Mechanical Force for the Transformation of Aziridine into Imine

Article abstract of DOI:10.1002/anie.202109358

Force-selective mechanochemical reactions may be important for applications in polymer mechanochemistry, yet it is difficult to achieve such reactions. This paper reports that cis-N-phthalimidoaziridine incorporated into a macromolecular backbone undergoes migration of N-phthalimido group to afford imine under mechanochemical condition and not thermal one. The imine is further hydrolyzed by water bifurcating into amine and aldehyde. These structural transformations are confirmed by ¹H NMR and FT-IR spectroscopic analyses. Computational simulations are conducted for the aziridine mechanophore to propose the mechanism of reaction and define the substrate scope of reaction.

Full text of DOI:10.1002/anie.202109358

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Accepted Article
Title: Mechanical Force for the Transformation of Aziridine into Imine
Authors: Sangmin Jung and Hyo Jae Yoon
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Mechanical Force for the Transformation of Aziridine into Imine
Sangmin Jung, and Hyo Jae Yoon*
Abstract: Force-selective mechanochemical reactions may be
important for applications in polymer mechanochemistry, yet it is
difficult to achieve such reactions. This paper reports that cis-N-
phthalimidoaziridine incorporated into a macromolecular backbone
undergoes migration of N-phthalimido group to afford imine under
mechanochemical condition and not thermal one. The imine is further
hydrolyzed by water bifurcating into amine and aldehyde. These
1
structural transformations are confirmed by H NMR and FT-IR
spectroscopic analyses. Computational simulations are conducted for
the aziridine mechanophore to propose the mechanism of reaction
and define the substrate scope of reaction.
Scheme 1. Mechanical force selective transformation of N-phthalimidoaziridine
into imine.
Polymer
mechanochemistry
permits
access
to
mechanism and the substrate scope of our mechanochemical
reaction.
mechanochemical activation of latent catalysts,[1] stress-
sensing,[ materials transfer, drug release, changes in optical
and electrical properties,[5] and degradation of polymer. It also
provides the ability to steer chemical reactions toward routes that
are forbidden under traditional photochemical and thermal
conditions.[ The utility and application scope of polymer
mechanochemistry depend on covalent mechanophores
designed to facilitate chemical transformations upon exposure to
external force.[8] Mechanochemical reactions that have been
2]
[3]
[4]
Our group has recently probed the potential of aziridine for a
mechanophore.[7a] The trivalency of nitrogen atom allows one to
[6]
[17]
fine-tune reactivity of aziridine. To design force-sensitive and
thermal-insensitive aziridine mechanophore, we focused on
aziridine with N-phthalimido moiety. Under thermal condition, N-
phthalimidoaziridine with C-aryl substituents undergoes 1,2-
7]
[18]
migration of N-phthalimido group to afford an imine. When aryl
group is substituted on the aziridine carbon, the azomethine ylide
intermediate formed under thermal conditions can be
stabilized.[18b] We hypothesized that, if the stabilizing aryl
substituents are replaced with alkyl ones, the aziridine are inactive
under thermal conditions.
[9]
reported thus far can be categorized as follows: i) retro-[2+2],
10]
[11]
[7a, 7b, 12]
[7c, 13]
[
4+2],[ and [4+4] cycloadditions, ii) 2π,
4π,
and
[14]
6
π
electrocyclic ring opening reactions,
iii) homolytic
[
15]
[16]
reactions and iv) heterolytic reactions.
As summarized in Table S1, many mechanophores respond
to both thermal and photochemical stimuli, yet the reaction routes
differ from each other. Such
We
synthesized
the
polymer
containing
cis-N-
phthalimidoaziridine (P in Figure 1A). An enantiopure eight-
membered cyclic monomer containing cis-N-phthalimidoaziridine
a universal sensitivity of
mechanophore makes it difficult to couple orthogonally
mechanochemical reactions with other reactions that proceed
under traditional thermal and photochemical conditions. A handful
of studies have reported on mechanical force-selective activation
of mechanophores (Table S2).
