Direct syntheses of styryl ethers from benzyl alcohols via ag nanoparticle-catalyzed tandem aerobic oxidation

Article abstract of DOI:10.1021/cs400295h

Ag nanoparticle-catalyzed aerobic oxidation of benzyl alcohols in basic DMSO gave efficient formation of styryl ethers, featuring single carbon transfer and Ci - C and C-O bond formation. A deuterium labeling experiment established that DMSO was the carbo

Full text of DOI:10.1021/cs400295h

Letter
pubs.acs.org/acscatalysis
Direct Syntheses of Styryl Ethers from Benzyl Alcohols via Ag
Nanoparticle-Catalyzed Tandem Aerobic Oxidation
Qi Zhang,† Shuangfei Cai,‡ Linsen Li,‡ Yifeng Chen,† Hongpan Rong,‡ Zhiqiang Niu,‡ Junjia Liu,§,⊥
Wei He,*,† and Yadong Li*,‡
‡
†Tsinghua-Peking Joint Center for Life Sciences and School of Medicine, Department of Chemistry, Tsinghua University, Beijing
100084, China
§Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
S
* Supporting Information
ABSTRACT: Ag nanoparticle-catalyzed aerobic oxidation of
benzyl alcohols in basic DMSO gave efficient formation of
styryl ethers, featuring single carbon transfer and CC and
C−O bond formation. A deuterium labeling experiment
established that DMSO was the carbon source. Further
experiments showed the reaction proceeded through a styryl
sulfoxide intermediate originating from basic DMSO and
transient benzaldehyde. Control reactions in the absence of the
Ag NPs or air indicated that oxidation of the styryl sulfoxide was required for the final C−O bond formation. This work
demonstrated that metal nanoparticles could be applied to tandem heterogeneous catalysis in organic chemistry.
KEYWORDS: styryl ether, Ag nanoparticle, aerobic oxidation, tandem catalysis
elective oxidation of benzyl alcohol to benzaldehyde is an
S
important process.1,2 In the past decade, methods utilizing
molecule oxygen as the sole oxidant have attracted much
attention.3,4 Remarkable progress has been made with
supported nanoparticle catalysts, including PI-CB/Au−Pt,5
Au/Al2O3,6 Au/CeO27 and so on. It is still desirable to develop
catalysts with lower cost and higher efficiency.
For their special roles in heterogeneous catalysis, metal
nanoparticles have been shown to catalyze many other
oxidation reactions in which the selectivity and efficiency
could be greatly enhanced. In this context, we recently
discovered that various metal nanoparticles (NPs), such as
Ag, Au, Pd, and Pt NPs were highly active in dioxygen
activation under ambient conditions. Oxidative coupling of
various anilines with 0.6 mol % Ag nanoparticles (∼12 nm) in
basic DMSO gave efficient and selective formation of
azobenzenes8 (eq 1). This spurred us to explore the selective
oxidation of alcohols in the Ag NPs/DMSO system.
Figure 1. TE images (a) TEM and (b) HRTEM of Ag NPs; (c) TEM
of the Ag/C.
are monodispersed spheres with a very narrow size distribution
(4 nm) (Figure 1a and b). After the Ag NPs were supported on
activated carbon, their morphology and size did not undergo
apparent change (Figure 1c).
To our surprise, when benzyl alcohol 1a was treated with 1
mol % Ag/C under air in basic DMSO, no benzaldehyde
(C7H6O) was observed. Instead, an unexpected product was
isolated in 93% yield (Figure 2a). High-resolution mass
spectroscopy indicated that the exact mass of the [M + H]+
was 211.1116, which fits with the formula C15H14O. This data
suggested that an extra single carbon had been incorporated
into the product. The 1H NMR shows olefinic protons (Figure
2b), indicative of a CC bond that was further confirmed by
13C NMR. Accordingly, the product was identified as the styryl
ether (E)-2aa (Figure 2a). Notably, styryl ethers are important
building blocks in organic syntheses and polymer chem-
Herein, we report that carbon-supported Ag nanoparticles
(Ag/C) catalyzed efficient direct formation of styryl ethers
from benzyl alcohols. Moreover, our mechanistic study
demonstrated that the ability of the Ag NPs in oxygen
activation orchestrated a tandem aerobic oxidation that
accounts well for the reaction outcome.
