Preparation of silyl esters by ZnCl2-catalyzed dehydrogenative cross-coupling of carboxylic acids and silanes

Article abstract of DOI:10.1055/s-2006-941576

An efficient route to silyl esters by dehydrogenative cross-coupling of silanes and carboxylic acids was investigated. Treatment of silanes and carboxylic acids in the presence of 15mol% of ZnCl₂ in DMF at 120 °C afforded the corresponding silyl esters in good yields under relatively mild conditions. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-941576

LETTER
1431
Preparation of Silyl Esters by ZnCl -Catalyzed Dehydrogenative
2
Cross-Coupling of Carboxylic Acids and Silanes
P
G
reparation of
S
ily
u
l
Esters o-Bin Liu*
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, P. R. of China
Fax +86(21)65642261; E-mail: liuguobin@fudan.edu.cn
Received 19 February 2006
chemical and physical properties as the desired unsaturat-
ed silyl esters. In this communication, we wish to report
Abstract: An efficient route to silyl esters by dehydrogenative
cross-coupling of silanes and carboxylic acids was investigated.
Treatment of silanes and carboxylic acids in the presence of
the first finding that ZnCl -catalyzed dehydrogenative
2
1
5mol% of ZnCl in DMF at 120 °C afforded the corresponding cross-coupling of carboxylic acids and silanes affords the
2
silyl esters in good yields under relatively mild conditions.
corresponding silyl esters selectively without the forma-
tion of any overreduced side products. The results are
summarized in Table 1 and Scheme 1.
Key words: silyl esters, zinc chloride, silanes, carboxylic acids
O
O
Silyl esters are important intermediates for the preparation
of many types of useful functional polymeric substrates
such as easily degradable poly(silyl ester)s, which are cur-
cat. ZnCl2
+
HSiR3
+
H2
R1
OH
R1
DMF, N , 120 °C
2
OSiR3
1
Scheme 1
rently much in demand. A lot of effort has been devoted
to developing a simple, practical and atom-economical
method for the preparation of stable and easily isolable
silyl esters. Usually, silyl esters are prepared from the
cross-coupling of carboxylic acids and chlorosilanes.2
Corrosive gaseous hydrogen chloride is unavoidably
produced in these reactions. Normally, a stoichiometric or
even an excess amount of base such as amines or ammo-
nia is needed to consume the HCl gas formed. Moreover,
chlorosilanes themselves are prepared by the chlorination
Dehydrocoupling reactions were carried out by heating a
mixture of the corresponding carboxylic acid, silane and
ZnCl (15 mol%) in N,N-dimethylformamide (DMF) at
2
1
20 °C for several hours under a N atmosphere (Table 1,
2
monitored by GC). In order to optimize the reaction, the
ZnCl -catalyzed coupling of acetic acid and triethylsilane
2
was performed under various reaction conditions. The de-
hydrocoupling was found to be complete after 24 hours at
1
c
1
20 °C, when using 15 mol% of ZnCl , giving the corre-
of silanes, either with chlorine gas or with hydrochloric
2
3
sponding triethylsilyl acetate in 83% yield (run 1). How-
ever, the optimal reaction time could not be shortened,
acid under Pd/C catalysis. Thus, usually two steps are
needed to prepare silyl esters from the corresponding si-
lanes. Therefore, it is of interest to look for effective meth-
ods to prepare silyl esters in one step from silanes.
Although many articles have disclosed transition metal-
catalyzed cross-coupling reactions of silanes with water
even when adding 20, 30, 40 or 50 mol% of ZnCl . These
2
results indicate that an excess amount of catalyst does not
have any effect on the acceleration of the dehydro-
coupling reaction. On the other hand, the coupling
4
progresses more slowly, when the amount of ZnCl is
or alcohols, only few papers have documented the de-
2
reduced (cf., to 5 mol%), where 25% of the triethylsilane
used (as measured by GC) remained even after 48 hours.
The reaction temperature affects the coupling reaction
greatly. The dehydrocoupling becomes slow, when the re-
action is carried out at 80 °C or 100 °C in DMF, where tri-
ethylsilane could still be detected in significant amounts
after 48 hours (82% [80 °C]; 64% [100 °C] according to
GC measurements). When the reaction was run in other
solvents, such as xylene, anisole, n-octane at 120 °C for
hydrogenative cross-coupling of carboxylic acids and
silanes. The dehydrocoupling reactions reported are cata-
5
lyzed by [(Ph P)CuH], HPtCl , Rh or Pd. Some short-
3
6
comings have been noted in these reported methods based
on practical and economical considerations: Transition
metals are generally expensive. Transition metal-cata-
lyzed cross-coupling reactions of unsaturated carboxylic
acids and silanes usually afford the desired unsaturated si-
lyl esters, along with amounts of the undesired saturated
esters as by-products, which are formed from hydrogen
addition to the unsaturated silyl esters. This is due to the
known ability of Rh, Pd or Pd/C to act as catalyst in the
hydrogenation of carbon–carbon double bonds. General-
ly, it is troublesome to separate the desired silyl esters
from the mixture as they often show almost the same
7
2 hours, the cross-coupling reaction was found to be
slower than in DMF and unreacted triethylsilane remained
52–71% according to GC). In other lower boiling sol-
(
vents such as toluene (110 °C), acetonitrile (81 °C), hex-
ane (69 °C), cyclohexane (80 °C), 1,4-dioxane (100 °C),
and benzene (80 °C), the dehydrocoupling reaction was
sluggish, where again triethylsilane remained unreacted
after 72 hours under reflux in the corresponding solvent
(50–72%, according to GC). These results reveal that
DMF is the most suitable solvent for this kind of dehydro-
SYNLETT 2006, No. 9, pp 1431–1433
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1.
0
6.
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Advanced online publication: 22.05.2006
DOI: 10.1055/s-2006-941576; Art ID: W03906ST
©
Georg Thieme Verlag Stuttgart · New York

