Stereospecific photochemistry of Δ2-1,2,3-triazolines in solution and in the solid state: Scope and mechanistic studies

Article abstract of DOI:10.1039/c7pp00187h

The stereospecific photochemistry of ten N-aryl-substituted cis- or trans-Δ²-1,2,3-triazolines to form the corresponding cis- or trans-aziridines was investigated both in solution and in the solid-state. We found that photochemical reactions in the solid state are more stereospecific than in solution for the 8 crystalline Δ²-1,2,3-triazolines. Additionally, triplet sensitization for some triazolines results in triplet biradicals, which provide the more thermodynamically favored trans-aziridine regardless of the starting triazoline stereochemistry. Product analyses as a function of temperature and solvent polarity suggest that the electronic excitation of the Δ²-1,2,3-triazolines results in the formation of a 1,3-biradical intermediate.

Full text of DOI:10.1039/c7pp00187h

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Stereospecific photochemistry of Δ2-1,2,3-triazo-
lines in solution and in the solid state: scope and
mechanistic studies†
Cite this: DOI: 10.1039/c7pp00187h
Tim S. Chung,
Yang Xue,
Alberto Carranza
and
Miguel A. Garcia-Garibay
*
The stereospecific photochemistry of ten N-aryl-substituted cis- or trans-Δ2-1,2,3-triazolines to form the
corresponding cis- or trans-aziridines was investigated both in solution and in the solid-state. We found
that photochemical reactions in the solid state are more stereospecific than in solution for the 8 crystal-
line Δ2-1,2,3-triazolines. Additionally, triplet sensitization for some triazolines results in triplet biradicals,
which provide the more thermodynamically favored trans-aziridine regardless of the starting triazoline
stereochemistry. Product analyses as a function of temperature and solvent polarity suggest that the
electronic excitation of the Δ2-1,2,3-triazolines results in the formation of a 1,3-biradical intermediate.
Received 20th May 2017,
Accepted 18th July 2017
DOI: 10.1039/c7pp00187h
rsc.li/pps
Introduction
Aziridines are nitrogen-containing three-membered hetero-
cycles widely explored for their interesting reactivity and bio-
logical potential.1–6 Some of the most common strategies for
the stereospecific synthesis of aziridines7 involve the addition
of nitrenes to alkenes, or the addition of carbenes into imine
double bonds. Unfortunately, improved selectivity requires the
use of expensive metal catalysts and toxic volatile solvents,
such that alternative strategies are desirable. Knowing that
solid-state photochemistry is a viable green-chemistry alterna-
tive for the preparation of valuable synthetic targets,8–13 we
recognized an opportunity to explore a two-step strategy for the
stereospecific synthesis of substituted aziridines. As illustrated
in Scheme 1, we proposed the use of (cis)- or (trans)-alkenes 1
to carry out 1,3-dipolar cycloadditions with azides 2 to form
(cis)- or (trans)-substituted 1,2,3-triazolines 3.8 The second step
would involve the photoinduced diastereospecific generation
of the desired aziridines 4 by the formation of an intermediate
1,3-alkylaminyl biradical in the crystals of the pure 1,2,3-triazo-
line 3.
Scheme 1
ing the compound where the N-aryl group is located vicinal to
the ester group. The crystallization of the two triazoline
isomers proceeded well, and the solid state photochemistry
occurred with high diastereomeric yields. In this article, we
increased the number of aryl azides studied to include phenyl
azide (2A), 2-naphthyl azide (2B), 4-biphenyl azide (2C), and
4-(triphenylmethyl)phenyl azide (2E). All the dipolar cyclo-
addition reactions were highly regioselective and stereo-
The viability of the strategy as shown in Scheme 1 was
recently reported in a brief letter.8 It was shown that the hydro-
gen-bond-catalyzed dipolar cycloadditions of trifluoromethyl
acrylates trans-1 and cis-1 with 2-fluorenyl azide 2D (Scheme 2)
are not only stereospecific, but remarkably regioselective, yield-
Department of Chemistry and Biochemistry, University of California, Los Angeles,
USA. E-mail: mgg@chem.ucla.edu
specific, each giving
a
single triazoline diastereomer
(Scheme 2). Eight out of the ten triazolines prepared turned
out to be crystalline solids and photochemical experiments
†Electronic supplementary information (ESI) available: Synthetic and photo-
chemical procedures, as well as IR, 1H and 13C NMR. See DOI: 10.1039/c7pp00187h
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Solution photochemistry
Triazoline (ca. 5 mg) was dissolved in 1 mL of either deute-
rated acetonitrile or acetone in an NMR tube and the solution
was freeze–pump–thawed for 3 cycles to ensure no gas was
present in the reaction. The NMR tube was photolyzed using a
medium-pressure Hg Hanovia lamp with a Pyrex emersion well
filter with a cutoff of λ ≤ 290 nm. The reactions were moni-
tored by 1H NMR and showed completion in less than one
hour.
