A one-pot strategy to synthesize β-ketophosphonates: Silver/copper catalyzed direct oxyphosphorylation of alkynes with H-phosphonates and oxygen in the air

Article abstract of DOI:10.1039/c4cc10312b

A highly efficient one-pot strategy has been developed for the synthesis of β-ketophosphonates directly from alkynes and dialkyl H-phosphonates in the presence of widely available AgNO₃/CuSO₄ and K₂S₂O₈ at room temperature under open-air conditions.

Full text of DOI:10.1039/c4cc10312b

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A one-pot strategy to β-ketophosphonates: sliver/copper catalyzed direct
oxyphosphorylation of alkynes with H-phosphonates and oxygen in the
air
Xin Chen, a Xu Li, a Xiao-Lan Chen, *,a Ling-Bo Qu, *, a,b Jian-Yu Chen,a Kai Sun,a Zhi-Dong Liu,a Wen-
Zhu Bi,a Ying-Ya Xia,a Hai-Tao Wu,a and Yu-Fen Zhao*,a,c
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
ketophosphonates, through direct oxyphosphorylation of alkenes
with dioxygen and H-phosphonates in the presence of Cu-Fe
cocatalysts.[15] However Ji’s method still suffers from several
55 drawbacks such as long reaction time (24 h), oxygen atomosphere,
and relatively poor product yields (Scheme 1d). Therefore,
development of convenient, efficient, and especially benign
methods to access β-ketophosphonates is still highly desirable in
synthetic chemistry.
A highly efficient one-pot strategy has been developed for the
synthesis of β-ketophosphonates directly from alkynes and
10 dialkyl H-phosphonates in the presence of widely available
AgNO3/CuSO4 and K2S2O8 at room temperature under open-
air conditions.
β-ketophosphonates are extremely versatile intermediates in
organic chemistry, especially for the construction of α,β-
15 unsaturated carbonyl compounds through the well-known
60
Herein, we disclose a more convenient synthetic approach by
which a series of β-ketophosphonates are readily prepared by
direct one-pot reaction of alkynes with dialkyl H-phosphonates
and oxygen in the air in the presence of silver/copper catalysts
and K2S2O8 at room temperature. This work is interesting because
Horner-Wadsworth-Emmons
(HWE)
reactions.[1]
β-
ketophosphonates exhibit a wide range of biological activities and
outstanding metal-complexing abilities.[2] β-ketophosphonates
can therefore be used for liquid-liquid extraction of metal ions.[3]
20 In addition, β-ketophosphonates can serve as useful precursors in
the synthesis of chiral β-amino and β-hydroxy phosphonic acids
as well, both of which are endowed with interesting biological
properties.[4] Recently, Nishibayashi’s group reported that, in the
presence of catalytic amounts of copper (II) and an optically
65 of the direct oxyphosphonylation that can be performed without
the need to prepare alkynylphosphonates.[13, 14] The methodology
is an extension of the work of the Zhao group,[13] however this
procedure is more powerful. Compared to Ji’s method, the most
prominent advantages of our method include a much reduced
70 reaction time (2-3 hours), open air and room temperature reaction
conditions, and excellent product yields.
