P-ligand-free, microwave-assisted variation of the Hirao reaction under solvent-free conditions; The P-C coupling reaction of >p(O)H species and bromoarenes

Article abstract of DOI:10.1016/j.tetlet.2013.05.111

Under microwave conditions, the P-C coupling of aryl bromides and >P(O)H species, such as dialkyl phosphites, alkyl aryl-H-phosphinates, or diphenylphosphine oxide takes place in the presence of Pd(OAc)₂ as the catalyst and triethylamine as the base without using any P-ligand or solvent. The method described is green and is of more general value.

Full text of DOI:10.1016/j.tetlet.2013.05.111

Accepted Manuscript
P-Ligand-free, microwave-assisted variation of the Hirao reaction under sol‐
vent-free conditions; the P–C coupling reaction of >P(O)H species and bro‐
moarenes
Erzsébet Jablonkai, György Keglevich
PII:
S0040-4039(13)00890-3
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.05.111
TETL 43011
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
22 April 2013
14 May 2013
24 May 2013
Please cite this article as: Jablonkai, E., Keglevich, G., P-Ligand-free, microwave-assisted variation of the Hirao
reaction under solvent-free conditions; the P–C coupling reaction of >P(O)H species and bromoarenes, Tetrahedron
Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.2013.05.111
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P-Ligand-free, microwave-assisted variation
of the Hirao reaction under solvent-free
conditions; the P–C coupling reaction of
>P(O)H species and bromoarenes
Leave this area blank for abstract info.
Erzsébet Jablonkai, György Keglevich*
O
P
MW
150−200 °C
O
P
Y1 OEt OBu OEt Ph
Y2 OEt OBu Ph Ph
Y1
Y2
Y1
Y2
+
ArBr
H
Ar
Pd(OAc)2
Et3N
solvent-free
Ar = Ph, 4-MeC6H4, 4-ClC6H4, 4-FC6H4, 4-EtO2CC6H4, 4-MeC(O)C6H4

1
Tetrahedron Letters
journal homepage: www.elsevier.com
P-Ligand-free, microwave-assisted variation of the Hirao reaction under solvent-free
conditions; the P–C coupling reaction of >P(O)H species and bromoarenes
Erzsébet Jablonkai, György Keglevich∗
Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, 1521 Budapest, Hungary
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Under microwave conditions, the P–C coupling of aryl bromides and >P(O)H species, such as
dialkyl phosphites, alkyl aryl-H-phosphinates or diphenylphosphine oxide takes place in the
presence of Pd(OAc)2 as the catalyst and triethylamine as the base without using any P-ligand or
solvent. The method described is green and is of more general value.
Available online
2013 Elsevier Ltd. All rights reserved.
Keywords:
P–C coupling
Hirao reaction
P-ligand-free
Microwave
Dialkyl phosphite
Aryl phosphonate
The synthesis of arylphosphonates and related compounds has
attracted significant attention. Perhaps the most suitable method
for their preparation is the Hirao reaction involving a P–C
coupling between aryl halides and H-phosphonate diesters
(dialkyl phosphites) in the presence of Pd(PPh3)4 as the catalyst,
and triethylamine as the base in suitable solvents.1 Many
applications and variations of the Hirao reaction have been
described.2 In one variation, the P–C coupling was promoted by
microwave (MW) irradiation, but this method did not bring a
breakthrough as Pd(PPh3)4 and THF had to be used.3 As in
another case,4 MW irradiation was only useful in accelerating the
P–C coupling. It was a turning point when attempts were made to
replace Pd(PPh3)4 with Pd(dba)2 (dba = dibenzylideneacetone),
Pd(OAc)2 or PdCl2. The most efficient catalytic system was
generated from Pd(OAc)2 in the presence of triphenylphosphine
systems were found to work well in P–C couplings; even
chloroarenes could be phosphinylated.7b In the coupling of
secondary phosphine oxides with iodobenzene, the in situ formed
Pd-catalyst was so active that the reaction took place at room
temperature.8 In another variation, Pd(OAc)2 was used together
with triphenylphosphine, but the base employed was K2CO3 and
