Silver-catalyzed highly regioselective phosphonation of arenes bearing electron-withdrawing groups

Article abstract of DOI:10.1002/ejoc.201300545

A highly efficient, Ag^I/K₂S₂O ₈-mediated regioselective phosphonation reaction has been developed by using electron-deficient directing groups. These phosphonation reactions were performed with N,N-dialkylbenzamides, N,N-dialkylbenzenesulfonamides, and nitrobenzene. This method has a broad substrate scope and offers facile construction of C-P bonds. Copyright

Full text of DOI:10.1002/ejoc.201300545

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SHORT COMMUNICATION
Phosphonation
X. Mao, X. Ma, S. Zhang, H. Hu,
C. Zhu,* Y. Cheng* .......................... 1–5
Silver-Catalyzed Highly Regioselective
Phosphonation of Arenes Bearing Elec-
tron-Withdrawing Groups
A highly regioselective phosphonation re-
action has been developed by using N,N-
dialkyl-substituted amides, N,N-dialkyl-
sulfonamides, and nitro groups as directing
groups (DGs); Ag2SO4 as a catalyst; and
K2S2O8 as a benign oxidant under mild re-
action conditions. This method has a broad
scope and offers facile construction of C–
P bonds.
Keywords: Synthetic methods / Dehydro-
genation
/ Cross-coupling / Phosphon-
ation / Silver
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X. Mao, X. Ma, S. Zhang, H. Hu, C. Zhu, Y. Cheng
SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300545
Silver-Catalyzed Highly Regioselective Phosphonation of Arenes Bearing
Electron-Withdrawing Groups
Xuerong Mao,[a] Xiao Ma,[a] Shuwei Zhang,[a] Hongwen Hu,[a] Chengjian Zhu,*[a] and
Yixiang Cheng*[a]
Keywords: Synthetic methods / Dehydrogenation / Cross-coupling / Phosphonation / Silver
A highly efficient, AgI/K2S2O8-mediated regioselective phos-
phonation reaction has been developed by using electron-
deficient directing groups. These phosphonation reactions
were performed with N,N-dialkylbenzamides, N,N-dialk-
ylbenzenesulfonamides, and nitrobenzene. This method has
a broad substrate scope and offers facile construction of C–P
bonds.
Mn(OAc)2/Co(OAc)2/O2 (Scheme 1, b).[10] In 2006, Zou
and Zhang successfully developed noncatalytic cross-cou-
pling reactions of heteroarenes with dialkyl phosphites by
using Mn(OAc)3 as an oxidant.[11] Notably, these transition-
metal-catalyzed dehydrogenative phosphonation reactions
of arenes were mainly focused on arenes bearing electron-
donating groups. However, there is no report on the dehy-
drogenative cross-coupling reactions of dialkyl phosphites
with arenes bearing strongly electron-deficient groups such
as N-alkyl-substituted amides, N,N-dialkyl benzenesulfon-
amides and nitro groups. Therefore, the development of
such a strategy is highly desirable. In this paper, we report
the silver-catalyzed phosphonation of arenes bearing elec-
tron-withdrawing groups (EWGs, Scheme 1, c).
Introduction
Phosphorus compounds are very important intermedi-
ates in organic synthesis, medicinal chemistry, and photo-
electric materials.[1] As aryl phosphonates and their deriva-
tives are of great importance in chemistry, the synthesis of
dialkyl arylphosphonates has drawn much attention, and
some efficient methods have been established.[2] Among
them, transition-metal-catalyzed coupling reactions serve as
an efficient protocol for the construction of C–P bonds, for
example, the Hirao reaction[3] and the transition-metal-cat-
alyzed cross-coupling of diaryl and dialkyl phosphites with
aryl halides, triflates, diazonium salts, tosylates, and so
on.[4] However, these strategies require prefunctionalized re-
actants, which limit their applications. In recent years,
cross-dehydrogenative-coupling (CDC) reactions have been
extensively applied in the formation of C–C[5] and C–het-
eroatom bonds,[6] as C–H bonds are ubiquitous in organic
molecules.[7] Our group recently reported an efficient oxi-
dative phosphonation of sp3 C–H bonds with various di-
arylphosphane oxides and dialkyl phosphites, which pro-
vides easy access to α-aminophosphonic compounds in
high yields with a broad reaction scope.[8] Herein, we wish
to report the phosphonation reaction of arenes.