(
M in Figure 1A) was prepared from cis,cis-1,5-cyclobutadiene in
one step. The desired chemical structure and stereochemistry
were confirmed by 1H and 13C NMR spectroscopies and high-
resolution mass spectrometry (HRMS). The proton resonance of
aziridine’s carbon at δ = 2.62 ppm was indicative of formation of
the desired aziridine ring (Figure 1B). Entropically driven ring
opening metathesis polymerization (ED-ROMP) of compound M
Here we show force-selective migration in N-
phthalimidoaziridine
incorporated
into
an
aliphatic
macromolecular backbone (Scheme 1). A pulling force along the
backbone transduced by ultrasonication activates the aziridine
ring structure and induces migration of N-phthalimido group,
yielding the corresponding imine. Once the reaction is coupled
with the scenario of hydrolytic cleavage in ambient conditions, the
polymer backbone bifurcates into amine and aldehyde. These
and cis-cyclooctene at 1:1 molar ratio in the presence of Grubbs
2nd generation catalyst yielded cis-N-phthalimidoaziridine
embedded copolymer (P). H NMR analysis indicated that the
molar ratio of monomers was reflected into the polymer chain
structure. The proton resonance of the aziridine carbon in the
copolymer P was observed at δ = 2.59 ppm (Figure 1B),
consistent with that of monomer M and indicative of the retention
of aziridine ring structure after the polymerization. Figure 1C
shows the size exclusion chromatography (SEC) trace of
1
sequential reactions are verified by chemical structure analysis of
1
polymers using
H
NMR and FT-IR spectroscopies.
Computational simulations are employed to understand the
S. Jung, and Prof. Dr. H. J. Yoon*
Department of Chemistry
Korea University
copolymer P; the number average molecular weight (M
n
), weight
average molecular weight (M ), and polydispersity index (PDI)
w
Seoul, 02841, South Korea
E-mail: hyoon@korea.ac.kr
were revealed to be 81.3 kDa, 487.7 kDa, and 6.0, respectively.
The Supporting Information contains detailed synthetic
procedures and analytical data.
Supporting information for this article is given via a link at the end of the
document
1
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Figure 1. A) Synthesis of cis-N-phthalimidoaziridine embedded copolymer (P)
from 8-membered cyclic monomer (M) through ED-ROMP. B) Comparison of
1
H
NMR spectra for monomer M and polymer P. C) Size exclusion
chromatography (SEC) trace of P.
For mechanochemical experiments, ultrasound (20 kHz, 30 %
amplitude; pulse sequence: 1 sec on / 1 sec off; power intensity:
2
1
0.26 W/cm ) was applied to the copolymer P dissolved in an-
o
atmosphere for 2 h. 1H NMR
hydrous THF at 4-8 C under N
2
analysis revealed the appearance of new proton resonances at δ
=
7.79, 7.40 and 7.33 ppm (d-f in Figure 2C). To confirm the
1
formation of imine, H NMR spectrum of P1 was compared with
that of model imine compound (Model 4): the f peak was
consistent with the imine proton at δ = 7.79 ppm, indicative of
imine formation (Figure 2C, D). Additional new peaks were
detected at δ = 9.76, 7.85, 7.76, and 5.24 ppm (orange squares
in Figure 2C), which were attributed to the formation of aldehyde
and amine species resulting from unexpected decomposition of
imine via hydrolysis with adventitious water. To evaluate the
thermal stability of copolymer P, its toluene solution was refluxed
1
for 24 h (P2 in Figure 2A). There was no detectable change in H
Figure 2. A) Mechanochemical and thermal reactions of P to yield products P1
and P2, respectively. B-E) Comparison of 1H NMR spectra for P, P1, Model 4,
and P2. Asterisk indicates the residual solvent peaks; black circles are attributed
to undesirable chain scission of polymer backbone (and/or its reaction with THF
solvent) by ultrasonication. Orange squares are peaks of hydrolysis products
NMR (Figure 2E) and FT-IR (Figure S4C in the Supporting
Information) spectra compared to the intact copolymer P. This
indicates the 1,2-dialkyl-substituted cis-N-phthalimidoaziridine is
reactive under mechanochemical condition and not thermal
condition, within the detection limits of NMR and FT-IR
spectroscopies.
(see Figure 3 for details).
