Copper-catalyzed decarboxylative C-P cross-coupling of alkynyl acids with H-phosphine oxides: A facile and selective synthesis of (E)-1-alkenylphosphine oxides

Article abstract of DOI:10.1021/ol502009b

A novel and efficient copper-catalyzed decarboxylative cross-coupling of alkynyl acids for the stereoselective synthesis of E-alkenylphosphine oxides has been developed. In the presence of 10 mol % of CuCl without added ligand, base, and additive, various alkynyl acids reacted with H-phosphine oxides to afford E-alkenylphosphine oxides with operational simplicity, broad substrate scope, and the stereoselectivity for E-isomers.

Full text of DOI:10.1021/ol502009b

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed Decarboxylative C−P Cross-Coupling of Alkynyl
Acids with H‑Phosphine Oxides: A Facile and Selective Synthesis of
(E)‑1-Alkenylphosphine Oxides
Gaobo Hu, Yuxing Gao,* and Yufen Zhao
Department of Chemistry and Key Laboratory for Chemical Biology of Fujian Province, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, Fujian, China
S
* Supporting Information
ABSTRACT: A novel and efficient copper-catalyzed decarboxylative
cross-coupling of alkynyl acids for the stereoselective synthesis of E-
alkenylphosphine oxides has been developed. In the presence of 10 mol %
of CuCl without added ligand, base, and additive, various alkynyl acids
reacted with H-phosphine oxides to afford E-alkenylphosphine oxides with
operational simplicity, broad substrate scope, and the stereoselectivity for E-isomers.
of alkenylphosphine oxides (Scheme 1b).8b Despite their
he utility of alkenylphosphine oxides has increased
usefulness, almost all of these methods need well-defined ligands
Tenormously over the past several years for their important
and excess bases or additives, and these protocols have common
problems such as relatively strict reaction conditions, excess
reagents, poor substrate scope, or lack of stereoselectivity, thus
increasing the cost and limiting the applications of these
methods.
roles. For example, they can be used as biologically active
compounds,1 and the main building blocks of phosphorus-
containing materials,2 and the key precursors for the preparation
of valuable phosphine ligands.3 Moreover, they could also
provide a variety of synthetically elaborated bifunctional adducts
by the addition of heteroatom nucleophiles,4 carbanion species,5
and carbon-centered radicals6 to the olefinic bonds. Recently, a
few transition-metal-catalyzed methods for the synthesis of
alkenylphosphine oxides involving the use of palladium,7
copper,8 nickel,9 rhodium,10 and ytterbiumimine complex11
based catalytic systems have continuously emerged. Among
them, transition-metal-catalyzed addition of P(O)H compounds
to alkynes has emerged as the most commonly used and powerful
approach for making alkenylphosphine oxides. In 2001 and 2004,
Han reported the Rh- and Ni-catalyzed additions of P(O)H
compounds to alkynes (Scheme 1a).10a,9a In 2007, Fu and co-
workers developed a CuI/EDA catalytic system for the synthesis
In recent years, transition-metal-catalyzed decarboxylative
coupling reactions as a new synthetic strategy have attracted
increasing attention in the formation of C−C and C−
heteroatom bonds.12 In particular, various valuable P-alkenylated
and P-alkynylated motifs can be readily obtained through
decarboxylative C−P cross-coupling.8d,13 In 2011, Yang’s group
first reported the synthesis of alkenylphosphine oxides via a Cu-
catalyzed decarboxylative coupling of alkenyl acid (Scheme
1c).8d It is noteworthy that as a practical alternative, using
arylpropiolic acids instead of terminal alkynes is safer and more
attractive because arylpropiolic acids are usually solids without an
unpleasant smell and are convenient to synthesize, store, and
transport.14 On the basis of this viewpoint, very recently, Wu’s
group13 and Yang’s group8d fulfilled the decarboxylative coupling
of arylpropiolic acids with P(O)H to construct a Csp−P bond
with the assistance of a Cu or Cu/Pd cocatalyst system,
respectively (Scheme 1d). Notably, these reactions only afforded
alkynylphosphorus products and did not produce alkenylphos-
phorus compounds. To the best of our knowledge, the example
of alkenylphosphine oxide formation via decarboxylative
coupling of alkynyl acids has yet to be reported (Scheme 1e).
