Metal-Free Synthesis of α-Aminophosphonates from Tertiary Amines and P(O)H Compounds via a Cross-Dehydrogenative Coupling Reaction

Article abstract of DOI:10.1055/s-0037-1610306

The various α-aminophosphonates have been prepared from tertiary aromatic and aliphatic amines with P(O)H compounds via a tert -butyl hydroperoxide mediated cross-dehydrogenative coupling reaction, eliminating the need for transition-metal catalysts. The

Full text of DOI:10.1055/s-0037-1610306

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© Georg Thieme Verlag Stuttgart · New York
2018, 29, A–D
letter
A
en
B. Lin et al.
Letter
Synlett
Metal-Free Synthesis of α-Aminophosphonates from Tertiary
Amines and P(O)H Compounds via a Cross-Dehydrogenative
Coupling Reaction
Binzhou Lin
R2
R1
TBHP
( 70% in water)
R4
O
P
Guozhang Lu
Rongcan Lin
Yiqun Cui
R4
R3
P
R1
R2
O
H
+
N
R3
N
R5
MeCN, Ar, 80 °C, 24 h
R5
dialkyl H-phosphonates
diaryl phosphine oxides
tertiary aromatic amines
tertiary aliphatic amines
metal-free
33 examples
up to 95% yield
Yan Liu
Guo Tang*
Yufen Zhao
Department of Chemistry, College of Chemistry and Chemical
Engineering, and the Key Laboratory for Chemical Biology of
Fujian Province, Xiamen University, Xiamen, Fujian 361005,
P. R. of China
t12g21@xmu.edu.cn
Received: 23.08.2018
Accepted after revision: 21.09.2018
Published online: 16.10.2018
is very elegant, the range of starting materials is limited to
tertiary arylamines.6d On the other hand, only dialkyl H-
phosphonate was competent for this conversion. Given the
numerous methods involving N-aryltetrahydroisoquinoline
derivatives, with aliphatic amines often being used as sacri-
ficial electron donors in photoredox reactions,8 direct con-
densation of simple tertiary aliphatic amines with H-phos-
phonates for the formation of N–C–P bond has rarely been
studied.9 In 2018, we developed the first Co(OAc)2/NHPI-
catalyzed oxidative phosphonylation of tertiary aliphatic
amines with P(O)H compounds.9
DOI: 10.1055/s-0037-1610306; Art ID: st-2018-w0533-l
Abstract The various α-aminophosphonates have been prepared from
tertiary aromatic and aliphatic amines with P(O)H compounds via a tert-
butyl hydroperoxide mediated cross-dehydrogenative coupling reac-
tion, eliminating the need for transition-metal catalysts. The oxidation
of tertiary amine by tert-butyl hydroperoxide generates an iminium cat-
ion intermediate. A further addition of P(O)H compound to iminium
cation gives the desired product.
Key words tert-butyl hydroperoxide, α-aminophosphonates, tertiary
The use of transition-metal catalysts in the synthesis of
fine chemicals and pharmaceutical intermediates has be-
come common in the last few decades. Transition-metal-
catalyzed synthesis of α-amino phosphonates has seen an
explosion of interest.3 The use of transition metals often en-
tails additional procedures to reduce the content of the re-
sidual metals and improve the quality of pharmaceutical
ingredients.10 As a continuation of our endeavor to develop
step-economical C–P formations,11 we wanted to explore
the possibility of α-aminophosphonylation of both simple
aliphatic amines and tertiary arylamines with P(O)H com-
pounds via a metal-free system.
