Electrochemical synthesis of 1,2-disilylethanes from α-silylacetic acids

Article abstract of DOI:10.1021/jo200254z

The synthesis of 1,2-disilylethanes [R₁R₂R ₃Si(CH₂)₂SiR₁R₂R ₃] is usually conducted by using noble metal reagents or catalysts. This work describes a new electrochemical synthetic method for their preparation in good yields by oxidation of α-silylacetic acids at Pt anodes (Kolbe electrolysis). Most of the reported synthesized 1,2-disilylethanes in this work are unknown.

Full text of DOI:10.1021/jo200254z

NOTE
pubs.acs.org/joc
Electrochemical Synthesis of 1,2-Disilylethanes from r-Silylacetic
Acids
Alex V. Shtelman and James Y. Becker*
Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel
S Supporting Information
b
ABSTRACT: The synthesis of 1,2-disilylethanes [R1R2R3Si-
(CH2)2SiR1R2R3] is usually conducted by using noble metal
reagents or catalysts. This work describes a new electrochemical
synthetic method for their preparation in good yields by oxidation of R-silylacetic acids at Pt anodes (Kolbe electrolysis). Most of the
reported synthesized 1,2-disilylethanes in this work are unknown.
Scheme 1
he well-known Kolbe electrolysis has been utilized as a useful
method for the synthesis of 1,2-disilylethanes of the type
T
R1R2R3Si(CH2)2SiR1R2R3 that have been prepared so far by
using noble metal reagents or catalysts. The anodic oxidation of
various R-silylacetic acids [(R1R2R3)SiCH2COOH] under basic
conditions at Pt anodes provided the corresponding homocou-
pling products, after decarboxylation, in moderate to good yields.
Organic electrosynthesis has been established for selective and
efficient chemical transformations.1 The first and best known anodic
electrosynthesis reaction is the Kolbe electrolysis, and it is still under
active investigation.2 It has been a simple and effective way of
preparing a variety of organic compounds, also in industry, in large
quantities.3 In principle, Kolbe electrolysis leads to oxidative
decarboxylation of carboxylate anions to generate radical intermedi-
ates that undergo CꢀC coupling to provide Kolbe “dimers”.
However, under certain conditions, the radical intermediate could
be further oxidized to the corresponding carbenium ion to afford the
so-called “non-Kolbe” products, as illustrated in Scheme 1.
The selectivity of the Kolbe reaction depends on the structural
parameters of the carboxylic acid and electrochemical conditions.
For instance, the Kolbe dimers are favored when a platinum
anode is used and the radical intermediate is either primary or
substituted by electron-withdrawing group(s).4 From this point
of view, it is interesting to investigate the Kolbe oxidation of
carboxylic acids with silyl substituents at the R-position because
of two contradictive effects. Previously, Wilt and co-workers5
demonstrated that R-silyl groups stabilize alkyl radicals to some
extent. A similar trend was also shown by gas phase kinetic
experiments.6 Therefore, it is reasonable to assume that this
property (known as the “R-effect”) may also contribute to the
stabilization of radicals generated in the Kolbe oxidation of
R-silylcarboxylic acids and promote the formation of “dimeric”
products. However, there is a contradictive effect that stems from
the polarity of the SiꢀC bond, causing the silyl group at the
R-position to be the electron-donating group in nature. This
property causes the carboxylic group to be less acidic and,
therefore, more reluctant to form a carboxylate anion, which is
necessary for the Kolbe reaction to take place.
