Zinc-Catalyzed Synthesis of Acylsilanes Using Carboxylic Acids and a Silylborane in the Presence of Pivalic Anhydride

Article abstract of DOI:10.1021/acs.orglett.9b04151

Zinc-catalyzed synthesis of acylsilanes using carboxylic acids and a silylborane has been achieved in the presence of pivalic anhydride. Various carboxylic acids were converted to the corresponding acylsilanes. The in situ formation of mixed anhydrides wa

Full text of DOI:10.1021/acs.orglett.9b04151

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Zinc-Catalyzed Synthesis of Acylsilanes Using Carboxylic Acids and a
Silylborane in the Presence of Pivalic Anhydride
Kenta Tatsumi, Sae Tanabe, Yasushi Tsuji, and Tetsuaki Fujihara*
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto
615-8510, Japan
S
* Supporting Information
ABSTRACT: Zinc-catalyzed synthesis of acylsilanes using
carboxylic acids and a silylborane has been achieved in the
presence of pivalic anhydride. Various carboxylic acids were
converted to the corresponding acylsilanes. The in situ
formation of mixed anhydrides was essential in the present
reaction.
Scheme 1. Transition-Metal-Catalyzed Synthesis of
Acylsilanes
cylsilanes are carbonyl compounds in which silicone and
oxygen atoms are directly bound to the same carbon
A
atom.1 As a result of this unique feature, acylsilanes are very
versatile and can be used in many organic transformations,
such as reactions with various nucleophiles via Brook
rearrangement,2 photochemical reactions,3 and transition-
metal-catalyzed reactions.4
Classically, acylsilanes are synthesized by silylation of
lithiated 1,3-dithianes followed by a deprotection step.5
Alternative preparation methods include the oxidation of
organosilicon compounds.6,7 Direct synthesis of acylsilanes via
silylation of carbonyl compounds using silyl metals such as
lithium,8 magnesium,9 or aluminum10 species was also
reported. However, functional group tolerance was quite low
due to their high reactivity. Although silyl copper species
provided acylsilanes with various functional groups,11
stoichiometric amount of copper wastes was generated after
the reaction.
a
a zinc catalyst, the reaction did not proceed at all (entry 1).
Employing ZnCl2 as the catalyst, an acylsilane (2a) was
obtained in 76% GC yield (entry 2). Zn(OAc)2 was found to
be less effective as the catalyst (entry 3), producing the
product 2a in 66% yield, while Zn(O2CtBu)2 showed high
catalytic activity (entry 4). A commercially available ZnEt2 was
also effective for the reaction, giving 2a in 86% yield (entry 5).
Lithium benzoate (1a−Li) and potassium benzoate (1a−K)
were not effective substrates for the reaction (entries 6 and 7).
In the case of benzoic acid (1a) as the substrate, the desired
product was not detected at all (entry 8). On the other hand,
in situ deprotonation using NaH dramatically improved the
yield of 2a (entry 9). Under the reaction conditions, pure 2a
was isolated in 82% yield. The reaction using 2.0 mol of 1a
(0.24 g) afforded 3a in 64% yield (0.31 g). Next, other
catalysts were tested and Pd(OAc)2, Cu(OAc), and AlCl3 were
found to be ineffective (entries 10−12). When other silyl
sources such as Et3Si−B(pin) or Me3Si−SiMe3 were tested in
In contrast to classical synthetic methods, transition-metal-
catalyzed preparation of acylsilanes using mild silylation
reagents is highly reliable. Yamamoto reported the palla-
dium-catalyzed synthesis of acylsilanes from acid chlorides and
disilanes (Scheme 1a).12 Later, Riant reported that acylsilanes
could be synthesized by the reaction of acid anhydrides with
silylboranes in the presence of Cu catalysts (Scheme 1b).13
These methods result in various acylsilanes; however, the
reaction starting materials, such as acid chlorides or acid
anhydrides, must be prepared from the corresponding
carboxylic acids before use. Herein, the Zn-catalyzed synthesis
of acylsilanes using carboxylic acids and a silylborane is
reported, where pivalic anhydride was found to be an
indispensable additive (Scheme 1c). The key to success of
this reaction is the in situ formation of a mixed anhydride.14
First, sodium benzoate (1a−Na) was reacted with a
silylborane, PhMe2Si−B(pin), using pivalic anhydride as an
additive in the presence of a zinc catalyst in 1:1 (v/v)
ClCH2CH2Cl/toluene at 90 °C for 18 h (Table 1).15 Without
Received: November 20, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04151
