Selective Oxidation of Activated Alcohols by Supported Gold Nanoparticles under an Atmospheric Pressure of O2: Batch and Continuous-Flow Studies

Article abstract of DOI:10.1002/cctc.201700179

In the hunt for a simple, mild, and scalable protocol for gold nanoparticle-catalyzed oxidation of benzylic and allylic alcohols under O₂, we have used commercially available gold nanoparticles supported on alumina to selectively oxidize a large range of activated alcohols to the corresponding carbonyl compounds in good yields (68–99 %) and with excellent selectivity (ca. 100 %). The true heterogeneous nature of the catalysis by gold was demonstrated, allowing us to further adapt this protocol to continuous-flow reactors by using the tube-in-tube technology, in which higher yields were obtained thanks to an improved oxygenation of the reaction medium.

Full text of DOI:10.1002/cctc.201700179

Accepted Article
Title: Selective oxidation of activated alcohols by supported gold
nanoparticles under an atmospheric pressure of O2: batch and
continuous flow studies.
Authors: Pascal Giorgi, Nelli Elizarov, and Sylvain Antoniotti
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Selective oxidation of activated alcohols by supported gold
nanoparticles under an atmospheric pressure of O2: batch and
continuous flow studies.
Pascal D. Giorgi,[a] Nelli Elizarov,[a] and Sylvain Antoniotti*[a]
Abstract: In the need of a simple, mild and scalable protocol for
gold nanoparticles-catalyzed oxidation of benzylic and allylic
alcohols under O2, we have used commercially available gold
nanoparticles supported on alumina to selectively oxidize a large
range of activated alcohols to the corresponding carbonyl
oxidizing system both specific of the allylic alcohol motif, and
selective for the formation of aldehydes. In addition, this ideal
oxidative catalytic system had to be compatible with the
multicatalytic setting. These prerequisites were found with Au
NPs operating under O2.[7]
compounds in good yields (68-99%) and with excellent selectivity (ca. In the present paper, we detail the study of the influence of
100%). The true heterogeneous nature of the catalysis by gold was
demonstrated, allowing to further adapt this protocol to continuous
flow reactors using the tube-in-tube technology, where higher yields
were obtained thanks to an improved oxygenation of the reaction
medium.
experimental parameters, the optimization both in batch and
continuous flow reactors, and the substrate scope of the
oxidation reaction. We have been extending the scope beyond
known substrates such as benzyl alcohol, which has been the
focus of most of the studies in the field. Conveniently, we show
how we could run these oxidation reactions with simple,
commercially available Au NPs and under an atmospheric
oxygen pressure, in contrast with most of procedures found in
the literature requiring high temperatures and/or pressures or
sophisticated materials as catalysts.
Introduction
In the 1980’s, metal nanoparticles were generally neglected by
chemists, and their properties barely understood. At the same
time, the potential of gold in catalysis had yet to be unveiled,
gold being considered an inert metal. In spite of these two major
difficulties, Hutchings and Haruta were the first to report their
results on the use of finely divided gold particles in catalysis, the
former on the hydrochloration of acetylene,[1] the latter on the
oxidation of CO at low temperature.[2] Not only that the
fundamental impact of their discoveries was huge, but so was
the industrial significance of their results, taking into account the
importance of PVC manufacture worldwide and of environmental
catalysis (as in catalytic converters for the automobile industry).
