Two new catalysts for the dehydrogenative coupling reaction of carboxylic acids with silanes - Convenient methods for an atom-economical preparation of silyl esters

Article abstract of DOI:10.1080/00397910701465669

Tris(triphenylphosphine)cuprous chloride [Cu(PPh3)3Cl] has been found to be an efficient catalyst for the dehydrosilylation of carboxylic acids with silanes. In the presence of 4 mol% Cu(PPh3)3Cl, dehydrosilylation reactions in acetonitrile afforded the corresponding silyl esters at 80°C in good yields. It was noted that triphenylphosphine itself also functions as an adequate catalyst for the reaction. Copyright Taylor & Francis Group, LLC.

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Two New Catalysts for the Dehydrogenative
Coupling Reaction of Carboxylic Acids
with Silanes—Convenient Methods for an
Atom‐Economical Preparation of Silyl Esters
Guo‐Bin Liu a , Hong‐Yun Zhao a & Thies Thiemann b
a Department of Chemistry , Fudan University , Shanghai, P.R.China
b Interdisciplinary Graduate School of Engineering Sciences, Kyushu
University , Kasuga-kohen, Kasuga, Fukuoka, Japan
Published online: 14 Aug 2007.
To cite this article: Guo‐Bin Liu , Hong‐Yun Zhao & Thies Thiemann (2007) Two New Catalysts for
the Dehydrogenative Coupling Reaction of Carboxylic Acids with Silanes—Convenient Methods for an
Atom‐Economical Preparation of Silyl Esters, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 37:16, 2717-2727, DOI: 10.1080/00397910701465669
To link to this article: http://dx.doi.org/10.1080/00397910701465669
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Synthetic Communicationsw, 37: 2717–2727, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701465669
Two New Catalysts for the Dehydrogenative
Coupling Reaction of Carboxylic Acids with
Silanes—Convenient Methods for an Atom-
Economical Preparation of Silyl Esters
Guo-Bin Liu and Hong-Yun Zhao
Department of Chemistry, Fudan University, Shanghai, China
Thies Thiemann
Interdisciplinary Graduate School of Engineering Sciences, Kyushu
University, Kasuga-kohen, Kasuga, Fukuoka, Japan
Abstract: Tris(triphenylphosphine)cuprous chloride [Cu(PPh3)3Cl] has been found to
be an efficient catalyst for the dehydrosilylation of carboxylic acids with silanes. In the
presence of 4 mol% Cu(PPh3)3Cl, dehydrosilylation reactions in acetonitrile afforded
the corresponding silyl esters at 808C in good yields. It was noted that triphenylpho-
sphine itself also functions as an adequate catalyst for the reaction.
Keywords: catalysts, dehydrosilylation, silyl esters, triphenylphosphine, tris(triphenyl-
phosphine)cuprous chloride
INTRODUCTION
Silyl esters are key intermediates for the preparation of functional polymeric
substrates such as easily degradable poly(silyl ester)s, widely utilized as
recyclable materials, gene delivery carriers, matrices for drug delivery, and
biodegradable surgical devices.[1] In this respect, unsaturated silyl esters are
important substrates in polymerization reactions. Much attention has been
Received October 17, 2006
Address correspondence to Guo-Bin Liu, Department of Chemistry, Fudan
University, 220 Handan Road, Shanghai 200433, P. R. China. E-mail: liuguobin@
fudan.edu.cn
2717

2718
G.-B. Liu, H.-Y. Zhao, and T. Thiemann
devoted to the development of a simple, practical, and atom-economical prep-
aration of stable and easily isolable silyl esters.
A conventional way to prepare silyl esters is the coupling reaction of
carboxylic acids and chlorosilanes.[2] Hydrogen chloride is unavoidably
produced in these procedures, and a stoichiometric or even excess
amount of base such as amines or ammonia is needed to neutralize the
HCl gas formed. As chlorosilanes themselves are made by the chlorination
of silanes, either with chlorine gas[1g] or with hydrochloric acid under Pd/
C catalysis,[3] the synthesis of silyl esters from the corresponding silanes
necessitates two reaction steps. Although a number of newer synthetic
methods for silyl esters have appeared and a lot of literature concerns
itself with the transition-metal-catalyzed coupling of OH-containing
compounds such as water and alcohols with silanes,[4] there are still
few examples of the dehydrogenative coupling reaction of carboxylic
acids with silanes. The reported dehydrosilylation reactions are exclu-
sively catalyzed by metal salts such as zinc chloride or, more frequently,
by transition metals and metal complexes such as [(Ph3P)CuH], H2PtCl6,
Rh, and Pd.[5] Transition metals are expensive. Catalysts such as
[(Ph3P)CuH] require a multiple-step synthesis and are very difficult to
obtain in high purity. Moreover, transition-metal-catalyzed coupling of
unsaturated carboxylic acids and silanes always results in the formation
of by-products because of the known ability of Rh, Pd, and Pd/C to act
as hydrogenation catalyst in the reduction of carbon–carbon double
bonds in the desired olefinic silyl esters, leading to troublesome
purification. Because of this, the screening of catalysts for this reaction
remains of interest.
Here, we report that both tris(triphenylphosphine)cuprous chloride
[Cu(PPh3)3Cl] and triphenylphosphine (Ph3P) catalyze the dehydrogenative
coupling of carboxylic acids with silanes, yielding the corresponding silyl
esters selectively, without formation of any reduced by-products in the case
of unsaturated silyl esters (Scheme 1).
RESULTS AND DISCUSSION
In the presence of 4 mol% Cu(PPh3)3Cl, the reaction was carried out in aceto-
nitrile by heating a mixture of the corresponding acid (1) and silane (2) at 808C
Scheme 1.

