Enhanced acyl radical formation in the Au nanoparticle-catalysed aldehyde oxidation

Article abstract of DOI:10.1039/b918200d

EPR spectroscopy and spin-trapping experiments showed that polymer-encapsulated Au nanoparticles promote aldehyde oxidation via a radical pathway by initiating formation of acyl radicals.

Full text of DOI:10.1039/b918200d

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Enhanced acyl radical formation in the Au nanoparticle-catalysed
aldehyde oxidationw
a
b
b
a
Marco Conte, Hiroyuki Miyamura, Sh u¯ Kobayashi and Victor Chechik*
Received (in Cambridge, UK) 4th September 2009, Accepted 7th October 2009
First published as an Advance Article on the web 19th October 2009
DOI: 10.1039/b918200d
EPR spectroscopy and spin-trapping experiments showed that
polymer-encapsulated Au nanoparticles promote aldehyde
oxidation via a radical pathway by initiating formation of acyl
radicals.
radical (DTBA), an impurity present in MNP samples
1
0
(see ESIw). However, as the amount of DTBA is constant
in all samples, it can be used to estimate the relative amount of
MNP–COR adduct formed under different conditions. When
PBN was used as a spin trap, a peroxyl PBN–OOCR adduct
3
1
1
Aldehyde oxidation is an important reaction for the manu-
facture of organic acids, peracids and anhydrides, which
find applications as precursors for resins and pharmaceutical
was observed (a
= 13.4, a = 1.72 G) in aerobic condi-
N H
1
1
tions, while a PBN–COR adduct (a
= 14.3, a = 4.57 G)
H
N
was observed in partially deoxygenated mixtures (details are in
the ESIw).
Observation of acyl and peroxyl adducts is consistent with
1
products. Aldehydes can autoxidise in air via a radical
2+
or
2
pathway; this reaction is facilitated by Mn , Cu
3+
2
+
3
salts, which promote the initiation step. Gold nano-
Co
the free radical mechanism of aldehyde autoxidation reported
2
in the literature. This mechanism includes: (i) an initiation
4
particles can also efficiently catalyse aldehyde oxidation,
providing the advantage of a heterogeneous system. However,
many mechanistic aspects of the catalytic cycle and possible
pathways, particularly in basic media, are not well understood.
This prompted us to explore aldehyde oxidation over polymer-
step which leads to acyl radical formation, (ii) addition of
oxygen to acyl radicals to form peroxyl radicals, (iii) abstraction
of a hydrogen atom from the aldehyde by the peroxyl radical
(giving a peracid) during the propagation step, and (iv)
carboxylic acid formation from the aldehyde and peracid via
5
incarcerated Au nanoparticles (PI-Au) in the absence and
6
presence of base using EPR spin trapping methodology.
2
Baeyer–Villiger reaction. Consequently, using different spin
Spin-trapping methodology relies on the fast selective
addition (trapping) of short-lived radicals to a diamagnetic
spin trap, usually a nitrone or a nitroso compound, such as
traps with different affinity for different radicals, we could
monitor the overall reaction.
When the catalyst PI-Au was introduced into the system,
two effects were observed: a detectable increase in the acyl
adduct intensity (by 32% relative to DTBA), and increased
sharpening of the spectral lines, which is diagnostic of oxygen
consumption (Fig. 1a). This is consistent with the catalytic
action of the PI-Au nanocomposite. Control tests confirmed
that the catalyst cannot oxidise solvent (toluene) and hence the
observed radicals originate from the substrate.
5
,5-dimethyl-1-pyrroline-N-oxide (DMPO). The product of
this addition (spin adduct) is a persistent free radical (nitroxide)
with sufficiently long lifetime to enable detection by conven-
tional EPR spectroscopy. Magnetic properties of the spin
adducts often make it possible to assign the structure of the
original short-lived radicals. Structures of spin traps used in
this study are shown in Scheme 1.
PI-Au is a catalyst containing gold nanoparticles trapped in
5
a polymer matrix. The catalyst is prepared by reduction of
We subsequently tested the oxidation of aliphatic aldehydes,
e.g., valeraldehyde and acetaldehyde, both in the presence and
absence of PI-Au. Enhanced acyl radical generation was
detected for valeraldehyde within the first 5 min of the reaction
(spin adduct intensity more than doubled, Fig. 1b). For
acetaldehyde, acyl radicals could be detected only in the
presence of PI-Au catalyst (Fig. 1c).
Au(I) with sodium borohydride in the presence of polymer
solution (full preparation details are in the ESIw). This material
is an efficient alcohol oxidation catalyst. Reaction of
7
benzaldehyde with air in toluene at 50 1C in the presence of
a DMPO or MNP spin trap, gave rise to a persistent free
radical which was identified by X-band EPR as an acyl adduct
In order to confirm that the intensity of the radical spin
adducts is related to the catalytic efficiency, bulk tests were
carried out. A detectable increase in the yield of carboxylic
by comparison with the literature data, e.g., DMPO–COR
8
= 15.73 G) or MNP–COR adduct
adduct (a
N
= 14.03 G, a
H
9
= 8.1 G). EPR spectra recorded with an MNP spin trap
(
a
N
additionally contain a triplet with coupling constant a
N
=
1
5.4 G, which is characteristic of the di-tert-butyl aminoxyl
a
b
Department of Chemistry, University of York, Heslington, York,
UK YO10 5DD. E-mail: vc4@york.ac.uk; Tel: +44 1904 434185
Graduate School of Pharmaceutical Sciences, The University of
Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
w Electronic supplementary information (ESI) available: Experimental
procedures, details of polymer incarcerated nanoparticle synthesis and
1
characterization, H-NMR data, oxygen consumption trend and EPR
spectra. See DOI: 10.1039/b918200d
Scheme 1 Chemical structure of DMPO, PBN and MNP spin traps
(top) and trapping of a free radical (bottom).
This journal is ꢀc The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 145–147 | 145

