Syntheses of 1, 1-organo-substituted silole derivatives. 1, 1-ethylboration, 1, 1-vinylboration and protodeborylation

Article abstract of DOI:10.1515/znb-2009-0902

1, 1-Organoboration of dialkyn-1-ylsilanes using triethylborane, BEt ₃, and 9-ethyl-9-borabicyclo [3.3.1]nonane, Et-9-BBN, was carried out at elevated temperatures, 100-120 °C. These reactions afforded selectively silole derivatives bearing the

Full text of DOI:10.1515/znb-2009-0902

Syntheses of 1,1-Organo-substituted Silole Derivatives. 1,1-Ethylboration,
1,1-Vinylboration and Protodeborylation
Ezzat Khana,b, Stefan Bayera, and Bernd Wrackmeyera
a Anorganische Chemie II, Universita¨t Bayreuth, 95440 Bayreuth, Germany
b Department of Chemistry University of Malakand, Chakdara, Dir(Lower), N. W. F. P., Pakistan
Reprint requests to Prof. Dr. B. Wrackmeyer. E-mail: b.wrack@uni-bayreuth.de
Z. Naturforsch. 2009, 64b, 995 – 1002; received June 9, 2009
1,1-Organoboration of dialkyn-1-ylsilanes using triethylborane, BEt3, and 9-◦ethyl-9-borabicyclo
[3.3.1]nonane, Et-9-BBN, was carried out at elevated temperatures, 100 – 120 C. These reactions
afforded selectively silole derivatives bearing the dialkylboryl group in 3-position. The siloles are
formed via intermolecular 1,1-alkylboration, followed by intramolecular 1,1-vinylboration. Two ex-
amples of boryl-substituted siloles were treated with an excess of acetic acid at ambient temperature
to afford the respective protodeborylated compounds. All new compounds were characterized in so-
lution by multinuclear magnetic resonance spectroscopy (1H, 13C, 11B and 29Si NMR).
Key words: Alkynylsilanes, Siloles, Organoboration, Triethylborane, NMR
Introduction
Dialkyn-1-ylsilanes [1 – 3] and triorganoboranes
[4 – 6] have numerous applications in chemistry. By
combining these useful classes of compounds, it
proved possible to develop a versatile synthesis of
siloles [7 – 9] via organoboration reactions [10, 11].
Recently, it has been shown that siloles can also
be obtained by using tris(pentafluorophenyl)borane,
B(C6F5)3, as the organoborating reagent [12]. Since
siloles have become increasingly important owing to
their peculiar photophysical properties [13– 15], fur-
ther efforts were made to use organoboration aim-
ing for a greater variety of substituents in all five
ring positions. So far, siloles of type A were ob-
tained in high yield from dialkyn-1-yldimethylsilanes
1 (Scheme 1a) [10a], and similarly siloles C were pre-
pared from B, however under less harsh conditions
(Scheme 1b) [10b].
Scheme 1. 1,1-Dimethylsilole derivatives obtained via 1,1-
organoboration as described in ref. [10].
(4a – d) are listed in Table 1, and for the other precursor
alkynes, such data will be described elsewhere [17].
The reaction of various dialkyn-1-yldiorganosilanes
2 – 5 with an excess of BEt3 is shown in Scheme 2.
The yields are essentially quantitative after prolonged
periods of heating. The products are the siloles 6 –
8 which were obtained as air-sensitive oily liquids
or waxy solids. Their purity and structure in solu-
tion follow from consistent sets of characteristic NMR
data (Table 2). This is illustrated by the 13C NMR
spectra in Fig. 1 which also shows the clean for-
mation of the corresponding protodeborylatetd silole
(vide infra).
In the present report, we describe the synthesis
of siloles analogous to A by varying the organyl
groups linked to silicon and also by using 9-ethyl-9-
borabicyclo[3,3,1]nonane (Et-9-BBN) instead of tri-
ethylborane, BEt3, as an organoborating reagent.
Results and Discussion
The dialkyn-1-ylsilanes 2 – 5 were prepared in the
usual way by the reactions of the respective alkynyl-
lithium reagents with diorganosilicon dichlorides [16].
