Studies on the thermal ring-opening reactions of cis-3,4-bis(organosilyl) cyclobutenes

Article abstract of DOI:10.1021/jo070039n

(Chemical Equation Presented) Three cis-3,4-bis(organosilyl)cyclobutenes were synthesized, and their thermal ring-opening reactions were studied. The ring-opening reaction of cis-3,4-bis(trimethylsilyl)cyclobutene proceeded remarkably faster than that of

Full text of DOI:10.1021/jo070039n

Studies on the Thermal Ring-Opening Reactions of
cis-3,4-Bis(organosilyl)cyclobutenes
Munehiro Hasegawa and Masahiro Murakami*
Department of Synthetic Chemistry and Biological Chemistry, Kyoto UniVersity,
Katsura, Kyoto 615-8510, Japan
murakami@sbchem.kyoto-u.ac.jp
ReceiVed January 8, 2007
Three cis-3,4-bis(organosilyl)cyclobutenes were synthesized, and their thermal ring-opening reactions
were studied. The ring-opening reaction of cis-3,4-bis(trimethylsilyl)cyclobutene proceeded remarkably
faster than that of cis-3,4-dimethylcyclobutene. The significant rate acceleration was explained by assuming
(i) stabilization of the transition state by electron delocalization from the cyclobutene HOMO to the
Si-CH3 σ* orbital, (ii) destabilization of the ground state by intramolecular interaction between the
C-Si σ orbitals and the π orbital of cyclobutene, and (iii) through-space steric repulsion of the two
bulky trimethylsilyl groups in a cis arrangement. The ring-opening reaction of unsymmetrical cis-3,4-
bis(arylsilyl)cyclobutenes having electronically different arylsilyl groups was also examined. The inward
preference increased in the order, p-CH3OC6H4-Si, C6H5-Si, p-CF3C6H4-Si, supporting the interpretation
of the origin of the inward preference of silyl substituents on the basis of a stabilizing interaction between
the cyclobutene HOMO and the Si-C σ* orbital at the transition state.
SCHEME 1. Ring-Opening Reaction of 3-Substituted
Cyclobutene
1. Introduction
An electrocyclic ring-opening reaction of cyclobutenes to
produce conjugated 1,3-dienes has been a source of continuing
interest in terms of the rotational behavior of the substituents.
Woodward-Hoffmann rules state that, under thermal conditions,
the substituents located at the 3- and 4-positions rotate in a same
direction, that is, in a conrotatory fashion.1 Another stereo-
chemical feature, which is also of fundamental importance,
concerns the direction of the rotation relative to the cleaving σ
bond; a substituent can move toward the breaking C3-C4 bond
(inward rotation) or away from it (outward rotation) during the
thermal ring-opening reaction. The selectivity of the rotational
direction has been termed “torquoselectivity” by Houk and
has been intensively studied theoretically and experimentally
since the 1980s (Scheme 1, the 1,3-butadiene-type products are
depicted in an s-cis conformation for convenience in this paper).2
Simple steric arguments would predict that outward rotation
is preferred over inward rotation because inward rotation suffers
from more steric repulsion than outward rotation. However, there
are examples which contradict this expectation. Substituents of
(2) (a) Durst, T.; Breau, L. In ComprehensiVe Organic Synthesis; Trost,
B. M., Fleming, I., Paquette, L. A., Eds.; Pergamon Press: Oxford, 1991;
Vol. 5, pp 675-697. (b) Dolbier, W. R., Jr.; Koroniak, H.; Houk, K. N.;
Sheu, C. Acc. Chem. Res. 1996, 29, 471.
(1) Woodward, R. B.; Hoffmann, R. The ConserVation of Orbital
Symmetry; Academic Press: New York, 1970.
10.1021/jo070039n CCC: $37.00 © 2007 American Chemical Society
Published on Web 04/14/2007
3764
J. Org. Chem. 2007, 72, 3764-3769

Ring Opening of cis-3,4-Bis(organosilyl)cyclobutenes
SCHEME 2. Ring-Opening Reaction of
3-Formylcyclobutene
SCHEME 4. Ring-Opening Reaction of 3-Silylcyclobutene
ing rotational direction. Thus, the stereochemical outcome of
the ring-opening reaction reflects the magnitude of rotational
preferences of the two substituents.
