Pd/C-Catalyzed Carbonylative Synthesis of 2-Aminobenzoxazinones from 2-Iodoaryl Azides and Amines

Article abstract of DOI:10.1021/acs.orglett.9b00966

A palladium-catalyzed carbonylative procedure for the synthesis of 2-aminobenzoxazinones from 1-azido-2-iodobenzenes and amines has been developed. A broad range of 2-aminobenzoxazinone derivatives were prepared in moderate to excellent yields by using Pd

Full text of DOI:10.1021/acs.orglett.9b00966

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Pd/C-Catalyzed Carbonylative Synthesis of 2‑Aminobenzoxazinones
from 2‑Iodoaryl Azides and Amines
Youcan Zhang, Zhiping Yin, Hai Wang, and Xiao-Feng Wu*
Leibniz-Institut fur Katalyse e.V. an der Universitat Rostock, Albert-Einstein-Straße 29a, 18059 Rostock, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A palladium-catalyzed carbonylative procedure for
the synthesis of 2-aminobenzoxazinones from 1-azido-2-iodoben-
zenes and amines has been developed. A broad range of 2-
aminobenzoxazinone derivatives were prepared in moderate to
excellent yields by using Pd/C as the catalyst under CO
atmosphere. Notably, by using organic azides as the substrates, external oxidant usage can be successfully avoided and only
forms N2 as the byproduct.
enzoxazinones are an important class of heterocyclic
Scheme 2. Procedures for the Synthesis of 2-
Aminobenzoxazines
B
scaffolds, which present widely in bioactive compounds
and natural products.1 These valuable heterocycles have been
commonly applied as precursors for the synthesis of
pharmaceutically active compounds as well.2 Among them, 2-
amino-substituted benzoxazinones have been frequently
applied for treating diseases in the past few decades.3 Some
representative examples are selected and shown in Scheme 1.
Scheme 1. Selected Bioactive 2-Aminobenzoxazines
On the other hand, transition-metal-catalyzed carbonylation
reactions using low cost and accessible carbon monoxide as the
C1 building block have been widely applied in organic
synthesis,11 and such types of reaction have been used for
constructing various biologically active heterocycles as well.12
Moreover, azides are interesting compounds and have been
broadly utilized in the synthesis of functionalized 1,2,3-
triazoles through click reaction.13 Compared with traditional
methods for generating 2-amino-substituted benzoxazinones,
using azides as the substrates to construct N-containing
heterocycles could avoid the use of external oxidants and only
forms N2 as the byproduct.10,14 Combining the recent work of
our group with the continued exploration of palladium-
catalyzed carbonylation reactions,15 herein we describe the
application of 2-iodoaryl azides and amines as substrates in
palladium-catalyzed carbonylative transformation. A variety of
2-amino-substituted benzoxazinones were isolated in moderate
to excellent yields with good functional group tolerance.
Compounds I as effective inhibitors of the complementary
enzyme C1r have been reported.4 Compounds II are good
inhibitors of HSV-1 protease.5 Derivatives of III showed strong
and highly specific inhibition of human sputum elastase
(HSE).6 Compound IV is a chemically stable PSA-specific
inhibitor, which shows potent PSA-inhibitory activity.7 Hence,
the development of new procedures for the synthesis of 2-
amino-substituted benzoxazinones has attracted a lot of
attention over the past decades. Ordinary methods for
constructing these heterocyclic compounds are based on 2-
aminobenzoic acid derivatives; however, some procedures
need to be performed in the presence of concentrated acid or
toxic catalyst or under oxidative conditions8 (Scheme 2). An
alternative procedure using palladium-catalyzed carbonylation
of 1,1-disubstituted 3-phenylureas9 or palladium-catalyzed
cross-coupling/cyclization of 2-azidobenzoic acids with iso-
cyanides has been developed as well.10
Received: March 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00966
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Initially, we selected 1-azido-2-iodobenzene 1a and aniline
2a as the model substrates to optimize the reaction conditions,
using Pd(OAc)2 (5 mol %) and XPhos (10 mol %) as the
catalytic system and Et3N (1.5 equiv) as the base in toluene (1
mL) under CO (5 bar) atmosphere at 80 °C. To our delight,
the desired product was obtained in 67% yield (Table 1, entry
1 bar, the reaction yields were reduced to 58% and 75%,
respectively (Table 1, entries 18 and 19). Satisfactorily, the
catalyst loading can be decreased to 1 mol % Pd/C and is still
able to provide the desired product 3a in 95% isolated yield
(Table 1, entry 20).
