Stereospecific Assembly of Fused Imidazolidines via Tandem Ring Opening/Oxidative Amination of Aziridines with Cyclic Secondary Amines Using Photoredox Catalysis

Article abstract of DOI:10.1021/acs.orglett.9b02957

Sustainable assembly of imidazolidines is accomplished via a sequential stereospecific ring opening and C-H amination using aziridines with secondary cyclic amines under visible light mediated indazoloquinoline photoredox catalysis at ambient conditions.

Full text of DOI:10.1021/acs.orglett.9b02957

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Stereospecific Assembly of Fused Imidazolidines via Tandem Ring
Opening/Oxidative Amination of Aziridines with Cyclic Secondary
Amines Using Photoredox Catalysis
Murugan Vijay,† Sundaravel Vivek Kumar,† Vanaparthi Satheesh,† Periyasamy Ananthappan,‡
Hemant Kumar Srivastava,† Sundaram Ellairaja,‡ Vairathevar Sivasamy Vasantha,‡
and Tharmalingam Punniyamurthy*,†
†Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India
‡School of Chemistry, Madurai Kamaraj University, Madurai 625001, India
S
* Supporting Information
ABSTRACT: Sustainable assembly of imidazolidines is accomplished via a sequential stereospecific ring opening and C−H
amination using aziridines with secondary cyclic amines under visible light mediated indazoloquinoline photoredox catalysis at
ambient conditions. Optically active aziridines are coupled with high enantiomeric purities. The computational studies provide
insights on the redox properties of the catalysts as well as a profile of the reaction.
Scheme 1. Outline of this Work
isible-light photoredox catalysis affords a powerful
Vsynthetic tool for the sustainable construction of
carbon−carbon and carbon−heteroatom bonds.1 In particular,
organo photoredox catalysis is a vibrant field, providing an
effective synthetic strategy for the conversion of the simple
substrates into complex molecules with structural diversity.2
Ideally, these catalysts are attractive, as they are cheap, are less
toxic, and have comparable redox potential as that of the
metal-based systems.3 However, synthetic application of the
organic dyes is still fairly limited, as few catalyst choices are
available. Development of organic photoredox catalysts with
broad redox competencies is thus valuable.4,5 Indazoloquino-
lines (IQ) are widely utilized as the organic light emitting
diodes (OLED).6 These robust fluorescent frameworks can
offer a unique stance to alter the redox properties by installing
diverse functional groups. In addition, fused imidazolines are
often found in natural products and bioactive compounds.7
Efforts are thus made on the development of synthetic
methods to construct these scaffolds.8 Recently, Seidel and co-
workers reported a benzoic acid catalyzed redox-neutral
annulation of amine with ketoamides (Scheme 1a),8f while
Meggers and co-workers demonstrated a chiral Ru-catalyzed
C−N cyclization of tetrahydroisoquinoline with 2-azidoaceta-
mides with up to 92% ee (Scheme 1b).8g Herein, we present a
stereospecific assembly of imidazolidines via a tandem ring
opening and an oxidative amination of aziridines with cyclic
secondary amines utilizing quinoline based IQ-A-E photoredox
catalysts in visible light (Scheme 1c). Optically active aziridines
can be reacted with 94−99% ee. Computational studies (DFT
and TD-DFT) afford insight of the redox properties of the
catalysts. To the best of our knowledge, this is the first example
available using IQ photoredox catalysis.
Rh-catalyzed regioselective C−H functionalization of 2-aryl-
2H-indazoles with alkynes produced indazoloquinolines IQ-A-
Received: August 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02957
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
D and indazolonaphthyridine IQ-E in 56−74% yields (Figure
1). Upon visible light irradiation, IQ-A-E showed a nano-
Table 1. Optimization of the Reaction Conditions
entry
deviation
3a (%)
4a (%)
1
2
3
4
5
6
7
8
9
10
None
86
69
61
72
55
40
46
n.d.
n.d.
n.d.
n.d.
10
19
12
31
45
33
87
85
88
IQ-A instead of IQ-D
IQ-B instead of IQ-D
IQ-C instead of IQ-D
IQ-E instead of IQ-D
Eosin Y instead of IQ-D
Rose bengal instead of IQ-D
Ru(bpy)3Cl2·6H2O instead of IQ-D
Ir(ppy)3 instead of IQ-D
Without IQ-D and light
a
Reaction conditions: 1a (0.30 mmol), 2a (0.25 mmol), [cat.] (3 mol
b
%), DMF (2 mL), air, rt, 24 h. Isolated yield. n.d. = not detected.
cyclization using the photoredox catalysis to furnish 3a as a
single diastereomer via an oxidative amination.
