Aerobic α,β-C(sp3)-H Bond Difunctionalization and C-N Bond Cleavage of Triethylamine: Difunctional Ammonium Iodide Enabling the Regioselective Synthesis of 4-Arylpyrimido[1,2- b]indazoles

Article abstract of DOI:10.1021/acs.orglett.9b02218

A novel method for the regioselective synthesis of 4-arylpyrimido[1,2-b]indazoles has been developed via the dual C(sp³)-H bond functionalization and C-N bond cleavage of triethylamine. The elusive acyclic enamine intermediates are effectively in situ generated and captured by aromatic aldehydes to form a wide array of tricyclic products from 3-aminoindazoles under the NH₄I-mediated aerobic oxidative conditions. This reaction features easily available feedstock, green and economic conditions, and valuable products.

Full text of DOI:10.1021/acs.orglett.9b02218

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
3
Aerobic α,β-C(sp )−H Bond Difunctionalization and C−N Bond
Cleavage of Triethylamine: Difunctional Ammonium Iodide Enabling
the Regioselective Synthesis of 4‑Arylpyrimido[1,2‑b]indazoles
,
†,§
‡,§
†
†
†
‡
Qinghe Gao,*
Xinya Han, Peiyuan Tong, Zhiang Zhang, Haotian Shen, Yanrong Guo,
and Suping Bai†
†
School of Pharmacy, Xinxiang Medical University, Xinxiang, Henan 453003, P. R. China
‡
School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan, Anhui 243002, P. R. China
S Supporting Information
*
ABSTRACT: A novel method for the regioselective synthesis of 4-arylpyrimido[1,2-b]indazoles has been developed via the
3
dual C(sp )−H bond functionalization and C−N bond cleavage of triethylamine. The elusive acyclic enamine intermediates are
effectively in situ generated and captured by aromatic aldehydes to form a wide array of tricyclic products from 3-
aminoindazoles under the NH I-mediated aerobic oxidative conditions. This reaction features easily available feedstock, green
4
and economic conditions, and valuable products.
5
used nitrogen tricyclic skeletons occur widely in natural
products and synthetic drugs. Among them, pyrimido-
1,2-b]indazole derivatives have shown prominent biological
properties, such as anticancer activity by inhibition of
makes this process extremely challenging. Also, over the past
1
F
few years, considerable effort has been dedicated to the
development of methods for the tertiary amine α-C(sp )−H
bonds functionalization via reactive intermediates such as
3
[
topoisomerase IIα, anti-infective activity of hepatitis C virus,
and protein kinase inhibiting activity. Unfortunately, despite
their importance and utility, little attention has been focused
toward constructing tricyclic pyrimido-fused indazole deriva-
tives. Recently, it was found that condensation of 1H-indazol-
iminium ions, α-amino organometallics, and α-amino radicals,
and this has enhanced their functional-group diversity. In
2
6
stark contrast, the direct β-functionalization of tertiary amines
7
is less established. Importantly, some pioneering examples of
the selective transformation of the inert aliphatic β-C−H
bonds of ethyl tertiary amines have been reported. In 2015, the
Pan group reported a Cu-catalyzed regioselective β-function-
alization of aliphatic tertiary amines with thiophenols through a
radical pathway to obtain a series of α,α-disulfenylated
aldehydes, in which the yields of aldehydes decreased as the
3
-amines with carbonyl compounds was the frequent and
efficient strategy toward a diverse range of substituted
3
pyrimido[1,2-b]indazoles. Generally, the free primary NH
2
group in 3-aminoindazoles was reacted with the carbonyl
compounds preferentially and then the cyclization reactions
took place. We have thus investigated the possibility of the
formation of a C−N bond from the carbonyl group with the
N2 in 3-aminoindazoles to furnish the annulation products in a
selective manner. The key to realize this transformation might
be the ability to adjust the interaction mode of the substrates.
