Dibromocarbene and bromofluorocarbene addition to substituted allylsilanes

Article abstract of DOI:10.1016/j.tetlet.2010.12.005

The dibromocarbene or bromofluorocarbene addition to substituted allyldisilanes afforded instable gem-dibromocyclopropanes or a gem-bromofluorocyclopropane, respectively. These gem-bromohalogenocyclopropanes undergo a spontaneous ring opening at room temp

Full text of DOI:10.1016/j.tetlet.2010.12.005

Tetrahedron Letters 52 (2011) 688–691
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Dibromocarbene and bromofluorocarbene addition to substituted allylsilanes
⇑
Chahinez Aouf, Maurice Santelli
Laboratoire Chimie Provence, associé au CNRS n° 6264, Université d’Aix-Marseille, Faculté des Sciences de St-Jérôme, Avenue Escadrille Normandie-Niemen,
13397 Marseille, Cedex 20, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The dibromocarbene or bromofluorocarbene addition to substituted allyldisilanes afforded instable gem-
dibromocyclopropanes or a gem-bromofluorocyclopropane, respectively. These gem-bromohalogenocy-
clopropanes undergo a spontaneous ring opening at room temperature to give halogenated dienylsilanes
or bromo dienes or dibromotetraenes.
Received 28 October 2010
Revised 29 November 2010
Accepted 1 December 2010
Available online 7 December 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Dibromocarbene
Bromofluorocarbene
gem-Dibromocyclopropanes
Dienylsilanes
Halogenated dienes
R1
R4
1. Introduction
R2
R3
R2
R1
R4
Br
Br
−
)
Br(
(+)
products
+
gem-Dibromocyclopropanes are known to give bromoallylic
bromides by heating.1 According to the Woodward–Hoffmann–
DePuy rule,2 a disrotatory electrocyclic ring opening of the cyclo-
propane with concomitant loss of the bromide anion affords an
allylic cation which then captures an anion (Scheme 1). Generally,
the ring opening occurs by thermolysis at a temperature >100 °C,
but in a few cases, concerning only strained polycyclic compounds,
the gem-dibromocyclopropane is not isolable and the obtained
products result from the opening of the cyclopropane followed
by the evolution of the bromoallylic cation.
Br
R3
R1, R4 > R2, R3
Scheme 1. Disrotatory electrocyclic ring opening of gem-dibromocyclopropanes.
CHBr3
t-BuOK
Br
Br
SiMe3
Me3Si
Previously, we have shown that the dibromocarbene addition to
the 1,4-bis(trimethylsilyl)but-2-ene 1 and thermolysis of the ad-
duct 2 under vacuum at 100 °C afforded 3-bromo-1-trimethylsilyl-
penta-2,4-diene 3 in 75% yield (Scheme 2).3
SiMe3
2, 93%
0 °C
pentane
Me3Si
1
Br
neat, 100°C
- Me3SiBr
Herein, we report on the dibromocarbene addition4 to the 2,3-di-
methyl-1,4-bis(trimethylsilyl)but-2-ene 4 and the 2,3,6,7-tetra-
methyl-1,8-bis(trimethylsilyl)octa-2,6-diene 5 resulting from a
reductive silylation of the 2,3-dimethylbuta-1,3-diene (Scheme 3).5
From 4, a very clean reaction occurred to give 9 in the good
yield of 81% (Scheme 4).6,7
3
Me3Si
, 75%
Scheme 2. Synthesis of the 3-bromo-1-trimethylsilylpenta-2,4-diene 3.
a spectacular ring opening. To the best of our knowledge,1 this
result corresponds to the first ring opening at room temperature
of a monocyclic gem-dibromocyclopropane.
This result prompted us to synthesise the corresponding
fluorine derivative. After addition of bromofluorocarbene to 4, we
have obtained the silylated fluoropentadiene 12 in 72% yield
(Scheme 5).9 There are substantially fewer reports in the litera-
ture of bromofluorocarbene.10 The unprecedented reaction of
commercially available ethyl dibromofluoroacetate with sodium
The synthesis of product 9 is explained by the instability of
adduct 6 and surprisingly a protolysis of 8. Previously, Magnus ob-
tained 9 by heating 1,1-dibromo-2,2,3,3-tetramethyl-cyclopropane
in PhNMe2 at 150 °C.8 Therefore, in comparison with 1, the pres-
ence of the two additional methyl groups in the disilane 4 induced
⇑
Corresponding author.