On the other hand, a ligand-free, base-free, and additive-free
catalysis system would be more attractive both from economic
and industrial points of view as compared to the ones-containing
system. As part of our ongoing efforts to develop environ-
mentally friendly new methodologies for the P−C bond
formation,9b,15 we herein report the first example of a single-
Scheme 1. Transition-Metal-Catalyzed C−P Bond-Forming
Reaction
Received: July 9, 2014
© XXXX American Chemical Society
A
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Organic Letters
Letter
step preparation of stereoselective (E)-alkenylphosphine oxides
by a simple copper-catalyzed decarboxylative coupling of various
alkynyl acids with P(O)H compounds under ligand-free, base-
free, and additive-free conditions.
Phenylpropiolic acid (1a) and H(O)PPh2 (2a) were chosen as
the model substrates to screen and optimize the catalysis
conditions as shown in Table 1. Initially, the reaction of 1a (1.5
that they could also promote the reaction efficiently to give 97%
and 95% yields, respectively (Table 1, entries 10 and 11). Thus,
we chose CuCl as the best catalyst owing to the lower price and
similar yield in contrast to CuI (Table 1, entry 10). Decreasing
the reaction temperature to 60 °C and rt would lead to poor
yields (Table 1, entries 12 and 13). Noting that decreasing the
load of 1a to 1.2 equiv was sufficient to produce an excellent yield
of 97% (Table 1, entry 14). However, using 1 equiv of 1a resulted
in a slight decrease of the yield (Table 1, entry 15). The CuCl
loading was also evaluated, and using 2 and 6 mol % of CuCl gave
3a in 55% and 92% yields, respectively (Table 1, entries 16 and
17). Under an open air atmosphere, the yield decreased to 76%
due to the oxidation of 2a to Ph2P(O)OH (Table 1, entry 18).
Notably, this novel decarboxylative coupling only produced the
E-isomer determined by in situ 31P NMR and 1H NMR analysis,
without the observation of other regio- or stereoisomers.
Moreover, the ligand, the base, and the additive are not needed,
and milder reaction conditions are possible.
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst
CuI
ligand
base
solvent
yield (E, %)
1
L-proline
L-proline
L-proline
L-proline
L-proline
Cs2CO3
t-BuOK
Et3N
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
toluene
dioxane
DMF
84
66
2
CuI
3
CuI
91
c
4
CuI
DMAP
90
5
CuI
92
Under the optimized conditions shown in footnote a, Scheme
2, the substrate scope of this coupling was surveyed. As shown in
Scheme 2, various arylpropiolic acids bearing different electron-
withdrawing and electron-donating substituents were all
6
CuI
96
7
CuI
0
8
CuI
trace
98
9
CuI
10
11
12
13
14
15
16
17
18
CuCl
Cu(OAc)2
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
DMF
97
Scheme 2. Decarboxylative Cross-Coupling of Alkyne Acids
with P(O)H
a,b
DMF
95
d
DMF
45
e
DMF
trace
f
DMF
97
g
DMF
94
h
DMF
55
i
DMF
92
j
DMF
76
a
Reaction conditions: phenylpropiolic acid 1a (0.75 mmol), H(O)-
PPh2 2a (0.5 mmol), catalyst (10 mol %), ligand (15 mol %), solvent
b
(2 mL), base (1.2 equiv) at 90 °C for 6 h under N2. Isolated yield
based on 2a. Only E-isomer 3a was detected by in situ 31P NMR and
c
1H NMR analysis of crude reaction mixtures. DMAP = (4-N,N-
d
e
f
dimethylpyridine). At 60 °C. At rt. Using 1.2 equiv of 1a (0.6
g
h
mmol). Using 1.0 equiv of 1a (0.5 mmol). Using 2 mol % of CuCl.
