A Silyl Substituent Can Dictate a Concerted Electrocyclic Pathway: Inward Torquoselectivity in the Ring Opening of 3-Silyl-1-cyclobutene

Full text of DOI:10.1002/1521-3773(20010105)40:1<189::AID-ANIE189>3.0.CO;2-L

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A Silyl Substituent Can Dictate a Concerted
Electrocyclic Pathway: Inward
140 °C, 1 h
m-xylene
n-C8H17
n-C8H17
Torquoselectivity in the Ring Opening of
1
2
3-Silyl-1-cyclobutene
Masahiro Murakami,* Yasufumi Miyamoto, and
Yoshihiko Ito*
SiMe2Ph
SiMe2Ph
140 °C, 1 h
SiMe2Ph
+
m-xylene
n-C8H17
n-C8H17
8 17
n-C H
A thermal conrotatory ring-opening reaction of cyclo-
butenes to produce 1,3-butadienes has been a source of
continuing interest from the viewpoints of both organic and
theoretical chemistry. The substituent effects on this reaction
have been well studied in terms of torquoselectivity, namely,
3
4
5
SiMe2Ph
n-C8H17
SiMe2Ph
n-C H
1
40 °C, 1 h
8
17
[
1]
toluene
inward versus outward rotation. Theoretical studies proved
that the stereochemical trend is subject to electronic control
rather than steric control. For example, outward rotation is
preferred by electron-donating substituents, such as a hydroxy
group at the 3-position. In contrast, it was predicted that an
electron-accepting substituent at the 3-position would prefer
6
7
Scheme 1. Ring-opening reactions of cyclobutenes 1, 3, and 6.
CMe3
CMe3
1
40 °C, 12 h
[
2]
inward rotation. However, experimentally verified exam-
ples of this phenomenon have been limited to the CHO,
m-xylene
R
R
R
9
8
[
3]
[4]
[5]
[6]
COCl, and CF3 substituents. Even CO Et and CN
2
SiMe3
SiMe3
groups favor outward rotation. On the other hand, it is known
140 °C, 9 h
m-xylene
SiMe3
that, while the silicon atom in a Si�C linkage is depleted of its
+
R
negative charge, silicon can accommodate electron density
from a neighboring p orbital in its low lying s* orbital.[ The
HOMO of the transition state of the ring-
R
11
12
10
7]
Scheme 2. Ring-opening reactions of cyclobutenes 8 and 10. R ˆ PhMe
2
C.
opening reaction of 1-cyclobutene is pri-
marily concentrated on the cleaving
suggests that the ratio was kinetically determined. The
preference for inward rotation was opposed to what might
be expected on steric grounds. Moreover, only the cis isomer 7
C3�C4 s orbital, which is a good electron
donor. Thus, we anticipated that a silyl
substituent at the 3-position would possi-
[9]
was produced by inward rotation of 6. The inward-selective
bly accept the HOMO electron of the transition state, to result
in its stabilization. Here, we report the interesting effects of
the silyl substitution on a concerted thermal ring-opening
reaction of 1-cyclobutene.
ring opening reaction of 6 also took place at 1108C in toluene
and completed in 1 h. These results suggested that a silyl
[10]
substituent at the 3-position of 1-cyclobutene accelerates the
ring-opening reaction and prefers inward, rather than out-
ward, rotation.
To get further experimental verification of this effect,
cyclobutenes 8 and 10 were prepared and their reactivities
compared. Cyclobutenes 8 and 10 have an identical structure
except the tertiary element, that is, a carbon atom of a tertiary
butyl group versus a silicon atom of a trimethylsilyl group.
When heated at 1408C, 8 underwent ring opening with
First, the reactions of 1-octyl-1-cyclobutene (1) and its silyl-
substituted analogues 3 and 6 were examined (Scheme 1).
After heating a solution of 1 at 1408C for 1 h, only 12% of 1
underwent a ring-opening reaction to produce 1,3-diene 2. In
[
8]
contrast, when a mixture of 3 and 6 (58:42) was heated under
the same conditions, complete conversion to the correspond-
ing 1,3-dienes (4, 5, and 7; 96% overall yield) was observed.