the
solid-liquid
reaction
was
promoted
by
triethylbenzylammonium chloride as the phase transfer catalyst.9
It was observed that the phosphinylation of aryl iodides also took
place in the presence of “phosphine-free Pd”, but aryl bromides
underwent the coupling reaction only in the presence of
triphenylphosphine.9 An example of the reaction of halogenated
benzoic acids with diphenylphosphine oxide is noteworthy for
three reasons: the coupling took place in the presence of Pd/C (in
the absence of any phosphine), in water as the solvent, and under
MW irradiation.10
as
the
P-ligand.5
Instead
of
triethylamine,
dicyclohexylmethylamine could also be applied as a suitable
base.6 Also, there have been attempts to use bidentate P-ligands,
such as dppf [1,1'-bis(diphenylphosphino)ferrocene], Xantphos
[4,5-bis(diphenylphosphino)-9,9-dimethylxanthene], dppp [1,3-
We wanted to study the P–C coupling between aryl bromides
and >P(O)H species in the presence of Pd(OAc)2 as the catalyst
and 1.1 equivalents of triethylamine under solvent-free MW
conditions, without the application of any P-ligand. The first
model system investigated was the MW-assisted reaction of
bromobenzene with 1.5 equivalents of diethyl phosphite (1a).11 In
the absence of Pd(OAc)2 or Et3N there was no reaction at 120 °C
(Table 1, entries 1 and 2). However, in the presence of 5% of
bis(diphenylphosphino)propane],
bis(diphenylphosphino)butane]
dppb
BINAP
[1,4-
[2,2'-
and
bis(diphenylphosphino)-1,1'-binaphthyl] together with Pd(OAc)2,
instead of triphenylphosphine, to form in situ catalysts.7 These
———
∗ Corresponding author. Tel.: +36 1 463 1111/5883; fax: +36 1 463 3648; E-mail address: gkeglevich@mail.bme.hu (G. Keglevich).

2
Tetrahedron Letters
Pd(OAc)2 and 1.1 equivalents of Et3N, the reaction took place
complete conversion (94%) required an irradiation time of 1.5
hours (Table 1, entry 15). The comparative thermal variation was
not so efficient (Table 1, entry 16) and to achieve a better
conversion, a heating for 3 hours was required (Table 1, entry
17). At 150 °C, conversions of 77% and 91% could be attained
after reaction times of 5 and 20 minutes, respectively (Table 1,
entries 18 and 19). The comparative thermal experiment led to a
conversion of only 76% after 20 minutes (Table 1, entry 20).
Ethyl phenyl-H-phosphinate (1c) was also tested in the coupling
reaction with bromobenzene. Comparable results were obtained
at 120 °C/ 0.5 hour and at 150 °C/ 5 minutes, the conversions
being 90% and 94%, respectively (Table 1, entries 21 and 22).
Finally, the experiments with diphenylphosphine oxide (1d)
demonstrated its higher reactivity, as at 120 °C, the coupling
reaction was complete after 15 minutes (Table 1, entry 23), and at
150 °C within 5 minutes (Table 1, entry 24).
within 0.5 hours and with a conversion of 97% (Table 1, entry 3).
Conventional heating led, under the same conditions, to a
conversion of only 52% (Table 1, entry 4). A longer reaction
time of 1.5 hours was useful in obtaining a conversion of 95%
(Table 1, entry 5). We also investigated the effect of the reaction
temperature. At 150 °C, there was no reaction without Pd(OAc)2
(Table 1, entries 6 and 7). In the presence of 5% Pd(OAc)2, and
in the absence of Et3N a conversion of only 16% was attained
after 6 hours (Table 1, entry 8). The use of 1.1 equivalent of Et3N
led again to a dramatic change, as the coupling was almost
quantitative after 5 minutes (Table 1, entry 9). Comparative
thermal experiments led to similar outcomes as those at 120 °C
(Table 1, entries 10 and 11). Decreasing the amount of the
catalyst to 3 mol%, the coupling was somewhat less efficient
than in the presence of 5 mol% (Table 1, entry 12). The thermal
experiment was not complete when using only 3 mol% of the
catalyst, even after prolonged heating (Table 1, entry 13). In the
next experiments, dibutyl phosphite (1b) was used as the P-
reactant.