E-mail address: m.santelli@univ-cezanne.fr (M. Santelli).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.12.005

C. Aouf, M. Santelli / Tetrahedron Letters 52 (2011) 688–691
689
CHBr3
t-BuOK
SiMe3
SiMe3
Me3Si
Me3Si
Li
4, 47%
0 °C
ClSiMe3
Me3Si
Br
pentane
SiMe3
Br
Br
SiMe3
5
THF
Me3Si
Br
5
, 43%
−
Br(
)
−
)
(+)
Br(
Br
Me3Si
Scheme 3. Reductive silylation of the 2,3-dimethylbuta-1,3-diene.
SiMe3
(+)
Br
Br
Br
CHBr3
t-BuOK
+
SiMe3
SiMe3
Me3Si
Me3Si
0 °C
pentane
Br
Br
Br
Br
14 52 %
15 32 %
4
Br(−)
6
Scheme 7. Dibromocarbene addition to the bis(allylsilane) 5.
(+)
SiMe3
Me3Si
SiMe3
Br
Br
8
7
protolysis
4-cyanobenzaldehyde, and in the presence of BF3ꢀEt2O, this dienylsi-
lane led to the Diels–Alder adduct 13 (one isomer) in 63% yield
(Scheme 6).12
Br
9, 81%
In order to extend these results, the bis(allylsilane) 55 was sub-
mitted to dibromocarbene addition (Scheme 7). Two dibromides
14 and 15 were obtained and 14 can be isomerized in more stable
tetraene 15.13 The structure of this latter compound has been con-
firmed by an X-ray crystallographic analysis, which showed that
the molecule is twisted (C2 axis) with a reduced conjugation
(Fig. 1).14
Scheme 4. Dibromocarbene addition to the 2,3-dimethyl-1,4-bis(trimethylsilyl)-
but-2-ene 4.
O
CFBr2
OEt
SiMe3
The very high stability of the allylic cations 7 and 11 which are bi-
tertiary and twice b-silylated15 explained the easy ring opening of 6
and 10. Calculations at the B3LYP/6-311++G(3d,3p) level of the the-
ory16 showed that 7 and 11 are very stable allylic cations compared
to the parent cation 16 and the bi-tertiary allylic cation 17 (Fig. 2).
Me3Si
4
F
MeONa
0 °C
pentane
Br
10
−
Br( )
Global electronic indexes, the chemical hardness g 17
,
the elec-
(+)
tronic chemical potential (negative of the electronegativity) l18
SiMe3
Me3Si
SiMe3
and the electrophilic power x 19
as defined within the density
,
F
functional theory (DFT) of Parr, Pearson, and Yang,20 are useful
tools to understand the reactivity of molecules in their ground
states (Table 1).21
F
12, 72%
11
Scheme 5. Preparation of the 3-fluoro-2,4-dimethyl-1-trimethylsilylpenta-2,4-
diene 12.
g
ꢁ ðELUMO ꢂ EHOMOÞ=2;
l
ꢁ ðELUMO þ EHOMOÞ=2;
x
ꢁ
l g
2=2
The global electrophilic power
x
measures the stabilization in
BF3 OEt2
energy when the system acquires an additional electronic charge
N from the environment. A comparison of the electrophilic power
values for 7, = 20.1 or 11, = 19.7 and allylic cation 16, = 33.7,
12
+
NC
CHO
D
CH2Cl2
78 °C to 25 °C
x
x
x
−
SiMe3
shows high values particularly for the allylic cation. Consequently,
we can deduce that 7 and 11 (more soft allylic cations) are more
stable than the allylic cation 16.
Me3Si
F
F
BF3
O
O
CN
H
2. Conclusion
13, 63%
CN
We have shown that from the silylation of 2,3-dimethyl-1,3-
butadiene5 followed by the dibromocarbene or bromofluorocar-
bene addition, we could obtain valuable halogenated dienes and
tetraenes in two steps.
Scheme 6. Addition of dienylsilane 12 to the p-cyanobenzaldehyde.
methylate, carried out in the presence of an alkene, afforded appro-
priate bromofluorocarbene in good yield.11 The presence of the
fluorine atom did not prevent the spontaneous ring opening of
the bromofluorocyclopropane 10 at room temperature. As previ-
ously,3 the more stable transoid configuration of the allylic cation
11 induced the configuration of the double bond of 12.