25 active
enantioselective alkylation with diaryl methanols to form α-alkyl-
β-ketophosphonates with
high enantioselectivity.[5] By
ligand,
trifluoromethanesulfonate
can
undergo
a
employing a palladium complex as chiral catalyst, Kim’s group
accomplished a highly efficient enantioselective fluorination of
30 various α-chloro-β-ketophosphonates with excellent yield and
enantioselectivity, etc.[6]
Generally, β-ketophosphonates are prepared by the reaction of
α-haloketones with trialkylphosphites (Arbuzov Reaction)
(Scheme 1a)[7] or acylation of alkylphosphonates with carboxylic
35 acid derivatives by employing stoichiometric amounts of
organometallic reagents (Scheme 1b).[8] Alternative procedures
include oxidation of β-hydroxyalkylphosphonates with inorganic
oxidants,[9] acylation of arenes with phosphonoacetic acids,[10]
and metal-mediated reactions of α-halophosphonates with
40 esters,[11] and so on.[12] However, almost all of these methods
suffer from limitations such as low atom economy, inaccessible
materials, tedious procedures, relatively harsh reaction conditions
and excess amounts of organometallic reagents. Recently, two
methodologies about hydration of alkynylphosphonates to β-
45 ketophosphonates have been developed, in which, relatively
expensive palladium(II) (Scheme 1c) and gold(I) catalysts were
employed to catalyse the corresponding hydration reactions,
Scheme 1. Comparison of previous work with this work
We initiated the meaningful study, starting with establishing
respectively.[13] The disadvantage of these methods is that one 75 optimal experimental conditions using the model reaction of
must synthesize the starting materials, alkynylphosphonates in
50 advance, using previously published methods.[14] In contrast, Ji’s
phenylacetylene (1a) with diethyl phosphonate (2a) under open-
air conditions for 3 h at room temperature. The screening results
group introduced
a relatively mild method to access β-
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DOI: 10.1039/C4CC10312B
are summarized in Table 1. Initially, the reaction of
phenylacetylene (1a) with diethyl phosphonate (2a) was
performed in CH2Cl2-H2O (v/v = 1/1) to examine the catalytic
The substrate scope of alkynes and dialkyl H-phosphonates
under the optimized conditions was then examined. As
demonstrated in Table 2. A large variety of β-ketophosphonates
activities of several relatively cheap transition-metal salts 40 was conveniently and efficiently obtained via this novel
5
including Fe, Ag, Cu and Mg salts (Table 1, entries 1-11). The
reluts showed that metal salts such as FeCl3•6H2O and Fe(NO3)3
failed to give the product 3a (Table 1, entries 1-2). Among the
silver salts (Table 1, entries 3-5), AgNO3 was found to be the
silver/copper catalyzed reaction. In general, both electro-rich and
electro-deficient aromatic alkynes were suitable for this protocol.
Starting from phenylacetylene and its ring-substituted derivatives,
the corresponding β-ketophosphonates were obtained in good to
most effective catalyst. Notably, the yield increased tremendously 45 excellent yields (3a-q). Propargyl aryl ethers, which could be
10 when CuSO4•5H2O was employed as the cocatalyst of AgNO3 in
the presence of K2S2O8 (Table 1, entry 6). Other transition metal
salts, such as Cu(OAc)2, CuI, CuBr, FeSO4 and MgSO4 were less
effective (Table 1, entries 7-11). It is worth noting that the model
thought of a type of aliphatic alkynes, were also well suitable for
the reaction, affording the expected β-ketophosphonates (3r-x) in
moderate to good yield. In addition, the method smoothly and
efficiently transformed two other aliphatic alkynes, cyclopropyl
reaction failed to give the product 3a when metals or K2S2O8 50 acetylene and 1-hexyne, to the corresponding β-ketophosphonates
15 were absent (Table 1, entries 12, 13). And also CuSO4•5H2O
alone cannot serve as a catalyst (Table 1, entry 14). Besides
K2S2O8, other oxidants such as TBHP, DTBP, H2O2 and m-
CPBA were also examined. The result showed that those oxidants
(3y-z). In comparison with the method developed by Ji’s group,[15]
our substrate scope of alkynes successfully extended to aliphatic
alkynes. Also, an internal aromatic alkyne was tolerated in this
process, leading to the desired product 3aa in a moderate yield. It
failed to produce the product 3a (entries 15-18). Thus the metal 55 can be seen here that the electronic effects of substituents
20 catalysts and K2S2O8 were indispensable for the reaction. The
solvent systems employed also notably affected the related
reaction efficiencies. Conducting the reaction in DMSO-H2O,
CH3CN-H2O and acetone-H2O failed to generate the product 3a
attached benzene ring had little influence on the efficiency of
oxyphosphorylation reactions (3e-q, 3u-x). Moreover, owing to
the mildness of the reaction conditions employed, the cyclopropyl
group (3y), a functionally unstable group, survived the reaction
(Table 1, entries 19-21), while the reaction conducted in CHCl3- 60 unchanged. With respect to organophosphorous reagents
25 H2O gave a moderate yield (entry 22). The investigations of
solvent systems (Table 1, entries 19-22) indicated that CH2Cl2-
H2O (v/v = 1/1) shown in entry 6 still brought about the best
result. Suitable amounts of AgNO3, CuSO4•5H2O and K2S2O8
were also extensively screened (Table 1, entries 23-30). The
30 amounts shown in entry 29 bought about the most satisfying yield.