The imine resulting from the force-induced migration reaction
underwent hydrolytic cleavage, bifurcating into amine and
aldehyde (Figure 3A). Upon addition of water (5.0 % v/v) into the
THF solution containing the imine product P1 (1 mg/mL) in
ambient conditions, the intensity of proton resonance at δ = 9.76,
which corresponded to the aldehyde proton, significantly
increased while the intensity of imine proton peak at δ = 7.79 ppm
The amine proton was difficult to observe in 1H NMR
spectroscopic analysis. The occurrence of amine was confirmed
through FT-IR spectroscopy (Figure 3F). The absorption peaks at
3
210, 1710, 1375 and 1310 cm-1 were attributed to –NH
2
,
phthalimido group’s C=O, aziridine’s C-N, and the C-N bond of
migrated phthalimido group, respectively (①-④ in Figure 3F).
Molecular weight analysis before and after the hydrolysis
1
was significantly reduced (f in Figure 3C). The H NMR spectrum
revealed reduced M
detectable changes in H NMR and FT-IR spectra when P2 was
n
by 28.0 % (Figure S9A). There were no
of hydrolytic product P1’ was further compared with those of
model aldehyde and amine compounds (Model 2 and 3 in Figure
1
exposed to the same hydrolytic condition (Figure S4B and S4C).
3
D and E, respectively). The comparison indicated that the g-j
peaks in P1’ corresponded to the aldehyde and amine species
Figure 3C-E).
(
2
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COMMUNICATION
increase in the relative energy of the system (Figure 4B) and
exponential increase in the N−N bond length and aziridine’s C-C
bond length (Figure 4C). At Δd = 1.50 Å (iii in Figure 4), the C-C
bond of aziridine was finally cleaved while the N-N bond was not,
suggesting the formation of ylide intermediate. Upon the C-C
bond cleavage, the natural atomic charge (NAC) of aziridine’s
carbons and phthalimido’s nitrogen showed sharp decrease and
increase (Figure 4D, E), indicative of electron-rich and deficient
charge states, respectively. This charge unbalance seems to
induce migration of phthalimido group (Figure 4F). Nonetheless,
we believe that further experimental and computational studies
are needed to clarify the detailed mechanisms of our migration
reactions: particularly, whether ylide is indeed formed in our
reaction, and, if yes, whether its structure is identical with the one
formed under thermal conditions.[
18a, 20]
Figure 4. Simulation of tensile stress-induced structural changes in cis-N-
phthalimidoaziridine. A) Increase in the distance between terminal methyl
groups (d, Å). B) Plots of relative energy as a function of the change in d relative
to intact aziridine (Δd, Å). C) Plots of the N-N bond length (filled circle) and
aziridine’s C-C bond length (open circle). D) Plots of NAC of the aziridine’s two
carbon atoms. E) Plots of NAC of the phthalimido group’s nitrogen (filled circle)
and aziridine’s nitrogen atoms (open circle). Note that the former bond persist
as an artifact in Gaussian after a reaction is predicted to occur. F) Plausible
mechanism for structural transformation of N-phthalimidoaziridine into imine.
Figure 3. A) Hydrolysis of P1 into P1’. B-E) Comparison of 1H NMR spectra
among P1, P1’, and model compounds (Model 2 and 3). F) Comparison of FT-
IR spectra among P, P1, and P1’. Asterisk indicates the residual solvent peaks;
black circles are attributed to undesirable chain scission of polymer backbone
(and/or its reaction with THF solvent) by ultrasonication.
The mechanism of our reaction was further examined by
constrained geometries simulate external force (CoGEF)
calculation using density functional theory (DFT) at the B3LYP/6-
We reconciled our simulation results with the literature. In our
simulation with the cis-aziridine, Fmax—the parameter enabling
relative comparison of mechanochemical reactivity between
different mechanophores[19b]—was revealed to be 4.34 nN.
Recently, theoretical work by Robb[19b] revealed that 2π
electrocyclic mechanophores such as aziridine and gem-
dichlorocyclopropane (gDCC) can undergo mechanochemical
3
1G* level with the model structure (cis-N-phthalimidoaziridine
with alkyl side chains; Figure 4A).[19] In simulation, the distance
between terminal methyl substituents (d, Å) gradually increased.
