Synthesis of α-silylcarboxylic acids

Article abstract of DOI:10.1016/j.tet.2010.12.020

New α-silylacetic acids have been prepared following a general and convenient procedure of a one-pot reaction between trimethylsilylacetate and different chlorosilanes. New derivatives of diphenyl(methyl)silylacetic acid were prepared by a direct alkylati

Full text of DOI:10.1016/j.tet.2010.12.020

Tetrahedron 67 (2011) 1135e1141
Contents lists available at ScienceDirect
Tetrahedron
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Synthesis of a-silylcarboxylic acids
*
Alex V. Shtelman, James Y. Becker
Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2010
Received in revised form 14 November 2010
Accepted 7 December 2010
Available online 13 December 2010
New a-silylacetic acids have been prepared following a general and convenient procedure of a one-pot
reaction between trimethylsilylacetate and different chlorosilanes. New derivatives of diphenyl(methyl)
silylacetic acid were prepared by a direct alkylation of its dianion with benzyl-, allyl-, and alkyl halides
and trimethylchlorogermane to afford new
a
-silylcarboxylic acids.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
a
-Silylcarboxylic acids
Chlorosilanes
-Silylacetic acid dianion
Alkylation
0-Disilyldicarboxylic acids
a
a,a
1. Intoduction
and relatively high cost of the reagents used. Recently, our group
developed a useful method for the preparation of various -silyl-
a
Over the past decades, organosilicon compounds have reached
considerable importance in synthetic organic chemistry. They are
readily available and fairly stable compared to other organometallic
compounds and are widely utilized not only as various functional
materials, but also as valuable synthetic organic reagents owing to
their unique chemical and physical properties.1 As a result, orga-
nosilicon compounds are widely used in polymer and dendrimer
synthesis,2 bioorganic and drug design chemistry,3,4 electrochem-
istry5 and also in new type of reactions, such as cross coupling,6
cross metathesis,7 and Lewis acid-catalyzed reactions.8
acetic acids of the type R1R2R3SiCH2CO2H by treating trimethylsi-
lylacetate with LDA followed by quenching with chlorosilanes.15
This method was found to be convenient, reproducible, efficient,
and quite general for using different kinds of silyl substituents, as
illustrated in Table 1 (entries 1e6).
Table 1
Synthesis of new a-silylacetic acids
Entry
Chlorosilane
Product
Yielda, %
1b
2b
3b
4b
5b
6b
7
8
9
10
11
12
13
Me3SiCl
n-Pr3SiCl
i-Pr3SiCl
Ph3SiCl
Me2(Ph)SiCl
Ph2(Me)SiCl
Me2(CH2Cl)SiCl
Me2(C6H11)SiCl
Me2(CH2Ph)SiCl
Me(Vinyl)PhSiCl
Ph2(Vinyl)SiCl
(Vinyl)3SiCl
[(Me)Si(Ph)Cl]2O
Me3SiCH2CO2H (1)
n-Pr3SiCH2CO2H (2)
i-Pr3SiCH2CO2H (3)
Ph3SiCH2CO2H (4)
Me2(Ph)SiCH2CO2H (5)
Ph2(Me)SiCH2CO2H (6)
Me2(CH2Cl)SiCH2CO2H (7)
Me2(C6H11)SiCH2CO2H (8)
Me2(CH2Ph)SiCH2CO2H (9)
Me(Vinyl)PhSiCH2CO2H (10)
Ph2(Vinyl)SiCH2CO2H (11)
(Vinyl)3SiCH2CO2H (12)
[(Me)Si(Ph)CH2CO2H]2O (13)
72
70
62
70
75
78
68
60
72
74
80
71
30
a
-Silylcarboxylic acids are important intermediates in organic
synthesis. They are widely used for various biological purposes,
such as preparing diethyl malonate labeled at each carboxyl car-
bon,9 synthesizing
a
,
b
-unsaturated thiol esters,10 and
a
-silyl thio-
esters.11 In addition, the dianion of trimethylsilylacetic acid
provides a highly efficient route for the preparation of -un-
saturated acids and butyrolactones.12 Recently,
-silylcarboxylic
a,b
a
acids have been utilized in the electrochemical synthesis of useful
disilylalkanes of type R3SiCH2CH2SiR3 by the Kolbe anodic oxida-
tion of their carboxylate anions.13
Since the first report of the synthesis of trimethylsilylacetic acid
by Sommer and co-workers in 1949,14 other attempts to synthesize
similar acids have been hampered by low yields, lack of selectivity,
a
Isolated yield.
Taken from Ref. 15.
b
2. Results and discussion
The present work describes the synthesis and characterization
of new -silylacetic acids and their utility in the preparation of
* Corresponding author. Tel.: þ972 8 6461197; fax: þ972 8 6472943; e-mail
a
address: becker@bgu.ac.il (J.Y. Becker).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.12.020

1136
A.V. Shtelman, J.Y. Becker / Tetrahedron 67 (2011) 1135e1141
novel derivatives of
a
-silylcarboxylic acids. The synthesis is based
phenyl or vinyl substituent. In addition, it can be observed (Table 1,
on the well-known property of trimethylsilyl esters of carboxylic
acids that they are readily hydrolyzed under very mild acidic con-
ditions.16 Trimethylsilylacetate (CH3CO2SiMe3) was chosen as the
starting material and was treated with 1 equiv of LDA to give the
anticipated lithium enolate salt. Apparently, the early work of
entries 2e4) that the reaction is not sensitive to the bulkiness of the
silyl substituents because the yields of the corresponding a-sily-
lated acetic acids were still good. Interestingly, it is quite remark-
able that the vinyl groups (entries 10e12) remain intact and do not
undergo polymerization. As a result, different kinds of a-silylacetic
Ainsworth and Kuo indicated that
a
ketoeenol equilibrium
acids were successfully synthesized and isolated by this new
method. Moreover, some of these compounds (entries 7, 10e12) are
functionalized, allowing further chemical transformations to new
organosilyl derivatives.
between two organolithium species (Scheme 1) was observed and
as a result, equal amounts of O- and C-silyl derivatives were formed
upon silylation.16
A successful attempt to prepare a new a,a
0-disilyldicarboxylic
acid under the same experimental conditions by the use of 1,3-bis
(dichlorosilyl)siloxane as reagent (entry 13) and 2 equiv of trime-
thylsilylacetate has been performed. Due to its relatively polar
property, 13 was easily separated from the other major product,
trimethysilylacetic acid, without further purification (see
Experimental section for compound 13). Apparently, its yield was
low (entry 13) due to an O/C migration of the trimethylsilyl group
to afford trimethysilylacetic acid. The reason for that is not clear yet.
