Topochemical Ene–Azide Cycloaddition Reaction

Article abstract of DOI:10.1002/anie.202109344

Topochemical reactions, high-yielding solid-state reactions arising from the proximal alignment of reacting partners in the crystal lattice, do not require solvents, catalysts, and additives, are of high demand in the context of green processes and enviro

Full text of DOI:10.1002/anie.202109344

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Title: Topochemical Ene-Azide Cycloaddition Reaction
Authors: Kana M Sureshan and Ravichandran Khazeber
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Topochemical Ene-Azide Cycloaddition Reaction
Ravichandran Khazeber, and Kana M. Sureshan*
[*]
R. Khazeber, Prof. Dr. K. M. Sureshan
Department: School of Chemistry
Institution: Indian Institute of Science Education and Research Thiruvananthapuram
Address: Thiruvananthapuram, Kerala-695551 (India)
E-mail: kms@iisertvm.ac.in
Homepage: http://kms514.wixsite.com/kmsgroup
Abstract: Topochemical reactions, high-yielding solid-state
reactions arising from the proximal alignment of reacting partners in
the crystal lattice, do not require solvents, catalysts, and additives
are of high demand in the context of green processes and
environmental safety. However, the bottleneck is the limited number
of reactions that can be done in the crystal medium. We present the
topochemical ene-azide cycloaddition (TEAC) reaction, wherein
alkene and azide groups undergo lattice-controlled cycloaddition
reaction giving triazoline in crystals. A designed monomer that
arranges in a head-to-tail manner in its crystals pre-organizing the
reacting groups of adjacent molecules in proximity undergoes
The broad applicability and success of this thermal topochemical
reaction and the advantages of topochemical reactions over
solution-phase reactions, in general, prompts the discovery and
development of novel thermal topochemical reactions. Here we
report the topochemical ene-azide cycloaddition (TEAC) reaction
as a novel thermal topochemical reaction that gives D2-1,2,3-
triazoline product, which, at will, can be modified to synthetically
versatile aziridine product (Figure 1b). We illustrate this concept
by the regio- and stereospecific polymerization of a dipeptide
monomer to a triazoline-linked polymer in a single-crystal-to-
single-crystal fashion and its post-synthetic backbone
modification to an aziridine-linked polymer.
spontaneous cycloaddition reaction in
a single-crystal-to-single-
crystal fashion, yielding the triazoline-linked polymer. A unique
advantage of this reaction is that the triazoline can be converted to
aziridine by simple heating, which we exploited for the otherwise
challenging post-synthetic backbone modification of the polymer.
This reaction may revolutionize the field of polymer science.
Thermal 1,3-dipolar alkyne-azide cycloaddition (AAC) reaction
giving 1,2,3-triazoles (Huisgen reaction) and ene-azide
cycloaddition (EAC) reaction giving Δ2-1,2,3-triazolines are
highly exothermic reactions with similar activation energies,[13]
and both give a mixture of products in the absence of any
catalyst.[14] While Cu(I) catalyzed AAC reaction (click chemistry)
is regiospecific and occurs under mild conditions,[15] no catalyst
has been developed so far for the EAC reaction to make it
universal or regio-/stereospecific. Furthermore, the solution-
phase EAC reaction is limited to only highly activated[16] or
strained alkene.[17] Moreover, it yields a mixture of regioisomers,
viz. 1,4-disubstituted and 1,5-disubstituted triazolines and other
products.[18] Hence, solution-phase EAC reactions are neither
widely applicable nor used for synthetic purposes. This
prompted us to investigate the EAC reaction under topochemical
conditions.