Therefore the best yield of 3a (92.1%) was obtained by
employing 5.0 mol% AgNO3, 10.0 mol% CuSO4•5H2O and 4
equivalent of K2S2O8 in CH2Cl2-H2O at room temperature for 3 h
(Table 1, entry 29).
employed, in addition to 2a, diisopropyl H-phosphonate (2b),
diphenylphosphine oxide (2c) and ethyl phenylphosphinate (2d)
were all suitable substrates, leading to the products 3b-d, f-h, j, n,
p, q, s, t, v, w and x in good to excellent yields.
65 Table 2 Scope of the oxyphosphorylation of alkynes.a, b
35 Table 1 Optimization of reaction conditions.a
Two more synthetic experiments were subsequently carried out
to deepen our understanding of the mechanism (Scheme 3). When
70 TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), a widely used
radical scavenger, was added into the reaction system,
2
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oxyphosphorylation reaction of phenylacetylene 1a with diethyl
H-phosphonate 2a was quenched (Scheme 3-a), suggesting that
the reactions proceed via a radical mechanism. When the reaction
was carried out under a nitrogen atmosphere, the corresponding
β-ketophosphonates 3a was not obtained (Scheme 3-b), indicating
that the copper/silver-catalyzed oxyphosphorylation of alkynes
required the presence of oxygen. A plausible mechanism is
proposed accordingly in Scheme 2. The dialkyl H-phosphonate
(RO)2P(O)H 2 exists in equilibrium with its tautomer dialkyl
5
10 phosphite (RO)2POH 2’.[16] Initially, Ag(I) cation is oxidized to
Ag(II) cation by peroxodisulfate.[16b, 17] Then diethyl phosphite 2’
is deprived of an electron by the Ag(II) ion, forming cation
Scheme 3 Preliminary mechanistic studies. Reaction conditions: (a) 1a
(0.3 mmol), 2a (0.45 mmol), AgNO3 (5.0 mol%), CuSO4•5H2O (10.0
mol%), K2S2O8 (4 equiv), CH2Cl2-H2O (v/v = 1/1) (5.0 mL) under
45 nitrogen atmosphere for 3 h at room temperature; (b) 1a (0.3 mmol), 2a
(0.45 mmol), AgNO3 (5.0 mol%), CuSO4•5H2O (10.0 mol%), K2S2O8 (4
equiv), CH2Cl2-H2O (v/v = 1/1) (5.0 mL), TEMPO (3 equiv) at room
temperature for 3 h.