SCHEME 3. Ring-Opening Reaction of Unsymmetrical
cis-3,4-Disubstituted Cyclobutenes
We recently discovered the interesting preference of silyl
groups for inward rotation (Scheme 4).5-8 This preference of
silyl substituents can be understood on the basis of the Houk’s
theory by invoking the electron-accepting nature of silyl substit-
uents. Whereas the σ orbital of a Si-C linkage is energetically
high-lying enough to donate its electron density to nearby vacant
orbitals, the antibonding σ* orbital of a Si-C linkage is ener-
getically low-lying.9 In addition, the Si-C σ* orbital is polarized
toward the silicon end due to the difference in electronegativity.
Therefore, the Si-C σ* orbital is potentially a good electron
acceptor, accommodating electron density on the silicon end.
At the inward transition state, the energetically low-lying σ*
orbital on silicon is in the vicinity of the HOMO of the opening
cyclobutene skeleton, that is, the breaking C3-C4 σ orbital
which is distorted by conrotation. Thus, overlap can occur
between the two orbitals, providing stabilization to the inward
transition state through the electron delocalization.
an electron-accepting nature rotate preferentially inward; that
is, they move into a significantly more congested environment.3
For example, the ring-opening reaction of 3-formylcyclobutene
produces (Z)-penta-2,4-dienal stereoselectively via inward rota-
tion of the formyl group (Scheme 2).3a An orbital interaction
theory proposed by Rondan and Houk provides a clear explana-
tion for the contrasteric rotational behavior of electron-accepting
substituents.4 During the thermal ring-opening reaction, the σ
orbital connecting the 3- and 4-carbons of cyclobutene breaks
up in a conrotatory fashion and the distorted σ orbital becomes
the HOMO at the transition state. The electron density of the
HOMO is mostly concentrated between the 3- and 4-carbons.
Likewise, the σ* orbital of the C3-C4 linkage, which has a
node in the middle, is distorted by conrotation to become the
LUMO of the transition state. When a substituent at the
3-position rotates inward, it approaches the distorted C3-C4
linkage of the parent cyclobutene skeleton, and at the transition
state, it comes into the vicinity of the HOMO, which is
potentially a good electron donor. If the rotating substituent
possesses an energetically low-lying vacant orbital, it accepts
electron density from the HOMO. Such electron delocalization
stabilizes the inward transition state relative to the outward
transition state.
It is interesting to examine the substituent effects on the ring-
opening reaction of cis-3,4-disubstituted cyclobutenes (Scheme
3). Since both substituents rotate in the same direction under
thermal conditions according to the Woodward-Hoffmann
rules, one substituent has to rotate outward and the other has to
rotate inward. If the two different substituents in a cis arrange-
ment possess opposite preferences in torquoselectivity, their
preferences reinforce the torquoselectivity. On the other hand,
if the two substituents possess the same torquoselective prefer-
ences, either outward or inward, their preferences for rotational
direction have to mismatch and competition arises in determin-
Our communication5a was followed by two papers, wherein
the preference of inward rotation of silyl substituents was studied
from a theoretical point of view. Whereas the theoretical study
by Houk’s group supports our interpretation based on the
HOMO-σ* interaction,10 Inagaki and his co-workers offer the
explanation of geminal σ-bond participation.11 If an electroni-
cally biased substituent, either electron-withdrawing or electron-
donating, is introduced on silicon at the 3-position of a cyclo-
butene, opposite electronic influences are excepted for the sub-
stituent depending on which electronic interaction, HOMO-
σ* interaction or geminal σ-bond participation, dominates at
the transition state (vide infra). In order to obtain experimental
information about the origin of the contrasteric inward prefer-
ence of silyl substituents, we synthesized one symmetrical and
(5) (a) Murakami, M.; Miyamoto, Y.; Ito, Y. Angew. Chem., Int. Ed.
2001, 40, 189. (b) Murakami, M.; Miyamoto, Y.; Ito, Y. J. Am. Chem.
Soc. 2001, 123, 6441. (c) Murakami, M.; Hasegawa, M. Angew. Chem.,
Int. Ed. 2004, 43, 4874.
