Metal-free relay oxidation: Valuable synthesis of acylsilane and ketones under aerobic oxidation

Article abstract of DOI:10.1055/s-0031-1289907

In this letter, an example of interesting metal-free relay air oxidation of -hydroxysilanes, promoted by the hydroperoxidated carbonyl compounds derived from the Michael reaction of 5,5-dimethylcyclohexane-1,3-dione and chalcone, is reported. A series of aromatic acylsilanes with TBDPS were obtained in promising isolated yields. In addition, as an extension of the relay oxidation under aerobic conditions, this catalyst-free relay oxidation induced by diketone can be applied to the oxidation of general aromatic alcohols (up to 75% yield). Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1289907

LETTER
3031
Metal-Free Relay Oxidation: Valuable Synthesis of Acylsilane and Ketones
under Aerobic Oxidation
Metal-Free
R
elay O
i
xidati
n
on g-Feng Bai,a Guang Gao,a Zhan-Jiang Zheng,a Fei Li,a Guo-Qiao Lai,a Kezhi Jiang,a Fuwei Li,b Li-Wen Xu*a,b
a
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University,
Hangzhou 310012, P. R. of China
Fax +86(571)28865135; E-mail: liwenxu@hznu.edu.cn; E-mail: licpxulw@yahoo.com
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, P. R. of China
b
Received 18 October 2011
– acylsilanes – by an interesting catalyst-free and metal-
free relay air oxidation.
Abstract: In this letter, an example of interesting metal-free relay
air oxidation of a-hydroxysilanes, promoted by the hydroperoxidat-
ed carbonyl compounds derived from the Michael reaction of 5,5-
dimethylcyclohexane-1,3-dione and chalcone, is reported. A series
of aromatic acylsilanes with TBDPS were obtained in promising
isolated yields. In addition, as an extension of the relay oxidation
under aerobic conditions, this catalyst-free relay oxidation induced
by diketone can be applied to the oxidation of general aromatic al-
cohols (up to 75% yield).
There are only a few known examples for the oxidation of
a-hydroxysilanes to the corresponding acylsilanes, utiliz-
ing KMnO4/Al2O3 and oxalyl choride/DMSO (Swern ox-
idation) as oxidants.16 However, the drawback of the
previous protocol is unavoidable and, in general, it led to
silanols and aldehydes as major products. Therefore it is
highly desirable to develop a novel and efficient approach
to acylsilanes via oxidation, which is also beneficial for
the chemistry of acylsilanes.
Key words: catalyst-free, oxidation, silane, ketone, relay chemistry
In an attempt to extend metal-free methodologies to the
synthesis of acylsilanes through oxidation, initially we in-
vestigated a variety of oxidants for a-hydroxysilane 1a,
such as tert-butyl hydroperoxide (TBHP), hydrogen per-
oxide, and pyridine–sulfur trioxide complex. However,
the desired acylsilane 2a was obtained in less than 10%
yield, probably due to the steric hindrance around the hy-
droxy group of the starting material (Scheme 1). Remark-
ably, the use of TBHP gave a promising yield (10%
isolated yield) of product, and only relatively few byprod-
ucts were obtained. However, the yield was low enough
that improvement in yield by the use of catalysts or the
modification of reaction conditions seemed possible.
Thus, various conditions with metal salts and Lewis bases,
such as CuBr2, InCl3, FeCl3, Et3N, DBU, etc., as catalysts
were further tested for the oxidation of a-hydroxysilane in
order to find an appropriate oxidative system for the selec-
tive oxidation. Unfortunately, no improvement in yield
was achieved and only trace amounts of product were ob-
tained.