Organosilicon chemicals and materials are attracting an in-
creasing amount of attention because of their versatile chemical
and physical properties. Over the past three decades, organosi-
licon chemistry has grown rapidly, and today organosilicon
compounds are widely utilized as various functional materials
and reagents.7 For example, symmetric 1,2-disilylethanes of the
type R3Si(CH2)2SiR3 (R = alkyl, Ph, or mixed) have been used as
precursors for a wide range of organosilicon derivatives, includ-
ing molecular,8 polymeric,9dendritic,10 and drug design11 enti-
ties. They are also used for the preparation of novel complexes12
and as new ligands for bioorganic13 and catalytic purposes.14
1,2-Disilylethanes are usually prepared by the use of expensive
noble metal reagents or catalysts. The most useful catalysts for
their preparation are based on platinum and have been developed
by Speier15 and Karstedt.16 Additional noble metal systems
predominantly contain Rh, Ru, Ir, and Pt centers and have been
used efficiently in specific reactions.17 Dunoges et al. synthesized
phenyl-substituted 1,2-disilylethanes by reductive silylation of
alkenes using magnesium and the carcinogenic solvent hexam-
ethylphosphoric triamide (HMPT).18 Fry et al. obtained 1,2-
bis(trimethylsilyl)-1,2-diphenylethanes among other products by
electrochemical reduction of R-trimethylsilylbenzyl bromide.19
This work demonstrates a new synthetic method for the
preparation of functional 1,2-disilylethanes in good yields by
Kolbe anodic oxidation of R-silylacetic acids prepared previously.20
Received: March 1, 2011
Published: April 18, 2011
r
2011 American Chemical Society
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NOTE
Scheme 2
Scheme 4
Scheme 3
Me3Si ∼ Me2(CH2Cl)Si ∼ Me2(Ph)Si ∼ Me(Ph)2Si > Ph3Si >
n-Pr3Si > Me2(cyclohexyl)Si.
Interestingly, these results are in good correlation with the
data reported in the literature. The relative steric bulk of
trialkylsilyl groups was described by Cheng and co-workers,24
and the same order of bulkiness for trialkylsilyl groups was
observed. Apparently, it seems that steric effects play an im-
portant role in the Kolbe decarboxylation of R-silylacetic acids.
To investigate the usefulness of the Kolbe electrolysis for
other types of silyl groups bearing unsaturated alkene(s), we used
R-silylacetic acids with silylvinyl moieties, as shown in Table 1
(entries 8ꢀ11). The results indicate that the Kolbe oxidation of
Me2(vinyl)silylacetic and (vinyl)3silylacetic acids (entries 8 and
9, respectively) affords Kolbe dimers in moderate yields (∼50%)
with only partial polymerization of the substrate (15ꢀ20%). The
isolated polymers (white powders) can be easily separated at the
end of the electrolysis by filtration because of their insolubility in
the CH3CN/MeOH reaction medium. The formation of poly-
mers is not surprising, because the Kolbe reaction includes
radical intermediates that could encourage polymerization of
the substrate via its vinyl moieties.
It is noteworthy that the Kolbe electrolysis of R-silylacetic
acids with silyl(vinyl)(phenyl) substituents (entries 10 and 11)
afforded mostly polymerization at the surface of the platinum
anode under similar experimental conditions. In attempts to
prevent the polymerization, we changed the electrolysis medium
by employing 1,2-dimethoxyethane (DME) (instead of CH3CN)
as a cosolvent with methanol. Indeed, the polymerization at the
substrate was prevented, and the desired Kolbe dimerswere formed
in moderate yields. Possibly, DME acts as a sacrificial solvent, being
electrolyzed by itself at the electrode surface, and competes
favorably with the polymerization reaction, or it is preferably
adsorbed at the electrode surface compared to the substrate.
A mixture of MeOH/DME in a 2/1 ratio (v/v) was found to
be the best choice for the Kolbe oxidation of such acids.
Interestingly, a larger volume of DME in the MeOH/DME
mixture decreases both the current density and yield of the
dimeric product but encourages the non-Kolbe process. On the
other hand, a larger volume of MeOH in the mixture causes the
polymerization process to become predominant. The overall
experimental conditions for the preparation of 1,2-disilylethanes
10 and 11 are illustrated in Scheme 4.
Scheme 2 outlines the favorable experimental conditions for
synthesizing different types of R-silylacetic acids.