Aiming at the challenges, we started our studies by ex-
ploring potential metal-free photoredox catalyst systems
for the oxidative phosphonylation of tributylamine (1) with
diethyl H-phosphonate (2) (Table 1, entries 1–4). Gratify-
ingly, when a mixture of 1 (1.5 mmol) and 2 (0.5 mmol) in
acetonitrile was stirred at room temperature in the pres-
ence of eosinY (5 mol%) and molecular oxygen (1 atm,
balloon) for 24 h, the desired α-aminophosphonate product
3 was obtained in 25% yield (entry 1). When rose bengal
was used, a moderate yield was obtained (entry 2). A com-
bination of rose bengal with two equivalents of TBHP pro-
vided the desired product in 83% yield (entry 3). While
most reactions still worked in air, we found that reaction
aromatic amines, tertiary aliphatic amines, dialkyl H-phosphonate
α-Aminophosphonates, known as analogues of amino
acids, play important roles in a variety of fields such as
medical chemistry and agrochemistry.1 Traditionally, α-
aminophosphonates are prepared by the nucleophilic addi-
tion of phosphites to imines either directly or indirectly,2
and Lewis acid/base catalysts are required in many situa-
tions.3 During the past decade, the following four novel
transition-metal-mediated approaches have received much
more attention. (1) Copper-catalyzed four-component reac-
tion from methyleneaziridines, the Grignard reagents, RBr,
and P(O)H compounds developed by Shipman and co-work-
ers.4 (2) The phosphonylation of amide carbonyls promoted
by Cp2ZrHCl, reported by our group.5 (3) The transition-
metal-catalyzed cross-dehydrogenative coupling (CDC) re-
action of tertiary arylamines with dialkyl H-phosphonates
has been studied extensively.6 (4) Gold-catalyzed amino-
phosphonylation of benzyl alcohol with aniline and di-
methyl phosphite reported by the Fan group, but only one
example was reported.7
In 2017, the Prabhu group developed the synthesis of a
variety of N-aryltetrahydroisoquinoline-derived α-amino-
phosphonates using air as the oxidant. Although this method
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D

B
B. Lin et al.
Letter
Synlett
with some P(O)H compounds led to substantial amounts of
byproducts. Therefore, we decided to run all reactions un-
der an atmosphere of argon. We evaluated several related
organic dyes (entries 3–5) and observed that rose bengal
gave more promising results. The choice of oxidant was
vital to this C–H phosphonylation reaction because P(O)H
compounds can easily be oxidized to P(O)OH. The reaction
efficiency was maintained when one equivalent of TBHP
was used at room temperature, furnishing the desired prod-
uct in 83% yield (entry 6). Serendipitously, we found that
TBHP alone promoted the reaction of 1 with 2 without any
catalyst (entry 7). The yields were observed to increase to
72% and 82% as the temperature was raised to 40 °C and
60 °C, respectively (entries 8 and 9). Screening other oxi-
dants, such as air, O2, BPO, DTBP and H2O2, revealed that
TBHP was the best choice (entries 9–14). We were pleased
to find that one equivalent of TBHP at 80 °C provided the
desired product in 90% yield (entries 15 and 16). The yield
decreased when the reaction was operated in open air
(entry 17). The reaction performed best in acetonitrile
(entry 18). Tokuyama reported that acetic acid caused a sig-
nificant acceleration effect in the oxidative functionaliza-
tion of the benzylic C–H bonds.12 No corresponding α-amino-
phosphonate was obtained when diethyl hydrogen phos-
phate [P(O)OH, 0.1 mmol] was added and the reaction was
heated at reflux under metal-free conditions and an oxygen
atmosphere for 16 h (entry 19). Thus, the optimal reaction
conditions are 1 (1.5 mmol), 2 (0.5 mmol), TBHP (0.5 mmol,
70% in water), and MeCN (2.5 mL) at 80 °C for 24 h under an
argon atmosphere.
Table 1 Optimization of Reaction Conditionsa
O
P
O
EtO
OEt
catalyst, oxidant
OEt
OEt
HP
+
Bu2N
temperature, MeCN
Bu2N
2
3
1
Entry
Catalyst
EosinY
Oxidant [equiv]
T [°C]
Yield [%]b
1
2
O2 (balloon)
O2 (balloon)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (1)
TBHP (1)
TBHP (1)
TBHP (1)
air
RT
RT
RT
RT
RT
RT
RT
40
60
60
60
60
60
60
80
80
80
80
80
25
49
83
45
52
83
52
72
82
0
rose bengal
rose bengal
EosinY
rodamin b
rose bengal
none
3
4
5
6
7
8
none
9
none
10
11
12
13
14
15
16
17
18
19
none
none
O2 (balloon)
BPO (1)
0
none
40
trace
43
90
87
60c
0–50d
0
none
DTBP (1)
H2O2 (1)
TBHP (1)
TBHP (2)
TBHP (1)
TBHP (1)
O2 (balloon)
none
none
none
none
none
P(O)OHe
With these preliminary results in hand, we next at-
tempted to explore the scope of the reaction (Scheme 1).