Acylsilanes are interesting functional molecules and syn-
thons exhibiting specific chemical properties and reactiv-
ities besides those that are common to carbonyl
derivatives, and thus allow for the generation of many
synthetically and pharmaceutically useful compounds.1,2
Because of the high steric hindrance of the silicon func-
tionality in acylsilanes, the synthesis of acylsilanes had
been a difficult task for a long time. Over the past decades,
numerous new types of acylsilanes have been developed.1
Among the presently known synthetic strategies for the
synthesis of acylsilanes, most studies have focused on the
multistep conversion starting from compounds as diverse
as aldehydes (Brook–Corey method),3 acyl chlorides,4
alkyl halides,5 and others.6 Acylsilanes, though sensitive
to light and strong basic media, behave frequently like a
ketone or as a synthetic equivalent of an aldehyde; for ex-
ample, they give a-silyl alcohols on treatment with
7
LiAlH4 or borane.8 Recently, acylsilanes have received
increasing research interest in organosilicon chemistry
and organic synthesis, and they have been recognized as
important synthons and valuable intermediates in the syn-
thesis of complex molecules via a number of synthetically
useful transformations,9–14 such as Brook reaction,9 alkyl-
ation,10 and especially C-acylation of aldehydes,11 im-
ines,12 or nitrones13, which leads to a-hydroxycarbonyl
compounds, a-amino ketones, etc.15 In the course of our
studies in relation to silicon-mediated organic synthesis,
we became very interested in the synthesis of acylsilanes.
In this paper we report the synthesis of functional silanes
OH
O
oxidant, r.t.
12–24 h
Si
Ph
1a
Si
Ph
Ph
Ph
2a
TBHP: 12%
H2O2: trace (< 5%)
pyridine–SO3: trace (<5%)
Scheme 1 Formation of acylsilane from a-hydroxysilane in the
presence of oxidant without any catalysts
Since the TBHP was effective and promising for the direct
oxidation of a-hydroxysilane, attention was turned toward
screeing organic hydroperoxides with different functional
substituents. Very recently, Wang et al.17d and others17a–c
SYNLETT 2011, No. 20, pp 3031–3035
Advanced online publication: 23.11.2011
DOI: 10.1055/s-0031-1289907; Art ID: W20211ST
© Georg Thieme Verlag Stuttgart · New York
x
x
.x
x
.2
0
1
1

3032
X.-F. Bai et al.
LETTER
O
O
O
O
OH
O
Michael
addition
O
O2 or air
O
+
O
DCE, 4–6 h,
97–98% yield
HO
O
3
4
Scheme 2 Formation of hydroperoxidated product in the presence of oxygen or air17d
reported the Michael reaction of 5,5-dimethylcyclohex- above-mentioned conditions. However, all these dike-
ane-1,3-dione and chalcones and the unexpected prepara- tones exhibited lower activities and resulted in only low to
tion of a type of hydroperoxidated carbonyl compounds in moderate isolated yields (30–50%) under the same condi-
air or O2 via autoxidation of the Michael adducts at room tions. Furthermore, additional studies of some general
temperature, in which the reaction conversion was in the metal-salt-based catalysts in this oxidation reaction, for
range of excellent to nearly quantitative. As shown in example, Cu and Fe, proved to be unsuccessful, thus dem-
Scheme 2, nearly quantitative yields were achieved when onstrating that the metal catalysts, which were suitable for
the reaction was conducted in DCE under aerobic condi- the general oxidation reaction, did not guarantee the pro-
tions. Inspired by this work, we envisaged that a hydro- motion of two different oxidations (sequential oxidation)
peroxide generated from the Michael adduct and simultaneously. In addition, using other solvents instead
molecular oxygen would react with a-hydroxysilanes to of DCE, such as toluene, ethyl acetate, and THF, did not
obtain the corresponding acylsilanes in better yields. If increase the yields, and poorer conversion or almost no re-
this were the case, the oxidation of the Michael adduct action occurred. Upon optimization experiments, the re-
could result in the formation of a novel oxidant and pro- sults from the screenings demonstrated that: 1) the relay
mote the oxidation of a-hydroxysilanes successfully.
