Palladium(II)-catalyzed hydration of alkynylphosphonates to β-ketophosphonates

Article abstract of DOI:10.1002/adsc.201200420

A highly efficient palladium(II)-catalyzed hydration of a wide range of alkynylphosphonates to the corresponding β-ketophosphonates has been developed to give high yields at 80 °C in 1, 4-dioxane, with no acidic or alkaline cocatalysts required. The described catalytic system should provide an efficient alternative to highly toxic mercury-catalyzed methodologies and be useful in synthetic programs. Copyright

Full text of DOI:10.1002/adsc.201200420

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DOI: 10.1002/adsc.201200420
Palladium(II)-Catalyzed Hydration of Alkynylphosphonates to
b-Ketophosphonates
Xiubin Li,a Gaobo Hu,a Ping Luo,a Guo Tang,a Yuxing Gao,a,* Pengxiang Xu,a,*
and Yufen Zhaoa,b
a
Department of Chemistry and Key Laboratory for Chemical Biology of Fujian Province, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, Peopleꢀs Republic of China
Fax : (+86)-592-218-5780; e-mail: gaoxingchem@xmu.edu.cn or xpengxiang@xmu.edu.cn
Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry of Education), Department of
b
Chemistry Tsinghua University, Beijing 100084, Peopleꢀs Republic of China
Received: May 12, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200420.
Abstract: A highly efficient palladium(II)-catalyzed
hydration of a wide range of alkynylphosphonates
to the corresponding b-ketophosphonates has been
developed to give high yields at 808C in 1, 4-diox-
ane, with no acidic or alkaline cocatalysts required.
The described catalytic system should provide an
efficient alternative to highly toxic mercury-cata-
lyzed methodologies and be useful in synthetic pro-
grams.
alkylphosphonates,[11] the oxyphosphorylation of al-
ACHTUNGTRENNUNG
kenes,[12] and so on.[13] Among these, the most envi-
ronmentally benign and economically attractive
method is the hydration reaction of a wide variety of
alkynylphosphonates, which can be conveniently pre-
pared by using our recently reported method.[14]
However, the procedure for the hydration of alk-
AHCTUNGTREGyNNUN nylphosphonates has remained unchanged since the
first report in 1966,[10,15] the potential of which in or-
ganic synthesis and/or in industrial processes has not
been conveniently exploited. The reason for that is
the catalysts known to date, HgSO4,[15,16] HgCl2,[17] and
HgO·BF3[18], are highly toxic mercury salts that, in ad-
dition, must be used in combination with either acids
or bases in a longer reaction time. Obviously, these
methodologies suffer from relatively strict reaction
Keywords: alkynylphosphonates; hydration; b-keto-
phosphonates; palladium(II) catalysis
The use of water as an inexpensive and environmen- conditions, requiring highly toxic mercury reagents,
tally benign reagent in organic synthesis is an impor- poor substrate scope or unsatisfactory yield. For ex-
tant topic, and the catalytic hydration of alkynes to ample, the conversion of sensitive enyne phospho-
synthesize valuable carbonyl compounds[1,2] such as b- nates into g,d-ethylene-b-ketophosphonates gave rela-
ketophosphonates, represents one of the most ideal tively low yields even using a sophisticated catalytic
models of environmentally benign and sustainable mixture of HgO/ClCH2CO2H/BF3·Et2O/ROH in re-
transformations in modern synthetic chemistry. b-Ke- fluxing EtOH.[18] Therefore, the development of a gen-
tophosphonates are an important class of oxygen-con- eral, efficient, economic, and especially, environmen-
taining, extremely versatile reagents in organic tally benign hydration procedure that can directly
chemistry, especially for the preparation of a,b-unsa- convert alkynylphosphonates into b-ketophospho-
turated carbonyl compounds via the well-known nates by using non-mercury catalysts is still highly de-
HWE (Horner–Wadsworth–Emmons) reaction,[3] and sirable in modern synthetic chemistry.
