Synthesis and NMR spectroscopic characterization of Si-substituted 1-silacyclobutene derivatives

Article abstract of DOI:10.2478/s11532-010-0122-z

Hydroboration of trialkyn-1-yl(organo)silanes with one equivalent and two equivalents of 9-borabicyclo[3.3.1]nonane, 9-BBN afford dialkyn-1-ylsilanes and alkyn-1-ylsilanes, respectively. The alkyn-1-ylsilane derivatives are stable at room temperature and can be store under dry argon for prolong period of time. These compounds are attractive materials for further rearrangements to afford novel 1-silacyclobutene derivatives bearing Si-alkenyl or Si-alkynyl functionalities. The hydroboration reaction is well controlled by the Si-R ¹ function, i.e., the starting silanes with R¹ = Ph selectively afford hydroboration of one Si-C≡C bond with one equivalent of 9-BBN, leaving the other two functionalities untouched. Under mild reaction conditions (25°C), the starting silanes with R¹ = Me lead to mixture containing dialkyn-1-ylsilane, alkyn-1-ylsilane and their respective 1-silacyclobutene derivatives. All new compounds are sensitive towards air and moisture and were studied by multinuclear magnetic resonance spectroscopy ( ¹H, ¹³C, ¹¹B, ²⁹Si NMR) in solution. Versita Sp. z o.o.

Full text of DOI:10.2478/s11532-010-0122-z

Cent. Eur. J. Chem. • 9(1) • 2011 • 126-132
DOI: 10.2478/s11532-010-0122-z
Central European Journal of Chemistry
Synthesis and NMR spectroscopic
characterization of Si-substituted
1-silacyclobutene derivatives
Research Article
Ezzat Khana,b, Bernd Wrackmeyera
aAnorganische Chemie II, Universität Bayreuth, D-95440 Bayreuth, Germany
bDepartment of Chemistry University of Malakand, Chakdara,
Dir(Lower), Khyber Pakhtunkhwa, Pakistan
Received 23 June 2010; Accepted 6 October 2010
Abstract:Hydroboration of trialkyn-1-yl(organo)silanes with one equivalent and two equivalents of 9-borabicyclo[3.3.1]nonane, 9-BBN afford
dialkyn-1-ylsilanes and alkyn-1-ylsilanes, respectively. The alkyn-1-ylsilane derivatives are stable at room temperature and can be
store under dry argon for prolong period of time. These compounds are attractive materials for further rearrangements to afford novel
1-silacyclobutene derivatives bearing Si-alkenyl or Si-alkynyl functionalities. The hydroboration reaction is well controlled by the Si-R1
function, i.e., the starting silanes with R1 = Ph selectively afford hydroboration of one Si-C≡C bond with one equivalent of 9-BBN,
leaving the other two functionalities untouched. Under mild reaction conditions (25°C), the starting silanes with R1 = Me lead to mixture
containing dialkyn-1-ylsilane, alkyn-1-ylsilane and their respective 1-silacyclobutene derivatives. All new compounds are sensitive
towards air and moisture and were studied by multinuclear magnetic resonance spectroscopy (1H, 13C, 11B, 29Si NMR) in solution.
Keywords: Alkynyl(organo)silanes • Hydroboration • Vinylboration • Silacyclobutenes • NMR.
© Versita Sp. z o.o.
1-silacyclobutene derivatives with exocyclic Si-alkenyl
or Si-alkynyl functions. In addition to Si-Me or Si-Ph
1. Introduction
group in the starting silanes, the influence of Si-C≡C-R
There is a considerable interest in the synthesis of
groups were investigated on the course of the 1,2-
1-silacyclobutene derivatives [1-7]. For the last few
hydroboration reactions.
years we are dealing with the combination of 1,2-
hydroboration and 1,1-organoboration and have applied
this approach to the synthesis of several heterocycles
including 1-silacyclobutene derivatives (A in Scheme 1)
2. Experimental Procedure
[8-12] and 4-silaspiro[3.3]hepta-1,5-diene derivatives
(B in Scheme 1) [13]. The 1-silacyclobutene derivatives
bear a variety of substituents at the silicon atom such
as Si-Cl, Si-H, Si-Ph, Si-Me as well as at ring position
4 (Me, Bu, Ph). The synthetic approach adopted by our
group has certain advantages, for instance, the reactions
are highly selective and side reactions appear to be
negligible [10].The reaction has been fully explored and
the mechanism is reasonably well understood.
