Iron-Catalyzed Acyl Migration of Tertiary α-Azidyl Ketones: Synthetic Approach toward Enamides and Isoquinolones

Article abstract of DOI:10.1021/acs.orglett.8b00409

This paper reports that tertiary α-azidyl phenyl ketones can be transformed into enamides by treatment with FeBr₂ at elevated temperature in DMF. The reaction proceeds via 1,2-benzoyl migration from α-carbon to the nitrogen atom, accompanied by expulsion of a nitrogen molecule. This protocol is suitable for the synthesis of N-(cyclopent-1-en-1-yl)benzamides, N-(cyclohex-1-en-1-yl)benzamides, and N-benzoyl-α-methyl enamines and provides a convenient approach toward isoquinolones.

Full text of DOI:10.1021/acs.orglett.8b00409

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iron-Catalyzed Acyl Migration of Tertiary α‑Azidyl Ketones: Synthetic
Approach toward Enamides and Isoquinolones
Tonghao Yang, Xing Fan, Xiaopeng Zhao, and Wei Yu*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou
730000, P. R. China
S
* Supporting Information
ABSTRACT: This paper reports that tertiary α-azidyl phenyl
ketones can be transformed into enamides by treatment with
FeBr2 at elevated temperature in DMF. The reaction proceeds
via 1,2-benzoyl migration from α-carbon to the nitrogen atom,
accompanied by expulsion of a nitrogen molecule. This
protocol is suitable for the synthesis of N-(cyclopent-1-en-1-
yl)benzamides, N-(cyclohex-1-en-1-yl)benzamides, and N-
benzoyl-α-methyl enamines and provides a convenient
approach toward isoquinolones.
At the initial stage of this study, we found that when treated
with 20 mol % of FeBr2 at 100 °C in DMF under an argon
atmosphere, compound 1a can be converted efficiently into
enamide 2a in high yield (Table 1, entries 3 and 4). Inferior
results were obtained at higher or lower temperature (entries 1
and 6). The optimal loading level of FeBr2 was found to be
20%. The reaction also took place in acetonitrile, but it hardly
occurred when toluene or DMSO was used as the solvent
(entries 8 and 9). Apart from FeBr2, other ferrous salts, such as
FeCl2, Fe(OTf)2, Fe(OAc)2, and Fe(acac)2, exhibited little or
no catalytic activity (entries 15−18). The reaction was also
explored in the presence of ligands such as L1, L2, or L3, but in
these cases, the yields of 2a were noticeably lower (entries 11−
13) compared with that under ligand-free conditions.
Enamides belong to an important class of organic
compounds that serve as versatile intermediates in organic
synthesis.9 Numerous studies have been made to gain access to
this useful structural motif.10−17 Synthesis of enamides mostly
involves (1) the reductive acylation of ketoximes,10 (2)
acylation of the in situ formed imine intermediates,11 (3)
condensation of amides with ketones,12 (4) transition-metal-
catalyzed C−N coupling of vinyl electrophiles13 or vinyl
nucleophiles14 with amidating agents, (5) isomerization of
allylamides,15 (6) hydroamidation of alkynes,16 and (7)
addition to ynamides.17 Several new strategies have also been
reported recently to achieve this goal.18 We envisioned that the
present iron-mediated rearrangement of α-azidyl ketones might
also be applied to the synthesis of enamides. In this regard, the
optimal conditions (Table 1, entry 5) were then used upon
differently substituted α-azidyl ketones, and the result is
illustrated in Scheme 2.
rganic azides are important compounds that have
1
Omanifold applications in organic synthesis.
Recently,
the transition-metal-catalyzed amination reactions with azides
as precursors have emerged to prominence because they
provide highly effective and atom-economic approaches toward
nitrogen-containing compounds.2 In this context, much
attention has been paid to azide-involved amination reactions
with iron as catalyst.3−6 From the point view of modern
synthesis and catalysis, iron constitutes an ideal catalyst
candidate because of its high earth abundance, low cost, and
nontoxicity, and the versatile capacities of iron salts and
complexes as catalyst have been well demonstrated by recent
studies.7 It has been proven that iron is highly capable of
catalyzing the C−H amination with azides as the nitrogen
source,3,4 and it can also enable the aminations of alkenes and
alkynes5 as well as thioethers and sulfoxides.6 As a continuation
of our interest in the azide-involved C−N forming reactions,8