Acknowledgements
This work has been financially supported by the CNRS and the
Ministère de l’Education Nationale. We thank Dr. M. Giorgi (Spec-
tropôle, Université d’Aix-Marseille) for the X-ray structure deter-
mination. Discussion with Dr. J.-L. Parrain has been helpful.
Attention was next directed to the Lewis acid promoted
addition reaction of 12 to aldehydes. Surprisingly, with the

690
C. Aouf, M. Santelli / Tetrahedron Letters 52 (2011) 688–691
C6
Br1
C7
C4
C5
C2
C1
C3
Figure 1. ORTEP drawing of (E,E)-3,8-dibromo-2,4,7,9-tetramethyldeca-2,4,6,8-tetraene 15.
(+)
J J
2 (CF) = 31.6 Hz), 136.8 (d, 2J(CF) = 30.2 Hz), 131.4 (q, 1 (CF) = 254.7 Hz), 116.7 (t),
Me3Si
SiMe3
(+)
(+)
21.8 (t), 20.9 (q), 20.1 (q), 0.9 (q); 19F NMR (CDCl3, 282.4 MHz) d ꢂ112.9 (s).
HRMS for [C10H19FSi]+, 186.1240; found: 186.1235.
10. (a) Brahms, D. L. S.; Dailey, W. P. Chem. Rev. 1996, 96, 1585–1632; (b) Dolbier,
W. R., Jr.; Battiste, M. A. Chem. Rev. 2003, 103, 1071–1098.
X
16
17
7, X = Br; 11, X = F
11. For substitution reactions on ethyl dibromofluoroacetate, see: Takeuchi, Y.;
Asahina, M.; Hori, K.; Koizumi, T. J. Chem. Soc., Perkin Trans. 1 1988, 1149–
1153.
Figure 2. Allylic cations.
12. 6-(4-Cyanophenyl)-3-fluoro-2,4-dimethyl-2-trimethylsilyl-methyl-5,6-dihydro-
2H-pyran (13). To 4-cyanobenzaldehyde (0.04 g, 0.32 mmol) in anhydrous
CH2Cl2 (5 mL) under argon, stirred to ꢂ78 °C, were slowly added BF3ꢀEt2O
(0.05 mL, 0.4 mmol) and then 12 (0.12 g, 0.64 mmol). After stirring for 1 h at
ꢂ78 °C and 18 h at rt, the mixture was filtered on CeliteÒ and the filtrate was
stirred with brine. After usual work-up (CH2Cl2), the crude product was flash
chromatographed (petroleum ether/ether, 80:20) to give 13 (0.064 g,
0.20 mmol, 63%). 1H NMR (CDCl3, 300 MHz) d 7.63 (d, J = 6.0 Hz, 2H), 7.49 (d,
J = 6.0 Hz, 2H), 4.80 (dd, J = 3.0, 9.0 Hz, 1H), 2.16 (s, 3H), 1.66 (s, 2H), 1.40 (s,
3H), 0.90 (dd, 3.0, 9.0 Hz, 2H), 0.05 (s, 9H); 13C NMR (CDCl3, 75 MHz) d 162.5
(s), 147.5 (s), 132.4 (d) (2C), 126.0 (d) (2C), 119.0 (s), 111.3 (s), 105.7 (q,
1J(CF) = 285.0 Hz), 73.4 (s), 70.0 (d), 38.1 (t), 29.8 (t), 26.7 (q), 13.8 (q), 1.1 (q).
Elemental Anal. Calcd for C18H24FNOSi (317.47): C, 68.10; H, 7.62. Found: C,
68.19; H, 7.71.