An NMR analysis of the product mixture revealed that the
ratio (4 ‡ 5):7 was identical with that of 3:6, to confirm that
the 1,3-dienes 4 and 5 arose from 3, and the 1,3-diene 7 from 6,
in a specific manner. The reactions of 3 and 6 are therefore
described separately in Scheme 1.
outward rotation to give exclusively the trans isomer 9
[9]
(
1
92%), which is in agreement with the result of 3-methyl-
-cyclobutene.[11] On the other hand, 10 furnished a mixture of
cis isomer 11 and trans isomer 12, with the former predom-
inating (11:12 ˆ 69:31; 95% overall yield). The reaction of 10
Interestingly, the cis isomer 4, formed by inward rotation,
was favored over the trans isomer 5, formed by outward
rotation (4:5 ˆ 83:17). The ratio 4:5 was constant through the
reaction and even after heating for a longer period, which
-1
-1
(
1
k ˆ 0.53 h at 1408C) was faster than that of 8 (k ˆ 0.32 h at
408C). The proportion of cis and trans isomers 11:12 was
again opposite to what would be predicted as a result of
simple steric argument. The contrasting results of 8 and 10
confirm that a silyl substituent prefers inward rotation.
For density functional theory calculations, 3-silyl-1-cyclo-
butene (13) was taken as a model and the transition states for
outward and inward rotations were computationally locat-
ed.[ The potential-energy diagram is shown in Figure 1. The
activation energy for inward rotation to cis product 15 is
[
*] Prof. M. Murakami, Prof. Y. Ito, Y. Miyamoto
Department of Synthetic Chemistry and Biological Chemistry
Kyoto University
Yoshida, Kyoto 606-8501 (Japan)
Fax : (‡ 81)75-753-5668
12]
E-mail: murakami@sbchem.kyoto-u.ac.jp
yoshi@sbchem.kyoto-u.ac.jp
�
1
1.67 kcalmol lower than that of outward rotation to trans
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product 14. In the case of 3-methyl-1-cyclobutene, the carbon
counterpart of 13, the calculated preference is just the
opposite, that is, the outward rotation is favored by
Experimental Section
1
1 and 12: A solution 10 (30.0 mg, 0.10 mmol) in m-xylene (2 mL) was
heated at 1408C for 9 h under an argon atmosphere. The cooled reaction
mixture was filtered through a pad of florisil and the filtrate was purified by
preparative thin-layer chromatography (silica gel, n-hexane) to afford 11
�
1 [13]
5
.3 kcalmol .
1
and 12 (69:31, 28.6 mg, 95%) as a colorless oil. 11: H NMR (200 MHz,
CDCl
.4 Hz, 1H), 5.43 (d, J ˆ 14.8 Hz, 1H), 6.36 (d, J ˆ 14.8 Hz, 1H), 7.18 ± 7.38
m, 5H); 13C NMR (75 MHz, CDCl
): d ˆ 0.3, 28.4, 42.8, 111.3, 125.9, 126.3,
28.1, 131.3, 147.2, 148.0, 156.2; elemental analysis: calcd for C16 24Si: C
8.62, H 9.90; found: C 78.86, H 10.14. 12: H NMR (300 MHz, CDCl ):
3
): d ˆ 0.12 (s, 9H), 1.42 (s, 6H), 5.09 (d, J ˆ 1.4 Hz, 1H), 5.15 (d, J ˆ
1
(
3
1
7
H
1
3
d ˆ 0.07 (s, 9H), 1.44 (s, 6H), 5.09 (s, 1H), 5.35 (s, 1H), 5.90 (d, J ˆ 18.6 Hz,
13
1
H), 6.09 (d, J ˆ 18.6 Hz, 1H), 7.11 ± 7.30 (m, 5H); C NMR (75 MHz,
CDCl
): d ˆ � 1.5, 28.9, 42.9, 109.8, 125.7, 126.3, 128.1, 130.9, 144.1, 148.3,
56.0; elemental analysis: calcd for C16 24Si: C 78.62, H 9.90; found: C
9.01, H 10.22.
3
1
7
H
Received: June 26, 2000 [Z15338]
[
1] W. R. Dolbier, Jr., H. Koroniak, K. N. Houk, C. Sheu, Acc. Chem. Res.