recently, we found that tertiary α-azidyl phenyl ketones can be
transformed into enamides by treatment with FeBr2 at elevated
temperature in DMF (Scheme 1, (1)). The reaction proceeded
via 1,2-benzoyl migration to the nitrogen atom, accompanied
by expulsion of a nitrogen molecule. This protocol can be
applied to the preparation of isoquinolone compounds
(Scheme 1, (2)). Herein, we report this result.
Scheme 1. Iron-Catalyzed 1,2-Acyl Migration of Tertiary α-
Azidyl Ketones
Received: February 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00409
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
As shown in Scheme 2, this protocol is applicabe to the
preparation of N-(cyclopent-1-en-1-yl)benzamides (2b−e) and
N-(cyclohex-1-en-1-yl)benzamides (2f). Open-chain tertiary α-
azidyl phenyl ketones with at least one α-methyl substituent
enamides (2g−j) also be converted into the corresponding
enamides, albeit in lower yield. The present method has the
merits in that the precursors are easily accessible, and the
reaction is atom-economical. However, this procedure did not
work for nontertiary α-azidyl phenyl ketones (2l), aliphatic α-
azidyl ketones (2m), or substrates lacking α-methyl substituent
(2k). It is interesting to see that for the reactions of 1h, 1i, and
1j the formation of the double bond is highly regioselective to
deliver predominantly the more substituted alkenes. The E-
selectivity was also observed for these reactions. When 2-azido-
2,3-dihydro-1H-indenone or 2-azido-3,4-dihydronaphthalen-
1(2H)-one was used as the substrate, the corresponding
isoquinolone (2n and 2o) and 2,5-dihydro-1H-benzo[c]azepin-
1-one (2p) products were generated in good yields. This
protocol was also applied to compound 1q, but in this case, no
enamide product was obtained; instead, imine 3q was
generated in 86% yield (Scheme 3).
Table 1. Screening of Reaction Conditions
b
[FeII]
temp
(°C)
time
(h)
yield of 2a
entry
(20 mol %)
solvent
(%)
1
FeBr2
FeBr2
FeBr2
FeBr2
DMF
CH3CN
DMF
DMF
DMF
DMF
CH3CN
toluene
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
80
80
16
18
8
14
43
78
83
83
40
63
2
3
100
100
100
120
100
100
100
100
100
100
100
100
100
100
100
100
100
4
6
c
5
FeBr2
6
6
FeBr2
2
7
FeBr2
8
d
8
FeBr2
8
4
d
9
FeBr2
8
9
10
11
12
13
14
15
16
17
18
19
FeBr2 (10)
FeBr2 (10)
FeBr2 (10)
FeBr2 (10)
FeBr2 (30)
Fe(OTf)2
FeCl2
8
62
e
8
21
f
8
52
g
8
27
6
56
h
8
NR
d
8
7
Scheme 3. Reaction of (1-Azidocyclobutyl)(phenyl)-ketone
Fe(OAc)2
Fe(acac)2
8
NR
NR
NR
8
8
This acyl-migration rearrangement is interesting from a
mechanistic point of view. It is analogous to the Schmidt
rearrangement in reaction pattern19 and might follow the
pathway shown in Scheme 4. However, our control experiment
a
The reaction was conducted on 0.2 mmol scale in 2 mL of solvent
b
under an argon atmosphere. 20 mol % of ferrous salt was used unless
otherwise indicated. The reaction was conducted on 0.5 mmol scale
in 5.5 mL of solvent. Most of 1a was recovered. In the presence of
10 mol % of L1. In the presence of 10 mol % of L2. In the presence
c
Scheme 4. Possible Schmidt-Type Mechanism for the
Present Reactions
d
e
f
g
h
of 10 mol % of L3. No reaction took place.
Scheme 2. Scope of the FeBr2-Catalyzed Reaction for the
Synthesis of Enamides from α-Azidyl Ketones
indicated that this transformation cannot be effected through
the action of commonly used Lewis acids, such as FeCl3, BF3·
OEt2, TiCl4, Zn(OTf)2, and Sc(OTf)3 (see Table S2), whereas
these acids should be superior to FeBr2 for the Schmidt
reaction. Besides the Schmidt-type mechanism, there is another
possibility that acyl migration takes place through an iron−
nitrene intermediate (Scheme 5). Recent studies demonstrate
that iron salts and complexes can form metal−nitrene species
with the azidyl group like intermediate A.3,4 These iron−
Scheme 5. Proposed Mechanism Involving Iron−Nitrene
Intermediate
a
The configurations were determined by comparing the NMR spectra
b
with those found in ref 15d. See the Supporting Information. The
configuration was determined on the basis of NOE of 1H NMR
c
d
spectra. No reaction took place. S. M. decomposed.
B
DOI: 10.1021/acs.orglett.8b00409
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
nitrene species are electrophilic-like nitrenes. As nitrenes have a
