Masked Alkyne Equivalents for the Synthesis of Mechanically Interlocked Polyynes**

Article abstract of DOI:10.1002/anie.202013623

Polyyne polyrotaxanes, encapsulated cyclocarbon catenanes and other fascinating mechanically interlocked carbon-rich architectures should become accessible if masked alkyne equivalents (MAEs) can be developed that are large enough to prevent unthreading of a macrocycle, and that can be cleanly unmasked under mild conditions. Herein, we report the synthesis of a new bulky MAE based on t-butylbicyclo[4.3.1]decatriene. This MAE was used to synthesize a polyyne [2]rotaxane and a masked-polyyne [3]rotaxane by Cadiot–Chodkiewicz coupling. Glaser cyclo-oligomerization of the [2]rotaxane gave masked cyclocarbon catenanes. The unmasking behavior of the catenanes and rotaxanes was tested by photolysis at a range of UV wavelengths. Photochemical unmasking did not proceed cleanly enough to prepare extended encapsulated polyyne polyrotaxanes. We highlight the scope and challenges involved with this approach to interlocked carbon-rich architectures.

Full text of DOI:10.1002/anie.202013623

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Accepted Article
Title: Masked Alkyne Equivalents for the Synthesis of Mechanically
Interlocked Polyynes
Authors: Przemyslaw Gawel, Steffen Woltering, Yaoyao Xiong, Kirsten
Christensen, and Harry Laurence Anderson
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202013623
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10.1002/anie.202013623
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RESEARCH ARTICLE
Masked Alkyne Equivalents for the Synthesis of Mechanically
Interlocked Polyynes
Przemyslaw Gawel, Steffen L. Woltering, Yaoyao Xiong, Kirsten E. Christensen, and Harry L.
Anderson*
Dedicated in memory of François Diederich (1952–2020)
[*]
[‡]
Dr. P. Gawel,‡ Dr. S. L. Woltering, Dr. Y. Xiong, Dr. K. E. Christensen, Prof. H. L. Anderson
Department of Chemistry, Oxford University
Chemistry Research Laboratory, Oxford, OX1 3TA (UK)
E-mail: harry.anderson@chem.ox.ac.uk
Current address: Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warsaw (Poland)
Supporting information for this article is given via a link at the end of the document.
Abstract: Polyyne polyrotaxanes, encapsulated cyclocarbon
catenanes and other fascinating mechanically interlocked carbon-
rich architectures should become accessible if masked alkyne
equivalents (MAEs) can be developed that are large enough to
prevent unthreading of a macrocycle, and that can be cleanly
unmasked under mild conditions. Here we report the synthesis of a
new bulky MAE based on t-butylbicyclo[4.3.1]decatriene. This MAE
was used to synthesize a polyyne [2]rotaxane and a masked-
polyyne [3]rotaxane by Cadiot-Chodkiewicz coupling. Glaser cyclo-
oligomerization of the [2]rotaxane gave masked cyclocarbon
catenanes. The unmasking behavior of the catenanes and rotaxanes
particularly using active metal templates,[12,13] but there is a need
for methods for threading many macrocycles onto a long
polyyne thread to form a polyrotaxane.
Masked alkyne equivalents (MAEs) have been used since
the 1980s to release polyyne chains from suitable precursors.
Unmasking is achieved using light,[14-16] heat,[17] chemical
reagents[18-21] and on-surface using voltage pulses.[22,23] MAEs
have opened synthetic avenues to cyclocarbons, rings
consisting entirely of sp-hybridized carbon atoms.[17-19,23] While
cyclocarbons were successfully studied on surfaces using
AFM,[23] they are only stable at low temperatures. One strategy
for making them stable under ambient conditions may be to
encapsulate them as catenanes,[24] by analogy with the
stabilization of cyclobutadiene inside a hemicarcerand, by Cram
and co-workers.[25] The synthesis of cyclocarbon catenanes
could be achieved using MAEs that act as stoppers in a
rotaxane intermediate. Such a rotaxane would be a versatile
building block for encapsulated cyclocarbons and carbyne
polyrotaxanes, as a form of insulated molecular wire.[26]
was tested by photolysis at
a range of UV wavelengths.
Photochemical unmasking did not proceed cleanly enough to
prepare extended encapsulated polyyne polyrotaxanes. We highlight
the scope and challenges involved with this approach to interlocked
carbon-rich architectures.
