Alkene functionalization for the stereospecific synthesis of substituted aziridines by visible-light photoredox catalysis

Article abstract of DOI:10.1039/c7cc09151f

A novel strategy involving visible-light-induced functionalization of alkenes for the synthesis of substituted aziridines was developed. The readily prepared N-protected 1-aminopyridinium salts were used for the generation of N-centered radicals. This approach allowed the synthesis of aziridines bearing various functional groups with excellent diastereoselectivity under mild conditions. Moreover, this protocol was successfully applied to prepare structurally diverse nitrogen-containing frameworks.

Full text of DOI:10.1039/c7cc09151f

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Alkene functionalization for the stereospecific synthesis of
substituted aziridines by visible-light photoredox catalysis
Wan-Lei Yua, Jian-Qiang Chena, Yun-Long Weia, Zhu-Yin Wanga and Peng-Fei Xua*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Yoon's work:ref 10
A novel strategy involving visible-light-induced functionalization of
alkenes for the synthesis of substituted aziridines was developed.
The readily prepared N-protected 1-aminopyridinium salts were
used for the generation of N-centered radicals. This approach
allowed for the synthesis of aziridines bearing various functional
groups with excellent diastereoselectivity under mild conditions.
Moreover, this protocol was successfully applied to prepare
structurally diverse nitrogen-containing frameworks.
Photosensitive generation of triplet nitrenes
O
photocatalyst
NTroc
NHTs
Cl3C
O
N3
visible-light
Akita's work:ref 13c
Aminohydroxylation of alkenes
OH
photocatalyst
N
NHTs
BF4
Ph
visible-light
acetone/H2O
Ph
Our previous work:ref 14
Me
Synthesis of imidazoline and oxazolidine
N
N
Ts
Ts
Three-membered ring heterocycles, especially aziridines are
ubiquitous structural motifs in natural products,
pharmaceuticals and biologically active molecules.1
Meanwhile, aziridines have been well known as a significant
class of synthons for the synthetic elaborations and functional
group transformations because of the intrinsic ring strain.2
Over the last decades, lots of attention has been focused on
exploring the new methodologies for aziridine ring-opening
reactions.3 However, the chemical synthesis of aziridines
seems to be neglected. The traditional process usually involves
multistep sequences starting from the preparations of amino
alcohols, azido alcohols or the addition carbene species to
imines.4 In particular, the transition metal complex mediated
method represents the most widely used and most powerful
approach for the synthesis of the aziridine rings.5 To this end,
iminoiodanes,6 haloamines7 and organic azides8 have acted as
alternative nitrene sources for their preparation. Despite
impressive progresses achieved in this field, new synthetic
strategies remain largely underdeveloped. Therefore, the
MeCN
Ar
Solvent
Ar
Ar
photocatalyst
visible-light
Me
O
N
NHTs
BF4
Me
acetone
N
This work:
mild conditions
photocatalyst
visible-light
R2
PG
N
PG
H
concise operation
N
N
insert solvent
R1
visible-light irradiation
high diastereoselectivity
R1=Aryl, R2=H, Alkyl, Ph R1
PG=SO2Ar
R2
BF4
Scheme 1. Stereospecific synthesis of substituted aziridines.
build various compounds.9 To date, however, there are only
few synthetic processes to build small organic heterocycles in
this field. More recently, the group of Yoon have reported a
visible-light-activated aziridination reaction which was
responsible for selectively producing triplet nitrenes from
aziriformates.10 In contrast to nitrenes, N-centered radicals11
can be more easily generated from the unactivated and stable
substrates in single-electron-transfer (SET) processes by
photoredox catalysis.12 One of the useful N-centered radicals
precursors i.e. N-protected aminopyridinium salts13 were used
in the aminohydroxylation reactions by the group of Akita.
Subsequently, our group confirmed that the generated NHTs
radicals had multiple reaction sites14, which were different
from other common radicals in photocatalysis reactions.