Received: April 19, 2013
Revised: June 11, 2013
Published: June 18, 2013
Ag NPs were synthesized using a protocol described
elsewhere.9−11 Figure 1 shows that the as-obtained Ag NPs
© 2013 American Chemical Society
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Letter
Scheme 1. The Carbon Source is DMSO
involving sulfur-,24 sulfoxide-,25 and sulfonium ylide26 have
been extensively reported, DMSO is rarely documented as a
single carbon source.
Next, we investigated the process associated with the CC
bond formation (Scheme 2). Close study of the reaction by GC
Scheme 2. Styryl Sulfoxide Is the Intermediate
1
Figure 2. Styryl ether formation: (a) reaction equation, (b) H NMR
of 2aa, and (c) deuterated 2aa.
unveiled an intermediate whose concentration decreased
gradually after 1 h. This intermediate was subsequently isolated
and identified as styryl sulfoxide 5a. Discrete sulfoxide 5a could
be cleanly converted to product 2aa under reaction conditions,
suggesting it is a likely precursor for the final product. Although
benzaldehyde was not detected in the reaction mixture, it could
react with basic DMSO under nitrogen to yield sulfoxide 5a
(Supporting Information), suggesting that the benzyl alcohol
1a was probably oxidized into transient benzaldehyde 3a prior
to sulfoxide 5a formation.
istry.12,13 Current syntheses of styryl ether rely on prefunction-
alized starting materials such as vinyl halides,14 allyl ethers,15
α,β-unsaturated carbonyls,16 and sulfones.17 Reported direct
synthesis of styryl ethers from alcohols requires harsh reaction
conditions (180−200 °C, 20−50 atm acetylene).18 The mild
conditions and high efficiency of our method were, indeed,
remarkable and intriguing.
To establish that the reaction is a Ag/C-catalyzed aerobic
oxidation, we carried out several control experiments
(Supporting Information). No reaction occurred under nitro-
gen atmosphere. Very low conversion (<5%) was observed
when Ag NPs was absent. In line with this Ag catalysis, the
reaction rate was inversely correlated to the size of the Ag NPs:
Ag NPs of smaller size have higher ability in dioxygen
activation.8 Further experiments suggested that the reaction
was catalyzed by Ag NPs but not leached Ag ions. The Ag/C
catalyst could be recycled for five runs without activity decay.
This evidence suggested a tandem catalysis featuring our Ag
NPs as the heterogeneous catalyst and oxygen as the oxidant.
Heterogeneous tandem catalysis is surprisingly rare in the
literature,19 with the few examples limited to “heterogenized”
homogeneous metal complexes.20−23
We then studied the C−O bond formation en route from
styryl sulfoxide 5a to the product 2aa (Scheme 3). Specifically,
Scheme 3. The C−O Bond Formation Requires Oxygen
The reaction raised three interesting questions: What was the
source of the extra carbon in the product (Figure 2a, green)?
How did the CC bond formation occur? What was the
mechanism of the C−O bond formation?
we wonder if Ag/C catalyst or oxygen was involved in this step.
To avoid background reaction of benzyl alcohol in DMSO, we
carried out reactions of 5a with benzyl alcohol in DMF under
air or nitrogen atmosphere (Scheme 3, eq 1) as a complement
to the more closely mimicked condition in which p-
methoxybenzyl alcohol 1b reacted with sulfoxide 5a in
DMSO (Scheme 3, eq 2). In both solvents, oxygen was
required for the full conversion of the sulfoxide 5a. Additional
experiments showed that Ag NPs were also required for this
step (Supporting Information). These observations suggested
that the oxidation of styryl sulfoxide is crucial to the C−O bond
formation, possibly via styryl sulfone, since discrete styryl 7a
sulfone was rapidly converted into the styryl ether 2aa under
We first set out to identify the source of the single carbon
(Scheme 1). From the observation that a similar reaction in
DMF under otherwise identical conditions gave only
benzaldehyde 3a (Scheme 1, eq 1), we suspected that DMSO
was the carbon source. Indeed, adding 3.0 equiv of DMSO to
the reaction in DMF furnished product 2aa in 48% isolated
yield (Scheme 1, eq 1). When the reaction was performed in
1
DMSO-d6 (Scheme 1, eq 2), H NMR of the final product
showed nearly 100% deuterium incorporation at the extra
carbon (Figure 2c). These experiments proved DMSO’s role as
the single carbon source. Although methylene transfer reactions
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Letter
the reaction conditions (Supporting Information). This
hypothesis was further substantiated by the fact that under
nitrogen, the sulfoxide 5a underwent slow disproportionation
to give the styryl ether 2aa together with approximately equal
molar sulfide 6a.
