Phosphonation of arenes with dialkyl phosphites catalyzed by Mn(II)/Co(II)/O2 redox couple

Article abstract of DOI:10.1021/ol052406s

Arylphosphonates were first synthesized through a catalytic phosphonation of various arenes with dialkyl phosphites under the influence of an Mn(OAc) ₂/Co(OAc)₂/O₂ redox couple. For instance, the reaction of benzene with d

Full text of DOI:10.1021/ol052406s

ORGANIC
LETTERS
2
006
Vol. 8, No. 3
07-409
Phosphonation of Arenes with Dialkyl
Phosphites Catalyzed by Mn(II)/Co(II)/O2
Redox Couple
4
Takashi Kagayama, Atsushi Nakano, Satoshi Sakaguchi, and Yasutaka Ishii*
Department of Applied Chemistry, Faculty of Engineering, Kansai UniVersity, Suita,
Osaka 564-8680, Japan
ishii@ipcku.kansai-u.ac.jp
Received October 4, 2005
ABSTRACT
Arylphosphonates were first synthesized through a catalytic phosphonation of various arenes with dialkyl phosphites under the influence of
an Mn(OAc) /Co(OAc) /O redox couple. For instance, the reaction of benzene with diethyl phosphite in the presence of Mn(OAc) (5 mol %)
and Co(OAc) (1 mol %) under a mixed gas of O (0.5 atm) and N (0.5 atm) at 45 C led to diethyl phenylphosphonate in 81% selectivity at
2% conversion. This is the first successful phosphonation of benzene with dialkyl phosphites through a catalytic radical process.
2
2
2
2
2
2
2
°
6
4 2 3 6 2 2 8 3
the influence of (NH ) [Ce(NO ) S O and AgNO
]6 or Na .7
In recent years, we have developed a novel catalytic method
Dialkyl arylphosphonates are important intermediates in the
synthesis of pesticides and biologically active compounds.
1
There have been several methods for the synthesis of dialkyl
for the addition of diethyl phosphite to alkenes by using Mn-
8
arylphosphonates. Among them, the Michaelis-Arbuzov
(II)/Co(II)/O
2
redox system. In this reaction, it is thought
2
reaction is frequently used for this purpose. The photolysis
that diethyl phosphate undergoes one-electron oxidation by
Mn(III) generated in situ from Mn(II) by the action of Co-
of iodobenzenes in the presence of trialkyl phosphates is
3
2
(II) and O to form phosphonyl radicals which then add to
reported to lead to arylphosphonates in good yields. Jason
alkenes leading to diethyl alkyphosphonates.
reported free-radical phosphonation of naphthalene with
diethyl phosphate using tert-butyl peroxide as a radical
In continuation of this study, we have now developed a
catalytic phosphonation of benzenes with dialkyl phosphites
4
initiator. Masui reported the formation of dialkyl aryl-
5
phosphonates by anodic oxidation. Chemical phosphona-
such as HP(O)(OEt)
2 2
by an Mn(II)/Co(II)/O redox system
tion of arenes with diethyl phosphite has been made under
under mild conditions (eq 1).
(1) (a) Organic Phosphorus Compounds; Kosolapoff, G. M., Maier, L.,
Eds.; Wiley-Interscience: New York, 1976. (b) Corbridge, D. E. C.
Phosphorus: An Outline of Its Chemistry, Biochemistry and Uses, 5th ed.;
Elsevier: Amsterdam, 1995. (c) Swaminathan, S.; Narayanan, K. V. Chem.
ReV. 1971, 71, 429. (d) Bhattacharya, A.; K.; Thyagarajan, G. Chem. ReV.
1
981, 81, 415.
2) (a) Michaelis, A.; Kaehne, R. Ber. 1898, 31, 1048. (b) Arbuzov, A.
(
E. J. Russ. Phys. Chem. Soc. 1906, 38, 687. (c) Gellespie, P.; Ramierz, F.;
Ugi, I.; Marquarding, D. Angew. Chem., Int. Ed. Engl. 1973, 12, 91. (d)
Burton, D. J.; Flymn, R. M. Synthesis 1979, 615. (e) Harwood: L. M.;
Julia, M. Synthesis 1980, 456. (f) Battagia, S.; Vyle, S. Tetrahedron Lett.
2
003, 44, 861.
3) (a) Obrycki, R.; Griffin, C. E. J. Org. Chem. 1968, 33, 632. (b)
Bunnet, J. F.; Creary, X. J. Org. Chem. 1974, 39, 3612.
(
(6) Kottman, H.; Skarzewski, J.; Effenberger, F. Synthesis 1987,
797.
(
4) Jason, E. F.; Fields, E. K. J. Org. Chem. 1962, 27, 1402.
(7) Effenberger, F.; Kottmann, H. Tetrahedron 1985, 41, 4171.
(8) Tayama, O.; Nakano, A.; Iwahama, T.; Sakaguchi, S. Ishii, Y. J.
Org. Chem. 2004, 69, 5494.
(5) Ohmori, H.; Nakai, S.; Masui, M. J. Chem. Soc., Perkin Trans. 1
1
979, 2024.
1
0.1021/ol052406s CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/13/2006

