Effect of various substituents on intramolecular 1, 1-vinylboration, synthesis of 1-silacyclobutene derivatives

Article abstract of DOI:10.3906/kim-0910-20

The reaction of 1-boryl-1-alkenyl chlorosilane derivatives with alkynyllithium reagents [Li-C = C-R³ (R³ = Ph, SiMe ₃)] at low temperature (-78 ° C) affords alkenyl(alkyn-1-yl) silanes. These compounds are precursors of 1-

Full text of DOI:10.3906/kim-0910-20

Turk J Chem
34 (2010) , 793 – 803.
¨
˙
c
ꢀ TUBITAK
doi:10.3906/kim-0910-20
Effect of various substituents on intramolecular
1,1-vinylboration, synthesis of 1-silacyclobutene
derivatives
Ezzat KHAN1,2, Bernd WRACKMEYER1
1Anorganische Chemie II, Universit¨at Bayreuth, D-95440 Bayreuth-GERMANY
2Department of Chemistry, University of Malakand, Chakdara, Dir(Lower),
North-West Frontier Province (N-W.F.P.), PAKISTAN
e-mail: b.wrack@uni-bayreuth.de, ekhan@uom.edu.pk
Received 08.10.2009
The reaction of 1-boryl-1-alkenyl chlorosilane derivatives with alkynyllithium reagents [Li-C≡C-R3
(R3 = Ph, SiMe3)] at low temperature (–78 ◦ C) affords alkenyl(alkyn-1-yl)silanes. These compounds
are precursors of 1-silacyclobutene derivatives, which are formed via intramolecular 1,1-vinylboration. This
reaction works for various groups at silicon (R1 /R2 : R1 = H, Me, Ph; R2 = Me, Ph) and at the C=C and
C≡C units (R/R3 : R = n Bu, Ph; R3
=
n Bu, Ph, SiMe3). The conversion into 1-silacyclobutene derivatives
is incomplete only in the case of R3 = SiMe3 . The reactions were monitored by NMR spectroscopy in order
to elucidate the reaction mechanism, and the proposed structures of all new compounds follow from consistent
sets of NMR parameters (1 H-, 13 C-, 11 B-, 29 Si-NMR).
Key Words: Alkynylsilanes, triorganoboranes, hydroboration, organoboration, silacyclobutenes, NMR
Introduction
Reactions involving 1,2-hydroboration and 1,1-organoboration have been widely used in organic1−4 as well as
in organometallic synthesis.5−7 Among organometallic compounds the alkynyl metal compounds of group 14
elements such as alkyn-1-ylsilanes have been used in hydroboration and organoboration reactions. Both of these
intermolecular reactions require totally different reaction conditions. For instance, 1,2-hydroboration of alkyn-1-
ylsilanes takes place under mild reaction conditions,8−16 in contrast to 1,1-organoboration, which requires more
harsh reaction conditions.5 Numerous novel organometallic compounds have been prepared taking advantage of
1,1-organoboration or 1,2-hydroboration.17−29 In this context, the combination of 1,2-hydroboration and 1,1-
793

Effect of various substituents on intramolecular 1,1-vinylboration..., E. KHAN, B. WRACKMEYER
organoboration has led to a diverse field of heterocyclic chemistry comprising simple silicon heterocycles30−33
as well as spirosilanes.34,35 This combination is primarily based on the fact that 1,2-hydroboration allows one to
introduce the boryl group into the molecule under mild reaction conditions. Then the activation energy for the
1,1-organoboration becomes lower, since this is now an intramolecular process. Therefore, these reactions take
place more readily at comparatively low temperature and in a short time. We have reported 1-silacyclobutene
derivatives A-D36−38 (Scheme 1). These derivatives have been studied in solution by NMR and the molecular
structure for one example has been determined by X-ray diffraction.36
R
H
R
H
n Bu
H
R
H
R1
R1
Cl
Cl
R1
Si
B
Si
BEt2
Si
BPr2
Si
B
R2
Cl
Cl
R
A
R
C
n Bu
D
R
B
R1 = H, Me, Ph, Cl
R = nBu, Ph
R1/R2 = Me, Ph
R = nBu, tBu, Ph, SiMe3
R1 = Me, Ph
R = nBu, Ph
Scheme 1. Examples of various substituted 1-silacyclobutene derivatives.