Convenient Methods for the Preparation of Silyl Esters
2719
under an N2 atmosphere (Table 1). To optimize the reaction conditions, the
coupling was performed with acetic acid (1a) and triethylsilane (2a) as sub-
strates under different reaction conditions. The desired coupling was found
to be complete after 4 h at 808C (4 mol% catalyst), affording the correspond-
ing triethylsilyl acetate (3a) in 90% yield (Table 1, run 1). Using Cu(PPh3)3Cl
as catalyst, the reaction time for the transformation is much shorter than when
using ZnCl2.[5g] The use of additional Cu(PPh3)3Cl (e.g., 8 mol%) did not
accelerate the reaction further, however. On the other hand, a decrease of
the amount of catalyst (to 2 mol%) led to a decrease in reaction rate, where
39% of the triethylsilane (2a) (as determined by GC) remained unreacted
even after the reaction mixture was heated for 12 h at 808C. Also, the
reaction temperature plays an important role. Thus, the transformation was
slow when the reaction was carried out at 408C or 608C, and significant
amounts of triethylsilane (2a) could still be detected after 48 h (78% at
408C, 49% at 608C, according to GC analysis). At room temperature, no
coupling reaction was observed, and the triethylsilane (2a) remained
unchanged. When the reaction was carried out in other solvents, such as in
hexane (at 698C), cyclohexane (at 808C), 1,4-dioxane (at 1008C), or
benzene (at 808C), the reaction was found to be slower than in acetonitrile,
where again triethylsilane (2a) remained unreacted after 24 h at reflux
temperature of the corresponding solvent (45–78%, as estimated by GC
analyses).
A wide range of trialkylsilanes (2) could be reacted successfully when
using the optimized conditions discussed previousy [run 1: 4 mol%
Cu(PPh3)3Cl, acetonitrile, 808C]. The relative reactivity of the silanes is of
the order Et3SiH (2a) . n-Pr3SiH (2b) . n-Bu3SiH (2c) (Table 1, runs 1
and 2 vs. 3). Also benzoic acid (1d) and substituted benzoic acids (1e–1g)
could be submitted to the reaction successfully (runs 7–14). It must be
noted that under these conditions also unsaturated silyl esters such as
acrylic (1h), methacrylic (1i), cinnamic (1j), and furylacrylic (1k) acids
could be reacted, leading to the desired monomers of poly(silyl ester)s.
Here, no hydrogenated by-products were formed, as is usually the case in
many of the Pd-, Pt- and Ru-based catalysts.
Also, triphenylphosphine (PPh3) itself was found to catalyze the dehydro-
genative reactions of carboxylic acids with silanes. Thus, when a mixture of
carboxylic acid (1), silane (2), and PPh3 (4 mol%) in N,N-dimethylformamide
(DMF) was heated under a nitrogen atmosphere at 1208C for 12–18 h, the
desired silyl esters were obtained in good yield (Table 2, reaction monitored
by GC). Again, the transformation of propionic acid (1b) with triethylsilane
(2a) was used to optimize the reaction conditions. Here, the dehydrogenative
coupling was found to be finished after 15 h at 120 ºC, in the presence of
4 mol% PPh3, affording the corresponding triethylsilyl propionate (3d) in
87% yield (Table 1, run 3). As with using Cu(PPh3)3Cl, an increase of
catalyst did not accelerate the reaction further, whereas a decrease in PPh3
(to 1 or 2 mol%) slowed the reaction significantly, and 48% and 30% of

