Electrophilic Cleavage of One Silicon-Carbon Bond of Pentacoordinate Tetraorganosilanes: Synthesis of Silalactones

Article abstract of DOI:10.1002/anie.200352705

Dealkylation of trialkylvinylsilanes: The direct electrophilic cleavage of one silicon-carbon bond of tetraorganosilanes activated by the intramolecular pentacoordination of a slightly nucleophilic ester or carboxylic acid group provides a novel route to

Full text of DOI:10.1002/anie.200352705

Communications
Synthesis of Silalactones
Electrophilic Cleavage of One Silicon–Carbon
Bond of Pentacoordinate Tetraorganosilanes:
Synthesis of Silalactones**
Mitsuru Shindo,* Kenji Matsumoto, and Kozo Shishido
The cleavage of silicon–carbon bonds is an important process
in synthetic organic chemistry.[1] The Tamao–Fleming oxida-
tion[2] is one general method for peroxide oxidation of
organosilicon bearing at least one heteroatom or aromatic
group, sometimes with the aid of a fluoride ion. In addition to
the several published reports on intramolecular nucleophilic
attack of an oxyanion on silicone leading to demethylation[3]
or dephenylation,[4] there have also been some examples of
electrophilic cleavage under strongly acidic conditions or by
Scheme 2. The unexpected formation of silalactone 4a by the cleavage
À
of a Si C bond.
silicon–carbon bond was cleaved electrophilically by iodine.
This is one of only a fewexamples of direct electrophilic
3
À
C(sp ) Si bond cleavage. These results prompted us to
examine the generality of this oxidation reaction by using
various types of (Z)-b-silyl-a,b-unsaturated acids.
using fluorosilicates.[5] However, the selective cleavage of one
3
À
carbon–silicon bond, especially the C(sp ) Si bond, in a
As shown in Table 1, the silalactones were obtained in
good to excellent yields using the acrylic acids bearing several
types of substituents, including o-silylbenzoic acids. When N-
iodosuccinimide (NIS) was used in place of iodine, the
reaction proceeded faster at room temperature (entry 2,
Method B), but in some cases, lower yields were obtained
(entry 8). The ethyl, isopropyl, and phenyl groups, as well as
the methyl group, were also cleaved (entries 4, 5 and 6). Since
the substrate that have a terminal alkenyl group also
furnished the silalactone 4h (entry 10), the cleavage of the
tetraorganosilane is a challenging problem that has yet to be
solved. Herein, we describe the direct electrophilic cleavage
of one silicon–carbon bond activated by intramolecular
pentacoordination of carbonyl oxygen, thus leading to the
novel synthesis of silalactones (Scheme 1).
À
Me Si bond proceeds faster than the iodination of the olefins.
The methoxy and siloxy substituents were stable under these
reaction conditions (entries 9 and 11). In the reaction of the
substrate bearing the tert-butyldimethylsilyl group, the methyl
group was selectively cleaved (entries 3 and 9). It is note-
worthy that the methyl group rather than the phenyl group
was selectively cleaved (entry 13).
À
Scheme 1. Cleavage of Si C bonds.
In the course of our studies on ynolates (1),[6] we found
that a highly stereoselective olefination of acylsilanes (2)
afforded (Z)-b-trialkylsilylacrylates (3).[7] When the desilyl-
iodination of the acrylic acid ester 3a upon treatment with
iodine was attempted, the desired b-iodoacrylate was not
produced at all, but the unexpected 2H-[1,2]oxasilol-5-one
(silalactone 4a) was obtained in good yield (Scheme 2).
Silalactone 4a was also obtained by using (Z)-b-trialkyl-
silylacrylic acid 5 as the substrate in the presence of iodine
and pyridine under reflux in CCl4 (Table 1). The fact that one
equivalent of iodomethane was generated indicated that the
The X-ray crystal structures[8] of (Z)-b-trimethylsilyl-a-
methylcinnamic acid (5 f) may explain why the carbon–silicon
bonds are unusually activated (Figure 1). The distance
À
between the silicon atom and the carbonyl oxygen (Si O2)
is 2.826 Š, which is shorter than the sum of the van der Waals
radii (3.35 Š) for the silicon and oxygen atoms, and the bond
À
À
À
length of C9 Si is longer than that of the C13 Si or C14 Si
bonds. The angles of C9-Si-C5, C9-Si-C13, and C9-Si-C14 are
all slightly smaller (105.2–105.8) than those of a tetrahedral
structure. From these values, it would seem that in the crystal
structure, a weak intramolecular coordination of the neutral
carbonyl oxygen to the tetraorganosilane, probably due to the
rigid geometry, gives rise to the hypervalent silicon struc-
ture,[9] in which O2 and C9 are at apical positions. In the
[*] Prof. Dr. M. Shindo,K. Matsumoto,Prof. Dr. K. Shishido
Institute for Medicinal Resources