In continuation of our previous work, here we report the effect of ≡C-R3 substituents on intramolecular
1,1-vinylboration for syntheses of 1-silacyclobutene derivatives. Various groups R3 =n Bu,Ph,SiMe3 were con-
sidered for this study and their effect on the course of intramolecular reaction was explored. The intramolecular
1,1-vinylboration to afford 1-silacyclobutenes was hindered by R3 = SiMe3 . Alkenyl(alkyn-1-yl)silane deriva-
tives instead of 1-silacyclobutenes were achieved in quantitative yield. In the case of R3= n Bu,Ph the reaction
led to quantitative formation of 1-silacyclobutene derivatives.
Experimental section
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. Dialkyn-1-ylsilanes1,239−41
and alkenyl(chloro)silanes 3-542,43 were prepared following the literature procedure. Trimethylsilylethyne, n-
butyllithium in hexane (1.6 m), and 9-borabicyclo[3.3.1]nonane (9-BBN) were commercial products and were
used without further purification. NMR spectra: Varian Inova 300 MHz and 400 MHz spectrometers (23
1
◦ C), both equipped with multinuclear units, using C6 D6 solutions, if not mentioned (ca. 10%-15% v/v) in 5
mm tubes. Chemical shifts are given with respect to SiMe4 [δ1 H (C6 D5 H) = 7.15, δ13 C (C6 D6) = 128.0,
δ29 Si = 0 for SiMe4 with Ξ(29 Si) = 19.867187 MHz], and δ11 B = 0 for BF3 -OEt2 with Ξ(11 B) = 32.083971
MHz. 29 Si-NMR spectra were recorded using the refocused INEPT pulse sequence with 1 H decoupling,44−47
based on 3J(29 Si-C=C-1 H) = 25-30 Hz or 1J(29 Si-1 H) = 180-200 Hz (after optimization of the respective
refocusing delays).
794

Effect of various substituents on intramolecular 1,1-vinylboration..., E. KHAN, B. WRACKMEYER
Reaction of Li-C≡C-R3 with alkenylchlorosilanes 3-5 to afford alkenyl(alkyn-1-yl)silanes 8-12.
A solution of the alkenylsilane, 3b (1.8g, 5.22 mmol) in hexane (5 mL) was prepared and slowly added to an
equimolar freshly prepared suspension of Li-C≡C-SiMe3 at –78 ◦ C in hexane (10 mL). The reaction mixture
was allowed to warm to room temperature, and was kept stirring for 3 h. Then solid materials, mainly LiCl,
were separated, and the solvent was removed in a vacuum. A colorless oily liquid was left, identified as a mixture
of 6b (borate) (≈ 20%, NMR data) and 9b. Other alkyn-1-ylsilanes (8c, 9c, 10 - 12) were obtained following
the same procedure. All the alkenyl(alkyn-1-yl)silanes were obtained in reasonably pure form except that the
compound 4c afforded a mixture of 7c (borate-like intermediate) and 11c.
6b: 13 C-NMR (75.4 MHz): δ [J(29 Si,13 C)] = 1.0 (SiMe3), 153.7 (=CH), 153.0br (BC=), 79.3 [95.6]
(Me3 Si-C≡), 106.8br (≡C-B), 135.8, 135.2, 130.0, 128.2 (Si-Ph), Bu and 9-BBN carbons were not assigned;
29 Si-NMR (59.6 MHz): δ = –19.9, –38.2; 11 B-NMR (96.2 MHz): δ = –16.3.
7c: 1 H-NMR (400 MHz): δ = 0.03 (s, 9H, SiMe3), 0.27 (s, 3H, SiMe), 1.07-1.98 (m, 14H, BBN), 6.93-
7.63 (m, 10H, SiPh, Ph), 8.04 (s, 1H, =CH, 3J(29 Si,1 H) = 17.6 Hz); 13 C-NMR (100.5 MHz): δ [J(29 Si,13 C)]
= 0.8 [57.3] (SiMe), –1.2 [54.9] (SiMe3), 155.8 (=CH), 151.2br (BC=), 34.7, 34.6, 31.9br , 23.7 (9-BBN), 107.9
[91.5] (Me3 Si-C≡), 106.2br (≡C-B), 138.2 [73.7, i] (SiPh), 140.5 [4.5, i] (Ph) other carbons were not assigned;