(6) A similar or related effect of silyl substituents was observed in other
electrocyclic reactions. A ring-opening reaction of silyloxetene: (a) Shindo,
M.; Matsumoto, K.; Mori, S.; Shishido, K. J. Am. Chem. Soc. 2002, 124,
6840. (b) Mori, S.; Shindo, M. Org. Lett. 2004, 6, 3945. Nazarov
cyclization: (c) Denmark, S. E.; Wallace, M. A.; Walker, C. B., Jr. J. Org.
Chem. 1990, 55, 5543. (d) Smith, D. A.; Ulmer, C. W., II. J. Org. Chem.
1993, 58, 4118.
(7) For inward preference of tin and boron substituents, see: (a)
Murakami, M.; Hasegawa, M.; Igawa, H. J. Org. Chem. 2004, 69, 587. (b)
Murakami, M.; Usui, I.; Hasegawa, M.; Matsuda, T. J. Am. Chem. Soc.
2005, 127, 1366.
(8) For an accelerating effect of (organosilyl)methyl substituents in the
ring opening of benzocyclobutenes, see: Matsuya, Y.; Ohsawa, N.; Nemoto,
H. J. Am. Chem. Soc. 2006, 128, 412.
(9) (a) Bock, H. Angew. Chem., Int. Ed. Engl. 1989, 28, 1627. (b)
Giordan, J. C. J. Am. Chem. Soc. 1983, 105, 6544. (c) Giordan, J. C.; Moore,
J. H. J. Am. Chem. Soc. 1983, 105, 6541.
(10) (a) Lee, P. S.; Zhang, X.; Houk, K. N. J. Am. Chem. Soc. 2003,
125, 5072. (b) Shindo, M.; Sato, Y.; Shishido, K. J. Org. Chem. 2000, 65,
5443.
(11) (a) Yasui, M.; Naruse, Y.; Inagaki, S. J. Org. Chem. 2004, 69, 7246.
(b) Ikeda, H.; Kato, T.; Inagaki, S. Chem. Lett. 2001, 270.
(3) (a) Rudolf, K.; Spellmeyer, D. C.; Houk, K. N. J. Org. Chem. 1987,
52, 3708. (b) Piers, E.; Lu, Y.-F. J. Org. Chem. 1989, 54, 2267. (c) Dolbier,
W. R., Jr.; Gray, T. A.; Keaffaber, J. J.; Celewicz, L.; Koroniak, H. J. Am.
Chem. Soc. 1990, 112, 363. (d) Jefford, C. W.; Bernardinelli, G.; Wang,
Y.; Spellmeyer, D. C.; Buda, A.; Houk, K. N. J. Am. Chem. Soc. 1992,
114, 1157. (e) Niwayama, S.; Houk, K. N. Tetrahedron Lett. 1993, 34,
1251 and references therein.
(4) (a) Rondan, N. G.; Houk, K. N. J. Am. Chem. Soc. 1985, 107, 2099.
(b) Kirmse, W.; Rondan, N. G.; Houk, K. N. J. Am. Chem. Soc. 1984, 106,
7989.
J. Org. Chem, Vol. 72, No. 10, 2007 3765

Hasegawa and Murakami
SCHEME 5. Synthesis of
SCHEME 6. Synthesis of Unsymmetrical
cis-3,4-Bis(trimethylsilyl)cyclobutene 7a
cis-3,4-Bis(organosilyl)cyclobutenes 16a
a DME ) 1,2-dimethoxyethane; cod ) 1,5-cyclooctadiene; PCy3
)
tricyclohexylphosphine; py ) pyridine; DMAP ) 4-dimethylaminopyridine.
two unsymmetrical cis-3,4-bis(organosilyl)cyclobutenes and
examined their ring-opening reactions. In this paper, we describe
the results of a systematic study of the ring-opening reactions
of cis-3,4-bis(organosilyl)cyclobutenes.
