Various arenes and olefins undergo the coupling reactions
t
2
using the Pd(OAc) /BQ/ BuOOH system, and the results are
Table 1. Optimization of Reaction Conditions of Pd-Catalyzed
Coupling of Benzene with (2E)-Ethyl Cinnamatea
shown in Table 2. All the reactions of arenes with the olefins
bearing an electron-withdrawing group at the R-carbon
selectively gave â-aryl trans-olefins (JH-H > 15 Hz). The
yields of the products ranged from low to good yields,
depending on the reactivity of the corresponding arenes and
olefins.
b
entry
catalyst (mol %)
Pd(OAc)2 (1)
oxidant
yield (%) TON
1
2
3
4
5
6
7
8
9
AgOAc
AgO2CPh
MnO2
30% H2O2
tBuOOH
tBuOOH
tBuOOH
tBuOOH
tBuOOH
tBuOOH
51
53
46
35
60
71
76
81e
70
56
25
43
51
53
46
35
60
71
76
81
Pd(OAc)2 (1)
Pd(OAc)2 (1)/BQ (5)
Pd(OAc)2 (1)/BQ (5)
Pd(OAc)2 (1)
Pd(OAc)2 (1)/BQ (5)
Pd(OAc)2 (1)/BQ (5)c
Pd(OAc)2 (1)/BQ (10)c
Pd(OAc)2 (0.5)/BQ(5)c,d
Pd(OAc)2 (0.2)/BQ(3)c,d
Good yields were obtained from the reaction of equimolar
furan (1h), methyl furan (1g), benzofuran (1i), or indole (1j)
with methyl/ethyl acrylate (2k/2f) or vinyl methyl ketone
t
(2g) using the Pd(OAc)
2
/BQ/ BuOOH system (entries 15-
140
280
25
1
7 and 20-26 in Table 2). A high TON (260) can be
obtained also from the reaction of 1g with 2f in the presence
of a small amount of Pd(OAc) (0.2 mol %, entry 16 in Table
). Dicoupling products (two olefins with one arene) such
as diethyl (2E,2′E)-3,3′-(2,5-furandiyl)bispropenoate (3s) or
(2E,2′E)-dimethyl 3,3′-(2,3-benzofurandiyl)bispropenoate (3u)8
were found as the minor products together with monocou-
pling products (2E)-ethyl 3-(2-furanyl)propenoate (3r) or
1
1
1
0
1
2
Pd(OAc)2 (1)/Cu(OAc)2(5)c tBuOOH
Pd(PPh3)4 (1)/BQ (15)c
tBuOOH
43
2
2
a
5
A similar procedure is used unless otherwise indicated. The quantity
b
of the catalyst is based on ethyl cinnamate. GC yield based on ethyl
cinnamate. Acetic anhydride (1 mL) was added. Ethyl cinnamate (30
mmol), BuOOH (60 mmol), benzene (150 mmol), AcOH (30 mL), and
Ac2O (7 mL) were employed, 90 °C, 15 h. 72% isolated yield.
c
d
t
e
(
2E)-methyl 3-(2-benzofuranyl)propenoate (3t) in the reac-
t
The results in Table 1 indicate that BuOOH is the most
tions of 1h with 2f, or 1i with 2k, respectively (entries 21
and 23). When 2 equiv of olefins 2f or 2k with BuOOH
t
powerful of the tested oxidants which include AgOAc,
AgOOCPh, MnO
to the catalytic system enhanced the reaction considerably,
and the yields increased with increasing quantity of BQ
2
, and H
O
2 2
(entries 1-5). Addition of BQ
were employed, dicoupling products 3s or 3u were isolated
as the major products (entries 22 and 24), respectively. The
reaction of either toluene (1b) or anisole (1c) with 2f gave
a mixture of p-, o-, and m-substituted isomers 3i or 3j,
respectively (entries 11 and 12), however favoring p-
substitution. The reaction of naphthalene (1f) with 2f gave
a mixture of (2E)-ethyl R- and â-naphthyl propenoates (3l)
(entry 14) in a 5/1 ratio of R-/â-substitution. The reactivity
of arenes follows the order furans > indole > naphthalene
> anisole > toluene, benzene. All these indicate that the
reaction is of the electrophilic nature to arenes used in the
present catalytic system. The formation of the σ-aryl-Pd-
(II) complexes via electrophilic metalation of aromatic C-H
(entries 5-8). BQ is believed to stabilize the Pd(0) species
by the in situ formation of Pd(0)-BQ complexes to prevent
the Pd(0) species from aggregation to Pd black and subse-
quently BQ to oxidize Pd(0) to Pd(II). The yields were also
improved by addition of acetic anhydride (entries 6-8),
6
presumably because it removed water from the peroxide and
7
c
facilitated the formation of tert-butyl peracetate. The
quantity of Pd(OAc) can be reduced to a very small amount
in this catalytic system (entry 10 in Table 1), and the highest
TON (280) was obtained. Cu(OAc) quickly decomposed
2
2
2
the peroxide and is apparently unsuitable as a cocatalyst in
this system (entry 11 in Table 1). Pd(II)/BQ/peroxides as an
efficient and environmentally clean system also has been
used in other reactions such as oxidative acetoxylation of
bonds is the rate-determining step.