All preparative work and handling of air sensitive
chemicals were carried out by observing precautions to
exclude oxygen and moisture. Dry solvents and oven-
dried glassware were used throughout. Silanes 1, 2
[14-19] and 3b [20] were prepared following the literature
H
R
H
R
R
R1
R2
B
Si
B
B
Si
R
Here we have extended our work [12] to study the
hydroboration of trialkyn-1-ylsilanes Me-Si(C≡C-R)3 1
and Ph-Si(C≡C-R)3 2 [R= nBu(a), Ph(b), 4-Me-C6H4(c)).
These silanes were treated with one equivalent and
two equivalents of 9-BBN, aiming for the synthesis of
R
A
B
H
R
R1/R2 = H, Me, Ph, Cl
= Bu, Ph
R = tBu, Ph, 4-Me-C6H5
R
Scheme 1. 1-Silacyclobutene, A and 4-silaspiro[3.3]hepta-1,5-
diene derivatives, B.
* E-mail: ekhan@uom.edu.pk
126

E. Khan, B.Wrackmeyer
29Si{1H} NMR spectra of a mixture of compounds 1b, 5b, 8b, 10b and 13b (lower trace). The upper trace shows quantitative formation
Figure 1.
of 6c and trace amount (ca. 5 %) of 9c. 13C satellites are marked by asterisks in expansion (6c) for 1J(29Si,13C) and 2J(29Si,13C). Values
against each signal (ppm) and in square brackets represent 29Si chemical shift and coupling constants, respectively.
29Si{1H} NMR of the mixture containing 7b and 12b. Expansions are given for corresponding signals showing 13C satellites marked by
asterisks representing 1J(29Si,13C) and 2J(29Si,13C). The satellites close to the parent signal of 7b, marked by arrows (5.9 Hz) may arise
due to nJ(29Si,13C) n ≥ 2.
Figure 2.
procedure.
Crystalline
9-borabicyclo[3.3.1]nonane
2.1. Preparation of trialkyn-1-ylsilanes 1
n
(9-BBN), 3,3-dimethylbutyne, BuLi in hexane (1.6 m)
were commercial products and were used as received.
NMR spectra: Varian Inova 300 MHz and 400 MHz
spectrometers (23±1°C), all equipped with multinuclear
units, using C6D6 solutions, if not mentioned (ca.
10-15%v/v)in5mmtubes.Chemicalshiftsaregivenwith
respecttoSiMe4 [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
and 2.
The silane derivatives 1 and 2 were prepared following
the literature procedure. They were purified by fractional
n
distillation (R= Bu(a)) or recrystallization (R= Ph(b),
4-Me-C6H4(c)) and their structures were determined by
NMR in solution and X-ray structural analysis in the solid
state [18].
2.2. Hydroboration of trialkyn-1-ylsilanes 1
and 2 using 9-BBN.
1
pulse sequence with H decoupling [21-24], based on
3J(29Si-C=C-1H) = 25-30 Hz or 2/3J(29Si-Me/Ph) ≈ 7 Hz A Schlenk tube was charged with a THF solution of
(after optimisation of the respective refocusing delays).
silane 1a (0.45 g, 1.57 mmol) and crystals of 9-BBN
(0.19 g, 1.56 mmol). The reaction mixture was stirred
127

Synthesis and NMR spectroscopic characterization
of Si-substituted 1-silacyclobutene derivatives
59.6 MHz 29Si{1H} NMR spectrum of the mixture containing 10b and 13b. Expansion for the signal corresponding to 10b is given. The
corresponding 1J(29Si,13C) and 2J(29Si,13C) coupling constants are listed in square brackets. 13C satellite signals marked by arrows close
to the parent peak may arise due to nJ(29Si,13C) = 4.5 Hz, n ≥ 2.