Using 6 mol % of CuCl. Under air.
i
j
equiv) with 2a (1 equiv) was performed in DMSO (2 mL) at 90
°C for 6 h under nitrogen in the presence of CuI (10 mol %), L-
proline (15 mol %), and Cs2CO3 (1.2 equiv). Gratifyingly, the
desired product (E)-styryl diphenylphosphine oxide (3a) was
obtained in a high yield of 84% (Table 1, entry 1). Encouraged by
this promising result, various bases were further investigated. It
was found that a strong base like t-BuOK afforded 3a in a lower
yield of 66% (Table 1, entry 2), but when some weak bases such
as Et3N and DMAP were used, the yield could increase to 91%
and 90% yields, respectively (Table 1, entries 3 and 4). In order
to test the role of the base, the reaction was carried out under
base-free conditions (Table 1, entry 5). To our delight, the
transformation could still proceed smoothly to provide 3a in an
excellent yield of 92%, clearly indicating that the base was not
essential to this coupling. Subsequently, we checked the effect of
the ligand, and without the ligand, the yield of 3a could be up to
96%, demonstrating that the addition of the ligand is less effective
(Table 1, entry 6). A screening of the solvents such as toluene,
1,4-dioxane, and DMF illustrated that DMF was the best choice
for this reaction and enhanced the yield up to 98% (Table 1,
entries 7−9). To advance the process further, other copper salts
including CuCl and Cu(OAc)2 were detected, with the finding
a
Reaction conditions: alkynyl acid 1 (0.6 mmol), P(O)H 2 (0.5
mmol), CuCl (10 mol %), and DMF (2.0 mL) at 90 °C for 6 h under
b
nitrogen. Isolated yield. Only E-isomer 3 was determined by in situ
31P NMR and H NMR analysis of crude reaction mixtures.
1
B
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Organic Letters
Letter
efficiently reacted with diphosphine oxide 2a via decarboxylative
cross-coupling reactions to afford anti-Markovnikov coupling
products (E)-1-alkenylphosphine oxides in moderate to good
yields of 55−98% with the stereoselectivity for E-isomers based
on the analysis of in situ 31P NMR and 1H NMR spectra (Scheme
2, 3a−r), revealing that this novel Cu-catalyzed decarboxylative
C−P cross-coupling is a general and powerful tool for the
synthesis of various valuable P-alkenylated motifs. Importantly, a
variety of functional groups, such as methyl, aryl, alkoxyl, fluoro,
chloro, bromo, iodo, trifluoromethyl, nitro, cyano, carbonyl, and
carboxyl groups, were all well tolerated, indicating that electronic
effects and steric hindrance are not evident in this trans-
formation. Notably, the propiolic acid moiety represented a
higher chemoselectivity over the chlorine, bromine, and reactive
iodine atoms as leaving groups under the standard reaction
conditions, producing the desired products (3i−k) in 55−81%
yields. Thus, the chemoselectivity may be further applied for the
preparation of more complex molecules through stepwise
coupling of arylpropiolic acids and halides. Interestingly, a
carboxylic acid substrate having a carboxyl unit without needing
any protection could also be used in the reaction to give the
desired product 3q in a good yield of 77%. The hetero-
cyclicpropiolic acid having a pyridine ring could also be
compatible for this reaction, affording the desired product 3s
in 71% yield. However, the sulfur-containing heterocyclicpro-
piolic acid could only provide the product 3t in 36% yield, which
may be attributed to the sulfur species dramatically deactivating
the catalyst. In addition, the present protocol is less effective for
aliphatic alkynyl acid; for example, 2-octynoic acid 2u only gave
3u in 30% yield.
