J . Org. Chem. 1999, 64, 4179-4182
4179
Sch em e 1
Rea ction Rou tes of Sp ecific
Deca r bon yla tive r,r-Dia r yla tion of
2-Meth oxyp r op a n oic Acid An a logu es in
P 2O5-MsOH a n d th e Rela ted Rea gen ts
Noriyuki Yonezawa,* Tetsuo Hino,† Masayuki Shimizu,‡
Kazuhisa Matsuda,‡ and Tomiki Ikeda†
Department of Material Systems Engineering,
Tokyo University of Agriculture and Technology, Koganei,
Tokyo 184-8588, J apan, Research Laboratory of Resources
Utilization, Tokyo Institute of Technology,
Nagatsuda, Midori-ku, Yokohama 226-8503, J apan, and
Department of Chemistry, Gunma University,
Kiryu, Gunma 376-8515, J apan
Received J uly 14, 1998
Ta ble 1. Rea ction s of 2-Meth oxyp r op a n oic Acid (2) w ith
Va r iou s Am ou n ts of An isole (4a ) in P 2O5-MsOH a t Room
Tem p er a tu r e for 24 h a
Recently, we found that 2-alkoxycarboxylic acids (1)
undertook specific double electrophilic aromatic substitu-
tion reactions in P2O5-MsOH (phosphorus pentoxide-
methanesulfonic acid mixture).1,2 The corresponding 1,1-
diarylalkanes (5) were obtained in good yields with
evolution of carbon monoxide instead of formation of the
suspected Friedel-Crafts acylation type adducts (Scheme
1).2,3
There have been some papers describing the decarbon-
ylations of carboxylic esters and free acids4,5 without
sufficient mechanistic clarification.6 As far as we know,
only two cases have been reported concerning the decar-
bonylative arylation of carboxylic acids or their deriva-
tives.7,8 The reaction of acids 1 described above is the first
example of decarbonylative R-arylation of free carboxylic
yieldb (%)
anisole/acid 2
runc
(mol/mol)
6a
7a
1
2
3
4
5
6f
2
5
10
20
40
10
40
43
52
66
85e
0
44
13
18
trd
0
78
a
Reaction conditions: for 1 mmol of acid 2, 4 mL of P2O5-
b
MsOH and 40 mmol of anisole (4a ) were used. Yields were
determined on the basis of 1H NMR spectra. c Methanesulfonylated
d
product (Ar-Ms) was also obtained except for run 1. Many
unidentified products were also obtained. e Reference 2. f Reaction
was carried out at 80 °C for 5 h.
acids.9 However, the mechanism, scope, and limitation
of this reaction have not been fully understood yet. In
this paper, the pathways of the decarbonylative R,R-
diarylation of carboxylic acids 1 with and without oxida-
tion are closely discussed.
Table 1 shows the results of the reactions of acid 2 with
various amounts of anisole (4a ) in P2O5-MsOH at room
temperature for 24 h. The yield of 1,1-diarylethane (6a )
increased with the amount of anisole, and that of 1,1-
diarylethene (7a ) was reduced in turn. When a 40-fold
excess of anisole to acid 2 was used, alkane 6a was solely
obtained in an 85% yield and alkene 7a was not observed
(run 5). When the excess of anisole was reduced to 2-20
times, alkene 7a was obtained as a byproduct (runs 1-4).
The use of an equimolar amount of anisole gave alkene
7a in the highest yield (44%, run 1). In particular, the
reaction with a 10-fold excess of anisole at 80 °C yielded
alkene 7a without alkane 6a (78%, run 6).
* To whom correspondence should be addressed. Tel: +81-42-388-
7053. Fax: +81-42-381-7979. E-mail: yonezawa@cc.tuat.ac.jp.
† Tokyo Institute of Technology.
‡ Gunma University.
(1) Eaton, P. E.; Carson, G. R.; Lee, J . T. J . Org. Chem. 1973, 38,
4071. The direct condensation of free carboxylic acids in P2O5-MsOH
is explained to be induced by formation of reactive mixed acid
anhydrides between phosphoric acid moieties and carboxyl groups. The
anhydride is regarded as an acyl cation equivalent giving a phenone
skeleton via a Friedel-Crafts acylation-type reaction.
(2) Yonezawa, N.; Tokita, Y.; Hino, T.; Nakamura, H.; Katakai, R.
J . Org. Chem. 1996, 61, 3551.
(3) Yonezawa, N.; Hino, T.; Tokita, Y.; Matsuda, K.; Ikeda, T.
Tetrahedron 1997, 53, 14287. The reaction of 2-methoxyacetic acid (25)
with anisole in P2O5-MsOH gave the corresponding phenone (29a ) in
addition to diarylmethane (28a ) (Scheme 3).
(4) March, J . Advanced Organic Chemistry, Reaction, Mechanisms,
and Structure, 3rd ed.; Wiley-Interscience: New York, 1985; pp 341-
342.
(5) (a) Liler, M. Reaction Mechanisms in Sulfuric Acid; Academic
Press: New York, 1971; pp 254-259. (b) Organic Reactions; Bittman,
R., Dauben, W. G., Fried, J ., Kende, A. S., Marshall, J . A., McKusick,
B. C., Meinwald, J ., Trost, B. M., Eds.; J ohn Wiley and Sons: New
York, 1972, Vol. 19, pp 295-299.
(6) According to ref 4, some decarbonylation reactions were ex-
plained by SN1 reaction through an acyl cation intermediate.
(7) DeHaan, F. P.; Djaputra, M.; Grinstaff, M. W.; Kaufman, C. R.;
Keithly, J . C.; Kumar, A.; Kuwayama, M. K.; Macknet, K. D.; Na, J .;
Patel, B. R.; Pinkerton, M. J .; Tidwell, J . H.; Villahermosa, R. M. J .
Org. Chem. 1997, 62, 2694. Decarbonylative mono- and diarylations
of methoxyacetyl chloride in the presence of Lewis acids were reported.
Diarylmethane was explained to be formed from a benzyl chloride type
intermediate, which was considered to be the primary product from
benzene and methoxyacetyl chloride with decarbonylation.
(8) Palmer, M. H.; McVie, G. J . J . Chem. Soc., C 1967, 343.
Decarbonylative diarylation of phenoxyacetyl chloride with an excess
amount of p-xylene in the presence of Lewis acids was reported.
However, the yield of the 1,1-diarylated product (ArCH2Ar) was very
low (1%), and the phenone (PhOCH2COAr) was predominantly ob-
tained (65%).
Table 2 shows the results of the reactions of acid 2 and
acid chloride 3 with anisole in some acidic media under
various conditions (Scheme 1). In P2O5-MsOH or poly-
phosphoric acid (PPA), acid 2 efficiently gave alkane 6a
(9) In the previous papers (refs 2 and 3), the following features were
found: (1) P2O5-MsOH and PPA effectively promote the reaction, (2)
an alkoxy group or a hydroxy one on the R-carbon of the carboxylic
acid is indispensable, (3) the reaction does not proceed when the free
carboxylic acid structure is converted to ester one, (4) the relative
reactivity and the selectivity are suspected to be primarily governed
by the electronic property of the R-substituents on the carboxylic acids
(1), and (5) the correlation between the structure of the arene
substrates and their reactivity is almost comparable to that observed
in conventional electrophilic aromatic substitution reactions.
10.1021/jo981379z CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/29/1999