Gold catalysis has now emerged as a mature area of research,[3]
as well as nanocatalysis, i.e. the use of metal nanoparticles in
catalysis.[4] Intersecting these two fields, gold nanoparticles (Au
NPs) have also been successfully used in various catalytic
carbon-carbon and carbon-heteroatom bond-forming reactions
and, thanks to their ability to activate molecular oxygen,
oxidation reactions of organic substrates.[4-5]
Results and Discussion
A preliminary screening of catalysts and reaction conditions
taught us that small Au NPs (2-3 nm of diameter), commercially
available under the tradename Aurolite, were suitable as
catalyst in toluene and THF.[6] Was also confirmed the useful
role of peroxide additives such as H2O2 (aqueous solution) and
TBHP (organic solution in decane). Peroxides are intermediates
in mechanistic events involving Au NPs and O2 such as gold
peroxo and hydroperoxo compounds,[8] and could act as
initiators of the catalytic cycle further only requiring O2 as
terminal oxidant.[9] Hydroperoxides have also been shown to
play the role of terminal oxidant in Au NPs-catalysed oxidation of
substituted phenols,[10] or in Au(III)-catalysed oxidation of
sulfides to sulfoxides,[11] for example. Au NPs could be obtained
supported on TiO2, Al2O3, and ZnO. Continued study was thus
performed with these catalysts in batch reactors on benzyl
alcohol 1a, a benchmark substrate for this type of oxidation
reaction. Benzyl alcohol 1a could typically form three oxidation
products, namely benzaldehyde 1b, benzoic acid 1c, and benzyl
benzoate 1d (Scheme 1). To avoid both energy consumption to
maintain pressurized reactors and safety issues, we have
chosen to work with a fixed pressure of O2 of 1 atm in batch
reactions.
We have recently reported in
a communication a novel
bicatalytic tandem oxidation/hetero-Michael addition/aldolisation
reaction in which allylic alcohols where oxidized selectively to
the corresponding ,-unsaturated aldehydes further undergoing
the above-mentioned sequence under base catalysis.[6] To
achieve this one-pot/4-steps transformation, we needed an
[a]
Mr P. Giorgi, Dr N. Elizarov, and Dr S. Antoniotti
Université Cote d’Azur, CNRS, Institut de Chimie de Nice, Parc
Valrose, 06108 Nice cedex 2, FRANCE.
E-mail: sylvain.antoniotti@unice.fr
Supporting information for this article is given via a link at the end of
the document.
Scheme 1. Benzyl alcohol 1a and its main oxidation products.
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oxidation reaction of toluene was observed, however leading to
negligible amounts of 1b (Table 2, entry 5). When the reaction
was performed in our optimized conditions but under a N2
atmosphere instead of O2, no reaction occurred (Table 2, entry
6).
Batch reaction: Influence of experimental parameters
Upon solvent screening, we found toluene as the most suitable
solvent for this reaction in our conditions, the oxidation
proceeding with 88% conversion and 87% yield of 1b with high
selectivity (see ESI). Increasing catalyst loading from 0.2 to 1.2
mol% linearly increased both conversion and yield up to an
optimum observed with 0.8 mol% (Table 1, entries 1-6). A too
large amount of catalyst resulted in total conversion but
increased degradation (Table 1, entry 7).
Table 1. Optimisation of the catalyst loading for the oxidation of
benzyl alcohol 1a[a]
TOF
Entry
Catalyst loading
Conversion[b]
1b yield[b]
(h-1)[c]
6.25
1
2
3
4
5
6
7
0.2 mol%
0.4 mol%
0.6 mol%
0.8 mol%
1.0 mol%
1.2 mol%
1 eq.
10%
57%
65%
99%
93%
97%
99%[e]
10%
57%
17.81
13.54
13.59
10.75
8.64
Figure 1. Effect of O2 bubbling on the oxidation of 1a to 1b
65%
(reaction conditions of Table 2, entry 4). Static O2 atmosphere, no
initiators (x), static O2 atmosphere, H2O2 as initiator (▲), pre-bubbling of O2,
static O2 atmosphere, TBHP as initiator (■), pre-bubbling of O2, static O2
atmosphere, H2O2 as initiator (●).
87%
86%[d]
83%[d]
60%
Table 2. Control experiments for the oxidation of benzyl alcohol
1a[a]
0.07
Yield
[a] Reaction conditions: 1a (1 mmol), toluene (2 mL), AuNPs/TiO2 (x mol%),
Catalyst
Additives
T °C
90
Conversion[b]
10%
Entry
of 1b[b]
H2O2 (5 mol%), O2 (1 bar), 8 hours, 95 °C. [b]Conversions and yields were
[c]
determined by GC-TCD analysis (external calibration).