Scheme 4. Mechanism Study Experiments
labeling study was also performed as shown in Scheme 4b. It was
found that when the substrate 2a′ (40% D) was treated with 1.2
equiv of (phenylethynyl)copper, almost all deuterium incorpo-
ration into 3a was observed (3a′, 38% D), clearly illustrating that
the hydrogen atom of the α-position of the product 3 came from
P(O)-H. On the basis of these experimental results and previous
reports,16 a plausible mechanism for this coupling reaction is
proposed (Scheme 5). First, the decarboxylative reaction of 1
Scheme 5. Plausible Reaction Mechanism
To further extend the scope of this reaction, the alkenylation of
various P(O)H substrates was also investigated (Scheme 2, 3v−
y). In regard to the H-phosphine oxides, apart from 2a, di-p-
tolylphosphine oxide (2b) and aliphatic dipentylphosphine oxide
(2c) were all suitable substrates, and the corresponding products
3v and 3w were obtained in 72% and 84% yields, respectively. In
addition, ethyl phenylphosphinate (2d) and 6H-dibenzo[c,e]-
[1,2]oxaphosphinine 6-oxide (2e) were also detected and
produced the desired products 3x and 3y in good yields.
However, H-phosphonates such as diethyl phosphonate only
provided a trace amount of the desired product in the present
catalytic system.
It is noteworthy that this Cu-catalyzed decarboxylative
coupling of alkynyl acids could also be effectively scaled up
with the high efficiency. For example, in the presence of CuCl
(10 mol %), 1a (12 mmol) reacted with 2a (10 mmol) in DMF at
90 °C for 12 h to afford the corresponding product 3a in 86%
yield (Scheme 3).
took place with the assistance of CuCl to form alkynyl copper
intermediate A and released one molecular CO2 and HCl. Then,
the coordination of 2 (in the form of the trivalent phosphine
oxide 2′) to A, affording B, was reasonable.15a Subsequently, a
proton transfer from oxygen atom to Csp atom led to generation
of a three-membered-ring transition state C. Next, C underwent
a four-centered transition state C′16f to give a thermodynamically
more stable alkenyl copper intermediate D with the E
configuration due to the steric hindrance and the cis-addition
of (Ph)2P(O)Cu to phenyl acetylene, which are the main reasons
for the selection of an all-trans configuration for the product 3.
Finally, the protonolysis of intermediate D with 1 or HCl
resulted in the generation of the desired product 3 and
catalytically active Cu(I) species to fulfill the catalytic cycle.
In conclusion, we have successfully developed the first facile
and efficient method for the preparation of (E)-1-alkenylphos-
phine oxides and (E)-1-alkenylphosphinate esters through
copper-catalyzed decarboxylative C−P cross-coupling of various
alkynyl acids with H-phosphine oxides and H-phosphinate
esters. Importantly, this novel method not only expands our
understanding of the decarboxylation but also affords a powerful
synthetic tool for simple synthesis of valuable (E)-P-alkenylated
motifs. Moreover, only in the presence of CuCl without needing
a base, a ligand, and an additive, the transformation could
proceed smoothly. In addition, the use of inexpensive CuCl
catalyst, using readily available and stable alkynyl acids only
producing CO2, the remarkable functional group tolerance and
high stereoselectivity for E-isomers mean that this facile protocol
will be attractive for academia and industry. Further mechanistic
investigations and synthetic applications are currently underway.
Scheme 3. Gram-Scale Synthesis of 3a
To understand the mechanism more clearly, some control
experiments were carried out (Scheme 4). When 10 mol % of
(phenylethynyl)copper instead of CuCl was employed as the
catalyst, a 64% yield of 3a was isolated, which demonstrated that
(phenylethynyl)copper might be a key intermediate in this
reaction and was essential for the transformation (Scheme 4a).
Indeed, the (phenylethynyl)copper as a yellow precipitate was
observed during the reaction process. In addition, a deuterium
C
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Organic Letters
Letter
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ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental procedure and characterization data of the
products. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: gaoxingchem@xmu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Chinese National Natural
Science Foundation (21202135), the Natural Science Founda-
tion of Fujian Province of China (No. 2013J05031), and the
National Basic Research Program of China (2012CB821600).
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