Oxidative heck-type reaction involving cleavage of a carbon-phosphorus bond of arylphosphonic acids

Article abstract of DOI:10.1021/ja026758v

Cleavage of a carbon-phosphorus bond is achieved under palladium catalysis in an oxidative Heck-type reaction which exploits arylphosphonic acids. The reaction of arylphosphonic acids with alkenes provides arylation products in good yields in the presence of TBAF with trimethylamine oxide as an oxidant. Copyright

Full text of DOI:10.1021/ja026758v

Published on Web 01/15/2003
Oxidative Heck-Type Reaction Involving Cleavage of a Carbon-Phosphorus
Bond of Arylphosphonic Acids
Atsushi Inoue, Hiroshi Shinokubo,* and Koichiro Oshima*
Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity, Kyoto 606-8501, Japan
Received May 2, 2002 ; E-mail: shino@fm1.kuic.kyoto-u.ac.jp; oshima@fm1.kuic.kyoto-u.ac.jp
Scheme 1
In recent years, cleavage of a carbon-phosphorus bond with
transition metals has been a topic in the organometallic chemistry.
Several reactions involving C-P bond fission have appeared in
the literature.1,2 However, application of such a process in organic
synthesis is not well documented. Furthermore, exploitation of
phosphonic acids in C-P bond cleavage remained unexplored.2,3
Scheme 2. Proposed Mechanism
The Heck reaction, in which aromatic halides are coupled with
alkenes under Pd(0) catalysis, is a powerful tool in organic
synthesis.4 Recently, oxidative Heck-type reactions, which exploit
arylmetals such as arylborane,5 -stannane,6 -silane,7 -bismuth,8 and
-antimony,9 have been extensively investigated.10 Herein we disclose
a Pd-catalyzed oxidative Heck-type reaction which makes use of
arylphosphonic acid as a source of an aryl group (Scheme 1). This
reaction involves cleavage of a carbon-phosphorus bond of an
arylphosphonic acid 1. To our best knowledge, this is the first report
on a C-P bond activation of arylphosphonic acid under transition
metal catalysis.
We investigated the Heck-type reaction of 4-methoxyphe-
nylphosphonic acid (1a) with styrene in the presence of a palladium
catalyst (10 mol %) in 1,4-dioxane at 100 °C (Table 1). Initially,
we found that the use of TBAF (3.0 equiv, THF solution) and Me3-
NO‚2H2O (3.0 equiv) afforded 4-methoxystilbene (2a) in 53% yield
(entry 1). Naturally, no reaction proceeded without a catalyst (entry
2). As a palladium source, Pd(OAc)2 proved to be most effective.
The use of PdCl2 or PdCl2(PPh3)2 lowered the yield (entries 3 and
4), and Pd(PPh3)4 is not effective at all (entry 5). A phosphine ligand
retards or inhibits the reaction. A choice of an oxidant is essential,
and Me3NO‚2H2O was the best among oxidants we examined
(entries 6-8). The use of TBAF is also crucial (entry 9). As the
reaction solvent, toluene is comparable to 1,4-dioxane (entry 10).
Although the reduced amount of Pd(OAc)2 (5 mol %) retarded the
reaction (entry 16), a prolonged reaction period (24 h) achieved
quantitative conversion (entry 17). After optimization, we picked
up the conditions of entry 17 as the standard. Interestingly, the use
of diethyl arylphosphonates instead of arylphosphonic acids pro-
vided none of expected products under the same reaction conditions.
With the optimized reaction conditions in hand, we examined
the reaction of a variety of arylphosphonic acids with alkenes. The
results are summarized in Table 2. Both electron-rich and -deficient
styrenes can be used as the alkene. Acrylic esters and amides also
provided good results. Phenyl-, 2-methylphenyl-, 4-methoxyphenyl-,
and 1-naphthylphosphonic acids react with alkenes to provide the