To test this hypothesis, we examined the relay oxidation
reaction of a-hydroxysilane in the presence of Michael
O
O
O
O
adduct 3 (1.5 equiv) obtained from 5,5-dimethylcyclohex-
ane-1,3-dione and chalcone.17 Gratifyingly, the one-pot
relay oxidation started from Michael adduct 3 and a-
hydroxysilane 1a to provide the desired acylsilane 2a in
good isolated yield (67%).
HO
O
HO
5
+
3
+
relay air oxidation
hydroperoxidated 4
OH
O
To further improve the conversion of the metal-free oxi-
dation of a-hydroxysilane 1a, as shown in Figure 1, a se-
ries of representative Michael adducts 3a–e17d of 5,5-
dimethylcyclohexane-1,3-dione and chalcones, with elec-
tron-withdrawing or electron-donating groups attached to
the aryl moiety, and simplified diketones 3f,g18 were in-
vestigated in this oxidation of a-hydroxysilane under the
Si
Ph
Si
DCE, r.t., 36 h,
67% yield
Ph
Ph
Ph
1a
2a
Scheme 3 Relay oxidation of a-hydroxysilane 1a in the presence of
compound 3
Cl
Cl
O
O
O
O
O
O
HO
3b
NO2
Cl
HO
3c
HO
OMe
t-Bu
O
O
O
O
O
O
HO
3d
HO
3e
HO
HO
3g
3f
Figure 1 The screening of compounds 3a–g in the relay oxidation of a-hydroxysilane 1a to acylsilane 2a
Synlett 2011, No. 20, 3031–3035 © Thieme Stuttgart · New York

LETTER
Metal-Free Relay Oxidation
3033
R1 R2
Si R3
R1 R2
Si R3
O
OH
OH
Cl
R1
Si
R3
THF, Et3N
s-BuLi, THF
–78 °C to r.t., 12 h
R4
R4
R2
R4
r.t., 12 h
6
7
1
51–85%
R1, R2, R3 = Me, t-Bu, Ph
4 = H, OMe, F, Me, etc.
R
Scheme 4 The preparation of a-hydroxysilane 1 through retro-Brook rearrangement (see Supporting Information)
oxidation may be conducted with Michael adducts and a- synthesis. The unique potential of the concept or phenom-
hydroxysilanes in the absence of catalysts or additives, enon of relay oxidation in the synthesis of carbonyl com-
such as TEMPO, and 2) the autoxidation of Michael ad- pounds is extremely attractive.
duct 3 to hydroxides and the next oxidation of a-hydroxy-
The success of our relay oxidation reaction has provided
us with the impetus to develop a general synthesis and ap-
plication of a broad range of acylsilanes. It is worth men-
silanes to acylsilanes occurred smoothly and completely.
However, silanols 6 and aldehydes as byproducts were de-
tected in the reaction of the one-pot relay oxidation, which
tioning that the acylsilanes function similarly to carbonyl
led to moderate to good isolated yields. Overall, the reac-
compounds, but the inherent striking differences between
tion conditions of the catalyst-free aerobic oxidation de-
carbon and silicon allows for silicon to have its own dis-
picted in Scheme 3 are still the best choice.
tinct chemistry.9–14,20
Encouraged by the above-described observations, we also
examined the oxidation reaction of various a-hydroxy-
The oxidation of an alcohol to the corresponding carbonyl
product is a very important and common transformation in
silanes, which were prepared from the retro-Brook rear-
synthetic organic chemistry.21,22 As an extension of the re-
rangement promoted by sec-butyllithium (Scheme 4).19
lay oxidation under aerobic conditions, we also explored
Some examples for the oxidation of a-hydroxysilanes
the oxidation of alcohols in the presence of compound 3.