Herein, we report that ammonium iodide is able to perform
regioselective synthesis of 4-arylpyrimido[1,2-b]indazoles via
α,β-difunctionalization and C−N bond cleavage of triethyl-
amine.
carbon chain length increased on the tertiary amines [Scheme
8
1
a(i)]. Subsequently, Yuan et al. established an interesting
route for the selective synthesis of β-arylsulfonyl enamines
from sodium sulfinate and Et N, and DMSO could promote
3
the β-C−H bond cleavage of the iminium ions for the in situ
9
generation of reactive enamines [Scheme 1a(ii)]. Later on,
Zhou described a Cu-catalyzed oxidative β-functionalization of
acyclic N,N-diethyl anilines with N-tosylaldimines as electro-
10
philes to produce 1,3-diamines [Scheme 1a(iii)]. Most
recently, Ma and co-workers found the B(C F ) -catalyzed
In recent years, transition-metal-catalyzed cleavage of the
C−N single bond in inactivated amines has been developed
into a convenient nitrogen and carbon source to construct
6 5 3
borrowing hydrogen strategy to be able to accomplish the
4
useful molecules. However, the high C−N bond dissociation
Received: June 27, 2019
energy and requirement for site selective functionalization
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02218
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Previous Overviews and New Challenge
Table 1. Optimization of the Reaction Conditions
b
entry
[I]
oxidant
solvent
temp (°C)
yield (%)
1
2
3
4
5
6
7
8
9
NIS
NH4I
I2
DTBP
DTBP
DTBP
DTBP
DTBP
TBHP
DPO
BPO
TBPB
CHP
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
toluene
dioxane
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
150
120
110
120
120
120
120
21
54
14
9
KI
TBAI
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
trace
31
43
63
66
trace
23
75
37
83
82
80
65
78
43
21
41
17
84
82
62
67
92
10
11
12
13
14
15
16
17
18
19
20
21
H O
2
2
DMSO
K S O
2
2
8
O2
air
air
air
air
air
air
air
air
air
air
air
air
air
air
air
CH CN
3
NMP
DCE
DMF
DMSO
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
3
functionalization of the C(sp )−H bond at the β-position of
Et N via conjugate addition to para-quinone methides
3
22
1
1
[
Scheme 1a(iv)]. Among these transformations, converting
2
2
2
2
2
2
2
3
4
5
6
7
the inert β-position of saturated amines to an active
nucleophilic position via deprotonation process to form
enamine intermediates was a crucial process. Thus, based on
this new variant of these transformations, we recognized an
opportunity to enable the α,β-difunctionalization of amines via
transient enamines under appropriate conditions. In this paper,
we disclose an appealing strategy for the realization of
c
d
e
g
8
f
92(78)
63
9
a
Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), 3a (2.0 mmol),
I] (0.5 mmol), oxidant (1.5 mmol), solvent (2 mL) for 18 h.
3
simultaneous C(sp )−H bond difunctionalization and C−N
[
b
c
d
e
bond cleavage of Et N to produce pyrimido-fused indazoles
3
Isolated yields. NH I (0.1 mmol). NH I (0.75 mmol). 3a (1.25
4
4
f
g
under the mild and metal-free conditions (Scheme 1b).
We initiated this novel sequential reaction of commercial
benzaldehyde (1a) with 1H-indazol-3-amine (2a) and triethyl-
amine (3a) in the presence of NIS and DTBP (di-tert-butyl
peroxide) in chlorobenzene at 130 °C for 18 h (Table 1). To
our delight, the desired 4-phenylpyrimido[1,2-b]indazole
mmol), NH I (0.75 mmol). 3a (0.5 mmol), NH I (0.75 mmol). 1a
4
4
(5 mmol), 2a (5 mmol), 3a (12.5 mmol).
Surprisingly, the yield of 4aa was improved to 92% when
increasing the amount of NH I to 1.5 equiv (entry 27). To our
delight, the yield of 4aa has no distinct influence when we used
4
(
4aa) was obtained in 21% yield (entry 1). On the basis of
this encouraging result, different kinds of iodine reagents such
as NH I, elemental iodine, KI, and TBAI were observed (entry
2
.5 equiv of Et N (entry 28).