29 Si-NMR (59.6 MHz): δ = –17.6, –11.6; 11 B-NMR (96.2 MHz): δ = –16.8.
9b: 1 H-NMR (300 MHz): δ = 0.3 (s, 9H, SiMe3), 0.8, 1.3-1.4, 2.2 (t, m, m, t, 9H, Bu), 1.3-2.2 (m, 14H,
9-BBN), 5.6 (s, 1H, 1J(29 Si,1 H) = 189.1 Hz, Si-H), 6.5 (t, 1H, 3J(1 H,1 H) = 7.5 Hz, =CH), 7.2-7.9 (m, 5H,
Si-Ph).
9c:1 H-NMR (300 MHz): δ = –0.1 (s, 9H, SiMe3), 1.2-2.0 (m, 14H, 9-BBN), 5.3 (s, 1H, 1J(29 Si,1 H) =
211.3 Hz, Si-H), 8.1 (s, 1H, 3J(29 Si,1 H) = 17.7 Hz, =CH), 6.8-7.6 (m, 10H, Si-Ph, Ph).
10b:1 H-NMR (300 MHz): δ = 0.6 (s, 3H, Si-Me), 0.8, 1.2-2.4 (t, m, m, 9-BBN, Bu), 7.0 (t, 1H,
3J(1 H,1 H) = 7.3 Hz, =CH), 7.2-7.7 (m, 10H, Ph, Si-Ph).
10c: 1 H-NMR (300 MHz): δ = 0.12 (s, 3H, Si-Me), 1.0-1.5 (m, 14H, 9-BBN), 6.5-7.4 (m, 15H, Ph,
Si-Ph), 7.8 (s, 1H, 3J(29 Si,1 H) = 17.9 Hz, =CH).
11b:1 H-NMR (300 MHz): δ = –0.1 (s, 9H, SiMe3), 0.5 (s, 3H, SiMe), 0.6, 1.0, 1.2, 2.3 (t, m, m, m, 9H,
Bu), 1.2-1.8 (m, 14H, 9-BBN), 6.9 (t, 1H, 3J(1 H, 1 H) = 7.3 Hz, =CH), 7.0, 7.5 (m, m, 5H, SiPh).
11c: 1 H-NMR (300 MHz): δ = 0.03 (s, 9H, SiMe3), 0.3 (s, 3H, SiMe), 1.07-1.98 (m, 14H, 9-BBN),
6.9-7.6 (m, 10H, SiPh, Ph), 8.00 (s, 1H, 3J(29 Si, 1 H) = 15.8 Hz, =CH).
12b: 1 H-NMR (300 MHz) = 0.2 (s, 9H, SiMe3), 0.8, 0.9-1.3, 2.3 (t, m, m, 9H, Bu), 1.3-1.8 (m, 14H,
9–BBN), 7.2 (t, 1H, 3J(1 H,1 H) = 7.6 Hz, =CH), 7.3-7.6 (m, 10H, SiPh2).
12c: 1 H-NMR (C6 D6) = 0.03 (s, 9H, SiMe3), 1.3-2.6 (m, 14H, 9-BBN), 6.9-7.8 (m, 15H, SiPh2 , Ph),
8.3 (s, 1H, 3J (29 Si, 1 H) = 22.3 Hz, =CH).
Conversion of alkenyl(alkyn-1-yl)silanes 8-11 into 1-silacyclobutene derivatives
Compound 8c was sealed as C6 D6 solution in an NMR tube and was kept at 80-120 ◦ C. The reaction was
continuously monitored by NMR spectroscopy (mainly 29 Si- and 1 H-NMR). The intramolecular rearrangement
795

Effect of various substituents on intramolecular 1,1-vinylboration..., E. KHAN, B. WRACKMEYER
was complete in 21 h and 1-silacyclobutene 15c was achieved in almost quantitative amount (ca. 90%). All
other 1-silacyclobutene derivatives were obtained in the same way, except that the time taken by each reaction
was slightly different (16b: 21 h, 16c 48 h, 17c: 12 h at 25 ◦ C).