With the optimized conditions, the scope of the procedure
was examined for the reaction of a series aziridines 2b−o using
1a as a standard substrate (Scheme 2). Modulation in the 2-
Figure 1. Redox potentials of IQ-A-E with the common catalysts (vs
SCE, all potentials mentioned hereafter against SCE). Computed
values are given in parentheses. For simplicity, only excited state redox
potentials are shown. a 1H NMR shows a 10:1 inseparable mixture of
rotomers. See SI for more details.
a b
,
Scheme 2. Scope of Aziridines
exp
second singlet excited state with the strong oxidative (E*red
= +1.65 V to +1.85 V, vs SCE and E*redDFT = +1.41 V to +1.65
exp
V, vs SCE) and reductive (E*ox = −1.73 V to −1.91 V, vs
DFT
SCE and E*ox
= −2.15 V to −2.62 V, vs SCE)
competencies that are comparable/superior to the common
photoredox organic dyes and metal complexes (Figure 1 and
SI). The electronic properties indicated that the HOMO−
LUMO gap of IQ (ΔEopt = 2.84 to 2.90 eV, Egexp = 2.48 to 2.84
eV and ΔEDFT = 3.96 to 4.05 eV; see SI) can facilitate the
formation of a radical through a single electron transfer (SET).
This wide window of strong redox potentials suggests that IQ-
A-E are most suitable to promote single electron transfer of
organic molecules under visible light.
a
Reaction conditions: Amine 1a (0.30 mmol), aziridines 2b−o (0.25
mmol), IQ-D (3 mol %), DMF (2 mL), white LED 14 W, rt, air.
Isolated yield.
b
Having the inspired redox results, we initiated the
optimization studies for the reaction of tetrahydroisoquinoline
1a with 2-phenyl-1-tosylaziridine 2a as the test substrates to
examine the feasibility of the hypothesis that can lead to
diverse fused imidazolidines (Table 1 and SI: Tables S1 and
S2). To our delight, the reaction occurred to produce 3a in
86% yield when the substrates were stirred with 3 mol % IQ-D
for 24 h in DMF under white LED 14 W irradiation (entry 1).
The reaction of IQ-A-C gave 61−72% yields, while IQ-E
furnished a 55% yield along with 4a in <31% yield (entries 2−
5). These results suggest that the polarizable planar and
extended conjugation of IQ-D may stabilize the radical anion
on the catalyst,3a whereas the common photoredox dyes, eosin
Y, rose bengal, methylene blue, alizarin red and metal-
complexes, Ru(bpy)3Cl2·6H2O, and fac-Ir(ppy)3, yielded
inferior results, which may be attributed to their lower redox
potential (entries 6−9). Control experiments confirmed that
1a reacts with 2a to allow the ring opening of 4a that leads to
aryl ring of 2b−o with electronically varied substituents was
well tolerated. For example, the substrates bearing 2-chloro 2b,
2-fluoro 2c, and 2-methyl 2d groups underwent reaction to
produce the heterocycles 3b−d in 64−86% yields. Similar
results were observed with the substrates containing 3-methyl
2e, 3-methoxy 2f, and 3-nitro 2g substituents, producing the
heterocycle scaffolds 3e−g in 56−82% yields, whereas the
reaction of the substrates having 4-bromo 2h, 4-chloro 2i, 4-
fluoro 2j, 4-methyl 2k, and 4-phenyl 2l groups proved to be
compatible giving 3h−l in 68−78% yields. Sterically
encumbered substrates, with 2,4-dimethyl 2m and 2,4,6-
trimethyl 2n substituents, as well as 2-naphthyl aziridine 2o
were tolerated to provide the desired 3m−o in 66−76% yields.
The heterocycles formed as a single diastereomer, and the
stereochemistry was confirmed using single crystal X-ray
diffraction analysis of 3n.
B
DOI: 10.1021/acs.orglett.9b02957
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
The electronic effect of the N-substituents in aziridines was
next tested (Scheme 3). N-(Phenylsulfonyl)aziridine 2p
furnished 3w and 3x in 69% and 87% yields, respectively.
