A new synthetic route for silacyclopropanes: Reactions of a bromosilylenoid with olefins

Article abstract of DOI:10.1021/om3005349

Stable 1-bromo-1-silacyclopropanes were synthesized in high yields from the reaction of the stable bromotrisylsilylenoid 1 (trisyl = C(SiMe ₃)₃) with olefins such as styrene, trimethylvinylsilane, triethylvinylsilane, and dimethylphenylvinylsilane, a new synthetic route for silacyclopropanes. The structure of bromosilacyclopropane 5 was confirmed by X-ray analysis. In the presence of excess MeOH the phenyl-substituted bromosilacyclopropane 2 underwent regioselective ring opening to give the corresponding product 6. In contrast, the reaction of silyl-substituted bromosilacyclopropane 5 with MeOH gave only the product of nucleophilic substitution at the silicon atom, 1-methoxy-1-silacyclopropane 7, without any ring opening. Furthermore, the bromosilacyclopropane 2 extruded bromosilylene 8 through a thermal cycloreversion reaction.

Full text of DOI:10.1021/om3005349

Communication
pubs.acs.org/Organometallics
A New Synthetic Route for Silacyclopropanes: Reactions of a
Bromosilylenoid with Olefins
Hyeon Mo Cho,† Kyongjin Bok,† Seo Hyeon Park,† Young Mook Lim,† Myong Euy Lee,*,†
Moon-Gun Choi,‡ and Kang Mun Lee§
†Research & Education Center for Advanced Silicon Materials, Department of Chemistry & Medical Chemistry, College of Science
and Technology, Yonsei University, Wonju, Gangwondo 220-710, Korea
‡Department of Chemistry & Molecular Structure Laboratory, Yonsei University, Seoul 120-749, Korea
§Department of Chemistry, KAIST, Daejeon 305-701, Korea
S
* Supporting Information
ABSTRACT: Stable 1-bromo-1-silacyclopropanes were synthesized in high
yields from the reaction of the stable bromotrisylsilylenoid 1 (trisyl =
C(SiMe3)3) with olefins such as styrene, trimethylvinylsilane, triethylvinylsi-
lane, and dimethylphenylvinylsilane, a new synthetic route for silacyclopro-
panes. The structure of bromosilacyclopropane 5 was confirmed by X-ray
analysis. In the presence of excess MeOH the phenyl-substituted
bromosilacyclopropane 2 underwent regioselective ring opening to give the corresponding product 6. In contrast, the reaction
of silyl-substituted bromosilacyclopropane 5 with MeOH gave only the product of nucleophilic substitution at the silicon atom,
1-methoxy-1-silacyclopropane 7, without any ring opening. Furthermore, the bromosilacyclopropane 2 extruded bromosilylene 8
through a thermal cycloreversion reaction.
ilylenoids (R2SiMX), silicon analogues of carbenoids, are
species in which an electropositive metal (M, usually an
Scheme 1. Synthesis of 2
S
alkali metal) and a leaving group (X, usually a halogen) are
bound to the same silicon atom and show amphiphilic
(nucleophilic and electrophilic) properties.1−7 Silylenoids and
silylenes (R2Si:) have been postulated as key intermediates in
oligosilane and polysilane syntheses.1 The reactivities of
silylenoids are often similar to those of silylenes (R2Si:). It is
well-known that silylenes react with carbon−carbon double
bonds to form silacyclopropanes,8 the corresponding [1 + 2]
cycloadducts.9 Only a few stable silylenoids, including (tBuO)-
Ph2SiLi,2 (Mes)2Si(SMes)Li,3 (Me3Si)2Si(OMe)K·crown
ether,4 (R3Si)2SiFLi5 (R3Si = t-Bu2MeSi), and the halosilyle-
noid TsiSiBr2Li6 (Tsi (trisyl) = C(SiMe3)3), and their
reactivities have been reported, but there has been no report
on their reactivities with olefins. In this paper we report
unprecedented examples of reactions of the stable bromo-
trisylsilylenoid 1 with olefins such as styrene, trimethylvinysi-
lane, triethylvinylsilane, and dimethylphenylvinylsilane, to yield
the corresponding 1-bromo-1-silacyclopropanes. Halogen-sub-
stituted silacyclopropanes could be very useful because of their
high synthetic potential. In addition, their reactivity toward
MeOH and their thermal stability are reported.