Previously, we investigated the preferred conditions for the
electrochemical decarboxylation of Ph2MeSiCH2CO2H (1) as a
model for R-silylacetic acids, by examining various parameters
(e.g., solvent, nature of base, concentration of base, currentdensity,
etc.).21 It turned out that such investigations were necessary and
unavoidable because the commonly used conditions for Kolbe
electrolysis of carboxylic acids were not useful for R-silylcar-
boxylic acids. For example, when initially Ph2MeSiCH2CO2H
was dissolved in methanol containing sodium methoxide or
potassium hydroxide to neutralize a small portion (∼2ꢀ5%)
of the acid and then electrolyze the solution between two
platinum electrodes at a high current density,22 the reaction
did not take place at all. Apparently, the weak nature of the
R-silylacetic acid did not allow the formation of the corresponding
carboxylate anion in a meaningful amount. Indeed, this assump-
tion is supported by the claim that R-silylacetic acids are weak
acids with pKa values of >5.22 (for comparison, the pKa of acetic
acid is 4.75).23 Obviously, the inductive effect exerted by the
silicon group decreases the strength of the acid.
In this respect, we have also examined Kolbe electrolysis by
using solid-supported bases (e.g., silica gel-supported piperidine),
following recent work by Fuchigami and co-workers.2a Again, the
basic power of the organic base was too weak to allow formation
of carboxylate anions and therefore caused a high cell voltage that
prevented a high current density, necessary to obtain Kolbe-type
products. Consequently, under these conditions, the electroche-
mical decarboxylation of Ph2MeSiCH2CO2H provided the de-
sired Kolbe dimer in low yields.
Finally, by adopting the “optimal” conditions reported previ-
ously21 for the Kolbe electrolysis of Ph2MeSiCH2CO2H to other
R-silylacetic acids, and especially when a high concentration of a
strong base was used (3 N KOH, 25%), the decarboxylation
process afforded dimeric products in moderate to good yields. A
summary of the optimized reaction conditions is illustrated in
Scheme 3, and the results of the Kolbe reaction are described in
Table 1. It appears that R-silylacetic acids with less hindered silyl
substituents [Me3Si, Me2(CH2Cl)Si, Me2PhSi, and MePh2Si]
undergo a smooth Kolbe anodic oxidation to give dimeric
products in good isolated yields (entries 1ꢀ4). In these cases,
it is likely that the R-silyl group favors stabilization of a carbon
radical rather than a carbenium ion and, consequently, increases
the yield of Kolbe dimers. However, when more sterically
hindered silyl substituents were employed (entries 5ꢀ7), the
yields of the dimeric products decreased in the following order:
It was claimed that the yields of Kolbe dimers are very low
when the R-position of the acid bears an electron-donating
substituent capable of stabilizing carbenium ions.4 In this respect,
the anodic oxidation of R-silylcarboxylic acids with alkyl sub-
situents at the R-position in addition to the silyl group at the
same position has been investigated as well. Although it has been
known that R-silyl groups stabilize alkyl radicals, as also demon-
strated above, an addition of an alkyl substituent should favor
carbenium ion stabilization and, at the same time, a reduction in
acid strength. Therefore, it is not surprising that no Kolbe dimers
were detected from electrolysis of such acids (entries 12 and 13).
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NOTE
Table 1. Synthesis of 1,2-Disilylethanes by Kolbe Anodic Oxidation of r-Silylacetic Acids
a Isolated yields. No attempts were made to optimize yields. b Conducted in a MeCN/MeOH mixture (3/1). c Conducted in a MeOH/DME mixture (2/1).
Unreacted starting material was left and minor non-Kolbe products were formed. d No dimer was found (see the text for more details).
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NOTE
Scheme 5
δ ꢀ2.2, 8.7; 29Si NMR (100 MHz, CDCl3) δ 3.4; GCꢀMS (m/z) (%)
1,2-Bis[dimethyl(chloromethyl)silyl]ethane (2): colorless oil; 1H
M
þ• 174 (16), 159 (25), 86 (33), 73 (100), 58 (5), 45 (5).