15c: 1 H-NMR (300 MHz): δ = 1.1-1.9 (m, 14H, 9-BBN), 4.8 (s, 1H, 1J(29 Si,1 H) = 196.9 Hz, Si-H),
6.0 (s, 1H, 3J (29 Si, 1 H) = 18.6 Hz, =CH), 6.7-7.6 (m, 15H, Si-Ph, Ph, Ph).
16b:1 H-NMR data (300 MHz): δ = 0.4 (s, 3H, SiMe), 0.9, 0.8-1.0, 1.7 (t, m, m, 9H, Bu), 1.0-1.7 (m,
14H, BBN), 5.8 (t, 1H, =CH, 3J(1 H, 1 H) = 7.2 Hz), 6.6-7.4 (m, 10H, Si-Ph, Ph).
16c: 1 H-NMR data (300 MHz): δ = 0.2 (s, 3H, Si-Me), 0.9-1.7 (m, 14H, 9-BBN), 6.7-7.4 (m, 15H,
Si-Ph, 2 × Ph), 7.9 (s, 1H, 3J(29 Si, 1 H) = 15.7 Hz, =CH).
17c:1 H-NMR (400 MHz): δ = 0.05 (s, 9H, SiMe3), 0.44 (s, 3H, Si-Me), 1.07-1.98 (m, 14H, BBN), other
signals were not assigned.
Reaction of dialkyn-1-ylsilanes, 1a, c, and 2b with 9-BBN to afford alkenyl(alkyn-1-yl)silanes,
13a,c, 14c, and their conversion into 1-silacyclobutenes 18c and 19c
A solution of silane 1a (0.50 g, 3.67 mmol) in C6 D6 (1.5 mL) was mixed with one equivalent of 9-BBN dimer
(0.448 g, 3.67 mmol). The mixture was heated to 80 ◦ C for 5 min to give 13a. During this time 9-BBN
was completely consumed (monitored by 11 B-NMR). The 1,2-hydroboration of 1c and 2b was carried out in
the same way leading to alkenyl(alkyn-1-yl)silanes 13c (after 5 min at 80 ◦ C) and 14c (after 10 min at 80
◦ C). The samples were further heated at the same temperature. In the case of 13a, heating caused extensive
decomposition and identification of products was not possible. Heating of the silanes 13c and 14c led to
1-silacyclobutene derivatives, 18c (1-2 h at 80 ◦ C) and 19c (8 h at 120 ◦ C), respectively.
13a: 1 H-NMR (400 MHz): δ = 0.3 (s, 6H, SiMe2), 1.5, 1.6 (s, s, 3H, 3H, 2Me), 1.4-1.9 (m, 14H, 9-BBN),
6.9 (q, 1H, =CH).
13c: 1 H-NMR (400 MHz): δ = 1.0-1.7 (m, 14H, 9-BBN), –0.05 (s, 6H, SiMe2), 6.5-7.0 (m, 10H, Ph,
Ph), 7.6 (s, 1H, =CH, 3J(29 Si, 1 H) = 17.5 Hz).
14b: 1 H-NMR (400 MHz): δ = 2.2, 1.2-1.1, 1.0, 0.59 (m, m, m, t, 9H, =C-Bu), 1.9, 1.2-1.1, 0.58 (m,
m, t, 9H, ≡C-Bu), 1.5-1.8 (m, 14H, 9-BBN), 7.1 (t, 1H, =CH, 3J(1 H, 1 H) = 7.2 Hz), 7.0-7.1, 7.74, 7.72 (m,
d, d, 10H, SiPh2).
1-Silacyclobutene derivatives
18c: 1 H-NMR (400 MHz): δ = 0.7 (s, 6H, SiMe2), 1.3-2.0 (m, 14H, 9-BBN), 6.8-7.3 (m, 1H, 10H, =CH, Ph,
Ph).