Tetrahydro-1H-benzo[b]azepine 1f reacted to furnish 3y as a
9:1 mixture of diastereomers in 77% yield. In addition,
tetrahydro-β-carbolines 1g−h and thiophene fused amine 1i
reacted to give the scaffolds 3z and 3aa−ab in 64−92% yields,
while the reaction of acyclic amines, N-methyl 1j and N-benzyl
1k amines, produced the ring opening 4ac−ad, which failed to
undergo cyclization.
a b
,
Scheme 3. Scope of N-Substituted Aziridines 2p−t
To ascertain the stereospecificity, we inspected the reaction
of optically active aziridines (Scheme 5).9 For instance, (R)-2-
Scheme 5. Enantiospecific Synthesis of Fused
a b
,
Imidazolidines
a
Reaction conditions: Amine 1a (0.30 mmol), aziridines 2p−t (0.25
mmol), IQ-D (3 mol %), DMF (2 mL), white LED 14 W, rt, air.
b
Isolated yield.
underwent reaction to produce 3p in 70% yield. The reaction
of the aziridines bearing 4-nitro 2q, 4-methoxy 2r, and 4-tert-
butyl 2s functional groups in the N-sulfonyl aryl ring furnished
3p−3s in 62−81% yields. In addition, N-(methylsulfonyl)-
aziridine 2t reacted to give 3t in 72% yield. These results
suggest that azirdindes having electronically varied N-sulfonyl
substituents can be successfully coupled.
The scope of the procedure was further extended to the
reaction of a series of cyclic amines 1b−k employing aziridine
2a as a standard substrate (Scheme 4). Tetrahydroisoquinoline
bearing 5-bromo 1b and 5-nitro 1c groups reacted to give 3u
and 3v in 74% and 78% yields, respectively. The reaction of the
substrates with 7-nitro 1d and 6,7-dimethoxy 1e substituents
a
a b
,
Reaction conditions: amine 1a (0.30 mmol), 2a′ (0.25 mmol), IQ-D
Scheme 4. Scope of Cyclic Amines 1b−m
b
c
(3 mol %), DMF (2 mL), white LED 14W, rt, air. Isolated yield. ee
was determined using HPLC.
phenyl-1-tosylaziridine (R)-2a′ underwent reaction with 1a to
give (S,R)-3a′ in 98% ee. Similar results was observed with 1c
and 1x providing 3v′ and 3x′ in 99% ee and 96% ee,
respectively. In addition, the reaction of N-phenylsulfonyl (R)-
2p′ and 4-(tert-butyl)phenyl)-sulfonyl (R)-2s′ aziridines with
1a produced 3p′ and 3s′ in 94 and 99% ee, respectively.
Further, (S)-2-phenyl-1-arylsulfonylaziridine (S)-2a′ was well-
suited, affording (R,S)-3a′ in >98% ee (Scheme 6). The
absolute configuration was determined using the single crystal
X-ray analysis of (R,S)-3a′. These results posited the initial ring
opening of the aziridine with amine takes place via the
stereospecific pathway.10
To gain insight in the mechanism, a series of control
experiments and theoretical studies were performed. Amine 1a
underwent reaction with 2a to give 4a in quantitative yield,
which showed no cyclization in the absence of the photoredox
catalyst. In addition, the compound 4a underwent C−H
amination to give 3a in 91% yield using IQ-D*, which suggests
that the reaction involves a tandem ring opening and oxidative
amination using photoredox catalysis (see SI). The Stern−
Volmer quenching studies posited that the electron transfer
occurs at 4a to IQ-D* (see SI). An alternative pathway,
proceeding via the electron transfer from 1a to IQ-D*, leads to
the formation of 3,4-dihydroisoquinoline and light-mediated
a
Reaction conditions: Amine 1b−m (0.30 mmol), aziridines 2a (0.25
mmol), IQ-D (3 mol %), DMF (2 mL), white LED 14 W, rt, air.
b
c
1
Isolated yield. Calculated using H NMR.
C
DOI: 10.1021/acs.orglett.9b02957
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Organic Letters
Letter
cyclization of c can lead to the formation of the target
compound 3. Formation of the single diastereomer suggests
that the cyclization can take place via the transition state TS c
compared to that of TS d, which can be attributed to the 1,3-
diaxial interaction. The proposed reaction pathway was further
examined by computational studies using M06/6-31+G(d,p)
(see SI). The reaction of 1a with 2a can produce 4 through the
TS-1 with an activation barrier of 37.30 kcal/mol (Figure 2).
