Sequential introduction of carbon nucleophiles onto silicon atoms using methyl as a leaving group

Article abstract of DOI:10.1246/cl.2009.116

A molecular structure, which has a trimethylsilyl group in close proximity to a hydroxy group spatially, converts methyl on silicon into a good leaving group. Copyright

Full text of DOI:10.1246/cl.2009.116

116
Chemistry Letters Vol.38, No.2 (2009)
Sequential Introduction of Carbon Nucleophiles onto Silicon Atoms
Using Methyl as a Leaving Group
Hiroaki Horie, Yuichi Kajita, and Seijiro Matsubaraꢀ
Department of Material Chemistry, Graduate School of Engineering, Kyoto University,
Kyoutodaigaku-katsura, Nishikyo-ku, Kyoto 615-8510
(Received November 11, 2008; CL-081066; E-mail: matsubar@orgrxn.mbox.media.kyoto-u.ac.jp)
CH3
O
Ts
CH2(ZnI)2
(2, 1.5 equiv)
A molecular structure, which has a trimethylsilyl group in
close proximity to a hydroxy group spatially, converts methyl
on silicon into a good leaving group.
Ts
N
Ts
N
N
H3C
H3C
0 °C
CH3
OH
NiCl2dppe
(7.5 mol%)
THF
25 °C, 6 h
I
SiMe3
SiMe3
H2C
IZnH2C
CH3
3
SiMe3
1
4 (65%)
+
Cleavage of Si–C bonds via a hypercoordinate silicate has
been well discussed and studied both from mechanistic and the
synthetic viewpoints.1 The carbanion equivalent, which is re-
leased from the hypercoordinate silicate atom, has been used
for transition-metal-catalyzed cross-coupling reactions.2 In the
migration of an organo-group from a hypercoordinate silicate,
aryl, benzyl, alkynyl, alkenyl, and allyl groups have higher
migration tendency than alkyl groups.3 Among saturated alkyl
groups, methyl is the most labile. In some specific substrates,
it had been reported that a methyl group on Si can be removed
by an intramolecular coordination of an anionic oxygen atom
such as metal alkoxide or carboxylate.4 To complete this proc-
ess, the oxygen atom should be placed in the proximity of silicon
atom spatially by molecular design. When the coordination is
well tuned, the methyl on Si can be replaced with an alkoxide
group specifically; an alkoxy group on a Si will be replaced easi-
ly with an organolithium or -magnesium reagent. As a result of
these transformations, a variety of substituents can be introduced
on the Si atom starting from trimethylsilyl-substituted com-
pounds as shown in eq 1. The transformation will be a conven-
ient route to prepare chiral trialkylsilyl halides or alkoxides
which have three different alkyl groups on the silicon atom.
25 °C
Ts
N
O
H3C
H3C
Ts
N
CH3
O
H3C
CH3
O
SiMe2
H2C
SiMe2
H2C
5 (86%)
5 (16%)
Scheme 1. Preparation of cyclic silyl ether using methyl group
as a leaving group.
Figure 1. Detailed structural data around Si atom of 4.
Mtl1
O
The X-ray analysis of a single crystal of 4 gave structural
information as shown in Figure 1.7 The notable stereochemical
feature of compound 4 is a pyrrolidine ring which has geminal
substituents: One is an ꢀ-trimethylsilylvinyl group and the other
is a 1-hydroxy-1-methylethyl group. Disfavored steric interac-
tion between the pyrrolidine ring and trimethylsilyl group en-
forces the silyl group to the exo-position. The steric bulkiness
of the trimethylsilyl group also effects the conformation of the
1-hydroxy-1-methylethyl group. The hydroxy group, which is
smaller than methyl group, will come closer to the trimethylsilyl
group. As shown in Figure 1, the data of bond angles around Si
atom implied an interaction of hydroxy group and Si atom.8 The
sum of three bond angle around Si atom was 338ꢁ, which is big-
ger than the typical tetrahedral Si case (328ꢁ). The distance be-
Mtl2
O
H
O
O
– MeMtl1
X–
+ RMt2
ð1Þ
X-SiRR'R"
SiMe3
SiMe2
SiMe2R
SiRR'R"
In the course of our synthetic studies using bis(iodozincio)-
methane,5 we have found that a nickel-catalyzed arylmetallation
to an alkyne, which is followed by a cross-coupling with bis-
(iodozincio)methane,6 affords an allylic zinc species with a tet-
rasubstituted alkene unit (eq 2).