We were delighted to find that the reaction worked well for
P(O)H compounds. Diethyl H-phosphonate, dimethyl H-
phosphonate, diisopropyl H-phosphonate, dibutyl H-phos-
phonate, and dibenzyl H-phosphonate all could be used as
P(O)H resources, generating the corresponding α-amino-
phosphonates (4–7) in good yields ranging from 60% to 78%,
and P(O)OH as the main byproduct. When diphenylphos-
phine oxide [Ph2P(O)H] reacted with tri-n-butylamine un-
der standard conditions, the desired product 8 was ob-
tained in merely 30% yield because diphenylphosphine
oxide can be easily converted into Ph2P(O)OH in the pres-
ence of oxidant.13
Oxidation of Ph2P(O)H in the presence of TBHP will in-
hibit the C–H activation step. Various tertiary amines were
subjected to the phosphonylation reactions with H-phos-
phonates, and representative results are shown in Scheme
1. For the tertiary aliphatic amines, tri-n-butylamine and
tri-n-propylamine both gave give the desired α-amino-
phosphonates (3, 9 and 10) in moderate to good yields.
More bulky substrates such as tri-n-hexylamine and tri-n-
octylamine also efficiently reacted with diethyl H-phos-
phonate (2) and afforded the corresponding α-aminophos-
a Reaction conditions: 1 (1.5 mmol), 2 (0.5 mmol), catalyst (0.05 mol %),
TBHP (70% in water) in MeCN (2.5 mL) stirring for 24 h. Reactions shown in
entries 3–16 were conducted under argon.
b Yield of the isolated products.
c Reaction in the open air.
d Reaction in DCE, DMF, cyclopentyl methyl ether, MeOH and H2O under an
argon atmosphere.
e P(O)OH: (EtO)2P(O)OH (0.1 mmol)
phonates 10 and 11 in 61% and 60% yields, respectively. An
increase in reaction efficiency was observed when two
equivalents of TBHP was used, furnishing the desired prod-
ucts in 86–90% yields (9–11). However, coupling of triethyl-
amine proved to be sluggish, giving an isolated yield of only
35%. N,N-Dimethylaniline (13) was also examined,14 but no
corresponding α-aminophosphonate was obtained and
P(O)H was oxidized to P(O)OH completely. In addition to
the simple aliphatic amines, a variety of N-aryltetra-
hydroisoquinolines proved to be good candidates for the
oxidative phosphonylation. N-Aryltetrahydroisoquinoline
derivatives bearing either electron-withdrawing or elec-
tron-donating groups all gave the desired products 15–22
in high yields. Notably, a variety of H-phosphonates and di-
phenylphosphine oxide [Ph2P(O)H] reacted with N-aryl-
tetrahydroisoquinoline smoothly in the presence of TBHP
(22–27). Diphenylphosphine (Ph2PH) was not competent in
this transformation and converted into Ph2P(O)OH com-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D

C
B. Lin et al.
Letter
Synlett
ing from phosphonylation of the methylene group of the N-
aryltetrahydroisoquinoline ring being obtained (28–31). To
our surprise, the bulky octyl and diphenylmethane groups
provided 29 and 30 in high yields. Tribenzylamine and 1-
phenylpiperidine were also examined. Unfortunately, only
trace amounts of the corresponding α-aminophosphonates
(32 and 33) were detected by 31P NMR spectroscopic analy-
sis.