At completion.
[d]Small amounts of benzyl benzoate 1d were formed (ca. 3-4%).
AuCl
(10 mol%)
H2O2
(5 mol%)
1
2
4%[c]
[e]Degradation occurred.
The catalytic system being based on the use of O2 as terminal
oxidant, the question of how O2 was supplied to the reaction was
of critical importance. The solubility of gases such as O2 in
organic solvents is low, particularly in toluene (10-20 mM)[12] and
tend to decline upon stirring and heating. A static oxygen
atmosphere in the reactor is not always sufficient to provide
catalytic system with O2. We tested the effect of O2 bubbling in
toluene prior running the reaction and during the reaction
compared with static oxygen atmosphere (Figure 1). This crucial
point will be further discussed in the section dealing with the
transposition of the reaction in continuous flow reactors. A
significant increase of both conversion and yield was observed
when the solvent was saturated with O2 prior to use (with TOF
up to 111 h-1 vs 29 h-1 after 30 minutes and 19.6 h-1 vs 11.4 h-1
at optimum). Additional bubbling during the reaction resulted
also in an improvement, though moderate (not shown).
AuCl3
(10 mol%)
H2O2
(5 mol%)
90
7%
3%[c]
0%
TBHP
(10 mol%)
3
-
-
90
0%
TBHP
(1,5 eq.)
4
90
35%
0%[d]
4%
Au NPs
/Al2O3
TBHP
(10 mol%)
5[e]
80
N.A.
Au NPs
/Al2O3
TBHP
(10 mol%)
6[f]
80
0%
0%
[a]Reaction conditions: 1a (1 mmol), toluene (2 mL), O2 (1 bar), 8 hours.
[b]Conversions and yields were determined by GC-TCD analysis (external
calibration). [c]The formation of benzyl chloride and dibenzyl ether was
observed. [d]Conversion of staring material was related to degradation. [e]No
substrate: 1b was formed upon toluene oxidation. [f]Reaction performed under
N2 (1 bar).
A series of control experiments was then performed to confirm
our understanding of this Au NPs/O2 catalytic oxidation reaction
(Table 2). First of all, homogeneous version of the reaction
conducted either with AuCl or AuCl3 (10 mol%) did not allow to
reach the performance of Au NPs, 1b being formed in only 3-4%
yield (Table 2, entries 1,2). In the absence of catalyst with 10
mol% of peroxide initiator, no reaction occurred indicating that
no direct oxidation of the substrate by TBHP could be of concern
here (Table 2, entry 3). Using superstoichiometric amount of
peroxide, 1b was not observed and degradation occurred (Table
2, entry 4). In the absence of substrate 1a, a background
Batch reaction: substrate scope
Satisfied by the efficiency of the reaction when performed in our
optimized conditions, we then focused on the substrate scope of
the reaction, initially with benzyl alcohols, then with primary and
secondary allylic alcohols, rarely considered in the literature
(Table 3).[7e]
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Table 3. Conversion and selectivity in the oxidation of various alcohols.[a]
Entry
1[b]
Substrate
Catalyst
Additives
Solvent
Toluene
T °C
95
Conversion
99%
Product
Isolated yield
87%
H2O2
(5 mol%)
Au NPs/TiO2
H2O2
(5 mol%)
2
3
Au NPs/TiO2
Toluene
Toluene
Toluene
Toluene
THF
110
80
80
80
60
80
99%
87%
99%
70%
70%
66%
86%
87% [c]
99%
Au
NPs/Al2O3
TBHP
(10 mol%)
Au
NPs/Al2O3
TBHP
(10 mol%)
4
TBHP
(10 mol%)
5
Au NPs/TiO2
Au NPs/ZnO
43%[d]
68%
TBHP
(10 mol%)
6[b]
Au
NPs/Al2O3
TBHP
(10 mol%)
66%[e]
(E/Z:4/6)
7[b]
Toluene
Au
NPs/Al2O3
TBHP
(10 mol%)
68%
(mainly E/E)[f]
8[b]
Toluene
80
68%
Au
NPs/Al2O3
TBHP
(10 mol%)
9[b]
THF
THF
THF
60
60
60
100%
100%
100%
k
99%
99%[g]
93%
Au
NPs/Al2O3
TBHP
(10 mol%)
10[b]
Au
NPs/Al2O3
TBHP
(10 mol%)
11[b]
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Au
NPs/Al2O3
TBHP
(10 mol%)
12
THF
60
70%
50%
Au
NPs/Al2O3
TBHP
(10 mol%)
13
14
Toluene
Toluene
80
80
90%
76%
53%[h]
68%
Au
NPs/Al2O3
TBHP
(10 mol%)
Au
NPs/Al2O3
TBHP
(10 mol%)
15
Toluene
80
100%
90%
[a]Reaction conditions: substrate (1 mmol), solvent (2 mL), AuNPs (0.8 mol%), additives, O2 (1 bar). [b]Taken from ref [6]
.