However, it should be noted that this type of migration was ob-
served in all other reaction but to a minor extent. In any case, the
reaction in entry 13 demonstrates the formation of a new class of
OLi
2LDA/THF/-78 oC
H2C
C
H2C CO2Li
Li
CH3CO2H
OLi
Scheme 1. Ketoeenol equilibrium between two organolithium species.
However, we have found in our laboratory that the yield of the
C-silylated products increases relative to the O-silylated one when
the lithium enolote salt is allowed to stand at ꢀ78 ꢁC for 2 h prior to
quenching it with an alkylchlorosilane. On the other hand, if the
enolate salt is immediately quenched at ꢀ78 ꢁC, the yield of the
C-silylated product is reduced significantly, whereas that of the
O-silylation product increases. We also found that after adding an
alkylchlorosilane, a continuous cooling of the reaction mixture at
ꢀ78 ꢁC for an additional 2 h was necessary to maintain a high yield
of the C-silylated products. It seems that the rate of silylation at the
carbon is slow at this low temperature, and therefore, a rapid
heating to room temperature shifts the equilibrium toward
O-silylation, resulting in a decrease in the yield of C-silylated
products. Progress toward understanding the reactivity of the
synthetically important lithium enolates relies on the ability to
characterize their aggregation states in solutions. Mechanistic
studies of enolate alkylations have afforded diverse hypotheses.
The reaction could proceed via dimer17 or tetramer-based18 forms.
a,
a
0-disilyldicarboxylic acids linked by a siloxyl moiety and no
further attempts were made to optimize its yield.
After the successful preparation of -silylacetic acids from tri-
a
methylsilylacetate, we attempted the reaction with trimethylsilyl
esters of other carboxylic derivatives of the type RCH2CO2SiMe3
(R¼Me, Cl, Ph) in order to prepare new derivatives of
a-silylcar-
boxylic acids. The -silylation of these esters was examined with
a
diphenylmethylchlorosilane, which was preferred over trimethyl-
chlorosilane due to its being a ‘softer’ acid (it has a stronger elec-
tronic withdrawing effect) and therefore, more reactive toward the
carbon terminus of the ambient nucleophile. Consequently,
C-silylation is predominant with diphenylmethylchlorosilane,
whereas O-silylation is favored with trimethylchlorosilane.22
However, there are other factors to be considered (inductive and
steric effects). For example, it is noteworthy that silylation of the
trimethylsilyl ester of propionic acid under the same experimental
conditions gave the desired product in a low yield (Scheme 3). This
trend could be explained either due to the inductive effect exerted
by the methyl group that destabilizes the carbanion intermediate,
or by a steric effect. On the other hand, it is conceivable that Cl and
Ph substituents in trimethylsilyl esters of chloroacetic and phe-
Recently, it has been shown that C-alkylation of b-amino esters
proceeds via hexameric enolates, which exist as such both in the
solid-state and in THF solutions.19
It is noteworthy that recent work has proven that the hexameric
enolate of b-amino esters undergo C-alkylation directly without
deaggregation.20 In addition, Hudrlik and co-workers reported on
an unusual O/C migration of the trimethylsilyl group under
similar experimental conditions.21 It is conceivable that the 2 h
reaction time before quenching (as described above) is required to
obtain the desired aggregation state that allows efficient C-silyla-
tion at a low temperature. By employing the experimental condi-
tions described above (Scheme 2), we have synthesized different
nylacetic acids should stabilize a carbanion at the a position, aiming
toward higher yields of the desired C-silylated products. Surpris-
ingly, the yield of the product from ClCH2CO2SiMe3 decreased
compared to that obtained from CH3CO2SiMe3, while that from
PhCH2CO2SiMe3 was less then 10% (Scheme 3).
types of known and novel
as is shown in Table 1 (entries 1e12).
a-silylacetic acids in good isolated yields,
1. LiN(iPr)2 / -78 0C / 2h
SiR'3
2. R'3SiCl / -78 0C / 2h
R-CH2CO2-SiMe3
R-CH-CO2H
3. H+
1. LiN(iPr)2 / -78 0C / 2h
2. R3SiCl / -78 0C / 2h
3. H+
R = H, R' = Ph2(Me)
R = Me, R' = Ph2(Me)
R = Cl, R' = Ph2(Me)
R = Ph, R' = Ph2(Me)
78%
25%
CH3CO2SiMe3
R3SiCH2CO2H
50%
Scheme 2. Experimental conditions for
a-silylation of trimethylsilylacetate.
<10%
Scheme 3.
a-Silylation of trimethylsilyl esters of carboxylic acids.
The observation that C-silylation of the tert-butylester of acetic
acid (analogue of trimethylsilylacetate) is quite sensitive to the
bulkiness of the silyl group or ‘softness’ of the electrophilic silicon
moiety is well known.22 Now it can be seen that by using trime-
thylsilylacetate as the starting material and applying the experi-
mental conditions mentioned above, the outcome of the reaction is
not sensitive to the nature of the silyl group, whether it is an alkyl,
In spite of the few examples, the above results could indicate
that in the latter two cases the inductive effect plays a minor role
compared with steric effects. As a consequence, it is possible that
the yield of the C-silylated product is dictated mostly by the bulk-
iness of the group attached to the carbon at the
a position that
increases in the order: H>Cl>Me>Ph.