radical 4, which subsequently loses a proton, giving phosphite
c, 18]
radical 5.[16b,
This process is similar to that proposed by
15 Huang’s group.[16b] The resulting radical 5 selectively adds to
alkyne 1, leading to an alkenyl radical 6. Oxygen is one of the
most important radicals. Molecular oxygen in the ground state is
a diradical with one unpaired electron on each oxygen. It is well
known that the copper active-oxygen complexes have been
20 demonstrated or suggested to be involved in copper proteins
(enzymes), playing key roles in the oxygen transport and the
oxidation/oxygenation reactions in biological systems, being one
of the most attractive research objectives of bioinorganic
chemistry. The copper active oxygen species are also very
initiate other chain reactions. Via initial homolysis of the oxygen-
50 oxygen bond, the hydroperoxide 9 reacts with another molecule 2
to produce a hydroxyl radical, an enol complex 10 as well as a
phosphite radical 5. The enolate 10 quickly undergoes
tautomerization to its keto form 3. Meanwhile, the hydroxyl
radical produced further reacts with (RO)2P(O)H 2 to give
55 another phosphite radical 5 (Scheme 2-II). Dialkyl phosphate 11
is formed via termination reaction of hydroxyl radical and
phosphite radical 5 in the reaction process (Scheme 2-III). 31P
NMR traced the progress of the reactions. Besides the main
product 3a, the 31P NMR stacks diagram shown in Figure 1
60 proved the formation of dialkyl phosphate 11a. In addition, 31P
NMR stacks diagram didn’t offer support for the formation of
alkynlphosphates in the reaction process. 31P NMR signal from
alkynylphosphate should appear at around -5 ppm.[14] But no
trace of such a signal was detected by 31P NMR. Zhao’ group has
65 reported that copper-catalyzed aerbic oxidative coupling of
terminal alkynes with H-phosphonates can lead to the formation
of alknylphosphonates in high yield.[14] However, with the same
materials, alknylphosphonates were not detected under our
different reaction conditions, implying that the target products, β-
70 ketophosphonates, were not possibly formed via the hydration of
alkynylphophonates in our cases. It is quite understandable,
because we have already known that this one-pot reaction
proceeds via a radical chain mechanism. The mechanism
proposed in Scheme 2 dose not support the formation of
75 alkynylphosphate in the reaction process. The last strong support
for our proposed mechaniem is from the formation of product 3aa,
which was synthesized starting from 1-phenyl-1-propyne, an
internal aromatic alkyne. It is clear that it is only terminal alkynes
that can react with H-phosphonates via an aerobic oxidative
80 coupling reaction to generate alkynylphosphonates.[14] The
formation of 3aa here once again offers support for the radical
mechanism shown in Scheme 2 and excludes the possibility of
25 important intermediates involved in a variety of copper-catalyzed
reactions in synthetic organic reactions.[19,
20]
O2 here reacts
further with the copper (II) to form an active-oxygen copper
complex 7. The alkenyl radical 6 is then attacked by this active-
oxygen copper complex 7, affording complex 8. Complex 8
30 quickly reacts with water, forming hydroperoxide 9 and a
copper(II) ion. The oxygen-oxygen bond of the hydroperoxide of
9 is quite weak, and it breaks and produces radicals that can
Scheme 2 Proposed reaction mechanism
formation of β-ketophosphonates via
alkynylphosphate.
a
hydration of
85 Conclusions
In conclusion, we have developed a straightforward method by
which a wide variety of β-ketophosphonates were prepared in a
one-pot aerobic reaction of alkynes with dialkyl H-phosphonates
in the presence of silver/copper catalysts and K2S2O8 at room
90 temperature within 3 h. The current substrate scope encompasses
a series of alkynes, including aromatic alkynes, aliphatic alkynes,
as well as a series of organophosphorous reagents, including
35
Figure 1 31P NMR stacks diagram for Scheme 2. Reaction conditions: 1a
(0.3 mmol), 2a (0.45 mmol), AgNO3 (5.0 mol%), CuSO4•5H2O (10.0
mol%), K2S2O8 (4 equiv), CH2Cl2-H2O (v/v = 1/1) (5.0 mL) at room
temperature for 3 h. The whole process was monitored by 31P NMR every
40 25 min (comp-2a, 7.1 ppm; comp-3a, 20.6 ppm; comp-11a, -0.8 ppm).
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DOI: 10.1039/C4CC10312B
G. Wang, Z. H. Peng, J. N. Xiang, W. M. He, Eur. J. Org. Chem.
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diphenylphosphine oxide and ethyl phenylphosphinate, and
dialkyl H-phosphonates. To the best of our knowledge, this
reaction is the first example using easily available
silver/copper/K2S2O8 catalytic system to perform the
oxyphosphorylation reaction. Compared with literature methods,
great advantages of this strategy include high efficiency, readily
available catalysts and starting materials, time-saving one-pot
procedure, and extremely mild and open air conditions. The
method will undoubtedly be a far superior alternative for the
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