Synthesis of 1-silacyclopent-2-ene derivatives using 1,2-hydroboration, 1,1-organoboration and protodeborylation

Article abstract of DOI:10.1002/aoc.3121

The reaction of di(alkyn-1-yl)vinylsilanes R¹(H ₂Ca?-CH)Si(C≡C - R)₂ (R¹ = Me (1), Ph (2); R = Bu (a), Ph (b), Me₂HSi (c)) at 25°C with 1 equiv. of 9-borabicyclo[3.3.1]nonane (9-BBN) affords 1-silacyclopent-2-ene derivatives (3a, 3b, 3c, 4a, 4b), bearing one Si - C≡C - R function readily available for further transformations. These compounds are formed by consecutive 1,2-hydroboration followed by intramolecular 1,1-carboboration. Treated with a further equivalent of 9-BBN in benzene they are converted at relatively high temperature (80-100°C) into 1-alkenyl-1-silacyclopent-2-ene derivatives (5a, 5b 6a, 6b) as a result of 1,2-hydroboration of the Si - C≡C - R function. Protodeborylation of the 9-BBN-substituted 1-silacyclopent-2-ene derivatives 3, 4, 5, 6, using acetic acid in excess, proceeds smoothly to give the novel 1-silacyclopent-2-ene (7, 8, 9, 10). The solution-state structural assignment of all new compounds, i.e. di(alkyn-1-yl)vinylsilanes and 1-silacyclopent-2-ene derivatives, was carried out using multinuclear magnetic resonance techniques (¹H, ¹³C, ¹¹B, ²⁹Si NMR). The gas phase structures of some examples were calculated and optimized by density functional theory methods (B3LYP/6-311+G/(d,p) level of theory), and ²⁹Si NMR parameters were calculated (chemical shifts δ²⁹Si and coupling constants ⁿJ(²⁹Si,¹³C)). Copyright

Full text of DOI:10.1002/aoc.3121

Full Paper
Received: 19 November 2013
Revised: 20 December 2013
Accepted: 22 December 2013
Published online in Wiley Online Library: 27 February 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3121
Synthesis of 1-silacyclopent-2-ene derivatives
using 1,2-hydroboration, 1,1-organoboration
and protodeborylation
Ezzat Khana*, Oleg L. Tokb and Bernd Wrackmeyerb
The reaction of di(alkyn-1-yl)vinylsilanes R1(H2C CH)Si(C≡C―R)2 (R1 = Me (1), Ph (2); R = Bu (a), Ph (b), Me2HSi (c)) at 25°C
with 1 equiv. of 9-borabicyclo[3.3.1]nonane (9-BBN) affords 1-silacyclopent-2-ene derivatives (3a–c, 4a,b), bearing
one Si―C≡C―R function readily available for further transformations. These compounds are formed by consecutive 1,2-
hydroboration followed by intramolecular 1,1-carboboration. Treated with a further equivalent of 9-BBN in benzene
they are converted at relatively high temperature (80–100°C) into 1-alkenyl-1-silacyclopent-2-ene derivatives (5a,b, 6a,b)
as a result of 1,2-hydroboration of the Si―C≡C―R function. Protodeborylation of the 9-BBN-substituted 1-silacyclopent-
2-ene derivatives 3–6, using acetic acid in excess, proceeds smoothly to give the novel 1-silacyclopent-2-ene (7–10).
The solution-state structural assignment of all new compounds, i.e. di(alkyn-1-yl)vinylsilanes and 1-silacyclopent-2-ene
derivatives, was carried out using multinuclear magnetic resonance techniques (1H, 13C, 11B, 29Si NMR). The gas phase
structures of some examples were calculated and optimized by density functional theory methods (B3LYP/6-311+G/(d,p)
level of theory), and 29Si NMR parameters were calculated (chemical shifts δ29Si and coupling constants nJ(29Si,13C)).
Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: vinylsilanes; hydroboration; carboboration; silacyclopentenes; protodeborylation
Introduction
Experimental
Alkynylsilanes and alkyn-1-yl(vinyl)silanes are readily available[1]
and have widespread applications in chemistry.[2] These silanes
are attractive starting materials for application of hydroboration,[3]
hydroalumination,[4] hydrogalation[5] and carboboration reac-
tions,[6] leading to numerous novel cyclic and non-cyclic silicon
compounds.[7,8] Among cyclic compounds, 1-silacyclopent-2-ene
derivatives are of particular importance as the selective
synthesis of these compounds in high yield has been a challeng-
ing goal. Very few derivatives of 1-silacyclopentene have been
All preparative work and handling of air- and moisture-sensitive
materials was carried out in an inert atmosphere (dry argon).
Dichloro(methyl)vinylsilane, dichloro(phenyl)vinylsilane, 1-hexyne,
ethynylbenzene, 3,3-dimethylbutyne, nBuLi in hexane (1.6 M)
and 9-borabicyclo[3.3.1]nonane (9-BBN) were used as commer-
cial products (Aldrich) without further purification. NMR spectra
were recorded at 23°C using Varian Inova 300 MHz and
400 MHz spectrometers, both equipped with multinuclear units,
using C6D6 solutions (~5–10% v/v) in 5 mm o.d. tubes. Chemical
shifts are given with respect to Me4Si [δ1H (C6D5H) = 7.15, δ13C
(C6D6) = 128.0), δ29Si = 0 for Me4Si with (29Si) = 19.867187 MHz],
and δ11B = 0 for BF3-OEt2 with (11B) = 32.083971 MHz. 29Si NMR
spectra were recorded using the refocused INEPT pulse
reported following
a
number of synthetic routes.[9–12]
Attempted synthesis either afforded a mixture of isomers or
led to compounds other than silacyclopentene derivatives, such
as siloles, silacyclobutenes, silacyclohexenes and open chain
silanes.[13–16] 1,2-Hydroboration of alkynyl(vinyl)silanes followed
by intramolecular rearrangement, typical of a 1,1-carboboration
reaction,[6d,e] provides an easy access to 1-silacyclopent-2-ene
derivatives. Using this synthetic approach, it proved possible to
obtain organo-substituted (A) as well as chloro-substituted
derivatives (B and C) with a variety of substituents in the 2-
position (Scheme 1).[3a,17]
1
sequence with H decoupling,[18] optimizing the delays based
on 2,3J(29Si,1H)vinyl = 12–15 Hz and 3J(29Si―C C1H) = 20–25 Hz.
