Hydrosilane and bismuth-accelerated palladium catalyzed aerobic oxidative esterification of benzylic alcohols with air

Article abstract of DOI:10.1039/c2cc34117d

In a palladium-catalyzed oxidative esterification, hydrosilane can serve as an activator of palladium catalyst with bismuth, thus leading to a novel ligand- and silver-free palladium catalyst system for facile oxidative esterification of a variety of benzylic alcohols in good yields.

Full text of DOI:10.1039/c2cc34117d

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COMMUNICATION
Hydrosilane and bismuth-accelerated palladium catalyzed aerobic
oxidative esterification of benzylic alcohols with airw
a
a
a
a
b
Xing-Feng Bai, Fei Ye, Long-Sheng Zheng, Guo-Qiao Lai, Chun-Gu Xia and
ab
Li-Wen Xu*
Received 8th June 2012, Accepted 6th July 2012
DOI: 10.1039/c2cc34117d
In a palladium-catalyzed oxidative esterification, hydrosilane
can serve as an activator of palladium catalyst with bismuth,
thus leading to a novel ligand- and silver-free palladium catalyst
system for facile oxidative esterification of a variety of benzylic
alcohols in good yields.
a simple and selective catalytic oxidative esterification of
benzylic alcohols promoted by palladium under mild conditions
with air. This procedure is rapid and highly efficient and
operates with a silver-free activator at ambient temperature
and pressure.
In palladium-catalyzed oxidation, it is well known that this
procedure involves reduction of a Pd(II) species to a Pd(0)
species, and then the oxidative addition of substrates oxidizes
In recent years, the development of palladium-catalyzed
oxidations that apply molecular oxygen has become a hot
1
topic in organic synthesis and homogeneous catalysis. Apart
4
palladium to regenerate a palladium species in catalytic cycle.
Inspired by previous studies on palladium catalysis, we
assumed that the two-electron reduction of Pd(II) to Pd(0)
through a hydridopalladium intermediate is crucial to the
palladium-catalyzed oxidative esterification, therefore we
supposed that the addition of hydrosilanes could promote
and accelerate the reduction of Pd(II) to Pd(0), which would be
useful in the enhancement of catalytic efficiency of palladium-
catalyzed oxidative esterification. Hydrosilanes are present in
many widely used compounds and used as an indispensable
reagent in the field of organosilicon chemistry and organic
synthesis. In the past decades, much effort has been given to
their synthetic application in the addition of the Si–H bond in
hydrosilanes across a CQC, CQO, CQN, or triple bond,
also named hydrosilylation of olefins, carbonyl compounds,
from direct oxidation of alcohols to aldehydes or ketones,
transition metal complex-catalyzed oxidative esterification of
alcohols remains of high interest to academic and industrial
2
chemists. Although palladium-catalyzed oxidation reactions
using oxygen as the terminal oxidant have been widely
investigated, so far relatively few works has been reported
on palladium-catalyzed aerobic oxidative esterification from
3
3a
3b
two different alcohols. In 2011, Beller’s and Lei’s groups
demonstrated at the same time the utility of a palladium
complex (Pd-phosphine) for the catalytic oxidative esterifica-
tions of primary alcohols with O
hexafluorophosphate (AgPF6) or silver tetrafluoroborate
AgBF ) respectively. These works constructed an important
2
in the presence of silver
(
4
milestone in the palladium-catalyzed oxidative esterification of
alcohols. However, the present synthetic methods still suffered
from one or more disadvantages, including slow rates with a
long reaction time and expensive silver salts (about 830 RMB
5
imines, or alkynes accordingly. Hydrosilanes are also stable
6
7
reducing and silylation agents and a transition metal or
strong Lewis acid catalyst is required to drive this reaction.
Their reaction by-products are safer and more easily handled
than other organometallic-based reducing agents. To the best
of our knowledge, no reports on the use of hydrosilanes in
oxidative esterification and similar oxidative transformations
of alcohols are known to date.
