3552 J . Org. Chem., Vol. 61, No. 10, 1996
Notes
Sch em e 3
We supposed that the first step of this reaction occurs
concertedly. The bonds of P-H (from COOH), Ar-C (CR),
and P-O-H (from ArH) are presumed to be formed and
the bonds of Ar-H, C(CR)-C(COOH), and O-H (of
COOH) to be cleaved simultaneously. At the same time,
the C-O single bond of COOH is suspected to be
transformed to a double bond of CO2 and the PdO double
bond to a P-O(-H) single bond. Indispensability of the
hydrogen of COOH of acid 1 is considered to be due to
completion of the electron transfer in this concerted
reaction. The preference of an alkoxy group rather than
a free hydroxy group on the 2-C carbon is interpreted to
arise from the difference in the stabilization of the
intermediate.
Contrarily, this tandem arylation is suspected to
proceed by a different mechanism than that for direct
condensation described above. If the reaction proceeded
through the corresponding R-methoxyphenone (4), the
aryl group in this phenone intermediate must shift to
R-carbon. Though the propagation of this demethoxyl-
ative rearrangement step still has possibility via forma-
tion of a hemiacetal-like intermediate formed from
R-methoxyphenones 4 and P2O5/PPA, we think such an
aryl 1,2-shift is less plausible. There are a few reports
about the acid-catalyzed or photochemical aryl 1,2-shift
of acetals having a leaving group on the R-position,5 but
the absence of the phenone products in this system
strongly opposes the intermediacy of such phenone
derivatives.
In addition, 2-phenylpropanoic acid (8) gave the cor-
responding condensation product (12) quantitatively.
This fact clearly demonstrates that the replacement of
the methoxy group of acid 1 by aryls 2 can not be the
preliminary reaction of this novel tandem arylation.
Accordingly, the decarboxylation and the first arylation
are considered to precede demethoxylation. Further, the
distinct dependency of the conversion on the nucleophilic
character of the aromatic compounds, as well as other
characteristic features of this reaction, indicates that the
critical step of this tandem reaction is the first arylation
step. In the second step, acid-catalyzed demethoxylation
is supposed to occur followed by second arylation accord-
ing to ordinary electrophilic aromatic substitutions. In
fact, benzyl methyl ethers 16 and 17, which are possible
intermediates for this reaction, gave diarylethane 3a d
and diarylmethane 20 quantitatively, as did benzyl
chloride (18) and benzyl alcohol (19) (Scheme 3).
The electrophilic attack of the R-carbon of acid 1 to the
aromatic ring implies that, in the course of the reaction,
the acid was oxidatively arylated. Acid 1 and aromatic
compound 2 lost, between them, one carboxyl group with
one electron and one proton. At this stage we speculate
the reaction mechanism as follows: The phosphorus of
P2O5/PPA is considered to be reduced, which could
provide the driving force of the reaction. This mechanism
coincides with the substrate specificity that this reaction
did not undergo when acid 1 was replaced with the
corresponding ester (15).
The reaction mechanism of this novel concurrent
oxidative decarboxylation and arylation reaction via
intermediacy of P2O5-MeSO3H or PPA will be discussed
with other possible reaction mechanisms such as evolu-
tion of carbon monoxide elsewhere.
Exp er im en ta l Section
Gen er a l. Purification of reagents was performed according
to the literature6 as occasion calls. NMR spectra were recorded
at 200 or 500 MHz for 1H and at 50 or 125 MHz for 13C in CDCl3
using TMS as an internal reference. The melting point is
uncorrected. P2O5-MeSO3H was prepared by stirring the 1:10
mixture of P2O5 and MeSO3H at rt according to the literature
method.1
Typ ica l P r oced u r e for Rea ction of Acid 1 a n d An isole
(2a ). P2O5-MeSO3H (6 mL, 6 mmol) was added to an ice-cooled
mixture of acid 1 (156 mg, 1.5 mmol) and anisole (2a , 6.48 g, 60
mmol) under vigorous stirring. The mixture was stirred at the
prescribed temperature for the prescribed time interval. Then
the mixture was poured into ice-water. The aqueous solution
was extracted with ether (40 mL × 2). The combined organic
layer was washed with saturated aqueous NaCl solution, dried
over MgSO4 overnight, and concentrated under reduced pres-
sure. Dianisylethane (3a ) was obtained as an isomeric mixture.
The major product is the isomer that has two methoxy groups
at 4- and 4′-positions. The product ratio of 4,4′-dianisylethane,
2,4′-dianisylethane, and 2,2′-dianisylethane is 78:20:2. By silica
gel column chromatography (eluent benzene:hexane ) 1:1 v/v)
1
each isomer was obtained H NMR spectrometrically pure. The
data for the major isomer of dianisylethane 3a are as follows:
mp 71.5-72 °C; 1H NMR (200 MHz) δ (CDCl3) 7.15 (pseudo d of
AA′MM′ pattern, 4H), 6.85 (pseudo d of AA′MM′ pattern, 4H),
4.10 (q, J ) 7.5 Hz, 1H), 3.80 (s, 6H), 1.60 (d, J ) 7.5 Hz, 3H);
13C NMR (125 MHz) δ (CDCl3) 157.7, 138.9, 128.4, 113.7, 55.1,
43.0, 22.2. Anal. Calcd for
Found: C, 79.20; H, 7.53.
C16H18O2: C, 79.31; H, 7.59.
The reactions using other condensation reagents (PPA,
MeSO3H) were undertaken in a similar manner. The reactions
of the acids (5, 6, 7, 8, 13, and 14), ester 15, benzyl ethers 16
and 17, benzyl chloride (18), and benzyl alcohol (19) were
undertaken in the essentially same manner. The structures of
1,1-diarylethanes 3 and other products were identified by 1H
NMR spectroscopy.
Ack n ow led gm en t. The authors thank Mr. Toshio
Kuroda for his excellent technical assistance with the
elemental analysis.
(4) Cablewski, T.; Gurr, P. A.; Raner, K. D.; Strauss, C. R. J . Org.
Chem. 1994, 59, 5814.
(5) (a) Honda, Y.; Ori, A.; Tsuchihashi, G.-i. Bull. Chem. Soc. J pn.
1987, 60, 1027. (b) Sonawane, H. R.; Nanjundiah, B. S.; Kulkarmi,
D. G.; Ahuja, J . R. Tetrahedron 1988, 44, 7319. (c) Sonawane, H. R.;
Bellur, N. S.; Kulkarmi, D. G.; Ayyangar, N. R. Tetrahedron 1994, 50,
1243.
J O960103T
(6) Perrin, D. D.; W. L. F. Purification of Laboratory Chemicals, 3rd
ed.; Pergamon Press: Oxford, 1988.
Yonezawa, Noriyuki