desired products in good yields (entries 1-13). Substituents at the
ortho position do not affect the reactivity (entries 6-8). An electron-
withdrawing group on the phenyl ring retards the reaction (entries
14-20), and prolonged reaction time is required to obtain satisfac-
tory yields. Bis-coupling proceeds with p-phenylenediphosphonic
acid (entries 19 and 20). Alkenylphosphonic acids such as (E)-â-
styrylphosphonic acid also react with alkenes under the same
conditions to afford conjugated dienes in good yields (entries 21
and 22).
Table 1. Optimization of the Reaction Conditionsa
oxidant
(equiv)
TBAF
(equiv)
time yield
(h)
entry
catalyst (mol %)
solvent
(%)b
1
2
3
4
5
6
7
8
Pd(OAc)2 (10)
none
PdCl2 (10)
Me3NO (3)
Me3NO (3)
Me3NO (3)
3
3
3
3
3
3
3
3
0
3
3
3
3
2
1
2
2
2
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
toluene
DMF
dioxane
dioxane
dioxane
dioxane
dioxane
2
2
2
2
2
2
2
2
2
2
2
8
8
8
8
8
53
0
45
33
0
PdCl2(PPh3)2 (10) Me3NO (3)
Pd (PPh3)4 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Me3NO (3)
none
CuCl2 (3)
NMO (3)
0
0
38
19
55
37
84
79
87
76
67
9
Me3NO (3)
Me3NO (3)
Me3NO (3)
Me3NO (3)
Me3NO (2)
Me3NO (3)
Me3NO (3)
Me3NO (3)
Me3NO (3)
Me3NO (3)
10
11
12
13
14
15
16
17
18
dioxane 24 100
toluene 24 99
a
The mixture of 4-methoxyphenylphosphonic acid (0.3 mmol), styrene
(0.36 mmol), palladium catalyst, oxidant, TBAF (1 M THF solution), and
b
solvent (3 mL) is stirred at 100 °C. NMR yields with dibenzyl ether as
an internal standard.
Formation of phosphoric acid in the reaction mixture was
confirmed with 31P NMR spectroscopy. Although the mechanism
of this reaction is not clear at this stage, we propose the following
mechanism as illustrated in Scheme 2. Due to the strong affinity
between fluoride and phosphorus,11,12 the nucleophilic attack of a
9
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J. AM. CHEM. SOC. 2003, 125, 1484-1485
10.1021/ja026758v CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Oxidative Heck-Type Reaction of Arylphosphonic Acidsa
provides the intermediate 4. An aryl transfer from phosphorus to
palladium yields arylpalladium species, which then reacts with an
alkene to afford the Heck product 2 and Pd(0) species via
â-elimination. Oxidation of Pd(0) with Me3NO regenerates Pd(II)
species.
In conclusion, we have found that the reaction of arylphosphonic
acids with a variety of alkenes provides the Heck-type adducts in
the presence of a catalytic amount of Pd(OAc)2. The reaction
requires an oxidant and TBAF as the activator and proceeds via
carbon-phosphorus bond cleavage.
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan. A.I. acknowledges JSPS
for financial support.
Supporting Information Available: General procedures (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
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a
The mixture of arylphosphonic acid (0.3 mmol), alkene (0.36 mmol),
Pd(OAc)2 (0.015 mmol), Me3NO‚2H2O (0.9 mmol), TBAF (0.6 mmol, 1
M THF solution), and 1,4-dioxane (3 mL) is stirred at 100 °C for 24 h.b
c
d
(14) An addition of TBAF (1.0 equiv) to a solution of phenylphosphonic acid
Pd(OAc)2 (0.03 mmol) is used. Reaction time is 64 h. Toluene is
employed as a solvent. Diphosphonic acid (0.15 mmol) is used.
e
in 1,4-dioxane causes an upfield shift of a signal by about 10 ppm in 31
P
NMR spectrum. However, no coupling between 31P and 19F could be
observed.
fluoride ion to an arylphosphonic acid generates a pentacoordinated
phosphorus compound 3.13,14 Ligand exchange on palladium
JA026758V
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J. AM. CHEM. SOC. VOL. 125, NO. 6, 2003 1485