Figure 3.
at room temperature for 12 h. After the reaction was
complete, all volatile materials were removed under
4b: proposed, not detected.
7b: 1H NMR (400 MHz): δ = 0.8 (s, 18H, tBu), 1.0, 1.6
reduced pressure (≈ 10-2 Torr) and the oily liquid left (t, m, 10H, BEt2), 6.9 - 7.7 (m, 11H, Si-Ph, Ph, =CH).
was identified as the mixture of silanes 5a and 8a (in
8a: 1H NMR (400 MHz): δ = 0.6 (s, 3H, Si-Me), 0.7,
the ratio of 8:2, respectively, 1H NMR). The other silane 0.9, 1.9 - 1.5, 2.4 (t, t, m, m, 27H, nBu), 1.4, 1.8 - 2.1 (m,
derivatives (6b,c and 9b,c) were synthesized at high 28H, 9-BBN), 6.9 (t, 2H, 3J(1H,1H) = 7.1 Hz, =CH).
temperature (80 - 100°C) using toluene or benzene as
8b: 1H NMR (400 MHz): δ = 0.6 (s, 3H, Si-Me), 1.2–
the solvent. The reaction period lasted from 10 - 30 min. 2.2 (m, 28H, 9-BBN), 6.7–7.2, 7.5 (m, m, 15H, Ph), 7.4
Compound 7b was prepared in a different way.Asolution (s, 2H, =CH).
1
of alkenylsilane 3b (1.86 g, 5.37 mmol) in hexane
9b: H NMR (C6D6): δ = 1.2–2.1 (m, 28H, 9-BBN),
(5mL)wasaddedtofreshlyprepared3,3-dimethylbutynyl 6.8–7.3, 7.6, 8.2 (m, m, m, 22H, Si-Ph, Ph, =CH).
lithium at -78°C. The reaction mixture was slowly allowed
to warm up to room temperature. Further stirring at room 2.3.
temperature for three hours was continued, all volatiles
were removed in a vacuum, and solid materials mainly
LiCl were separated. The oily product left was identified
1,1-Vinylboration of alkenyl(dialkyn-1-
yl)silanes 5-7 and dialkenyl(alkyn-1-yl)
silanes 8 and 9
The mixture containing silanes 5b and 8b was heated
at 80-100°C in an NMR tube for 2-4 h in C6D6. The
reaction was monitored by 29Si NMR spectroscopy
to show the presence of 10b and 13b. They could be
distinguished by their characteristic NMR data. The
same experimental procedure was followed for the
formation of 1-silacyclobutene derivatives 11b,c, 12b
and 14b. A slight variation in the reaction time was
observed, rearrangement of silanes 6, 7 and 9 took 24 h
to afford heterocycles 11, 12 and 14, respectively.
10b: 1H NMR (400 MHz): δ = 0.2 (s, 3H, Si-Me), 1.6
- 1.9 (m, 14H, 9-BBN), 6.7 (s, 1H, 3J(29Si,1H) = 14.6 Hz,
=CH), 6.6 - 7.1 (m, 15H, Ph).
as 7b (Fig. 2) containing reasonable amount of 12b vide
infra.
5a: 1H NMR (400 MHz): δ = 0.6 (s, 3H, Si-Me), 0.7,
0.9, 1.9 - 1.5, 1.9, 2.6 (t, t, m, m, t, 27H, Bu), 1.4, 1.8
3
- 2.1 (m, 14H, 9-BBN), 7.0 (t, 1H, J(1H,1H) = 7.2 Hz,
=CH).
5b: 1H NMR (400 MHz): δ = 0.4 (s, 3H, Si-Me), 1.2–
2.2 (m, 14H, 9-BBN), 6.7 - 7.2, 7.5 (m, m, 15H, Ph), 8.0
(s, 1H, 3J(29Si,1H) = 18.8 Hz, =CH).