NMR spectroscopic data of the new alkyn-1-ylsilanes (≈ 80 %) the silole 9 along with a small amount
In the case of 5, the reaction afforded mainly
c
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E. Khan et al. · Syntheses of 1,1-Organo-substituted Silole Derivatives
Table 1. 13C and 29Si NMR dataa for the silane derivatives 4a – d.
δ
29Si
δ
13C(Si-C≡)
δ
13C(≡C)
δ
13C(Si(CH2)3)
δ
13C(R)
4a 80.7 [96.1]
4b 78.8 [95.7]
4cb 89.7 [93.4]
110.5 [18.8]
118.3 [18.0]
108.6 [18.3]
17.1 [50.4], 18.7 [18.8]
17.3 [51.3], 18.1 [18.8]
17.2 [50.6], 19.2 [18.5]
13.6, 19.8, 22.1, 30.6 (R = nBu)
28.4, 30.7 (R = tBu)
−37.7
−37.1
122.9, 128.7, 132.6, 129.5 (i, o, m, p) (R = Ph)
−0.4 [56.2] R = SiMe3)
−35.8
4d 108.1 [86.2] 117.7 [13.9] [75.1] 16.6 [49.9], 18.9 [18.7]
−39.2, −17.9
Measured in C6D6 at 23 1 ◦C, coupling constants 1J(29Si,13C) and 2J(29Si, 13C) [Hz] are given in brackets; b measured in CDCl3.
a
Table 2. 11B, 13C and 29Si NMR dataa for the siloles 6 – 11 and A [10a] for comparison.
δ
13C
C-4
δ
29Si
δ
11B
C-2
C-3
C-5
A (R = Bu)
6ab (Bu)
6cc (Ph)
7ad (Bu)
8ae (Bu)
8bf (tBu)
8cg (Ph)
8dh (SiMe3)
9i
140.8 [64.1]
140.9 [64.5]
140.9 [58.2]
140.0 [65.9]
137.1 [59.0]
147.7 [59.7]
142.2
142.1 [38.6, 63.6]
133.9 [62.9, 51.5]
149.7 [62.8]
165.8 (br)
168.1 (br)
170.4 (br)
169.6 (br)
167.4 (br)
164.4 (br)
169.3 (br)
185.1 (br)
176.1 (br)
169.7 (br)
153.6 [10.7]
155.9 [11.2]
157.9 [10.1]
157.6 [11.3]
155.5 [10.4]
154.5 [11.6]
157.7 [9.5]
171.7 [8.1, 11.1]
171.8 [5.1]
165.4 [8.8]
136.2 [68.2]
136.5 [69.9]
134.7 [65.6]
134.8 [67.5]
132.8 [63.1]
141.1 [n. d.]
141.0
134.0 [43.0, 62.2]
135.8 [60.5]
137.6 [66.3]
5.2
−0.4
1.9
86.7
88.1
87.0
86.5
88.3
89.0
89.2
88.5
87.6
87.8
−4.6
18.4
22.7
19.9
40.2
17.5, −11.4
11cj (Ph)
6.9
a
Measured in C6D6 at 296 1 K; coupling constants 1J(29Si,13C) and 2J(29Si,13C) [Hz] are given in brackets; br denotes broad 13C NMR
signal owing to partially relaxed 13C-11B spin-spin coupling [25];
other 13C NMR data: δ [J(29Si,13C)] = −5.5 [49.4, Si-Me], 24.7, 14.2
b
c
(Et), 22.8 (br), 9.4 (BEt2), 34.0, 33.4, 32.7, 28.8, 23.3, 23.3, 14.1, 14.2 (Bu), 136.3 [62.9], 134.5, 128.3, 129.5 (i, o, m, p, Si-Ph);
other
13C NMR data: δ [J(29Si,13C)] = −6.4 [51.4, Si-Me], 24.8, 14.5 (Et), 22.3 (br), 9.9 (BEt2), 140.1 [65.5], 134.8, 128.7, 130.0 (i, o, m, p,
d
Si-Ph), 145.0, 141.9, 128.7, 128.6, 128.2, 127.7, 125.9, 126.4 (i, i, o, o, m, m, p, p, Ph);
other 13C NMR data: δ [J(29Si,13C)] = 34.0,
e
33.4, 33.2, 29.1, 23.3, 14.2, 14.1 (Bu), 24.9, 14.0 (Et), 22.9 /br), 9.5 (BEt2), 135.8 [62.1], 135.9, 128.4, 129.8 (i, o, m, p, SiPh2);
13C NMR data: δ [J(29Si,13C)] = 14.0, 32.6 (Et), 22.8(br), 9.2 (BEt2), 14.8, 23.6, 28.4, 33.7, 34.0 (Bu), 14.3, 19.1 ((CH2)3);