Solid-state photochemistry
Triazoline (ca. 3 mg) was crushed between two microscope
slides and exposed to light from
a medium-pressure
Hg Hanovia lamp with a Pyrex emersion well filter with a
cutoff of λ ≤ 290 nm. The reaction was monitored by H NMR
Scheme 2
1
and was completed in less than one hour.
showed that the previously observed results are robust, with
solid state diastereomeric ratios that are consistently greater
(99 : 1 to 87 : 13) than those observed in acetonitrile solution
(85 : 15 to 66 : 34).
Results and discussion
Synthesis and characterization
To establish the stereospecificities of product formation in
terms of the ratios of trans- and cis-aziridine products, various
1,2,3-triazolines were synthesized using a 1,3-dipolar cyclo-
addition of five different aryl azides (2A–2E) and the two geo-
metric isomers of dipolarophile 1 (Table 1). The aryl groups
used in this study contain phenyl (2A), 2-naphthyl (2B),
4-biphenyl (2C), 2-fluorenyl (2D), and 4-trityl phenyl (2E) as
illustrated in Table 1.
Experimental
Chemicals
All of the aryl amines and solvents employed were commer-
cially available and used without further purification.
Synthesis of aryl azides (2A–2E)
Commercially available aryl amines were easily converted to
their corresponding aryl azides (2A–E) under standard
conditions in good yields ranging from 74–99% (Table 1).12
The newly formed azide products were characterized by
1H, and 13C NMR spectroscopy, which matched the reported
spectra from the literature, and the azido functional groups
were shown to display the characteristic IR stretch at ca.
2200 cm−1 (see the ESI†). With the aryl azides in hand for the
1,3-dipolar cycloaddition, we used commercially available ethyl
The following general protocol was used to synthesize the aryl
azides. Aniline (6 mmol) was dissolved in 5 mL of trifluoro-
acetic acid. The solution was cooled to 15 °C and kept at this
temperature throughout the reaction. An aqueous solution
(2 M) of sodium nitrite was slowly added (32 mmol) and the
reaction mixture was stirred for 30 min. Subsequently, an
aqueous solution of sodium azide (32 mmol, 2 M) was added
dropwise. The resulting mixture was allowed to warm to room
temperature for one more hour. The reaction mixture was
neutralized to pH 7 and extracted three times with 20 mL of di-
chloromethane. The crude mixture was purified by column
chromatography using 1 : 7 (ethyl acetate : hexanes).
Table 1 Isolated yields of aryl azides (2A–E) and aryl triazolines (trans-
3A–E and cis-3A–E)
Synthesis of Δ2-1,2,3-triazolines
Structure
Azide yields
Triazoline yields
All triazolines were synthesized using the following general
procedure: ethyl 4,4,4-trifluorocrotonate (0.3 mmol), 2A
(0.3 mmol), and N,N′-dimethylurea (0.03 mmol) were dissolved
in 1.5 mL of dry toluene and the reaction mixture was stirred
for 32 hours at 60 °C under an argon atmosphere. After the
initial 32 hours, an additional 0.3 mmol of ethyl 4,4,4-trifluoro-
crotonate was added to the reaction mixture. After 33 more
hours of heating, the reaction was cooled when the dis-
appearance of azide by TLC was observed. The crude mixture
was rotary evaporated under vacuum and the reaction mixture
was purified by column chromatography using an ethyl
acetate : hexane mixture (1 : 7) as the eluent.
