Visible-Light-Driven Aryl Migration and Cyclization of α-Azido Amides

Article abstract of DOI:10.1021/acs.orglett.1c01120

This paper reports two new visible-light-promoted radical reactions of α-azido amides. By catalysis of [Ir(ppy)2(dtbbpy)]PF6 with i-Pr2NEt as the reducing agent, N-aryl α-azido tertiary amides were first converted to the corresponding aminyl radicals through reduction of the azido group; the aminyl radicals then underwent N-to-N aryl migration to give α-anilinyl-functionalized amides. α-Azido secondary amides, on the other hand, reacted with the solvent ethanol and i-Pr2NEt to afford the imidazolinone products.

Full text of DOI:10.1021/acs.orglett.1c01120

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Letter
Visible-Light-Driven Aryl Migration and Cyclization of α‑Azido
Amides
Siyu Liang, Kaijie Wei, Yajun Lin, Tuming Liu, Dian Wei, Bing Han, and Wei Yu*
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ABSTRACT: This paper reports two new visible-light-promoted radical
reactions of α-azido amides. By catalysis of [Ir(ppy)2(dtbbpy)]PF6 with i-
Pr2NEt as the reducing agent, N-aryl α-azido tertiary amides were first
converted to the corresponding aminyl radicals through reduction of the
azido group; the aminyl radicals then underwent N-to-N aryl migration to
give α-anilinyl-functionalized amides. α-Azido secondary amides, on the
other hand, reacted with the solvent ethanol and i-Pr2NEt to afford the
imidazolinone products.
olecular rearrangements are a fundamental class of
organic transformations that are indispensable tools in
of this methodology has not been sufficiently investigated, and
new reaction patterns need to be explored to expand the scope
of radical amination reactions of azides.
M
organic synthesis.1 The Smiles rearrangement, which involves
intramolecular migration of an aryl group between two
nucleophilic centers, has long been of great interest to
chemists because of its synthetic usefulness in the function-
alization of aromatic rings.2 Although Smiles rearrangements
are traditionally categorized as intramolecular nucleophilic
aromatic substitution (SNAr) reactions, the aryl migration can
be more conveniently implemented under mild conditions
through radical pathways. Many studies have been devoted
recently to the radical Smiles-type rearrangement, and as a
result, a variety of new methods have been developed for aryl
migration between a carbon atom and a heteroatom atom as
well as between two carbon atoms.3 Despite these advances,
however, there are certain issues concerning this radical
rearrangement that still remain to be addressed. For example,
aryl migrations between two electronegative atoms such as
oxygen or nitrogen atoms have only scarcely been reported,4,5
and there are no literature examples involving aminyl-radical-
mediated migration between two nitrogen atoms.6
Recently, in the course of our investigation of the iron-
catalyzed amination reactions of α-azido carbonyl com-
pounds,17,18 we found that iron(II) salts or complexes can
enable the transformation of α-azido amides into imidazoli-
nones via intramolecular C(sp3)−H insertion mediated by
iron−imido species.18,19 We envisioned that by conversion of
the azido group into an aminyl radical, a different reaction
pathway would be opened for these precursors. The method of
visible-light photoredox catalysis20 would provide a convenient
means to generate aminyl radicals from α-azido amides.
Indeed, after some exploration of the reaction conditions, we
found that N-phenyl α-azido amides can be converted readily
into the corresponding aminyl radicals via single electron
transfer (SET) under blue-light irradiation with [Ir-
(ppy)2(dtbbpy)]PF6 as the photocatalyst and i-Pr2NEt as the
reductant. The thus-formed aminyl radical attacks the N-
phenyl group to engender 1,4-phenyl transfer from the amido
nitrogen to the azido nitrogen with high efficacy in ethanol.
Apart from this rearrangement, α-azido amides can also react
with the solvent ethanol and i-Pr2NEt to afford imidazolinone
products if there is a hydrogen atom on the amido nitrogen.
The present reactions reveal some new aspects concerning the
reactions of aminyl radicals that will have implications in
organic synthesis.