Introduction
Most reported MAEs are too small to act as stoppers in
rotaxanes. Here we explore
a
MAE based on
a
bicyclo[4.3.1]decatriene pioneered by Tobe,[15] but with a t-butyl
group on the indane ring to increase the steric bulk. When
irradiated with UV light, bicyclo[4.3.1]decatrienes 1 generate a
carbene, which rearranges rapidly to form an alkyne 2 (Scheme
1).[15] Here we demonstrate that this masking group is large
enough to prevent slipping of a small phenanthroline macrocycle
3[27] by synthesizing rotaxane 4 (Scheme 2). We demonstrate
that this MAE can be used to synthesize masked polyyne
polyrotaxanes and masked cyclocarbon catenanes, and we test
photochemical unmasking of these compounds.
The synthesis of polyynes has attracted attention ever
since the invention of the Glaser and Cadiot-Chodkiewicz
coupling reactions.[1] Polyynes are models for the elusive carbon
allotrope carbyne,[2] which consists of infinite linear chains of sp-
hybridized carbon atoms. The electronic and mechanical
properties predicted for this material have inspired generations
of experimentalists and theorists,[3] but its synthesis is precluded
by the tendency of long polyyne chains to undergo exothermic
crosslinking reactions.[4] Bulky end groups have been shown to
stabilize carbon chains of up to 48 atoms.[5,6] Other strategies for
stabilizing carbon chains include rotaxane formation[7-9] and
helical wrapping.[10] Stabilization has been achieved by
encapsulation inside carbon nanotubes, but this strongly
modifies the properties of the 1D carbon chain.[11]
Polyrotaxane formation is an appealing approach to
stabilizing polyynes because the macrocyclic components could
cover the whole length of the carbon chain, shielding regions
that are unaffected by bulky end groups; in principle, this
strategy could stabilize polyynes of any length. Synthetic
approaches to polyyne [2]rotaxanes are well developed,[7,8]
Scheme 1. Photochemical unmasking of bicyclo[4.3.1]decatriene
subsequent rearrangement of the carbene to form TMS-triyne 2.[15]
1 and
1
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Scheme 2. Proposed synthetic route to cyclocarbon catenanes and carbyne polyrotaxanes. (PG = protecting group.)
S1 (which was identified by single-crystal X-ray analysis;
Supporting Information S7). Use of elemental bromine in carbon
tetrachloride at 0 °C gave the desired tetrabromide 8, which was
immediately submitted for elimination using DBU to furnish 9 in
40% yield over two steps. Intermediate 9 can be prepared in
large quantities and stored without special precautions, and its
structure was confirmed by X-ray crystallography (see
Supporting Information S7).
When 9 is treated with t-BuLi, one bromine is exchanged
for lithium and the carbanion can be captured with ketone 10[30]
to install alkyne units in the periphery of the MAE, to give 11,
after quenching with TMS-chloride. Another lithium-bromine
exchange with t-BuLi eliminates the OTMS group to give
masked triyne 12. This elimination is accompanied by a skeletal
rearrangement: the [4.3.1]propellane undergoes an electrocyclic
reaction to form the bicyclo[4.3.1]decane, as revealed by the fact
that the bridgehead carbon signal is shifted from 47 ppm in 11 to
140 ppm in 12. This is in accord with previous reports[15,31] and
the rearrangement is probably driven by the strain in the
cyclopropane subunit with an exocyclic double bond.
Results and Discussion
1. [2]Rotaxane Synthesis
Designing a bulky photocleavable MAE was the first
challenge in this project. The most promising MAE was identified
as Tobe’s bicyclo[4.3.1]decatriene. In contrast to the other
photocleavable
MAE
developed
by
Tobe’s
group
([4.3.2]propellatriene),[14] it is not fused as an annulated ring but
attached to a double bond that is cross-conjugated with the
acetylene network; as a result, it protrudes further from the
polyyne chain, increasing the steric bulk. However, model
studies
using
polyynes
masked
with
unsubstituted
bicyclo[4.3.1]decatriene and macrocycle 3 did not yield
rotaxanes, indicating that the macrocycle can slip over this MAE.
This problem was solved by adding a t-butyl group.
Masked triyne 13 is synthesized as shown in Scheme 3.
Friedel-Crafts alkylation of indane gives 5.[28] Subsequent Birch
reduction yields cyclohexadiene 6. Dibromocarbene attacks 6
predominantly on the central double bond to yield 7, whereas in
absence of the t-butyl group both double bonds react.[29]
Reaction conditions for addition of bromine to 7 to give
tetrabromide 8 had to be optimized, because Tobe’s procedure
(using pyridinium tribromide)[15] gave an overbrominated product
2
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Scheme 3. Synthesis of bulky masked alkyne equivalent building blocks 16 and 17.