At the initial stage (i-ii region in Figure 4), the increase in Δd (an
increase in d relative to the intact structure) led to a gradual
3
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transformation reactions in the range of 3.2 - 5.7 nN. Our
the aziridine’s C-C bond, implying its low mechanochemical
reactivity. Such a competition between desired and undesired
covalent bonds has been previously observed in epoxide
mechanophores.[7b, 21] It is noteworthy that, due to this reason, the
mechanochemical ring opening reaction of epoxide
mechanophores cannot be simulated by CoGEF.[19b] Despite the
high value of Fmax (6.18 nN; Table 1), trans-Azi PA exhibited
cleavage of the aziridine’s C-C bond, which indicates that the Fmax
value required for the mechanochemical 2π electrocyclic ring
opening reactions of three-membered ring mechanophores may
have a value greater than what Robb has proposed (5.7 nN).[
calculated
Fmax value fell into the range, proving the
mechanochemical activation at the C-C bond of aziridine.
19b]
Figure 5. N-Phthalimidoaziridine derivatives that can be derived from widely
used synthetic olefinic polymers.
Table 1. CoGEF simulation results for the aziridines presented in Figure 5.
Activation of
aziridine C-C bond
Stereochemistry
Structure
Fmax (nN)
Azi PB
Azi PA
Azi PI
O
O
O
O
O
O
O
O
O
X
4.34
2.77
3.70
3.70
2.54
5.61
6.18
5.45
5.07
-
cis
Azi PC
Azi DP
Azi PB
Azi PA
Azi PI
trans
Azi PC
Azi DP
The substrate scope of our reaction was evaluated by
conducting CoGEF calculations over N-phthalimidoaziridine
derivatives that can be formed on widely used synthetic olefinic
polymers—polybutadiene (PB), polyacetylene (PA), polyisoprene
(
PI), polychloroprene (PC), and diphenyl polyene (DP). If aziridine
can be introduced into the synthetic polymers through top-down
approach and coupled with the repertoire of simple hydrolysis, our
migration reaction can be a new mechanochemical gate for
developing degradable or activatable polymers. We performed
CoGEF simulations over N-phthalimidoaziridine derivatives
shown in Figure 5 and obtained Fmax values. As summarized in
Table 1, all the aziridines exhibited higher Fmax values (by 1.27 –
Figure 6. A) Aziridination of cis-PB polymer and sequential mechanochemical
1
and hydrolysis reactions of cis-Azi PB polymer. B) The
H NMR
characterization of activated species in cis-Azi PB polymer. Asterisk indicates
the residual solvent peaks; black circles are attributed to undesirable chain
scission of polymer backbone (and/or its reaction with THF solvent) by
ultrasonication.
3
.41 nN) in trans-isomers than cis-isomers, which is consistent
[7c, 19b]
with the findings by Craig and Robb.
The Fmax values of our
aziridines were lower than 5.7 nN, proving their potential as a
mechanophore, except the trans-Azi PA and trans-Azi DP.
In the simulation, trans-Azi DP underwent a bond scission in
the C-C bond of alkyl substituent and aziridine carbon, rather than
To verify the simulation results, cis-PB polymer (Figure 6)
was post-modified to have N-phthalimidoaziridine through
oxidative addition of N-aminophthalimide. Force was applied to
4
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COMMUNICATION
the resulting cis-Azi PB polymer under the same conditions as
those of compound P. The H NMR spectrum showed the imine
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polymer1 was hydrolyzed, the imine peak (f’) disappeared, and
peak intensities of protons j’ and g’ in amine and aldehyde
species were increased (cis-Azi PB polymer1’ in Figure 6B).
Each peak shown in Figure 6B was assigned based on the
comparison with the model compounds. Molecular weight
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analysis indicated reduced M upon the hydrolysis (Figure S9C).
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phthalimidoaziridine into imine via 1,2-migration of the N-
phthalimido group, triggered by force and not heating. The force
activates the bond whose axis is orthogonal to the direction of
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5
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Entry for the Table of Contents
(
1.51 cm × 5.5 cm)
When cis-N-phthalimidoaziridine is incorporated into a macromolecular backbone, it undergoes structural transformation into an imine
by mechanical-force-induced 1,2-migration of the N-phthalimido group. The migration occurs under mechanochemical conditions but
not thermal conditions. The imine is further hydrolyzed in the presence of water under ambient conditions, resulting in an amine and
aldehyde.
6
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