All quantum chemical calculations were carried out using
the Gaussian 09 program package.[19] Optimized geometries at
the B3LYP/6-311+G(d,p) level of theory[20] were found to be
minima by the absence of imaginary frequencies. NMR parameters
were calculated at the same level of theory. Calculated nuclear
In continuation of our previous work,[8e,17a] here we report 1-
silacyclopent-2-ene derivatives bearing Si-alkynyl or Si-alkenyl
functions. The protodeborylation of all novel heterocycles
was successfully carried out without affecting other function-
alities within the molecules. Considering the presence of 1H,
11B, 13C and 29Si nuclei, application of multinuclear magnetic
resonance techniques was indicated, collecting numerous
complementary NMR data, to confirm the solution-state
structural assignments.
*
Correspondence to: Ezzat Khan, Department of Chemistry, University of
Malakand, Chakdara 18550, Dir (Lower), Khyber Pakhtunkhwa, Pakistan.
E-mail: ekhan@uom.edu.pk
a Department of Chemistry, University of Malakand, Chakdara 18550, Dir (Lower),
Khyber Pakhtunkhwa, Pakistan
b Anorganische Chemie II, Universität Bayreuth, D-95440, Bayreuth, Germany
Appl. Organometal. Chem. 2014, 28, 280–285
Copyright © 2014 John Wiley & Sons, Ltd.

1-Silacyclopent-2-enes and facile protodeborylation
1
3
4.2 (sep., 2H, J(29Si,1H)= 203.5 Hz, J(1H,1H)= 3.9 Hz, 2 × SiH), 5.9
(m, 1H, CH), 6.1 (m, 2H, CH2); 13C NMR: δ [J(29Si,13C)] = 114.3 [77.7,
15.1, R―C≡],110.2 [92.8, 12.6, Si―C≡],136.1 (C ), 133.6 [79.9,
Si―C ]; À2.9 [56.1, Si―Me2], À0.4 [63.4, Si―Me], 29Si NMR: δ =
À49.3, -38.3 (2J(29Si,29Si) = 1.5 Hz, Me2HSi).;
2a:, 1H NMR (400 MHz): δ = 0.6, 1.2, 1.9 (t, m, t, 18H, J(1H,1H)= 7.2,
6.8, 2 × Bu), 6.0 (dd, 1H, J(1H,1H)= 4.6, 13.2Hz, CH2), 6.1 (dd, 1H,
J(1H,1H) = 4.6, 20.1 Hz, CH2), 6.2 (dd, 1H, J(1H,1H) = 13.2, 20.1Hz,
HC ), 7.1, 7.9 (m, m, 5H, Si―Ph); 13C NMR: δ [J(29Si,13C)] = 111.3
[20.2, R―C≡], 79.1 [107.2, Si―C≡], 135.5 (C ), 134.3 [80.4, Si―C ],
30.6, 22.1, 19.9, 13.7 (Bu), 137.1, 134.9, 128.3, 130.2 (i, o, m, p,
Si―Ph); 29Si NMR: δ = À51.8.;
2b:, 1H NMR (400 MHz): δ = 6.2 (m, 2H, J(1H,1H)= 8.8Hz, CH2), 6.0
(m, 1H, J(1H,1H) = 8.8Hz, HC ), 6.8–6.9, 7.3 (m, m, 15H, Si―Ph, 2 ×
Ph); 13C NMR: δ [J(29Si,13C)] = 109.1 [20.1, R―C≡], 88.0 [105.3,
Si―C≡], 136.9 (C ), 135.1 [81.0, Si―C ], 132.9 [84.1, i], 122.6
(i, ≡C―Ph), 132.7, 132.4, 130.7, 129.4, 128.6, 128.6 (aromatic carbons
without assignment); 29Si NMR: δ = À50.0.
Hydroboration of 1a–c and 2a,b Using 1 and 2 Equiv. of 9-BBN
as Hydroborating Reagent. Syntheses of 1-Silacyclopent-2-ene
Derivatives 3–6
Scheme 1. Heterocycles obtained via consecutive 1,2-hydroboration
and 1,1-carboboration of alkynyl(vinyl)silanes (A and B). Compound C
was obtained as a result of subsequent protodeborylation of B type
compounds.
Pure silane 2a (0.57 g; 2.47 mmol) was dissolved in THF (10 ml)
and 9-BBN (0.32 g; 2.47 mmol) was added as a solid in one
portion. The reaction mixture was stirred at 25°C; after 30 min
all volatile materials were removed under reduced pressure,
and the colorless oily liquid left was identified as 3a. The same
experimental procedure was followed for the preparation of 3b,c
and 4a,b. The products obtained were reasonably pure (>95 %,)
and were studied by NMR spectroscopy. The oily compound 3a
was dissolved in C6D6 and one further equivalent of 9-BBN
(2.47 mmol) was added. The reaction mixture was heated
(80–100°C) for 40 min. The progress of the reaction was constantly
monitored by 11B NMR, and heating was stopped when all 9-BBN
was consumed. The reaction solution showed quantitative
formation of the 1-silacyclopent-2-ene derivative 5a (NMR data).