Table 1. CuCl(Ph3P)3-catalyzed dehydrocoupling of carboxylic acids and silanesa
Yield
(%)b
Run
Acid
Silane
Time (h)
Product
1
2
3
4
5
6
7
8
9
CH3CO2H (1a)
1a
Et3SiH (2a)
n-Pr3SiH (2b)
4
4
4
4
6
7
4
6
6
6
6
7
7
CH3CO2SiEt3 (3a)
CH3CO2SiPrn3 (3b)
90[5c]
88[6]
86[6]
84[7]
76[8]
79[9]
80[10]
81[11]
80[9]
85[9]
84[12]
82[12]
81[12]
1a
CH3CH2CO2H (1b)
n-Bu3SiH (2c)
2a
CH3CO2SiBun3 (3c)
CH3CH2CO2SiEt3 (3d)
CH3CH2CO2SiBun3 (3e)
CH3CH2CO2SiPri3 (3f)
C6H5CO2SiPrn3 (3g)
C6H5CO2SiBun3 (3h)
C6H5CO2SiPri3 (3i)
1b
CH3(CH2)8CO2H (1c)
2c
i-Pr3SiH (2d)
C6H5CO2H (1d)
1d
2b
2c
1d
1d
2d
t-BuMe2SiH (2e)
10
C6H5CO2SiMe2But (3j)
3-BrC6H4CO2SiPri3 (3k)
3-BrC6H4CO2SiMe2But (3L)
3-ClC6H4CO2SiMe2But (3m)
11
12
13
3-BrC6H4CO2H (1e)
1e
3-ClC6H4CO2H (1f)
2d
2e
2e

14
15
4-O2NC6H4CO2H (1g)
2e
2a
8
5
4-O2NC6H4CO2SiMe2But (3n)
79[13]
83[14]
16
17
18
1h
1h
2b
2c
2a
5
6
5
78[15]
74[16]
83[17]
19
20
1i
1i
2d
2c
6
6
76[15]
79[5b]
21
22
23
2a
2d
2e
5
6
8
87[18]
81[18]
72[12]
1j
1j
24
2a
6
86[19]
aCarboxylic acid (20 mmol), silane (20 mmol), CuCl(PPh3)3 (0.8 mmol, 4 mol%).
bIsolated yield.

Table 2. Ph3P-catalyzed dehydrocoupling of carboxylic acids with silanesa
Run
Acid
Silane
Time (h)
Product
Yield (%)b
1
2
CH3CO2H (1a)
1a
1a
Et3SiH (2a)
n-Pr3SiH (2b)
12
12
14
15
14
14
15
14
16
15
14
18
13
18
14
CH3CO2SiEt3 (3a)
CH3CO2SiPrn3 (3b)
91[5c]
88[6]
86[6]
84[7]
76[8]
85[6]
79[9]
83[9]
77[9]
80[12]
81[12]
77[12]
85[20]
75[13]
80[7]
3
n-Bu3SiH (2c)
2a
CH3CO2SiBun3 (3c)
4
CH3CH2CO2H (1b)
1b
1b
CH3CH2CO2SiEt3 (3d)
CH3CH2CO2SiBun3 (3e)
CH3CH2CO2SiPrn3 (3-I)
CH3(CH2)8CO2SiPri3 (3f)
C6H5CO2SiPri3 (3i)
5
2c
2b
6
7
CH3(CH2)8CO2H (1c)
C6H5CO2H (1d)
1d
i-Pr3SiH (2d)
2d
t-BuMe2SiH (2e)
8
9
C6H5CO2SiMe2But (3j)
3-BrC6H4CO2SiPri3 (3k)
3-ClC6H4CO2SiPri3 (3y)
3-ClC6H4CO2SiMe2But (3j)
4-ClC6H4CO2SiEt3 (3z)
4-O2NC6H4CO2SiMe2But (3n)
C6H5CO2SiEt3 (3o)
10
11
12
13
14
15
3-BrC6H4CO2H (1e)
3-ClC6H4CO2H (1f)
3-ClC6H4CO2H (1f)
4-ClC6H4CO2H (IL)
4-O2NC6H4CO2H (1g)
C6H5CH2CO2H (1m)
2d
2d
2e
2a
2e
2a

16
2a
12
79[19]
17
18
2d
2d
14
14
78[9]
74[12]
19
20
21
2a
2d
2a
13
15
13
82[18]
75[18]
80[19]
1j
22
23
2b
2c
14
15
86[21]
78[5c]
1i
aCarbolic acid (20 mmol), silane (20 mmol), PPh3 (0.8 mmol, 4 mol%).
bIsolated yield.