À1
À1
or 132 USD g for AgPF
AgBF ). Therefore the development of new palladium-based
6
and 280 RMB or 45 USD g for
4
oxidation chemistry and novel palladium-based catalytic
systems with higher reaction efficiency, shorter reaction times
and good selectivity remains a challenging task in this reac-
tion, especially using an air atmosphere directly as the terminal
oxidant and silver-free reaction conditions. Herein, we report
Upon the hypothesis that hydrosilanes could improve
and accelerate the palladium-catalyzed oxidation of alcohols,
the palladium-catalyzed oxidation of benzylic alcohol was
performed with reaction conditions similar to Beller’s work
3a
(Pd(OAc) in methanol) employing various hydrosilanes in
2
a
Key Laboratory of Organosilicon Chemistry and Material
Technology of Ministry of Education, Hangzhou Normal University,
Hangzhou 310012, P. R. China. E-mail: liwenxu@hznu.edu.cn;
Fax: +86 2886 5135; Tel: +86 2886 5135
the absence of any ligands and additives. Interestingly, a
methanol solution containing benzylic alcohol (1a) and
triethylsilane in a 1 : 1.5 molar ratio was heated at 40 1C for
b
State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou, 730000, P. R. China.
E-mail: licpxulw@yahoo.com
w Electronic supplementary information (ESI) available. See DOI:
0.1039/c2cc34117d
24 h in the presence of a catalytic amount of Pd(OAc)
2
and
Na CO (2 eq.), causing the formation of mixtures of benzyloxy-
3
2
triethylsilane (2a), benzaldehyde (2b), and methyl benzoate (2d).
1
However, the reaction proceeded in a nonselective manner and
8
592 Chem. Commun., 2012, 48, 8592–8594
This journal is c The Royal Society of Chemistry 2012

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Table 1 Silane-initiated ligand-free palladium catalyzed oxidation of
benzylic alcohol in methanol
a
Conversion (%) 2a/2b/2c/2d
b
Entry Base
Silane (x eq.)
1
2
3
4
5
6
7
8
9
1
1
1
Na
—
2
CO
3
3
Et
—
—
Et
Et
3
SiH (1.5)
91
14
29
56
84
78
9
92
18/60/0/13
0/2/12/0
0/21/0/8
1/11/43/1
0/23/60/1
0/20/58/0
0/9/0/0
12/57/0/16
4/16/0/80
0/6/0/94
0/0/0/100
0/30/0/70
c
c
c
Na
—
2
CO
c
3
SiH (1.5)
SiH (0.2)
c
Scheme 1 Proposed mechanism of palladium-catalyzed oxidative
—
—
3
c
esterification of benzyl alcohol in the presence of PMHS.
PhMe SiH (0.2)
2
TEOS (0.1)
Na
Na
Na
Na
Na
Na
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
3
Et
Et
3
SiH (1.0)
SiH (0.1)
ester and the palladium hydride intermediate (I) which under-
goes reductive elimination to regenerate the Pd(0) species.
Having established the oxidative esterifcation of alcohol by
palladium with PMHS, we turned to evaluate the scope of
3
>99
0
1
2
2
(EtO) SiHMe (0.2) >99
PMHS (0.2)
PMHS (0.1)
>99
>99
3a,b
a
benzylic alcohols. Unfortunately, similar to previous reports,
Reaction conditions: 0.5 mmol of benzylic alcohol, 5 mol%
, 2 eq. Na CO , 2 mL of MeOH, 40 1C, in air, for 24 h.
Pd(OAc)
b
2
2
3
the oxidative esterification was very sensitive to the substituent
on the phenyl ring of the benzylic alcohol (see Table S1, ESIw).
For most substituted benzylic alcohols, the aldehyde was the
major product and the yield of the ester was poor. For the
example of o-tolylmethanol, a trace ester product was detected
while 2-methylbenzaldehyde was observed in only 43% yield.