19b: 1 H-NMR (400 MHz): δ = 0.5, 1.3-1.0, 2.0, 2.4 (t, m, m, m, 18H, Bu, Bu), 1.7-1.8 (m, 14H, 9-BBN),
5.9 (t, 1H, =CH), 6.9-7.7 (m, 10H, SiPh2).
796

Effect of various substituents on intramolecular 1,1-vinylboration..., E. KHAN, B. WRACKMEYER
Results and discussion
The alkenylsilanes 3-5 bearing Si-Cl function are useful synthons for further transformations.36,48 They were
prepared by the reaction of the respective alkyn-1-yl(chloro)silanes with 9-borabicyclo[3.3.1]nonane, adopting
the literature procedure.42,43 Alkenylsilanes analogous to 3-5 have been studied in solution and solid state by
Table 1. 11 B-, 13 C-, and 29 Si-NMR dataa of alkyn-1-ylsilanes 9-14.
δ13C (BC=) δ13C (=C)
δ13C (Si-C≡)
δ13C (≡C)
δ29Si
δ11
B
9bb
9cc
143.8br
141.6br
141.2br
147.8br
143.9br
147.7br
142.8br
144.8br
162.1
159.4
161.3
155.7
161.5
154.1
162.8
158.0
152.3
153.3
161.8
93.9
111.8
–17.5, –53.2 82.6
109.4 [83.8] [12.4] 118.7 [76.3] [12.7] –18.7, –51.8 82.4
10bd
10ce
11bf
11cg
12bh
12ci
94.0 [87.8]
93.7 [88.8]
107.8 [16.9]
108.4 [16.5]
–32.6
–31.8
81.6
82.4
113.5 [81.1] [12.4] 116.1 [77.5] [12.0] –19.4, –34.1 80.9
113.2 [81.9] [12.3] 116.9 [77.1] [12.4] –19.2, –33.1 82.9
112.3 [84.3] [11.4] 118.0 [76.7] [12.3] –18.6, –37.3 81.2
111.2 [86.1] [12.6] 118.1 [76.8] [12.8] –18.9, –36.6 82.2
13aj 148.3br [62.7]
84.8 [88.4]
95.6 [85.0]
82.8 [95.7]
103.6 [17.3]
106.9 [15.9]
111.2 [17.2]
–30.4
–28.2
–36.8
81.0
82.9
83.5
13ck 150.6br [62.3]
14bl
143.9br
br
a Measured in C6 D6 at 23 ◦ C, coupling constants J (29 Si,13 C) [ 0.4 Hz] are given in square brackets,
denotes a
broad 13 C resonance signal as the result of partially relaxed scalar 11 B-13 C spin-spin coupling.52
b other 13 C-NMR data: δ = 0.5 (SiMe3 ), 137.8, 135.1, 129.2, 129.3 (Si-Ph), Bu carbons could not be assigned.
c other 13 C-NMR data: δ [J (29 Si,13 C)] = –0.3 [55.9, SiMe3 ], 34.5, 34.3, 31.2br , 23.7 (9-BBN), 139.5, 134.2 [74.9],
135.3, 130.0, 128.4, 129.9, 129.2, 128.2 (Si-Ph, Ph).
d other 13 C-NMR data: δ [J (29 Si,13 C)] = 0.8 [58.6, Si-Me], 34.7, 34.6, 31.8br , 23.7 (9-BBN), 35.1, 27.2, 23.0, 14.4
(Bu), 140.5, 138.5 [74.0], 134.8, 132.2, 128.4, 128.1, 129.6, 123.7 (Si-Ph, Ph).
e other 13 C-NMR data: δ [J (29 Si,13 C)] = 0.1 [58.3, Si-Me], 34.4, 34.4, 31.4br , 23.6 (9-BBN), 138.2 [71.9], 134.6, 134.6,
132.3, 132.2, 129.6, 129.8, 123.2 (Si-Ph, Ph).
f other 13 C-NMR data: δ [J (29 Si,13 C)] = –0.2 [56.3, SiMe3 ], 1.0 [56.5, Si-Me], 34.4, 31.3br , 23.7 (9-BBN), 35.6, 31.7,
22.9, 14.3 (Bu), 138.0 [72.8], 134.6, 129.6, 128.3 (i, o, m, p, Si-Ph).