13C NMR data: δ [J(29Si,13C)] = 14.9, 31.7 (Et), 22.5 (br), 10.2 (BEt2), 32.9, 30.8 (t Bu), 16.0 [39.2], 18.6 [15.5] ((CH2)3);
other
other
other
f
g
13C NMR data: δ [J(29Si,13C)] = 13.9, 31.2 (Et), 22.0 (br), 9.4(BEt2), 128.4, 128.8, 126.9, 129.3 (i, o, m, p, Ph), 13.9 [40.3], 19.3 [16.4
h
(CH2)3);
19.0 [15.5] (CH2)3);
other 13C NMR data: δ [J(29Si,13C)] = 15.3, 30.0 (Et), 23.0 (br), 9.4 (BEt2), 1.6 [51.3, SiMe3], 1.7 [51.6, SiMe3], 14.3 [38.8],
i
the silole 9 is numbered in this way for consistency of the data; other 13C NMR data: δ [J(29Si,13C)] = −3.3 [48.8,
j
SiMe3], 1.5 [51.3, SiMe2], 9.2, 21.0 (br) (BEt2), 14.4, 30.4 (Et);
(br), 21.7, 30.6, 34.8, 35.0 (br) (BEt, BC8H14), 134.3 [6.9, i], 141.6 [5.8, i], 128.7, 128.6, 128.0, 127.8, 126.6, 126.7 (2,5-Ph2).
other 13C NMR data: δ [J(29Si,13C)] = −3.7 [49.3, SiMe2], 11.2, 22.7
Scheme 2. 1,1-Ethylboration
of the dialkyn-1-ylsilanes 2 – 5
using BEt3.
(< 5 %) of the silole 9ꢀ, which results from intermolec- step. In contrast, 9 is formed by intermolecular 1,1-eth-
ular 1,1-ethylboration of the Si–C≡CH unit in the first ylboration of the more reactive Si–C≡C–SiMe3 unit
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E. Khan et al. · Syntheses of 1,1-Organo-substituted Silole Derivatives
997
1
Fig. 1. Part of the 100.5 MHz 13C{ H} NMR spectrum of the 3-diethylborylsilole derivative 6a (upper inset). The same
spectral region is shown for the protodeborylated silole 12a (middle trace: J-modulated experiment in the middle and normal
1
13C{ H} NMR in the lower trace). 29Si satellites (marked by asterisks in expansions) indicate coupling constants 1J(29Si,13C)
and 2J(29Si,13C) (values are given in brackets; see also Tables 2 and 3).
Scheme 3. [4+2]Cycloaddition
reactions of the silole 9.
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E. Khan et al. · Syntheses of 1,1-Organo-substituted Silole Derivatives
1
Fig. 2. 59.6 MHz 29Si{ H}
NMR spectrum (INEPT, re-
focused [23]) of the reac-
tion mixture after 2 d of
heating 5 in BEt3 as a sol-
vent at 110 ◦C. The mix-
ture contains 5, 9, 9ꢀ, 9ꢀꢀ,
and 9ꢀꢀꢀ.
in the first step, followed by fast intramolecular 1,1-
vinylboration of the Si–C≡C–H unit. Under the re-
action conditions, the silole derivative 9 reacts fur-
ther (Scheme 3) via [4+2]cycloadditions, either with
the starting silane 5 or with one of its kind to afford
the 7-silanorbornadiene 9ꢀꢀ (≈ 5 %) and a tricyclic 7-
silanorbornene derivative 9ꢀꢀꢀ, respectively. The NMR
Scheme 4. Side products from the 1,1-organoboration of 4b,
spectroscopic data, in particular those obtained from c, unsuitable for further rearrangements to afford the corre-
29Si NMR spectra, are very helpful (Fig. 2) in the iden-
sponding silole derivatives.