3
4
Based on the optimized conditions described above, we
2
−5), which showed that NH I enhanced the yield of 4aa and
4
further examined another variety of ethyl amines (3b−f) as the
carbon source to react with benzaldehyde (1a) and 1H-
indazol-3-amine (2a) for the construction of tricyclic
pyrimido-fused indazoles. As shown in Scheme 2, only
diethylamine (3f) gave 4aa in only 27% yield, and other
tertiary ethyl amines were ineffective. These results clearly
TBAI quenched the desired transformation. To improve the
reaction efficiency, by replacing DTBP with BPO (benzoyl
peroxide), TBPB (tert-butyl peroxybenzoate), DMSO, or
dioxygen, the desired product was isolated in 63%, 66%,
7
5%, and 83% yields, respectively, and other peroxide oxidants
were unsuitable for the reaction (entries 6−14). Gratifyingly,
the air atmosphere also worked most efficiently (entry 15).
Next, we screened the solvent effects on this transformation
Scheme 2. Representative Ethyl Amines
(
entries 16−22), and it turned out chlorobenzene was still the
best choice. Then, the temperature was tested (entries 23−25),
which revealed that the reaction could proceed successfully as
well at 120 °C (entry 24), and a dramatic decrease in yield was
observed when the temperature was reduced to 110 °C (entry
2
5). In addition, only 67% yield of 4aa was obtained when 0.2
equiv (catalytic amount) of NH I was used (entry 26).
4
B
DOI: 10.1021/acs.orglett.9b02218
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
show that the choice of amines is crucial for this cascade
process.
Scheme 4. Substrate Scope of 3-Aminoindazoles
Subsequently, we explored the generality and scope of this
aerobic NH I-mediated three-component [3 + 2 + 1]
4
annulation under the optimized reaction conditions. A wide
range of aromatic aldehydes were found to display comparable
reactivities to benzaldehyde (Scheme 3). The benzaldehydes
Scheme 3. Substrate Scope of Aldehydes
methoxy, fluoro, chloro, bromo, and iodo at the C-4 position
showed good reactivity in this transformation, leading to the
corresponding products (4ab−af) in 77−95% yields. Similarly,
3-aminoindazoles halo-substituted at the C-5, C6, and C7
positions were also found to be effectual for generating
pyrimido[1,2-b]indazole derivatives in moderate yields (72−
9
1%; 4ag−ai, 4ak−am). Notably, halogen atoms, including F,
Cl, Br, and I, provided a convenient handle for further
synthetic elaboration of products. Furthermore, a strong
electron-withdrawing group (5-NO ) rendered moderately
2
lower yields presumably because of the lowered nucleophil-
icity. Compared with 3-aminoindazoles, 1H-pyrazolo[3,4-
b]pyridin-3-amines 2n and 2o remained unaffected under
these reaction conditions to produce 4an and 4ao in 85% and
8
6% yields, respectively.
With the scope of the method established, preliminary
mechanistic investigations were carried out. The addition of
commonly used radical scavengers TEMPO or BHT did not
dramatically affect the yields of 4aa under the standard
conditions. This means a nonradical pathway might not be
involved in the process (Scheme 5a). A reaction performed
with cinnamic aldehyde (5) and 3-aminoindazole (2a) could
provide the pyrimido[1,2-b]indazole product in 41% yield
(Scheme 5b). Since it is known that triethylamine generates
with electron-neutral and electron-donating groups on the
aromatic rings such as methyl, tertiary butyl, and methoxyl
delivered to the corresponding pyrimido[1,2-b]indazoles in
moderate to good yields (49−90%; 4ba−ha). Notably, the
halo groups including fluoro-, chloro-, bromo-, and trifluor-
omethyl could be incorporated into the phenyl ring of 4-
phenylpyrimido[1,2-b]indazoles, and products 4ia−ma are
obtained in 45−81% yields. In particular, these biologically
relevant functionalities such as methyl, methoxyl, and
trifluoromethyl highlighted the potential medicinal use of the
products. These results indicated the steric effect played a role
in the reaction (4db and 4ka), and multisubstituted aldehydes
effciently reacted as well (4ea, 4ha, and 4ka). Meanwhile, the
benzaldehydes with the strongly electron-withdrawing group
nitro could be accessed through this route to generate the
targeted products in acceptable yield (63%; 4na). Moreover,
the reactions with 2-naphthaldehyde (1o), 1-naphthaldehyde
12
13
the acetaldehyde or N,N-diethylethenamine, it is possible
that they might be the intermediates in this reaction. However,
no conversion occurred when Et N was replaced by metal-
3
dehyde and enamine (6) (Scheme 5c and 5d). These results
strongly demonstrated that the imine intermediate from Et N
3
was preferentially captured by 3-aminoindazoles and prevented
from converting to its enamine form under the NH I-mediated
4
14
aerobic oxidative conditions. Based on the blank experi-
ments, it was confirmed that NH I and molecular oxygen were
4
indispensable for this transformation (Scheme 5e and 5f).