[c]DMF was added as cosolvent to ensure
vanillic alcohol solubilisation. [d]The high mass loss upon work-up was attributed to the volatility of the product and its tendency to polymerize. [e]When the reaction
was performed starting with geraniol (E isomer), 71% of aldehyde was obtained in a 6/4 E/Z ratio. [f]Mixture E/E // Z/E:75/25. [g]Similar results were obtained in
toluene at 80 °C. [h]Significant mass loss presumably due to the volatility of the product.
Benzylic alcohols 1a-4a with various substitution patterns were
converted with excellent conversions (87-99%) and selectivities
in favor of the corresponding aromatic aldehydes, obtained in
86-99% isolated yields (Table 3, entries 1-4). With an electron-
withdrawing group as in the case of para-nitrobenzyl alcohol, the
reaction proceeded with 53% conversion and ca. 50% yield of
aldehydes (see Supporting information). Aliphatic allylic alcohols
5a-8a were similarly oxidized with high selectivities, but with
lower conversions (66-70%) and isolated yields from 43% for the
volatile and unstable acrolein 5b to 68% for (E,E)-farnesal 8b
(Table 3, entries 5-8). Worthnotingly, nerol 6a, featuring a Z
double bond, was converted into a mixture of E and Z
stereoisomers (Table 3, entry 7), indicating that isomerization
could occur in our conditions. Cinnamyl alcohol 9a and
mechanistic features. The first one consisted in a hot filtration
test and the second one consisted in the titration of residual
soluble gold species in the reaction medium by ICP-AES.
The hot filtration test was performed as follows. The reaction
was carried out with 1a in our optimized conditions under GC-
TCD analysis (external calibration method). The conversion of
1a was thus monitored with the analysis of aliquots withdrawn
from the reaction medium versus the course of time. After
sufficient time allowing for a ca. 70% conversion, the reaction
mixture was filtered over a fritted glass heated at 80 °C and the
filtrate was stirred at 80 °C under O2 and the monitoring
continued. The results are summarized in Figure 2.
1
derivatives
10a-12a
were
efficiently
converted
to
0,9
cinnamaldehyde and related products 9b-12b with good to
excellent conversions (70-100%) and often quantitative isolated
yields (Table 3, entries 9-12). Secondary allylic alcohols 13-14a
reacted less efficiently in our conditions but could be converted
into ketones 13-14b in 53-66% isolated yields (Table 3, entries
13-14). Perillyl alcohol 15a, exhibiting a cyclic core with an
exocyclic hydroxymethyl substituent, was totally converted to
perillaldehyde 15b in 90% yield (Table 3, entry 15).
0,8
Filtration
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
The products were isolated by a simple filtration over a pad of
celite to remove the solid catalyst and subjected when
necessary to column chromatography. Interestingly, 9 out of
these 15 products have an industrial significance in the field of
fragrance chemistry (see SI).