2. Results and Discussion
2.1. Synthesis of cis-3,4-Bis(organosilyl)cyclobutenes. The
synthesis of cis-3,4-bis(trimethylsilyl)cyclobutene 7 is shown
in Scheme 5. This represents a general method for the synthesis
of cyclobutenes having two organosilyl groups at the 3- and
4-positions in a cis arrangement. Cyclobutenedione 3 was
prepared from 1,2-bis(trimethylsilyl)ethyne 2 according to a
literature procedure.12 The dione 3 was then reduced to the cis-
diol 4 with NaBH4/CeCl3‚H2O.13 The olefinic diol 4 was
subjected to hydrogenation using a cationic iridium catalyst
under homogeneous conditions.14 The two hydroxyl groups
coordinate to iridium to cause syn hydrogenation from the same
face, thus providing the two trimethylsilyl groups in a cis
relationship. Next, the cis-1,2-diol unit of 5 was converted into
a carbon-carbon double bond by the Corey-Hopkins method.15
A reaction of 5 with thiophosgene afforded the cyclic thiocar-
bonate 6. Reductive deoxygenation at room temperature using
1,3-dimethyl-2-phenyl-1,3,2-diazaphospholidine produced the
desired cis-3,4-bis(trimethylsilyl)cyclobutene 7.
Unsymmetrical cis-3,4-bis(organosilyl)cyclobutenes 16a and
16b were synthesized according to an analogous procedure
starting from unsymmetrically substituted 1,2-bis(organosilyl)-
ethynes 10a and 10b, respectively (Scheme 6). The unsym-
metrical 1,2-bis(organosilyl)ethynes 10a and 10b were prepared
from the corresponding substituted phenyl Grignard reagents
and ethynyldimethyl(phenyl)silane. Unlike the synthesis of 7,
H2SO4 was not used for hydrolysis of these R-dichloro-
cyclobutenones 11 since concomitant cleavage of the Si-aryl
linkages was noted. Instead, a silver salt (AgOCOCF3) could
be employed in the conversion of 11a and 11b to the
a See Scheme 5 for abbreviations.
SCHEME 7. Synthesis of (E)-1-Bromo-2-silylethenes
(24-26)
corresponding R-diketones 12a and 12b,16 although the yield
was marginally acceptable.
2.2. Synthesis of Authentic Samples of the Ring-Opening
Products. It is not straightforward to identify the stereochem-
istries of compounds which result from the conrotatory ring-
opening reaction of unsymmetrical cyclobutenes 16a and 16b.
Therefore, stereochemically authentic samples of the products
31, 32, 33, and 34 were independently prepared by cross-
coupling reactions, as shown in Scheme 7 and Table 1.17 The
stereochemistries of the ring-opening products were determined
by comparison of the 1H NMR spectra with those of the
authentic samples.
2.3. Thermal Ring-Opening Reactions of cis-3,4-Bis-
(organosilyl)cyclobutenes. cis-3,4-Bis(trimethylsilyl)cyclobutene
(12) Zhao, D. C.; Allen, A. D.; Tidwell, T. T. J. Am. Chem. Soc. 1993,
115, 10097.
(13) Luche, J.-L. J. Am. Chem. Soc. 1978, 100, 2226.
(14) (a) Crabtree, R. H.; Davis, M. W. Organometallics 1983, 2, 681.
(b) Crabtree, R. H.; Davis, M. W. J. Org. Chem. 1986, 51, 2655.
(15) Corey, E. J.; Hopkins, P. B. Tetrahedron Lett. 1982, 23, 1979.
(16) Liu, R.; Marra, R. M.; Tidwell, T. T. J. Org. Chem. 1996, 61, 6227.
(17) Andreini, B. P.; Carpita, A.; Rossi, R.; Scamuzzi, B. Tetrahedron
1989, 45, 5621.
3766 J. Org. Chem., Vol. 72, No. 10, 2007

Ring Opening of cis-3,4-Bis(organosilyl)cyclobutenes
TABLE 1. Synthesis of Authentic Samples of the Ring-Opening
Products (31-34)
enyne (yield)
diene (yield)
Ar ) Ph, Ar′ ) p-CH3OC6H4
(from 18 and 25)
27 (72%)
31 (78%)
Ar ) p-CH3OC6H4, Ar’ ) Ph
28 (79%)
29 (52%)
30 (65%)
32 (70%)
33 (73%)
34 (77%)
(from 19 and 24)
Ar ) p-CF3C6H4, Ar’ ) Ph
(from 20 and 24)
Ar ) Ph, Ar’ ) p-CF3C6H4
(from 18 and 26)
FIGURE 1. Three factors accelerating the ring-opening reaction of
cis-3,4-bis(trimethylsilyl)cyclobutene 7.