tification of such products. For compound 9ꢀꢀ, all the
29Si chemical shifts [δ29Si = 79.0 (7-SiMe2), −3.9
(SiMe3), −19.7 (C≡C–SiMe3), −28.3 ppm (SiMe2)]
and relevant 1H NMR signals [e. g. δ1H(=CH) =
6.92 ppm (d, J(1H,1H) = 1.5 Hz)] can be found in the
expected regions [18]. Numerous additional 29Si NMR
signals of low intensities grow with time, and from
Scheme 5. 9-Ethyl-9-borabicyclo[3.3.1]nonane, 9-Et-9-
these it appears that 9 undergoes [4+2]cycloadditions
BBN, is used as an organoborating reagent to afford the
with one of its kind to give 9ꢀꢀꢀ, for which at least the
polycyclic silole 11c.
29Si NMR signal of the 7-silanorbornene unit (δ29Si =
41.5) [18]) as well as the 29Si NMR signal of the SiMe2
group as part of the silacyclopent-2-ene ring (δ29Si =
23.2) are typical. Other spurious signals may arise by
1,1-ethylboration of the Si–C≡C–H unit in 9ꢀꢀ or by
the – likely – formation of other isomers in the [4+2]
cycloaddition reactions. Owing to the low concentra-
tion of the compounds 9ꢀꢀ, 9ꢀꢀꢀ and others, we did not
allow for a rearrangement towards ring closure. The
corresponding Z-isomers were not detected.
The rate-determining step of the silole formation
via 1,1-organoboration is the intermolecular 1,1-ethyl-
boration in all cases, followed by the fast intramolec-
ular 1,1-vinylboration, if the stereochemistry of the
alkenyl-alkyn-1-ylsilane is favorable. In our previ-
ous work it was found that BEt3 can react with cer-
tain alkyn-1-ylsilanes as a 1,2-hydroborating reagent
[19, 20]. This was not observed in the present work.
The harsh reaction condition required for the 1,1-
1
attempt to assign H and 13C NMR signals to these
compounds i. e., 9ꢀꢀ and 9ꢀꢀꢀ.
In the cases of the 1,1-dialkyn-1-ylsilacyclobutane
derivatives 4b and 4c, the reactions with BEt3 lead
to the siloles 8b, c along with side products 10b, c organoboration of dialkyn-1-ylsilanes restrict these
(Scheme 4), for which the stereochemistry does not reactions to thermally stable triorganoboranes. This
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E. Khan et al. · Syntheses of 1,1-Organo-substituted Silole Derivatives
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Table 3. 13C and 29Si NMR dataa for the siloles 12a, c.
12a
12c
δ
δ
δ
δ
13C(C-2)
13C(C-3)
13C(C-4)
13C(C-5)
145.2 [63.6]
144.2 [8.3]
153.9 [8.7]
134.9 [67.8]
13.7, 23.8
143.8 [64.2]
143.3 [7.2]
155.3 [7.9]
138.1 [67.4]
13.8, 24.8
δ13C(Et)
δ13C(Si-Me) −6.2 [49.7]
−6.2 [51.9]
13C(Si-Ph) 135.5 [64.1], 134.5, 128.3, 129.6 (i, o, m, p)
140.4 [5.8], 138.9 [5.9], 134.2 [66.5], 134.8, 130.1,
129.0, 128.7, 128.6, 128.3, 127.3, 126.9, 126.1b
1.0
R = Bu (a)
R = Ph (c)
δ
δ
δ
13C(R)
29Si
14.1, 14.1, 22.9, 23.2, 28.7, 32.3, 32.8, 33.3
−1.6
Measured in C6D6 at 23 1 ◦C, coupling constants 1J(29Si,13C) and 2J(29Si,13C) [Hz] are given in brackets; b Ph carbons without assign-
a
ment.