In 1985, Effenberger reported the formation of C–P
bonds by coupling arenes with diethyl phosphite by using
Na2S2O8 and AgNO3, but only arenes bearing electron-do-
nating groups (EDGs) were mainly suitable (Scheme 1, a).[9]
In addition, Ishii’s group realized the dehydrogenative syn-
thesis of arylphosphonates under a redox system of
Scheme 1. Transition-metal-catalyzed phosphonylation reactions.
[a] Key Lab of Mesoscopic Chemistry of MOE, School of
Chemistry and Chemical Engineering
Nanjing University, Nanjing 210093, China
E-mail: yxcheng@ nju.edu.cn
E-mail: cjzhu@nju.edu.cn
Results and Discussion
Homepage: http://hysz.nju.edu.cn/yxcheng/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300545.
At the outset, N,N-diethylbenzamide (1a) and diethyl
phosphite (2a) were selected as model substrates under the
2
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Ag-Catalyzed Phosphonation of Arenes Bearing EWGs
conditions of Pd(OAc)2 (10 mol-%) and K2S2O8 (3.0 equiv.) used as catalysts, only a trace amount of product 3a was
in CH3CN at 90 °C.[6i] To our delight, we obtained diethyl detected (Table 1, entries 17 and 18). Therefore, the optimal
(2-diethylcarbamoylphenyl)phosphonate (3a) in 15% yield reaction conditions for regioselective C–P bond formation
(Table 1, entry 1). Encouraged by this result, we investi- are Ag2SO4 (10 mol-%), K2S2O8 (3.0 equiv.) as the oxidant,
gated the influence of the catalyst, oxidant, and solvent on and CH3CN/H2O (1:1) as the solvent at 90 °C.
this dehydrogenative cross-coupling reaction. A screening
To explore the reaction scope, N-alkyl-substituted aro-
of the conditions is summarized in Table 1. If AgI com- matic amides (Scheme 2, 1a–f) and aryl-substituted N,N-di-
pounds were chosen as catalysts, the yield of 3a was greatly methylbenzamides (Scheme 2, 1g–j) were chosen as sub-
improved (Table 1, entries 4–7). Other metal salts such as strates for phosphonation. As shown in Scheme 2, N-alkyl-
CuI (Table 1, entry 2) and FeCl2 (Table 1, entry 3) turned substituted amides such as N-branched amides and cyclo-
out to be much less active catalysts under the same condi- alkane-substituted amides were efficiently coupled to afford
tions, and this indicates that AgI is the best catalyst in this the expected products (Scheme 2, 3a–f) in high yields (54–
reaction system. Subsequently, various oxidants, including 82%) under the standard conditions. As for aryl-substituted
tert-butyl hydroperoxide (TBHP), PhI(OAc)2, and di-tert- N,N-diethylbenzamides, electron-donating groups and most
butyl peroxide (DTBP; Table 1, entries 12–14), were also in- of the electron-withdrawing groups on the aromatic rings
vestigated, but expected product 3a was not obtained, were compatible with the C–P coupling reaction. Moreover,
which thus demonstrates that K2S2O8 plays a critical role aryl-substituted N,N-diethylbenzamides bearing electron-
in this C–P coupling reaction. Interestingly, 3a could be ob- donating groups showed higher activity than those contain-
tained in a high yield of 81% by using Ag2SO4 (10 mol- ing electron-deficient groups (i.e., 3g). Interestingly, a mod-
%) and K2S2O8 (3.0 equiv.) in CH3CN/H2O (1:1; Table 1, erate yield of 55% was provided by using N,N-diethyl-2-
entry 8), and it was obtained in 47% yield in CH2Cl2/H2O naphthamide as a substrate (Scheme 2, 3j). Furthermore,
(1:1; Table 1, entry 15). Notably, 3a was obtained in 64 and replacement of chlorine with bromine as a substituent
81% yield if 5 and 20% Ag2SO4 was loaded (Table 1, en- slightly enhanced the reactivity of the substrate in the phos-
tries 9 and 10). Moreover, the yield decreased if the reaction phonation reaction; diethyl [5-chloro-2-(diethylcarbamoyl)-
time were prolonged (24 h). Under AgNO3 catalysis, 3a was phenyl]phosphonate (3h) and diethyl [5-bromo-2-(diethyl-
obtained in 76% yield in CH2Cl2/H2O (1:1; Table 1, en- carbamoyl)phenyl]phosphonate (3i) were obtained in 61
try 16).[12] If Mn(OAc)2/Co(OAc)2 and Mn(OAc)3 were and 64% yield, respectively. Beyond diethyl phosphonate
(2a), phosphonates 3k–m were also effective.