996, 29, 471.
2] W. Kirmse, N. G. Rondan, K. N. Houk, J. Am. Chem. Soc. 1984, 106,
989.
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708; b) R. Hayes, S. Ingham, S. T. Saengchantara, T. W. Wallace,
1
[
[
7
3
Figure 1. Potential-energy diagram of ring opening reactions of 3-silyl-1-
cyclobutene 13.
Tetrahedron Lett. 1991, 32, 2953.
4] W. R. Dolbier, Jr., H. Koroniak, D. J. Burton, A. R. Bailey, G. S. Shaw,
[
S. W. Hansen, J. Am. Chem. Soc. 1984, 106, 1871.
[
[
5] E. Piers, Y.-F. Lu, J. Org. Chem. 1989, 54, 2267.
6] C. W. Jefford, G. Bernardinelli, Y. Wang, D. C. Spellmeyer, A. Buda,
K. N. Houk, J. Am. Chem. Soc. 1992, 114, 1157.
For the inward transition state of 13, the angle C4-C3-Si5 is
9
28 and the dihedral angle C4-C3-Si5-H1 is 1708. These
[7] a) M. A. Brook, Silicon in Organic, Organometallic, and Polymer
Chemistry, Wiley, New York, 2000, pp. 27 ± 38; b) J. C. Giordan, J. H.
Moore, J. Am. Chem. Soc. 1983, 105, 6541; c) J. C. Giordan, J. Am.
Chem. Soc. 1983, 105, 6544.
structural features indicate that the Si5�H1 s* orbital eclipses
well with the breaking C3�C4 s orbital, where the HOMO
electron is primarily concentrated, and thus accommodates
some of the HOMO electron density. For the outward
transition structure, in which the angle C4-C3-Si5 is as much
as 1328, an analogous stabilizing interaction of the silicon s*
orbital with the HOMO is smaller. Thus, the observed inward
preference and accelerating effect of a silyl substituent can be
explained by assuming electron-accepting interactions of the
rather low lying s* orbital of the silicon atom with the HOMO
[
8] A 58:42 mixture of 3 and 6, prepared by the reaction of dimethyl-
phenylsilyllithium with 3-bromo-1-octyl-1-cyclobutene, proved diffi-
cult to separate by chromatographic means and was used as the
mixture.
[9] No formation of the other stereoisomer was observed through the
reaction.
[
10] No appreciable reaction of 1 occurred at 1108C. The reaction of 3 at
1108C was slower than that of 6 and took 3 days to reach completion.
The ratio of 4:5 formed at 1108C was 89:11.
11] a) E. Gil-Av, J. Shabtai, J. Org. Chem. 1964, 29, 257; b) H. M. Frey,
Trans. Faraday Soc. 1964, 60, 83.
12] The DFT calculations were performed with the Gaussian 98 program
(Gaussian 98, Revision A.6, Gaussian, Inc., Pittsburgh, PA, USA,
1998) with a CRAY Origin 2000 in the Supercomputer Laboratory,
Institute for Chemical Research, Kyoto University. Geometry opti-
mization and energy calculations were carried out at the B3LYP/
[
[
14]
orbital of the inward transition state.
The substitution effects of silicon are now widely utilized in
[
[
15]
a variety of organic reactions: The stabilization of b-cations
in electrophilic reactions of organosilicon compounds is one
of the most exploited. The present study discloses an
interesting influence of silyl substituents on a concerted
electrocyclic reaction. Further investigations of the effects of
silyl substituents on other concerted electrocyclic reactions
are underway.
6
± 31G(d) level.
[
13] S. Niwayama, E. A. Kallel, D. C. Spellmeyer, C. Chimin, K. N. Houk,
J. Org. Chem. 1996, 61, 2813.
14] Inagaki et al. obtained similar results by performing DFT calculations
on 13. They explained the calculated preference for inward rotation by
assuming geminal s bond participation. H. Ikeda, T. Kato, S. Inagaki,
Org. Lett., submitted.
[
[15] E. W. Colvin, Silicon Reagents in Organic Synthesis, Academic Press,
London, 1989.
190
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