Facile hydroboration of (Z)-1-trimethylsilyl-1-alkenes with dichloroborane-dioxane complex: An easy access to gem-dimetalloalkanes containing boron and silicon

Article abstract of DOI:10.1016/j.jorganchem.2005.12.043

(Z)-1-trimethylsilyl-1-alkenes easily prepared by the hydroboration of the corresponding 1-trimethylsilyl-1-alkynes followed by protonolysis with acetic acid, readily react with dichloroborane-dioxane complex in dichloromethane for 6 h. The resulting solution is then treated with 1,3-propane diol in dichloromethane at 0 °C for half an hour to provide the corresponding gem-dimetalloalkanes containing boron and silicon. These α- trimethylsilylalkylboronate esters are purified by vacuum distillation in high yields (72-84%) and the structures of these novel intermediates are further confirmed by selective oxidation with alkaline hydrogen peroxide to provide the corresponding alcohols containing α-trimethylsilyl group in 78-88% isolated yields.

Full text of DOI:10.1016/j.jorganchem.2005.12.043

Journal of Organometallic Chemistry 691 (2006) 1298–1300
www.elsevier.com/locate/jorganchem
Note
Facile hydroboration of (Z)-1-trimethylsilyl-1-alkenes with
dichloroborane–dioxane complex: An easy access to
gem-dimetalloalkanes containing boron and silicon
*
Narayan G. Bhat , Mary A. Villanueva
Department of Chemistry, University of Texas-Pan American, 1201 West University Drive, Edinburg, TX 78541, USA
Received 24 June 2005; received in revised form 18 December 2005; accepted 19 December 2005
Available online 23 January 2006
Abstract
(Z)-1-Trimethylsilyl-1-alkenes easily prepared by the hydroboration of the corresponding 1-trimethylsilyl-1-alkynes followed by pro-
tonolysis with acetic acid, readily react with dichloroborane–dioxane complex in dichloromethane for 6 h. The resulting solution is then
treated with 1,3-propane diol in dichloromethane at 0 ꢀC for half an hour to provide the corresponding gem-dimetalloalkanes containing
boron and silicon. These a-trimethylsilylalkylboronate esters are purified by vacuum distillation in high yields (72–84%) and the struc-
tures of these novel intermediates are further confirmed by selective oxidation with alkaline hydrogen peroxide to provide the corre-
sponding alcohols containing a-trimethylsilyl group in 78–88% isolated yields.
ꢁ 2006 Elsevier B.V. All rights reserved.
Keywords: Hydroboration; gem-Dimetalloalkanes; Dichloroborane–dioxane complex; 1-Trimethylsilylalkynes; (Z)-1-trimethylsilyl-1-alkenes
1. Introduction
quently, we undertook a methodology involving (Z)-1-
trimethysilyl-1-alkenes via the hydroboration-alcoholysis
sequence to achieve the synthesis of gem-dimetalloalkanes
containing boron and silicon. In this report, we reveal the
results of our investigation.
Hydroboration of 1-trimethylsilyl-1-alkynes with di-
cyclohexylborane [1] is well documented in literature. The
protonolysis of the resulting boron intermediates with ace-
tic acid [2] provided the corresponding (Z)-1-trimethylsilyl-
1-alkenes in high stereochemical purities. The preparation
of different gem-dimetallics containing aluminum and zir-
conium [3], zinc and zirconium [4], zinc and magnesium
[5], zinc and lithium [5], zinc and boron [6], copper and
boron [6], boron and lithium [7], boron and zirconium
[8], boron and silicon [9,10], and boron and boron [11]
and their synthetic importance are well documented in lit-
erature [3–11]. We wanted to utilize the recently discovered
dichloroborane–dioxane complex [12] as an hydroborating
agent to achieve a highly regioselective synthesis of gem-
dimetalloalkanes containing boron and silicon. Conse-
2. Results and discussion
We reacted terminal alkynes with n-butyllithium at
ꢀ78 ꢀC for 1 h followed by treatment with chlorotrimeth-
ylsilane at ꢀ78 ꢀC for 1 h and allowed the reaction mixture
to stir overnight. The resulting 1-trimethylsilyl-1-alkynes
were isolated and kept in refrigerator under nitrogen atmo-
sphere for further use. They were characterized by
300 MHz NMR spectral data.
The hydroboration of 1-trimethylsilyl-1-alkynes with
dicyclohexylborane [13] (freshly prepared by mixing cyclo-
hexene with borane–methyl sulfide complex) in tetrahydro-
furan proceeded cleanly at 0 ꢀC, for 3 h followed by stirring
for 1 h at room temperature. The resulting clear solution
*
Corresponding author. Tel.: +1 956 381 3373; fax: +1 956 384 5006.
E-mail address: nbhat@panam.edu (N.G. Bhat).
0022-328X/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.12.043