Given the above results, we postulated the reaction
mechanism to be as depicted in Scheme 4. First, Ag/C-
activation of molecule oxygen as well as in the oxidation of the
benzyl alcohol.
With the above results in hand, we set out to examine the
synthetic utility of the Ag/C catalyst. Table 1 shows that several
substituted benzyl alcohols are compatible with the reaction
conditions, giving the corresponding homocoupling products in
excellent yields.
a
Scheme 4. Proposed Mechanism
Table 1. Reactions of Substituted Benzyl Alcohols
catalyzed aerobic oxidation of the benzyl alcohol 1a led to the
benzaldehyde 3a, followed by rapid condensation with DMSO
to give the styryl sulfoxide 5a. A second Ag/C-catalyzed aerobic
oxidation gave rise to the styryl sulfone 7a, which is the rate-
limiting step. Finally, addition of an alkoxide anion to the α-
position of sulfone 7a formed the benzylic anion that
underwent elimination to furnish the final product 2aa.
Because the Ag/C-catalyzed aerobic oxidations of both the
benzyl alcohol and the styryl sulfoxide are crucial for the
outcome of the reaction, we performed a series EPR
experiments to further elucidate these oxidation steps (Figure
3). No superoxide anion radical (O2−•) was observed in the
a
Reaction conditions: 1 (0.5 mmol), KOH (1 equiv), Ag/C (1 mol
%), DMSO (2 mL), air (1 atm), rt, 4 h. Isolated yields.
As we showed that the sulfoxide was a viable precursor,
synthesis of asymmetric styryl ethers is possible with our Ag/C
catalyst. As illustrated in Table 2, reactions between various
a
Table 2. Reaction of Sulfoxide 5a with Alcohols
Figure 3. EPR spectra using DMPO (0.02 mmol) as the spin trap.
Conditions: alcohol (0.15 mmol), Ag/C (0.0015 mmol) in DMSO
(0.5 mL) at room temperature.
a
Reaction conditions: 5a (0.25 mmol), 1 (0.25 mmol), KOH (1
equiv.), Ag/C (1 mol %), DMSO (1 mL), air (1 atm), r.t, 3 h. Isolated
yields.
blank solvent (Figure 3a), but KOH was found to promote the
oxygen activation to a small extent (Figure 3b). In the presence
of Ag/C, a persistent signal of the superoxide anion radical was
recorded (Figure 3c). Importantly, the oxygen activation by the
Ag/C was not enhanced by the KOH (Figure 3c, d). When
benzyl alcohol was added into the DMSO/KOH system, only
benzyl cation radical was observed (Figure 3e). This result
suggests that the superoxide anion radical was rapidly
consumed in the reaction. Introduction of Ag/C into the
reaction caused an obvious shift of the benzyl cation radical
signal (Figure 3f), indicating that this radical might be
associated with the Ag/C catalyst. This evidence collectively
confirmed that the Ag/C catalyst plays an important role in the
alcohols, including secondary alcohol 1e and aliphatic alcohol
1f, and styryl sulfoxide 5a furnished the desired products in
high efficiency. It is expected that this method could find
further application in the preparation of styryl ether with
different substitutions.
In conclusion, we discovered that Ag NPs are an efficient
catalyst for direct synthesis of styryl ethers from benzyl alcohols
under ambient conditions. Our mechanistic study revealed
reaction processes accounting well for the single carbon transfer
as well as CC and C−O bond formations. Most importantly,
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Letter
(15) Crivello, J. V.; Kong, S. J. Org. Chem. 1998, 63, 6745−6748.
(16) Jones, D. E.; Morris, R. O.; Vernon, C. A.; White, R. F. J. Chem.
Soc. 1960, 2349−2366.
both control experiments and EPR study proved that the Ag
NPs’ ability in dioxygen activation was crucial for the success of
this tandem aerobic oxidation reaction. We further showed that
the Ag NP catalyst is applicable to the syntheses of structurally
diverse styryl ethers. This work demonstrates that applying
nanoparticle catalysts to organic reactions could lead to rapid
formation of molecular complexity rivaling that of the
homogeneous catalysis, which is a highly desired goal in
heterogeneous catalysis.
(17) Surprenant, S.; Chan, W. Y.; Berthelette, C. Org. Lett. 2003, 5,
4851−4854.