NMR (500 MHz, CDCl3) δ 0.11 (s, 12H), 0.55 (s, 4H), 2.80 (s, 4H);
13C NMR (125 MHz, CDCl3) δ ꢀ5.2, 5.4, 29.8; 29Si NMR (100 MHz,
CDCl3) δ 4.6; MS (MALDI-TOF) m/z calcd for C8H20Cl2Si2 [M þ
H]þ 243.056, found 243.120.
1,2-Bis[dimethyl(phenyl)silyl]ethane (3):26 mp 57ꢀ59 °C; 1H NMR
(200 MHz, CDCl3) δ 0.28 (s, 12H), 0.70 (s, 4H), 7.34ꢀ7.58 (m, 10H);
13C NMR (125 MHz, CDCl3) δ ꢀ3.6, 7.8, 127.7, 128.8, 133.6, 139.3;
29Si NMR (100 MHz, CDCl3) δ ꢀ1.2; GCꢀMS (m/z) (%) Mþ• 298
(8), 283 (12), 135 (100), 120 (5), 105 (5).
In fact, electrolysis of Ph2MeSiCH(n-Pr)CO2H afforded 50% of
the recovered starting material and non-Kolbe products, with no
evidence of a dimeric product, as shown in Scheme 5, where only
the major non-Kolbe product is described. On the other hand,
electrolysis of Ph2MeSiCH(benzyl)CO2H led to a complete
desilylation under the same experimental conditions.
In summary, an electrochemical method has been utilized for the
synthesis of symmetric 1,2-disilylethanes of the type R3Si(CH2)2SiR3
in good isolated yields, based on Kolbe anodic oxidation of
various R-silylacetic acids. Most of the dimeric products are new, and
three of them (9ꢀ11) could be potentially useful as precursors
for preparing novel polymers and dendrones.
1
1,2-Bis[diphenyl(methyl)silyl]ethane (4): mp 85ꢀ87 °C; H NMR
(200 MHz, CDCl3) δ 0.62 (s, 6H), 1.10 (s, 4H), 7.35ꢀ7.56 (m, 20H);
13C NMR (125 MHz, CDCl3) δ ꢀ5.2, 6.3, 127.8, 129.1, 134.5, 139.0;
29Si NMR (100 MHz, CDCl3) δ ꢀ5.4; GCꢀMS (m/z) (%) Mþ• 422
(1), 394 (6), 197 (100), 165 (5), 105 (11). Anal. Calcd for C28H30Si2: C,
79.56; H, 7.15. Found: C, 79.27; H, 7.32.
1
1,2-Bis(triphenylsilyl)ethane (5): mp 207ꢀ209 °C; H NMR (200
MHz, CDCl3) δ 1.46 (s, 4H), 7.31ꢀ7.57 (m, 30H); 13C NMR (125
MHz, CDCl3) δ 5.6, 127.8, 129.4, 134.8, 135.7; 29Si NMR (100 MHz,
CDCl3) δ ꢀ10.7; MS (MALDI-TOF) m/z calcd for C38H34Si2 [M þ
Ag]þ 653.125, found 653.137. Anal. Calcd for C38H34Si2: C, 83.46; H,
6.27. Found: C, 83.28; H, 6.32.
1,2-Bis(tri-n-propylsilyl)ethane (6): colorless oil; 1H NMR (500
MHz, CDCl3) δ 0.37 (s, 4H), 0.48ꢀ0.52 (m, 12H), 0.94ꢀ0.98 (t, J =
7.0, 18H), 1.28ꢀ1.34 (m, 12H); 13C NMR (125 MHz, CDCl3) δ 4.4, 14.9,
17.5, 18.7; 29Si NMR (100 MHz, CDCl3) δ 5.5; MS (MALDI-TOF) m/z
calcd for C20H46Si2 [M þ Ag]þ 449.219, found 449.461. Anal. Calcd for
C20H46Si2: C, 70.08; H, 13.52. Found: C, 70.15; H, 13.31.
