Synthesis of aryl phosphines by phosphination with triphenylphosphine catalyzed by palladium on charcoal

Article abstract of DOI:10.1016/S0040-4039(01)00890-5

The palladium-catalyzed phosphination of aryl bromides and triflates by phosphination with triphenylphines to yield aryl phosphines was catalyzed by the thermally stable catalyst palladium on charcoal.

Full text of DOI:10.1016/S0040-4039(01)00890-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 4883–4885
Synthesis of aryl phosphines by phosphination with
triphenylphosphine catalyzed by palladium on charcoal
Chi Wai Lai, Fuk Yee Kwong, Yanchun Wang and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong
Received 28 March 2001; accepted 24 May 2001
Abstract—The palladium-catalyzed phosphination of aryl bromides and triflates by phosphination with triphenylphines to yield
aryl phosphines was catalyzed by the thermally stable catalyst palladium on charcoal. © 2001 Elsevier Science Ltd. All rights
reserved.
A synthesis of aryl phosphines has been recently
reported which involves the phosphination of aryl
bromides1 and triflates2 with triarylphosphines using 10
mol% of palladium catalysts such as Pd(OAc)2 and
Pd(dba)2. This method is direct and applicable to func-
tionalized substrates using air-stable and economical
starting materials.3 However, the use of these homoge-
neous palladium catalysts suffers from frequent decom-
position at elevated temperatures in the course of the
reactions to form catalytically inactive palladium black
precipitates. No further catalysis proceeds. Since het-
erogeneous palladium catalysts4 including palladium on
glass,5 charcoal and even carbon nanofiber6 have been
extensively employed as efficient catalysts in catalysis as
well as catalytic hydrogenation, we would like to
employ these heterogeneous catalysts in phosphination
to improve the efficiency of the process. Palladium on
charcoal is particularly attractive since it is commer-
cially available and has been extensively applied in
homo-,7 Sonogashira,8 Stille,9 Suzuki,10 and Negishi11
cross-couplings as well as in Heck reactions12 as an
air-stable and easily removable catalyst with compara-
ble activity with that of Pd(OAc)2 and Pd(Ph3P)4. An
added advantage is the easy removal of palladium after
the reaction by its simple filtration with Celite and the
possibility of its re-use. We now report the use of the
Pd/C catalyst in the successful phosphination of aryl
bromides and triflates with triphenylphosphine by
virtue of its large surface area and thermal stability.
and/or yield of the reaction while less than 1 mol%
increased the reaction time significantly and lowered the
yield slightly. The use of a higher reaction temperature
of 160°C allowed a lower catalyst loading of just 1%
making this process more attractive compared to the 10
mol% of Pd(OAc)2 catalyst necessary at 110°C.1,2
Table 2 shows the successful phosphination of function-
alized aryl bromides, triflates and nonaflate using the
optimized conditions. In general, aryl triflates reacted
faster than aryl bromides.1,2b In some cases (entries
14–17), only the aryl triflates reacted. A variety of
functional groups such as ketone, aldehyde, ester, pyri-
dine and cyanide were compatible with the reaction
without the need of a protective group as neutral
reaction media were employed.1,2 Electron-deficient
substrates (entries 1, 2, 6 and 7) gave higher yields than
electron-rich substrates (entries 8 and 9). Presumably,
the diphenylarylphosphine products formed from elec-
tron-rich substrates may undergo rapid further phos-
phination.2b This is consistent with the observation that
Table 1. Effect of Pd/C catalyst loading in phosphination
O
O
2.5 eq. PPh3, DMF
160-165 oC
Br
PPh2
Entry
Pd/C (mol%)
Time (h)
Yield (%)
1
2
3
4
5
6
10
5
1
0.5
0.2
0.1
30
32
32
96
168
288
45
44
46
39
37
28
Table 1 shows the effect of Pd/C catalyst loading on the
phosphination of 4-bromoacetophenone with 2.5 equiv.
of Ph3P in DMF at 160°C.1,2 Amounts of Pd/C catalyst
from 1 to 10 mol% had little difference on the rate
* Corresponding author. E-mail: ksc@cuhk.edu.hk
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00890-5

4884
C. W. Lai et al. / Tetrahedron Letters 42 (2001) 4883–4885
Table 2. Palladium-catalyzed phosphination of aryl bromides, triflates and nonaflate with triphenylphosphine
2.5 eq. PPh3
X
PPh2
1 mol% Pd/C
160-165 oC
Fn
Fn
X = Br, OTf, ONf
Entry
Substrate
X
Product
Time (h) Yield (%)
O
O
1
2
X = Br
X = OTf
32
20
46
39
X
X
PPh2
PPh2
O
No reaction after 9 days
O
3
4
5
X = Br
X = OTf
X = ONf
42
12
20
35
MeO
MeO
6
7
X = Br
X = OTf
73
47
53
44
NC
X
X
X
X
NC
PPh2
PPh2
PPh2
PPh2
37
15
33
29
8
9
X = Br
X = OTf
MeO
OHC
MeO
OHC
10
11
X = Br
X = OTf
75
46
31
20
12
13
X = Br
X = OTf
52
20
35
30
MeO
MeO
14
15
X = Br
X = OTf
No reaction after 5 days
X
OMe
X
PPh2
26
24
OMe
PPh2
16
17
X = Br
X = OTf
No reaction after 10 days
168
16
CN
X
CN
PPh2
N
N
18
X = OTf
29
33
the electron-rich MeO-substituted substrates (entries 8
and 9) reacted faster than electron-poor ones. For
sterically hindered 2-substituted aryl bromides and tri-
flates, the phosphination required longer reaction times
and lower yields were obtained.
ligated by phosphines catalyzes the reaction through
the formation of phosphonium salts,14 arylꢀPd/arylꢀP
exchange and CꢀP activation.15,16
In conclusion, a variety of functionalized phosphines
were prepared by the operationally simple phosphina-
tion of aryl bromides and triflates using triphenylphos-
phine and a Pd/C catalyst.
Preliminary experiments showed that Pd/C could be
used twice without diminishing its activity by loading
the catalyst in a thimble. During the third run, no
catalysis occurred. Presumably, the palladium species
which dissolved in the course of reaction did not re-
deposit on charcoal after the reaction13 in a sufficient
amount, i.e. the leaching rate of the palladium was too
fast to allow multiple re-use.
Experimental: In a typical experiment, 4-bromoace-
tophenone (199 mg, 1 mmol), 10% palladium on char-
coal (10 mg, 0.01 mmol) and triphenylphosphine (655
mg, 2.5 mmol) were dissolved in DMF (4 ml) in a
Teflon screw-capped flask under nitrogen. The reaction
mixture was heated to 160°C for 32 h. The reaction was
cooled down and dissolved in a minimal amount of
dichloromethane, and the product was purified by
column chromatography on silica gel using hexane/
The mechanism of the phosphination is likely to be
similar to our suggested mechanism1,2 in which palla-
dium, either on the charcoal surface, or in solution,

C. W. Lai et al. / Tetrahedron Letters 42 (2001) 4883–4885
4885
ethyl acetate (15/1) as the eluent to obtain the 4-
(diphenylphosphino)acetophenone in 46% yield as a
white solid.
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Acknowledgements
The authors would like to thank the Direct Grant of
the Chinese University of Hong Kong for financial
support.
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