Therefore, it was reasonable that the formation of NHTs
radicals might offer an opportunity to constitute a new
cascade way of making desired aziridines. Based on this
scenario, we explored diverse alkenes as potential substrates
and N-protected aminopyridinium salts as the nitrogen radical
sources to furnish a range of aziridines under very mild
conditions.
development of
a
conceptually novel approach for
transforming the unactivated alkenes directly into aziridines
with new nitrogen sources will have important research value
and promising application prospect.
Visible-light-induced photocatalysis has recently emerged as
a simple yet robust technique applied in organic synthesis to
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Table 1. Optimization of the reaction conditionsa
entry photocatalyst
base
NaOAc
NaOAc
NaOAc
NaHCO3
K2HPO4
CsF
solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
time (h)
yield (%)b
56
1
2
3
4
5
6
7
I
II
III
II
II
II
II
12
3
72
24
3
trace
76
78
61
44
Scheme 2. Scope of nitrogen radical precursors in the aziridination reaction. Reaction
condition: substrate (E)-1a (0.1 mmol), substrate 2 (0.15 mmol), K2HPO4 (0.15 mmol),
3
Ir(ppy)2(dtbbpy)PF6 (0.001 mmol) in CH2Cl2 (anhydrous,
2
ml) at 25 oC under the
3
irradiation of a 25 W blue LED strip for 6-10 h. aIsolated yields. Diastereomeric ratio
determined by 1H NMR analysis of products.
DABCO
12
2,6-
8
II
CH2Cl2
12
50
lutidine
control experiments verified the necessity of both the
photocatalyst and irradiation (entries 15-16).
9
II
II
II
II
II
II
-
K2HPO4
K2HPO4
K2HPO4
K2HPO4
K2HPO4
K2HPO4
K2HPO4
K2HPO4
DCE
3
12
24
3
52
61
NR
71
57
78
NR
NR
10
CHCl3
DMF
Having identified the optimal conditions for this
aziridination protocol, the scope of nitrogen-centered radical
precursors was investigated in detail. As illustrated in Scheme
2, various N-protected 1-aminopyridium salts containing either
electron-withdrawing or electron-donating groups were
11
12c
13d
14e
15
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
3
3
reacted to generate the corresponding aziridine products (3ac
-
3
3ah) in 52-76% yields. It can be seen that a variety of
16f
II
3
substituent groups on the aromatic ring were well tolerated
aUnless otherwise noted, reaction conditions are as follows: 1a (0.1 mmol), 2a (0.15 and the electron-donating groups such as methoxyl (2c) and
mmol), photocatalyst (0.001mmol), base (0.15 mmol), solvent (2mL), 25 W blue LED
tert-butyl (2d) group gave better yields (3ac, 73% and 3ad
,
strip, 25 oC under N2 atmosphere. bIsolated yield, dr > 20:1. c2a (0.2 mmol). d2a (0.1
mmol). eE/Z-1a mixtures (1.6:1) was used. fReaction carried out in the dark.
76%). When electron-neural benzenesulfonyl group (2b) was
used, the yield of the product was slightly improved to 80%
(
3ab). Additionally, the N-naphathenesulfonyl protected 1-
We initiated our investigation using (E)-β-methylstyrene (1a
)
aminopyridium salts (2i) also afforded the anticipated product
as the alkene substrate and N-Ts-protected 1-aminopyridinium
as the radical source in the presence of 1 mol% of fac-Ir(ppy)3
(
3ai) in 55% yield.
Next, the scope of the alkenes was examined. As shown in
(I
), NaOAc (1.5 equiv.), dichloromethane, and a 25 W blue LED
strip as the visible light source. To our delight, the desired
trans-1-phenyl-2-methylaziridine 3aa as single
diastereoisomer (dr > 20:1) was afforded in 56% yield.