1
432
G.-B. Liu
LETTER
Table 1 ZnCl -Catalyzed Dehydrocoupling of Carboxylic Acids and Silanesa
2
Run
1
Acid
Silane
Time (h)
24
Product
Yield (%)b
MeCO H
Et SiH
MeCO SiEt
835c
806
756
847
796
768
859
8010
7411
7712
2
3
2
3
2
MeCO H
n-Pr SiH
24
MeCO Si-n-Pr
2 3
2
3
3
MeCO H
n-Bu SiH
36
MeCO Si-n-Bu
2
3
2
3
4
EtCO H
Et SiH
24
EtCO SiEt
3
2
3
2
5
EtCO H
n-Pr SiH
26
EtCO Si-n-Pr
2 3
2
3
6
EtCO H
n-Bu SiH
38
EtCO Si-n-Bu
2
3
2
3
7
PhCO H
Et SiH
25
PhCO SiEt
3
2
3
2
8
PhCO H
n-Pr SiH
25
PhCO Si-n-Pr
2 3
2
3
9
PhCO H
n-Bu SiH
36
PhCO Si-n-Bu
2
3
2
3
1
1
1
1
0
1
2
3
Et SiH
24
3
CO2H
CO2SiEt3
n-Pr SiH
24
48
24
7413
7314
7815
3
CO2Si-n-Pr3
CO2Si-n-Bu3
n-Bu SiH
3
Et SiH
3
CO2H
CO2SiEt3
1
1
4
5
n-Pr SiH
26
48
7616
735b
3
CO2Si-n-Pr3
CO2Si-n-Bu3
n-Bu SiH
3
a
Carboxylic acid (20 mmol), silane (20 mmol), ZnCl (3 mmol, 15 mol%).
Isolated yield after distillation under reduced pressure.
2
b
coupling reaction. Triethylsilane, tri-n-propylsilane and esters in good yields. In the case of unsaturated carboxylic
tri-n-butylsilane were easily transformed to the corre- acids, no by-products such as overreduced silyl esters
sponding silyl esters in good yield. Tri-n-propylsilyl and were detected, since a hydrogenation of the products
tri-n-butylsilyl acetate were formed in 80% and 75% cannot occur under these reaction conditions. We believe
yield, respectively, by reaction of tri-n-propylsilane and that the ZnCl -catalyzed cross-coupling of silanes and
2
tri-n-butylsilane and acetic acid (runs 2 and 3). In the case carboxylic acids provides another quite useful protocol
of using tri-n-butylsilane the cross-coupling was slower as for a one-step, highly selective, atom-economical and
compared to that with triethylsilane or tri-n-propylsilane. efficient synthetic way towards silyl esters, a protocol that
The relative reactivity of silanes decreases in the order of is characterized by easy manipulation and easy product
Et SiH > n-Pr SiH > n-Bu SiH (runs 1 and 2 vs. 3). Under separation. Moreover, ZnCl is commercially readily
3
3
3
2
similar reaction conditions, the desired unsaturated silyl available and cheaper than Pt, Pd and Rh. This method can
esters were produced by dehydrocoupling of silanes with be considered as a promising alternative to transition
acrylic acid or methacrylic acid without the formation of metal-catalyzed cross-coupling techniques. Further work
hydrogenated by-products. Undoubtedly, this is due to the on the dehydrosilylation of carboxylic acids and silanes
fact that ZnCl does not catalyze the hydrogenation of and related compounds is currently underway in our labo-
2
unsaturated silyl esters (runs 10–15).
ratory, and the results will be published in due course.
In conclusion, the dehydrogenative cross-coupling of
silanes and carboxylic acids can be achieved in the
Acknowledgment
presence of a catalytic amount of ZnCl under relative
2
1
7,18
The author thanks Prof. T. Thiemann (Kyushu University, Japan)
for helpful discussions and reading the manuscript.
mild reaction conditions.
The dehydrocoupling reac-
tions proceed efficiently to give the corresponding silyl
Synlett 2006, No. 9, 1431–1433 © Thieme Stuttgart · New York