The major challenge in designing a topochemical reaction lies in
the crystal engineering to place the reacting groups in the crystal
lattice at a distance and geometry suitable for their reaction with
minimal movement. Generally, small peptides showing
hydrogen-bonded packing in their crystals align head-to-tail in a
direction perpendicular to the H-bonding direction (Figure 1c).[19]
Thus, a sheet-forming peptide terminally modified with two
mutually complementary reacting groups (CRGs) would arrange
in head-to-tail fashion in its crystal, placing the CRGs of adjacent
molecules at proximity. Diphenylalanine-based peptides are
known to adopt hydrogen-bonded packing in their crystals. [12g,20]
We anticipated that the diphenylalanine-based peptide
(monomer) 1 would exhibit an H-bonded packing in its crystal
with proximally placed azide and ene groups between molecules
of adjacent H-bonded arrays.
Topochemical reactions, the reactions that occur in the crystal
lattice due to the proximal alignment of reacting groups, attract
much attention due to their high yield, regio/stereo-specificity,
green reaction conditions, and ability to give products that are
not conceivable by conventional solution-phase chemistry.[1]
Since the discovery of light-induced topochemical [2+2]
cycloaddition in the 1960s,[2] this field has been growing, and
even today, it continues to elicit interest. A few more light-driven
topochemical reactions such as [4+4] cycloaddition,[1d,1e,3]
polymerization of diynes,[4] bisindenones,[5] dienes,[1a,1g,6] and
quinodimethanes[1h,7] have been discovered (Figure 1a), but
[2+2] cycloaddition continues to be the most commonly exploited
topochemical reaction.[1i,8] One commonly encountered
disadvantage of light-induced topochemical reactions is the low
yield due to the poor phototransmittance through the initially
formed product-phase, especially when it is not crystalline,
leaving the inner part of the crystal unreacted.[1g,9] Though a few
thermal topochemical reactions are known,[1h,10] they are seldom
used for reaction designs. A real breakthrough came when we
furtherance the topochemical alkyne-azide cycloaddition (TAAC)
reaction (Figure 1a), [11] wherein proximally placed azide and
alkyne groups undergo regiospecific 1,3-dipolar cycloaddition
reaction under thermal conditions, leading to the formation of
1,2,3-triazoles in the crystal-state. We have exploited this
thermal TAAC reaction for the synthesis of several polymers.[12]
1
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Figure 1. a) Common topochemical reactions. b) Topochemical ene-azide cycloaddition reaction. c) Schematic representation of the crystal packing of small
peptides showing their head-to-tail arrangement perpendicular to the H-bonding direction.
2
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Figure 2. a) Chemical structure of monomer 1. b) Twist-stacking of monomer 1 along the ‘c’ axis via NH…O hydrogen bonding. Orange dotted lines represent
hydrogen bonding. c) View of the monomer 1 along the ‘c’ axis showing a twist-stacked packing. d) Parallelly arranged linear CH…N hydrogen-bonded chains of
monomer 1 in the ‘ab’ plane showing the proximal arrangement of azide (magenta) and alkene (sky blue) units. Maroon dotted-line indicate the proximity of azide
and alkene, and purple dotted-lines indicate the CH…N hydrogen bond. (Hydrogen atoms and phenyl rings are hidden for clarity)
We have synthesized monomer 1 (Figure 2a, Figure S1) and
crystallized from a mixture of ethyl acetate and petroleum ether.
Single-crystal X-ray diffraction (SCXRD) analysis revealed that
monomer 1 adopts the hexagonal packing in the P65 space
group (Table S1). NH…O Hydrogen bonds connect the
molecules, forming a twisted sheet along the ‘c’ direction (Figure
2b). Adjacent H-bonded molecules are aligned at an angle of 60o
along the six-fold screw axis and show
a left-handed
supramolecular helicity (Video S1). Perpendicular to the helix
axis, molecules are head-to-tail arranged and are connected
3
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Figure 3. a) Chemical structure of polymer 2. b) Twist-stacking of polymer 2 along ‘c’ axis via NH…O hydrogen bonding; Orange dotted lines represent hydrogen
bonding. c) View of the polymer 2 along the ‘c’ axis showing a twist-stacked packing. d) Parallel arrangement of 1,4-triazoline linked polymer chains in ‘ab’-plane.