R
R
R
R
– Ni(0)
Ar
Ni-CH2ZnI
R
R
Ni(0)
Ar-I
Ar-Ni-I
CH2(ZnI)2
CH2–ZnI
ð2Þ
Ar
Oxidative
Addition
Reductive
Eliminatiom
Carbometallation
When this reaction was applied to N-(2-iodophenyl)-4-
˚
methyl-N-[3-(trimethylsilyl)prop-2-ynyl]benzenesulfonamide (1),
an intramolecular carbometallation lead to the formation of cy-
clic allylic zinc 3, which added to acetone to afford the alcohol
4 in 65% yield at 0 ꢁC accompanying a formation of a cyclic silyl
ether 5 in 16% yield (Scheme 1). When the reaction temperature
was kept at 25 ꢁC for 2 h after addition of acetone, the cyclic silyl
ether 5 was obtained in 86% yield. The formation of 5 was con-
sidered to be a result of a ring closure of zinc alkoxide of 4 by
releasing a methyl from Si atom.
tween Si and O atom was 2.97 A, which is shorter than the sim-
˚
ple addition of van der Waals radius of Si and O atoms (3.35 A).
Once the hydroxy group is converted into zinc–alkoxide, the cor-
responding pentacoordinate silicate will be formed smoothly.
As shown in eq 3, alkyne 6, which is a carbon analog of
compound 1, was also examined in the nickel-catalyzed reaction
followed by treatment with acetone at 50 ꢁC resulting formation
of the corresponding cyclic ether 7 in 78% yield.9 When the re-
action was performed at 25 ꢁC, the simple adduct 8 was obtained
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.2 (2009)
117
in 80% yield. Treatment of the intermediary allylic zinc species
with acetaldehyde at 50 ꢁC afforded the alcohol 9 in 98% yield
without the formation of the cyclic ether. Even after treatment
for 6 h at 50 ꢁC, the cyclic silyl ether was not detected (eq 4).
in the presence of acid catalyst and gave 16 in 39% overall
yield.11
Me
t-Bu
Me
Ph
OH
t-BuOK
Si Et
O
Bu
Ph
OH
Bu
ð9Þ
cat. TfOH
Bu
- acetone
Si
CH2(ZnI)2
(2, 1.5 equiv)
Si
Me
Me
Me
Me
14
15
16 (39% from 14)
Acetone
t-Bu
Et
t-Bu
Et
I
ð3Þ
NiCl2dppe
(7.5 mol%)
25 °C, 6 h
50 °C, 2 h
OH
O
Si
Me
SiMe3
SiMe3
6
Me
Me
Me
H+
7 (78%)
8
ð10Þ
O–K
R3
R2
15
R3
R2
R3
R2
Si
Si
Si
CH2(ZnI)2
(2, 1.5 equiv) CH3CHO
isomerization
retro-allylation
R1
R1
R1
Me
H
I
ð4Þ
NiCl2dppe
50 °C, 2 h
In this study, three methyl groups on a Si atom can be used
as leaving groups, although sp3 carbon–Si bonds is more stable
under various reaction conditions. In the present study, we can-
not show the preparation of an optically active trialkylsilyl alk-
oxide, which possesses high potential for organic synthesis.12
Proper design of a molecule which has similar structure to our
device will give a route to them.13
OH
(7.5 mol%)
25 °C, 6 h
SiMe3
SiMe3
6
9 (98%)
As shown in eq 5, the cyclic silyl ether 7 which was formed
from 6 in eq 3 was treated with butyllithium to afford the corre-
sponding ring-opening product 10 in 82% yield. When the alco-
hol 10 was treated with methyllithium and zinc(II) chloride in
THF at 50 ꢁC, the cyclic silyl ether 11, which was formed by
elimination of a methyl group, was obtained in 82% yield. The
product has two stereogenic centers and obtained as a mixture
of diastereomers in 1:1 ratio. In this transformation, the cyclic
ether 7, which would be formed by butyl group elimination from
10, was not detected. In this case also, methyl group was substi-
tuted selectively to give 11. Without isolation of the alcohol 10,
the sequential treatment with butyllithium, zinc(II) chloride, and
ethylmagnesium bromide in THF afforded the corresponding or-
ganosilane 12 carrying butyl, ethyl, and methyl groups in 53%
overall yield (eq 6).
References and Notes
1
2
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Hudrlik, Tetrahedron Lett. 1992, 33, 6747. d) P. F. Hudrlik, J. O. Arango,
Y. M. Hijji, C. O. Okoro, A. M. Hudrlik, Can. J. Chem. 2000, 78, 1421.
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Oshima, S. Matsubara, Bull. Chem. Soc. Jpn. 2001, 74, 753.