Other 1-silacyclopent-2-ene derivatives 5b and 6a,b were
prepared in the same way as described for 5a. When the
corresponding silane was mixed with 2 equiv. of 9-BBN the
same products were obtained. All derivatives obtained were
colorless, oxygen- and moisture-sensitive oily liquids.
magnetic shielding constants σ(29Si) were converted to chemical
shifts δ29Si by δ29Si (calcd) = σ(29Si, TMS) À σ(29Si), with σ(29Si,
TMS)= +340.1 and [δ29Si (TMS)= 0].
Synthesis of Alkyn-1-ylvinylsilanes 1 and 2 Used as Starting
Materials
A suspension of Li―C≡C―Bu (19.5 mmol) was freshly prepared
in hexane (~80 ml) and the solution was cooled to À78°C.[21]
Dichloro(methyl)vinylsilane was slowly added with constant
stirring (the molar ratio was kept at 2:1 with respect to
lithiumalkynyl and corresponding silane). The reaction mixture
was slowly warmed to room temperature and stirred for 3–4 h.
After separating insoluble materials, all readily volatile materials
were removed under reduced pressure (~10À2 torr). The colorless
oily residue left was identified as Me(vinyl)Si(C≡C-Bu)2, 1a. The
silane was obtained in reasonably pure form (>95% from NMR
data), and Me(vinyl)(Cl)Si(C≡C―Bu) was obtained as a side
product in the reaction mixture. A pure sample (>99%) of 1a
was obtained by fractional distillation.[17a] An identical work-up
procedure was followed for the syntheses of 1b,c and 2a,b, and
the same purity in case of all derivatives was achieved.
3a:, 1H NMR (400MHz): δ = 0.4 (s, 3H, 2J(29Si,1H)= 7.0Hz, Si―Me),
0.8, 0.9, 1.3, 2.1 (t, t, m, t, 18H, J(1H,1H)= 7.2, 7.1, 6.9, 2 × Bu), 1.2–2.0
(m, 14H, 9-BBN), 0.8 (ddd, 1H, J(1H,1H)= 14.9, 4.6, 9.0 Hz, C5H2), 1.2
(ddd, 1H, J(1H,1H)= 14.9, 4.6, 9.0 Hz, C5H2), 2.5, 2.7 (m, m, 2H,
C4H2); 13C NMR: δ [J(29Si,13C)] = 148.0 [67.5, C-2], 167.7br (C-3), 34.2
(C-4), 11.1 [58.6, C-5], À0.4 [54.3, Si―Me], 32.8 [6.4], 33.6, 31.0,
22.2, 20.0, 14.4, 13.7 (2 × Bu), 33.8, 32.3br, 23.6 (BBN), 109.5 [15.9,
≡C], 83.4 [84.7, Si―C≡]; 11B NMR: δ = 85.6; 29Si NMR: δ = À6.0.;
3b:, 1H NMR (400 MHz): δ =0.3 (s, 3H, 2J(29Si,1H) = 7.1 Hz, Si―Me),
1.2–1.6 (m, 14H, 9-BBN), 0.9 (ddd, 1H, J(1H,1H) = 15.0, 5.4, 8.9, C5H2),
1.3 (m, 1H, C5H2), 2.7 (m, 2H, C4H2), 6.7–7.2 (m, 10H, 2 × Ph); 13C
NMR: δ [J(29Si,13C)] = 147.7 [67.9, C-2], 172.9br (C-3), 34.8 [8.1, C-4],
11.5 [58.9, C-5], À1.0 [55.8, Si―Me], 34.6, 34.3, 32.5br, 23.6 (BBN),
108.1 [16.0, ≡C], 92.8 [83.1, Si―C≡], 142.9 [6.0, i, C2―Ph), 123.6
(i, ≡C―Ph), 132.3, 128.8, 128.5, 128.5, 128.4, 126.6 (aromatic carbons
without assignment); 11B NMR: δ = 86.1; 29Si NMR: δ = À4.1.;
1a:, 1H NMR (400MHz): δ = 0.3 (s, 3H, 2J(29Si,1H)= 7.5 Hz, Si―Me),
2.0, 1.2, 0.7 (t, m, t, 18H, J(1H,1H) = 7.3, 6.9, 2 × Bu), 6.0 (dd, 1H, J
(1H,1H)= 4.5, 13.4Hz, CH2), 6.1 (dd, 1H, J(1H,1H)= 4.5, 20.0 Hz,
CH2), 6.2 (dd, 1H, J(1H,1H)= 13.4, 20.0Hz, Si―CH); 13C NMR: δ
[J(29Si,13C)] = 109.4 [20.0, R―C≡], 80.4 [103.4, Si―C≡], 134.0 (C ),
135.4 [79.4, Si―C ], 0.03 [63.3, Si―Me], 30.7, 22.1, 19.8, 13.7 (Bu);
29Si NMR data: δ = À48.4.;
1b:, 1H NMR (400 MHz): δ = 0.5 (s, 3H, 2J(29Si,1H)= 7.6 Hz, Si―Me),
6.0 (dd, J(1H,1H) = 4.6, 13.2Hz, CH2), 6.2 (dd, 1H, J(1H,1H) = 4.6,
20.1 Hz, CH2), 6.3 (dd, 1H, J(1H,1H)= 13.2, 20.1Hz, Si―CH), 6.9–7.4
(m, 10H, 2 × Ph); 13C NMR: δ [J(29Si,13C)] = 107.7 [19.6, R―C≡], 89.4
[101.2, Si―C≡], 135.4 (C ), 134.0 [80.4, Si―C ], 0.5 [63.9, Si―Me],
123.0 [2.1], 132.4, 128.5, 129.2 (i, o, m, p, Ph); 29Si NMR: δ = À46.2.;
3c:, 1H NMR (400 MHz): δ = 0.08 (d, 3H, J(1H,1H) = 3.9 Hz, 2J(29Si,1H) =
7.5 Hz, Me2Si), 0.1 (d, 3H, J(1H,1H) = 3.9 Hz, 2J(29Si,1H) = 7.5 Hz, Me2Si),
0.3 (d, 3H, J(1H,1H) = 3.6 Hz, 2J(29Si,1H) = 7.0 Hz, Me2Si); 0.4 (s, 3H,
2J(29Si,1H)=7.0Hz, MeSi), 0.5 (d, 3H, J(1H,1H) = 3.6 Hz, 2J(29Si,1H) = 7.0
1
2
2
1c:, H NMR (400 MHz): δ = 0.01 (d, 12H, J(29Si,1H)= 7.6 Hz, J
(1H,1H)= 3.9 Hz, 2 × Me2Si), 0.30 (s, 3H, 2J(29Si,1H)= 7.6 Hz, Si―Me),
Appl. Organometal. Chem. 2014, 28, 280–285
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