SCHEME 8. Ring-Opening Reaction of
cis-3,4-Bis(trimethylsilyl)cyclobutene 7 and
cis-3,4-Dimethylcyclobutene 36
FIGURE 2. σ-Bond interaction between a Si-C bond and an allylic
π bond.
7 was heated in C6D6 at 80 °C (Scheme 8, eq 1). It underwent
a ring-opening reaction in a conrotatory fashion to afford
(1Z,3E)-1,4-bis(trimethylsilyl)butadiene 35.17 Remarkably, the
half-life at 80 °C was 37 min. The kinetics of the ring-opening
reaction were investigated at varying temperatures, and activa-
tion parameters k ) 109.5exp(-20.8/RT) s-1 were obtained from
the Arrhenius plot.18 The activation parameters of the ring
opening of cis-3,4-dimethylcyclobutene 36 are reported as k )
1013.9exp(-34.0/RT) s-1 (Scheme 8, eq 2).19 The activation
energy of 7 is smaller than that of 36 by more than 13 kcal/
mol. The half-life for 36 at 80 °C was 4500 times longer than
that for 7.
Thus, it has a huge effect on the ring-opening reactivity to
place the two trimethylsilyl groups at the 3- and 4-positions.
The ring-opening reaction of 7 (k ) 78.1 h-1 at 140 °C) was
significantly fast even if compared with those of 1-(1-methyl-
1-phenylethyl)-3-(trimethylsilyl)cyclobutene 1 (k ) 0.530 h-1
at 140 °C)5a and trans-3,4-bis(trimethylsilyl)cyclobutene (k )
5.04 h-1 at 140 °C).5c
We assume that the following three factors are responsible
for the significant acceleration. The first one is the electronic
participation of the trimethylsilyl group which stabilizes the
transition state (Figure 1a). One of the two trimethylsilyl groups
rotates inward to approach the HOMO of the opening cyclo-
butene skeleton. The energetically low-lying σ* orbital of the
Si-CH3 linkage accepts electron density from the HOMO at
the transition state. The transition state is stabilized by this
electron delocalization.
(Figure 1b). The two trimethylsilyl groups of 7 are located at
the allylic positions of the carbon-carbon double bond and
aligned to interact well with the π orbital of the carbon-carbon
double bond. The σ orbitals of the C3-Si and C4-Si linkages
are energetically higher and, therefore, energetically closer to
the π orbital than the corresponding C-CH3 σ orbitals of 36
(Figure 2). The intramolecular interaction between the C-Si σ
orbitals and the π orbital, which are both occupied, results in
destabilization of the ground state.9c,20
The third factor is through-space steric repulsion between the
two sterically bulky trimethylsilyl groups, which also destabi-
lizes the ground state of 7 (Figure 1c). The two trimethylsilyl
groups are aligned on the cyclobutene ring such that they eclipse
each other. A through-space repulsive interaction arises among
them to destabilize the ground state.
We assume these three factors, one stabilizing the transitions
state of the ring-opening reaction and the other two destabilizing
the ground state of the starting material 7, provide an explanation
for the significant decrease of the activation energy observed
in the reaction of 7.
Next, we examined the thermal ring-opening reaction of
unsymmetrical cyclobutenes 16a and 16b having different
(18) The log A values of the ring-opening reaction of cyclobutenes are
normally in the range of 11.7-14.0 s-1 4b
The observed log A value of 7
.
was considerably smaller, although the reason was unclear.
(19) (a) Srinivasan, R. J. Am. Chem. Soc. 1969, 91, 7557. (b) Winter,
R. E. K. Tetrahedron Lett. 1965, 6, 1207.
The second factor is the electronic participation of the
trimethylsilyl groups which destabilizes the ground state of 7
(20) Yoshida, J.; Nishiwaki, J. J. Chem. Soc., Dalton Trans. 1998, 2589.
J. Org. Chem, Vol. 72, No. 10, 2007 3767

Hasegawa and Murakami
SCHEME 9. Ring-Opening Reaction of Unsymmetrical
cis-3,4-Bis(organosilyl)cyclobutenes 16a and 16b
dimethyl(p-methoxyphenyl)silyl substituent with a ratio 31/32
) 53:47 ((1) (see Experimental Section for determination of
the ratio).