6a: 1H NMR (400 MHz): δ = 0.7, 0.8, 1.9 - 1.5, 2.6 (t,
t, m, m, 27H, Bu), 1.4 - 2.1 (m, 14H, 9-BBN), 7.2, 7.7 (m,
m, 6H, Si-Ph, =CH).
6b: 1H NMR (400 MHz): δ = 1.2 - 1.9 (m, 14H, 9-BBN),
11b: 1H NMR (400 MHz): δ = 1.4-2.1 (m, 14H,
9-BBN), 6.8–8.2 (m, 21H, Si-Ph, Ph, =CH).
3
6.6 - 7.9 (m, 20H, Si-Ph, Ph), 8.0 (s, 1H, J(29Si,1H) =
11c: 1H NMR (400 MHz): δ = 1.4-2.1 (m, 14H,
9-BBN), 1.8, 1.8, 6.5–8.2 (s, s, m, 27H, 4-Me-C6H4, Si-
Ph, =CH).
22.1 Hz, =CH).
6c: 1H NMR (400 MHz): δ = 1.3 - 2.0 (m, 14H,
9-BBN), 1.9, 1.9, 2.0, 6.4 - 8.2 (s, s, s, m, 27H, Si-Ph,
4-Me-C6H4, =CH).
1
12b: H NMR (400 MHz): δ = 1.0, 1.1 (s, s, 18H,
128

E. Khan, B.Wrackmeyer
3
1
tBu), 1.1, 1.4 (t, m, 10H, BEt2), 6.3 (s, 1H, J(29Si,1H) = all signals in the NMR spectra (13C and H). The 29Si
18.6 Hz, =CH), 6.8–8.1 (m, 10H, Si-Ph, Ph).
NMR spectroscopy proved to be a powerful technique
13b: 1H NMR (400 MHz): δ = 0.5 (s, 3H, Si-Me), 1.6- for proposing the structures of various intermediates
3
1.9 (m, 28H, 9-BBN), 6.6 (s, 1H, J(29Si,1H) = 13.4 Hz, and final products (Figs. 1, 2). All the related compounds
=CH), 6.6-7.1, 7.4 (m, m, 15H, Ph), 7.8 (s, 1H, 3J(29Si,1H) e.g. 5b, 8b and 1-silacyclobutenes derivatives, 10b,13b
= 15.6 Hz, =CH).
possess characteristic 29Si chemical shifts and they
14b: 1H NMR (400 MHz): δ = 1.4-2.1 (m, 28H, could readily be identified in the mixtures. The same
9-BBN), 6.8-8.2 (m, 21H, Si-Ph, Ph, =CH), 8.3 (s, 1H, reaction was carried out for silane 2 with one equivalent
3J(29Si,1H) = 17.6 Hz, =CH).
of 9-BBN. It was observed that the 1,2-hydroboration
takes place less readily when compared with silane 1.
Thus, silane derivatives 6a-c were formed in almost
quantitative yield (≈ 95%) along with a very small amount
of 9b,c (≈ 5%). The 29Si NMR did not show even traces
of 1-silacyclobutenes at this stage of the reaction. The
3. Results and Discussion
Trialkyn-1-ylsilanes were prepared following the reported
synthetic procedure. They were purified and structurally
characterized using modern analytical techniques (NMR
and X-ray structural analysis) [18]. These silanes are
useful synthons for further transformations, and they
can be converted into alkenylsilanes (5,6,8,9) and
various 1-substituted 1-silacyclobutene derivatives
(10,11,13,14). The trihexyn-1-yl(methyl)silane 1a was
treated with 9-BBN in 1:1 molar ratio. The reaction
proceeded selectively at room temperature and
afforded a mixture of products 5a and 8a (Scheme 2).
These silanes were accompanied by some rearranged
products (respective 1-silacyclobutene derivatives, vide
infra) at the same temperature. Owing to more than two
fractions in the mixture it was not easy to assign correctly
Table 1. 11B, 13C and 29Si NMR data[a] of alkenyl(dialkyn-1-yl)silanes 5 - 9.