(Triazoline rings are highlighted in ball and stick model. Hydrogen atoms and phenyl rings are hidden for clarity).
through a CH…N hydrogen bond between the alkene-H and a-N
of the azide (Figure 2d). Such CH…N hydrogen-bonded chains
of molecules are parallelly arranged in the ‘ab’ plane, and by
virtue of the six-fold screw axis, such parallelly arranged chains
are twist-stacked (twist angle of 60o) along the ‘c’ direction
(Figure 2c).
Though the molecules are arranged in a head-to-tail fashion in
‘ab’ planes, the azide and alkene of adjacent molecules are not
in the parallel orientation required for their cycloaddition reaction
(Fig 2d). However, the presence of voids around the azide and
alkene groups and the flexible linkages connecting these groups
in monomer 1 would allow rotation of these complementary
reactive groups to reach a reactive parallel orientation (Figure
S3). Hence, they could undergo topochemical cycloaddition
reaction forming a triazoline-linked polymer (Figure 3a). Crystals
of many molecules having an unfavorable arrangement of their
reacting groups undergo topochemical reactions due to the
4
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Figure 4. a) Chemical transformation of polymer 2 to polymer 3 upon heating. b) Crystal kept in the immersion oil on heating at the range of 175-190 oC releases
N2 as bubbles. c) TGA comparison of monomer 1, polymer 2 and polymer 3 (Inset: ~7% weight loss corresponding to N2) d) DSC comparison of monomer 1,
polymer 2 and polymer 3.
transient attainment of reactive orientation via molecular
motion.[12g]
newly generated chiral center in the triazoline ring suggests that
the observed spontaneous solid-state reaction is both
regiospecific and stereospecific. Though this spontaneous
reaction at room temperature takes about 60 days for
completion, the reaction gets accelerated at higher
We found that the crystals react spontaneously at room
temperature. We have systematically studied the reactivities of
these crystals by various time-dependent analytical techniques.
For this, we kept 100 mg of crystals at room temperature and
withdrawn small portions at different intervals, and analyzed by
using IR, NMR, and PXRD measurements. The time-dependent
IR spectroscopy reveals that the peak corresponding to the
azide stretching at ~2117 cm-1 diminishes progressively with
time, suggesting the gradual consumption of azide presumably
due to the ene-azide cycloaddition reaction (Figure S4). After
60d, the azide signal disappeared completely, suggesting the
completion of the reaction. Similarly, the time-dependent 1H
NMR spectroscopy also proved the gradual progress of the
reaction. The signals corresponding to the alkene protons at
5.72 and 5.06-5.01 ppm diminished with time and the new signal
corresponding to the proton at the newly formed stereogenic
center of the triazoline grew concomitantly (Figure S5). The
presence of only one signal corresponding to the proton at the
o
temperatures; at 50 C, the reaction completes within 64 h and
at 100 oC, it completes in 4h. We recorded the PXRD profiles of
the sample at different stages of the reaction. At all stages of the
reaction, the PXRD profile exhibited sharp reflection peaks,
suggesting the crystallinity of the sample throughout the reaction.
As the reaction progressed, some peaks shifted, some peaks
vanished, and some new peaks have appeared, suggesting a
smooth transition from reactant crystal to product crystal. Thus,
the time-dependent PXRD study revealed that the reaction is a
crystal-to-crystal reaction (Figure S6).
As the morphology and the crystallinity of the sample were
intact (Figure S7), we determined the crystal structure of a fully
reacted crystal. As expected, ene-azide cycloaddition reaction
happened in a single-crystal-to-single-crystal (SCSC) manner,
5
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yielding triazoline-linked linear polymer without affecting the
crystal symmetry. The cycloaddition reaction was both
regiospecific and stereospecific to give 1,4-disubstituted
triazoline having (S)-stereochemistry for the newly generated
chiral center. The polymer chains are parallelly arranged in ‘ab’
planes (Figure 3d) and such layers are twist-stacked along the
screw axis with a twist of 60o (Video S2, Figure 3b,c). Aligning
polymers in such an interesting topology is not possible by
conventional solution-phase polymerization. GPC analysis
revealed the presence of polymers with a weight averaged
molecular weight (Mw) of around 170 kDa (Figure S8).
