7
96
Chemistry Letters Vol.37, No.7 (2008)
Palladium-catalyzed Cross-coupling of Benzylic Carbonates with Organostannanes
ꢀ
Masato Ohsumi and Ryoichi Kuwano
Department of Chemistry, Graduate School of Sciences, Kyushu University,
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581
(Received April 11, 2008; CL-080376; E-mail: rkuwano@chem.kyushu-univ.jp)
The cross-coupling of benzylic carbonates with arylstan-
3
Table 1. Effect of phosphine ligand on the reaction of 1a
a
nanes proceeded in the presence of [Pd(ꢀ -C3H5)Cl]2–DPPPent
catalyst, affording the desired diarylmethanes in good yield.
with 2a
5.0% [Pd]
Bu Sn
5.5% Ligand
3
OCOMe
O
+
DMF
Palladium-catalyzed cross-coupling of organohalides with
organostannanes, i.e. the Kosugi–Migita–Stille coupling, is one
of the reliable methods for selective formation of carbon–carbon
1
a
2a
Ph P(CH ) PPh
2
80 °C, 24 h
3a
2
2 n
1
bond in organic synthesis. The catalytic reaction often employs
sulfonates in place of halides as leaving groups on electrophilic
DPPHex (n = 6) DPPPent (n = 5)
O
DPPB (n = 4)
DPPE (n = 2)
DPPP (n = 3)
Ph P
PPh2
2
2
substrates. Use of carboxylate leaving group, which is easier to
handle and more accessible than sulfonate, has been unexplored
DPEphos
b,c
Entry [Pd]
Ligand
Solvent Yield/%
3
–5
except allylic substrates. Recently, we have devoted our ef-
forts to develop the palladium-catalyzed reactions involving
3
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
[Pd(ꢀ -C3H5)Cl]2 DPPPent DMF
65 (69)
64 (45)
56 (56)
80 (56)
0
1
31
0
0
4
0
3
6
,7
[Pd(ꢀ -C H )Cl]
DPEphos DMF
3
5
2
benzylic carbonates. As a part of the research program, we
report herein the use of benzylic carbonates as electrophilic
3
[Pd(ꢀ -C3H5)Cl]2 DPPHex
DMF
DMF
DMF
DMF
DMF
3
8
–11
[Pd(ꢀ -C H )Cl]
DPPB
substrates in the Kosugi–Migita–Stille coupling.
In our initial attempt, a mixture of benzyl methyl carbonate
3
5
2
3
[Pd(ꢀ -C3H5)Cl]2 DPPP
[Pd(ꢀ -C3H5)Cl]2 DPPE
[Pd(ꢀ -C3H5)Cl]2 PPh3
3
(
8
1a) with tributylphenylstannane (2a) was treated in DMF at
ꢁ
d
d
d
3
0 C with 5 mol % of the palladium complex generated in situ
3
3
from [PdCl(ꢀ -C3H5)]2 and DPPPent, which was the most effec-
tive catalyst for the Suzuki–Miyaura coupling of 1a.7 The re-
sulting mixture afforded diphenylmethane (3a) in 65% yield at
[Pd(ꢀ -C3H5)Cl]2 P(o-Tol)3 DMF
a
3
[Pd(ꢀ -C3H5)Cl]2 P(t-Bu)3
DMF
DPPPent DMF
Pd(dba)2
Pd(OAc)2
24 h (Table 1, Entry 1). As the result of further screening of
DPPPent DMF
3
phosphine ligands, the catalytic cross-coupling was found to pro-
ceed in moderate yield by using the bisphosphines possessing
larger bite angle than DPPP (Entries 2–4). In contrast, DPPP-
or DPPE-ligated palladium failed to catalyze the cross-coupling
of 1a with 2a (Entries 5 and 6). To our surprise, a certain level of
production of 3a was observed in the reaction using triphenyl-
phosphine–palladium catalyst, which was ineffective for the cor-
[Pd(ꢀ -C3H5)Cl]2 DPPPent DMSO
73 (51)
27
28
3
[Pd(ꢀ -C H )Cl]
[Pd(ꢀ -C H )Cl]
[Pd(ꢀ -C H )Cl]
3
5
2
2
2
DPPPent THF
DPPPent dioxane
DPPPent toluene
ꢁ
3
3
5
3
3
5
18
a
Reactions were conducted in DMF at 80 C for 24 h. The molar
ratio of 1a:2a:[Pd]:ligand was 20:24:1.0:1.1 unless otherwise
b
c
noted. GC yield (average of two runs). The yields in parenthe-
ses were GC yields at 72 h in the reaction with 1.0% catalyst
loading. The molar ratio of [Pd]:ligand was 1.0:2.2.
7
a
responding Suzuki–Miyaura coupling (Entry 7). However, the
reaction of 1a scarcely proceeded with sterically congested and
electron-donating monophosphines, which are regarded as better
ligands for many palladium-catalyzed cross-couplings than tri-
d
pling product was remarkably enhanced by the electron-donat-
ing methoxy group of 1c (Entry 3). Electron-withdrawing sub-
stituents did not disturb the formation of 3 significantly
(Entry 4). Chloro group, which is possible to undergo an oxida-
tive addition to palladium(0), was compatible with the catalytic
cross-coupling, affording diarylmethane 3e in 73% yield
1
2
phenylphosphine in general (Entries 8 and 9). The palladium
catalysis is sensitive to palladium catalyst precursor. Use of
3
Pd(dba)2 or Pd(OAc)2 in place of [Pd(ꢀ -C3H5)Cl]2 resulted
in little or no formation of 3a (Entries 10 and 11). The catalytic
reaction worked well in DMSO as well as in DMF (Entry 12).
Use of a less polar solvent such as THF, 1,4-dioxane, and toluene
caused significant decrease of the yield of 3a (Entries 13–15).
Bidentate bisphosphines affording 3a in over 50% yield in the
above ligand screening were reexamined on the reaction with
0
(Entry 5). In contrast, p-bromobenzyl carbonate 1e selectively
reacted with 2a at its bromo group and was transformed into
biaryl 4 in 87% yield (Scheme 1). The observations indicate that
the benzylic C–O bond of 1 possesses the reactivity between aryl
C–Cl and C–Br bond. The o-methyl group of 1f did not hinder
the catalytic reaction (Entry 6). Benz-fused benzylic carbonates,
1g and 1h, worked as good electrophilic substrates in the cross-
coupling with 2a (Entries 7 and 8). However, use of benzylic
substrates bearing a coordinative functional group, such as nitro
or alkoxycarbonyl, resulted in no production of the desired di-
arylmethanes. Electronic property of the para substituent of aryl-
1% palladium loading. DPPPent gave the highest yields (69%)
of the desired coupling product. The yield of 3a was improved
to 78% when the molar ratio of 1a to 2a was changed from
1:1.2 to 1.5:1 (Table 2, Entry 1).
As shown in Table 2, the cross-couplings of various benzyl-
ic carbonates 1 and organostannanes 2 were attempted under the
reaction conditions optimized above. The yield of the cross-cou-
Copyright Ó 2008 The Chemical Society of Japan
Ohsumi, Masato