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Optimization of Reaction Conditions for Zn-
Catalyzed Synthesis of an Acylsilane 2a
Table 2. Scope of Carboxylic Acids for the Synthesis of
a
a
Acylsilanes
b
entry
catalysts
M
yield of 2a (%)
1
2
3
4
5
6
7
8
9
10
11
12
none
ZnCl2
Na
Na
Na
Na
Na
Li
K
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
0
76
66
86
86
56
13
0
Zn(OAc)2
Zn(O2CtBu)2
ZnEt2 (1.0 M in hexane)
ZnEt2
ZnEt2
ZnEt2
c
d
ZnEt2
86 (82)
c
Pd(OAc)2
Cu(OAc)
AlCl3
0
0
0
c
c
a
Reaction conditions: 1a (0.40 mmol), Me2PhSi−B(pin) (0.52
mmol), catalyst (0.020 mmol, 5.0 mol %), (tBuCO)2O (0.60
mmol) in ClCH2CH2Cl/toluene (1:1 (v/v), 2.0 mL) at 90 °C for
18 h. GC yield by an internal standard method. NaH (1.1 equiv)
b
c
d
was added. Isolated yield of 2a.
place of Me2PhSi−B(pin), the desired products were not
obtained at all.15
Next, the substrate scope was examined using various
carboxylic acids in the presence of NaH (Table 2). Aromatic
carboxylic acids bearing electron-donating (1b and 1c) and
electron-withdrawing groups (1d−g) on the phenyl ring
afforded the corresponding acylsilanes (2b−g) in moderate-
to-good yields (entries 1−6). In the reaction, bromo and iodo
groups were intact (entries 4 and 5). A boronic ester moiety
was also tolerated (entry 7). A sterically hindered substrate
(1i) was smoothly converted to the product (2i) in 80% yield
(entry 8). 1-Naphthoic acid (1j) also gave the product (2j) in
good yield (entry 9). Aliphatic carboxylic acids (1k−n) also
afforded the corresponding products (2k−n) (entries 10−13).
The ester functional group in 1m was tolerated (entry 12).
Cyclohexanecarboxylic acid (1n) also reacted with silylborane,
giving 2n in moderate yield (entry 13). On the other hand,
bulky pivalic acid 1o did not afford the corresponding product
at all (entry 14). Other carboxylic acids having a chiral center
at the α-position such as N,N-dibutylalanine were not suitable
substrates.
To obtain insights into the reaction mechanism, some
control experiments were carried out (Scheme 2). When
sodium benzoate (1a−Na) was reacted with PhMe2Si−B(pin)
in the absence of pivalic anhydride, the product 2a was not
obtained at all (Scheme 2a). Thus, carboxylate was not directly
silylated, and the formation of a mixed anhydride by the
reaction of 1a−Na and pivalic anhydride (in equilibrium) is
indispensable. Next, benzoic anhydride (3) as an alternative
substrate for the mixed anhydride reacted with PhMe2Si−
B(pin). Although 2a was obtained, the yield was low (Scheme
2b). In order to generate the mixed anhydride, sodium pivalate
was added to the reaction with 3 (Scheme 2c). As a result, the
yield of 2a was increased to 52% yield, indicating in situ
generated mixed anhydride is a key intermediate for the
present reaction.
a
Reaction conditions: 1 (0.40 mmol), Me2PhSi−B(pin) (0.52 mmol),
ZnEt2 (1.0 M in hexane, 0.020 mmol, 5.0 mol %), (tBuCO)2O (0.60
mmol), NaH (0.44 mmol) in ClCH2CH2Cl/toluene = 1:1 (v/v, 2.0
b
c
mL) at 90 °C for 18 h. Isolated yield of 2. ZnEt2 (10 mol %) was
used.
A plausible catalytic cycle for the reaction discovered in this
work is presented in Scheme 3. First, ZnEt2 reacts with pivalic
anhydride, giving zinc carboxylate A. Next, transmetalation
with silylborane affords silylzinc intermediate B, which was
recently reported by Uchiyama.16 Then, B reacts with in situ
generated mixed anhydride C to afford an intermediate D. In
B
DOI: 10.1021/acs.orglett.9b04151
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Control Experiments To Elucidate the Reaction
Mechanism
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant Number
18H04257 in Precisely Designed Catalysts with Customized
Scaffolding (T.F.) and 17H03096 in Grant-in-Aid for Scientific
Research B (Y.T.) from MEXT, Japan. T.F. also acknowledged
the Tokuyama Science Foundation for financial support.
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S
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04151.
Experimental procedures and characterization of the
products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tfuji@scl.kyoto-u.ac.jp.
ORCID
Yasushi Tsuji: 0000-0001-6733-0539
Tetsuaki Fujihara: 0000-0002-6687-2528
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.9b04151
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Letter
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DOI: 10.1021/acs.orglett.9b04151
Org. Lett. XXXX, XXX, XXX−XXX