A.V. Shtelman, J.Y. Becker / Tetrahedron 67 (2011) 1135e1141
1137
Table 2
Synthesis of new
In order to synthesize a-substituted silylcarboxylic acids in good
yields, we have utilized an alternative route of direct alkylation of
their dianions. The dianion of a carboxylic acid is a good nucleo-
a
-silylcarboxylic acids
Entry
RX
Product
Yielda, %
phile and the most nucleophilic site is at the
a carbanionic position.
However, in general, the limitation in alkylation of metalated
straight-chain acids lies in their partial solubility in THF. In order to
overcome this problem, hexamethylphosphoramide (HMPA) was
used by others, previously.23 Interestingly, we have found that the
dianion of diphenyl(methyl)silylacetic acid is readily dissolved in
1
(14)
90
2
3
(15)
(16)
85
81
THF. Moreover, a silyl group at the
electron accepting effect and therefore should stabilize it (‘the
effect’).24 Consequently, such an effect contributes to the stabili-
zation of the -carbanions of -silylcarboxylic acids, and this could
a position to carbanions has an
a
-
a
a
cause them to be more nucleophilic and reactive toward electro-
philes. Earlier it was reported that the dianion of trimethylsilyl-
acetic acid is readily alkylated with benzyl, allyl, and primary alkyl
halides.12 Similarly, we have found that the dianion of diphenyl
(methyl)silylacetic acid [Ph2(Me)SiCH2CO2H] is also readily alky-
lated to give new a-silylcarboxylic acids in good yields, as illus-
trated in Scheme 4 and Table 2 (entries 1–5). Alkylation of the
dianion of diphenyl(methylsilyl)acetic acid can be representative
4
(17)
(18)
72
60
5b
for other type of
phenylsilyl moieties) and may be utilized for the preparation of
new -silylcarboxylic acid derivatives.
a-silylacetic acids (especially for those containing
a
1. LiN(iPr)2 / 0 0C / 1h
R
6
7
(19)
(20)
75c
2. RX / RT
Ph2(Me)SiCHCO2H
(60-90%)
Ph2(Me)SiCH2CO2H
3. H+
[RX = benzyl, allyl, alkyl, Me3Ge halides]
Scheme 4. Alkylation of dianion of Ph2(Me)SiCH2CO2H.
87
Thus, as it is shown inTable 2, the dianion of diphenyl(methylsilyl)
acetic acid is readily alkylated with benzyl- and allylbromides to give
a
Isolated yield.
An excess of alkyl iodide was used.
This product involves a mixture of two isomers in 3/1 ratio and one of them was
b
c
a
-silyl products in excellent yields (entries 1 and 2). Also, good results
are obtained with common primary alkyl iodides including less re-
active -branched primary alkyl iodides (Table 2, entries 3 and 4).
characterized by X-ray (vide infra).
b
However, its reaction with isopropyl iodide provides only 40% of the
desired alkylated product. In order to improve the yield of this
product, anexcessofisopropyl iodide(5 equiv)wasused to obtainthe
same product in moderate yield (60%, entry 5).
The decrease in the yield of the alkylated product upon using
a secondary alkyl iodide is not surprising since it is well established
that alkylation of acid dianions with secondary and tertiary halides
mostly causes elimination.25 However, in our case, interestingly, the
major product from alkylation of the dianion with a secondary alkyl
1H and 13C NMR spectroscopy. Both isomers have the same mo-
lecular ion mass, as detected by MALDI-TOF measurements.
Therefore, it is reasonable to assume that the two isomers are di-
astereoisomers of the obtained dicarboxylic acid derivative, which
contains two chiral centers (however, the possibility that the cis
isomer is also formed cannot entirely be ruled out). Re-
crystallization of 19 from a mixture of hexane/ethyl acetate gave
good quality crystals of a major isomer (19a) whose structure has
been determined by X-ray analysis (vide infra) and 1H and 13C NMR
spectroscopy.
halide is an a-alkylated product.
Previously, it was shown that the orientation of O- and C-silylation
of dilithiated carboxylic acids depends on the steric bulk of the alkyl
group attached to the
a
position of the carboxylate anion.26 Indeed,
2.1. Molecular structure of compound 19a
treatment of the dianion of diphenyl(methyl)silylacetic acid with
2 equiv of trimethylchlorosilane at ꢀ78 ꢁC afforded only O-silylated
product. However, germylation of the same dianion using 2 equiv of
trimethylchlorogermane under the same experimental conditions
yielded the C-alkylation product in a good isolated yield, as is illus-
trated inTable 2 (entry7). Thisinterestingcompound is an example of
Colorless crystals suitable for X-ray crystallography were
obtained by dissolving a mixture of the two isomers in ethyl ace-
tate, followed by addition of hexane at ambient temperature and
slow evaporation of the solvents. Single crystal X-ray diffraction
analysis reveals that the major isomer (19a) in the solid-state packs
in the monoclinic P2/n space group with crystal cell parameters:
producing new class of
After the successful preparation of a new generation of
carboxylic acids by alkylation of its dianion, we have attempted to
prepare new
0-disilyldicarboxylic acid by the same method. For
a-silyl-a-germylacetic acids.
ꢀ
ꢀ
ꢀ
a
-silyl-
Z¼4, dc¼1.186; a¼14.302 (15) A, b¼14.388 (15) A, c¼15.819 (17) A,
a
¼90ꢁ,
b
¼103.64 (6)ꢁ,
g
¼90ꢁ, R1¼0.0574.
a,
a
Interestingly, the X-ray structure shows that 19a crystallizes in
this purpose (E)-1,4-dibromo-2-butene was used as the alkylating
agent, as illustrated in Table 2 (entry 6) and in Scheme 5.
After the alkylation and work up procedure, a mixture of two
isomers of 19 (a and b) was obtained in 3/1 ratio, as determined by
the unit cell as a mixture of trans-enantiomers (R,R) and (S,S)
interlinked by OeH/O intermolecular hydrogen bonds. The mo-
lecular structure of 19a is shown in Fig. 1 with selected bond
lengths and angles given in the figure caption.

1138
A.V. Shtelman, J.Y. Becker / Tetrahedron 67 (2011) 1135e1141
1. LiN(iPr)2 / 0 0C / 1h
Br
CO2H
Ph2(Me)Si
HO2C
Br
Si(Me)Ph2
2.