N.G. Bhat, M.A. Villanueva / Journal of Organometallic Chemistry 691 (2006) 1298–1300
1299
was treated with acetic acid and the stirring continued for
6 h at room temperature. The solvents were removed and
replaced by n-pentane. Monoethanolamine [14] was added
to remove the boron by-product. Representative (Z)-1-
trimethylsilyl-1-alkenes (Table 1) were prepared (Eq. (1))
and characterized by 300 MHz NMR data
These novel gem-dimetalloalkanes were reacted with
alkaline hydrogen peroxide at room temperature for 3 h
and the resulting alcohols containing a-trimethylsilyl group
(Eq. (3)) were purified by column chromatography over
alumina
RCH CHSiMe
2
3
RCH CHSiMe
2
3
NaOH
H O
R
SiMe
3
Chx BH
2
AcOH
2
2
B
C
RC
SiMe
3
C
C
OH
78%-88%
ð1Þ
O
O
ð3Þ
H
H
The (Z)-1-trimethylsilyl-1-alkenes thus prepared were
I
II
subjected to hydroboration with dichloroborane–dioxane
complex in dichloromethane at room temperature for 6 h.
The resulting solution was reacted with 1,3-propane diol
in dichloromethane at 0 ꢀC for half an hour. The solvents
were pumped off and the resulting liquids were purified
by high vacuum distillation. The representative selection
of gem-dimetalloalkanes was prepared (Table 2, Eq. (2))
using the above procedure
3. Experimental
The preparation of B-2-(1-trimethylsilyl-1-hexyl)-1,3,2-
dioxaborinane and 1-trimethylsilyl-1-hexanol is represen-
tative: In an oven dried 100 mL side-arm round bottom
flask equipped with a septum inlet was placed (Z)-1-trim-
ethylsilyl-1-hexene (10 mmol, 1.56 g) under nitrogen
atmosphere. It was cooled to 0 ꢀC and the dichlorobo-
rane–dioxane complex (3 M solution, 10 mmol, 3.3 mL)
in methylene chloride was added dropwise. The resulting
mixture was stirred for 1 h at 0 ꢀC followed by 6 h at
room temperature. The reaction mixture was cooled to
0 ꢀC. To this cooled solution was added 1,3-propane diol
(10 mmol, 0.76 g) dropwise and the resulting reaction mix-
ture was stirred at 0 ꢀC for 0.5 h. The methylene chloride
layer containing the desired product was separated and
the solvent was removed to provide the B-2-(1-trimethylsi-
lyl-1-hexyl)-1,3,2-dioxaborinane in 84% (2.0 g) yield. The
NMR spectral data indicated that the compound was
the desired product. PMR (CDCl3/without TMS): d 0.02
(9H, s), 0.86–1.88 (14H, m), and 3.95 (4H, m), CMR
.
BHCl
2
R
SiMe
RCH CHSiMe
O
3
2
3
O
C
C
.
B
O
O
CH Cl
2
2
H
Cl
Cl
H
ð2Þ
RCH CHSiMe
2
HO (CH ) OH
2 3
B
O
O
72%-84%
I
Table 1
Synthesis of 1-trimethylsilyl-1-alkenes
Yielda (%)
(CDCl3/without TMS):
d
ꢀ0.46,13.95, 14.21, 14.55,
No.
Eq. (1) R=
22.69, 25.59, 27.79, 32.02, 33.16, and 61.59 ppm. To a
solution of B-2-(1-trimethylsilyl-1-hexyl)-1,3,2-dioxabor-
inane (10 mmol, 2.42 g) in tetrahydrofuran (10 mL) was
added at 0 ꢀC the sodium hydroxide (3 M, 5 mL) followed
by 30% hydrogen peroxide slowly (25 mmol, 2.5 mL). The
reaction mixture was allowed to come to room tempera-
ture and the stirring was continued for 4 h at room tem-
perature. The compound was then extracted with ether
(2 · 25 mL) and the ether layer was washed with water
(2 · 10 mL). It was then dried over anhydrous sodium sul-
fate. The removal of the solvent followed by chromatogra-
phy over alumina provided 1-trimethylsilyl-1-hexanol in
80% (1.39 g) isolated yield. The compound was character-
ized by spectral data. IR (neat): m 3398 cmꢀ1, PMR
(CDCl3/without TMS): d ꢀ0.20 (9H, s), 0.84 (3H, m),
1.14–1.45 (9H, m), and 03.45 (1H, s) ppm; CMR
(CDCl3/without TMS): d ꢀ3.92, 14.03, 22.35, 25.24,
25.81, 34.15, and 65.85 ppm.
1
2
3
4
6
7
a
n-C4H9
n-C5H11
n-C6H13
–(CH2)3Cl
–CH2CH2CH(CH3)2
–CH2CH2Ph
82
78
86
78
79
80
All of the reactions were carried out on a 50 mmol scale. The com-
pounds were purified by chromatography over alumina followed by vac-
uum distillation.
Table 2
The synthesis of gem-dimetalloalkanes containing boron and silicon
No.
Eq. (2) R=
Yielda (%)
1
2
3
4
6
7
a
n-C4H9
n-C5H11
n-C6H13
–(CH2)3Cl
–CH2CH2CH(CH3)2
–CH2CH2Ph
84
82
80
74
80
72
The spectral data for (1–7, Table 2) are consistent with
the reported data in the literature [13].
Product 1, Table 2. PMR (CDCl3/without TMS): d
ꢀ0.03 (s, 9H), 0.88 (m, 3H), 1.24–1.87 (m, 11H), and
All of the reactions were carried out on a 10 mmol scale. The com-
pounds were purified by vacuum distillation. The carbon skeletons present
in these compounds were confirmed by selective oxidation to alcohols
containing a-trimethylsilyl group in >78% isolated yields (Eq. (3)).