Hence, it was evident that the P–C coupling reaction of
bromobenzene with a series of >P(O)H reactants could be
accomplished under solvent-free MW conditions in the presence
of a P-ligand-free Pd(OAc)2 catalyst and 1.1 equivalents of Et3N.
The reactions were not so efficient under traditional thermal
conditions.
Species 1b was of lower reactivity then diethyl phosphite, as
demonstrated by the fact that after reaction at 120 °C for 0.5
hours, the conversion was only 57% (Table 1, entry 14). More
Table 1
Palladium-catalyzed coupling of >P(O)H species with bromobenzene
O
P
O
MW/Δ
Y1
OEt OBu
OEt Ph
Y2 OEt OBu Ph Ph
Temp., time
P
Y1
Y2
Y1
Y2
+
PhBr
H
Ph
Pd(OAc)2
Et3N (1.1 equiv.)
solvent-free
a
b
c
d
2
1
n equiv.
Entry
1
2
3
4
5
6
7
8
Y1Y2P(O)H
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1c
1c
1d
1d
n
Pd(OAc)2 (%) Et3N (1.1 equiv.)
Temp. (°C) Time
Mode of heating
Conversiona,b (%)
−
−
97
52
95
−
Yield (%)
−
−
90 (2a)
41 (2a)
85 (2a)
−
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1
−
5
5
5
5
−
−
5
5
5
5
3
3
5
5
5
5
5
5
5
5
5
5
5
+
−
+
+
+
−
+
−
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
120
120
120
120
120
150
150
150
150
150
150
150
150
120
120
120
120
150
150
150
120
150
120
150
6 h
6 h
0.5 h
0.5 h
1.5 h
6 h
MW
MW
MW
Δ
Δ
MW
MW
MW
MW
Δ
6 h
6 h
−
−
16
99
47
97
93
71
57
94
48
90
77
91
76
90
94
98
100
10 (2a)
93 (2a)
27 (2a)
88 (2a)
84 (2a)
65 (2a)
44 (2b)
84 (2b)
38 (2b)
86 (2b)
69 (2b)
77 (2b)
68 (2b)
77 (2c)
85 (2c)
90 (2d)
88 (2d)
9
5 min
5 min
15 min
5 min
45 min
0.5 h
1.5 h
1.5 h
3 h
5 min
20 min
20 min
0.5 h
5 min
15 min
<5 min
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Δ
MW
Δ
MW
MW
Δ
Δ
MW
MW
Δ
MW
MW
MW
MW
1
1
1
a On the basis of GC analysis.
b Average from 3 parallel reactions.

3
Table 2
Palladium-catalyzed coupling of different aryl bromides with diethyl phosphite
O
Br
P(OEt)2
MW
Temp., time
O
P
+
EtO
EtO
Y= Me (a), Cl (b), F (c),
CO2Et (d), C(O)Me (e)
H
Pd(OAc)2
Et3N (1.1 equiv.)
solvent-free
Y
1.5 equiv.
Y
3
4
Entry
1
2
3
4
5
6
7
8
ArBr
3a
3a
3a
3a
3a
3b
3b
3b
3b
3b
3b
3b
3c
Pd(OAc)2 (%)
5
10
5
10
5
Temp. (°C)
150
150
175
175
200
150
150
150
175
175
175
200
150
175
150
150
175
150
150
175
Time (min)
5
5
5
10
2
5
10
5
5
5
10
2
5
5
15
15
15
5
5
5
Conversiona,b (%)
Yield (%)
49 (4a)
55 (4a)
52 (4a)
73 (4a)
62 (4a)
45 (4b)
51 (4b)
62 (4b)
50 (4b)
71 (4b)
83 (4b)
95 (4b)
85 (4c)
91 (4c)
50 (4d)
82 (4d)
89 (4d)
41 (4e)
78 (4e)
87 (4e)
62c
69c
68c
86c
74
5
5
52
58c
74c
59c
80
10
5
10
10
10
5
5
5
10
5
5
9
10
11
12
13
14
15
16
17
18
19
20
95
100
92
3c
99
3d
3d
3d
3e
3e
3e
63c
95
100
61c
90
10
5
100
a On the basis of GC analysis.
b Average from 3 parallel reactions.
c No change on further irradiation.