Ph2(Me)SiCH2CO2H
3. H+
Scheme 5. Alkylation of dianion by (E)-1,4-dibromo-2-butene.
R1R2R3SiCH2CO2H. Diisopropylamine (24.0 mmol, 3.4 mL) and an-
hydrous tetrahydrofuran (40 mL) were added to an oven-dried
round-bottomed flask (250 mL) equipped with a magnetic stirring
bar. The mixture was cooled to ꢀ78 ꢁC prior to drop-wise addition
of n-butyllithium 1.6 M (24.0 mmol, 15.0 mL). The mixture was
warmed at room temperature for 15 min and cooled again to
ꢀ78 ꢁC. Trimethylsilylacetate (CH3CO2SiMe3) (21.0 mmol, 3.2 mL)
was added drop-wise to the cooled solution of LDA over 15 min and
the reaction mixture was stirred for 2 h at ꢀ78 ꢁC. Then diphe-
nylmethylchlorosilane (24.0 mmol, 5.0 mL) in dry THF (5 mL) was
added drop-wise to the solution over 10 min. The reaction mixture
was then stirred at ꢀ78 ꢁC for two additional hours and allowed to
reach room temperature overnight. A solution of brine (30 mL) was
added and the reaction mixture was carefully acidified with 1 N HCl
to pH¼1. The aqueous layer was extracted with diethyl ether
(30 mLꢂ3) and the combined organic extracts were washed with
water (20 mLꢂ2), dried over anhydrous MgSO4, filtered, and con-
centrated in vacuo. The residual product was crystallized from
hexane to give diphenylmethylsilylacetic acid (6) (4.2 g, 78%). Mp:
104e105 ꢁC. NMR and MS spectrums of acids (1e6) were published
in our previous work (see Ref. 15). The above procedure for 6 was
also applied to 7e13.
ꢀ
Fig. 1. ORTEP diagram of major isomer 19a. Selected bond lengths [A] and bond angels
[deg]: C4_Si1 1.933(3); C4_C6 1.500(4); C6_O4 1.247(4); C6_O1 1.317(4); C3_C4 1.538(4);
C(3)_C(30) 1.527(5); C30_C32 1.294(5); C32_C30_C3 128.4(3); C30_C3_C4 115.8(3);
C6_C4_C3 112.6(3); C6_C4_Si1 111.0(2); C3_C4_Si1 108.7(2); O4_C6_O1 122.1(3);
O4_C6_C4 121.9(3); O1_C6_C4 121.9(3).
In summary, we have successfully prepared new
acids in good isolated yields by a convenient and reproducible
method. Novel -alkyl, -silylcarboxylic acids were prepared by
alkylation of the dianions of -silylacetic acids that should allow
the easy way for the synthesis of highly branched -silylcarboxylic
0-disilyldicarboxylic acids
a-silylacetic
a
a
a
a
acids. In addition, two new types of
were synthesized and characterized.
a,a
3.2.1. Dimethyl(chloromethyl)silylacetic acid (7). Trimethylsilylacetate
(20 mmol, 3.0 mL) was reacted with LDA (24 mmol) and dimethyl
(chloromethyl)chlorosilane (24 mmol, 3.1 mL). After standard work
up procedure the crude product was purified by a silica gel flash
column chromatography by eluting with hexane/acetone (9/1e8/2; v/
v) that afforded product 7 (2.3 g, 68%) as a white solid. Mp: 42e44 ꢁC.
3. Experimental section
3.1. General
1H NMR (500 MHz, CDCl3)
d 0.28 (s, 6H, Si(CH3)2), 2.06 (s, 2H, SiCH2
-
CO2H), 2.89 (s, 2H, SiCH2Cl), 10.92 (br, 1H, CO2H). 13C NMR (125 MHz,
CDCl3) ꢀ4.60 (Si(CH3)2), 24.30 (SiCH2CO2H), 29.21 (SiCH2Cl), 179.35
Reagents and solvents were used as received from commercial
sources (SigmaeAldrich, BioLab, Gelest (chlorosilanes)). Anhydrous
tetrahydrofuran was dried over sodiumebenzophenone under ni-
trogen and fractionally distilled. Diisopropylamine was dried over
CaH2 andfractionallydistilled.Chloroaceticacidwascrystallizedfrom
chloroform and dried over P2O5 in a vacuum desiccator. Reactions
were conducted under an atmosphere of dry nitrogen in over-dried
glassware. All air-sensitive liquids were transferred using standard
syringe needle techniques under an atmosphere of argon. NMR
spectra 1H (500.13 MHz), 13C (125.76 MHz), and 29Si (99.36 MHz)
were collected using a Bruker Avance DMX-500 MHz or Bruker ARX-
200 MHz (for 1H (200 MHz)) spectrometers. The chemical shifts are
given in parts per million relative to the residual proton signal of the
solvent (CDCl3) 7.26 ppm (1H) and CDCl3 77.0 ppm (13C) and then
referenced to (CH3)4Si (0.00 ppm); J values are given in Hertz. Mo-
lecular mass of materials was determined by (HRESI MS) Thermo
Scientific LTQ Orbitrap XL ETD equipped with Electrospay Ionization
(ESI-MS) and by (MALDI-TOF) Bruker Daltonics (Reflex IV). Mass
spectral datawere reported inunits ofmass to charge (m/z). IRspectra
were recorded using Impact 410 spectrometer. Single crystal X-ray
diffractionmeasurementswereperformedonaBrukerSmartApexon
D8-Goniometer at low temperature (114 K).
(SiCH2CO2H). 29Si NMR (100 MHz, CDCl3) 5.92. IR (KBr):
n (SiCH2CO2H)
at 1690 and 3150 cmꢀ1. HRMS (ESI positive mode): m/z calcd for
C5H11ClO2Si [MþNa]þ: 189.0109; found 189.0109.