Formation of 4 is favored, as the stabilization is much more
than the activation barrier. When the relative stability of 4 was
compared with 4′, 4′ is around 2 kcal/mol less stable than 4,
and thus the formation of 4′ is ruled out. SET from the catalyst
to 4 can give a, which is 2.80 kcal/mol more stable than 4.
Interaction of O2•− with a through TS-2 can form b, and this
step is the rate-determining step as the barrier for the
formation is highest (38.55 kcal/mol). This finding is in
accordance with the observed experimental results. The
abstraction of a proton from b can form c through TS-3,
which is a quick reaction with the barrier of 1.55 kcal/mol and
the stabilization of more than 12 kcal/mol. Cyclization of c can
lead to the target 3, which is very fast, as the activation barrier
TS-4 is less than 1 kcal/mol. The relative stability of 3 and 3′
were compared to prove that the formation of 3 is favored over
3′.
Scheme 6. Proposed Mechanism and Stereochemical Model
ring opening of aziridine, which is unlikely because no
quenching was observed at IQ-D* by 1a or 2a. Next, we
exp
measured the oxidation potential, Eox = 0.92 to 1.05 V and
DFT
Eox
= +1.14 to +1.26 V, for a series of electronically varied
intermediates 4a, 4j−k, and 4p−q as the representative
examples, which suggest that they can be oxidized by the
excited IQ-D* E*redexp = +1.72 V and E*redDFT = +1.52 V (see
SI). In addition, the reaction generates H2O2, which was
confirmed using the iodide test (KI in acetic acid) which
suggests that O2 plays an important role in the catalysis (see
SI). Further, the one-pot ([PH/PD] = 3.3) and parallel (kH/kD
= 4.3) kinetic isotope experiments suggest that the hydrogen
atom transfer (HAT) can be the rate-determining step (see
SI). These experimental results and literature reports1,2 suggest
that the stereospecific ring opening of aziridine 2 with 1 can
produce 4 with an inverted stereochemistry (Scheme 5). The
visible light irradiation of IQ-D can produce the excited IQ-
D*, which can undergo single electron transfer (SET) with 4
to afford the radical cation a and radical anion IQ-D•−. The
Finally, the scale-up (1 mmol) of the procedure was studied
using 1a with 2a as the representative substrates (Scheme 7).
The reaction occurred to give the target heterocycle 3a in 79%
yield.
Scheme 7. Scale-up Reaction
•−
latter can react with oxygen (air) to generate IQ-D and O2
In conclusion, the stereospecific assembly of fused imidazo-
lines is developed from N-sulfonyl aziridines and secondary
cyclic amines via a tandem ring opening/C−H amination using
via the SET. Hydrogen atom transfer (HAT) from the radical
cation a to O2•− can yield HO2− and imine b. HO2− mediated
Figure 2. Calculated energy profile of tandem ring opening/C−H amination reaction at M06/6-31+G(d,p) level of theory. “Numbers in blue
colors” depict the relative energies (kcal/mol), “numbers in bold” show the activation barrier, and “numbers in italics” show the stabilization. All
energy values are given in kcal/mol. Important interatomic distances in transition state structures are given in green color in Å unit. Red color
numbers and cross sign (X) show the unfavored intermediate or product.
D
DOI: 10.1021/acs.orglett.9b02957
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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CCDC 1937814 and 1937816 contain the supplementary
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Corresponding Author
*E-mail: tpunni@iitg.ac.in.
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Sundaravel Vivek Kumar: 0000-0002-8054-730X
Hemant Kumar Srivastava: 0000-0001-6589-6854
Vairathevar Sivasamy Vasantha: 0000-0001-8391-8682
Tharmalingam Punniyamurthy: 0000-0003-4696-8896
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Science and Engineering Research Board (SERB) for
the research grant (CRG/2018/000406) to T.P., National
Postdoctoral Fellowship (PDF/2016/001460) to S.V.K., and
Ramanujan Grant (DST-SERB SB/S2/RJN-004/2015) to
H.K.S. We thank Dr. S. C. Pan, IIT Guwahati for the HPLC
facility.
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