The reaction of 16b proceeded faster (k ) 1.50 h-1) than
that of 16a to produce a mixture of 1,3-butadienes 33 and 34
(Scheme 9, eq 2). The 1,3-butadiene 33 which resulted from
inward rotation of the dimethyl(p-trifluoromethylphenyl)silyl
substituent predominated over the 1,3-butadiene 34 which
resulted from inward rotation of the dimethylphenylsilyl sub-
stituent with a ratio of 33/34 ) 56:44 ((1) (see Experimental
Section for determination of the ratio).
The stereochemical deviations observed with the products of
the ring-opening reactions of 16a and 16b are consistent with
expectations based on the stabilizing HOMO-σ* interaction;
the magnitude of inward preference increased in the order,
p-CH3OC6H4-Si, C6H5-Si, p-CF3C6H4-Si. Although differ-
ences in activation energies are relatively small (0.084 kcal/
mol for 31 vs 32; 0.17 kcal/mol for 33 vs 34), these small
differences are the consequence of the small structural variation
that is given to the pair of competitive substituents in order to
minimize influences of other factors. These experimental results
supported the interpretation that the origin of the inward
preference of silyl substituents is attributed to the electron
delocalization from the HOMO to the Si-C σ* orbital.
organosilyl substituents at the 3- and 4-positions in a cis
arrangement. The unsymmetrical cyclobutene 16a is equipped
with dimethylphenylsilyl and dimethyl(p-methoxyphenyl)silyl
substituents. Another unsymmetrical cyclobutene 16b is sub-
stituted by dimethylphenylsilyl and dimethyl(p-trifluorometh-
ylphenyl)silyl groups. These organosilyl groups would all prefer
inward rotation rather than outward rotation. The key feature
in these substrates is that although the different aryl groups on
silicon are approximately the same size they are different enough
in electronic nature to extend their electronic influence to the σ
and σ* orbitals of the C-Si linkage of the organosilyl sub-
stituents. The σ* energy levels of the aryl-Si linkages decrease
in the order, p-CH3OC6H4-Si, C6H5-Si, p-CF3C6H4-Si.21
Likewise, the polarization of the aryl-Si σ* orbital toward
the silicon end increases across the series, p-CH3OC6H4-Si,
C6H5-Si, p-CF3C6H4-Si. The stabilizing interaction of the
cyclobutene HOMO occurs more efficiently with an energeti-
cally closer and more polarized σ* orbital, both of which should
be strongest with the p-CF3C6H4-Si σ* orbital among the series.
Therefore, if the origin of the inward preference of silyl
substituents is related to transition state interactions of
HOMO-σ*, the magnitude of the preference for inward rotation
should increase in the order, p-CH3OC6H4-Si, C6H5-Si,
p-CF3C6H4-Si. On the contrary, the electron-donating capability
of the σ orbital connecting the C3 carbon and the silicon would
decrease in the order, p-CH3OC6H4Si-C3, C6H5Si-C3,
p-CF3C6H4Si-C3, Therefore, the interpretation based on the
geminal σ-bond participation would predict that the magnitude
of inward preference increases in the reverse order.
3. Conclusion
cis-3,4-Disubstituted cyclobutenes are useful substrates to
compare the torquoselectivity of the two different substituents
in conrotatory ring-opening reactions. We synthesized the cis-
3,4-bis(organosilyl)cyclobutenes and examined their ring-open-
ing reactions. The degrees of inward preferences of the
p-CF3C6H4Me2Si, C6H5Me2Si, and p-CH3OC6H4Me2Si groups
were compared in the ring-opening reaction of unsymmetrical
cis-3,4-bis(aryldimethylsilyl)cyclobutenes. The magnitude of
inward preference increases in the order, p-CH3OC6H4-Si,
C6H5-Si, p-CF3C6H4-Si. This electronic trend provides im-
portant experimental support for explanation of the origin of
inward preferences by silyl groups based on the HOMO-σ*
interaction.