Examination of three common photocatalysts revealed that
Ir(ppy)2(dtbbpy)PF6 II) was superior with respect to the
Scheme 3, generally, this transformation had good substrate
compatibility and excellent diastereoselectivity. We first
examined a styrene derivative bearing an electron-donating
group (1b, 4-methyl) on the aromatic ring, which produced the
corresponding aziridine (3ba) in 35% yield but as a mixture of
two diastereomers (dr = 1.5:1). It is worth mentioning that the
introduction of a stronger electron-donating group such as a
methoxyl group at para-position had a detrimental effect on
the transformation. Styrenes bearing electron-withdrawing
groups were well accommodated in this conversion. Different
halosubstituted derivatives bearing fluoro, chloro, bromo and
(
)
a
(
reaction efficiency (Table 1, entry 2). It was also found that the
base additive had a crucial impact on the reaction outcome.
The choice of inorganic bases such as NaHCO3 and CsF instead
of organic bases would facilitate the reaction, and K2HPO4 gave
the best results (entries 4-8). In addition, solvent screening
showed that nonpolar solvent CH2Cl2 was the optimal medium
while polar solvents such as DMF inhibited the reaction
(entries 9-11). Furthermore, the optimal equivalent ratio of the
N-centered radical source to the alkene was 1:1.5 in this case
(entries 12-13). More importantly, the reaction was found to
be diastereoconvergent, and the E,Z-β-methylstyrene mixtures
iodo afforded products (3ca-3ja) in good to high yields (54-
83%). Other styrenes with electron-withdrawing groups at
para-position such as a cyano (1k), ester (1l), trifluoromethyl
(
1m), and Bpin (1n; boronic acid pinacol ester) group led to the
corresponding aziridines 3ka 3na in 57-77% yields. In
addition, dihalosubstituted styrenes (1o 1p) also afforded 3oa
(
-
)
,
(E/Z = 1.6:1) provided the identical trans-diastereoisomer (3aa
without loss of selectivity (entry 14).15 Finally, additional
)
and 3pa smoothly in 76% and 80% yield, respectively. Having
verified that the electronic property of the aromatic moiety
2 | J. Name., 2012, 00, 1-3
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Ts
N
II (1 mol%)
R
N
NHTs
BF4
Ar
K2HPO4, CH2Cl2, 25oC
blue LED strip, N2
R
Ar
3[a]
1a-w
2a
Ts
N
Ts
N
Ts
N
Ts
N
Me
Me
Me
Me
3ba, 35%b
3ca, 54%
dr > 20:1
Cl
Me
3da, 80%
dr > 20:1
F
3aa, 78%
dr >20:1
dr = 1.5:1
Ts
N
Ts
N
Ts
N
Ts
N
Cl
Cl
Me
Me
Me
Me
I
3ha, 78%
dr > 20:1
Br
3ga, 81%
dr > 20:1
3fa, 75%
dr > 20:1
3ea, 82%
dr > 20:1
Ts
N
Ts
N
Ts
N
Ts
N
Br
Br
Me
Me
Me
Me
NC
MeO2C
3ia, 83%
dr > 20:1
3ja, 78%
dr > 20:1
3la, 77%
dr > 20:1
3ka, 57%
dr > 20:1
Ts
N
Ts
N
Ts
N
Ts
N
F
Cl
Me
Bpin
Me
Br
Me
Me
F3C
Cl
3ma, 68%
dr > 20:1
3oa, 76%
dr > 20:1
Ts
3pa, 80%
dr > 20:1
3na, 69%
dr > 20:1
3qa, R = H, 44%
Ts
N
Ts
N
3ra, R = Et, 75%, dr > 20:1
N
3sa, R = -Pr, 71%, dr > 20:1
n
Me
Me
R
3ta, R = CH OTBS, 47% dr > 20:1
,
2
3ua, R = CH2Br, 55%c, dr > 20:1
3va, R = Ph, 25%d, dr > 20:1
3va, R = Ph, 23%e, dr > 20:1
3wa, 47%
3xa, 34%
Scheme 4. Synthetic utility of the methodology
To gain more mechanistic insights into this reaction, several
control experiments were conducted. When the reaction was
performed in the presence of 2.0 equivalents of TEMPO under
the standard reaction conditions (Scheme 4e), no
corresponding product was produced, which implied that the
reaction is a radical process. A radical clock experiment18 was
also performed, which further confirmed the existence of N-
centered radicals (Scheme 4f). In this experiment, the six-
Scheme 3. Scope of alkenes in the aziridination reaction. Reaction condition: substrate
1 (0.1 mmol, E- and Z-alkene mixtures), substrate 2a (0.15 mmol), K2HPO4 (0.15 mmol),
Ir(ppy)2(dtbbpy)PF6 (0.001 mmol) in CH2Cl2 (anhydrous, 2 ml) at 25 oC under the
irradiation of a 25 W blue LED strip for 3-12 h. aIsolated yields. Diastereomeric ratio
determined by 1H NMR analysis of the products. bMajor cis-diastereoisomer.