LETTER
Preparation of Silyl Esters
1433
References and Notes
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(
17) Typical Procedure.
To a mixture of AcOH (20 mmol, 1.20 g), and TES (20
mmol, 2.32 g) in DMF (10 mL) was added ZnCl (3 mmol,
2
0
.40 g, 0.15 equiv, dried over P O under reduced pressure
2 5
(
b) Fujino, J.; Taniguchi, H.; Mori, K.; Funaoka, S.; Ito, M.
at r.t. for 24 h) under a nitrogen atmosphere at r.t. The
reaction mixture was stirred at 120 °C for 24 h (monitored by
GC). The desired triethylsilyl acetate was obtained as a
colorless oil (yielding: 80%) after distillation under reduced
pressure (run 1).
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5c
Triethylsilyl acetate: IR (neat): 686, 742, 883, 920, 1056,
(
(
–1 1
1
278, 1357, 1463, 1702, 2868, 2945 cm . H NMR (400
MHz, CDCl ): d = 0.72 (6 H, q, J = 8.0 Hz), 0.93 (9 H, t,
3
1
297.
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13
J = 8.0 Hz), 2.05 (3 H, s). C NMR (100 MHz, CDCl ): d =
.21, 6.30, 22.35, 171.20.
3
(
(
4
2
(
18) All of the silyl esters are known compounds and were
compared with authentic samples (prepared by cross-
coupling of carboxylic acids and chlorosilanes in the
presence of Et N in CH Cl ). Identification was based on
3
2
2
8
1153.
10) Frainnet, E.; Paul, M. C. R. Seances Acad. Sci., Ser. C 1967,
65, 1185; Chem. Abstr. 1968, 68: 59653.
1
their IR, H NMR and GCMS spectral data.
(
2
Synlett 2006, No. 9, 1431–1433 © Thieme Stuttgart · New York