R = Bu (a), Ph (b), 4-Me-C6H4 (c)
Si R1
R
3
R1 = Me (1), Ph (2)
2 9-BBN
9-BBN
H
H
R
R
R1
R1
B
Si
B
Si
2
R
R
R
8a, b 9b,c
R1 = Me,
Ph
5a, b 6a, b, c
1 = Me,
Ph
R
Scheme 2. Reaction of trialkyn-1-ylsilanes with 9-BBN as
hydroborating reagent.
δ13C (BC=)
δ13C (C=)
δ13C (Si-C≡)
δ13C (≡C)
δ29Si
δ11B
5ab
5bc
144.2br [70.4]
160.4
83.3 [99.5]
108.6 [19.5]
-54.1
82.3
146.5br [70.1]
155.7
92.4 [98.6]
107.5 [18.6]
-51.1
86.4
6ad
6be
141.2br
143.8br
164.1
158.1
91.2 [101.2]
90.8 [102.8]
108.3 [18.8]
108.7 [20.0]
-55.8
-55.2
82.7
84.3
6cf
152.8br [71.9]
158.3
90.3 [102.9]
108.8 [19.5]
-55.2
84.5
7bg
8ah
8bI
9bj
144.2br [83.1]
148.1br
150.1
157.4
151.7
158.7
87.8 [104.6]
85.8
108.9 [19.2]
108.8
-61.4
-41.1
-39.4
-29.1
85.6
82.7
86.4
84.0
149.5br
96.3 [85.5]
108.8 [15.8]
143.9br
89.3 [86.2]
108.6 [16.3]
[a] Measured in C6D6, coupling constants J(13C,29Si) [± 0.4 Hz] are given in square brackets, superscript ‘br’ denotes a broad 13C resonance signal
[29,30].
[b] other 13C data: δ [J(29Si,13C)] = 2.9 [60.6, Si-Me], 34.4, 31.1br, 23.7 (9-BBN), 35.1, 31.1, 23.0, 14.3 (=C-Bu), 30.7, 22.2, 20.0, 13.7 (C≡C-Bu).
[c] other 13C data: δ [J(29Si,13C)] = 2.0 [63.0, Si-Me], 35.1, 31.8br, 23.8 (9-BBN), Ph carbons without assignment.
[d] other 13C data: δ = 34.5, 31.6br, 23.7 (9-BBN), Bu carbons without assignment;
[e] other 13C data: δ = 34.6, 31.8br, 23.6 (9-BBN), 139.7 [4.8, i], 135.8 [83.2, i], 135.3, 132.4, 129.0, 128.5, 128.4, 128.1 (Ph, Si-Ph) 123.2, 132.5, 128.3,
130.6 (i, o, m, p, ≡C-Ph).
[f] other 13C data: δ = 34.7, 31.8br, 23.7 (9-BBN), 21.3, 21.3 (Me, Me, 4-Me-C6H4) other carbons are not assigned, Si-Ph and 4-Me-C6H4.
[g] other NMR data not assigned.
[h] other 13C data: δ = 3.3 (Si-Me), 34.4, 31.4br, 23.8 (9-BBN), 35.5, 31.9, 23.1, 14.3 (C=C-Bu), 30.9, 22.2, 19.8, 13.7 (C≡C-Bu).
[i] other 13C data. δ [J(29Si,13C)]= 4.2 [56.8, Si-Me], 34.8, 31.9br, 23.8 (9-BBN).
[j] other 13C data: δ = 34.7, 32.0br, 23.5 (9-BBN).
129
129

Synthesis and NMR spectroscopic characterization
of Si-substituted 1-silacyclobutene derivatives
H
Ph
Ph
Si
H
H
Ph
Si
Ph
Li
tBu
Li
tBu
Ph
Cl
BEt2
t Bu
Ph
Cl
Si
BEt2
tBu
BEt2
- LiCl
- LiCl
Cl
t Bu
3b
7b
4b (not observed)
Scheme 3. Conversion of alkenylsilane into alkenyl(dialkyn-1-yl)silane as a result of Si-Cl function replacement.