g other 13 C-NMR data: δ [J (29 Si,13 C)] = –0.003 [55.6, SiMe3 ], –0.2 [56.4, Si-Me], 34.5, 34.6, 31.7br , 23.7 (9-BBN),
140.7 [64.9], 141.4 [4.2] other carbons are without assignment.
h other 13 C-NMR data: δ [J (29 Si,13 C)] = –0.2 [56.4, SiMe3 ], 34.3, 31.4br , 23.6 (9-BBN), 36.0, 27.2, 22.8, 14.1 (Bu),
136.2 [72.6], 135.6, 129.8, 129.3 (i, o, m, p, SiPh2 ).
i other 13 C-NMR data: δ [J (29 Si,13 C)] = –0.2 [56.4, SiMe3 ], 34.5, 31.9br , 23.6 (9-BBN), 136.3 [75.6], 135.6, 130.4,
128.2 (i, o, m, p, SiPh2 ), 125.7, 135.4, 129.7, 139.5, (i, o, m, p, Ph).
j other 13 C-NMR data: δ [J (29 Si,13 C)] = 1.8 [55.6, SiMe2 ], 4.8 [8.6, C2–CH3), 4.7 (CH3). kother 13 C-NMR data:
[J (29 Si,13 C)] = 1.9 [56.5, SiMe2 ], 141.2, 132.1, 129.7, 129.2, 129.0, 128.6, 128.5, 123.9 (Ph carbons without assignment).
l other 13 C-NMR data: δ [J (29 Si,13 C)] = 35.9, 33.8, 31.5, 22.9, 22.2, 20.1, 14.1, 13.7 (Bu), 137.1 [74.5, i], 135.6 (o),
129.6 (p), 128.2 (m) (SiPh2 ).
797

Effect of various substituents on intramolecular 1,1-vinylboration..., E. KHAN, B. WRACKMEYER
NMR spectroscopy and X-ray diffraction, respectively.42,43 These silanes bear 2 electrophilic centres, one at
silicon and the other at boron, and treatment with alkynyllithium reagents at low temperature (–78 ◦ C) should
afford either the borate-like intermediates, 6, 7 and/or the alkenyl(alkyn-1-yl)silanes, 8-12 (Figure 1). Borate-
like intermediates were detected in 2 cases, 6 and 7, and their relevant NMR data were collected (Experimental
section). It turned out that the borate-like intermediates are slowly converted into alkenyl(alkyn-1-yl)silanes by
elimination of LiCl. The progress of the reaction becomes evident by 11 B-NMR spectroscopy. The 11 B-NMR
signal at –16
+82 1 ppm is increasing. The intermediates, 8-12, were stable at room temperature and the relevant NMR
data were collected (Table 1).
1 ppm (typical region for tetraorganoborates49) decreases in intensity, whereas the signal at
Figure 1
The alkenyl(alkyn-1-yl)silanes, 8-12, obtained in Scheme 2 could be converted into useful products.
They were heated at 80-100 ◦ C for some time (0.5-48 h) and 1-silacyclobutene derivatives were achieved in
reasonably pure form (> 90%) via intramolecular 1,1-vinylboration. The effect of C≡C-R3 group (R3 =n Bu,
Ph, SiMe3) was studied on the course of intramolecular 1,1-vinylboration. In the case of C≡C-n Bu and C≡C-
Ph functionalities the reactions led to quantitative formation of 1-silacyclobutene derivatives (Figure 2). On
the other hand, the C≡C-SiMe3 group did not allow the reaction to afford reasonable amounts (Figure 1, ca.
5%) of the desired 1-silacyclobutenes. It is known that alkenes bearing 2 silyl groups and 1 boryl group undergo
1,1-deorganoboration upon heating.50 This would account for the observation of only a small amount of 17c
(Scheme 3). Further attempts to drive the equilibrium towards 17c finally lead to decomposition.
