Cross-coupling reaction of allylic and benzylic carbonates with organo[2-(hydroxymethyl)phenyl]dimethylsilanes

Article abstract of DOI:10.1246/cl.2007.606

The title reaction is found to proceed in the presence of a palladium catalyst and in the absence of any activator. Various functional groups are tolerated to give a diverse range of 1,4-diene and diarylmethane products, which are ubiquitous units of natural products and pharmaceuticals. Copyright

Full text of DOI:10.1246/cl.2007.606

606
Chemistry Letters Vol.36, No.5 (2007)
Cross-coupling Reaction of Allylic and Benzylic Carbonates
with Organo[2-(hydroxymethyl)phenyl]dimethylsilanes
Yoshiaki Nakao,ꢀ Shiro Ebata, Jinshui Chen, Hidekazu Imanaka, and Tamejiro Hiyamaꢀ
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510
(Received February 1, 2007; CL-070123; E-mail: nakao@npc05.kuic.kyoto-u.ac.jp, thiyama@npc05.kuic.kyoto-u.ac.jp)
The title reaction is found to proceed in the presence of a
our silicon reagents with these carbonate esters under neutral
conditions (Scheme 1).6 This has turned out to be the case as
reported herein.
We first examined the reaction of (E)-[2-(hydroxymethyl)-
phenyl]dimethyl(1-octenyl)silane (1a: 1.0 mmol) with allyl
tert-butyl carbonate (2a: 1.0 mmol) in the presence of 0.5
mol % of Pd2(dba)3 and 2.0 mol % of (2-thienyl)3P (2-Th3P) as
a ligand7 in THF at 50 ꢁC to obtain (E)-1,4-undecadiene (4aa)
in 85% yield after 2 h (Entry 1 of Table 1).8 Excellent chemose-
lectivity was observed with (E)-alkenylsilanes having a func-
palladium catalyst and in the absence of any activator. Various
functional groups are tolerated to give a diverse range of 1,4-
diene and diarylmethane products, which are ubiquitous units
of natural products and pharmaceuticals.
The structures of non-conjugated 1,4-diene, 3-arylpropene,
and diarylmethane are found in many natural products and
pharmaceuticals, and are often constructed by the cross-coupling
reactions of alkenyl- or arylmetallic reagents with allyl or benzyl
electrophiles.1 Whereas silicon-based approach2 toward these
frameworks is apparently a promising alternative to the conven-
tional ones1 in view of high stability and non-toxicity associated
with organosilicon reagents, only a few protocol has been
available to date using moisture and acid/base sensitive organo-
(fluoro)silanes3 and organo(trialkoxy)silanes.4
We have recently disclosed that organo[2-(hydroxymethyl)-
phenyl]dimethylsilanes (1) behave as a new class of silane
coupling reagents for the palladium-catalyzed cross-coupling re-
action.5 The reagents allow chemically stable tetraorganosilicon
compounds to participate in the cross-coupling chemistry under
fluoride-free conditions for the first time with excellent chemo-
selectivities. The proximal hydroxy group is supposed to be
converted to an alkoxide upon treatment with a mild base, such
as K2CO3, and coordinate to the nearby silicon atom to produce
a requisite five-membered penta-coordinated silicate species.