3.2.2. Dimethyl(cyclohexyl)silylacetic acid (8). Trimethylsilylacetate
(20 mmol, 3.0 mL) was reacted with LDA (24 mmol) and dimethyl
(chloromethyl)chlorosilane (23 mmol, 4.3 mL). After standard work
up procedure the crude product was purified by a silica gel flash
column chromatography by eluting with hexane/acetone (9/1e8/2;
v/v) that afforded product 8 (2.4 g, 60%) as a white solid. Mp:
52e54 ꢁC. 1H NMR (500 MHz, CDCl3)
d 0.09 (s, 6H, Si(CH3)2),
0.70e0.75 (m, 1H, cyclohexyl), 1.06e1.13 (m, 2H, cyclohexyl),
1.18e1.24 (m, 3H, cyclohexyl), 1.68e1.75 (m, 5H, cyclohexyl), 1.90 (s,
2H, SiCH2CO2H), 10.47 (br, 1H, CO2H). 13C NMR (125 MHz, CDCl3)
ꢀ5.13 (Si (CH3)2), 24.13 (SiCH2CO2H), 25.29, 26.75, 26.96, 27.81 (Si-
cyclohexyl), 180.47 (SiCH2CO2H). 29Si NMR (100 MHz, CDCl3) 5.41. IR
(KBr):
n
(SiCH2CO2H) at 1686 and 2913 cmꢀ1. HRMS (ESI positive
mode): m/z calcd for C10H20O2Si [MþNa]þ: 223.1130; found 223.1123.
3.2.3. Dimethyl(benzyl)silylacetic acid (9). Trimethylsilylacetate (20
mmol, 3.0 mL) was reacted with LDA (24 mmol) and dimethyl
(benzyl)chlorosilane (23 mmol, 4.5 mL). After standard work up
procedure the crude product was purified by a silica gel flash col-
umn chromatography by eluting with hexane/acetone (9/1e8/2;
v/v) that afforded product 9 (2.98 g, 72%), as a white solid. Mp:
3.2. General procedure for the synthesis of a-silylacetic acids
A general procedure for the synthesis of diphenyl(methyl)silyl-
acetic acid (6) is representative for all -silylacetic acids of type
a

A.V. Shtelman, J.Y. Becker / Tetrahedron 67 (2011) 1135e1141
1139
45e47 ꢁC. 1H NMR (500 MHz, CDCl3)
d
0.24 (s, 6H, Si(CH3)2), 2.03 (s,
2H, SiCH2CO2H), 7.33e7.40 (m, 6H, 2Ph), 7.58e7.60 (m, 4H, 2Ph),
10.96 (br, 1H, CO2H). 13C NMR (125 MHz, CD3OD) ꢀ0.71 (SiCH3),
28.72 (SiCH2CO2H), 128.93, 131.11, 134.42, 137.58 (Ph), 175.96
2H, SiCH2CO2H), 2.34 (s, 2H, SiCH2Ph), 7.15e7.22 (m, 3H, Ph),
7.32e7.36 (m, 2H, Ph), 10.97 (br, 1H, CO2H). 13C NMR (125 MHz,
CDCl3) ꢀ3.49 (Si (CH3)2), 25.09 (SiCH2Ph), 25.14 (SiCH2CO2H),
124.45, 128.23, 128.38, 138.66 (Ph), 179.53 (SiCH2CO2H). 29Si NMR
(SiCH2CO2H). 29Si NMR (100 MHz, CD3OD) ꢀ4.94. IR (KBr):
n
(SiCH2CO2H) at 1690 and 2972 cmꢀ1 (SiOSi) 1090 cmꢀ1
, . MS
(100 MHz, CDCl3) 4.55. IR (KBr):
n
(SiCH2CO2H) at 1691 and
(MALDI-TOF): m/z calcd for C18H22O5Si2 [MþNaþ]: 397.090; found
2967 cmꢀ1. HRMS (ESI positive mode): m/z calcd for C11H16O2Si
[MþNa]þ: 231.0812; found 231.0804.
397.061; [MþKþ]; 413.064, found 413.027.
3.3. General procedure for the synthesis of alkylated
3.2.4. Methyl(phenyl)vinylsilylacetic acid (10). Trimethylsilylacetate
(20 mmol, 3.0 mL) was reacted with LDA (24 mmol) and methyl
(phenyl)vinylchlorosilane (23 mmol, 4.1 mL). After standard work
up procedure the crude product was purified by a silica gel flash
column chromatography by eluting with hexane/acetone (9/1; v/v)
that afforded product 10 (3.05 g, 74%), as a colorless oil. 1H NMR
a
-silylcarboxylic acids
General procedure for the synthesis of a-diphenyl(methyl)silyl-
hydrocynnamic acid (14) is representative for all a-silylcarboxylic
acids of type Ph2(Me)SiCH(R)CO2H.
Diisopropylamine (14.8 mmol, 2.1 mL) and anhydrous tetrahy-
drofuran (30 mL) were added to an oven-dried round-bottomed
flask (100 mL) equipped with a magnetic stirring bar. The mixture
was cooled to ꢀ78 ꢁC prior to drop-wise addition of n-butyllithium
1.6 M (14.8 mmol, 9.3 mL). The mixture was warmed to room
temperature for 15 min and cooled to 0 ꢁC. Diphenyl(methyl)silyl-
acetic acid (6.75 mmol, 1.73 g) in dry THF (5 mL) was added drop-
wise over 10 min to the LDA solution and the mixture was stirred
for 1.5 h at the same temperature to complete the formation of the
dianion. Then benzylbromide (7.42 mmol, 0.88 mL) was added
drop-wise over 2 min to the dianion solution and the mixture was
stirred overnight at room temperature. The reaction mixture was
diluted with hexane (20 mL) and the organic phase washed twice
with a solution of brine (30 mLꢂ2). The aqueous phase was care-
fully acidified with 1 N HCl to pH¼2 and then extracted with diethyl
ether (20 mLꢂ3). The combined organic extracts were washed with
water (30 mL), dried over anhydrous MgSO4, filtered, and concen-
trated in vacuo. The residual product was crystallized from hexane
(500 MHz, CDCl3)
d
0.54 (s, 3H, SiCH3), 2.23 (d, 2J¼16.0 Hz, 1H,
SiCH2CO2H), 2.26 (d, 2J¼16.0 Hz, 1H, SiCH2CO2H), 5.87 (dd,
3J¼3.5 Hz, 2J¼20.0 Hz, 1H, SiCH]CH2), 6.18 (dd, 3J¼3.5 Hz,
2J¼14.5 Hz, 1H, SiCH]CH2), 6.34 (dd, 2J¼14.5 Hz, 2J¼20.0 Hz, 1H,
SiCH]CH2), 7.39e7.42 (m, 3H, Ph), 7.57e7.58 (m, 2H, Ph), 10.91 (br,
1H, CO2H). 13C NMR (125 MHz, CDCl3) ꢀ4.67 (SiCH3), 25.28
(SiCH2CO2H), 128.04, 129.85, 134.08, 134.22, 134.73, 135.33
(PhþVinyl), 179.38 (SiCH2CO2H). 29Si NMR (100 MHz, CDCl3)
ꢀ11.07. IR (neat):
n
(SiCH2CO2H) at 1696 and 3053 cmꢀ1. HRMS (ESI
positive mode): m/z calcd for C11H14O2Si [MþNa]þ: 229.0655;
found 229.0648.