’ EXPERIMENTAL SECTION
General. R-Silylacetic acids were prepared according to our previously
published report (see ref 20). All other reagents and solvents were purchased
from commercial suppliers and used without further purification.
1,2-Bis[dimethyl(cyclohexyl)silyl]ethane (7): mp 52ꢀ53 °C; 1H
NMR (500 MHz, CDCl3) δ ꢀ0.10 (s, 12H), 0.35 (s, 4H), 0.59ꢀ0.65
(m, 2H), 1.05ꢀ1.27 (m, 10H), 1.62ꢀ1.75 (m, 10H); 13C NMR (125
MHz, CDCl3) δ ꢀ5.9, 5.4, 25.0, 27.1, 27.6, 28.2; 29Si NMR (100 MHz,
CDCl3) δ 4.9; MS (MALDI-TOF) m/z calcd for C18H38Si2 [M þ Na]þ
333.241, found 333.576. Anal. Calcd for C18H38Si2: C, 69.59; H, 12.33.
Found: C, 69.72; H, 12.62.
1H NMR spectra were recorded for all samples in CDCl3. Chemical
shifts for 1H NMR spectra are reported as δ in parts per million relative
to the signal of chloroform-d (δ, 7.26, singlet). The number of protons
(n) for a given resonance is indicated by nH. Coupling constants are
reported as J values in hertz. 13C NMR spectra are reported as δ in parts
per million relative to the signal of chloroform-d (δ, 77.03, triplet).
29Si NMR spectra are reported as δ in parts per million. Mass spectral
data are reported in units of mass to charge (m/z). Kolbe electrolyses
were performed in an undivided cell, using a Potentiostat/Galvanostat.
Procedures and Spectral Data of Products. General Proce-
dure for the Synthesis of Dimer 4 as a Representative for 1,2-
Disilylethanes 1ꢀ9. A mixture of Ph2MeSiCH2CO2H (2.5 mmol)
and 3 N KOH (0.63 mmol) was electrolyzed in a wall-jacketed (to
allow a steady flow of tab water during electrolysis) undivided cell,
equipped with two platinum plates electrode (2 cm ꢁ 1.2 cm), in 20 mL
of a MeOH/MeCN solvent mixture (1/3, v/v). In a typical experiment,
after the desired electricity was consumed, a few drops of concentrated
acetic acid were added to acidify the mixture, and the solvents were
removed by evaporation. The crude product was dissolved in diethyl
ether and washed with a 5% sodium bicarbonate solution and finally with
brine. After phase separation, the ethereal solution was dried over
magnesium sulfate and filtered. Evaporation of the ether gave a crude
dimeric nonpolar product that was simply purified with a silica gel
column via elution with a hexane/acetone mixture (95/5, v/v, %).
Evaporation of the solvents afforded a pure dimeric product 4, after
separation from unreacted starting material.
1,2-Bis[dimethyl(vinyl)silyl]ethane (8):27 colorless oil; 1H NMR
(200 MHz, CDCl3) δ 0.07 (s, 12H), 0.48 (s, 4H), 5.68 (d ꢁ d, 2H,
2J = 4.8, 3J = 19.6), 5.91ꢀ6.25 (m, 4H); 13C NMR (125 MHz, CDCl3)
δ ꢀ4.0, 7.4, 131.5, 139.0; 29Si NMR (100 MHz, CDCl3) δ ꢀ5.1; GCꢀMS
(m/z) (%) Mþ• 199 (1), 171 (71), 133 (85), 97 (31), 85 (100).