E. Khan et al.
Hz, Me2Si), 0.7 (ddd, 1H, J(1H,1H)=5.0, 8.9, 14.9Hz, Si―C5H2), 1.1
(ddd, 1H, J(1H,1H)=5.0, 9.4, 14.9Hz, Si―C5H2), 1.4–2.0 (m, 14H,
Protodeborylation of 1-Silacyclopent-2-ene Derivatives 7–10
Compound 3a (0.38 g, 1.07 mmol) was dissolved in pentane and
acetic acid (approximately twofold excess) was slowly added at
25°C. The reaction afforded the desired protodeborylated silane
7a and the oxygen–boron bicyclic compound 11. The reaction
mixture was heated at 100–120°C (oil bath temperature) under
reduced pressure for 1–2 h; compound 11 sublimed and
accumulated along the walls of the Schlenk tube as colorless crys-
tals, leaving 7a as an oily liquid in pure form (>95% from NMR
data). All other reactions were performed in exactly the same way.
7a: 1H NMR (300 MHz): δ = 0.3 (s, 3H, 2J(1H,1H)= 7.3Hz, Si―Me),
0.7, 0.9, 1.3–1.5, 2.1, 2.3 (t, t, m, t, m, 18H, J(1H,1H) = 7.2, 6.9, 6.8,
C2Bu, ≡C―Bu), 1.0 (ddd, 1H, J(1H,1H) = 15.1, 4.7, 9.0 Hz, C5H2), 1.3
(m, 1H, C5H), 2.3 (m, 2H, C4H2), 6.3 (m, 1H, 3J(29Si,1H)= 14.6Hz,
C3H); 13C NMR: δ [J(29Si,13C)] = 143.3 [68.1, C-2], 146.7 [13.3, C-3],
32.7 [5.9, C-4], 10.4 [58.1, C-5], -0.7 [55.3, Si―Me], 32.4 [7.2], 30.3,
23.0, 14.3 (Bu), 31.0, 22.2, 20.0, 13.7 (≡C―Bu), 83.0 [86.4, Si―C≡],
109.3 [16.3, ―C]; 29Si NMR: δ = À8.6.
1
BBN); 2.7 (m, 2H, C4H2). 4.3 (sep., 1H, J(29Si,1H) = 201.3 Hz, Si―H),
4.6 (sep., 1H, 1J(29Si,1H) = 178.8 Hz, Si―H....B); 13C NMR: δ [J(29Si,13C)] =
137.5 [62.2, 53.1, C-2], 192.8br (C-3), 39.2 [11.3, 7.7, C-4], 10.7 [57.3,
C-5], À2.6 [55.5, Me2Si], À2.6 [55.3, Me2Si], À1.0 [50.8, Me2Si], À0.9
[51.5, Me2Si], 0.7 [54.8, Me2Si], 34.4, 34.4, 33.2br, 24.1 (BBN), 115.8
[73.8, 12.7, ≡C], 113.8 [79.2, 11.5, Si―C≡]; 11B NMR was not measured;
29Si NMR: δ [J(29Si,29Si)] = 2.9 [10.9,1.6], À39.1 [1.6, Me2HSi―C≡],
À22.3 [10.9, Me2HSi―C ].;
4a:, 1H NMR (400 MHz): δ = 0.7, 1.2, 2.0 (t, m, t, 9H, J(1H,1H) = 7.3,
6.9, ≡-Bu), 0.7, 1.5, 2.7 (t, m, m, J(1H,1H)= 7.0, 9H, C2―Bu), 1.2–1.8
(m, 14H, 9-BBN), 1.0, 1.3 (m, m, 2H, C5H2), 2.4, 2.5 (m, m, 2H, C4H2),
7.2, 7.8 (m, m, 5H, Si―Ph); 13C NMR: δ [J(29Si,13C)] = 146.3 [68.9,
C-2], 170.1br (C-3) 34.0 (C-4), 11.5 [59.7, C-5], 34.0, 32.9, 23.4, 14.3
( C―Bu), 30.9, 22.3, 20.1, 13.7 (≡C―Bu), 33.8, 32.4br, 23.6 (BBN),
81.2 [89.6, Si―C≡], 111.7 [16.7, ≡C], 136.8 [71.9], 134.9, 128.2,
129.8 (i, o, m, p, Si―Ph); 11B NMR: δ = 85.1 ; 29Si NMR: δ = À9.1.;
7b: 1H NMR (400 MHz): δ = 0.6 (s, 3H, Si―Me), 0.8 (ddd, 1H,
J(1H,1H) = 15.3, 4.7, 9.4, C5H2), 1.3 (ddd, 1H, J(1H,1H) = 15.3, 4.7, 9.4,
C5H2), 2.4, 2.5 (m, m, 2H, C4H2), 6.8 (t, 1H, C3H), 6.9, 7.2, 7.3, 7.7 (m, m, m,
m, 10H, C2Ph, ≡C―Ph); 13C NMR:δ [J(29Si,13C)] = 141.6 [69.1, C-2], 148.3
[12.7, C-3], 31.0 (C-4), 10.1 [59.1, C-6], À0.9 [56.4, Si―Me], 92.6 [84.4,
Si―C≡], 108.1 [16.1, ≡C], 123.4, 132.3, 129.9, 128.5 (i, o, m, p, ≡C―Ph),
139.7 [5.6], 128.9, 127.2, 127.0 (i, o, m, p, Ph); 29Si NMR: δ =À6.9.