2724
G.-B. Liu, H.-Y. Zhao, and T. Thiemann
Et3SiH (2a) (GC ratio) was found to remain unreacted after 48 h at 1208C.
Also, the reaction progressed more slowly when carried out at 808C or
1008C, with 72% of the triethylsilane (2a) remaining unreacted at 808C and
51% at 1008C after 48 h. Solvents other than DMF, such as xylene and
anisole, were studied but found to be inadequate, as even after 72 h at
1208C significant amounts of triethylsilane (2a) remained unreacted (62%
and 86%, respectively). Phosphines other than triphenylphosphine were also
tested as catalysts, such as tri-n-butylphosphine, tri-tert-butylphosphine, tricy-
clohexylphosphine, tris(2-methylphenyl)phosphine, and 1,2-bis(diphenylpho-
sphino)-ethane (all at 4 mol%, DMF, 1208C), but triphenylphosphine was
found to give the best results.
As with Cu(PPh3)3Cl, the use of PPh3 as catalyst allowed the transform-
ation of a large number of substrates, both of silanes such as n-Pr3SiH (2b),
n-Bu3SiH (2c), and t-BuMe2SiH (2e) and of acids such as benzoic acid (1d)
or substituted benzoic acids (1e–1g, 1l) (all reactions run with 4 mol%
PPh3, DMF, 1208C). Again, the desired unsaturated silyl esters could be
obtained by dehydrogenative coupling reaction of the silanes with cinnamic
acid (1j), furylacrylic acid (1k), 2,4-hexadienoic acid (1o), and methacrylic
acid (1i). In none of the cases was the formation of overreduced by-products
observed (Table 2, runs 16–23).
From these results, it can be seen that although both Cu(PPh3)3Cl and
PPh3 can be used as catalyst in the dehydrogenative preparation of silyl
esters, PPh3 necessitates higher reaction temperatures and longer reaction
times, where the product yields are comparable to those with Cu(PPh3)3Cl
as catalyst. Also, CuCl (at 4 mol% or 20 mol%) was examined as a
catalyst for the reaction but was found to be less reactive, leaving 78% (at
4 mol%) and 56% (at 20 mol%) triethylsilane (2a) unreacted after 12 h in
refluxing acetonitrile.
CONCLUSION
Cu(PPh3)3Cl as well as PPh3 are efficient catalysts for the dehydrogenative
coupling of carboxylic acids (1) with silanes (2), where the corresponding
silyl esters (3) are formed in good yields. No overreduction occurs when
reacting bromo- (1e), chloro- (1f and 1L), or nitrobenzoic (1g) acids with
silanes (2) under the conditions. Especially interesting is the transformation
of unsaturated carboxylic acids under this protocol, leading to unsaturated
silyl esters, where no hydrogenation products are observed. This type of dehy-
drogenative coupling reaction of carboxylic acids with silanes provides
another important example of a one-step, highly selective, atom-economical,
and efficient synthetic method, which in the case of using PPh3 can also be run
under metal-free conditions. Further work on silylation of carboxylic acids is
in progress in our laboratory.

Convenient Methods for the Preparation of Silyl Esters
EXPERIMENTAL
2725
Typical Procedure with Cu(PPh3)3Cl as Catalyst
To a mixture of propionic acid (1b) (40 mmol, 2.96 g) and triethylsilane (2a)
(40 mmol, 4.64 g), Cu(PPh3)3Cl (0.8 mmol, 710 mg, 0.04 eq.) was added at
room temperature under a nitrogen atmosphere. The reaction mixture was
stirred at 808C for 4 h. The desired triethylsilyl propionate (3d) was
obtained as a colorless oil (yield: 84%) after distillation under reduced
pressure (Table 1, run 4).
Typical Procedure with PPh3 as Catalyst
To a mixture of propionic acid (1b) (40 mmol, 2.96 g) and triethylsilane (2a)
(40 mmol, 4.64 g) in DMF (20 ml), triphenylphosphine (1.6 mmol, 0.42 g,
0.04 eq) was added at room temperature under a nitrogen atmosphere. The
reaction mixture was stirred at 1208C for 15 h (monitored by GC). The
desired triethylsilyl propionate (3d) was obtained (yield: 87%) after distilla-
tion under reduced pressure (Table 2, run 3).
Triethylsilyl Propionate (3d)7
IR (neat): 684, 740, 824, 998, 1062, 1241, 1410, 1464, 1714, 2870,
3
1
2954 cm21. H NMR (400 MHz, CDCl3): d 0.72 (6H, q, J 7.8 Hz), 0.95
3
3
3
(9H, t, J 7.8 Hz), 1.14 (3H, t, J 7.6 Hz), 2.36 (2H, q, J 7.6 Hz). 13C NMR
(100 MHz, CDCl3): d 4.42, 6.46, 9.34, 28.46, 175.20.
All of the silyl esters are known compounds and were compared with
authentic samples [prepared by coupling of carboxylic acids and chlorosilanes
in the presence of a base such as triethylamine or imidazole (t-butylsilyl
1
esters) in dichloromethane] and were identified on the basis of their IR, H
NMR, 13C NMR, and GC-MS spectral data.
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