Fig. 3. Monitoring of the
reaction of di(phenyl-
ethynyl) dimethylsilane
1c with Et-9-BBN by
29Si NMR spectroscopy
(INEPT, refocused [23]).
The 29Si NMR signal
for
1c
disappeared
after 7 d. Expansions are
shown for the two signals
corresponding to 1c and
the silole 11c, and 13C
satellite signals corre-
sponding to coupling
constants 1J(29Si,13C)
and 2J(29Si,13C) are
marked by asterisks and
arrows,
respectively.
All values (Table 2) are
also evident from the
respective 13C NMR
spectra.
condition is fulfilled by 9-ethyl-9- borabicyclo[3.3.1]
As an example for the protodeborylation of the
nonane, Et-9-BBN, as shown in Scheme 5. The reac- siloles reported here, the siloles 6a,c were treated with
tion proceeds by twofold ring expansion of the bicyclic an excess of acetic acid (Scheme 6). These reactions
system to give selectively the polycyclic silole 11c. afforded smoothly in essentially quantitative yield the
During the course of the reaction no intermediates or siloles 12a, c (see NMR data in Table 3, and also Fig. 1
1
other products were observed (Fig. 3).
for the 13C{ H} NMR spectrum of 12a), along with
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E. Khan et al. · Syntheses of 1,1-Organo-substituted Silole Derivatives
◦
was heated at 100 – 110 C (oil bath temperature), and the
progress of the reaction was monitored by 29Si NMR spec-
troscopy. After 3 d, when the reaction was complete, all read-
ily volatile materials were removed in vacuo, and the oily
yellowish residue was identified as the pure (> 97 % accord-
1
ing to H NMR) silole 6a. Except for the reaction time, the
experimental procedure was identical for all other members
Scheme 6. Protodeborylation of two siloles using acetic acid.
of the series, 6c (10 d), 7a (23 d), 8a (10 h), 8b (7 d), 8c
the known bicyclic boron-oxygen compound 13 [21]. (11 d), and 8d (6 h). Siloles 8b and 8c were accompanied by
alkenyl(alkyn-1-yl)silanes 10b and 10c for which the loss in
Interestingly, the siliole 11c does not react under the
stereoselectivity does not allow for further rearrangement.
same conditions.
6a: 1H NMR (400 MHz): δ = 0.5 (s, 3H, 2J(29Si,1H) = 6.5
Hz, Si-Me), 2.2, 1.0 – 0.9, 0.5, 0.5 (m, m, t, t, 18H, Bu), 1.8,
0.6 (m, t, 5H, Et), 1.8 – 1.9, 0.7 (m, t, 10H, BEt2), 6.9 – 7.4
(m, 5H, Si-Ph).
Conclusions
The synthesis of siloles via 1,1-organoboration of
dialkyn-1-ylsilanes allows not only for variations of
substituents in the positions 2 – 5 but also for intro-
ducing various organyl groups to silicon, including the
access to chiral siloles. Particularly noteworthy in this
context are also the new spirosilanes containing silacy-
clobutane units.
6c: 1H NMR (400 MHz): δ = 0.6 (s, 3H, Si-Me), 2.4, 0.9
(m, t, 5H, Et), 1.5, 1.1 (m, t, 10H, BEt2), 7.3 – 7.7 (m, 15H,
Si-Ph, Ph).
1
7a: H NMR (400 MHz): δ = 2.1 – 1.9, 1.3, 0.9, 0.5, 0.4
(m, m, m, t, t, 18H, Bu), 2.2, 0.8 (m, t, 5H, Et), 1.1, 0.9 (m, t,
10H, BEt2), 6.9 – 7.7 (m, 10H, SiPh2).
8a: Yield = 98 %. – 1H NMR (250 MHz): δ = 0.9, 2.0 (t,
q, 5H, Et), 1.0, 1.4 (t-br, q-br, 10H, BEt2), 0.9, 1.3 – 1.5, 2.2
(t, m, t, 18H, Bu), 1.6, 2.4 (t, quint, 6H, (CH2)3).
