COMMUNICATION
worked and indeed afforded trans-alkenylphosphine oxide
2a in 33% yield after 6 h (Table 1, entry 2). Encouraged by
this result, we further optimized the reaction conditions. The
AgOAc loading was increased (3.0 equiv), but no distinct
change was detected and the yield of 2a only increased to
38% (Table 1, entry 3). A screening of different Cu salts
(Table 1, entries 4–6) revealed Cu2O as the best catalyst to
afford 77% yield of isolated 2a. Other additives were also
evaluated and the results indicated that the use of Ag2O
(3.0 equiv) improved the reaction yield of 2a to 94%. Sol-
vent screening (Table 1, entries 11–12) showed that NMP
was the best choice. The single-crystal structure of ortho-
selective and chemoselective. Compared with previous
methods, it is simple and works under relatively mild condi-
tions.
Phosphorus-based ligands are important to many metal-
catalyzed organic transformations, including many chiral re-
actions.[9] Thus, effective and efficient ways to construct
these ligands are highly desirable. The alkenyldi-
AHCTUNGTREG(NNUN phenyl)phosphine oxides are important skeletons that have
been extensively applied to synthesize various phosphorus
ligands. We choose product 2a to react with 1,3-cyclopenta-
diene through 1,4-cycloaddition to yield phosphorus ligand
2al in 90% high yield, showing the utility of our chemistry
(Scheme 2).
methylcinnamic diACHTUNGTRENNUNG(phenyl)phosphine oxide (2d) confirmed
the identity of the product.[8]
Under the optimized reaction conditions (Table 1,
entry 9), various cinnamic-acid derivatives were surveyed
(Table 2). The corresponding alkenylphosphine oxides were
Table 2. Decarboxylative coupling with different substrates.[a,b]
Scheme 2. Phosphorus-ligand synthesis by 1,4-cycloaddition.
Inspired by these results, we wished to extend this method
to the synthesis of alkynyl phosphonates. Alkynyl di-
AHCTUNGTREG(NNNU phenyl)phosphine oxides are important precursor to syn-
Product
Ar
R1
R2
Yield[b][%]
2a
2b
2c
2d
2e
2 f
2g
2h
2i
Ph
Ph
Ph
o-CH3Ph
m-CH3Ph
p-CH3Ph
m-OCH3Ph
p-OCH3Ph
3,4-CH3Ph
3,5-OCH3Ph
2,5-OCH3Ph
b-naphthalene
3,4-methylenedioxy
p-ClPh
m-BrPh
p-FPh
p-CNPh
2,4-ClPh
Ph
Ph
OiPr
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
OiPr
Ph
94%
50%
76%
78%
88%
60%
92%
70%
81%
76%
78%
90%
86%
76%
60%
71%
63%
20%
60%
51%
62%
OiPr
OEt
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
thesize various (P^O) or (P^N) bidentate ligands.[10] Synthe-
sis of alkynylphosphanes is usually accomplished by treating
PIII-halides with acetylenides of sodium, lithium, magnesium,
or titanium.[11] A significant advance to simplify the process
was achieved by Han and co-workers, who developed an ef-
ficient copper-catalyzed oxidative coupling of alkynes with
(iPrO)2P(O)H. However, under similar reaction conditions,
Ph2P(O)H as substrate did not afford the oxidative-coupling
product, because Ph2P(O)H was oxidized to Ph2P(O)OH.[12]
If our assumption of copper-catalyzed decarboxylative cou-
pling of alkyne acid with Ph2P(O)H for the synthesis of al-
2j
2k
2l
2m
2n
2o
2p
2q
2r
2s
2t
kynyl diACHTNUTRGNE(UNG phenyl)phosphine oxides was achieved, it would be
beneficial in general and broadly applicable. At the outset,
we examined the reaction of 3-phenylpropiolic acid (3b)
with Ph2P(O)H under the decarboxylative reaction condi-
tions. The reaction occurred and the desired product of al-
furan
3,4-methylenedioxy
3,4-methylenedioxy
Ph
OiPr
OEt
2u
[a] All reactions were carried out under the optimal conditions reported
above for 6 h. [b] Yield of isolated product.
kynyl diACTHNUTRGENUG(N phenyl)phosphine oxides (4b) was obtained in
23% yield and the structure was confirmed by single-crystal
diffraction.[13] To optimize the reaction, we discovered that
the Cu/Pd-bimetallic catalysis system was the best choice.
produced with moderate to good yields and high selectivity,
regardless if electron-withdrawing or electron-donating
groups were introduced on the phenyl ring of the olefinelic
acid (Table 2, products 2d–s). Electronic effects are not evi-
dent in this reaction. However, steric hindrance will lead to
lower yields. Different R2P(O)H were also examined and
the corresponding products were obtained in moderate
yields (Table 2, products 2b–c and 2t–u). In addition, we at-
tempted to expand our substrates to vinyl carboxylic acids
and benzoic acids, but this failed and the aspired products
were not observed. In conclusion, this methodology is regio-
When we added PdACHTNGUTERNNU(G acac)2 (5% mol, acac=acetylacetonate)
and PPh3 (10% mol) to the previously described reaction
system, and selected AgOAc as the base, the reaction pro-
ceeded smoothly to yield 80% of 4b (Scheme 3).
The cross-reactions of different R2P(O)H compounds was
investigated and the corresponding alkynylphosphonates
were produced in moderate to good yields (Table 3, en-
tries 6–20). Notably, diACTHUNRGTNE(NUG phenyl)phosphine oxide exhibited
higher activity than others. Different alkyne acids were also
screened and the results illustrate that electronic effects and
steric hindrance play the key roles in this reaction; electron
Chem. Eur. J. 2011, 17, 5516 – 5521
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5517