798

Effect of various substituents on intramolecular 1,1-vinylboration..., E. KHAN, B. WRACKMEYER
R
R
H
B
H
B
R1
R1
+
Li C
C
R3
Si
Si
Li
Cl
Ph
Ph
Cl
3
H
4
5
R3
R1
Me Ph
6b
H
7c
Me
- LiCl
Li C
Ph
C
R3
R1
- LiCl
R3 SiMe3 SiMe3
R
H
R = Bu (b), Ph (c)
R1
Si
B
R3
8c 9b,c 10b,c 11b,c 12b,c
R1
H
H
Me
Me
Ph
R3 Ph SiMe3 Ph
Scheme 2. Reactions of alkyn-1-yllithium reagents with alkenyl(chloro)silanes.
SiMe3 SiMe3
H
R
H
R = Bu (b), Ph (c)
R
R1
R1
1,1-vinylboration
B
Si
Si
B
Ph
Ph
R3
R3
15c 16b,c 17c
8c, 9,10
R1
H
Me
Me
SiMe3
R3 Ph Ph
Scheme 3. Formation of 1-silacyclobutene derivatives.
The desired 1-silacyclobutene derivatives could also be obtained by the reaction of dialkyn-1-ylsilanes with
one equivalent of 9-BBN (Scheme 4). Hydroboration of one alkyn-1-yl group affords selectively intermediates
13 and 14 (Figure 2, upper spectrum). On heating, these intermediates rearrange in the same way as observed
for 8-12, and some 1-silacyclobutene derivatives were formed in almost quantitative yield (> 90%; see Figure
2). Surprisingly the C≡C-Me group did not favour the formation of 1-silacyclobutene and decomposition was
observed at 80 ◦ C immediately after formation of alkenyl(propyn-1-yl)silane 13a.
R = Me (a), Bu (b), Ph (c)
H
B
H
R
R
9-BBN
1,1-vinylboration
R12Si
R12Si
R
2
R12Si
B
R1 = Me (1), Ph (2)
R
R
R1 = Me (18c), Ph (19b)
R1 = Me (13a,c), Ph (14b)
Scheme 4. Reaction of dialkyn-1-ylsilanes with one equivalent of 9-BBN. 1,2-Hydroboration is followed by 1,1-
vinylboration.
799

Effect of various substituents on intramolecular 1,1-vinylboration..., E. KHAN, B. WRACKMEYER
Table 2. 11 B-, 13 C-, and 29 Si-NMR dataa of 1-silacyclobutene derivatives 15-19.
δ13C (HC=)
140.0
δ13C (C-2)
147.6 [54.3]
δ13C (C-3)
180.4br
177.9br
178.7br
194.9br
175.9br
178.6br
δ13C (C-4)
162.5 [55.4]
159.8 [54.9]
163.3 [55.2]
δ29Si
–10.5
3.2
δ11
B
15cb
16ac
16cd
17ce
18cf
19bg
85.0
87.2
86.0
140.1
147.0 [55.1]
140.8
148.8 [53.0]
7.9
139.8
151.7 [48.7] [15.1]
150.2 [52.1]
166.8 [38.2] [63.3] –11.6, –12.9 88.2
132.4
164.6 [53.6]
166.9 [53.4]
11.5
–0.3
87.4
86.7
135.6
146.0 [54.9]
a Measured in C6 D6 at 23 ◦ C, coupling constants J(29 Si,13 C) are given in square brackets [ 0.4 Hz], n.m. means
br
not measured, superscript
coupling.52
denotes a broad 13 C resonance signal as the result of partially relaxed scalar 11 B-13
C
b other 13 C-NMR data: δ [J(29 Si,13 C)] = 34.4, 32.4br , 23.5 (9-BBN), 134.2 [64.8], 135.9, 128.7, 130.6 (i, o, m, p, Si-Ph),
135.5, 132.6, 130.7, 128.8, 128.5, 128.4, 127.6, 127.2 (Ph).
c other 13 C-NMR data: δ [J(29 Si,13 C)] = –3.1 [64.8, Si-Me], 34.2, 31.1br , 23.6 (9-BBN), 35.0, 32.6, 22.7, 14.2 (Bu),
136.5 [62.8], 134.6, 128.5, 130.5 (i, o, m, p, Si-Ph), 132.5, 128.6, 128.3, 127.1 (i, o, m, p, Ph).