3.2.5. Diphenyl(vinyl)silylacetic acid (11). Trimethylsilylacetate (20
mmol, 3.0 mL) was reacted with LDA (24 mmol) and diphenyl(vi-
nyl)chlorosilane (23 mmol, 5.1 mL). After standard work up pro-
cedure the residual product was crystallized from hexane to give
product 11 (4.3 g, 80%) as a white solid. Mp: 95e97 ꢁC. 1H NMR
(500 MHz, CDCl3)
d
2.49 (s, 2H, SiCH2CO2H), 5.80 (dd, 3J¼3.5 Hz,
to give
a-diphenyl(methyl)silylhydrocynnamic acid (14) (2.1 g, 90%),
2J¼20 Hz, 1H, SiCH]CH2), 6.24 (dd, 3J¼3.5 Hz, 2J¼14.5 Hz, 1H,
SiCH]CH2), 6.48 (dd, 2J¼14.5 Hz, 2J¼20.0 Hz, 1H, SiCH]CH2),
7.30e7.53 (m, 6H, 2Ph), 7.54e7.55 (m, 4H, 2Ph),10.75 (br,1H, CO2H).
13C NMR (125 MHz, CDCl3) 24.18 (SiCH2CO2H), 127.93, 129.95,
132.34, 132.58, 135.15, 137.35 (Phþvinyl), 178.88 (SiCH2CO2H). 29Si
as a white solid. Mp: 119e121 ꢁC. 1H NMR (500 MHz, CDCl3)
d
0.72
(s, 3H, SiCH3), 2.76 (dd, 2J¼14.5 Hz, 3J¼2.5 Hz, 1H, SiCHCH2), 2.93
(dd, 2J¼12.0 Hz, 3J¼2.5 Hz, 1H, SiCHCH2), 3.11 (dd, 1H, 2J¼12.0 ,
14.5 Hz, 1H, SiCHCH2), 7.09e7.62 (m, 15H, 3Ph), 11.35 (br, 1H, CO2H).
13C NMR (125 MHz, CDCl3) ꢀ5.25 (SiCH3), 33.08 (PhCH2Si), 38.32
(SiCHCO2H), 126.14, 127.98, 128.06, 128.39, 129.87, 129.93, 133.49,
133.91, 134.74, 134.80, 134.92, 141.53 (2Ph), 179.50(CO2H). 29Si NMR
NMR (100 MHz, CDCl3) ꢀ16.11. IR (KBr):
n (SiCH2CO2H) at 1696 and
2957 cmꢀ1. MS (MALDI-TOF): m/z calcd for C16H16O2Si [MþNaþ]:
291.082; found 291.750; [MþKþ]: 307.056; found 307.671.
(100 MHz, CDCl3) ꢀ5.29. IR (KBr):
n (SiCHCO2H) at 1683 and
3055 cmꢀ1. HRMS (ESI positive mode): m/z calcd for C22H22O2Si
[MþNa]þ: 369.1287; found 369.1275. The above procedure for 14
was also applied to 15e20.
3.2.6. Trivinylsilylacetic acid (12). Trimethylsilylacetate (20 mmol,
3.0 mL) was reacted with LDA (24 mmol) and trivinylchlorosilane
(23 mmol, 3.6 mL). After standard work up procedure the crude
product was purified by a silica gel flash column chromatography
by eluting with hexane/acetone (9/1; v/v) that afforded product 12
3.3.1.
a-Diphenyl(methyl)silyl-4-pentenoic acid (15). Diphenyl
(methyl)silylacetic acid (6.98 mmol, 1.79 g) was reacted with LDA
(15.4 mmol) and allylbromide (7.68 mmol, 0.66 mL). After stan-
dard work up procedure the crude product was crystallized from
hexane to give acid (15) (1.76 g, 85%) as a white solid. Mp:
(2.39 g, 71%), as a colorless oil. 1H NMR (500 MHz, CDCl3)
d 2.13 (s,
2H, SiCH2CO2H), 5.81e5.94 (m, 3H, Si(CH]CH2)3), 6.13e6.19 (m,
6H, Si(CH]CH2)3), 10.36 (br, 1H, CO2H). 13C NMR (125 MHz, CDCl3)
23.98 (SiCH2CO2H), 132.16 (Si(CH]CH2)3), 136.28 (Si(CH]CH2)3),
115e117 ꢁC. 1H NMR (500 MHz, CDCl3)
d 0.68 (s, 3H, SiCH3),
178.86 (SiCH2CO2H). 29Si NMR (100 MHz, CDCl3) ꢀ21.76. IR (neat):
n
2.16e2.21 (m, 1H, CH2]CHCH2CH), 2.51e2.59 (m, 1H, CH2]
CHCH2CH), 2.71 (dd, 3J¼7.5 , 9.0 Hz, 1H, CH2]CHCH2CH),
4.97e5.04 (m, 2H, CH2]CHCH2), 5.75e5.83 (m, 1H, CH2]CHCH2),
7.26e7.45 (m, 10H, 2Ph), 11.59 (br, 1H, CO2H). 13C NMR (125 MHz,
CDCl3) ꢀ5.26 (SiCH3), 31.29 (CH2]CHCH2CH), 35.85 (CH2]
CHCH2CH), 115.27 (CH2]CH), 127.94, 127.99, 129.84, 133.57,
133.92, 134.43, 134.72, 134.88, 137.27 (2PhþCH2]CH), 180.53
(SiCH2CO2H) at 1690 and 2950 cmꢀ1. HRMS (ESI positive mode): m/
z calcd for C8H12O2Si [MþNa]þ: 191.0499; found 191.0497.