with compound 3 under the optimized reaction conditions
Under the above-described reaction conditions, the scope
and limitations of hydroperoxide-induced relay oxidation
of electronically activated, nonactivated, or deactivated
aromatic alcohols, such as diarylmethanol, 1-aryletha-
are shown in Scheme 5. For various a-hydroxysilanes 1,
the sequential reaction proceeded in moderate to good iso-
lated yields, except for (tert-butyldimethylsilyl)(phe-
nyl)methanol (1f). a-Hydroxysilanes with all-alkyl-
nols, and indenols, were investigated, using compound 3
substituted silicon-based groups for this oxidation, such as
as initiator. As shown in Scheme 6, relay oxidation of ar-
the TMS or the TBS group, but the yields of the corre-
omatic alcohols induced by the oxidation of compound 3
sponding acylsilanes were poor; starting material was
proceeded with moderate yields in most cases. Notably,
consumed but competitive oxidative and decomposed
the oxidation of aromatic alcohols with electron-with-
drawing groups was more difficult than that of nonactivat-
ed or deactivated aromatic alcohols. Interestingly, for the
products, such as silanols, were formed as side-products.
The use of phenyl-substituted a-hydroxysilanes was also
important according to these results. Although the results
same substituents, the different positions of the substitu-
reported here are not perfect, this is, to the best of our
ents in the aromatic ring, combined with different elec-
knowledge, the first and interesting example of a metal-
tronic and steric effects, resulted in different reactivities.
free relay oxidation under aerobic conditions in organic
As shown in the last column of Scheme 6, the yields in-
creased from 9% to 75%, which may be due to the sensi-
O
OH
tive steric effect in this relay oxidation system.
Accordingly, this result showed that this organic and bio-
mimetic oxidation system is similar to enzyme catalysis in
that it is sensitive to the structure of substrates. Although
the true mechanistic activation is unclear, the oxidation
induced by the air-oxidation of 3-hydroxy-5,5-dimethyl-
2-(3-oxo-1,3-diphenylpropyl)cyclohex-2-enone (3), this
new relay oxidation concept is very interesting.
R1
R2
R1
R2
3 (1.5 equiv)
Si
R3
Si
R3
R4
R4
DCE, air, r.t., 24–36 h
1
2
O
O
O
Si
Si
Ph
Si
Ph
Ph
Ph
Ph
Ph
F
2a: 67%
2b: 43%
2c: 52%
In conclusion, we have reported here the first example of
an interesting catalyst-free and metal-free relay air oxida-
tion of a-hydroxysilanes promoted by hydroperoxidated
carbonyl compounds derived from the Michael reaction of
5,5-dimethylcyclohexane-1,3-dione and chalcone. A se-
ries of aromatic acylsilanes with TBDPS group could be
obtained in promising isolated yields. In addition, as an
O
O
O
MeO
F
Si
Si
Si
Ph
Ph
Me
Ph
Ph
Me
2d: 40%
2e: 53%
2f: 9%
Scheme 5 Relay oxidation of a-hydroxysilane to acylsilane in air
Synlett 2011, No. 20, 3031–3035 © Thieme Stuttgart · New York

3034
X.-F. Bai et al.
LETTER
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O
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compound 3 (1.5 equiv)
R1
R1
no catalyst
R2
R2
Cl
DCE (2 mL), r.t., 24–36 h
O
O
O
O
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54%
58%
30%
44%
14%
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60%
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Scheme 6 Relay oxidation of aromatic alcohols to ketones in air
extension of the relay chemistry23 and the relay oxidation
under aerobic conditions, we also found that this catalyst-
free relay oxidation induced by diketone could be applied
to the oxidation of general aromatic alcohols (up to 75%
yield). Further research on the relay chemistry and air ox-
idation is currently under way in our laboratory.
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
Financial support by the National Natural Science Founder of China
(No. 20973051 and 21173064), Program for Excellent Young
Teachers in Hangzhou Normal University (HNUEYT, JTAS 2011-
01-014), and Zhejiang Provincial Natural Science Foundation of
China (Y4090139) is appreciated.
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