74% (2A)
22% (trans-3A)
12% (cis-3A)
14% (cis-3B)
98% (2B)
80% (trans-3B)
83% (2C)
99% (2D)
51% (trans-3C)
65% (trans-3D)
21% (cis-3C)
18% (cis-3D)
86% (2E)
74% (trans-3E)
22% (cis-3E)
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Table 2 Ratios of cis- and trans-aziridines from triazolines in solution
(E)-4,4,4-trifluorobut-2-enoate (trans-1) as the dipolarophile for
the cycloaddition. Alkene trans-1 was selected due to its inher-
ently electron poor nature resulting from the effects of the
ethoxy ester and trifluoromethyl groups. Ethyl (Z)-4,4,4-trifluoro-
but-2-enoate (cis-1) was obtained by isomerizing trans-1 under
UV irradiation to afford the photostationary state of 3 : 1
(trans-1 : cis-1). The samples of cis-1 were subsequently purified
by column chromatography. Triazolines were synthesized by
performing 1,3-dipolarr cycloadditions by heating azides 1A–E
with either trans-1 or cis-1 in toluene with 10 mol% of dimethyl
urea, which served as a hydrogen bond-donor catalyst, to
obtain triazolines trans-3A–E and cis-3A–E in low to modest
yields. The regiochemistry and stereochemistry of the triazo-
lines were assigned based on 1H and 13C, which matched
with the previous reports on Δ2-1,2,3-triazolines cis-3D and
trans-3D, which included their single crystal X-ray structures.8
Furthermore, the differential scanning calorimetric analysis of
triazoline trans-3D shows a relatively sharp endothermic
transition which correlated with the melting temperature
determined visually, and was immediately followed by a broad
exothermic transition that corresponds to the thermal denitro-
genation. This is illustrated in Fig. 1 with the DSC data from
trans-3D, which melts at 131 °C and has a minimum for the
decomposition peak at 171 °C.
and solid statea
CD3CN
trans : cis
(CD3)2CO
trans : cis
Crystals
trans : cis
Entry
Compound
1
2
3
4
5
6
7
8
9
trans-3A
trans-3B
trans-3C
trans-3D
trans-3E
cis-3A
cis-3B
cis-3C
cis-3D
cis-3E
85 : 15
83 : 17
82 : 18
76 : 24
73 : 27
29 : 71
21 : 79
34 : 66
32 : 68
28 : 72
79 : 21
75 : 25
99 : 1
99 : 1
97 : 3
52 : 48
43 : 57
96 : 4
99 : 1
43 : 57
n.a.b
n.a.b
90 : 10
83 : 17
83 : 17
1 : 99
6 : 94
17 : 83
10 : 90
12 : 88
10
Photochemical studies
a Product ratios determined in triplicate with estimated errors of 3%.
b Triazolines were liquids at 300 K.
Triazolines trans-3A–E and cis-3A–E were irradiated in solution
using either acetonitrile-d3 or acetone-d6, or in the neat solid-
state, using a medium-pressure Hg lamp (450 watts) equipped
with a Pyrex filter to provide λ ≥ ca. 290 nm. To our delight,
only the corresponding aziridine photoproducts were observed
both in solution and in the solid state (Table 2). Solution
experiments were carried out with ca. 5 mg ml−1 triazoline
solutions that had been subjected to three freeze–pump–thaw
cycles to ensure that photoreactions occurred free from
oxygen. Solid state experiments were carried out with ca. 3 mg
of finely powdered crystals sandwiched between two micro-
trans- to cis-aziridines identified by the different 1H coupling
constants, Jtrans ≈ 2.3 Hz and Jcis ≈ 6.5 Hz.14
The photochemical results summarized in Table 2 include
the product ratios obtained in triplicate with all five trans-
triazolines in the top five entries, and those determined with
the cis-triazolines in the bottom five. It is estimated that errors
in the table are in the range of 3%. The results obtained
under direct excitation in acetonitrile solutions, upon triplet
sensitization in acetone, and by direct irradiation in the crys-
talline solid state, are included in the left, middle, and right
columns, respectively. The results of experiments carried out
in acetonitrile revealed a high level of stereospecificity, with
each triazoline isomer resulting in the preferential formation
of the corresponding same isomeric aziridine product. As in
the previous examples, this result implies that the biradical
intermediates resulting from the denitrogenation of the five-
membered heterocycle are formed in the singlet state and have
no time to equilibrate before the bond formation to form the
three-membered ring. Thus, the irradiation of trans-triazolines
resulted in trans : cis ratios ranging from 85 : 15 to 73 : 27
(Table 2, third column, entries 1–5) and the direct photo-
excitation of cis-triazolines resulted in analogous but opposite
selectivities, from 21 : 79 to 34 : 66 (Table 2, third column,
entries 6–10).