for the synthesis of chiral b-hydroxy[4] and b-amino[5]
Several transition metal catalysts containing Au,[19]
phosphonic acids, which exhibit numerous interesting Ru,[20] Rh,[21] and Pt[22] have been extensively investi-
biological properties. Some b-ketophosphonates gated for the regioselective hydration of alkynes into
themselves are endowed with wide-ranging biological ketones in the past. A few references[23] have reported
activities[6] and eminent metal-complexing abilities.[7] the work done with Pd(II)-catalyzed alkyne hydra-
These compounds can be prepared by several meth- tion, but the Pd(II)-catalyzed hydration of alkynyl-
ods such as the classical Arbuzov reaction,[8] the acy- phosphonates was not investigated. Herein we dis-
lation of alkyl phosphonate anions,[9] the hydration of close a new methodology for the highly efficient prep-
alkynylphosphonates,[10] the oxidation of b-hydroxy- aration of b-ketophosphonates from readily available
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Table 1. Optimization of the reaction conditions.[a]
A screening of the catalysts showed that PdCl2 was
a good catalyst for this hydration. A low loading of
PdCl2 (1 mol%) was sufficient to produce an excellent
yield (entry 15). Without PdCl2, the hydration reac-
tion did not occur (entry 18), indicating that a PdCl2
catalyst was essential to achieve a high yield of prod-
Entry
Catalyst
Solvent
Yield [%][b]
uct 2a. Pd
and gave 41% yield (entry 14). It should be noted
that Pd(PPh3)4 and Pd/C did not afford the desired
ACHTUNGRTENN(UNG OAc)2 was less effective for this hydration
1
ZnI2
1,4-dioxane
0
2
3
4
5
FeCl3
FeCl2
CuCl2
MnCl2
NiCl2
0
0
0
0
AHCTUNGTRENNUNG
product at all, clearly demonstrating that Pd(II) not
Pd(0) could catalyzes this hydration (entries 12 and
13). Although FeCl3 was found to be very effective in
the hydration of terminal alkynes to corresponding
methyl ketones,[24] it was not active in the hydration
of alkynylphosphonates and gave no product
(entry 2). HAuCl4, NaAuCl4 and PtCl2 afforded the
desired product in 10%, 1% and 53% yields, respec-
tively (entries 9–11). Under similar reaction condi-
tions, other metal salts, such as ZnI2 (entry 1), FeCl2
(entry 3), CuCl2 (entry 4), MnCl2 (entry 5), NiCl2
(entry 6), CoCl2 (entry 7) and RuCl3 (entry 8), did not
catalyzed this reaction. The type of solvent also
highly affected the yield of this reaction. Various sol-
vents, such as 1,4-dioxane (entry 15), chloroform
(entry 19), dimethyl sulfoxide (entry 20), toluene
(entry 21), acetonitrile (entry 22) and ethanol
(entry 23), were investigated. It was found that 1,4-di-
6
0
7
8
9
CoCl2
RuCl3
HAuCl4
NaAuCl4
PtCl2
0
0
10
1
53
0
0
41
98 (96)
85 (80)
10
0
70
23
75
87(81)
24
10
11
12
13
14
15
16[c]
17[d]
18
19
20
21
22
23
Pd-C
Pd
Pd
U
PdCl2
PdCl2
PdCl2
–
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
chloroform
DMSO
toluene
acetonitrile
ethanol
oxane,
a
water-soluble solvent, was the best
(entry 15). The choice of temperature was also vital
to the present catalytic reaction (entries 15–17). The
hydration proceeded slowly at room temperature
(entry 17), however, the yield of product 2a increased
greatly when the temperature was raised to 508C
(entry 16) and the catalytic reaction could be
achieved quickly within 2 h and afforded the corre-
sponding product 2a in a quantitative yield when the
temperature was raised to 808C. However, under sim-
[a]
Reaction conditions: 1a (0.5 mmol), H2O (3 mmol), cata-
lyst (1 mol%), solvent (0.6 mL), 808C, 2 h.