Table 1. Optimization of the phosphonation conditions.[a]
Entry Catalyst
Oxidant
Solvent
Time Yield[b]
[h]
[%]
1
2
3
4
5
6
7
8
Pd(OAc)2
CuI
FeCl2
Ag2SO4
Ag2CO3
Ag2O
AgOAc
Ag2SO4
Ag2SO4
Ag2SO4
Ag2SO4
Ag2SO4
Ag2SO4
Ag2SO4
Ag2SO4
AgNO3
Mn(OAc)3
Mn(OAc)2
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
TBHP
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
10
24
24
24
24
24
24
24
1
1
10
4
4
4
15
10
5
41
28
19
32
81
64
81
73
trace
trace
trace
47
76
trace
trace
CH3CN/H2O[c]
CH3CN/H2O[c]
CH3CN/H2O[c]
CH3CN/H2O[c]
CH3CN/H2O[c]
9[d]
10[e]
11[f]
12
13
14
15
16
17
18
PhI(OAc)2 CH3CN/H2O[c]
DTBP
K2S2O8
K2S2O8
–
CH3CN/H2O[c]
CH3CN/H2O[c]
CH2Cl2/H2O[c]
HOAc
24
1
24
24
[g]
Co(OAc)
HOAc
[a] The reaction was carried out with 1a (0.1 mmol), 2a (0.3 mmol),
catalyst (10 mol-%), and oxidant (0.3 mmol) in solvent (2 mL) at
90 °C in air. [b] Yield of 3a. [c] v/v = 1:1. [d] Catalyst loading:
5 mol-%. [e] Catalyst loading: 20 mol-%. [f] Under catalyst (10 mol-
%) for 24 h. [g] 3.0 equiv.
Scheme 2. The scope of the P-arylation reaction. Reaction condi-
tions: 1 (0.1 mmol), 2 (0.3 mmol), Ag2SO4 (10 mol-%), K2S2O8
(3.0 equiv. relative to 1), CH3CN (1 mL), H2O (1 mL), in air, 90 °C.
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X. Mao, X. Ma, S. Zhang, H. Hu, C. Zhu, Y. Cheng
SHORT COMMUNICATION
We also investigated the scope of the C–P coupling reac-
The proposed mechanism for regioselective C–P bond
tion by changing the directing groups and substrates. formation is shown in Scheme 5. The reaction rate and yield
Whereas the N,N-dialkylsulfonamide was varied as the di- dramatically decreased under our standard coupling condi-
recting group, the reaction time for each substrate was set tions if a radical scavenger [i.e., (2,2,6,6-tetramethylpiper-
to 2 h by using Ag2SO4 (10 mol-%) and K2S2O8 (3.0 equiv.) idin-1-yl)oxidanyl, (TEMPO)] was added to the N,N-dieth-
in CH3CN/H2O (1:1) in air at 90 °C. As shown in Scheme 3, ylbenzamide and diethyl phosphonate system, which is in-
N,N-dialkylbenzenesulfonamides 4 also successfully under- dicative of a free-radical process. According to some pre-
went the CDC reactions with dialkyl phosphites, and highly vious reports and our experimental results, we herein pro-
regioselective C–P bond formation was achieved in moder- pose that the AgI salt and the persulfate anion forms un-
ate to good yields (Scheme 3; 5a–i, 59–82%).