Table 1
Computation of global electronic indexes for 7, 11, 16 and 17
Cation E (au)
EHOMO
(au)
ELUMO
(au)
g
(au)
l
(au)
x
(eV)
16
17
7
ꢂ117.006676 ꢂ0.578
ꢂ274.389914 ꢂ0.471
ꢂ3665.442780 ꢂ0.381
ꢂ1191.181743 ꢂ0.388
ꢂ0.387
ꢂ0.295
ꢂ0.247
ꢂ0.249
0.096 ꢂ0.483 33.7
0.088 ꢂ0.383 22.6
0.067 ꢂ0.314 20.1
0.070 ꢂ0.318 19.7
11
13. 3,8-Dibromo-2,4,7,9-tetramethyldeca-1,3,7,9-tetraene
(14)
and
(E,E)-
References and notes
3,8-dibromo-2,4,7,9-tetramethyldeca-2,4,6,8-tetraene (15). Following the
procedure described for 9, to t-BuOK (3.3 g, 29.1 mmol) in pentane
(40 mL) and disilane
5 (4.34 g, 14 mmol) was slowly added bromoform
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(2.6 mL, 29.1 mmol). After stirring for 1 h at 0 °C and 2 h at rt, usual work-
up and flash chromatography on silica gel (petroleum ether), gave 14
(2.52 g, 7.3 mmol, 52%) as an oil and 15 (1.55 g, 4.5 mmol, 32%) as white
crystals. Compound 14, 1H NMR (CDCl3, 300 MHz) d 4.92 (s, 2H), 4.88 (s,
2H), 2.28 (t, J = 3.8 Hz, 4H), 1.88 (s, 6H), 1.85 (s, 6H); 13C NMR (CDCl3,
75 MHz) d 144.2 (s), 144.0 (s), 133.9 (s), 116.6 (t), 35.4 (t), 21.3 (q), 20.1 (q).
15, mp 92 °C; 1H NMR (CDCl3, 300 MHz) d 6.15 (s, 2H), 1.91 (s, 6H), 1.87 (s,
6H), 1.79 (s, 6H); 13C NMR (CDCl3, 75 MHz) d 137.8 (s), 131.3 (s), 126.2 (d),
122.8 (s), 24.8 (q), 22.2 (q), 16.5 (q).
5. Aouf, C.; El Abed, D.; Giorgi, M.; Santelli, M. Tetrahedron 2009, 65, 5563–5570.
6. General: All reactions were performed under an argon atmosphere in oven-
dried glassware. NMR spectroscopy: Bruker AC 300 (1H = 300 MHz,
13C = 75 MHz); chemical shift d in parts per million relative to CDCl3 (signals
for residual CHCl3 in the CDCl3: 7.26 for ppm for 1H NMR and 77.00 (central) for
13C NMR). Carbon–proton couplings were determined by DEPT sequence
experiments. Compounds 4 and 5 were prepared according to the procedure
previously reported.5
14. X-ray Crystallography: CCDC-794285 (for 15), contain the supplementary
crystallographic data for this paper. These data can be obtained free of charge
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Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A.
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equipped with a thermometer, a septum cap, a magnetic stirring bar, and an
argon outlet was charged with potassium tert-butoxide (11.8 g, 105 mmol) in
anhydrous pentane (60 mL). The mixture was cooled to 0 °C and
4 (8 g,
35 mmol) was added. Then, bromoform (9.4 mL, 26.6 mmol) was slowly added
with a syringe. After 1 h of stirring at 0 °C and 1 h at rt, the mixture was filtered
on CeliteÒ and the filtrate was stirred with brine. After usual work-up (Et2O),
the crude product was flash chromatographed (petroleum ether) to give 9
(37.5 g, 21.5 mmol, 81%). 1H NMR (CDCl3, 300 MHz) d 5.02 (s, 1H), 4.89 (s, 1H),
1.88 (s, 3H), 1.87 (s, 3H), 1.79 (s, 3H); 13C NMR (CDCl3, 75 MHz) d 144.2 (s),
131.0 (s), 120.3 (s), 116.6 (t), 25.4 (q), 24.6 (q), 21.9 (q).
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pentane (15 mL) under argon, stirred to 0 °C, was slowly added ethyl
dibromofluoroacetate (1.2 mL, 13.2 mmol). After stirring for 8 h at 0 °C and
12 h at rt, the mixture was filtered on CeliteÒ and the filtrate was stirred with
brine. After usual work-up (pentane), the crude product was flash
chromatographed (petroleum ether) to give 12 (0.88 g, 4.75 mmol, 72%) as an
oil. 1H NMR (CDCl3, 300 MHz) d 5.09 (s, 1H), 4.97 (s, 1H), 1.86 (s, 3H), 1.72 (s,
3H), 1.69–1.67 (m, 2H), 0.03 (s, 9H); 13C NMR (CDCl3, 75 MHz) d 153.8 (d,
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