TEMPO-Catalyzed Aminophosphinoylation of Ethers via Tandem C(sp3)-H and C(sp3)-O Bond Cleavage

Article abstract of DOI:10.1021/acs.orglett.9b01081

A new TEMPO-catalyzed aminophosphinoylation of ethers with amines and H-phosphine oxides was developed for the synthesis of α-aminophosphine oxides. This metal-free aminophosphinoylation reaction could be conducted under mild conditions through tandem C(sp³)-H and C(sp³)-O bond cleavage. The present method offers a facile and efficient approach to broad range of α-aminophosphine oxide derivatives in moderate to good yields with excellent functional group tolerance.

Full text of DOI:10.1021/acs.orglett.9b01081

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
TEMPO-Catalyzed Aminophosphinoylation of Ethers via Tandem
3
3
C(sp )−H and C(sp )−O Bond Cleavage
†
,‡
†,‡
†,‡
∥
,†
,§
Qiang Huang, Kaikai Dong, Wenjing Bai, Dong Yi, Jian-Xin Ji,* and Wei Wei*
†
Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China
‡
University of Chinese Academy of Sciences, Beijing 100049, China
§
School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China
∥
School of Pharmacy, Southwest Medical University, Luzhou 646000, China
*
S Supporting Information
ABSTRACT: A new TEMPO-catalyzed aminophosphinoyla-
tion of ethers with amines and H-phosphine oxides was
developed for the synthesis of α-aminophosphine oxides. This
metal-free aminophosphinoylation reaction could be con-
3
ducted under mild conditions through tandem C(sp )−H and
3
C(sp )−O bond cleavage. The present method offers a facile
and efficient approach to broad range of α-aminophosphine
oxide derivatives in moderate to good yields with excellent
functional group tolerance.
rganic phosphorus compounds have attracted consid-
erable attention of chemists owing to their wide
Scheme 1. Synthetic Strategies toward α-Aminophosphine
Oxides via C(sp )−H Functionalization
3
O
applications as ligands in synthetic chemistry and their
1
important biological and pharmacological characteristics. In
particular, α-aminophosphine oxides exhibit various intriguing
2
3
biological activities such as antiviral, antitumor, antibacte-
4
5
rial, and enzyme-inhibiting properties. Consequently, much
effort has been devoted to develop efficient methods for the
construction of α-aminophosphine oxides. Generally, α-
aminophosphine oxides are prepared by the Lewis acid
catalyzed nucleophilic addition reaction of H-phosphine oxides
6
with imines (Pudovik reaction) and the three-component
reaction of aldehydes, amines, and H-phosphine oxides
7
(
Kabachnik−Fields reaction). Nevertheless, these traditional
the organic framework via C−H functionalization of
methods usually encountered limitations on the substrate
scope or reaction applicability because of the poor availability
and stability of imines. To overcome this problem, the
oxidative α-C(sp )−H phosphinoylation of tertiary amines is
undoubtedly the most direct protocol for the synthesis of α-
aminophosphine oxides via direct C−P bond formation.
Since the pioneering work of Li and co-workers, various
transition-metal-catalyzed oxidative phosphinoylations of C-
10,11
ethers.
So far, a few synthetic methods have been reported
for difunctionalization of ethers via tandem C−H and C−O
bond cleavage, in which transition-metal catalysts, strong
oxidants, or high reaction temperature are usually re-
3
12−15
8
,9
quired.
For example, Li and co-workers described Fe-
8
catalyzed reactions of indoles with ethers for the synthesis of
1
,1-bis-indolylmethane derivatives via C−H bond oxidation
13
3
and C−O bond cleavage in the presence of (t-BuO) . Wang
(
sp )−H adjacent to the N atom of amines with H-phosphine
2
9
reported Mn-catalyzed three-component synthesis of 1,5-
amino/keto alcohols from Grignard reagents, imines/nitriles,
and tetrahydrofuran (THF) through C−H and C−O bond
oxides or H-phosponates have been reported (Scheme 1a).