We next studied the P–C coupling reactions of 4-substituted
bromobenzenes (3a-e) with diethyl phosphite under similar
conditions to those applied above (Table 2).12 It was shown that
the MW-assisted reaction of bromobenzene with diethyl
phosphite at 150 °C was complete after 5 minutes in the presence
of 5 mol% of Pd(OAc)2 and 1.1 equivalent of Et3N (Table 1,
entry 9). Under similar conditions, the reaction of 4-
bromotoluene (3a) with diethyl phosphite gave diethyl 4-
methylphenylphosphonate (4a) in a conversion of only 62%
(Table 2, entry 1). It was not possible to attain much better
conversions by using 10 mol% Pd(OAc)2 or by increasing the
temperature to 175 °C (Table 2, entries 2 and 3). However, in the
presence of 10 mol% of the catalyst at a temperature of 175 °C
and after a 10 minutes reaction time, the conversion was 86%
(Table 2, entry 4). Carrying out the P–C coupling at 200 °C for 2
minutes, product 4a was obtained in a conversion of 74% (Table
2, entry 5). 4-Bromotoluene (3a) was obviously a less reactive
starting material in the studied reaction. The outcomes with 4-
chlorobromobenzene (3b) were quite similar: the couplings were
incomplete at 150 °C under MW irradiation (Table 2, entries 6–
8). Increasing the temperature to 175 °C did not help much
(Table 2, entry 9), but also increasing the quantity of Pd(OAc)2 to
10 mol%, the conversion was 80% (Table 2, entry 10). Better
results were obtained after a reaction time of 10 minutes (Table
2, entries 11). At 200 °C, a quantitative reaction took place after
2 minutes without any decomposition (Table 2, entry 12). Diethyl
4-chlorophenylphosphonate (4b) was isolated in a yield of 95%.
4-Fluorobromobenzene (3c) was much more reactive than 4-
chlorobromobenzene (3b). After a reaction at 150 °C in the
presence of 5 mol% of Pd(OAc)2 for 5 minutes, diethyl 4-
fluorophenylphosphonate (4c) was formed in a conversion of
92% (Table 2, entry 13). Elevating the temperature to 150 °C, the
coupling was almost quantitative and product 4c was isolated in a
yield of 91% (Table 2, entry 14). Ethyl 4-bromobenzoate (3d)
displayed somewhat decreased reactivity. At 150 °C, an increase
in the amount of catalyst from 5 mol% to 10 mol% had a
significant impact on the outcome, as the conversion increased
from 63% to 95% (Table 2, entries 15 and 16). However, at 175
°C, complete reaction occurred using only 5 mol% of the catalyst
(Table 2, entry 17). Arylphosphonate 4d was isolated in a yield
of 89%. Finally, the results of the experiments with 4-
bromoacetophenone (3e) suggested that this model compound
was more reactive than 3d. Although at 150 °C the coupling was
efficient only in the presence of 10 mol% of the catalyst (Table 2,
entries 18 and 19), at 175 °C, quantitative conversion took place
in the presence of 5 mol% of the catalyst (Table 2, entry 20).
Arylphosphonate 4e was prepared in a yield of 87%.
The above experiments show that 4-substituted aryl bromides
can undergo P–C coupling with diethyl phosphite under MW and
solvent-free conditions in the presence of Pd(OAc)2 as the
catalyst, and Et3N as the base. It can be seen that bromobenzene
and 4-fluorobromobenzene are the best model compounds. Both
electron-donating and electron-withdrawing substituents at
position 4 decrease the reactivity. The reason for this observation
and the mechanism of the coupling reaction will be the subject of
a future detailed study.