Organic azides are versatile compounds that have manifold
applications in the preparation of nitrogen-containing com-
pounds.7 Although the chemistry of organic azides is mostly
exploited with regard to their ionic properties, they are also
useful precursors to nitrogen-centered radicals.8 For instance,
the azido group can react with tributyltin radical or indium
hydride to form the corresponding N-(tributylstannyl)aminyl
radicals9 and indium−aminyl radicals.10 In 2011, Liu reported
an elegant visible-light photocatalytic method for reducing
alkyl or aryl azides to amines via the intermediacy of aminyl
radicals.11 That study paved a new pathway for the generation
of aminyl radicals from simple azides, which has recently been
employed to effect azide-involved C−N coupling12−14 and P−
N coupling15 under visible-light photoredox catalysis.16
Despite these achievements, however, the synthetic usefulness
Received: April 2, 2021
Published: May 27, 2021
https://doi.org/10.1021/acs.orglett.1c01120
© 2021 American Chemical Society
Org. Lett. 2021, 23, 4527−4531
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Initially, to explore the radical reaction of N-phenyl-α-azido
amides, compound 1a was chosen as the model compound for
screening of the reaction conditions for photoredox catalysis.
[Cu(DPEphos)(bcp)]PF6 was first employed as the photo-
catalyst. Under blue-light irradiation, this complex can be
reduced by Et3N or i-Pr2NEt to the [Cu0] form, which is a
powerful SET reductant.21 It was hoped that the azido group
of 1a could be reduced by this [Cu0] species to the
corresponding aminyl radical. We found that when 1a was
irradiated with blue LEDs in acetonitrile in the presence of 2
mol % [Cu(DPEphos)(bcp)]PF6 and 2.0 equiv of Et3N under
an argon atmosphere, the phenyl migration product 2a was
obtained in 32% yield. The structure of 2a was confirmed by
X-ray crystallographic analysis (CCDC 2016463). Encouraged
by the initial success, more photocatalytic conditions were
explored, and the results are summarized in Table 1.
[Ir(ppy)2(dtbbpy)]PF6 proved to be a more powerful
photoctalyst than [Cu(DPEphos)(bcp)]PF6 (entry 2), and in
subsequent experiments, [Ir(ppy)2(dtbbpy)]PF6 was fixed as
the photocatalyst to optimize the solvent. Ethanol and
methanol were found to be suitable solvents for the reaction,
in which 2a can be generated in high yield. When Et3N was
replaced with i-Pr2NEt, the yield of 2a was raised to 90% in
ethanol. A 1 mol% loading of [Ir(ppy)2(dtbbpy)]PF6 was
enough to guarantee a high yield (entry 9). Notably, the
reaction proceeded equally well under an aerobic atmosphere
(entry 10), and the yield did not decrease on a preparative
scale (5 mmol; entry 11). Control experiments indicated that
light, photocatalyst, and trialkylamine are all necessary for the
reaction to take place (entries 15−17). Additional information
about screening of the reaction conditions is provided in Table
S1.
The optimized conditions (Table 1, entry 10) were then
applied to differently substituted 2-azido-2-methyl-N,N-diphe-
nylpropanamides (1) to reveal the influence of the electronic
nature of the N-phenyl ring on its migration aptitude. As
shown in Table 2, for substrates bearing two identical aryl rings
Table 2. Influence of Electronic Effects on the Migration
Aptitude
a
Table 1. Screening of the Reaction Conditions
substrate
R1
R2
product(s), yield (%)
1b
1c
1d
1e
1f
1g
1h
1i
p-Me
p-OMe
p-Br
p-Ph
m-Me
H
H
H
H
H
p-Me
p-OMe
p-Br
2b, 84
2c, 38
2d, 51
2e, 93
2f, 71
2g + 2g′ (1:1.2), 87
2h + 2h′ (1:1), 72
2i + 2i′ (1:1), 88
2j + 2j′ (1:0.8), 91
a
p-Ph
b
photocat.
(mol %)
reductant
(equiv)
time
(h)
yield of 2a
m-Me
p-Me
p-F
c
entry
solvent
(%)
b
[CuI] (2)
[IrIII] (2)
[IrIII] (2)
[IrIII] (2)
[IrIII] (2)
[IrIII] (2)
[IrIII] (2)
[IrIII] (2)
Et3N (2.0)
Et3N (2.0)
Et3N (2.0)
Et3N (2.0)
Et3N (2.0)
Et3N (2.0)
Et3N (2.0)
MeCN
MeCN
DCE
THF
DMF
MeOH
EtOH
EtOH
24
24
24
24
24
24
24
24
32
49
43
35
b
1
2
3
4
5
6
7
8
b
p-Br
1j
1k
1l
p-CF3
m-Me
o-Me
b
2k + 2k′ (1:0.8), 92
2l + 2l′ (1:0.8), 48
d
b
trace
H
80
78
90
a
b
50% of 1c was recovered. The ratio was determined on the basis of
1H NMR spectra.