In a hope to achieve better substrate conversion and selectivity,
we examined other inorganic or organic bases when the
palladium-catalyzed oxidation of o-tolylmethanol was used a
model reaction. Gratifyingly, t-BuONa was found to be an
effective base for the Pd-catalyzed oxidation. However, the
selectivity of ester to aldehyde was low (24 : 70) and 2-methyl-
benzaldehyde was observed in 70% yield (Table S2, ESIw).
c
The selectivities were determined by GC analysis. No addition of
silanes or any additives.
the aldehyde was a major product while the ester was detected
with by-product in a promising yield. In the absence of
hydrosilanes and ligands, poor conversions of alcohol were
determined by GC (Entries 2 and 3, Table 1), which prompted
us to investigate the effect of hydrosilanes on the palladium-
catalyzed oxidation of alcohols. As shown in Table 1, different
hydrosilanes were tested in the absence or presence of base, and
consequently, these hydrosilanes exhibited promising activation
in the palladium-catalyzed oxidative transformation of benzylic
alcohol. The anticipated ester 2d was not formed in the absence
of base while the aldehyde 2b and corresponding acetal product
3
Consistent with Lei and Beller’s results, the present catalyst
system still exhibited limited ability in the key step of
b-hydride elimination of the hemiacetal intermediate while
the corresponding 2-methylbenzaldehyde was obtained in high
yield. Meanwhile, we found several silver salts showed poor
selectivity in this reaction, and the desired ester was obtained
in low yield under the PHMS-initiated Pd-catalyzed oxidation
(see ESIw). Thus we investigated the effect of various metal
salts on the palladium-catalyzed oxidative esterification of
o-tolylmethanol. To our surprise, employing bismuth halides
as cocatalyst resulted in completed conversion and the
selectivity is excellent (>99% of ester). Notably, in fact almost
2
c were obtained in good yields (Entries 4–6, Table 1). In this
reaction, when tetraethoxysilane without a Si–H moiety was
used as an additive, poor conversion of benzylic alcohol was
detected. Fortunately, the selectivity of oxidation with respect
to the ester product increased from 16 to 100% in the four
mixtures (Entries 8–12). To our delight, a highly chemoselective
oxidation took place in the presence of diethoxy(methyl)silane
or polymethylhydrosiloxane (PMHS) (Entries 10 and 11).
Control experiments proved that a catalytic amount of poly-
methylhydrosiloxane (PMHS) was enough to guarantee the
activation ofthe palladium catalyst in the oxidative esterification
of benzylic alcohol.
no aldehyde was detected when BiCl was used a cocatalyst. It
3
should be noted that the short reaction time (2 hr) and milder
conditions at room temperature also led to complete conversion
(>99%) and excellent isolated yield (87%). Other bismuth
Therefore, these facts in the model reaction suggest that
PMHS accelerated the reduction of Pd(II) to Pd(0) and then formed
8
polysiloxane-stabilized palladium colloids (Pd@polysiloxane),
salts, except BiI , were also effective for the Pd-catalyzed
3
which generated a highly efficient palladium catalyst with high
9
selectivity. As shown in Scheme 1, on the basis of previous
oxidative esterification of o-tolylmethanol (Table S3, ESIw).
In comparison to the control experiments without PMHS, a
decreased yield (73%) was obtained with longer reaction times
(8 h), which suggests that PMHS accelerates the formation of
the activated Pd(0) species and that bismuth promotes the
formation of the hemiacetal or b-hydride elimination of inter-
mediate IV or VI (Scheme 1) efficiently to corresponding
3
,10
studies
and this work, we propose that the palladium-
catalyzed oxidative esterification proceeds through a two step
catalytic cycle: 1) oxidative addition and alcoholysis of Pd(0)
intermediate (II) with alcohol subsequently to generate intermediate
(IV) and then afford the aldehyde via b-hydride elimination; and 2)
insertion-elimination of b-hydride of hemiacetal (V) to generate the
aldehyde and ester respectively. Accordingly, BiCl is one of
3
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8592–8594 8593

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oxidative esterification is also successful in an intramolecular
manifold. The reaction provides a catalytic route toward the
construction of 3-arylphthalides.
In summary, we have developed a novel palladium catalyst
system for oxidative esterification reactions of primary
benzylic alcohols that focus on the findings of catalytic activity
of bismuth and PMHS in this reaction. This silver-free catalyst
system for the oxidative esterification has several advantages
of high yields, ligand-free, environmentally benign, direct use
of air atmosphere as terminal oxidant or avoiding the use of
oxygen balloon, and the reaction efficiency is high.