To confirm an optimum reaction condition for the phos-
phonation of arenes with phosphites, the reaction of benzene
In the present reaction, we observed a similar acceleration
effect of AcOK on the phosphonation of 1a with 2a. Thus,
when AcOK was added to the reaction system, the amounts
(1a) with diethyl phosphite (2a) was carried out under various
conditions (Table 1).
2 2
of Mn(OAc) and Co(OAc) could be reduced to 0.5 and
0
.1 mol %, respectively (runs 9 and 10). Surprisingly, the
reaction was found to be induced even at room temperature
by adding AcOK to this system to give 3aa in 92%
selectivity, although the conversion was low (25%) (run 10).
When 2,6-di-tert-butyl-4-methylphenol (BHT) (0.1 mmol,
Table 1. Phosphonation of Benzene (1a) with Diethyl
Phosphite (2a) by Mn(OAc) /Co(OAc) /O System under
2 2 2
Various Conditionsa
5
mol %) was added to the reaction system, no reaction took
conv
(%)
selectivityb
(%)
place at all (run 11). This fact indicates that the present
phosphonation proceeds through a radical process. Like 2a,
dimethyl phosphite (2b) reacted also with 1a to form
dimethyl phenylphosphonate (3ab) in 87% selectivity at 68%
conversion of 1a, but diisopropyl phosphite (2c) was less
reactive than 2a to give diisopropyl phenylphosphonate (3ac)
in 83% selectivity at 30% conversion of 1a (runs 12 and
metal (mmol)
run Mn(OAc)2 Co(OAc)2 T (°C) time (h) 1a 2a 3aa 4aa
1
2
3
4
5
6
7
8
9
0
1
2
3
0.1
0.1
-
0.02
-
0.1
45
45
45
45
45
45
60
25
45
25
45
45
45
5
5
5
5
5
5
5
5
5
5
5
15
15
62 59 81
18 48 62
9
n.d.
7
1
7
9
n.d. n.d.
trace n.d.
c
0.1
0.1
0.1
0.1
0.1
0.01
0.01
0.1
0.1
0.1
0.02
0.02
0.02
0.02
0.02
0.002
0.002
0.02
0.02
0.02
d
e
38 33 89
80 79 41
69 74 80
trace
6
8
n.d.
5
n.d.
1
3).
On the basis of these results, a variety of substituted
benzenes were allowed to react with 2a under these condi-
tions (Table 2).
6
22 33
f
55 46 85
25 25 92
NR
68 66 87
30 61 83
f
1
1
1
1
g
h
i
8
n.d.
Table 2. Phosphonation of Substituted Benzenes with Diethyl
a
2 2 2
Phosphate (2a) by Mn(OAc) /Co(OAc) /O
a
1a (2 mmol) was reacted with 2a (6 mmol) in the presence of Mn(OAc)2
and Co(OAc)2 in EtCO2H (1 mL) under O2 (0.5 atm)/N2 (0.5 atm) at 25 or
b
c
d
4
5 °C for 5 h. Based on 1a reacted. Under Ar (1 atm). Under air (1
e
f
g
atm). Under O2 (1 atm). AcOK (1 mmol) was added. 2,6-Di-tert-butyl-
-methylphenol (BHT) (0.1 mmol) was added. HP(O)(MeO)2 (2b) was
used, and 3ab and 4ab were obtained. HP(O)( PrO)2 (2c) was used, and
h
4
i
i
3
ac was obtained.
The reaction of 1a (2 mmol) with 2a (6 mmol) in the
presence of Mn(OAc)
under O (0.5 atm) and N
C for 5 h gave diethyl phenylphosphonate (3aa) in 81%
selectivity and a small amount of disubstituted product 4aa
9%), which was formed by the further reaction of the
resulting 3aa with 2a at 62% conversion of 1a (run 1). The
yield of 3aa was considerably decreased when Co(OAc)
was removed from the catalytic system (run 2), while no
phosphonation took place in the absence of Mn(OAc) (run
). The same reaction under Ar (1 atm) resulted in recovery
2
(5 mol %) and Co(OAc)
2
(1 mol %)
2
2
(0.5 atm) in propionic acid at 45
°
(
2
2
3
of the starting 1a (run 4). Under air (1 atm), however, 3aa
was obtained in high selectivity (89%), although the conver-
sion of 1a was low (38%) (run 5). On the other hand, under
pure oxygen (1 atm), the selectivity of the reaction was
considerably lowered (run 6). The selectivity of the reaction
at 60 °C was almost the same as that at 45 °C (run 7). The
reaction did not proceed appreciably at 25 °C (run 8). In a
previous paper, we showed that the addition of activated
methylene compounds such as ethyl cyanoacetate to alkenes
a
Substituted benzene (2 mmol) was reacted with 2a (6 mmol) in the
presence of Mn(OAc) (0.1 mmol) and Co(OAc) (0.02 mmol) under O
2
2
2
b
(
0.5 atm)/N2 (0.5 atm) at 45 °C for 3-15 h. Based on substituted benzene
was markedly accelerated by adding AcOK to the Mn(OAc)
2
/
reacted. The selectivity (%) stands for the yield (%) of 3 based on 1
Co(Ac) /O
2
2
redox system.10
consumed. c AcOK (1 mmol) was added.
(
9) Hirase, K.; Iwahama, T.; Sakaguchi, S.; Ishii, Y. J. Org. Chem. 2002,
7, 970.
10) Kagayama, T.; Fuke, T.; Sakaguchi, S.; Ishii, Y. Bull. Chem. Soc.
Jpn. 2005, 78, 1673.
6
The reaction of p-xylene (1c) with 2a afforded diethyl 2,5-
dimethylphenylphophonate (3ca) in good selectivity (90%)
(
408
Org. Lett., Vol. 8, No. 3, 2006