i-Pr2NEt
(2.0)
9
10
11
12
13
14
15
[IrIII] (1)
[IrIII] (1)
[IrIII] (2)
[IrIII] (1)
[IrIII] (1)
[IrIII] (1)
−
i-Pr2NEt
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
5
5
90
(1b−f), the substituent has a large impact on the yield of the
rearrangement products. The introduction of methoxy or Br at
the para position reduced the yield substantially. In cases
where only one phenyl ring bears a substituent at the para or
meta position (1g−k), the reaction delivered two products in a
ratio varying between 0.8 and 1.2, indicating that the electronic
nature of the substituent does not influence the migration
selectivity very much. This result is consistent with a
mechanism of radical-involved phenyl migration. o-Methyl-
substituted 1l also reacted to give the phenyl migration
product as a pair of isomers (2l and 2l′) in a combined yield of
48%.
This protocol works equally well when one N-phenyl ring in
1 was replaced with an alkyl group. Compounds 3a−n
underwent rearrangement smoothly to give products 4a−n in
good yields (Scheme 1). Compounds incorporating a ring (4o
and 4p) or having only one alkyl group at the α-position (4q−
aa) can be converted in the same way. Notably, pyridyl can
also migrate (4ac), and its migration aptitude is stronger than
that of the phenyl group (4ad and 4ad′). However, the yield
was considerably lower in the case of N-phenyl-2-
(phenylamino)hex-5-enamide (4u), and the reaction was also
less efficient for 2-azido-N,N-diphenylacetamide (3ab).
(2.0)
e
i-Pr2NEt
(2.0)
i-Pr2NEt
(2.0)
i-Pr2NEt
(1.5)
i-Pr2NEt
(1.0)
i-Pr2NEt
(0.5)
i-Pr2NEt
(2.0)
−
i-Pr2NEt
(2.0)
90
f
24
5
93
80
5
51
5
25
5
N.R.
N.R.
16
17
[IrIII] (1)
[IrIII] (1)
EtOH
EtOH
5
5
g
N.R.
a
The reaction was conducted on a 0.2 mmol scale in 2 mL of solvent
under an argon atmosphere at ambient temperature (30−35 °C),
unless otherwise indicated. A 10 W blue LED strip was used as the
b
light source. [CuI] = [Cu(DPEphos)(bcp)]PF6; [IrIII] = [Ir-
c
d
(ppy)2(dtbbpy)]PF6. Isolated yields. Most of 1a was recovered.
The reaction was conducted under an aerobic atmosphere. The
reaction was conducted on a 5 mmol scale. Control experiment in
the dark.
e
f
g
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Letter
Scheme 1. Reaction Scope
These 1,4-aryl migration reactions can be rationalized with
the mechanism illustrated in Scheme 2. The reaction is
rather than abstracting a hydrogen atom from the amido-
nitrogen-attached C(sp3)−H bond (4a−n).
When the standard conditions were applied to 2-azido-2-
methyl-N-phenylpropanamide (7a), however, no phenyl
migration product was obtained; instead, the reaction afforded
imidazolidin-4-one 8a in 68% yield (Scheme 3). The same
transformation also took place for other α-azido amides
bearing only one substituent on the amido nitrogen (8b−d).
Scheme 2. Proposed Mechanism for 1,4-Aryl Migration
Scheme 3. Reaction of Substrates Monosubstituted at the
Amido Nitrogen
Compounds 8 apparently derived from the reaction of 7
with EtOH or i-Pr2NEt, both of which have an ethyl moiety
incorporated into their structures. To elucidate where the
CHCH3 group in 8 came from, control experiments were
conducted in which the solvent and reductant were changed,
and the results are presented in Scheme 4. It can be seen that
both the tertiary amine and the solvent can participate the
reaction. When butan-1-ol was used as the solvent, the reaction
of 7a delivered 9a as well as 8a. By contrast, only 8a was
generated when the reaction was conducted in propan-2-ol. On
the other hand, replacing i-Pr2NEt with tripropylamine
resulted in the formation of 10a as well as 8a, and the
reaction afforded only 8a when 1-methylpiperidine was used in
place of i-Pr2NEt.