Determined by 31P NMR. Yields are based on 1a. Yields
in parentheses are isolated yields after chromatography
on a silica column.
[b]
[c]
[d]
Run at 508C.
At room temperature.
alkynylphosphonates through Pd(II)-catalyzed hydra- ilar reaction conditions, when nitrogen ligands or
tion with complete atom economy, excellent yield and bases such as pyridine, Et2NH, Et3N or K2CO3, were
broad substrate applicability. To the best of our added respectively, only a trace of the desired product
knowledge, this method is the first example of the was formed.
non-mercury-catalyzed direct hydration of alkynyl-
phosphonates to b-ketophosphonates, and does not [a] of Table 1, we next examined the hydration of var-
require the use of a base, an acid, or an additive. ious substituted alkynylphosphonates to understand
Under the optimized conditions shown in footnote
This Pd-catalyzed hydration depends greatly on the the scope of the reaction. As demonstrated in Table 2,
reaction conditions. To optimize the conditions, we this novel Pd-catalyzed hydration of alkynylphospho-
examined the effect of catalyst, solvent, and tempera- nates is a general and practically useful method for
ture on the reactivity (Table 1). The hydration of di- the preparation of b-ketophosphonates. In all cases,
ACHTUNGTRENNUNGisopropyl 2-phenylethynylphosphonate (1a) to diiso- the corresponding hydration products were detected
propyl 2-oxo-2-phenylethylphosphonate (2a) was se- in 91–98% yields by 31P NMR. Thus, both an aromatic
lected as the standard reaction. The yield of 2a was alkynylphosphonate (entry 1) and an aliphatic alky-
determined based on the 31P NMR signal integration nylphosphonate (entry 2) gave the corresponding hy-
method. When a mixture of 1a (0.5 mmol) and H2O dration products in excellent yields. A variety of func-
(3 mmol) was heated in the presence of PdCl2 tionalities, such as hydroxy (2c, 2d and 2e), chloro (2f
(0.005 mmol) at 808C under an air atmosphere for and 2q), carboxyl (2g), cyano (2h), alkoxy (2i) and
2 h, 2a was obtained in 98% yield (entry 15), as deter- carbonyl (2i) groups, were all tolerated under the re-
mined by 31P NMR spectroscopy.
action conditions, and the corresponding hydration
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Palladium(II)-Catalyzed Hydration of Alkynylphosphonates to b-Ketophosphonates
Table 2. Palladium-catalyzed hydration of alkynylphosphonate to b-ketophosphonates.[a]
Entry
1
Substrate
Product
Yield [%][b]
96
2
95
85
86
3[c]
4[c]
5[c]
94
6
7
8
9
94
94
95
93
10[c]
92
11
12
13
95
94
93
14
15
88
93
16[d]
17[d]
95
94
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Table 2. (Continued)
Entry
Substrate
Product
Yield [%][b]
94
18[d]
19[d]
20[d]
21[e]
93
94
95
94
22[e]
[a]
Reaction conditions: substrate (0.5 mmol), PdCl2 (0.005 mmol), H2O (3 mmol), solvent (0.6 mL), 2 h, 808C.
Isolated yield.
PdCl2 (0.01 mmol), 12 h.
PdCl2 (0.01 mmol), 6 h.
PdCl2 (0.01 mmol), H2O (6 mmol), 6 h.