8a: 1H NMR (400 MHz): δ = 0.7, 0.8, 1.2, 2.1, 2.3 (t, t, m, t, m, 18H,
J(1H,1H)=7.3, 7.0, 6.9, C2Bu, ≡C―Bu), 1.0 (m, 1H, C5H), 1.2 (m, 1H,
C5H2), 2.4 (m, 2H, C4H2), 6.4 (m, 1H, 3J(29Si,1H) = 15.9 Hz, C3H), 7.2, 7.7
(m, m, 5H, Si―Ph); 13C NMR: δ [J(29Si,13C)] = 142.5 [69.4, C-2], 148.6
[13.4, C-3], 32.6 (C-4), 10.8 [58.4, C-5], 32.1 [8.7], 30.9, 30.7, 22.9, 22.2,
20.0, 14.2, 13.7 (2 × Bu), 80.8 [90.Si―C≡], 111.7 [16.2, ≡C], 136.3
[73.1], 134.9, 128.2, 129.8 (i, o, m, p, Si―Ph); 29Si NMR: δ = À11.5.
1
4b., H NMR (400 MHz): δ = 1.2 (m, 2H, CH2), 2.8 (m, 2H, CH2),
1.2–1.9 (m, 14H, BBN), 6.8–7.8 (m, 15H, Si―Ph, C5―Ph, ≡C―Ph);
13C NMR: δ [J(29Si,13C)] = 145.9 [69.6, C-2], 175.3br (C-3), 35.2 (C-4),
12.1 [60.0, C-5], 34.6, 34.5, 32.6br, 23.6 (BBN), 90.6 [88.6, Si―C≡],
109.8 [16.4, ≡C], 142.5 [6.1, i], 135.6 [73.7, i], 123.2 (i), 135.0, 132.4,
130.2, 129.1, 128.6, 128.5, 128.4, 128.4, 126.6 (Si―Ph, 2 × Ph); 11B
NMR: δ = 84.5; 29Si NMR: δ = À8.3.;
5a:, 1H NMR (400 MHz): δ = 0.5 (s, 3H, 2J(29Si,1H)= 6.5 Hz, Si―Me),
0.9, 0.9, 1.4, 2.5 (t, t, m, m, 18H, J(1H,1H) = 7.1, 7.3, 2 × Bu), 1.6–2.0 (m,
28H, 2 × 9-BBN), 0.9 (m, 1H, C5H2), 1.1 (ddd, 1H, J(1H,1H) = 15.0, 5.5,
8.0 Hz, C5H2), 2.3, 2.8 (m, m, 2H, C4H2), 6.9 (t, 1H, J(1H,1H)= 7.1Hz,
C3H); 13C NMR: δ [J(29Si,13C)] = 151.1 [60.6, C-2], 166.5br (C-3), 32.2
(C-4), 12.4 [53.9, C-5], 1.5 [49.3, Si―Me], 33.81, 33.80, 33.79, 33.78,
32.3 (br), 23.6, 23.7 (2 × 9-BBN), 35.2, 34.9, 33.7, 33.4, 24.3, 23.8,
14.4, 14.3 (2 × Bu), 148.9br [53.9, (B)C ], 157.4 ( C); 11B NMR: δ
= 84.4; 29Si NMR: δ = 7.3.;
1
8b: H NMR (400 MHz): δ = 1.1, 1.3 (m, m, 2H, C5H2), 2.5 (m, 2H,
3
C4H2), 7.0 (t, 1H, J(1H,1H) = 3.0 Hz, C3H), 6.9, 7.1, 7.2, 7.4, 7.6, 7.9 (m,
m, m, m, m, 15H, Si―Ph, 2 × Ph); 13C NMR: δ [J(29Si,13C)] = 140.4
[70.5, C-2], 150.0 [12.9, C-3], 31.3 (C-4), 10.8 [59.7, C-5], 90.3 [89.6,
Si―C≡], 109.9 [16.7, ≡C], 139.2 [5.7, i], 135.2 [74.5, i], 123.1 (i), 134.9,
132.4, 130.0, 129.1, 128.8, 128.5, 128.5, 127.4, 127.1 (Si―Ph, 2 × Ph);
29Si NMR: δ = À10.5.