strong tendency to undergo 1,2 migration from the carbon to
nitrogen atom, it is reasonable to assume that analogous
migration process could occur for intermediate A. The kinetic
isotope experiment showed that when compound d3-1g was
subjected to the reaction conditions, it reacted to afford two
products, d3-2g-1 and d3-2g-2, in a ratio of 5:1 (Scheme 6).
Subsequent exploration of the reaction conditions (see Table
S3) showed that when ligand L2 was used together with FeBr2
the yield of 6a can be considerably improved. As such, 6a was
obtained in 80% yield at the catalyst loading of 5 mol % when
the reaction temperature was raised to 120 °C. The latter
conditions are applicable to the preparation of isoquinoline-3-
carboxylates and isoquinoline-3-carboxamides, and the yields
were good in general (Scheme 9). It is noteworthy that this
Scheme 6. Kinetic Isotope Experiment with d3-1g
Scheme 9. Preparation of Substituted Isoquinolones with the
Modified Protocol
This result is in accordance with both of these two mechanisms.
However, the reaction of 1q, as shown in Scheme 3, lent
support to the iron−nitrene mechanism as in this case, it is the
methylene, rather than the acyl group, that migrated to the
nitrogen atom, probably because of the ring strain. Imine 3q
cannot be formed via Schmidt reaction. Further evidence
supporting the iron−nitrene mechanism came from the
reaction of 1r (Scheme 7). It was anticipated that if the
Scheme 7. Evidence Supporting the Iron−Nitrene
Mechanism
protocol can be applied to the preparation of isoquinolones
bearing a substituent varying in position at the phenyl ring,
which may be difficult with other methods. In addition to
isoquinoline compounds, functionalized 2-benzazepin-1-ones22
can be prepared in an analogous manner, albeit in lower yields
(Scheme 10).
Scheme 10. Preparation of 2-Benzazepin-1-ones
reaction proceeded through an iron-nitrene intermediate,
intramolecular C−H insertion would occur for compound 1r
to provide a five-membered ring. Indeed, when 1r was
subjected to the reaction conditions, 4r was generated along
with enamide products.
As illustrated in Scheme 2, when 2-azido-2,3-dihydro-1H-
indenones were used as the substrates, isoquinolone products
were generated in good yields. Isoquinolone is an important
structural motif that appears in many natural products and
pharmaceuticals,20 and its construction has long been of
interest to synthetic organic chemists.21 It was expected the
present method would provide a new approach toward
isoquinolone compounds. With this consideration in mind,
we applied this protocol to 2-methoxycarbonyl-substituted 5a
to examine its scope (Scheme 8). As expected, the isoquinolone
product 6a was obtained, but the yield was not high.
To further examine the scope of this method, the conditions
of FeBr2/L2 were next applied to several other 2-azidyl 1,3-
dicarbonyl compounds (9). As shown in Scheme 11, among the
substrates tested, 9a−e were transformed into the correspond-
ing enamides in moderate yields, while the desired reaction did
not occur for other substrates. The low yield for 10a−d might
be caused by the decomposition of these compounds, as
benzamide was generated as a byproduct in all these cases. The
low yield of 10e, on the other hand, is due to incomplete
conversion as well as its decomposition.
In summary, we have demonstrated that tertiary α-azidyl
ketones can be converted into enamides via denitrogenative
1,2-acyl migration by the catalysis of FeBr2. The reactions
provide a convenient approach toward N-(cyclopent-1-en-1-
yl)benzamides, N-(cyclohex-1-en-1-yl)benzamides, and N-
benzoyl α-methyl enamines. This method is well suitable for
Scheme 8. Reaction of 4a under the Catalysis of FeBr2
C
DOI: 10.1021/acs.orglett.8b00409
Org. Lett. XXXX, XXX, XXX−XXX

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General experimental procedures, tables illustrating
screening of the reaction conditions, characterization
1
data for the substrates and products, H NMR and 13C
NMR spectra of the substrates and products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
* E-mail: yuwei@lzu.edu.cn
ORCID
Wei Yu: 0000-0002-3131-3080
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
(Nos. 21772077 and 21372108) for financial support.
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