Deprotection of the TMS-alkynes with aqueous LiOH yields
masked triyne 13, which is desymmetrized by statistically
protecting one of the terminal acetylenes with acetone, to give
14. The two alkynes in 13 are nonequivalent, due to the
presence of the t-Bu substituent, and this reaction gives an
inseparable mixture of the E and Z stereoisomers of 14 in a ratio
of 2:3. It was not possible to assign the stereoisomers, as the
distance between t-butyl group and terminal acetylene or
acetonate group is too far to observe NOEs by 1H NMR
spectroscopy and attempts at crystallizing these compounds
were unsuccessful. Glaser coupling with TMS-acetylene yields
the masked tetrayne 15 and separation of the
E and Z
stereoisomers was achieved at this stage, although it was still
not possible to assign the isomers. The TMS group of 15 is
removed by treatment with aqueous potassium carbonate to
give the terminal acetylene 16, which is converted to the
corresponding bromide 17 with NBS in the presence of AgNO3.
Both 16 and 17 are unstable and quickly decompose (even at
low temperature) in pure form. Therefore, these compounds
were prepared immediately before use.
Scheme 4. Cadiot-Chodkiewicz coupling of 16 and 17 in the cavity of 3 yields
[2]rotaxane 18, which is deprotected with K2CO3 to give 4.
1
The H NMR spectrum of [2]rotaxane 4 is compared with
the spectra of free macrocycle 3 and free thread 19 in Figure 1.
As the axle is formed as a mixture of diastereomers, additional
splittings for the macrocycle peaks are observed. The
macrocycle also affects the peaks of the axle and increases the
splitting between the E and Z isomers; for example, the signal
for the proton of the terminal acetylene (d in Figure 1) appears
as a singlet in free thread 19, but it is split in [2]rotaxane 4. The
effect of interlocking is stronger for proton a (shielded by
0.2 ppm in the [2]rotaxane) of the cycloheptatriene ring
compared to signals b and c, which are almost unaffected by the
threaded macrocycle. [2]Rotaxanes such as 4 have potential as
building blocks for numerous systems involving encapsulated
alkynes. To demonstrate this, we show how interlocked systems
of higher order can be derived from this subunit.
With compounds 16 and 17 in hands, we screened
conditions for [2]rotaxane formation. Homocoupling of alkyne 16
in the presence of macrocycle 3, mediated by Cu(I)[12] gave
traces of interlocked product that could be detected by mass
spectrometry, but the rotaxane could not be isolated. In contrast,
Cadiot-Chodkiewicz coupling between the terminal alkyne 16
and bromide 17 gives [2]rotaxane 18 in 22% yield (Scheme 4).
Deprotection of the acetone groups with K2CO3/18-crown-6[32]
gives [2]rotaxane 4 in 60% yield. This [2]rotaxane is the first
example of an interlocked compound in which a masked alkyne
acts as a stopper. The interlocked character is unambiguously
1
confirmed by ESI-MS and H NMR spectroscopy. Tightly fitting
macrocycles can sometimes enhance the yield of rotaxanes,[33]
but the synthesis of 18 is less efficient than similar reactions
using larger macrocycles.[7,8]
3
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RESEARCH ARTICLE
Figure 1.1H NMR spectra (CDCl3, 298 K, 400 MHz) of macrocycle 3, rotaxane 4 and free thread 19. The signals of the macrocycle are strongly split because the
thread is a mixture of diastereomers.
2. [3]Rotaxane Synthesis
Synthesis of a [3]rotaxane was investigated as a step
towards the preparation of carbyne polyrotaxanes. An
asymmetric [2]rotaxane 21, with the MAE stopper on one side
and
a bulky supertrityl stopper on the other side, was
synthesized via a Cadiot-Chodkiewicz coupling between bromo
alkyne 17 and supertrityl triyne 20[5] in the presence of
macrocycle 3 (Scheme 5). In an attempt to keep the NMR
spectra simple, pure samples of diastereomer A of 16 (unknown
E or Z stereochemistry) were used in this line of experiments.
The bulky supertrityl stoppers were chosen to ensure that the
interlocking of components is maintained after unmasking of the
MAE. Removal of the acetonide protecting group with K2CO3/18-
crown-6 yields 22. Oxidative Glaser homo-coupling of 22 gives
[3]rotaxane 23 in 51% yield. The structure of 23 was
unambiguously confirmed by NMR spectroscopy and mass
spectrometry.
At
this
stage,
the
[3]rotaxane
is
compartmentalized: the macrocycles are trapped between the
supertrityl stopper unit and the MAE and cannot shuttle freely
along the polyyne thread. These findings show that it is possible
to synthesize masked encapsulated polyyne chains using this
approach, which opens avenues to the synthesis of
encapsulated carbyne (Scheme 1).
Scheme 5. Synthesis of [3]rotaxane 23.