1
2
5b:, H NMR (400MHz): δ = 0.4 (s, 3H, J(29Si,1H) = 6.4Hz, Si-Me),
1.5–2.1 (m, 28H, 2 × 9-BBN), 0.6 (ddd, 1H, J(1H,1H) = 14.2, 5.5,
8.0 Hz, C5H2), 0.8 (ddd, 1H, J(1H,1H)= 14.2, 5.5, 8.0 Hz, C5H2), 3.1 (m,
2H, C4H2), 6.5–6.9 (m, 10H, 2 × Ph), 7.6 (s, 1H, J(29Si,1H) = 16.1 Hz,
3
9a: 1H NMR (400 MHz): δ = 0.3 (s, 3H, Si―Me), 0.8, 0.9, 1.3, 2.1 (t, t, m,
m, 18H, J(1H,1H)=7.1, 7.3, 2 × Bu), 1.0 (m, 2H, C5H2), 2.4 (m, 2H, C4H2),
5.6 (d, 1H, J(1H,1H) = 13.9 Hz, CH), 6.3 (m, 1H, C3H), 6.3 (dt, 1H, J
(1H,1H) = 7.3 Hz, 13.8 Hz, C(Bu)H); 13C NMR: δ [J(29Si,13C)] = 144.7
[62.4, C-2], 145.6 [11.3, C-3], 32.2 (C-4), 10.8 [53.7, C-5], À0.6 [50.3,
Si―Me], 33.8, 32.9, 32.5, 30.6, 23.1, 22.8, 14.2 (2 × Bu), 127.1 [64.8,
Si―C ], 150.8 ( C); 29Si NMR: δ = 2.7.
C3H); 13C NMR: δ [J(29Si,13C)] = 151.6 [61.1, C-2], 171.1br (C-3), 33.8
(C-4), 13.0 [53.0, C-5], 1.0 [51.7, Si―Me], 34.8, 34.5, 34.5, 34.4,
32.4br, 23.7 (2 × 9-BBN), 152.5br ((B)C ), 152.9 ( C), 143.6 (i), 141.3
(i), 128.6, 128.3, 128.2, 128.16, 129.3 (p), 126.2 (p) (2 × Ph); 11B
NMR: δ = 85.3, 29Si NMR: δ = 9.0.;
6a:, 1H NMR (400MHz): δ = 0.7, 0.8, 1.3–1.4, 2.1, 2.5 (t, t, m, m, m,
18H, J(1H,1H) = 7.2, 7.3, 2 × Bu), 1.3–1.9 (m, 28H, 2 × 9-BBN), 1.2
(m, 2H, C5H2), 2.8 (m, 2H, C4H2), 7.1–7.6 (m, 6H, Si―Ph, C3H); 13C
NMR: δ [J(29Si,13C)] = 148.6 [61.8, C-2], 169.1br (C-3), 32.0 (C-4), 12.2
[53.4, C-5], 34.6, 34.4, 34.0, 33.9, 32.3br, 31.5br, 23.7, 23.6 (2 × 9-
BBN), 36.0, 35.0, 34.1, 33.7, 23.6, 23.0, 14.2, 14.2 (2 × Bu), 146.5br
((B)C ), 159.9 ( C), 140.5 [64.1], 134.6, 128.1, 129.0 (i, o, m, p, Si―Ph);
11B NMR: δ = 85.6, 29Si NMR: δ = 3.1.;
9b: 1H NMR (300 MHz): δ = 0.3 (s, 3H, 2J(29Si,1H) = 6.5 Hz, Si―Me),
0.8–0.9 (m, 2H, C5H2), 2.4 (m, 2H, C4H2), 6.8 (t, 1H, J(1H,1H) = 3.1 Hz,
3J(29Si,1H) = 13.7 Hz, C3H), 6.1 (d, 1H, J(1H,1H) = 15.1 Hz, CH), 7.4
(d, 1H, J(1H,1H) = 15.1 Hz, CH), 7.0–7.3, 7.4 (m, m, 10H, 2 × Ph); 13C
NMR: δ [J(29Si,13C)] = 144.3 [63.1, C-2], 147.0 [11.2, C-3], 31.2 (C-4),
10.1 [54.2, C-5], À0.6 [54.0, Si―Me], 130.1 [63.6, Si―C ], 148.5 (C ),
140.4, 140.1, 128.8, 128.4, 128.2, 127.9, 127.0, 126.7 (2 × Ph); 29Si
NMR: δ = 5.1.
6b:, 1H NMR (400 MHz): δ = 1.4–1.9 (m, 28H, 2 × 9-BBN), 1.3 (m, 2H,
C5H2), 2.9 (m, 2H, C4H2), 7.0–7.2, 7.6, 7.6 (m, m, m, 15H, Si―Ph, 2 ×
10a: 1H NMR (300 MHz): δ = 0.7, 0.7, 1.0-1.3, 2.0, 2.2 (t, t, m, m, t,
18H, J(1H,1H) = 7.3, 7.1, 6.8, 2 × Bu), 0.9 (m, 2H, C5H2), 2.5 (m, 2H,
C4H2), 5.9 (d, 1H, J(1H,1H) = 14.0Hz, CH), 6.4 (m, 1H, C3H), 6.5 (dt,
1H, J(1H,1H)= 7.3, 14.0Hz, C(Bu)H), 7.1, 7.2, 7.5 (m, m, 5H, Si―Ph);
13C NMR: δ [J(29Si,13C)] = 143.7 [63.8, C-2], 147.1 [11.7, C-3], 30.9
(C-4), 11.1 [53.1, C-5], 34.5, 32.8, 32.2, 32.0, 23.0, 22.8, 21.7, 14.2,
14.2 (2 × Bu), 124.1 [67.5, Si―C ], 152.5 (C=), 138.2 [65.5], 134.7,
3
Ph), 8.3 (s, 1H, J(29Si,1H)= 16.9Hz, CH); 13C NMR: δ [J(29Si,13C)] =
149.1 [62.1, C-2], 174.6br (C-3), 34.5 (C-4), 11.8 [53.9, C-5], 34.8,
34.7, 34.4, 34.3, 32.5br, 32.1br, 23.7, 23.7 (2 × 9-BBN), 149.4br ((B)C ),
155.6 ( C), 144.1 [5.9. i], 140.7, 140.6, 134.8, 134.3, 129.8, 128.5,
128.5, 128.4, 128.4, 128.3, 128.1 (Si―Ph, 2 × Ph); 11B NMR:
δ = 85.4; 29Si NMR: δ = 4.1.