3
4
5
Me
Me
Me
Me
Me
BuLi
MeLi / ZnCl2
THF
Me
ð5Þ
ð6Þ
O
THF, -78 °C
OH
Bu
O
Si
Si
Si
Me
Me
Me
Me
Me
Bu
7
10 (82%)
11 (82%)
a) Y. Yamamoto, Y. Takeda, K. Akiba, Tetrahedron Lett. 1989, 30, 725.
b) W. Kirmse, F. Sollenbohmer, J. Chem. Soc., Chem. Commun. 1989,
1) BuLi /
Me
Me
EtMgBr
THF, 25 °C
THF, -78 °C
Me
Me
Me
Me
¨
¨
2) ZnCl2 / 50 °C
774. c) C. Eaborn, P. B. Hitchcock, J. Chem. Soc., Perkin Trans. 2 1991,
1137. d) H. Taguchi, K. Ghoroku, M. Tadaki, A. Tsubouchi, T. Takeda,
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OH
Si
O
O
Si
Si
Me
Bu
Me Bu
Et
Me
Me
7
11
12 (53% oveall)
Treatment of alcohol 12 with butyllithium and zinc(II) chlo-
ride gave the cyclic ether 13 via a selective elimination of methyl
(eq 7). Nucleophilic attack of organolithium reagent onto 13 af-
forded the corresponding ring-opening product 14. Considering
the structure of the starting material 6, the production of 14
showed that three methyl groups on Si atom can be used as leav-
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H. Yoshino, N. Toda, M. Kobata, K. Ukai, K. Oshima, K. Utimoto, S.
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6
7
For the X-ray diffraction, a crystal was mounted on a glass fiber coated with
epoxy resin. Measurements were made on a Rigaku Mercury charge-coupled
deveice (CCD) system with graphite monochromated Mo Kꢀ radiation. Crys-
tal data: 4, Mr ¼ 429:64, monocliꢁnic C2=c, a ¼ 33:095ð6Þ, b ¼ 10:9080ð19Þ,
3
˚
˚
Me
Me
Me
Me
c ¼ 13:108ð2Þ A, ꢁ ¼ 95:469ð3Þ , V ¼ 4710:4ð14Þ A , Z ¼ 8, ꢂ ¼ 1:212
BuLi / ZnCl2
3
Mg/m , ꢃ(Mo Kꢀ) = 0.71073 A, T ¼ 296 K, 2ꢄmax ¼ 54:0ꢁ, R ¼ 0:0765
˚
ð7Þ
THF, -78 to 50 °C
O
OH
Si
for 12076 refrections (I > 2ꢅðIÞ).
Si
Et
Me
Bu
12
Bu
Et
8
9
K. Tamao, T. Hayashi, Y. Ito, M. Shiro, Organometallics 1992, 11, 2099.
This reaction was monitored by 1H NMR using THF-d8 as a solvent. Cyclic
ether was formed in situ before aqueous work-up.
13 (91%)
Me
Me
Me
Me
10 R. A. Benkeser, M. P. Siklosi, E. C. Mozdzen, J. Am. Chem. Soc. 1978, 100,
2134.
t-BuLi
ð8Þ
THF, -78 °C
O
OH
Bu
Si
Si
Et
t-Bu
11 a) T. Morita, Y. Okamoto, H. Sakurai, Tetrahedron Lett. 1980, 21, 835. b) A.
Hosomi, H. Sakurai, Chem. Lett. 1981, 85. c) G. A. Olah, A. Husain, B. G. B.
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12 a) L. H. Sommer, C. L. Parker, K. W. Michael, J. Am. Chem. Soc. 1964, 86,
3271. b) M. Oestreich, U. K. Schmid, G. Auer, M. Keller, Synthesis 2003,
2725.
Bu
Et
13
14 (70%)
As shown in eq 9, a device for methyl group migration can
be dismantled as follows. Treatment of 14 with t-BuOK trig-
gered a retro-allylation (eq 10)10 accompanying a loss of ace-
tone, followed by an isomerization, and afforded an allylsilane
derivative 15. The allyl moiety in 15 was removed with alcohol
13 Supporting Information is available electronically on the CSJ-journal Web
site, http://www.csj.jp/journals/chem-lett/index.html.
Published on the web (Advance View) December 27, 2008; doi:10.1246/cl.2009.116