The structure of silacyclopropane 2 was characterized by 1H,
13C, and 29Si NMR, GC/MS, and HRMS. In 1H and 13C NMR,
resonances (1H NMR, 2.69, 1.46 ppm; 13C NMR, 22.7, 10.1
ppm) derived from CH and CH2 of the three-membered ring
were observed. The 29Si NMR signal of the center silicon atom
in 2 was observed at −38.0 ppm, which is in the same range as
reported values (−48.9, −43.9, −33.8 ppm) for 1,l-di-tert-butyl-
2-benzyl-1-silacyclopropane,10 1,l-di-tert-butyl-trans-2,3-dimeth-
yl-1-silacyclopropane,1b and (Z)-1,2-dimethyl-4,4,7,7-tetrakis-
(trimethylsilyl)-3-silaspiro[2.4]heptane.11
The syntheses were successfully extended to the cyclo-
propanation of silyl-substituted olefins. Treatment of 1 with
trimethylvinylsilane, triethylvinylsilane, and dimethylphenylvi-
nylsilane furnished cyclic products 3−5 in 90%, 58%, and 91%
yields, respectively (Scheme 2). Cyclopropanes 3−5 were
The stable bromosilylenoid 1, having a bulky Tsi group, was
prepared by the reduction of tribromotrisylsilane with 2 equiv
of lithium naphthalenide.6 To the solution was added an excess
of styrene at −78 °C. When the reaction mixture was slowly
warmed to room temperature, no reaction took place.
However, stirring for 1 h at reflux temperature successfully
gave 1-bromo-2-phenyl-1-silacyclopropane 2 in 72% yield
(Scheme 1).
Scheme 2. Synthesis of 3−5
Received: June 15, 2012
Published: July 17, 2012
© 2012 American Chemical Society
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Organometallics
Communication
characterized by multinuclear NMR, GC/MS, and HRMS. The
29Si NMR signals attributed to the center silicon atoms in
compounds 3−5 resonate at −41.3, −41.2, and −42.4 ppm,
respectively. To the best of our knowledge, only two examples
of stable silacyclopropanes having halogen(s) substituted at the
silicon atom have been reported until now. Seyferth and
Duncan synthesized 1,1-difluoro-2,2,3,3-tetramethyl-1-silacyclo-
propane via a ring-closing reaction,12 and Tokitoh and co-
workers obtained 7-bromo-7-Bbt-7-silabicyclo[4.1.0]heptanes
(Bbt = 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)-
methyl]phenyl) from the reaction of 1,2-diaryl-1,2-dibromodi-
silene with cyclohexene.8b
Colorless crystals of bromosilacyclopropane 5 were obtained
from a cold hexane solution (−40 °C), and the structure was
confirmed by X-ray analysis,13 giving the first molecular
structure of a halosilacyclopropane (Figure 1). The endocyclic
Figure 2. ORTEP drawing of 6 with 30% probability thermal
ellipsoids. Hydrogen atoms and the disorder of three trimethylsilyl
groups are omitted for clarity. Selected bond lengths (Å) and angles
(deg): Si1−Br = 2.265(2), Si1−O = 1.621(6), Si1−C1 = 1.885(7),
Si1−C9 = 1.890(7), C9−Si2 = 1.951(8), C9−Si3 = 1.928(8), C9−Si4
= 1.943(8); C1−Si1−C9 = 118.8(3), Si1−O−C19 = 134.4(5).
Scheme 3. Reactions of 2 and 5 with MeOH
Figure 1. ORTEP drawing of 5 with 30% probability thermal
ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Si1−Br1 = 2.235(1), Si1−C1 =
1.825(4), Si1−C2 = 1.831(4), Si1−C5 = 1.860(4), C1−C2 =
1.601(5), Si3−C5 = 1.923(3); C1−Si1−C2 = 52.0(2), Si1−C2−C1
= 63.7(2), Si1−C1−C2 = 64.3(2), Si1−C2−Si2 = 131.9(2), Br1−
Si1−C2 = 111.6(1), Br1−Si1−C1 = 109.7(1), Br1−Si1−C5 =
114.5(1).
Si−C bond to form 6. Regioselective rupture of the Si−C bond
could be explained by previous reports that the less hindered
silicon−carbon bond in a three-membered ring was preferen-
tially cleaved by MeOH.16
1
In the H NMR of 6, the resonance derived from OMe was
observed at 2.48 ppm, shifted unexpectedly upfield in
comparison with the normal chemical shift (3.0−3.5 ppm) of
SiOMe groups. The abnormal shift may be understood from
the X-ray crystallographic characterization of 6. The molecular
structure of 6 (Figure 2) shows that the methyl group of OMe
is below the plane of the phenyl group. This arrangement
possibly caused the shielding of methyl protons in the OMe
group due to the aromatic ring current. All trimethylsilyl groups
in Tsi appeared to be disordered with nearly equivalent site
occupation factors (0.51/0.49).
bond angles of Si1, C2, and C1 (52.0(2), 63.7(2), and
64.3(2)°) imply the strained nature of the silacyclopropane.