1
1,2-Bis(trivinylsilyl)ethane (9): colorless oil; H NMR (500 MHz,
CDCl3) δ 0.71 (s, 4H), 5.77 (d ꢁ d, 6H, 2J = 5.5, 3J = 19.0), 6.06ꢀ6.18
(m, 12H); 13C NMR (125 MHz, CDCl3) δ 5.3, 127.9, 129.4, 134.2, 135.0,
135.3, 136.0; 29Si NMR (100 MHz, CDCl3) δ ꢀ12.2; MS (MALDI-TOF)
m/z calcd for C14H22Si2 [M þ Ag]þ 353.031, found 353.303. Anal. Calcd
for C14H22Si2: C, 68.21; H, 9.00. Found: C, 68.54; H, 8.90.
1,2-Bis[methyl(vinyl)phenylsilyl]ethane (10): colorless oil; 1H NMR
(500 MHz, CDCl3) δ 0.43 (s, 6H), 0.80 (s, 4H), 5.77 (d ꢁ d, 2H,
3
2
3
2J = 3.5, J = 20), 6.11 (d ꢁ d, J = 3.5, J = 14.5, 1H), 6.27 (d ꢁ d,
3J = 14.5, 3J = 20.0, 1H), 7.33ꢀ7.50 (m, 10H); 13C NMR (125 MHz,
CDCl3) δ ꢀ5.6, 6.2, 127.8, 129.0, 133.6, 134.2, 136.5, 137.3; 29Si NMR
(100 MHz, CDCl3) δ ꢀ6.6; MS (MALDI-TOF) m/z calcd for
C20H26Si2 [M þ Ag]þ 429.062, found 429.315. Anal. Calcd for
C20H26Si2: C, 74.46; H, 8.12. Found: C, 74.66; H, 7.96.
1
1,2-Bis[diphenyl(vinyl)silyl]ethane (11): mp 95ꢀ97 °C; H NMR
(500 MHz, CDCl3) δ 1.24 (s, 4H), 5.80 (d ꢁ d, 2J = 3.0, 3J = 20.5, 2H),
6.28 (d ꢁ d, 2J = 3.5, 3J = 14.5, 2H), 6.54 (d ꢁ d, 3J = 20.5, 3J = 14.5, 2H),
7.37ꢀ7.54 (m, 20H); 13C NMR (125 MHz, CDCl3) δ 5.28, 127.9,
129.4, 134.2, 135.0, 135.3, 136.0; 29Si NMR (100 MHz, CDCl3)
δ ꢀ12.2; MS (MALDI-TOF) m/z calcd for C30H30Si2 [M þ Ag]þ
553.094, found 553.078. Anal. Calcd for C30H30Si2: C, 80.66; H, 6.77.
Found: C, 80.35; H, 6.80.
Electrolysis of substrates 10 and 11 was conducted in 20 mL of a
MeOH/DME solvent mixture (2/1, v/v) (DME = 1,2-dimethoxy-
ethane). Constant-current electrolysis was conducted with magnetic
stirring. The conversion of the substrate to the Kolbe dimer was mon-
itored by 1H NMR and TLC, using hexane as the eluent.
1,2-Bis(trimethylsilyl)ethane (1):25 colorless oil; 1H NMR (200 MHz,
CDCl3) δ ꢀ0.04 (s, 18H), 0.36 (s, 4H); 13C NMR (125 MHz, CDCl3)
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NOTE
’ ASSOCIATED CONTENT
(13) Daiss, J.; Burschka, C.; Mills, J.; Montana, J.; Showell, G.;
Fleming, I.; Gaudon, C.; Ivanova, D.; Gronemeyer, H.; Tacke, R.
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(16) Karstedt, B. D. U.S. Patent 226,928, 1972.
Supporting Information. Copies of H, 13C, and 29Si
1
S
b
NMR spectra of products. This material is available free of charge
via the Internet at http://pubs.acs.org.
(17) (a) Kobayashi, T.; Hayashi, T.; Yamashita, H.; Tanaka, M.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: becker@bgu.ac.il.
’ ACKNOWLEDGMENT
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This work was supported by the Israel Science Foundation
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