4. Experimental Section
(1R*,2S*)-3,4-Bis(trimethylsilyl)cyclobut-1-ene-1,2-diol (4). To
a solution of 3 (5.00 g, 22.1 mmol) and CeCl3‚H2O22 (11.7 g, 44.2
mmol) in MeOH (470 mL) was added NaBH4 (2.20 g, 58.2 mmol)
in several portions at 0 °C. The reaction mixture was stirred at 0
°C for 2 h and then quenched with saturated NH4Cl aqueous
solution. The mixture was extracted with AcOEt (100 mL) 10 times.
The combined organic extracts were washed with brine, dried over
Na2SO4, and evaporated. The residue was purified by column
chromatography on silica gel (hexane/AcOEt ) 3:1) to provide 4
(3.82 g, 16.6 mmol, 75%): 1H NMR (300 MHz) δ 0.17 (s, 18H),
2.25 (d, J ) 6.4 Hz, 2H), 4.76 (d, J ) 6.4 Hz, 2H); 13C NMR δ
-0.9, 76.0, 172.9; HRMS (EI) calcd for C10H22O2Si2 (M+)
230.1158, found 230.1153.
(1R*,2S*,3R*,4S*)-3,4-Bis(trimethylsilyl)cyclobutane-1,2-di-
ol (5). A mixture of 4 (947 mg, 4.11 mmol) and [Ir(cod)PCy3(py)]-
PF623 (98.8 mg, 0.12 mmol, 3 mol %) in CH2Cl2 (8 mL) was stirred
under a hydrogen atmosphere at room temperature. After being
stirred for 12 h, the reaction mixture was concentrated and passed
through a short column of Florisil eluting with Et2O. The residue
The ring-opening reaction of the unsymmetrically disubsti-
tuted cyclobutene 16a and 16b was carried out at 80 °C in C6D6.
The reaction of 16a proceeded with k ) 1.28 h-1 to afford a
mixture of 1,3-butadienes 31 and 32 (Scheme 9, eq 1). The 1,3-
butadiene 31 which resulted from inward rotation of the
dimethylphenylsilyl substituent predominated over the 1,3-
butadiene 32 which resulted from inward rotation of the
(21) Computational studies [B3LYP/6-31G(d)] were carried out on
p-CF3C6H4SiH3, C6H5SiH3, and p-CH3OC6H4SiH3. The natural bond orbital
(NBO) analysis showed that the energy levels of the C-Si σ* orbitals
increase in this order, p-CF3C6H4-Si (131.2 kcal/mol), C6H5-Si (139.9
kcal/mol), and p-CH3OC6H4-Si (144.3 kcal/mol).
(22) Takeda, N.; Imamoto, T. Org. Synth. 1999, 76, 228.
(23) Brown, J. M. Angew. Chem., Int. Ed. Engl. 1987, 26, 190.
3768 J. Org. Chem., Vol. 72, No. 10, 2007

Ring Opening of cis-3,4-Bis(organosilyl)cyclobutenes
was purified by column chromatography on silica gel (hexane/
AcOEt ) 2:1) to afford 5 (615 mg, 2.65 mmol, 64%): 1H NMR
(300 MHz) δ 0.06 (s, 18H), 1.82 (d, J ) 4.8 Hz, 2H), 2.78 (br,
2H), 4.14 (d, J ) 4.2 Hz, 2H); 13C NMR δ -1.1, 33.0, 71.8; HRMS
(FAB) calcd for C10H23OSi2 (M+ - OH) 215.1287, found 215.1287.
(1R*,2S*,6R*,7S*)-6,7-Bis(trimethylsilyl)-2,4-dioxabicyclo-
[3.2.0]heptane-3-thione (6). To a mixture of 5 (360 mg, 1.54 mmol)
and DMAP (455 mg, 3.78 mmol) in CH2Cl2 (10 mL) was added
thiophosgene (0.14 mL, 1.84 mmol) slowly. After stirring at 0 °C
for 2 h, silica gel was added. The mixture was passed through a
short column of Florisil eluting with Et2O. The filtrate was
evaporated to afford 6 (418 mg, 1.52 mmol, 99%): 1H NMR (300
MHz) δ 0.13 (s, 18H), 2.43-2.44 (m, 2H), 5.06-5.07 (m, 2H);
13C NMR δ -1.6, 33.0, 83.8, 193.7; HRMS (FAB) calcd for
C11H23O2SSi2 (M+ + H) 275.0957, found 275.0957.