3.2.7. 1,3-Bis-(1,3-diphenyl-1,3-dimethyl)siloxanedioic acid (13).
Trimethylsilylacetate (16 mmol, 2.4 mL) in dry THF (30 mL) was
reacted with LDA (18 mmol), and 1,3-dichloro-1,3-diphenyl-1,3-
dimethyl-disiloxane (7.3 mmol, 2.1 mL), in dry THF (5 mL), added
drop-wise to a cooled solution of enolate. After standard work up
procedure the crude product was purified by washing it in the
solution of hexane/ethyl acetate (95/5; v/v). The pure product
precipitated as small white crystals, which were collected by vac-
uum filtration and dried to give product 13 (0.82 g, 30%), as a white
(CO2H). 29Si NMR (100 MHz, CDCl3) ꢀ5.29. IR (KBr):
n (SiCHCO2H)
at 1679 and 2962 cmꢀ1. HRMS (ESI positive mode): m/z calcd for
C18H20O2Si [MþNa]þ: 319.1130; found 319.1120.
3.3.2.
a-Diphenyl(methyl)silylvaleric acid (16). Diphenyl(methyl)
silylacetic acid (4.36 mmol, 1.12 g) was reacted with LDA
(9.6 mmol) and propyliodide (4.80 mmol, 0.47 mL). After standard
work up procedure the crude product was crystallized from
solid. Mp: 117e120 ꢁC. 1H NMR (500 MHz, CD3OD)
d 2.13 (s, 6H,
2SiCH3), 2.20 (d, 2H, 2J¼12.0 Hz, SiCH2CO2H), 2.26 (d, 2J¼12.0 Hz,

1140
A.V. Shtelman, J.Y. Becker / Tetrahedron 67 (2011) 1135e1141
hexane to give acid (16) (1.05 g, 81%), as a white solid. Mp:
C
34H36O4Si2 [MꢀHþNa]þ: 586.197; found 586.529; [MꢀHþKþ]:
82e83 ꢁC. 1H NMR (500 MHz, CDCl3)
d
0.67 (s, 3H, SiCH3), 0.85 (t,
602.171, found 602.479. For isomer 19b: 1H NMR (500 MHz, CDCl3)
3J¼7.5 Hz, 3H, CH2CH2CH3) 1.23e1.30 (m, 1H, CHCH2CH2CH3),
1.37e1.47 (m, 2H, CHCH2CH2CH3), 1.81e1.88 (m, 1H,
CHCH2CH2CH3), 2.60 (dd, 3J¼12.0 , 11.5 Hz, CHCH2CH2CH3),
7.26e7.42 (m, 6H, 2Ph), 7.53e7.57 (m, 4H, 2Ph), 11,11 (br, 1H,
d 0.65 (s, 6H, SiCH3), 2.03e2.08 (m, 2H, CH2CH, overlapped with
19a), 2.34e2.40 (m, 2H, CH2CH, overlapped with 19a), 2.67 (dd,
2J¼2.0 Hz, 3J¼10.0 Hz, 2H, CH2CH), 5.48 (t, 3J¼3.0 Hz, 2H, CH]CH),
7.26e7.51 (m, 20H, 4Ph, overlapped with 19a), 10.57 (br, 2H,
CO2H). 13C NMR (125 MHz, CDCl3) ꢀ5.29 (SiCH3), 29.41 (CH2CH),
36.40 (CH2CH), 127.90, 129.72, 129.94, 129.91, 133.62, 133.74,
134.08, 134.72, 134.89, 134.92 (2Ph, CH]CH), 180.80 (CO2H). 29Si
NMR (100 MHz, CDCl3) ꢀ10.29.
CO2H). 13C NMR (125 MHz, CDCl3) ꢀ5.34 (SiCH3), 13.61 (CH2CH3)
,
23.54 (CH2CH3), 29.56 (CH2CH2CH3), 36.17 (CHCO2H), 127.86,
127.93, 129.70, 129.74, 134.01, 134.35, 134.73, 134.88 (2Ph), 180.86
(CO2H). 29Si NMR (100 MHz, CDCl3) ꢀ5.76. IR (KBr):
n (SiCHCO2H)
at 1667 and 2975 cmꢀ1. HRMS (ESI positive mode): m/z calcd for
C18H22O2Si [MþNa]þ: 321.1287; found 321.1275.
3.3.6.
a-Trimethylgermyl-a-diphenyl(methyl)silylacetic acid (20). A
procedure similar for that for 14 was used with the following
modifications. Diphenyl(methyl)silylacetic acid (2.02 mmol, 0.52 g)
was reacted with LDA (4.44 mmol) in dry THF (20 mL). The gener-
ated dianion was cooled to ꢀ78 ꢁC. Then trimethylchlorogermane
(4.44 mmol, 0.55 mL) was added drop-wise to the solution over
10 min. The reaction mixture was stirred at ꢀ78 ꢁC for 1.5 h and
allowed to reach room temperature overnight. A solution of brine
(30 mL) was added and the reaction mixture was carefully hydro-
lyzed with 1 N HCl to pH¼1. The aqueous layer was extracted with
diethyl ether (30 mLꢂ3) and the combined organic extracts were
washed with water (30 mL), dried over anhydrous MgSO4, filtered,
and concentrated in vacuo. The residual product was crystallized
from hexane to give acid (20) (0.67 g, 87%). Mp: 68e70 ꢁC. 1H NMR
3.3.3.