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(
Table 1, entries 1–4). The turnover in the chain reactions may
be determined by the rate of termination rather than that
of initiation. For instance, simply replacing glass reaction
vials with polyethylene vials led to significantly increased
conversions (67% when PI-Au was used, see ESIw). This
suggests that termination can be controlled by the walls of
1
3
the reaction vessel. A related phenomenon was observed
1
4
when ZnO-supported gold catalyst (Au/ZnO) was used.
The oxidation in the presence of this catalyst was partially
suppressed. The conversion under conditions reported in
Table 1, was ca. 15%, which is much lower than that in a
catalyst-free reaction. We believe this is due to the termination
of radicals by the catalyst. This effect was confirmed by the
very low conversion observed when Au-free ZnO was added to
the uncatalysed reaction (7%, see ESIw). ZnO is a material rich
Fig. 1 Spin adducts formed from MNP in an oxidation reaction in
the absence (top) and presence (bottom) of PI-Au with: (a) benzaldehyde,
15
in neutral oxygen vacancies that could possibly quench
(b) valeraldehyde and (c) acetaldehyde. The three lines with bigger
peroxyl radicals. In fact, spin trapping experiments in the
presence of ZnO did not reveal any significant amount of
peroxyl adduct.
coupling constant in all spectra are due to di-tert-butyl
aminoxyl radicals (DTBA).
acids was observed for PI-Au catalysed benzaldehyde and
valeraldehyde oxidations, consistent with the EPR results
In order to further test the catalytic efficiency of PI-Au in
aldehyde oxidation, we carried out reactions in sealed vials in
the presence of TEMPO. The sensitivity of TEMPO EPR
(
Table 1, entries 1–4; full experimental details are in the ESIw).
These data suggest that the PI-Au catalyst can promote the
1
6
signals to the oxygen concentration makes it possible to
assess the conversion from the EPR data as TEMPO line
broadening is linearly proportional to the oxygen concentration.
The presence of excess TEMPO efficiently breaks the radical
chain process, so that the rate of deoxygenation is determined
by the initiation reaction. The rate of deoxygenation in the
presence of catalyst was ca. 2-fold higher than that in
the absence of catalyst, thus providing strong evidence that
the catalyst enhances the rate of initiation in aldehyde
oxidations (see ESI for further detailsw).
initiation step of the aldehyde oxidation by increasing the rate
of acyl radical formation. In order to confirm that the
catalysed oxidation indeed occurs via a free radical mecha-
0
0
nism, we carried out oxidations in the presence of 2,2 ,6,6 -
tetramethylpiperidine N-oxyl (TEMPO). While relatively
unreactive towards oxygen centred radicals, TEMPO is a
1
2
highly efficient quencher for carbon centred radicals.
TEMPO efficiently suppressed oxidation both in the presence
and absence of catalyst. For instance, for both benzaldehyde
and valeraldehyde, conversion values in the presence of
TEMPO dropped from ca. 50% to ca. 3%. (Table 1,
entries 5–8). Control tests using catalyst pre-treated with
TEMPO, showed a very similar activity to the untreated
catalyst (e.g., conversion of benzaldehyde in a 24 h reaction
with treated and untreated catalysts was ca. 56 and 59%,
respectively). Thus, the suppression of oxidation by TEMPO is
not due to the catalyst deactivation by the nitroxide. We
conclude therefore that the catalytic oxidation occurs via a
radical mechanism.
The overall reaction mechanism can thus be proposed as
follows (Scheme 2). It is likely that the reaction occurs via H
abstraction from the C–H bond by O2 activated over the
ꢁ
catalyst surface, probably via AuOO species. Activation of
molecular oxygen by Au catalysts has been proposed in other
6
similar reactions.
,17
The activation of the C–H bond would also be consistent
with a recent kinetic study of aldehyde oxidation over
1
9
Au/TiO2, via activated oxygen over the catalyst surface,
which illustrates that activation of the C–H bond takes place
in the rate-determining step.
The radical chain mechanism explains the rather moderate
increase in conversion in catalytic vs. non-catalytic reactions
Interestingly, different results were obtained when the
oxidation was carried out in the presence of base. Base is
2
often added to Au-catalysed oxidation reactions. Oxidation
0
Table 1 Conversion of benzaldehyde and valeraldehyde under
different experimental conditions
a
b
c
d
Entry Substrate
Catalyst
Reaction medium
Acid (%)
1
2
3
4
5
6
7
8
Benzaldehyde
—
Toluene
Toluene
Toluene
Toluene
Toluene/TEMPO
Toluene/TEMPO
Toluene/TEMPO
Toluene/TEMPO
29
50
46
54
1.2
1.5
3.6
4.4
Benzaldehyde PI-Au
Valeraldehyde
Valeraldehyde PI-Au
Benzaldehyde
Benzaldehyde PI-Au
Valeraldehyde
Valeraldehyde PI-Au
—
—
—
a
Reaction conditions: substrate (0.5 ml), solvent (0.5 ml), T = 50 1C,
P = 1 atm, reaction time 5 h. Catalyst (5 mg). TEMPO (100 ppm)
b
c
d
in toluene. Determined by H-NMR spectroscopy.
1
18
Scheme 2 Proposed aldehyde oxidation pathway.
1
46 | Chem. Commun., 2010, 46, 145–147
This journal is ꢀc The Royal Society of Chemistry 2010