A plausible mechanism is proposed to account for the
formation of imidazolidinones 8 (Scheme 5). In the reaction of
7a as an example, the excited catalyst [IrIII]* is first reduced by
i-Pr2NEt to [IrII]; the latter then reduces 7a to aminyl radical
A. Because of the unfavorable conformational effect driving the
phenyl group away from the nitrogen radical center, A does not
undergo phenyl migration; instead, it abstracts a hydrogen
atom from the solvent (or from i-Pr2NEt) to form amine D
initiated by SET between visible-light-excited [IrIII]* and i-
Pr2NEt, which gives rise to the reduced [IrII] and i-Pr2NEt
radical cation. Compound 1 or 3 is then reduced by [IrII] to
the corresponding aminyl radical A after extrusion of
dinitrogen and protonation, with [IrIII] being regenerated at
the same time. Ethanol facilitates the generation of A by
providing a proton. The involvement of this reduction process
is confirmed by the reaction of 2-azido-N,N-dibenzyl-2-
methylpropanamide (5) under the same conditions, which
gave the reduction product 2-amino-N,N-dibenzyl-2-methyl-
propanamide (6) in 85% yield (Scheme S1). A subsequently
undergoes aryl migration via intermediacy of azaspirocyclohex-
adienyl radical B to afford amidyl radical C, from which 2 or 4
is generated by accepting a hydrogen atom.
Theoretical (DFT) analysis indicates that phenyl migration
from the amidyl nitrogen to the azido nitrogen is
thermodynamically favorable (Figure S3). Interestingly,
under the current circumstance, no fused heterocyclic products
were obtained, in contrast with analogous reactions involving
attack of iminyl radicals.22 Moreover, from Scheme 1 it can be
seen that the aminyl radical preferably attacks the phenyl ring
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reacted with i-Pr2NEt and the solvent ethanol to generate
imidazolidin-4-one products. The present reactions not only
broaden the scope of the radical Smiles rearrangement but also
reveal new aspects to exploit the synthetic utility of aminyl
radicals in organic synthesis.
Scheme 4. Control Experiments with Variation of the
Tertiary Amine and Solvent
ASSOCIATED CONTENT
■
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01120.
General experimental procedures, characterization data,
copies of 1H and 13C NMR spectra, computational
details, and crystallographic data for compound 2a
(PDF)
Accession Codes
CCDC 2016463 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.
Scheme 5. Plausible Mechanism for the Formation of
Imidazolidin-4-ones
AUTHOR INFORMATION
■
Corresponding Author
Wei Yu − State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou 730000, China; orcid.org/0000-
0002-3131-3080; Email: yuwei@lzu.edu.cn
Authors
Siyu Liang − State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China
Kaijie Wei − State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China
Yajun Lin − State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China
Tuming Liu − State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China
Dian Wei − State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China
and α-hydroxy ethyl radical (or F). α-Hydroxy ethyl radical is
electron-rich and thus can be oxidized by [IrIII] to give
acetaldehyde. The latter then reacts with D to afford
imidazolidin-4-one 8a.23 Apart from this pathway, 8a can
also be formed through the reaction of D with iminium G,
which in turn comes from i-Pr2NEt radical cation following
deprotonation and a second SET oxidation.
Bing Han − State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China; orcid.org/
0000-0003-0507-9742
The involvement of hydrogen atom transfer (HAT) from
the solvent to the aminyl radical was further confirmed by the
reaction of 7a in deuterated ethanol. Under the conditions of
CD3CD2OD and 1-methylpiperidine, 8a-D was obtained in a
moderate yield of 19% (Scheme 4, eq 5), which is much lower
than that in ethanol under otherwise identical conditions.
In summary, we have realized 1,4-aryl migration from the
amido nitrogen in N-substituted-N-aryl α-azido amides to the
azido group under visible-light irradiation by employing
[Ir(ppy)2(dtbbpy)]PF6 as the photocatalyst and i-Pr2NEt as
the reductant. The reaction proceeds via the intermediacy of
an aminyl radical derived from SET reduction of the azido
group. This migration allows a wide variety of N-aryl-α-azido
amides to be converted to α-anilinyl-functionalized amides in
good yields. α-Azido secondary amides, on the other hand,
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01120
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Natural Science Foundation of
China (21772077) and the State Key Laboratory of Applied
Organic Chemistry for financial support.
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