Experimental Section
All preparative work and handling of air-sensitive chem-
icals were carried out by observing precautions to ex-
clude oxygen and moisture. Dichloromethylphenylsilane,
dichlorodiphenylsilane, 1,1-dichloro-silacyclobutane, 1-hex-
yne, 3,3-dimethyl-but-1-yne, ethynylbenzene, trimethylsilyl-
ethyne, n-butyllithium in hexane (1.6 M), and triethylbor-
ane (BEt3) were commercial products and were used with-
out further purification. 9-Etyl-9-borabicyclo[3.3.1]nonane,
Et-9-BBN, was prepared according to the literature proce-
dure [22]. The dialkyn-1-ylsilanes 2 – 5 were prepared in the
usual way [16], and details including NMR spectrosciopic
data will be reported elsewhere [17]. NMR spectra: Bruker
ARX 250 MHz or Varian Inova 300 MHz and 400 MHz
spectrometers, all equipped with multinuclear units; mea-
surements at 23 1 ◦C in 5 mm (o. d.) tubes, using C6D6 solu-
tions (ca. 10 – 15 % v/v), if not mentioned otherwise. Chem-
ical shifts are given relative to SiMe4 [δ1H (C6D5H) = 7.15,
δ13C (C6D6) = 128.0, δ29Si = 0 for SiMe4 with Ξ(29Si) =
19.867187 MHz], and δ11B = 0 for BF3-OEt2 with Ξ(11B) =
32.083971 MHz. 29Si NMR spectra were recorded using the
refocused INEPT pulse sequence with 1H decoupling [23],
based on 2/3J(29Si,1H) ≈ 7 – 10 Hz (after optimization of the
respective refocusing delays).
8b: 1H NMR (250 MHz) data: δ = 0.9, 2.1 (t, q, 5H,
Et), 1.1, 1.35 – 1.45 (t-br, m-br, 10H, BEt2), 1.2, 1.3 (s,
t
18H, Bu), 1.7, 2.5 (dt, quint., 6H, J(1H,1H) = 2.3, 8.5 Hz,
(CH2)3).
8c: 1H NMR (250 MHz): δ = 1.0, 1.5 (t-br, q-br, 10H,
BEt2), 1.2, 2.3 (t, q, 5H, Et), 1.5, 2.1 (t, quint., 6H, (CH2)3),
7.2 – 7.3, 7.3 – 7.5 (m, m, 10H, Ph).
8d: Yield = 98 %. – 1H NMR (250 MHz): δ = 0.2, 0.3
(s, s, 18H, SiMe3), 1.0, 2.2 (t, q, 5H, Et), 1.0, 1.4 (t-br, q-br,
10H, BEt2), 1.6, 2.5 (t, quint., 6H, (CH2)3).
1
9: H NMR (400 MHz): δ = 0.2 (s, 9H, SiMe3), 0.2 (s,
6H, SiMe2), 0.9, 2.4 (t, q, 5H, Et), 1.0, 1.3 (t, q, 10H, BEt2),
5.9 (s, 1H, 2J(29Si,1H) = 15.0 Hz, C5H).
9ꢀ: 1H NMR (400 MHz): δ = 5.8 (t, 1H, 2J(29Si,1H) =
15.3 Hz, 4J(1H,1H) = 1.9 Hz, C5H); 29Si NMR (59.6 MHz):
δ = −11.3, +15.4 ppm.
t
10b: 1H NMR (250 MHz): δ = 1.1 (s, 9H, Bu), 1.4
(s, 9H, tBu). – 13C NMR (62.8 MHz): δ = 82.8 (≡C),
116.3 (Si-C≡), 140.6 (=C), 168.4 (br, (B)C=), 9.8, 22.4br
(BEt2), 13.4, 14.2, 21.1 (Si(CH2)3), 31.0 (tBu). – 29Si NMR
(49.7 MHz): δ = −12.9.