With the optimized reaction conditions in hand (Table 1,
entry 20), we next set out to explore the substrate scope of this
reaction with a range of amines. As shown in Scheme 3,
a
Table 1. Optimization of Reaction Conditions
Scheme 3. Synthesis of 2-Aminobenzoxazinones from
a
Amines
b
entry
catalysts
ligands
XPhos
XPhos
XPhos
PPh3
solvents
yield (%)
1
2
3
4
5
6
7
Pd(OAc)2
Pd(PPh3)4
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
-
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DMF
1,4-dioxane
DCE
CH3CN
THF
THF
THF
67
74
91
trace
0
dppp
dppf
0
tBu3P·HBF4
XPhos
XPhos
XPhos
XPhos
XPhos
XPhos
XPhos
XPhos
-
12
44
0
60
92
33
72
99
0
c
8
d
9
10
11
12
13
14
15
16
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
0
0
e
17
XPhos
XPhos
XPhos
XPhos
THF
THF
THF
THF
f
18
19
58
75
g
h
i
20
98 (95)
a
Reaction conditions: 1a (0.15 mmol), 2a (0.18 mmol), catalysts (5
mol %), ligands (10 mol %), and Et3N (1.5 equiv) in solvent (1 mL)
b
at 80 °C for 14 h under CO (5 bar). Yields were determined by GC-
c
FID analysis using n-hexadecane as an internal standard. K2CO3 (1.5
d
e
f
g
h
equiv). DBU(1.5 equiv). No base. 60 °C. CO (1 bar). Pd/C (1
i
mol %), XPhos (2 mol %) Isolated yield. XPhos = 2-
dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl. BuPAd2 = di(1-
adamantyl)-n-butylphosphine. dppp = 1,3-bis(diphenylphosphino)-
propane. dppf = 1,1′-bis(diphenylphosphino)ferrocene. DBU = 1,8-
diazabicyclo[5.4.0]undec-7-ene. DMF = N,N-dimethylformamide.
DCE = 1,2-dichloroethane.
1). The yields of the desired product can be further improved
when other catalysts such as Pd(PPh3)4 (85% yield was
obtained with Pd2(dba)3) or Pd/C were used (Table 1, entries
2 and 3). In the case of using Pd/C as the catalyst, the desired
product 3a was formed in 91% yield. Different ligands were
tested subsequently; however, regardless of monodentate
a
Reaction conditions: 1a (0.3 mmol, 1.0 equiv), 2 (0.36 mmol, 1.2
equiv), Pd/C (3.2 mg, 1 mol %), XPhos (2 mol %), and Et3N (1.5
equiv) in solvent (2 mL) at 80 °C for 14 h under CO (5 bar), isolated
yield. Used 3.0 equiv of Et3N.
b
t
ligands such as PPh3 and Bu3P·HBF4 or bidentate ligands
such as dppp and dppf, all showed ineffectiveness for the
generation of the desired product (Table 1, entries 4−7). The
bases had a crucial effect on this transformation, and when
K2CO3 and DBU were used as the bases, the yield of 3a
decreased to 40% and 0%, respectively (Table 1, entries 8 and
9). Solvent screening revealed that THF is the best medium for
this transformation and gave the target product 3a in 99% yield
(Table 1, entries 10−14). Control experiments indicated that
no target product could be detected in the absence of Pd/C,
XPhos, or Et3N (Table 1, entries 15−17). Undesirably, when
we changed the temperature to 60 °C or the pressure of CO to
anilines with a series of different functional groups including
electron-donating (Ph, MeO), electron-neutral (Me), and
electron-withdrawing (F, Br, CF3) all showed good reaction
activities, forming the corresponding products in 49−96%
yields (Scheme 3, 3a−3i), and even sterically hindered 2-
phenylaniline could transform into the desired product 3b in
an excellent yield of 94%. These results of reactions indicated
that the aromatic ring with electron-donating functional groups
had a positive effect on the efficiency of this transformation.
Notably, there are disubstituted and trisubstituted functional
groups on the aromatic ring of aniline, and the reaction can be
B
DOI: 10.1021/acs.orglett.9b00966
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
performed smoothly, leading to the desired products in 60%
and 80% yields, respectively (Scheme 3, 3j, 3k). Next, different
aliphatic amines were investigated. Interestingly, the aliphatic
amine bearing group of terminal alkyne 1l, terminal olefin 1m,
furan 1n, and pyridine were well-tolerated, giving the desired
products 3l−3o in 25−81% yields.