0
50
100
200
250 450
500
Reaction time (min)
Figure 2. Conversion of 1a vs time and hot filtration experiment.
It is clear that, upon the removal of the solid catalyst by filtration
the reaction stopped, indicating that no soluble catalytically
active species were present in the liquid phase. In comparison, 8
hours were necessary to get 99% conversion and 87% isolated
yield of 1b in the same conditions (illustrated by the dashed lines
along the discontinued scale). Further evidences were obtained
by performing the titration of soluble gold species in the reaction
medium after 40 min and after prolonged reaction times by ICP-
AES. Residual gold was found to be 10 pg/ml and 20 pg/ml (
True heterogeneous nature of the catalysis
When dealing with supported metal catalysis, either in the form
of metals salts, complexes or nanoparticles, an obvious question
concerns the true heterogeneous nature of the catalysis. This
point is of crucial importance when the catalytic system is to be
applied in continuous flow to avoid washing of the active metal
species away from the support. Two independent types of
experiments were performed to gain insights on these
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5%, triplicates) at 40 and 960 minutes respectively. These
minute quantities could not account for the catalysis observed in
our conditions.
state. Moreover, the low loading of TBHP does not correlate with
the yield difference observed, i.e. 10% vs 28% (Table 4, entries
4,5). An effect on maintaining Au NPs in their most active form
should be invoked here.
Transposition in continuous flow reactors
With these results in hands, we could start to study the reaction
in continuous flow. The merit of flow chemistry has been largely
demonstrated and several advantages could be offered to the
synthetic chemist.[13] Heterogeneous catalysis is well suited for
being used in flow since catalytic reactors as simple as a column
filled with the solid catalytic material could be used. In the
particular case of reactions taking place in gas-liquid biphasic
medium, the flow chemistry setup is often outperforming batch
protocols.[14] Strategies to attach gold catalysts to insoluble
supports have been developed and applied in microchannel
reactors.[15] Recent examples of allylic and benzylic alcohols in
flow were given with Au NPs supported Fe3O4@SiO2
particles.[16] Optimized conditions required the use of benzene
as the solvent (90 mM), 7 bars of O2 pressure and heating of the
catalytic reactor at 150 °C. These conditions were not meeting
our expectations both in terms of sustainability, ease of access
to the catalyst and possible practical applications. We therefore
envisaged transposing our batch conditions, efficient in mild
conditions with a simple catalyst, in continuous flow. The
technology used was based on catalytic fixed-bed reactors
containing Au NPs supported on Al2O3 and a tube-in-tube
gas/liquid device.[14a] A solution of 1a in toluene was therefore
exposed to an O2 pressure along a 15 ml tubing before entering
the catalytic column charged with Au NPs/Al2O3 (Figure 3). O2
was supplied directly from a cylinder at controlled pressure of 2
bars. A back pressure regulator (BPR) set at 8 bars was placed
along the flow. A temperature of 80 °C was applied to the
system.
Table 4. Yield of 1b in continuous flow experiments. [a]
Residence
Flow rate
Yield
Entry
[1a]
Time[b]
(min)
c]
(mL.min-1)
of 1b[
1
2
500 mM
500 mM
500 mM
50 mM
50 mM
50 mM
0,327
0,150
0,222
0,444
0,444
0,444
5.1
11.1
7.5
28%
38%
7%
3
4
3.7
99%
71%
99%
5[d]
3.7
6[e]
9.2
[a]Reaction conditions: catalytic reactor charged with Au NPs/Al2O3 (1 g), 1a (2
mmol), TBHP (10 mol%) relative to 1a, toluene, PO2=2 bars. [b]Isolated yield of
1b collected during the steady state. [c]Within the catalytic reactor. [d]No TBHP
was used. [e]Performed on 10 mmol, catalytic reactor charged with Au
NPs/Al2O3 (4 g). See SI for details.