One advantage of triazolines over triazoles is that they can be
converted to other functional groups.[21] This allows the unique
possibility of post-polymerization modification of the backbone
linkage of the triazoline-linked polymers. Garcia-Garibay et al.
have reported that the crystalline triazoline derivatives undergo
denitrogenation upon photoirradiation or heating to yield
aziridine derivatives.[22] Encouraged by this report, we heated
the polymer crystals anticipating the conversion of triazoline-
linked polymer to aziridine-linked polymer (Figure 4a). From 175
oC onwards, we observed the evolution of nitrogen as bubbles
during the heating, and this bubbling continued till the complete
melting (190 oC) of the polymer (Figure 4b, Video S3). The
denitrogenation happens with retention of crystallinity. We
inferred this from the birefringence pattern of the crystal when
viewed through a polarising microscope (Figure S9, Video S4).
Thermogravimetric analysis (TGA) of the polymer crystals
indicated a weight loss of around 7%, in the temperature range
189-193 oC and this corresponds to the weight percentage of
nitrogen (7.4%) that can be evolved as N2 molecule in the
denitrogenation of the triazoline-based polymer (Figure 4c). This
suggests that denitrogenation is complete by 193 oC.
Figure 5. 13C NMR spectra of polymers in DMF-d7 before (polymer 2) and after (polymer 3) heating.
o
The NMR analysis of polymer 2 heated till 193 C confirmed its
conversion to aziridine-linked polymer. Differential Scanning
Calorimetry (DSC) analysis of the triazoline-linked polymer
showed an exothermic peak at 191 oC (Figure 4d), which
of backbone modification to access a diverse range of
functionalized polymers through the nucleophilic opening of the
aziridine ring.[23] The GPC analysis of the aziridine-linked
polymer showed a weight-average mass of 149 kDa (Figure
coincides with the denitrogenation temperature observed in TGA. S10).
The denitrogenation can also be done in solution. Thus, heating
o
a solution of polymer 2 in DMF at 90 C for 12 h resulted in its
smooth denitrogenation to the aziridine-linked polymer 3. This is
evident from the shift of carbon signals in the 13C NMR spectra
of polymers in DMF-d7 before (polymer 2) and after (polymer 3)
heating (Figure 5). Notably, the carbon signals corresponding to
C5 (76.41 ppm) and C6 (46.69 ppm) in the triazoline ring of
polymer 2 underwent an upfield shift to 70.21 ppm (C5) and
35.19 ppm (C6) respectively after it turned into an aziridine ring.
This smooth conversion to aziridine opens up further possibilities
In conclusion, we have developed a new topochemical
cycloaddition reaction between ene and azide groups that leads
to the regiospecific and stereospecific formation of substituted
triazolines. When a monomer containing these complementary
reacting groups pre-organizes in a ready-to-react head-to-tail
arrangement in its crystals, it can undergo TEAC reaction to
yield
a triazoline-linked polymer. The crystal confinement
ensures regiospecificity and stereospecificity for such a reaction.
We have demonstrated the utility of TEAC reaction in
6
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Acknowledgements
KMS thank the Department of Science and Technology (DST)
for a SwarnaJayanti Fellowship.
Conflict of interest
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Keywords: Topochemical reaction• single-crystal-to-single-
crystal • click chemistry • triazoline-linked polymer • aziridine-
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Entry for the Table of Contents
The rationally designed monomer having reactive groups, azide and alkene, adopt suitable crystal packing for a novel Topochemical
Ene-Azide Cycloaddition reaction (TEAC). Spontaneously, the monomer undergoes polymerization to yield triazoline-linked polymer
in a single-crystal-to-single-crystal fashion. The triazoline-linked polymer is transformed into a value-added aziridine-linked polymer
by simple heating. This work may generate a new class of crystalline polymers with futuristic applications.
8
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