cIr(ppy)2(dtbbpy)PF6 (2 mol%) was used. dE-stilbenes was used. eZ-stilbenes was used.
had a direct influence on the outcome of the aziridination
reaction, we then evaluated the effect of β-alkyl substitutions
membered tetrahydropyridine derivate (
yield of 30%.
9) was formed with a
of the styrenes. It was found that both terminal alkene (1q
and internal alkenes (1r 1s) could be applied to this reaction.
Both of the tert-butyldimethylsilyl protected cinnamyl alcohol
)
,
Based on above experimental results and fluorescence
quenching studies (see the Supporting Information), a putative
reaction mechanism is illustrated in Scheme 5.19 First, the
photocatalyst Ir(ppy)2(dtbbpy)PF6 (IrIII) undergos a metal-to-
ligand charge-transfer (MLCT) process under visible-light
irradiation to form the excited state (*IrIII), which is then
oxidatively quenched by N-Ts-protected 1-aminopyridium 2a
to generate the nitrogen-centered radical species (10) and
Ir(ppy)2(dtbbpy)+ (IrIV). The electrophilic radical then adds to
the alkene to form intermediate 11, which is oxidized by IrIV to
form the stabilized carboncation intermediate 12 and
(
(
1t) and cinnamyl bromide (1u) group were well tolerated
3ta 3ua). In order to further confirm the trans selectivity, E-
,
and Z-stilbenes were used under same conditions separately,
which give the identical trans-diastereoisomer (3va). The
corresponding aziridine bearing a quaternary carbon atom was
also obtained in moderate yield (3wa, 47%). Moreover, cyclic
alkene (1x; 1,2-dihydronaphthalene) was also suitable for this
transformation (3xa, 34%).
To highlight the generality and synthetic value of this
catalytic protocol, we performed a one-pot, three-component
reaction. As shown in the top of Scheme 4, with different
nucleophilic reagents also added to the reaction,
iminothiazolidine (
obtained. When N-Pfb-protected reagents
pentafluorobenzoyl) was used instead of 2a, five-membered
oxazoline (
) was afforded exclusively.16 In addition, further
transformations of the reaction product were studied using 2-
phenyl-1-tosylaziridine (3qa) as the substrate. Compound
4) and α-amino ketone (
5) were readily
(
2j, Pfb
=
6
7
was smoothly obtained by the Et3N-promoted ring-opening
reaction (Scheme 4d, left: 61% yield). As expected, Sc(OTf)3
catalyzed [3+3]-annulation reaction was also workable to give
compound 8
(Scheme 4d, right: 70% yield).17
Scheme 5. Proposed mechanism
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regenerate IrIII. Finally, the nitrogen of the sulfonamide as a
nucleophile initiates the intramolecular nucleophilic reaction,
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In conclusion, we have developed a novel and concise
approach to achieve the stereospecific aziridination of alkenes
by photoredox catalysis. The transformation features excellent
stereocontrol. A variety of functional groups can be tolerated
and the reaction occurs under mild reaction conditions.
Additionally, this reaction model has been successfully
10 S. O. Scholz, E. P. Farney, S. Kim, D. M. Bates and T. P. Yoon,
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expanded to synthesize
structures.
a series of nitrogen-containing
We are grateful to the NSFC (21632003, 21372105, and
21572087), the “111”program from the MOE of P. R. China,
and Syngenta Company for financial support.
Conflicts of interest
There are no conflicts to declare.
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4 | J. Name., 2012, 00, 1-3
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