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Appl. Organometal. Chem. 2014, 28, 280–285

1-Silacyclopent-2-enes and facile protodeborylation
128.2, 129.4 (i, o, m, p, Si―Ph); 29Si NMR: δ = À1.6.
10b: 1H NMR (300 MHz): δ = 1.0 (m, 2H, C5H2), 2.4 (m, 2H, C4H2),
6.3 (d, 1H, J(1H,1H)= 15.4Hz, CH), 6.9 (t, 1H, J(1H,1H)= 3.2 Hz, C3H),
7.5 (d, 1H, J(1H,1H) = 15.4Hz, CH), 7.0–7.2, 7.3, 7.4, 7.7 (m, m, m, m,
15H, Si―Ph, 2 × Ph); 13C NMR: δ [J(29Si,13C)] = 142.9 [64.8, C-2],
148.5 [11.3, C-3], 31.4 (C-4), 10.2 [56.8, C-5], 126.1 [66.4, Si―C ],
149.8 (C ), 140.0 [5.4, i], 139.5 (i), 137.8 [67.0, i], 134.9, 129.8, 128.8,
128.54, 128.52, 128.3, 128.1, 127.2, 126.8, (Si―Ph, 2 × Ph); 29Si
NMR: δ = 1.4.
Results and Discussion
The di(alkynyl)vinylsilane derivatives 1 and 2 were obtained by
following the literature procedure.[22] These silanes are fairly
stable at high temperature (up to 120°C under reduced pressure,
10À2 torr) and lighter fractions were removed at elevated
temperature under reduced pressure.[22] The desired silanes were
studied with the help of NMR spectroscopy (see Fig. 1 for a
representative 29Si NMR spectrum, and full set of data is summa-
rized in the Experimental section).
Scheme 2. Reaction of silanes 1 and 2 with one equivalent of 9-BBN
resulting into the syntheses of 1-silacyclopent-2-ene derivatives.
Hydroboration of Silanes 1 and 2
accompanied by 29Si satellites for 1/2J(29Si,13C). All the reactions in
this series were highly selective and led to formation of the desired
compounds in high purity (i.e. 90–95%). A consistent set of 1H, 11B,
13C and 29Si NMR data for 1-silacyclopent-2-ene derivatives 3 and 4
are summarized in the Experimental section.
Silanes 1 and 2 bear two functions; Si-vinyl and Si-alkynyl. These
functions are readily available for hydroboration when treated
with 9-BBN. It is well known that 9-BBN prefers the vinyl group
over the alkynyl function.[23] Reactions between silanes 1, 2
and 9-BBN were carried out in equimolar ratios at ambient
temperature in THF (Scheme 2). The hydroborating reagent
instantly gets attached to the terminal vinyl carbon (D), followed
by intramolecular 1,1-carboboration via ultimate ring closure to
afford 1-silacyclopent-2-ene derivatives 3 and 4. No attempt has
been made to isolate intermediates of type D. The activation of the
Si―C bond, prior to final 1,1-carboboration, in D by the neighbored
electron-deficient boron atom is indicated by a dashed line.
The proposed structure of the 1-silacyclopent-2-ene derivatives
follows conclusively from NMR data. All 13C nuclei in the five-
membered ring show coupling with 29Si, and the 13C(3) NMR
signals are typically broadened due to partially relaxed one-bond
13C―11B spin–spin coupling.[24] They could easily be identified
and were found in the same range as reported for analogous
heterocycles.8e,17a These compounds contain the Si―C≡C―R
functionality, and both alkynyl 13C nuclei are readily assigned. In
addition to their 13C chemicals shifts, these signals are
Hydroboration of 1-Alkynyl-1-silacyclopent-2-ene Deriva-
tives 3 and 4
Compounds obtained as a result of onefold 1,2-hydroboration of si-
lanes 1 and 2 were characterized as 1-alkynyl-1-silacyclopent-2-ene
derivatives. These compounds bear an Si-alkynyl function and in-
vite further chemical study. They were treated with 1 equiv. of 9-
BBN in benzene at a relatively high temperature (80–100°C) for
20 min and were completely converted into the 1-alkenyl-1-
silacyclopent-2-ene derivatives 5 and 6 (Scheme 3). The same
series of compounds were obtained as a result of twofold
hydroboration of the starting precursors 1 and 2. The first 1,2-
hydroboration reaction selectively takes place at the vinyl
group followed by intramolecular 1,1-vinylboration to afford 1-
alkynyl-1-silacyclopent-2-ene derivatives 3 and 4. The second
hydroboration requires relatively harsh reaction conditions and
Figure 1. 59.57 MHz 29Si{1H} NMR spectrum of silane 1b, showing 13C
satellites.
Scheme 3. Syntheses of silanes 5 and 6 by two possible routes.