Finally, we investigated the coupling of bromobenzene with
dibenzo[c,e][1,2]oxaphosphinine oxide (5) under MW conditions
using ligand-free Pd(OAc)2.13 At 175 °C, using 10 mol% catalyst,
Et3N as the base and acetonitrile as the solvent, 2-phenyl-
dibenzooxaphosphinine oxide 6 was obtained in a yield of 84%
(Scheme 1). In another variation, 5 mol% catalyst was used at
150 °C, again in acetonitrile, but with 1 equivalent of K2CO3 as
the base to afford the coupled product 6 in a yield of 91%
(Scheme 1).

4
Tetrahedron
MW
175 °C/15 min
Pd(OAc)2 (10%)
Et3N (1.1 equiv.)
MeCN
Zhu, J. Synthesis 1987, 726. (l) Xu, Y.; Li, Z.; Xia, J.; Guo, H.;
Huang, Y. Synthesis 1984, 781.
3. Kalek, M.; Ziadi, A.; Stawinski, J. Org. Lett. 2008, 10, 4637.
4. Villemin, D.; Jaffres, P.-A.; Simeon, F. Phosphorus, Sulfur,
Silicon 1997, 130, 59.
5. Kalek, M.; Stawinski, J. Organometallics 2007, 26, 5840.
6. (a) Gooßen, L. J.; Dezfuli; M. K. Synlett 2005, 445. (b)
Bessmertnykh, A.; Douaihy, C. M.; Guilard, R. Chem. Lett. 2009,
38, 738.
7. (a) Belabassi, Y.; Alzghari, S.; Montchamp, J.-L. J. Organomet.
Chem. 2008, 693, 3171. (b) Deal, E. L.; Petit, C.; Montchamp, J.-
L. Org. Lett. 2011, 13, 3270. (c) Kalek, M.; Jezowska, M.;
Stawinski, J. Adv. Synth. Catal. 2009, 351, 3207.
or
+
PhBr
O
H
O
MW
P
P
150 °C/5 min
Pd(OAc)2 (5%)
K2CO3 (1.1 equiv.)
MeCN
O
O
Ph
5
6
1 equiv.
84−91%
8. Bloomfield, A. J.; Herzon, S. B. Org. Lett. 2012, 14, 4370.
9. (a) Kabachnik, M. M.; Solntseva, M. D.; Izmer, V. V., Novikova,
Z. S.; Beletskaya, I. P. Russ. J. Org. Chem. 1998, 34, 93. (b)
Beletskaya, I. P.; Karlstedt, N. B.; Nifant’ev, E. E.; Khodarev, D.
V.; Kukhareva, T. S.; Nikolaev, A. V.; Ross, A. J. Russ. J. Org.
Chem. 2006, 42, 1780. (c) Novikova, Z. S.; Demik, N. N.;
Agarkov, A. Yu.; Beletskaya, I. P. Russ. J. Org. Chem. 1995, 31,
129. (d) Beletskaya, I. P.; Kazankova, M. A. Russ. J. Org Chem.
2002, 38, 1391.
Scheme 1. The coupling reaction of bromobenzene and
dibenzo[c,e][1,2]oxaphosphinine oxide 5.
In all cases, the corresponding products 2a-d, 4a-e and 6 were
isolated by flash column chromatography. The yields were in
accord with the conversions. The best experiments with respect
to the conversion are marked by entries 9 (2a), 15 (2b), 22 (2c)
and 24 (2d) in Table 1 and by entries 4 (4a), 12 (4b), 14 (4c), 17
(4d) and 20 (4e) in Table 2. The products 2a-d, 4a-e and 6 were
10. Rummelt, S. M.; Ranocchiari, M.; van Bokhoven, J. A. Org. Lett.
2012, 8, 2188.