Given the importance of the aforementioned transformations,
we hypothesized that a palladium alkoxide species generated
by oxidative addition of allylic and benzylic carbonates 2 to
Pd0 would act as a base to allow the cross-coupling reaction of
Table 1. Cross-coupling reaction of organo[2-(hydroxy-
methyl)phenyl]dimethylsilanes (1) with allylic tert-butyl car-
bonates 2
Pd2(dba)3 (0.5 mol %)
HO
O
2-Th3P (2 mol %)
R1 R2
R2
R1 Si
Me2
+
THF, 50 °C
t-BuO
O
4
1 (1.0 mmol)
2 (1.0 mmol)
R2 = allyl (2a); (E)-cinnamyl (2b); 2-cyclohexenyl (2c);
crotyl (methyl carbonate, 2d)
Entry
1
2
Time/h
Product
Yield/%a
1
2
3
4
5
2
4
2
2
3
1a 2a
1b 2a
1c 2a
85 (4aa)
85 (4ba)
72 (4ca)
74 (4da)
85 (4ea)
Hex
NC
TBDMSO
Cl
1d
2a
PhthN
HO
1e 2a
6
7
8
1f
2a
2a
4
2
2
53 (4fa)
91 (4ga)
92 (4ha)
Me Me
1g
Ph
Ph
1h 2a
O
R1
R2O
O
9
1i
2a
3
81 (4ia)
Pd(0)
Ph
CO2
Pr
10
11
24
3
1j
2a
2b
78 (4ja)
R1
Pd
OR2
Pd
Pr
1a
93 (4ab)b
Hex
Ph
12
1a 2c
1g
1k 2b
4
92 (4ac)
HO
Hex
Ph
O
+
R1 Si
7
6
13
2d
68 (4gd)c
73 (4kb)
Si
Ph
Ph
Ph
Me2
Me2
3
1
14d
+ HOR2
R1= (E)-HexCH=CH (1a)
(E)-NC(CH2)3CH=CH (1b)
(E)-PhCH=CH (1g)
(Z)-PhCH=CH (1h)
15d 1k 2c
6
75 (4kc)
Ph
(E)-TBDMSO(CH2)3CH=CH (1c) H2C=CPh (1i)
(E)-Cl(CH2)3CH=CH (1d)
(E)-PhthNCH2CH=CH (1e)
(E)-HOCMe2CH=CH (1f)
(E)-PrCH=CPr (1j)
Ph (1k)
4-MeO−C6H4 (1l)
aIsolated yields based on 1. bCyclic silyl ether 3 was also isolated
in 80% yield on a 10 mmol scale. The ratio of isomers was
determined to be 73:27 by H NMR. The reaction was carried
out at 70 ꢁC using CuOAc (50 mmol) as a co-catalyst.
c
1
d
Scheme 1.
Copyright ꢀ 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.5 (2007)
607
Table 2. Cross-coupling reaction of organo[2-(hydroxy-
methyl)phenyl]dimethylsilanes (1) with arylmethyl methyl
carbonates 5
cross-coupling chemistry by employing a Pd/bisphosphine sys-
tem.1a We also surveyed several combinations of a palladium
catalyst and a bisphosphine ligand and identified the combina-
tion of (ꢃ5-cylcopentadienyl)(ꢃ3-allyl)palladium [Cp(allyl)Pd]
and 1,10-bis(diphenylphosphino)ferrocene (dppf) was optimum.
Thus, the reaction of (E)-1-octenylsilane 1a with benzyl methyl
carbonate (5a) in the presence of 5 mol % of Cp(allyl)Pd and
dppf in THF at 80 ꢁC for 18 h gave (E)-1-phenyl-2-nonene
(6aa) in 87% yield (Entry 1 of Table 2). To our knowledge, this
is the first example of the silicon-based cross-coupling reaction
with a benzylic electrophile. Under the identical conditions, 4-
methoxyphenyl (5b), 2,4,6-trimethylphenyl (5d), and 2-pyridyl
(5e) carbonate also cross-coupled with 1a to give various (E)-
1-aryl-2-nonenes in good yields (Entries 2–4). The aryl-benzylic
coupling reactions with CuOAc as a CuI co-catalyst also
met with success. Arylsilanes cross-coupled successfully
with a range of arylmethyl carbonates including heteroarenes
to allow the silicon-based access to various diarylmethanes
(Entries 5–10).2
In summary, we have demonstrated that organo[2-(hydroxy-
methyl)phenyl]dimethylsilanes undergo the cross-coupling
reaction with allylic and benzylic carbonates to give a variety
of 1,4-dienes, 3-arylpropenes, and diarylmethanes in a highly
chemoselective manner. The use of readily accessible, highly
stable, and recyclable tetraorganosilicon compounds under mild
conditions free of an activator is definitely an attractive feature
that may replace the conventional protocols. Other catalytic
and non-catalytic transformations of the silane reagents are
currently under investigations in our laboratory.
Cp(allyl)Pd (5 mol %)
HO
O
dppf (5 mol %)
R1 Ar
R1 Si
Me2
+
THF, 80 °C
MeO
O
Ar
6
1 (1.0 mmol)
5 (1.0 mmol)
Ar = Ph (5a); 4-MeO−C6H4 (5b); 4-Cl−C6H4 (5c); 2,4,6-Me3−C6H2 (5d);
2-pyridyl (5e); 3-pyridyl (5f); 2-furyl (5g); 2-thienyl (5h)
Entry
1
1
5
Time/h
18
Product
Yield/%a
1a
5a
87 (6aa)
Hex
Ph
Hex
Hex
2
3
1a
1a
10
10
89 (6ab)
86 (6ad)
5b
5d
OMe
Me
Me
Me
Hex
4
1a
1k
1l
5e
5b
5c
40
8
75 (6ae)
92 (6kb)
70 (6lc)
N
Ph
5b
6b
OMe
13
MeO
Cl
Me
7b
8b
1k
1k
5d
5f
8
8
87 (6kd)
78 (6kf)
Ph
Me
Me
Ph
We thank Prof. Ryoichi Kuwano (Kyushu University) for
helpful suggestions. This work has been supported financially
by Grant-in-Aid for Creative Scientific Research, No.