Appl. Organometal. Chem. 2014, 28, 280–285
Copyright © 2014 John Wiley & Sons, Ltd.
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E. Khan et al.
7a: R = Bu, R1 = Me
7b: R = Ph, R1 = Me
8a: R = Bu, R1 = Ph
8b: R = R1 = Ph
AcOH (excess)
11
R1
the mixture was warmed to 40–50°C
for 1-2 h. The products were obtained
with the expected geometry with respect
to = C-H and =C-BBN as reported for 1,2-
hydroboration of other alkynylsilanes.[25]
Again the proposed structures of
these derivatives are supported by
NMR spectroscopy, and all the impor-
tant data are summarized in the
Experimental section.
Si
3, 4
Me
R
R
O
B
O
C8H15
B
C8H15
O
O
O
R1
9a: R = Bu, R1 = Me
9b: R = Ph, R1 = Me
10a: R = Bu, R1 = Ph
10b: R = R1 = Ph
Me
11
Si
5, 6
H
R
AcOH (excess)
2 x 11
R
H
Scheme 4. Protodeborylation of 1-silacyclopent-2-ene derivatives 3–6 (C8H15 = cyclooctyl).
Protodeborylation Reactions
Typical of triorganoboranes, all 1-silycy
clopent-2-enes 3–6 are sensitive to air.
Clearly, the B–C functions invite numer-
ous transformations.[26] Previously we
have successfully substituted this func-
tion with hydrogen and have explored
the boron–oxygen compound.[27] The
same attempt for the removal of boryl
groups by protodeborylation reaction
was made for the 1-silacyclopent-2-ene
derivatives 3–6. Treatment of the
compounds 3–6 with copious amounts
of acetic acid turned out to be the
successful approach (Scheme 4). The
NMR data of bicyclic boron–oxygen
compound 11 formed during the course
of these reactions is in full agreement
with the literature.[27] All other functions
of the 1-silacyclopent-2-enes remained
unaffected. The protodeborylation of all
boron-containing 1-silacyclopent-2-ene
derivatives afforded the desired products
in essentially quantitative yield (>90%).
The structural assignment of the 1-
silacyclopent-2-ene derivatives 7–10
was confirmed by consistent NMR data
sets (Experimental section). A representa-
tive 29Si NMR for 1-silacyclopent-2-ene
derivative 7a is shown in Fig. 2.
Figure 2. 59.57 MHz 29Si{1H} NMR spectrum of 1-methyl-1-(1-hexynyl)-2-butyl-1-silacyclopent-2-ene
7a (in C6D6) 13C satellites marked by different signs correspond to 1J(29Si,13C) and 2J(29Si,13C) spin–spin
coupling.
The gas phase geometries of the 1-
silacyclopent-2-ene derivatives 7a,b and
10a were optimized by calculations at
the B3LYP/6-311+G(d,p) level of the-
ory,[20] and 29Si nuclear shielding[28] and
coupling constants nJ(29Si,13C) (n = 1,
2)[29] were calculated at the same level
(Fig. 3). The calculated data are close
to those experimentally observed.[30]
Apparently, the sign of 2J(29Si,13C) is
negative (reduced coupling constants
2 K(29Si,13C) > 0), as expected for the
sum of two coupling pathways in the
five-membered rings. In 10a, the small
2
magnitude of J(29SiC 13C) for the exo-
cyclic alkenyl group is reproduced by
the calculations. Frequently this cou-
pling constant is too small for experi-
mental observation, and it may be of
either sign.[23]
Figure 3. Calculated chemical shifts δ29Si and optimized [B3LYP/6-311+G(d,p)] gas phase geometries
of 7b and 10a; selected bond lengths (pm) and bond angles (°). 7b: C2―Si 188.6, C5―Si 189.8, C(Me)
―Si 188.1, ≡C―Si 184.1, C2―C3 134.7, C3―C4 151.1, C4―C5 155.4, C≡C 121.6, C2―Si―C5 94.0,
≡C―Si―C(Me) 107.9. 10a: C2―Si 188.6, C5―Si 189.8, C(Ph) ―Si 189.7, C―Si 188.5, C2―C3 134.3,
C3―C4 151.4, C4―C5 155.6, CH CH 134.2; C2―Si―C5 93.90, CH―Si―C(Ph) 106.9.
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1-Silacyclopent-2-enes and facile protodeborylation
Wrackmeyer, R. Kempe, Eur. J. Inorg. Chem. 2008, 5367–5372. f)
B. Wrackmeyer, E. Khan, R. Kempe, Appl. Organomet. Chem.
2008, 22, 383–388.
Conclusions
The stereo- and regioselective 1,2-hydroboration of the vinyl
group in di(alkynyl)vinylsilanes proceeds at ambient temperature,
and is followed by intramolecular 1,1-vinylboration to afford
selectively 1-alkynyl-1-silacyclopent-2-ene derivatives in high
yield. Further 1,2-hydroboration of the alkynyl function gives
1-alkenyl-1-silacyclopent-2-ene derivatives. During the course of
these reactions no side products or isomers were detected. The
protodeborylation with an excess amount of acetic acid is of
particular interest, since it cleaves solely the B―C bond(s),
leaving all other bonds unaffected. The NMR data contain useful
information on 29Si―13C coupling constants, confirmed by den-
sity functional theory calculations, and are diagnostic in the
structural elucidation of such types of organosilicon compounds.
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Acknowledgements
Financial support from the Higher Education Commission
(HEC) Pakistan under National Research Program for Universities
(NRPU) No. 20-1488/R&D/09-5432 is highly acknowledged. O.L.T
and B.W. thank the Deutsche Forschungsgemeinschaft for support.
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