d other 13 C-NMR data: δ [J(29 Si,13 C)] = –1.3 [52.3, Si-Me], 34.7, 32.4br , 23.6 (9-BBN), 140.7 [65.1], 134.6, 128.3, 129.1
(i, o, m, p, Si-Ph), 132.5, 132.3, 128.8, 128.5, 128.2, 128.1, 127.1, 126.8 (Ph).
e other 13 C-NMR data: δ [J(29 Si,13 C)] = –2.1 (SiMe3), 0.3 (Si-Me), 33.8, 33.7, 32.4br , 23.5 (9-BBN), Ph and Si-Ph
carbons are without assignment.
f other 13 C-NMR data: δ [J(29 Si,13 C)] = –0.01 [46.3, SiMe2], 34.3, 32.1br , 23.6 (9-BBN), 140.5 (i), 140.1 (i), 128.9,
128.6, 128.2, 127.4, 127.0, 126.6 (Ph).
g other 13 C-NMR data: δ [J(29 Si,13 C)] = 33.4, 31.9br , 23.5 (9-BBN), 35.1, 34.4, 33.1, 32.6, 23.1, 22.7, 14.2, 14.0 (Bu),
135.5 [62.7], 135.7, 130.2, 128.4, (i, o, m, p, SiPh2 ).
NMR spectroscopic studies
The 11 B-, 13 C-, and 29 Si-NMR data for alkenyl(alkyn-1-yl)silanes (9-14) and 1-silacyclobutene derivatives
(15-19) are listed in Tables 1 and 2, respectively. The data for borate-like intermediates (6 and 7) and 1 H-
NMR data for all the new compounds are collected in the Experimental section. The data sets compare well
with the previously reported data36−38 and are in full agreement with the proposed structures. All compounds,
i.e. alkenyl(alkyn-1-yl)silanes, borate-like intermediates, and 1-silacyclobutene derivatives, could be identified
from their characteristic NMR parameters (see Figures 1 and 2). The 11 B chemical shifts for alkenyl(alkyn-
1-yl)silanes and 1-silacyclobutenes cover a narrow range (δ 11 B = 82-89 ppm), typical of triorganoboranes
without significant BC(pp) π interactions.51 For all products the 13 C-NMR data are useful to corroborate the
proposed structures. Many 13 C-NMR signals could be readily assigned by their 29 Si satellites [1J(29 Si,13 C) and
2J(29 Si,13 C)] or by the typical increase in the line widths owing to partially relaxed one-bond 13 C–11 B spin-
spin coupling.52 The 29 Si-NMR spectra are helpful in monitoring of the reactions, and δ 29 Si data are markedly
different for starting silanes (1, 2), alkenylsilanes (3-5), borates (6, 7), alkenyl(alkyn-1-yl)silanes (8-14), and
800

Effect of various substituents on intramolecular 1,1-vinylboration..., E. KHAN, B. WRACKMEYER
1-silacyclobutene derivatives (15-19). In the 1 H-NMR spectra a singlet for the olefinic proton of the C=CH(R)
group is accompanied by 29 Si satellites [3J(29 Si,1 H) ≈ 25 Hz], which shows that 1,2-hydroboration has taken
place. The value of 3J(29 Si,1 H) coupling constants is helpful in identification of products (1-silacyclobutenes)
and their precursors, the alkenyl(alkyn-1-yl)silanes.
Figure 2
Conclusions
We have shown that various 1-silacyclobutene derivatives are accessible via 1,2-hydroboration followed by 1,1-
vinylboration. The synthetic approach is efficient and a variety of functionalities such as Si-organyl as well as
Si-H can be included. The synthetic route is limited to R3 = SiMe3 which afforded alkenyl(alkyn-1-yl)silanes
instead of 1-silacyclobutenes. The new compounds alkenyl(alkyn-1-yl)silanes bear numerous reactive sites such
as C=C bond, C≡C bond, the boryl group, and the Si-H function, which are expected to possess further utility
and can be used for development of suitable chemistry.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft. E. K. is grateful to DAAD (Germany) and
HEC (Pakistan) for a scholarship (code A/04/30788).
801

Effect of various substituents on intramolecular 1,1-vinylboration..., E. KHAN, B. WRACKMEYER
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