Despite some great advantages, the harsh reaction conditions
and the use of toxic metal catalysts and stoichiometric
quantities of oxidants have stimulated chemists to develop
more mild, safe, and efficient approaches.
1
4
cleavage of THF. Sun presented the three-component
reaction of 1-(2-aminoaryl)pyrroles, elemental sulfur, and
ethers to access N-heterocycle-fused 1,3,6-benzothiadiazepines
Recently, the cross-coupling reaction involving α-C−H
functionalization of ether has emerged as a powerful and
attractive protocol for construction of the C−X (X = C, S, N,
1
5
in the presence of the n-Bu NI/TBHP system. As a
4
1
0
P) bond. In most cases, the C−O bond cleavage of ether
Received: March 27, 2019
does not occur, and the ether structural unit was introduced to
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01081
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a,b
continuation of our interest in constructing phosphorus-
Scheme 2. Scope of Amines and H-Phosphine Oxides
16
17
containing compounds and green organic synthesis, here,
we report a new and efficient method for the synthesis of
various α-aminophosphine oxide derivatives via TEMPO-
catalyzed aminophosphinoylation of ethers with amines and H-
phosphine oxides, in which a mild and selective cleavage of the
3
3
C(sp )−H and C(sp )−O bonds of ethers is achieved under
metal- and external oxidant-free conditions.
Initially, the reaction of aniline (1a) and diphenylphosphine
oxide (2a) was investigated in tetrahydrofuran (THF, 3a) at
room temperature under air. Interestingly, the product 4a was
formed in 23% isolated yield (Table 1, entry 1). We then tried
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst (mol %)
T (°C)
yield (%)
1
2
3
4
5
6
7
8
9
25
25
25
25
25
25
25
25
25
25
25
25
40
50
60
40
23
6
11
9
CuBr (20)
2
FeBr (20)
3
Cu(OAc) ·H O (20)
2
2
AgNO (20)
24
19
82
60
67
83
87
83
95
85
76
3
AgOAc (20)
TEMPO (20)
TEMPO (50)
TEMPO (100)
TEMPO (15)
TEMPO (10)
TEMPO (5)
TEMPO (10)
TEMPO (10)
TEMPO (10)
TEMPO (100)
10
11
12
13
14
15
16
a
Reaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), THF 3a (2 mL),
b
c
TEMPO (10 mol %), 12 h, air, 40 °C. Isolated yield based on 1. 1a
c
46
(2 mmol), 2a (4.0 mmol), THF 3a (8 mL).
a
Reaction conditions: 1a (0.5 mmol), 2a (1.0 mmol), TEMPO (5−
b
1
00 mol %), THF 3a (2 mL), 12 h, air, 25−60 °C. Isolated yield
c
based on 1a. Under N .
2
examined in this aminophosphinoylation reaction. Generally,
para-substituted anilines bearing electron-rich groups or
electron-withdrawing groups on the benzene rings reacted
readily with 2a, affording the corresponding products 4b−m in
moderate to good yields. It is worth noting that a wide range of
functionalities such as halogen, trifluoromethyl, cyano, nitro,
ester, and carbonyl groups were well tolerated in this
procedure to give the desired products 4f−m, which could
be utilized for further modification. Meta-substituted anilines
also worked well in this reaction to deliver the products 4n−q
in good yields. The steric hindrance of substituents on the
benzene ring had a slightly effect on the reaction efficiency, and
the corresponding products 4r and 4s were obtained in
moderate yields. Notably, aromatic heterocyclic amine (i.e.,
pyridin-3-amine) and some complex amines (i.e., 5-amino-
isoindoline-1,3-dione and 3-amino-2H-chromen-2-one) were
also suitable substrates to generate the products 4t−v.
Furthermore, various aliphatic primary amines could also
react smoothly to yield the corresponding products 4w−za.