For olefins, the reaction of active olefins such as (2E)-
ethyl cinnamate (2a) and (2E)-4-phenyl 3-buten-2-one (2b)
with benzene gave good yields of coupling products 3a and
7
olefins.
3
b, respectively (entries 1 and 2 in Table 2). The olefins
bearing a nitrile or carboxyl substituent (2c, 2l, and 2d)
somehow exhibited low reactivity (entries 3, 18, and 4),
possibly because of strong coordination of these groups to
palladium, which blocks the coordination of olefins to Pd.
The reaction of the olefins having an aldehyde substituent
(
4) (a) Shue, R. S. Chem. Commun. 1971, 1510. (b) Tsuji, J.; Nagashima,
H. Tetrahedron 1984, 2699.
5) Typical experimental procedure (entry 8 in Table 1): (2E)-ethyl
cinnamate (0.54 g, 3 mmol), benzene (1.2 g, 15 mmol), Pd(OAc)2 (6.7 mg,
.03 mmol), benzoquinone (32 mg, 0.3 mmol), 3 mL of acetic acid, and 1
(
0
mL of acetic anhydride were mixed in a 25 mL reaction tube equipped
with a magnetic stirrer, and the tube was sealed with a rubber septum. The
reaction mixture was heated to 90 °C (oil bath), and 80% BuOOH (0.4 g,
.9 mmol) was injected within 1 h. The reaction mixture was kept at the
same temperature for another 11 h. The reaction was monitored by GC
with diethyl 1,4-benzenedicarboxylate as internal standard) and TLC
(2e or 2m) gave low to fair yields of coupling products
t
(entries 5 and 19), the main reason being that the aldehydes
3
are easily oxidized to carboxylic acid. The reactivity of
(
analysis of the reaction mixture. After the usual workup of the reaction
mixture, column chromatography with hexane/ethyl acetate (10/1) as eluent
gave ethyl 3-phenyl cinnamate in 72% yield. Its structure was confirmed
(8) (2E,2′E)-Dimethyl 3,3′-(2,3-benzofurandiyl)bispropenoate (3u) was
isolated as yellow crystals: mp 119.5-220.6 °C (recrystallized from hexane/
1
13
1
by H and C NMR.
EtOAc 5/1). H NMR (300 MHz, CDCl3) δ 3.85 (s, 3H, CH3O), 3.86 (s,
(
6) The formation of Pd(0)-BQ complexes and acid-induced transforma-
3H, CH3O), 6.65 (d, J ) 15.9 Hz, 1H, vinyl), 6.70 (d, J ) 15.3 Hz, 1H,
vinyl), 7.35 (td, J ) 1.5, 8.4 Hz, 1H, aryl), 7.45 (td, J ) 1.5, 8.1 Hz, 1H,
aryl), 7.52 (d, J ) 8.4 Hz, 1H, aryl), 7.82 (d, J ) 15.3 Hz, 1H, vinyl), 7.85
tion of the Pd(0)-BQ complexes to Pd(II) have been reported. Grennberg,
H.; Gogoll, A.; Backvall, J. E. Organometallics 1993, 12, 1790.
1
3
(
7) (a) Jia, C.; Mimoun, H.; Mueller, P. J. Mol. Catal., A Chem. 1995,
(d, J ) 8.1 Hz, 1H, aryl), 7.95 (d, J ) 15.9 Hz, 1H, vinyl). C NMR (75
MHz, CDCl3) δ 51.90, 52.04, 111.83, 119.13, 120.39, 120.85, 121.58,
124.23, 125.75, 127.39, 127.77, 133.68, 152.81, 155.22, 166.66, 167.06.
1
01, 127-136; European Patent 685450, 1995. (b) Akermark, B.; Larsson,
E. M.; Oslob, J. D. J. Org. Chem. 1994, 59, 5729. (c) Strukul, G., Ed.
Catalytic Oxidations with Hydrogen Peroxide as Oxidant; Kluwer Academic
Publishers: Dordrecht, 1992.
-
1
IR (KBr, cm ): 1720 (CdO), 1737 (CdO). Anal. Calcd for C16H14O5:
C, 67.13; H, 4.93. Found: C, 66.96; H, 4.90.
2098
Org. Lett., Vol. 1, No. 13, 1999