1
scope slides. The reaction progress was monitored by H NMR
spectroscopy, which was also used to determine the ratio of
Fig. 1 Differential Scanning Calorimetry (DSC) of triazoline trans-3D.
A sharp peak at 131 °C indicates melting and a broad peak at 171 °C
shows denitrogenation of triazoline to form aziridine.
Experiments carried out in acetone, which is used both as a
solvent and triplet sensitizer, are expected to reveal the reactiv-
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ity of the longer-lived triplet state biradicals. In this case, the cis-aziridine as the preferred photoproduct with yields that
unable to form the singlet state photoproduct before intersys- range from 83% (cis-3C) to 99% (cis-3A). While a higher stereo-
tem crossing to the singlet state, triplet biradicals are expected specificity was expected in the solid state compared to solu-
to have a greater opportunity for conformational equilibration. tion, it appears that hindered bond rotation in the 1,3-bi-
Thus, at a first approximation, one may expect the stability of radical intermediate is not large enough to guarantee a very
the triplet state biradical to parallel the stability of the high specificity in all cases. The lack of correlation with
product, such that the trans-aziridine isomers should be pre- respect to the size of the N-aromatic substituent suggests that
ferred in the triplet-sensitized reaction. To verify this we per- isomerization occurs by bond rotation about the biradical
formed calculations on the ground state energies of the equili- ArNC(CO2Et)–CHCF3 bond. This hypothesis is suggested by a
brium conformers of trans-4A–D and cis-4A–D at the B3LYP/ previously reported example where the stereospecificity of a
6-31G* level of theory. We confirmed that the trans aziridines 4-methyl-4-trifluoromethyl triazoline analog was shown to be
are indeed more stable than the cis aziridines. Energy differ- quantitative.8
ences of 2.6 and 2.3 kcal mol−1 were observed, respectively, for
Mechanistic analysis
the N-phenyl (4A) and N-(4-phenyl) (4C) compounds, with the
corresponding values of 6.3 and 6.4 kcal mol−1 for the analo-
gous 2-naphthyl (4B) and 2-fluorenyl (4D) structures. We
omitted aziridines 4E due to their large size and long comput-
ing time, assuming that similar results should be observed.
While our expectations on the relative energies of the cis and
trans N-aryl aziridines were confirmed, we discovered that the
experimental results of reactions carried out in acetone are not
straightforward, as shown in the fourth column in Table 2.
On one hand, one can see that five entries, including trans-3C,
trans-3D, trans-3E, cis-3C and cis-3D, yield the trans-aziridine
isomer in yields greater than 96% and conform to our original
expectations. However, the stereoselectivities observed with
compounds trans-3A and trans-3B are diminished to ca. 75 and
79% trans, respectively, and those observed from cis-3A, cis-3B
and cis-3E are closer to 50 : 50. Additional measurements
revealed that the product ratios are not dependent on the
extent of conversion or irradiation time, and experiments
carried out with other triplet sensitizers, including benzophe-
none, xanthone and biacetyl, all using the same freeze–pump–
thaw method, showed that the products in the table do reflect
the reactivity of the triplet excited state. The fact that different
product ratios are observed from triplet sensitized experiments
that start from the cis- and trans-isomers of the same triazoline
indicates that biradical equilibration does not take place. For
example, trans-3A and cis-3A give aziridine 4A in trans : cis
ratios that are 79 : 21 and 52 : 48, respectively. This observation
indicates that intersystem crossing and ring closure compete
with the rate of conformational equilibration, a fact that has
been shown for a number of short chain 1,3-biradicals and
1,4-biradicals.15–18
To explore the type and range of experimental parameters that
affect the photoreaction, and perhaps the subtle differences
between the product ratios of trans : cis aziridine in solution
and in the solid-state, we first analysed the role of solvent
polarity. While we postulated a biradical intermediate resulting
from a homolytic cleavage reaction (Fig. 2), we reasoned that a
zwitterionic intermediate would have different stabilizing
interactions in different solvents, which may result in different
trans : cis aziridine ratios.19 The results of photochemical
experiments using triazoline trans-3C in methanol, acetonitrile
and carbon tetrachloride, which range in ET(30) values from
32–55 kcal mol−1 20
(Table 3).