1300
N.G. Bhat, M.A. Villanueva / Journal of Organometallic Chemistry 691 (2006) 1298–1300
3.95 (m, 4H) ppm. CMR (CDCl3/without TMS): d ꢀ0.46,
14.21, 22.70, 25.60, 27.79, 32.03, 33.16, and 61.59 ppm.
Product 2, Table 2. PMR (CDCl3/without TMS): d
0.03 (s, 9H), 0.82 (m, 3H), 1.18 –1.82 (m, 13H), and
3.91 (m, 4H) ppm. CMR (CDCl3/without TMS): d
ꢀ0.58, 14.07, 22.72, 25.60, 27.78, 29.34, 29.72, 33.51,
and 61.46 ppm.
Product 3, Table 2. PMR (CDCl3/without TMS): d 0.04
(s, 9H), 0.82–1.86 (m, 15H), 0.84 (m, 3H), and 3.92 (m, 4H)
ppm. CMR (CDCl3/without TMS): d ꢀ1.43, 14.15, 22.72,
25.59, 27.78, 29.73, 32.01, 33.69, and 61.54 ppm.
Product 4, Table 2. PMR (CDCl3/without TMS): d 0.05
(s, 9H), 0.78–2.16 (m, 9H), and 3.41–3.90 (m, 6H) ppm.
CMR (CDCl3/without TMS): d ꢀ0.57, 24.81, 27.70,
31.00, 32.43, 34.11, and 66.83 ppm.
4. Conclusions
In summation, we have developed for the first time a sim-
ple, and efficient conversion of 1- trimethylsilyl-1-alkynes
into the corresponding gem-dimetalloalkanes containing
boron and silicon in high yields. These gem-dimetalloalk-
anes were isolated and characterized using 300 MHz
NMR spectral data for the first time. The carbon skeletons
present in these intermediates are further confirmed
by selective oxidation studies with alkaline hydrogen perox-
ide to provide the corresponding alcohols containing
a-trimethylsilyl group. We are presently utilizing these valu-
able compounds for further synthetic transformations.
Acknowledgments
Product 5, Table 2. PMR (CDCl3/without TMS): d 0.05
(s, 9H), 0.83 (m, 6H), 1.11–1.85 (m, 13H), and 3.90 (m, 4H)
ppm. CMR (CDCl3/without TMS): d ꢀ1.45, 14.07, 22.36,
22.84, 25.78, 27.87, 31.26, 39.10, and 61.49 ppm.
Product 6, Table 2. PMR (CDCl3/without TMS): d 0.08
(s, 9H), 0.95–2.65 (m, 9H), 3.97 (m, 4H), and 7.20–7.29 (m,
5H) ppm. CMR (CDCl3/without TMS): d ꢀ1.27, 14.22,
22.49, 25.58, 26.00, 27.82, 35.47, 36.20, 61.61, 62.69,
66.90, 102.29, 125.57, 128.47, and 143.15 ppm.
We are highly grateful to the Robert A. Welch Founda-
tion of Texas for generous financial support under Grant
No. BG-1387.
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a-Trimethylsilyl-4-phenyl-1-butanol. PMR (CDCl3/with-
out TMS): d 0.05 (s, 9H), 1.21–2.65 (m, 12H), 3.48 (m,
1H), and 7.20 (m, 5H) ppm. CMR (CDCl3/without
TMS): d ꢀ3.70, 28.89, 33.34, 36.00, 66.98, 125.85, 128.46,
128.58 and 142.66 ppm.