OMe
O
OPr-i
O
OBu-n
O
i-PrO
N
n-BuO
N
MeO
C3H7
C3H7
C3H7
C3H7
C3H7
C3H7
P
P
P
N
C3H7
C3H7
C3H7
4, 65%
5, 65%
Ph
6, 78%
EtO
N
OBn
Ph
OEt
BnO
C3H7
C3H7
C3H7
C2H5
C2H5
P
P
P
O
O
O
N
N
C3H7
C3H7
C3H7
C2H5
9, 65% (86%)a
7, 60%
8, 30%
To demonstrate the practical application of this method,
a gram-scale reaction was conducted. Tri-n-butylamine
(5.55 g) reacted with diethyl H-phosphonate (1; 1.38 g, 10
mmol) in the presence of TBHP (1.30 g) at 80 °C for 24 h un-
der an argon atmosphere and delivered 3 in 70% yield
(Scheme 2).
OEt
OEt
OEt
EtO
EtO
C7H15
EtO
N
C5H11
C5H11
P
P
P
O
O
O
N
N
C7H15
C7H15
C5H11
10, 61%(90%)a
11, 60% (88%)a
12, 35%
OEt
EtO
P
N
N
O
N
EtO
EtO
P
O
EtO
P
O
n-Bu
TBHP
CF3
O
P
N
OEt
OEt
N
OEt (70% in water, 1.30 g)
n-Bu
EtO
H
13, trace
14, 90%
15, 65% (80%)a
+
OEt
MeCN (10 mL),
80 °C, 24 h
P
O
1
2
3
OEt
Yield (31P NMR): 73%
Isolated yield: 2.25 g, 70%
N
N
N
30 mmol
5.55 g
10 mmol
1.38 g
EtO
EtO
P
O
P
O
EtO P O
OMe
Br
Scheme 2 Gram-scale preparation of 3
OEt
OEt
OEt
18, 80%
O
16, 50%
17, 85%
F
Br
To understand the mechanism, further three experi-
ments were conducted (Scheme 3). When a stoichiometric
amount of 1,1-diphenylethylene, N,N-diallyl-4-methylben-
zenesulfonamide, or norbornene was added into the reac-
tion mixture under the standard conditions, the radical ad-
dition product was not detected and the desired product 3
was obtained in good yield, which suggested that this trans-
formation does not involve a radical process.
N
N
N
P
O
EtO
P
O
EtO P O
OEt
19, 85%a
OEt
20, 85%
OEt
21, 86%
N
N
N
EtO
P
O
n-BuO
P
O
BnO P O
Br
OEt
On-Bu
OBn
22, 85%
23, 90%
24, 91%
O
Ph
Ph
EtO
OEt
Ph
standard
conditions
O
P
OEt
+
1
1
1
+
+
+
2
2
2
+
+
+
P
N
N
N
Ph
Bu2N
OEt
3
72%
i-PrO
P
O
MeO
P
O
Ph
P
O
not detected
OPr-i
OMe
Ph
25, 65%
26, 92%
27, 85%
standard
conditions
O
OEt
N
Ts
+
+
P
3
Ts
N
OEt
N
75%
not detected
N
N
EtO
P O
O
EtO
P
O
EtO P O
OEt
OEt
P
OEt
28, 93%a
OEt
standard
conditions
29, 80%a
30, 95%a
OEt
3
Ph
76%
not detected
N
N
Ph
N
Scheme 3 Control experiments
EtO
P
O
EtO
P
O
EtO P O
OEt
OEt
32, trace
OEt
33, trace
31, 85%a
Based on the above experimental results and on previ-
ous reports, a plausible mechanism is proposed (Scheme 4).
Initially, the reaction of TBHP with tertiary amines A gener-
ates iminium cation intermediate B with the release of HO–
and tert-butanol. A further addition of anion C to iminium
cation B gives the desired product.
a TBHP (70% in water) (1.0 mmol).
Scheme 1 Reaction of tertiary amines with P(O)H compounds
pletely. In the case of N-alkyl, N-benzyl or even N,N-diben-
zyl-tetrahydroisoquinolines transformation, the desired
products were achieved in high yields (28–31). All reactions
showed very high regioselectivity, with only products aris-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D