The authors gratefully thank the financial support of the
National Natural Science Foundation of China (NSFC, No.
2
0973051 and 21173064) and Program for Excellent Young
Teachers in Hangzhou Normal University (HNUEYT, JTAS
011-01-014).
2
Notes and references
1
2
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412–9423; (b) K. M. Gligorich and M. S. Sigman, Chem.
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M. Akita, Y. Tanigaki, S. Takizawa and H. Sasai, Org. Lett., 2011,
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Scheme 2 Oxidative esterification of benzylic alcohols catalyzed by
palladium/bismuth in the presence of PMHS.
13, 3506–3509; (e) T. Yasukawa, H. Miyamura and S. Kobayashi,
Chem.–Asian J., 2011, 6, 621–627.
3
(a) S. Gowrisankar, H. Neumann and M. Beller, Angew. Chem.,
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Y. Deng, Y. Li and A. W. Lei, Angew. Chem., Int. Ed., 2011, 50,
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Scheme 3 Synthesis of 3-aryl phthalides via oxidative esterification.
À1
the cheapest metal salts (0.9 RMB or 0.0014 USD g ), which
4
5
Handbook of Organopalladium Chemistry for Organic Synthesis, ed.
E. i. Negishi, Wiley-Interscience, New York, 2002, p. 2853.
(a) D. Troegel and J. Stohrer, Coord. Chem. Rev., 2011, 255,
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Chem., Int. Ed., 2011, 50, 6004–6011.
would make the present palladium catalyst system more
practically useful in organic synthesis.
With the optimized conditions, a variety of benzylic
alcohols were examined by using bimetallic Pd/Bi in the
presence of PMHS. To our delight, the oxidative esterification
of various benzylic alcohols proceeded with remarkably high
yields and excellent selectivities in 2 hrs. The scope of
the bismuth and PHMS-accelerated Pd-catalyzed oxidative
esterification is depicted in Scheme 2. Unsubstituted benzylic
alcohol gave the desired product 2d smoothly and excellent
isolated yield was obtained (84%). No aldehdye was observed
in this case. For electron-donating group substituted aromatic
alcohols, all runs demonstrated complete oxidative esterifica-
tion in excellent selectivities to provide the corresponding
esters (up to 95%). For the strong electron-withdrawing
groups, such as a nitro moiety, the conversion was incomplete
in two hours and the isolated yield of the corresponding
ester 3d was moderate (58%). In addition, halide-containing
benzylic and allylic alcohols also resulted in complete conversion
with excellent selectivities.
6
7
Selected examples, (a) L. H. Sommer and J. E. Lyons, J. Am.
Chem. Soc., 1967, 89, 1521–1522; (b) L. H. Sommer and
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7
061–7067; (d) M. A. Brook, Silicon in Organic, Organometallic,
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8
9
8
493–8500.
The excellent catalytic activity of Pd@polysiloxane in this oxida-
tion reaction maybe due to the structural specificity of the possible
8
‘‘polysiloxane-palladium’’ nanocomposite (also see Fig. S1, ESIw).
1
0 (a) C. Liu, S. Tang, L. Zheng, D. Liu, H. Zhang and A. Lei,
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Phthalides are important building blocks in the field of
11 Recent examples: (a) J. Liu, F. Li, E. L. Kim, J. L. Li, J. Hong,
K. S. Bae, H. Y. Chung, H. S. Kim and J. H. Jung, J. Nat. Prod.,
1
1
medicinal chemistry and organic synthesis. Thus we expanded
the scope of this oxidative esterification to include diols that led
to selective oxidation and intramolecular esterification. As
shown in Scheme 2 (3l) and Scheme 3, this palladium-catalyzed
2
011, 74, 1826–1829; (b) D. H. T. Phan, B. Kim and V. M. Dong,
J. Am. Chem. Soc., 2009, 131, 15608–15609; (c) Z. S. Ye, G. L. Lv,
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8
594 Chem. Commun., 2012, 48, 8592–8594
This journal is c The Royal Society of Chemistry 2012