a-Diphenyl(methyl)silyl-4-methylvaleric acid (17). Diphenyl
(methyl)silylacetic acid (4.98 mmol, 1.28 g) was reacted with LDA
(11.0 mmol) and 1-iodo-2-methylpropane (5.48 mmol, 0.63 mL). Af-
ter standard work up procedure the crude product was crystallized
from hexane to give acid (17) (1.12 g, 72%), as a white solid. Mp:
84e86 ꢁC. 1H NMR (500 MHz, CDCl3)
d 0.06 (s, 3H, SiCH3), 0.25 (d,
3J¼7.0 Hz, 3H, CH(CH3)2), 0.26 (d, 3J¼7.0 Hz, 3H, CH(CH3)2), 0.62e0.67
(m,1H, CH2CH(CH3)2),1.04e1.08 (m, 1H, CH2CH(CH3)2),1.36e1.43 (m,
1H, CH2CH(CH3)2), 2.28 (dd, 3J¼13.0 ,13.0 Hz,1H, CHCO2H), 7.35e7.45
(m, 6H, 2Ph), 7.56e7.61 (m, 4H, 2Ph), 11.05 (br, 1H, CO2H). 13C NMR
(125 MHz, CDCl3) ꢀ5.39 (SiCH3), 21.19 (CH(CH3)2), 22.99 (CH(CH3)2),
28.60 (CH2CH(CH3)2), 34.36 (CH2CH(CH3)2), 36.19 (CHCO2H), 127.85,
127.94, 129.69, 129.75, 133.91, 134.35, 134.72, 134.88 (2Ph), 181.52
(500 MHz, CDCl3) d 0.09 (s, 9H, Ge(CH3)3), 0.77 (s, 3H, SiCH3), 2.27 (s,
(CO2H). 29Si NMR (100 MHz, CDCl3) ꢀ5.15. IR (KBr):
n
(SiCHCO2H) at
1H, SiCHGe), 7.31e7.60 (m, 10H, 2Ph), 9.47 (br, 1H, CO2H). 13C NMR
(125 MHz, CDCl3) ꢀ3.51(SiCH3), ꢀ0.44 (Ge(CH3)3), 27.84 (SiCHGe),
127.74, 127.86, 129.30, 129.52, 134.49, 134.59, 136.06, 136.56 (2Ph),
1676 and 2957 cmꢀ1. HRMS (ESI positive mode): m/z calcd for
C19H24O2Si [MþNa]þ: 335.1443; found 335.1434.
181.23 (CO2H). 29Si NMR (100 MHz, CDCl3) ꢀ13.78. IR (KBr):
n (Si(Ge)
3.3.4.
a-Diphenyl(methyl)silyl-3-methyl-butanoic acid (18). Diphenyl-
CHCO2H) at 1653 and 3045 cmꢀ1. HRMS (ESI positive mode): m/z
calcd for C18H24GeO2Si [MþNa]þ: 395.0660; found 395.0648.
(methyl)silylacetic acid (2.14 mmol, 0.55 g) was reacted with LDA
(4.7 mmol) and excess of 2-iodo-propane (10.7 mmol, 1.1 mL). After
standard work up procedure the crude product was purified by
a silica gel flash column chromatography by eluting with hexane/
ethyl acetate (7/3; v/v) that afforded product 18 (0.64 g, 60%), as
Acknowledgements
This research was supported (in part) by the Israel Science
Foundation (grant No. 317/07). The authors are thankful to Mrs. E.
Solomon for technical assistance.
a white solid. Mp: 130e131 ꢁC. 1H NMR (200 MHz, CDCl3)
d 0.73 (s,
3H, SiCH3), 0.83 (d, 3J¼6.6 Hz, 3H, CH(CH3)2), 1.02 (d, 3J¼6.4 Hz, 3H,
CH(CH3)2), 2.14e2.25 (m, 1H, CH(CH3)2), 2.40 (d, 3J¼10.2 Hz, 1H,
SiCH), 7.33e7.40 (m, 6H, 2Ph), 7.58e7.63 (m, 4H, 2Ph), 10.94 (br, 1H,
CO2H). 13C NMR (125 MHz, CDCl3), ꢀ4.42 (SiCH3), 22.95 (CH (CH3)2),
23.45 (CH(CH3)2), 28.51 (CH(CH3)2), 45.02 (SiCH), 127.76, 127.80,
127.85,129.55,134.61,134.71,134.78,134.82 (2Ph),181.55 (CO2H). 29Si
Supplementary data
CIF files giving crystallographic data including a full list of in-
teratomic bond lengths and angles for the reported isomer 19a are
deposited at CCDC 796633. Copies of 1H NMR, 13C NMR, and 29Si
NMR spectra of compounds 7e20 are provided. Supplementary
data associated with this article can be found in the online version,
at doi:10.1016/j.tet.2010.12.020. These data include MOL files and
InChIKeys of the most important compounds described in this
article.
NMR (100 MHz, CDCl3) -11.35. IR (KBr):
n (SiCHCO2H) at 1678 and
2955 cmꢀ1. MS (MALDI-TOF): m/z calcd for C18H22O5Si2 [MþNa]:
321.129; found 321.184.
3.3.5. (E)-2R(S),7R(S)-Bis(diphenyl(methyl)silyl)oct-4-enedioic acid
(19a). Diphenyl(methyl)silylacetic acid (2.83 mmol, 0.73 g) was
reacted with LDA (6.23 mmol) in dry THF (20 mL) and trans-1,4-
dibromo-2-butene (1.32 mmol, 0.28 g) in dry THF (3 mL), added
drop-wise over 5 min to a cool solution of dianion. After standard
work up procedure the crude product was washed with warm
hexane (20 mL), then collected by vacuum filtration and dried to
give a mixture of two isomers in 3/1 ratio (0.55 g, 75%), as de-
termined by 1H NMR. Recrystallization of this compound from
a mixture of hexane/ethyl acetate gave a good quality of colorless
crystals of major isomer 19a. Mp: 184e185 ꢁC. 1H NMR (500 MHz,
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