R = Ph (b), 4-Me-C6H4 (c)
8, 9
5, 6
7b
R
H
H
Ph
H
R
R1
Ph
R1
Si
BEt2
B
B
B
Si
Si
tBu
tBu
12b
R
R
R
H
R
13b, 14b
R1 = Me, Ph
10b, 11b, c
R1 = Me, Ph
Scheme 4. The 1-silacyclobutene derivatives bearing Si-C≡C and Si-C=C functional groups.
product distribution in the mixture depends upon various by 29Si NMR. The intramolecular 1,1-vinylboration
factors, e.g. reaction temperature and nature of the leads to 1-(alkyn-1-yl)-1-silacyclobutenes 10-12 and
groups R and R1. It was concluded that the Si-Ph group 1-(alkenyl)-1-silacyclobutenes 13,14. In the case of
reduces the reactivity, hence, allowing for quantitative C≡C-nBu group, the 1,1-vinylboration was extremely
hydroboration of only one Si-C≡C- bond in the presence slow even at elevated temperature and therefore, no
of one equivalent of 9-BBN. The same was true for data was collected. Moreover the silanes were stable
reaction of silane 2 with two equivalents of 9-BBN.
at that temperature and they did not show marked
Alkenyl silane 3b and its analogues are useful decomposition as has been reported for R = SiMe3 and
synthons for further transformations [9,20]. The R1 = Me groups [12]. No reaction with threefold 1,2-
chlorosilane 3b was treated with Li-C≡C-tBu at –78°C hydroboration was observed. The 1-silacyclobutene
according to literature procedure. The reaction mixture heterocycles achieved in these reactions are oily liquids
contained alkyn-1-ylsilane 7b precursor of 12b and and could readily be identified from their characteristic
some unidentified side products (Scheme 3). Again 29Si NMR data (11B, 13C, 1H and 29Si).
NMR was fruitful in the identification of various fractions
(Fig. 2). Although the chlororsilane, 4b was not detected
3.2. NMR spectroscopic studies
The NMR spectroscopic data (11B, 13C and 29Si) for
silanes 5 - 9 are listed in Table 1 and the respective
1H NMR data are collected in the Experimental section.
The NMR data set is consistent with the proposed
structures and the data show close resemblance to the
but can be proposed as an immediate precursor of
7b, Scheme 3. In the reaction mixture there were no
indications of borate-like intermediates (11B NMR) [9].
3.1. 1-Silacyclobutene derivatives
The alkenyldi(alkyn-1-yl)silanes 5-7 and dialkenyl(alkyn- reported alkynylsilane derivatives [11,26-28]. Structural
1-yl)silanes 8,9 were heated at 80 –120°C for several assignments were readily based on coupling constants
hours (Experimental section) in an NMR tube. The J(29Si,13C) especially for the Si-C≡C- unit and the broad
progress of the reaction was constantly monitored 13C (BC=) NMR signal [29,30]. The coupling constants
130
130

E. Khan, B.Wrackmeyer
Table 2. 11B, 13C and 29Si NMR data[a] for 1-silacyclobutene derivatives 10 - 14.
δ
13C (C-2)
δ
13C (C-3)
δ
13C (C-4)
δ
13C (C=)
δ
29Si
δ
11B
10bb
147.2 [57.3]
177.4br
161.4 [59.4]
132.4
-18.1
88.2
11bc
11cd
12be
146.0 [58.3]
145.4 [58.2]
146.0 [56.5]
181.6br
180.7br
179.5br
160.2 [60.0]
160.1 [60.2]
170.1 [59.6]
138.9
139.6
n.a.
-23.5
-24.0
-24.8
85.8
87.9
87.3
13bf
14bg
151.3 [52.7]
150.7 [n.o.]
178.8br
180.4br
168.2 [52.1]
164.9 [n.o.]