The torsion angles C5−Si1−C2−Si2 = 137.0(2)° and Br1−
Si1−C2−Si2 = 11.2(3)° indicate the trans orientation of the
Tsi group and the dimethylphenylsilyl group and cis orientation
of the bromine atom and the dimethylphenylsilyl group,
respectively. This stereoselectivity is possibly due to the bulky
substituent. The Si1−C2 bond length (1.831(4) Å) is within
the range of reported values (1.825−1.880 Å).14,15 The C1−C2
bond length (1.601(5) Å) in the three-membered ring is longer
than reported distances (1.508−1.576 Å)14 in 2,3-alkyl- or
hydrogen-substituted 1-silacyclopropanes, except for that
(1.643 Å)15 in 2,3-silyl-substituted 1-silacyclopropane. The
Si1−Br1 (2.235(1) Å), Si1−C5 (1.860(4) Å), and Si1−C2
bond lengths (1.831(4) Å) are shorter than the corresponding
Si1−Br (2.265(2) Å), Si1−C9 (1.890(7) Å), and Si1−C1
lengths (1.885(7) Å) in acyclic compound 6 (Figure 2).
In order to investigate the reactivity of silacyclopropanes
toward nucleophiles, excess methanol was added to 2 at room
temperature. The reaction mixture gave the ring-opened and
methoxy-substituted product 6 in 80% yield (Scheme 3). As
expected, the Si−C bond was cleaved due to ring strain in the
silacyclopropane, resulting in the insertion of MeOH into the
To compare the reactivity of a 2-silyl-substituted 1-
silacyclopropane with that of a 2-phenyl-substituted 1-
silacyclopropane, the reaction of 5 with MeOH in THF was
carried out. Interestingly, the reaction mixture afforded only
methoxy-substituted silacyclopropane 7 (Scheme 3). This is the
first example of nucleophilic substitution on a silacyclopropane
without ring cleavage. Compound 7 at room temperature was
stable even in the air for a few days.17 Those results indicate
that the silyl substituent increased the stability of silacyclopro-
pane toward MeOH in comparison with the phenyl group.18
Silacyclopropane 2 was thermally stable in THF or
chloroform solvent at room temperature. However, about
50% of compound 2 decomposed after heating at 80 °C for 60
1
1
min in toluene solution, monitored by H NMR. H NMR
signals of styrene also appeared downfield, and the intensities of
1H NMR signals due to CH and CH2 of the three-membered
ring decreased (Figure S2, Supporting Information). This
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Communication
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observation implies that styrene was thermally eliminated from
the silacyclopropane to afford bromotrisylsilylene 8.19 In order
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80 °C for 1 h to afford the Br(Tsi)silylene-trapping product 9
(Scheme 4).6 This result suggests strongly that halosilacyclo-
propanes can be promising precursors for halosilylenes.20
Scheme 4. Thermal Cycloreversion Reaction of 2
Our study demonstrated that stable silacyclopropanes at
room temperature were synthesized from the reaction of the
bromotrisylsilylenoid 1 with olefins, which extruded the
halosilylene 8 through a thermal cycloreversion reaction.
These results provide new synthetic pathways for synthesizing
halosilacyclopropanes and generating halosilylenes. No ring
cleavage of bromosilacyclopropane 5 in the presence of
methanol was observed. Further studies on the reactivity of
functionalized silacyclopropanes for synthetic applications are
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ASSOCIATED CONTENT
* Supporting Information
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10659−10663.
■
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(11) Kira, M.; Ishida, S.; Iwamoto, T.; de Meijere, A.; Fujitsuka, M.;
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CIF files giving crystal data for 5 and 6 and text, tables, and
figures giving details of the synthesis and characterization data
of 2−9 and crystallographic data of 5 and 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
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(13) CCDC 878575 (for compound 5) and CCDC 872079 (for
compound 6) contain supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
AUTHOR INFORMATION
Corresponding Author
*E-mail: melgg@yonsei.ac.kr.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and
Technology (2010-0024877). We thank Professor Robert
West, distinguished Professor in the WCU program, for
valuable discussions.
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