stable AgII, which further reacts with diethyl phosphite to
afford intermediate cation radical 10. Then, 10 may lead to
a free phosphonyl radical after losing a proton. It is impor-
tant that electrophilic addition to N,N-diethylbenzamide
leads to intermediate 11. Upon considering that the aryl
radical can be stabilized by a withdrawing group through
the captodative effect, the formation of radical 11 is favor-
able.[13] Subsequently, 11 is converted into 12 by a single-
electron transfer (SET), followed by deprotonation to yield
3a.
Scheme 3. Phosphonation of N,N-dialkyarenesulfonamides. Reac-
tion conditions: 4 (0.1 mmol), 2 (0.3 mmol), Ag2SO4 (10 mol-%),
K2S2O8 (3.0 equiv.), CH3CN (1 mL), H2O (1 mL), in air, 90 °C, 2 h.
Recognizing that the scope of the phosphonation reac-
tion was not just limited to these two types of substrates,
we also performed the CDC of diethyl phosphonate with
N-phenylacetamide and nitrobenzene. Under identical reac-
tion conditions, both N-phenylacetamide and nitrobenzene
reacted with diethyl phosphonate to provide the corre-
sponding phosphonation product in moderate yield
(Scheme 4).
Scheme 5. Proposed mechanism of the phosphonation reaction.
Conclusions
In summary, we developed a highly regioselective AgI-
catalyzed cross-dehydrogenative-coupling reaction for the
direct phosphonation of arenes bearing an electron-with-
drawing group. A series of coupling products could be syn-
thesized with inexpensive Ag2SO4 as the catalyst and
K2S2O8 as the oxidant. This method has a broad scope and
offers facile construction of C–P bonds.
Experimental Section
General Procedure:
A
solution of the dialkyl phosphite
(0.30 mmol), tertiary amide (0.10 mmol), catalyst (10 mol-%), and
oxidant (0.3 mmol) in solvent (2.0 mL) was heated at 70–90 °C.
After completion of the reaction, the mixture was poured into
water and extracted with EtOAc (3ϫ 8 mL). After the solvent was
removed, the residue was purified by flash column chromatography
Scheme 4. The scope of the phosphonation reactions. Reaction
conditions: 6 or 8 (0.1 mmol), 2a (0.3 mmol), Ag2SO4 (10 mol-%),
K2S2O8 (3.0 equiv. relative to 6 or 8), CH3CN (1 mL), H2O (1 mL),
in air, 90 °C.
4
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Ag-Catalyzed Phosphonation of Arenes Bearing EWGs
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Supporting Information (see footnote on the first page of this arti-
cle): General information, optimization of the reaction conditions,
general procedure for the phosphonation of amides, characteriza-
1
tion data, and copies of the H NMR, 13C NMR, and 31P NMR
spectra.
Acknowledgments
The authors gratefully acknowledge the National Natural Science
Foundation of China (NSFC) (grant numbers 21074054, 51173078,
21172106) and the National Basic Research Program of China
(grant number 2010CB923303).
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[12] At first, N,N-diethylbenzamide (1a) and diethyl phosphite (2a)
were selected as model substrates in the presence of AgNO3
(10 mol-%) with K2S2O8 (3.0 equiv.) as the oxidant in CH3CN/
H2O in air at 90 °C. Upon completion of the phosphonation
reaction of the amide with diethyl phosphite, we found that
AgNO3 was not the best catalyst for the system. The following
is the reaction:
When Ag2SO4 was used to replace AgNO3 as the catalyst under
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Received: April 15, 2013
Published Online: ᭿
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