Moreover, the speculation that the α-C and β-C of
triethylamine was integrated into the final tricyclic pyrimidines
has been demonstrated when 3a-d15 was used under the
standard conditions (Scheme 5g).
(
(
1p), nicotinaldehyde (1q), and thiophene-2-carbaldehyde
1r) also reacted smoothly to afford 4oa−4ra in 55−90%
yields, respectively. In the case of substrate (1s), no annulation
product (4sa) can be detected under the standard conditions.
We attribute the poor reaction efficiency to alkyl aldehyde. In
addition, the structure of 4ja was unambiguously confirmed by
single-crystal X-ray diffraction (CCDC 1923241).
Nevertheless, a similar ammonium salt NH Br or NH Cl
4
4
resulted in a lower yield and NH OAc gave 4aa in 47% yield
4
(Scheme 6a). In comparison, this transformation led to the
formation of 4aa only in 26% yield in the presence of HOAc. It
We next focused on the scope with respect to the
representative 3-aminoindazoles in this dehydrogenative
reaction. A series of 3-aminoindazoles bearing various groups
at all positions of the phenyl rings were proven to be good
candidates (Scheme 4). For instance, 3-aminoindazoles
substituted with a variety of functional groups such as
still remains unclear, however, why NH I plays such a critical
4
role in this oxidative annulation reaction. We believe that HI
and I are the real forms of NH I that participated in this
2
4
reaction. Compared to I , 4aa was produced in a higher yield
2
when the reaction was run with HI. Additionally, different
ratios of HI and I have a great effect on this reaction (Scheme
2
C
DOI: 10.1021/acs.orglett.9b02218
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 5. Control Experiments
Scheme 7. Plausible Mechanism
In summary, we have developed an appealing and versatile
strategy for the regioselective synthesis of pyrimido[1,2-
b]indazole derivatives from readily available building blocks
under air conditions. The key success of this reaction is the
generation of the elusive acyclic enamine enabled by the
3
cleavage of the C(sp )−H bond and C−N bond in
triethylamine. Significantly, NH I plays a dual-function role
4
in this sequential process, serving as an iodine source to enable
the oxidation reaction and as an acid to facilitate the
condensation reaction.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02218.
Scheme 6. Control Experiments
General experimental procedures and spectroscopic data
for the corresponding products (PDF)
Accession Codes
CCDC 1923241 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: gao_qinghe@xxmu.edu.cn.
ORCID
6
b). These results suggested the concentration of HI was well
controlled in the case of NH I since its decomposition was a
Qinghe Gao: 0000-0002-6207-8933
4
15
reversible process.
On the basis of the above observations and previous
Author Contributions
§
Q.G. and X.H. contributed equally.
1
0,11,16
reports,
a plausible mechanism for the formation of
Notes
pyrimido[1,2-b]indazoles is illustrated in Scheme 7. First, heat
splits NH I into NH and HI, and HI is further oxidized by air
The authors declare no competing financial interest.
4
3
to produce I . Subsequently, the oxidations of 3a with I and
air would form the imine-type intermediate B, which reacts
with the nucleophilic 3-aminoindazole to provide the
2
2
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (No. 21672182), National Key R&D Program of China
(No. 2017YFE0118200), the University Key Research Projects
of Henan Province (No. 19A350010), Anhui Key Research
and Development Program (No. 201904b11020025), Natural
Science Research Project of Anhui Higher Education
Institution (Key Project, No. KJ2018A0040), the Open Project
Program of State Key Laboratory of Microbial Technology,
intermediate C. Elimination of Et NH from C generates the
2
key enamine intermediate D in situ. Then, nucleophilic
addition of D to the aldehyde and dehydration affords the
intermediate F, which isomerizes to the intermediate G.
Finally, intramolecular Michael addition of G renders the
intermediate H followed by oxidative aromatization to furnish
the desired product 4aa.
D
DOI: 10.1021/acs.orglett.9b02218
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
and Shandong University (No. M2017-11) for financial
support.
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