137.8
139.8
-1.6
88.2
85.8
-11.2
[a] Measured in C6D6 at 23 ˚C, some coupling constants 1J(29Si,13C) are given in square brackets, ‘n.a.’ means not assigned and ‘n.o.’ means not observed,
due to signals crowed, superscript ‘br’ denotes a broad 13C resonance signal as a result of partially relaxed scalar 13C-11B coupling [29,30].
[b] other 13C data: δ [J(29Si,13C)] = 0.07 [53.0, Si-Me], 34.0, 34.1, 31.9br, 23.7 (9-BBN), 90.7 [82.7, Si-C≡], 110.2 [16.0, ≡C].
[c] other 13C data: δ [J(29Si,13C)] = 34.4, 32.3br, 23.5 (9-BBN), 88.2 [86.6, Si-C≡], 111.6 [16.3, ≡C].
[d] other 13C data: δ [J(29Si,13C)] = 34.4, 34.4, 32.2br, 23.6 (9-BBN), 88.1 [86.4, Si-C≡], 112.1 [16.4, ≡C], 21.4, 21.2 (Me, 4-Me-C6H4) other carbons are without
assignment. [e] other 13C data: δ [J(29Si,13C)] = 36.0 [4.3], 30.9, 30.6, 28.7 (tBu), 21.9br, 9.6 (BEt2), 78.3 [86.7, Si-C≡], 120.7 [15.9, ≡C], 139.8 [70.9, i].
[f] other 13C data: δ [J(29Si,13C)] = -0.01 [47.8, Si-Me], 33.8, 33.9, 34.1, 34.2, 31.9br, 32.0br, 23.5, 23.7 (9-BBN), 153.8 (=C), 150.1br (BC=).
[g] other 13C data: δ [J(29Si,13C)] = 33.9, 33.9, 32.2br, 23.8, 23.6 (9-BBN), 157.1 (=C), 146.5br (BC=).
J(29Si,13C) in 13C NMR spectra could be confirmed by
4. Conclusions
Theprocessof1,2-hydroborationand1,1-organoboration
observing 13C satellite signals in the respective 29Si
NMR spectra. The 29Si NMR is the only access to the
coupling constant value 1J(29Si,13C(B)) as can be seen in
can be applied separately to the synthesis of a large
Figs. 1 and 2. In the mixtures, it can be deduced that all
compounds possess distinct 29Si chemical shifts and the
reaction mechanism and various intermediates could
be proposed (Fig. 1). These facts make the mechanism
of the reaction well understandable, the data listed in
Table 1 are quite sufficient for structural elucidation of
the title compounds in solution state.
The NMR data (11B, 13C and 29Si) of 1-silacyclobutene
derivatives are listed in Table 2. The NMR data set is
consistent with the proposed structures. The 29Si NMR
spectroscopy serves as the main tool for assignments.
The data obtained mainly for the four-membered ring
(1-silacyclobutene) are well comparable with that
of 1-silycyclobutenes [9] and analogous spirosilane
number of organometallic compounds. The reaction in
which 1,2-hydroboration is followed by 1,1-vinylboration
have proved to be a facile route for the synthesis
of 1-silacyclobutene derivatives. Here, further new
examples can be contributed to this class of silicon
heterocycles. The synthetic route is preferred over other
methods [1-7] owing to its simplicity and efficiency.
The variety of functional groups on silicon atom is of
prime importance. The groups considered in this study
such as Si-C≡C and Si-C(B)=C can be used for further
reactions.
Acknowledgements
derivatives [13]. In 13C NMR spectra, the substituents on This work was supported by the Deutsche
the silicon atom Si-(B)C=C and Si-C≡C- can be readily Forschungsgemeinschaft. E. K. is grateful to DAAD
assigned, based on 29Si satellites for the 13C(Si-C≡C) and HEC (Pakistan) for a scholarship (Code No.
signals,whereasasharp13C(=CH)andabroad13C(B-C=) A/04/30788).
signal [29,30] at high frequency are characteristic of the
exocyclic Si-C(B)=C group. Other NMR data are within
the expected range.
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