Scheme 5. Synthesis of 2-Aminobenzoxazinone in the 1.5
mmol Level
a
Furthermore, amino acid ester hydrochlorides 2p and 2q
could react with 1a to give the desired 3p and 3q in 94% and
85% yields, respectively. Satisfactorily, aliphatic amines with
alkyl chains of different lengths all engaged in this reaction
smoothly to deliver the corresponding 2-amino-substituted
benzoxazinones 3r−3ad in moderate to good yields (48−
93%). Particularly, for sterically hindered alkylamines such as
1-adamantanamine 1aa, tert-butylamine 1ab, and cyclohexane-
amine 1ac, all could be converted into the corresponding
products in good yields. In addition, octylamine 1ad
containing eight carbons could give 3ad in 61% yield. The
relative configuration of the desired product was certificated by
X-ray diffraction analysis of 3d.
a
Reaction conditions: 1a (1.5 mmol), 2a (1.65 mmol), Pd/C (1 mol
%), XPhos (2 mol %), and Et3N (1.5 equiv) in THF (5 mL) at 80 °C
for 14 h under CO (5 bar).
Scheme 6. Reaction with 1-Iodo-2-isocyanatobenzene
intermediate as well, as it will produce 1H-quinazoline-2,4-
dione in the presence of CO via 2-isocyanato-N-phenyl-
benzamide.16 Hence, we believe azide and then isocyanate
coordinated to the benzoyl palladium complex should be the
intermediate.
Subsequently, various 1-azido-2-iodobenzenes were pre-
pared and reacted with aniline and tert-butylamine under the
standard conditions. As shown in Scheme 4, the corresponding
On the basis of these results and previous literature,10,14b,17
we proposed a simplified possible reaction mechanism
(Scheme 7). Initially, 1-azido-2-iodobenzene undergoes
Scheme 4. Synthesis of 2-Aminobenzoxazinones from 1-
a
Azido-2-iodobenzenes
Scheme 7. Proposed Reaction Mechanism
a
Reaction conditions: 1 (0.3 mmol, 1.0 equiv), 2 (0.36 mmol, 1.2
equiv), Pd/C (3.2 mg, 1 mol %), XPhos (2 mol %), and Et3N (1.5
equiv) in solvent (2 mL) at 80 °C for 14 h under CO (5 bar), isolated
yield.
products 4a−4d were formed in good to excellent yields with
the tested substrates. Furthermore, 1-azido-2-iodobenzene 1d
bearing a stronger electron-withdrawing (CF3) group can be
well transformed into the reaction and provide the
corresponding products 4e and 4f in 81% and 90% isolated
yields, respectively. Disubstituted 1-azido-2-iodobenzene 1e
engaged in the reaction well, affording the desired product 4g
in 70% yield.
Based on the optimal reaction conditions, we scaled up the
reaction to a 1.5 mmol level (Scheme 5). To our delight, the
reaction of 1a (1.5 mmol) and 2a (1.65 mmol) was performed
in 5 mL of THF under the standard conditions, giving the
desired product 3a in 91% isolated yield.
oxidative addition with ligand-coordinated Pd(0) and insertion
of CO to afford intermediate I. Subsequently, a Pd-coordinated
intermediate II was formed after the release of N2 and another
CO insertion. Then complex III is formed by nucleophilic
attack of amine at isocyanate C in the presence of Et3N.
Finally, the terminal benzoxazinone products were eliminated
from complex III after reductive elimination and meanwhile
regenerate the active Pd(0) species which is ready for the next
catalytic cycle.
Finally, in order to further prove the usefulness of this
procedure, the synthesis of bioactive 2-aminobenzoxazine was
carried out (Scheme 8). By using 2,6-diethylaniline as the
substrate, the desired 2-((2,6-diethylphenyl)amino)-4H-benzo-
[d][1,3]oxazin-4-one, which is a PSA-specific inhibitor, was
isolated in 88% yield.
In order to get some insight into the reaction pathway, 1-
iodo-2-isocyanatobenzene was prepared and tested with aniline
under our standard conditions (Scheme 6). However, to our
surprise, no desired product could be detected. On the other
hand, 2-nitro-N-phenylbenzamide can be excluded as an
C
DOI: 10.1021/acs.orglett.9b00966
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Accession Codes
CCDC 1900942 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, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xiao-feng.wu@catalysis.de.
ORCID
Zhiping Yin: 0000-0002-4296-8824
Xiao-Feng Wu: 0000-0001-6622-3328
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Chinese Scholarship Council for
financial support. The analytical support from Dr. Anke
Spannenberg, Dr. W. Baumann, Dr. C. Fischer, S. Buchholz,
and S. Schareina is gratefully acknowledged (all in LIKAT).
We also appreciate the general support from Professor Armin
̈
Borner and Professor Matthias Beller in LIKAT.
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DOI: 10.1021/acs.orglett.9b00966
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