University of Tokushima
Sho-machi 1,Tokushima 770-8505 (Japan)
Fax: (+81)88-633-7294
CDCl3 solution structure, however, the 29Si NMR spectra did
E-mail: shindo@ph2.tokushima-u.ac.jp
[10]
not showa significant upfield shift,
which would generally
be seen in a pentacoordinate silane,[11] relative to that
observed in the corresponding tetrahedral analogues. Since
[**] This work was supported in part by a Grant-in-Aid for Scientific
Research in the Priority Area (A) “Exploitation of Multi-Element
Cyclic Molecules” from the Ministry of Education,Culture,Sports,
Science and Technology,Japan,and PRESTO,JST. K.M. thanks the
Japan Society for the Promotion of Science (JSPS) for a research
fellowship.
even at À908C in CD2Cl2 only one singlet was observed in the
1
À
H NMR spectrum for the Me3Si group, the C5 Si bond
rotates freely. As a result, these compounds are not a perfect
trigonal-bipyramidal pentacoordinated structure,[12] corre-
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
104
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DOI: 10.1002/anie.200352705
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Angewandte
Chemie
À
Table 1: Cleavage of Si C bonds of (Z)-b-silylacrylic acids 5 to provide silalactones 4.
cleavage. Because the electron defi-
cient alkene bears the ester group,
the electrophilic iodine attacks only
the
(Figure 2).
The reactivity and synthetic util-
R group, not the alkene
ity of the silalactones are not well
known.[14] Reduction by LiAlH4
gave the cyclic silyl ether 6 in good
yield. Grignard reagents attack the
silicon selectively to afford the (Z)-
b-silylacrylic acids (Scheme 3). Pal-
ladium catalyzed cross-coupling
reactions (Hiyama coupling)[15] of
the silalactone with aryl iodides
furnishes the (Z)-b-arylacrylic acid
in good yield without the use of a
fluoride source. These transforma-
tions indicate the synthetic utility of
the silalactones.
Entry
R1
R2
R3
R4
Method[a]
t [h]
4
Yield [%]
1
2
3
4
5
6
7
8
Me
Me
Me
Me
Me
Me
Me
Me
Bn
Bn
Bn
Bn
Bn
Bn
Ph
Ph
Me
Me
Me
Et
iPr
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
tBu
Et
iPr
Ph
Me
Me
tBu
Me
Me
Me
Ph
A
B
A
A
A
A
A
B
A
A
A
A
A
29
1
4a
4a
4b
4c
4d
4e
4 f
4 f
4g
4h
4i
90[b]
96
81
94
74
87
95
65
85
65
64
81
90
18
28
35
34
20
1.5
19
28
50
28
23
9
Me
Me
MeOC2H4
4-pentenyl
H
10
11
12
13
PhCH(OTBS)
o-silylbenzoic acid
o-silylbenzoic acid
4j
4k
[a] Method A: I2,pyridine in CCl under reflux. Method B: NIS in CH2Cl2 at RT. Usually 0.1–0.5 mmol
scale. [b] 82% yield for 10 mmol scale.
4
In conclusion, we have found a
newmethodol-
ogy
for
the
direct electro-
À
philic cleavage of C Si bonds activated
by the formation of pentacoordinate orga-
nosilanes. This can be considered as an
À
activation method for stable C Si bonds.
Figure 2. The
push–pull mecha-
nism.
The silalactones would be useful for the
preparation of multisubstituted alkenes
and silicon-containing compounds.
Figure 1. ORTEP drawing of the X-ray crystal structure of 5 f (thermal
ellipsoids set at the 50% probability level) selected distances [Š] and
angles [8]: Si1-O2,2.826(1); Si1-C9,1.875(2); Si1-C13,1.862(2),Si1-
C14,1.865(2); Si1-C5,1.922(2); C5-Si1-C9,105.26(9); C9-Si1-C13,
105.8(1); C9-Si1-C14,105.5(1).
Scheme 3. Transformations of silalactone 4a.
sponding to an intermediate at the early stage of the SN2
reaction at the silicon.
From this spectral information, the reaction mechanism
can be elucidated based on the cooperative push–pull
Received: August 22, 2003 [Z52705]
À
mechanism, which involves the double activation of the C9
Keywords: cleavage reaction · hypervalent compounds · iodine ·
silalactones · silanes
.
Si bond mainly by the electrophilic iodine (pull) and
secondarily by the nucleophilic oxygen (push). Since the
methyl group was preferentially cleaved rather than the
phenyl group, the electrophile would directly attack the
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3
À
C(sp ) Si s bond bearing the higher HOMO energy rather
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[13]
À
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Angew. Chem. Int. Ed. 2004, 43, 104 –106
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
105

Communications
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À
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[8] CCDC-218067 (5 f) contains the supplementary crystallographic
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À
[13] The electrophilic selective cleavage of the Ph Si bond would be
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À
À
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106
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