(
run 2). Similarly, mesitylene (1d) reacted with 2a to give
Therefore, it is reasonable to assume the following reaction
+
-
diethyl mesitylphosphonate (3da) in 82% selectivity at 59%
conversion (run 3). In the reaction of chlorobenzene (1e)
and trifluoromethylbenzene (1f) bearing an electron-with-
drawing group with 2a, longer reaction time was needed to
obtain diethyl phophonates, 3ea and 3fa, in satisfactory
conversions and selectivities, although it was difficult to
determine the ratio of ortho, meta, and para isomers (runs 4
and 5). Naphthalene (1g) underwent phophonation with 2a
to form a regioisomeric mixture of diethyl naphthylphos-
phonate (3ga) (R/â ) 87/13) (Run 6).
path involving the abstraction of the H by AcO from the
diethyl phosphonate cation radical (A) generated by the one-
electron oxidation by Mn(III) of 2a (Scheme 1).
Scheme 1. Plausible Reaction Path for Phosphonation of 1a
with 2a
To obtain the information on the reaction pathway in the
present phosphonation, a 1:1 mixture of benzene 1a and
deutrated benzene (1a-d ) was reacted with 2a under the
6
same conditions as run 1 in Table 1 (eq 2). The ratio of the
resulting phophonated product 3aa to the deutrated product
3
aa-d
5
was found to consist of an almost 1:1 mixture of 3aa
(eq 2). No isotope effect was observed in the
and 3aa-d
5
present reaction. This shows that the loss of hydrogen atom
or proton from benzene is not involved in the rate-
determining step.
In conclusion, we have successfully achieved the catalytic
phosphonation of arenes with dialkyl phosphites through a
radical process by using a Mn(II)/Co(II)/O redox couple.
2
This method provides a simple route to arylphosphonates
which are difficult to prepare so far.
Acknowledgment. This work was partially supported by
a Grant-in-Aid for Scientific Research (S) from the Ministry
of Education, Culture, Sports and Technology (MEXT),
Japan.
Supporting Information Available: Experimental pro-
1
13
cedures and copies of H and C NMR spectra of the
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
As shown in Table 1 (run 8), the fact that the reaction
was markedly accelerated by the addition of a weak base
like AcOK indicates that the abstraction of proton by the
-
AcO in the sequential reaction setps may be involved.
OL052406S
Org. Lett., Vol. 8, No. 3, 2006
409