16GS0209, Scientific Research on Priority Areas ‘‘Reaction
Control of Dynamic Complex,’’ and COE Research on ‘‘United
Approach to New Material Science’’ from MEXT.
N
O
9b
1k
1k
75 (6kg)
71 (6kh)
9
8
5g
5h
Ph
Ph
S
10b
aIsolated yields. The reaction was carried out using Cu(OAc)
b
(50 mmol) as a co-catalyst.
References and Notes
1
For selected recent examples, see: a) R. Kuwano, M. Yokogi, Org.
Lett. 2005, 7, 945. b) M. J. Schnermann, D. L. Boger, J. Am. Chem.
Soc. 2005, 127, 15704. c) G. W. Kabalka, M. Al-Masum, Org. Lett.
2006, 8, 11. d) K. A. Keaton, A. J. Phillips, J. Am. Chem. Soc. 2006,
128, 408.
For synthesis of diarylmethanes using (2-pyridyl)silylmethylstan-
nane as a coupling partner, see: K. Itami, M. Mineno, T. Kamei,
J. Yoshida, Org. Lett. 2002, 4, 3635.
a) J. Yoshida, K. Tamao, H. Yamamoto, T. Kakui, T. Uchida, M.
Kumada, Organometallics 1982, 1, 542. b) H. Matsuhashi, S. Asai,
K. Hirabayashi, Y. Hatanaka, A. Mori, T. Hiyama, Bull. Chem.
Soc. Jpn. 1997, 70, 1943. c) M.-R. Brescia, P. DeShong, J. Org.
Chem. 1998, 63, 3156.
a) R. Correia, P. DeShong, J. Org. Chem. 2001, 66, 7159. b) G. W.
Kabalka, G. Dong, B. Venkataiah, C. Chen, J. Org. Chem. 2005, 70,
9207.
Y. Nakao, H. Imanaka, A. K. Sahoo, A. Yada, T. Hiyama, J. Am.
Chem. Soc. 2005, 127, 6952.
a) F. Guibe, Y. S. M’leux, Tetrahedron Lett. 1981, 22, 3591. b) J.
Tsuji, I. Shimizu, I. Minami, Y. Ohashi, T. Sugiura, K. Takahashi,
J. Org. Chem. 1985, 50, 1523.
The reaction using (2-furyl)3P as a ligand resulted in a slightly lower
yield.
In all cases in this study, O-allylation of 1 was observed <5%, if any.
Supporting Information is available electronically on the CSJ-
Journal Web site; http://www.csj.jp/journals/chem-lett/.
tional group, such as cyano, siloxy, chloro, or phthalimide,
giving various 1,4-dienes in good yields (Entries 2–5). Free
hydroxy, which serves as a nucleophile in ꢀ-allylpalladium
chemistry, was also compatible (Entry 6). (E)- and (Z)-ꢁ-
Styryl-, ꢂ-phenylvinyl-, and (E)-4-octenylsilanes participated
in the reaction with 2a in good yields with perfect regio- and
stereospecificities (Entries 7–10). (E)-Cinnamyl (2b) carbonate
similarly reacted with 1a to give linear (E)-1-phenyl-1,4-
undecadiene (4ab) in 93% yield irrespective of a reaction scale
(1 and 10 mmol) (Entry 11). The reaction on a 10 mmol scale
allowed isolation of cyclic silyl ether 3, a silicon residue, in
80% yield, which is reusable for the synthesis of the silane
reagents.5 tert-Butyl 2-cyclohexenyl carbonate (2c) readily
participated in the reaction to give the corresponding coupling
product in 92% yield (Entry 12), whereas crotyl methyl carbon-
ate (2d) gave a mixture of linear and branched products
(Entry 13). Phenylsilane 1k underwent the allyl coupling reac-
tion with 2b and 2c by using a 5.0 mol % of CuI co-catalyst
in a THF solvent at 70 ꢁC (Entries 14 and 15).
2
3
4
5
6
7
We then turned our attention to the cross-coupling reaction
with benzylic carbonate. Recently, Kuwano and Yokogi have
revealed that benzylic carbonates serve as electrophiles for the
8
9
Published on the web (Advance View) March 31, 2007; doi:10.1246/cl.2007.606