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catalyst, strongly suggests that the radical pathway is
dominant for both reactions in the presence of base.
In conclusion, gold catalyses aldehyde oxidation by
accelerating the initiation step in the radical pathway leading
to enhanced formation of acyl radicals. In the presence of
base, hydride transfer from the geminal diol is possible,
however, this is likely to be a minor secondary pathway as
the radical route appears to be dominant.
Funding for this work was provided by the EPSRC
(
(
grant EP/E001629/1). The authors thank Dr Moray Stark
University of York) for helpful discussions.
Notes and references
1
2
3
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Fig. 2 DMPO spin adducts formed in a reaction of acetaldehyde in
basic media. (a) DMPO–H formation in the presence of PI-Au and
(
b) DMPO–CH
R = COH).
2
R adduct formation in the absence of catalyst
4 (a) C. Marsden, E. Taarning, D. Hansen, L. Johansen,
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(
of benzaldehyde, valeraldehyde and acetaldehyde in the
presence of NaOH, PI-Au and DMPO gave a weak DMPO–H
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1
N
adduct (a = 14.49, aH(1) = aH(2) = 18.69 G). In the
6
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2
8
fied (aN = 14.55, aH = 21.07 G), (Fig. 2 and ESIw). To
explore the origin of the DMPO–H adduct, perdeuterated
7
8
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catalyst. A DMPO–D adduct (a
N
= 14.54, a
.84 G) was observed, thus suggesting that the hydrogen
adduct is formed as a consequence of C–H bond cleavage
H D
= 18.79, a =
2
2
2
9 M. Novak and B. A. Brodeur, J. Org. Chem., 1984, 49, 1142.
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1
1
(
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1
1
3 B. J. Hwang, Ind. Eng. Chem. Res., 1994, 33, 1897.
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2
3
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1
5 F. Leiter, H. Alves, D. Pfisterer, N. G. Romanov, D. M. Hofmann
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2
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1
7 D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu,
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8 Decomposition of AuOOH species, leading to water, can possibly
occur via hydrogen peroxide through oxygen reduction reaction, see:
2
4
oxidation to a nitroxide (not detected for benzaldehyde
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1
In order to probe whether hydride transfer from the geminal
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(
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2
2
2
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(
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70% (the by-product was benzyl alcohol which was formed
2
2
2 P. Maillard and C. Giannotti, Can. J. Chem., 1982, 60, 1402.
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4
via Cannizzaro reaction). The significant suppression of
2
4 P. Ionita, B. C. Gilbert and A. C. Whitwood, J. Chem. Soc., Perkin
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reaction by TEMPO, both in the presence and absence of
This journal is ꢀc The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 145–147 | 147