3. Synthesis of Masked Cyclocarbon Catenanes
Catenanes were prepared from [2]rotaxane 4 (consisting
exclusively of diastereomer B) by Glaser cyclo-oligomerization
under dilute conditions. A range of conditions was screened, and
4
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only coupling with copper(I) chloride in pyridine under oxygen
yielded catenanes (Scheme 6). [3]Catenane 24 and [4]catenane
25 were separated by recycling gel permeation chromatography
and isolated in 5% and 4% yield, respectively. The constitution
of 24 and 25 was demonstrated by MALDI mass spectrometry,
but we were unable to record informative NMR spectra of these
catenanes; they gave broad and poorly resolved spectra, which
probably reflects the fact that they are complex mixtures of
diastereomers. (Even using a single diastereomer of 4 is
expected to give 7 stereoisomers of 24 and 24 stereoisomers for
25).
signal-to-noise-ratio decreases over time. This contrasts with the
photochemistry of unsubstituted 1, in which an isomerized
product is formed in 30% yield via a 1,5-sigmatropic shift.[34]
Figure 2. 1H NMR spectra (CDCl3, 298 K, 400 MHz) of UV unmasking
(250 nm) of 12 in CDCl3.
Using adamantane as an internal 1H NMR standard, the yield for
5 and 2 from irradiation of 12 was determined to be 37% and
47%, respectively (with an uncertainty of about 3%), which is
consistent with the isolated yields (23% 5, 36% 2). The
unmasking performance could not be improved by switching to
longer wavelengths (300 nm, 350 nm) or by varying
concentration, or changing the solvent to cyclohexane.
Unfortunately, the photochemical performance of this MAE is not
suitable for the unmasking of extended systems with multiple
MAE units, because the yield is expected to drop exponentially
with the number of masking units present in the molecule. In
spite of this, we tested the unmasking of [3]rotaxane 23,
[3]catenane 24 and [4]catenane 25. In the case of
[3]rotaxane 23, evidence for unmasking was observed on
irradiation at 250 nm (Scheme 7). Mass spectrometric analysis
of the reaction mixture showed strong peaks for the partially
unmasked [3]rotaxane 26 and weak signals for fully unmasked
[3]rotaxane 27. UV-vis analysis of the reaction mixture confirmed
that unmasking was achieved, and the characteristic peaks of a
tetradecayne stoppered by supertrityl groups were observed at
428, 394, and 378 nm (see Supporting Information S6),[5] but the
unmasked rotaxanes 26 and 27 were not isolated.
Scheme 6. Synthesis of masked cyclocarbon catenanes 24 and 25 via
copper-mediated cyclo-oligomerization of rotaxane 4.
3. Photochemical Unmasking
Tobe reported photochemical unmasking of compound 1 to
give triyne 2 in 43% yield (Scheme 1),[15] so we decided to
investigate whether the additional t-butyl group in the present
system influences its photochemical reactivity. TMS-protected
MAE 12 was chosen as a model to study the photochemistry of
this system.
The 1H NMR spectra of compound 12 before and after
irradiation at 250 nm in CDCl3 are compared in Figure 2. Under
the conditions of our experiment, unmasking was complete after
4 h of irradiation. The unmasking reaction appears to proceed
cleanly to the corresponding TMS-triyne 2 and t-butyl indane 5,
but decomposition via an unknown pathway takes place and the
5
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Scheme 7. Photochemical unmasking of [3]rotaxane 23. When irradiated with UV light (250 nm) partially unmasked rotaxane 26 can be identified in the reaction
mixture by ESI-MS. The UV-vis of the reaction mixture shows signature peaks for a supertrityl-stoppered tetradecayne, suggesting the formation of 27.
Unmasking was not efficient enough to isolate 26 or 27.
These findings suggest that unmasking of the catenanes
24 and 25 is unlikely to be viable using this MAE. Irradiation with
UV light (254 nm or 350 nm) in either CDCl3 (at 20 °C) or n-
hexane (at –80 °C) was monitored by mass spectrometry, but no
peaks for fully or partially unmasked cyclocarbon catenanes
were observed. Based on the performance of the model
compound, the theoretical yield for complete unmasking would
be 4–6% for the [3]catenane and up to 1.5% for the [4]catenane.
Keywords: polyynes • rotaxanes • masked alkyne •
supramolecular encapsulation • photochemistry
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Acknowledgements
We thank Daniel R. Kohn and Alexandru Vlasceanu for helpful
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10.1002/anie.202013623
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
Text for Table of Contents: Active metal template synthesis has been used to prepare a [2]rotaxane in which the whole dumbbell
component is a masked C16H2 polyyne. t-Butyl substituents make the photo-labile terminal groups bulky enough to act as stoppers
and prevent unthreading of the macrocycle. Rotaxanes of this type are promising intermediates in the synthesis of mechanically
interlocked carbon-rich architectures, such as cyclocarbons catenanes and carbyne polyrotaxanes.
Institute and/or researcher Twitter usernames: @HLAGroupOx
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