(3R*,4S*)-3,4-Bis(trimethylsilyl)cyclobutene (7). To a solution
of 6 (678 mg, 2.47 mmol) in benzene (1.2 mL) was added 1,3-
dimethyl-2-phenyl-1,3,2-diazaphospholidine (1.4 mL, 7.41 mmol)
at room temperature. The reaction mixture was stirred at room
temperature for 12 h and then directly subjected to chromatography
on silica gel (hexane) to afford 7 (409 mg, 2.06 mmol, 83%): 1H
NMR (300 MHz) δ 0.04 (s, 18H), 2.62 (s, 2H), 6.10 (s, 2H); 13C
NMR δ -1.0, 39.4, 136.3; HRMS (EI) calcd for C10H22Si2 (M+)
198.1260, found 198.1259.
The ring-opening reaction of 16a was carried out at 80 °C twice.
The ratio of the first-run ring-opening reaction was estimated to
1
be 31/32 ) 52.7:47.3 based upon H NMR analysis. The ratio of
the second-run ring-opening reaction was estimated to be 31/32 )
53.4:46.6 by GC analysis and 31/32 ) 52.4:47.6 by 1H NMR
analysis. Thus, the ratio was determined to be 31/32 ) 53:47 ((1).
The authentic sample of 33 (7.46 mg) and the authentic sample
of 34 (7.46 mg), both prepared by the cross-coupling reaction, were
mixed. The mixture had a weight ratio 33/34 ) 50.0:50.0. The
1
mixture was analyzed by H NMR. The integral values of the
Me2ArSi singlet signals showed the ratio 33/34 ) 49.3:50.7. The
integral values of the Si-CHdCH-CHdCH-Si signals showed
1
the ratio 33/34 ) 49.6:50.4. Thus, the two ratios estimated by H
NMR analysis were in good accord with the weight ratio.
The ring-opening reaction of 16b was carried out at 80 °C twice.
The ratio of the first-run ring-opening reaction was estimated
to be 33/34 ) 55.1:44.9 based upon the integral values of the
Me2ArSi singlet signals and 33/34 ) 55.2:44.8 based upon the
integral values of the Si-CHdCH-CHdCH-Si signals. The ratio
of the second-run ring-opening reaction was estimated to be 33/34
) 56.2:43.8 based upon the integral values of the Me2ArSi singlet
signals and 33/34 ) 55.8:44.2 based upon the integral values of
the Si-CHdCH-CHdCH-Si signals. Thus, the ratio was deter-
mined to be 33/34 ) 56:44 ((1).
General Procedure for Thermal Ring-Opening Reaction. A
solution of cyclobutene (0.10 mmol/portion) in degassed C6D6 (0.25
mL/portion) was put into a Schlenk flask and then heated at the
described temperature in the dark. A portion of the mixture was
taken out of the flask by a syringe at intervals. The reaction was
Acknowledgment. We thank the Sumitomo Foundation for
financial support. M.H. thanks the Japan Society of Promotion
of Science for a fellowship.
1
monitored by H NMR.
Determination of the Ratios 31/32 and 33/34. The authentic
sample of 31 (7.25 mg) and the authentic sample of 32 (6.77 mg),
both prepared by the cross-coupling reaction, were mixed. The
mixture had a weight ratio 31/32 ) 51.7:48.3. GC analysis of the
mixture showed the ratio 31/32 ) 51.4:48.6. The mixture was
Supporting Information Available: Additional experimental
procedures and characterization data for compounds 4-7, 10,
1
12-14, 16, and 18-34. Copies of H and 13C NMR spectra for
compounds 10, 12-14, 16, and 18-34. Tables of atom coor-
dinates and total energies of p-CF3C6H4SiH3, C6H5SiH3, and
p-CH3OC6H4SiH3. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
analyzed also by H NMR. The integral values of the Me2ArSi
singlet signals showed the ratio 31/32 ) 52.0:48.0. Thus, the ratios
1
estimated by GC analysis and by H NMR analysis were in good
accord with the weight ratio.
JO070039N
J. Org. Chem, Vol. 72, No. 10, 2007 3769