Nevertheless, none of the desired products were detected when
secondary amines were used in this reaction system (4zb and
4zc). In addition, the methyl- and chlorine-substituted H-
phosphine oxides were also examined under the optimized
conditions, and the corresponding products 4zd and 4ze could
be isolated in moderate yields. Unfortunately, H-phosphonates
to optimize the reaction conditions to improve the yield of 4a
(Table 1). As shown in entries 2−6, when various transition-
metal catalysts such as Cu, Fe, and Ag salts (20 mol %) were
added in the model reaction system, the reaction efficiency was
not obviously improved. To our delight, significantly improved
reaction efficiency was obtained when TEMPO was used as the
catalyst (Table 1, entry 7). Encouraged by this result, the
effects of TEMPO loading were further examined (Table 1,
entries 8−12). The increase of TEMPO loading to 50 or 100
mol % did not improve the reaction efficiency (Table 1, entries
8
and 9). In contrast, the reaction efficiency was slightly
improved along with the reduction of catalyst loading, and 10
mol % of TEMPO was found to be the best choice (Table 1,
entry 11). Further investigation on the effect of the reaction
temperature found that the highest isolated yield (95%) was
obtained when reaction was conducted at 40 °C (Table 1,
entry 13). Moreover, the desired product 4a was only obtained
in 46% yield when the reaction was carried out in the presence
of stoichiometric amounts of TEMPO under N (Table 1,
2
entry 16).
Under the optimized conditions, the substrate scope of
various amines and H-phosphine oxides were investigated. As
summarized in Scheme 2, various aniline derivatives were first
B
DOI: 10.1021/acs.orglett.9b01081
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
such as diethyl phosphite and dimethyl phosphite failed to
provide the corresponding aminophosphonylation products.
Moreover, when other nucleophiles such as indole, 1-
cyclohexenylpiperidine, and 1,2,3-trimethoxybenzene were
examined under the standard conditions, none of the desired
product was detected.
Scheme 4. Formation of Product 5h in the Absence of
TEMPO
Subsequently, the scope of ethers was examined under the
optimal reaction conditions. As shown in Scheme 3, in
a,b
Scheme 3. Scope of Ethers
Scheme 5. Control Experiments
a
Conditions: 1a (0.5 mmol), 2a (1.0 mmol), 3 (1 mL), TEMPO (10
b
c
mol %), 12 h, 40 °C, air. Isolated yield based on 1. dr valueswere
determined by H NMR analysis of the mixture. 3f (2 mL), 60 °C.
1
d
possible intermediate (Scheme 5c). In addition to the product
4
a, an imine intermediate derived from the corresponding
+
aminal was detected (calcd for C H NO [M + H] ,
1
0
14
addition to tetrahydrofuran, other cyclic ethers such as
tetrahydro-2H-pyran, 2-methyltetrahydrofuran, and 1,4-diox-
ane were also suitable for this protocol, and the corresponding
aminophosphinoylation products were obtained in moderate
to good yields (5b−d). Moreover, linear ethers such as
dibenzyl ether 3e and dibutyl ether 3f were compatible with
this reaction, leading to the desired products 5e and 5f. It was
found that the product 5e was selectively obtained in 92%
yield, and no product 5g was detected when asymmetric ethyl
benzyl ether 3g was employed as the substrate.
1
64.1075; found, 164.1076).
On the basis of the above observations and previous
11c,12a,18
reports,
a plausible mechanism for the present
transformation is described in Scheme 6. First, α-alkoxyalkyl
Scheme 6. Possible Reaction Mechanism
Notably, 2-(furan-2-yl)-1,3-dioxolane could react with
aniline 1a and diphenylphosphine oxide 2a leading to the
corresponding product 5h in 66% yield in the absence of
TEMPO catalyst (Scheme 4). The product 5h might be
generated from the addition of diphenylphosphine oxide 2a
with imine intermediate A that was detected by HRMS
analysis.
Several control experiments were performed to understand
the possible reaction process, and the results are compiled in
Scheme 5. First, when the model reaction was carried out
under N , only a trace amount of 4a was detected, indicating
2
that O in air is essential for the present transformation
radical 6a is produced via the hydrogen-transfer reaction
2
18a
(Scheme 5a). Subsequently, the intermolecular kinetic isotope
between THF and TEMPO.