(18) Reppe, W. Ann. Chim. 1956, 601, 84−111.
(19) Sun, H.; Su, F. Z.; Ni, J.; Cao, Y.; He, H. Y.; Fan, K.-N. Angew.
Chem., Int. Ed. 2009, 48, 4390−4393.
(20) Cohen, B. J.; Kraus, M. J.; Patchornik, A. J. Am. Chem. Soc. 1981,
103, 7620−7629.
(21) Gelman, F.; Blum, J.; Avnir, D. J. Am. Chem. Soc. 2000, 122,
11999−12000.
ASSOCIATED CONTENT
* Supporting Information
Preparation of Ag nanoparticles, procedure for catalytic
reactions, and characterization of products. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
(22) Choudary, B. M.; Chowdari, N. S.; Madhi, S.; Kantam, M. L.
Angew. Chem. 2001, 113, 4755; Angew. Chem., Int. Ed. 2001, 40, 4619−
4623.
S
(23) Poe, S. L.; Kobasl
2006, 128, 15586−15587.
(24) Weber, L.; Luecke, E. Organometallics 1986, 5, 2114−2149.
(25) Corey, E. J.; Chaykovsky, M. J. Am. Chem. Soc. 1965, 87, 1353−
1364.
̌
ija, M.; McQuade, D. T. J. Am. Chem. Soc.
AUTHOR INFORMATION
Corresponding Author
*E-mail: (W.H.) whe@tsinghua.edu.cn, (Y.L.) ydli@mail.
tsinghua.edu.cn.
■
(26) Johnson, C. R.; Schroeck, C. W.; Shanklin, J. R. J. Am. Chem.
Soc. 1973, 95, 7424−7431.
Present Address
⊥(Junjia Liu) Eastman Chemical Company, P.O. Box 1972,
Kingsport, TN 37662, USA.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
W. He gratefully acknowledges financial support from the
Tsinghua-Peking Joint Center for Life Sciences, the National
key Basic Research Program of China (2012CB224802). Y. Li
thanks the State Key Project of Fundamental Research for
Nanoscience and Nanotechnology (2011CB932401 and
2011CBA00500) and the National Natural Science Foundation
of China (Grant No. 21221062 and 21131004).
REFERENCES
■
(1) Kroschwitz, J. I. Kirk-Othmer Encyclopeida of Chemical Technology,
4th ed.; Wiley-Interscience Publications: New York, 1992; Vol. 4.
̀
(2) Centi, G.; Cavani, F.; Trifiro, F. Selective Oxidation by
Heterogeneous Catalysis; Kluwer Academic/Plenum Publishers: New
York, 2001.
(3) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.;
Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.;
Hutchings, G. J. Science 2006, 311, 362−365.
(4) Turner, M.; Golovko, V. B.; Vaughan, O. P. H.; Abdulkin, P.;
Berenguer-Murcia, A.; Tikhov, M. S.; Johnson, B. F. G.; Lambert, R.
M. Nature 2008, 454, 981−983.
(5) Kaizuka, K.; Miyamura, H.; Kobayashi, S. J. Am. Chem. Soc. 2010,
132, 15096−15098.
(6) Zhao, G.; Ling, M.; Hu, H.; Deng, M.; Xue, Q.; Lu, Y. Green
Chem. 2011, 13, 3088−3092.
(7) Tanaka, A.; Hashimoto, K.; Kominami, H. J. Am. Chem. Soc.
2012, 134, 14526−14533.
(8) Cai, S.; Rong, H.; Yu, X.; Liu, X.; Wang, D. S.; He, W.; Li, Y. D.
ACS Catal. 2013, 3, 478−486.
(9) Wang, D. S.; Xie, T.; Peng, Q.; Li, Y. D. J. Am. Chem. Soc. 2008,
130, 4016−4022.
(10) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437,
121−124.
(11) Wang, D. S.; Peng, Q.; Li, Y. D. Nano Res. 2010, 3, 574−580.
(12) Maercker, A. Org. React. 1965, 14, 270−490.
(13) Trost, B. M.; Fleming, I. Comprehensive Organic Synthesis:
Pergamon Press: Oxford, 1991; Vol. 9.
(14) Wan, Z.; Jones, C. D.; Koenig, T. M.; Pu, Y. J.; Mitchell, D.
Tetrahedron Lett. 2003, 44, 8257−8259.
1684
dx.doi.org/10.1021/cs400295h | ACS Catal. 2013, 3, 1681−1684