D
B. Lin et al.
Letter
Synlett
(4) Mumford, P. M.; Tarver, G. J.; Shipman, M. J. Org. Chem. 2009, 74,
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t-BuOH
–OH
O
t-BuOOH
R1
N
O
HP R5
R4
R5
base
R3
R5
H
C
H2
C
P
R4
H
R1
R1
R2
R2
R3
N
C
C
N
P
R4
O
R3
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R2
A
Product
B
Scheme 4 A tentative mechanistic pathway
In summary, we have developed a tert-butyl hydroper-
oxide mediated α-aminophosphonylation of tertiary
amines, which represents the first metal-free cross-de-
hydrogenative coupling reaction between α-C–H bonds of
tertiary aliphatic amines and P(O)H compounds.15 Notably,
using commercially available and inexpensive tert-butyl
hydroperoxide as oxidant, this transformation is performed
under a metal-free system. Compared with traditional
cross-dehydrogenative coupling reaction of tertiary aryl-
amines with dialkyl H-phosphonate, the most representa-
tive feature is that both tertiary aliphatic amines and N-
aryltetrahydroisoquinolines are good candidates. Further-
more, H-phosphonates and diphenylphosphine oxide are
quite competent to react with N-aryltetrahydroisoquino-
lines in the presence of tert-butyl hydroperoxide.
(7) Sun, H.; Su, F. Z.; Ni, J.; Cao, Y.; He, H. Y.; Fan, K. N. Angew. Chem.
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Chem. 2018, 83, 6754.
(10) Garrett, C. E.; Prasad, K. Adv. Synth. Catal. 2004, 346, 889.
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2054. (b) Gao, Y.; Lu, G.; Zhang, P.; Zhang, L.; Tang, G.; Zhao, Y.
Org. Lett. 2016, 18, 1242. (c) Chen, S.; Zhang, P.; Shu, W.; Gao, Y.;
Tang, G.; Zhao, Y. Org. Lett. 2016, 18, 5712. (d) Zhang, P.; Gao, Y.;
Zhang, L.; Li, Z.; Liu, Y.; Tang, G.; Zhao, Y. Adv. Synth. Catal. 2016,
358, 138.
(12) Ueda, H.; Yoshida, K.; Tokuyama, H. Org. Lett. 2014, 16, 4194.
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Funding Information
We acknowledge financial support from the NSFC (21772163,
21778042, 41876072), NFFTBS (J1310024) and the Fundamental
Research Funds for the Central Universities (20720160030).
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1610306.
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(14) Pacheco, J. C. O.; Lipp, A.; Nauth, A. M.; Acke, F.; Dietz, J.-P.;
Opatz, T. Chem. Eur. J. 2016, 22, 5409.
(15) Experimental Procedure for the Synthesis of α-Aminophos-
phonate 3:
References and Notes
Tributylamine (1; 1.5 mmol, 3 equiv), diethyl H-phosphonate
(2; 0.5 mmol, 1 equiv), TBHP (1 or 2 equiv, 70% in water) and
MeCN (2.5 mL) were sequentially placed in a round-bottom
flask at room temperature. The reaction mixture was heated at
80 °C with stirring under an argon atmosphere for 24 hours.
Upon completion, the reaction mixture was concentrated under
vacuum. The residue was purified by silica gel column chroma-
tography using petroleum ether/EtOAc (20:1 to 2:1, v/v) as
eluent to give the corresponding product 3 (CAS no: 875228-
32-3) (ref. 9) as a light-yellow oil (144.5 mg, 90%). 1H NMR (500
MHz, CDCl3): δ = 4.07–3.98 (m, 4 H), 2.89–2.83 (m, 1 H), 2.63–
2.49 (m, 4 H), 1.55–1.48 (m, 3 H), 1.33–1.17 (m, 15 H), 0.87–
0.80 (m, 9 H). 13C NMR (125 MHz, CDCl3): δ = 61.5 (d, J = 7.4 Hz),
60.8 (d, J = 7.8 Hz), 58.3 (d, J = 134.0 Hz), 51.7 (d, J = 3.5 Hz), 31.7
(s), 29.9 (d, J = 7.2 Hz), 20.4 (d, J = 12.7 Hz), 20.3 (s), 16.6 (d, J =
5.4 Hz), 16.5 (d, J = 5.9 Hz), 14.1 (s), 13.9 (s). 31P NMR (202 MHz,
CDCl3): δ = 29.7. HRMS: m/z [M+Na]+ calcd for C16H36NNaO3P+:
344.2325; found: 344.2326.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D