[b]
[c]
[d]
[e]
products were obtained in high yields. Interestingly, phine oxide (entry 21) and an aliphatic dialkynylphos-
the ethynylferrocene substrate (1j, entry 10) was also phine oxide (entry 22) reacted with water to afford
successfully hydrated to give the corresponding ferro- the corresponding hydration products 2u and 2v in 95
cene product in 92% yield, which can be readily ap- and 94% yield, respectively.
plied in the synthesis of novel ferrocenyl phospho-
It is reasonable to assume that this palladium-cata-
nates with great applications in modern catalysis.[25] In lyzed alkynylphosphonate hydration should proceed
regard to the reactant, in addition to diisopropyl (1a– via a mechanism similar to that of the transition
1h), diethyl (1k), dibutyl (1l), and dibenzyl alkynyl- metal-catalyzed alkyne hydration[19c,22b,d]. We propose
phosphonates (1m) all could be used as the substrate, that the hydration proceeds through enol and keto
generating the corresponding products 2k–2m in 95%, tautomers of organometallic intermediates with
94% and 93% yields, respectively (entries 11–13). No- metal-carbon bonds, and the key enol intermediate
tably, a senstive enyne substrate having a reactive can immediately rearrange to the corresponding keto
enyne unit could also be used in the Pd-catalyzed al- tautomers.[27] Further mechanistic details and synthet-
kynylphosphonate hydration to give the desired g,d- ic applications are currently under investigation.
ethylene-b-ketophosphonate selectively without dam-
In summary, we have successfully developed
aging the other functionalities in good yield (entry 14, a simple and highly efficient palladium(II)-catalyzed
88%). Compared to the previous method,[18] this method for the synthesis of b-ketophosphonates by
method should be more convenient and practical. It is the hydration of readily available alkynylphospho-
noteworthy that when the highly functionalized alk- nates. The hydration reactions of alkynylphospho-
ACHTUNGTRENNUNGynylphosphonate 1o, a nucleotide analogue prepared nates were firstly performed without the need for
from unique biologically activity ribonucleoside,[26] highly toxic mercury catalysts in air and, importantly,
was used as the substrate (entry 15), a water molecule a base, an acid, a ligand, or an additive was not neces-
could easily be selectively added to the triple bond of sary. The high atom-economy, the remarkable func-
1o, clearly demonstrating the great potential of this tional group tolerance and operational simplicity of
new methodology to allow access to highly functional- the procedure mean that this reaction will find wide
ized targets. Various phenyl alkynylphosphinates with applications in various fields.
isopropoxy (1p and 1q), ethoxy (1r) and n-butoxy
groups (1s) were investigated; the respective products
(2p–2s) were obtained in high yield (93–95%, en-
Experimental Section
tries 16–19). Diphenyl phenylethynylphosphine oxide
1t could also be used as the substrate to give the de-
sired product in 95% yield (entry 20). The present
catalytic reaction was also successfully applied to di-
General Procedure for the Synthesis of 2a–2v
Alkynylphosphonates (0.5 mmol), water (3 mmol) and PdCl2
(0.005 or 0.01 mmol) were dissolved in 1,4-dioxane (0.6 mL)
ACHTUNGTRENNUNGalkynylphosphine oxide substrates with high yields
(entries 21 and 22). Thus, an aromatic dialkynylphos- and stirred at 808C for the indicated time in an air atmos-
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Palladium(II)-Catalyzed Hydration of Alkynylphosphonates to b-Ketophosphonates
phere. The resulting mixture was concentrated under
vacuum and the crude product was purified by silica gel
chromatography using a mixture of petroleum ether and
ethyl acetate as the eluent.
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Acknowledgements
We acknowledge financial support from the National Basic
Research Program of China (973 Program, No.
2012CB821600) and the Chinese National Natural Science
Foundation (21173178, 21075103), TJAB-2009-023.
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Palladium(II)-Catalyzed Hydration of Alkynylphosphonates
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to b-Ketophosphonates
Adv. Synth. Catal. 2012, 354, 1 – 7
Xiubin Li, Gaobo Hu, Ping Luo, Guo Tang, Yuxing Gao,*
Pengxiang Xu,* Yufen Zhao
Adv. Synth. Catal. 0000, 000, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
7
ÞÞ
These are not the final page numbers!