10c: 1H NMR (250 MHz): δ = 1.0, 1.3 (t, q, 10H, BEt2),
1.1, 2.0 – 2.5 (t, m, 5H, Et), 1.5, 2.0 – 2.5 (t, m, 6H, (CH2)3),
7.1 – 7.5 (m, 10H, Ph). – 13C NMR (62.8 MHz): δ = 87.4
Reaction of dialkyn-1-ylsilanes 2 – 5 with triethylborane
(typical procedure)
A Schlenk tube was charged with the dialkyn-1-ylsilane (≡C), 106.7 (Si-C≡), 136.4 (=C), 165.4br ((B)C=), 14.0,
2a (0.5 g; 1.77 mmol) where an excess of BEt3 (4 mL) 26.4 (Et), 9.1, 22.0br (BEt2), 13.7, 14.4, 18.2 (Si(CH2)3). –
served as reagent as well as solvent. The reaction mixture 29Si NMR (49.7 MHz): δ = −9.8.
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Reaction of di(alkyn-1-yl)dimethylsilane 1c with 9-ethyl-9- and the bicyclic boron-oxygen compound 13 (1H, ◦11B, 13C
borabicyclo[3.3.1]nonane, Et-9-BBN
and 29Si NMR). The mixture was heated to 100 C under
vacuum (ca. 10−2 Torr) for 1 h, adopting the literature proce-
dure [24] for separation of the boron-oxygen product 13. The
boron-oxygen compound was collected as a solid along the
walls of the Schlenk tube. The oily residue contained mainly
the desired protodeborylated silole 12a (> 95 %; Fig. 1). The
identical experimental procedure was adopted for protodebo-
rylation of 12c. The silole derivatives 12a and 12c are color-
less oily liquids, stable in dry air, and they were fully charac-
terized by multinuclear NMR spectroscopy (Table 3).
A few crystals (0.20 g; 0.77 mmol) of the silane 1c dis-
solved in C6D6 (ca. 0.5 mL) and Et-9-BBN (0.5 mL, in ex-
cess) were given into an NMR tube which was sealed un-
◦
der argon. The reaction mixture was heated to 102 5 C
(oil bath temperature), and changes were monitored by 29Si
NMR spectroscopy (Fig. 3). After 7 d the NMR tube was
cooled in liquid N2 and opened carefully. The contents of
the tube were dissolved in pentane (5 mL). After removing
all volatiles including the excess of Et-9-BBN (b. p. = 40 –
42 ◦C/ 0.9 Torr) in vacuo, the pure silole (> 97 % according
to 1H NMR) 11c was obtained as a waxy solid.
12a: 1H NMR (400 MHz): δ = 0.5 (s, 3H, 2J(29Si,1H) =
6.5 Hz, Si-Me), 1.1, 2.2 (t, q, 5H, Et), 0.7, 0.8, 1.1, 1.3, 2.3
3
(t, t, m, m, t, 18H, Bu), 6.5 (s, 1H, J(29Si,1H) = 13.4 Hz,
11c: 1H NMR (400 MHz): δ = 0.2 (s, 6H, SiMe2), 0.8,
1.1, 1.5, 1.7 – 1.8, 2.0, 2.1, 3.2 (t, b, m, m, m, m, m, 19H,
B-Et, BC8H14), 6.8 – 7.1, 7.2, 7.3 (m, m, m, 10H, Ph).
C4H), 7.5, 7.1 (m, m, 5H, Si-Ph).
12c: 1H NMR (400 MHz): δ = 0.6 (s, 3H, 2J(29Si,1H) =
6.6 Hz, Si-Me), 1.0, 2.4 (t, q, 5H, Et), 6.9 – 7.1, 7.4, 7.6,
7.9 (m, m, m, m, 15H, Si-Ph, Ph), 7.3 (s, 1H, 3J(29Si,1H) =
13.1 Hz, C4H).
Protodeborylation of siloles 6a and 6c (typical procedure)
The silole 6a (1.0 g) was dissolved in pentane (10 mL),
and glacial acetic acid (1.0 mL, in excess) was slowly added.
The reaction mixture was stirred at r. t. After 40 min all read-
ily volatile materials were removed under reduced pressure.
The oily residue was identified as a mixture of the silole 12a
Acknowledgements
This work was supported by the Deutsche Forschungsge-
meinschaft. E. K. is grateful to DAAD and HEC (Pakistan)
for a scholarship.
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