1
characterized by 31P, 13C and H NMR spectral data, as well as
11. General procedure for the reaction of bromobenzene and
>P(O)H species 1a-d: To 0.21 ml (2.0 mmol) of bromobenzene
was added 3.0 mmol of dialkyl phosphite [0.39 ml of diethyl
phosphite (1a) or 0.59 ml of dibutyl phosphite (1b)], or 2.0 mmol
of ethyl phenylphosphinate (1c) (0.30 ml) or diphenylphosphine
oxide (1d) (0.40 g), 0.31 ml (2.2 mmol) of Et3N and 0.022 g (0.10
mmol) [or 0.014 g (0.06 mmol)] of Pd(OAc)2, and the resulting
mixture was irradiated in a closed vial in a CEM Discover (300
W) microwave reactor at the temperature and for the time shown
in Table 1. The mixture was passed through a thin (ca. 1–1.5 cm)
layer of silica gel using EtOAc. Volatile components were
removed in vacuo to give the products as oils (2a-c), or as crystals
(2d). Mp for 2d: 156 °C, mp14d: 156.6–157.4 °C.
MS data, and the NMR data of the products were compared with
those described in the literature.14 Some of the compounds were
prepared by other approaches, but only products 2d, 4a, 4b, 4e
and 6 were synthesized by the Hirao reaction. In these cases, with
one exception (4b), the yields were comparable with those
obtained by us. Compound 4b was previously isolated in a yield
of only 44%.14a
In conclusion, the first P-ligand-free variant of the Hirao
reaction has been described in this report. The reaction is of
general value and environmentally friendly.
12. General procedure for the reaction of substituted
bromobenzenes 4a-d and diethyl phosphite: To 2.0 mmol of 4-
substituted bromobenzene [0.34 g of 4-bromotoluene (3a), 0.38 g
of 4-bromochlorobenzene (3b), 0.22 ml of 4-fluorobenzene (3c),
Acknowledgments
The above project was supported by the Hungarian Scientific
and Research Fund (OTKA K83118). GK thanks Professor Dr
Harry R. Hudson (London Metropolitan University, London,
UK) for useful discussions.
0.32 ml of ethyl 4-bromobenzoate (3d) or 0.40
g of 4-
bromoacetophenone (3e)] was added 0.39 ml (3.0 mmol) of
diethyl phosphite, 0.31 ml (2.2 mmol) of Et3N and 0.022 g (0.10
mmol) or [0.044 g (0.20 mmol)] of Pd(OAc)2, and the resulting
mixture was irradiated in a closed vial in the MW reactor outlined
above at the temperature and for the time shown in Table 2. The
work-up was similar as in the previous case to give products 4a-e
as oils.
Supplementary data
NMR spectral and HR-MS data for all compounds associated
with this article can be found in the online version, at
http://............
13. 2-Phenyl-dibenzo[c,e][1,2]oxaphosphinine oxide (6): To 0.11
ml (1.0 mmol) of bromobenzene was added 0.22 g (1.0 mmol) of
dibenzooxaphosphinine oxide (5), 1.1 mmol of base (1: 0.15 ml of
Et3N, or 2: 0.15 g of K2CO3), Pd(OAc)2 [1: 0.022 g (0.10 mmol),
or 2: 0.011 g (0.05 mmol)] and 1 ml of MeCN, and the resulting
mixture was irradiated in a closed vial in the MW reactor at 1: 175
°C/15 min, or 2: 150 °C/5 min. The work-up was similar as above
to give product 6 as crystals (1: 84%, 2: 91%), mp: 160–161 °C,
mp14h: 156–158 °C.
References and notes
1. (a) Hirao, T.; Masunaga, T.; Ohshiro, Y.; Agawa, T. Tetrahedron
Lett. 1980, 21, 3595. (b) Hirao, T.; Masunaga, T.; Ohshiro, Y.;
Agawa, T. Synthesis 1981, 56. (c) Hirao, T;. Masunaga, T.;
Yamada, N.; Ohshiro, Y.; Agawa, T. Bull. Chem. Soc. Jpn. 1982,
55, 909. (d) Prim, D.; Campagne, J.-M.; Joseph, D.; Andrioletti,
B. Tetrahedron 2002, 58, 2041. (e) Tappe, F. M. J.; Trepohl, V.
T.; Oestreich, M. Synthesis 2010, 3037.
2. (a) Machnitzki, P.; Nickel, T.; Stelzer, O.; Landgrafe, C. Eur. J.
Inorg. Chem. 1998, 1029. (b) Johansson, T.; Stawinski, J. Chem.
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