski, R. R. Carbon-rich nanostructures: the conversion of acety-
lenes into materials. J. Phys. Org. Chem. 2013, 26, 742–749.
(8) Zhang, Y.; Su, Y.; Wang, L.; Kong, E. S.-W.; Chen, X.-S.;
Zhang, Y. A one-dimensional extremely covalent material: monatom-
ic carbon linear chain. Nanoscale Res. Lett. 2011, 6, 577.
ACKNOWLEDGMENT
We thank Prof. Yoshito Tobe for providing us with a sample of
dichloride 3, Lorel M. Scriven for providing us with a sample of
macrocycle 15, and the Leverhulme Trust for funding. P.G.
acknowledges receipt of Postdoc.Mobility fellowships from the
Swiss National Science Foundation (P300P2_177829). We
acknowledge Diamond Light Source for time on Beamline I19
under Proposal MT20876.
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as per the SI (CIF). Full refinement details are given in the Supporting
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clization
to
Form
Carbon
Cages:
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Cambridge Crystallographic Data Centre (CCDC 2003495-99) and
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