The formed TEMPOH is
18b
experiment (KIE) was carried out under the standard reaction
oxidized by O in air to regenerate TEMPO. Then the α-
2
conditions, and the kinetic isotope effect (k /k = 2.6) was
alkoxyalkyl radical 6a undergoes further oxidation to give an
H
D
11c,12a
obviously observed (Scheme 5b). This result demonstrated
that the C−H bond cleavage in tetrahydrofuran might be
involved in the rate-determining step of this reaction (see the
Supporting Information). Moreover, an HRMS experiment on
the model reaction mixture was performed to detect the
electrophilic α-alkoxyalkyl cation 7a.
Next, the nucleo-
philic attack of 1a to 7a affords unstable aminal 8a, which will
form imine 9a owing to the electron-donating property of the
12a
nitrogen atom. Finally, the desired product 4a was produced
by nucleophilic addition of 2a to 8a or 9a.
C
DOI: 10.1021/acs.orglett.9b01081
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
In summary, we have successfully developed a new TEMPO-
catalyzed aminophosphinoylation of ethers with amines and H-
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AUTHOR INFORMATION
Corresponding Authors
■
*
*
S. D. Chem. Commun. 2017, 53, 4473−4476. (f) Xie, L.-Y.; Qu, J.;
Peng, S.; Liu, K.-J.; Wang, Z.; Ding, M.-H.; Wang, Y.; Cao, Z.; He,
W.-M. Green Chem. 2018, 20, 760−764. (g) Lin, B.; Shi, S.; Lin, R.;
Cui, Y.; Fang, M.; Tang, G.; Zhao, Y. J. Org. Chem. 2018, 83, 6754−
E-mail: weiweiqfnu@163.com.
E-mail: jijx@cib.ac.cn.
ORCID
6761. (h) Niu, L.; Wang, S.; Liu, J.; Yi, H.; Liang, X. A.; Liu, T.; Lei,
A. Chem. Commun. 2018, 54, 1659−1662. (i) Zhu, Z. Q.; Xiao, L. J.;
Guo, D.; Chen, X.; Ji, J. J.; Zhu, X.; Xie, Z. B.; Le, Z. G. J. Org. Chem.
2
(
Wei Wei: 0000-0002-0015-7636
Notes
019, 84, 435−442.
10) (a) Batra, A.; Singh, P.; Singh, K. N. Eur. J. Org. Chem. 2017,
017, 3739−3762. (b) Guo, S. R.; Kumar, P. S.; Yang, M. Adv. Synth.
The authors declare no competing financial interest.
2
Catal. 2017, 359, 2−25. (c) Wu, X. F. Solvents as Reagents in Organic
Synthesis: Reactions and Applications; Wiley: New York, 2018.
ACKNOWLEDGMENTS
■
(d) Muhammad, M. H.; Chen, X.-L.; Yu, B.; Qu, L.-B.; Zhao, Y.-F.
This work was supported by the National Natural Science
Foundation of China (Nos. 21572217 and 21302109),
Institute of Drug Innovation, Chinese Academy of Sciences
Pure Appl. Chem. 2019, 91, 33−41. (e) Li, J. S.; Xue, Y.; Fu, D. M.; Li,
D. L.; Li, Z. W.; Liu, W. D.; Pang, H. L.; Zhang, Y. F.; Cao, Z.; Zhang,
L. RSC Adv. 2014, 4, 54039−54042. (f) Sun, K.; Wang, X.; Li, G.;
Zhu, Z.-H.; Jiang, Y.-Q.; Xiao, B.-B. Chem. Commun. 2014, 50,
(
CASIMM0420152013 and CASIMM0420151009), Strategic
Biological Resources Service Network Program of Chinese
Academy of Sciences (ZSTH-001), and the Natural Science
Foundation of Shandong Province (ZR2018MB009).
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