,
showed no changes in stereospecificity
Fig. 2 Plausible biradical intermediate versus zwitterionic intermediate
to form aziridines.
Table 3 Solution-state photochemistry of triazoline trans-3C in sol-
vents with different polaritiesa
Photochemical experiments in the crystalline solid state
included the samples of trans-triazolines 3C–E and cis-triazo-
lines 3A–E. The samples of trans-(N-phenyl)- and trans-N-
(2-naphthyl)-triazolines trans-3A and trans-3B were liquids
under ambient conditions. We were able to confirm that, as
expected, stereospecificities obtained in the solid state are
greater than those obtained under direct irradiation in solu-
tion. For example, the reaction of N-(4-biphenyl)-trans-triazo-
line 3C improved the formation of trans-aziridine 4C from 82%
in solution to 90% in the solid state (entry 3, columns 3 and
5). Similar observations were made with trans-triazolines 3D
and 3E, and with all cis-triazolines, which systematically give
Solvent
ET(30) (kcal mol−1
)
trans-4C (%) cis-4C (%)
Methanola,b
Acetonitrilea
55
46
80
82
81
20
18
19
Carbon tetrachloridea 32
a Product ratios determined in triplicate with estimated errors of 3%.
b Products showed both methyl ester and ethyl ester.
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A plausible mechanism to account for different trans : cis
aziridine ratios under direct excitation in solution and in
solids would be that the trans-triazolines form stereospecifi-
cally to the trans-aziridine product, but continuous irradiation
may provide a pathway for isomerization. However, experi-
ments carried out with pure aziridine trans-4C in solution and
in the solid state for several hours under the previously
utilized reaction conditions led to no observable change,
ruling out the possibility of trans-aziridine isomerization. In
order to determine whether one can steer the ratios of cis- to
trans-aziridines in the solid state, we analysed the effects of
temperature from 25 °C to −196 °C. We observed a decrease in
the stereospecificity of the reaction through this temperature
range with the trans : cis ratio changing from 84 : 16 at 25 °C to
66 : 33 at −40 °C and below, showing that the optimum tem-
perature to provide the highest trans/cis aziridine ratios is
around room temperature.
3 B. Zwanenburg and P. ten Holte, Topics in Current
Chemistry, in Stereoselective Heterocyclic Synthesis III, ed.
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Natural Product Synthesis, 2011, p. 3.
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Conclusions
In conclusion, we have synthesized ten different diastereomeri-
cally pure triazolines and investigated their stereospecificities
to form aziridines in solution and in the solid state. We have
shown that the photochemistry of Δ2-1,2,3-triazolines are
moderately stereospecific in solution and more so in the solid
state. The solid-state photochemistry of triazolines provides an
efficient and solvent-free method to synthesize their corres-
ponding aziridines. Specifically, we found that the photochem-
istry of Δ2-1,2,3-triazolines in singlet excited states would
afford the kinetically controlled trans- to cis-aziridine ratios,
whereas efficient triplet sensitization would form the thermo-
dynamically controlled product ratios. Using various solvent
studies, variable temperature studies, and product analysis, we
propose that the photochemistry of Δ2-1,2,3-triazolines to
form aziridines follows a homolytic cleavage to expel nitrogen
and form the 1,3-biradical species.
Conflicts of interest
12 D. de Loera and M. A. Garcia-Garibay, Efficient Aziridine
The authors declare no competing financial interest.
Synthesis
in
Metastable
Crystalline
Phases
by
Photoinduced Denitrogenation of Crystalline Triazolines,
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13 M. A. Sinnwell and L. R. MacGillivray, Halogen-Bond-
Templated [2+2] Photodimerization in the Solid State:
Directed Synthesis and Rare Self-Inclusion of a Halogenated
Product, Angew. Chem., Int. Ed., 2016, 55, 3477–3480.
Acknowledgements
This work was made possible by the support from NSF Grant
CHE1566041.
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