The protocol in continuous flow was then tested with other
substrates (Table 5). Interestingly, the reaction was much more
efficient in flow than in batch with aliphatic substrates 6a and 7a.
The corresponding aldehydes 6b and 7b were indeed isolated in
84 and 87% yield respectively (Table 5, entries 2,3) instead of
66 and 68% in batch (Table 3, entries 7,8). A better relative O2
concentration could probably account for this improvement, as
well as the limited residence time of the product in the reactor,
thereby limiting the risk of degradation at 80 °C. The ability to
perform the oxidation in flow will open new possibilities in
multistep reactions in flow starting with or including an efficient,
specific and selective oxidation step operating in mild and simple
conditions with O2 as the terminal oxidant.
Reaction mechanism of Au NPs-catalyzed aerobic oxidation of
alcohols is difficult to address with accuracy because of the co-
existence of solid, liquid and gaseous substances. Based on
existing knowledge,[7d, 17] we consider that the alcohol substrate,
supposedly in the form of an alcoholate, should be adsorbed on
the Au NP surface. Deprotonation of the alcohol could be either
favored by Al2O3, the preferred support for Au NPs in our
conditions, or by hydroxide ions formed upon O2 reduction
further in the catalytic cycle. Upon a redox reaction, the
expected aldehyde could be formed by cleavage of the C-H
bond which is considered to be the kinetically limiting step,[18]
together with the formation of a gold-hydride intermediate. The
latter could then react with O2 and eventually lead to a re-
oxidised Au NP surface and HO-. At this occasion, peroxo- and
hydroperoxogold species could be formed, and the supply of
peroxides from external sources could help to provide the
catalytic cycle with active Au NPs to start with. Minor secondary
pathways could also start from this point leading to toluene by
reduction or to benzoic acid by oxidation. These events were
found to be negligible in our conditions and typically limited to
benzyl alcohol derivatives.
Figure 3. Oxidation of 1a in a continuous flow reactor.
Optimization of the operating conditions was realized mostly by
varying the substrate concentration and the flow rate (Table 4).
At 0.5 M, yields of 1b remained moderate (Table 4, entries 1-3).
At a lower concentration (50 mM) and at 0.444 mL.min-1, 1b was
obtained with 100% selectivity and quantitative isolated yield in
3.7 min residence time (Table 4, entries 4,6). These conditions
allowed a better matching of substrate and O2 concentrations.
The use of peroxide initiator, i.e. TBHP, was not mandatory but
better results were obtained with. This result was rather
surprising, since as initiator, the use of TBHP should not
influence the outcome of the reaction observed at the steady
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Table 5. Conversion and selectivity in the oxidation of various alcohols by Au NPs/O2 in continuous flow.[a]
Flow rate
(mL.min-1)
Residence
time (min)
Isolated
yield[b]
Entry
Substrate
Product
1
1a
0,444
0,444
3.7
3.7
99%
84%
(E/Z:1/1)[c]
2
3
6a
7a
8a
87%
(mainly E/E)[d]
0,444
0,222
3.7
7.5
4[e]
91%
96%
5[e]
10a
0,444
3.7
[a]Reaction conditions: catalytic reactor charged with Au NPs/Al2O3 (1 g), TBHP (10 mol%), substrate concentration 50 mM, PO2=2 bars. [b]Isolated yield of product
collected during the steady state. [c]Possible chromatography effect. Should not be compared with the ratio presented in Table 4, entry 7. [d]Mixture E/E //
Z/E:86/14. [e]THF as the solvent.
10-50% diethyl ether/petroleum ether gradient solvent, affording the
corresponding aldehyde.
Conclusions
General procedure for benzylic and allylic alcohols oxidation by Au
In summary, we have adapted a selective and efficient oxidation
reaction of benzylic and allylic alcohols to the corresponding
carbonyl compounds both in batch and in flow using
commercially available nanocatalysts. The reaction proceeds in
relatively mild conditions and under an atmospheric pressure of
O2 and was showed to be truly heterogeneous. A large substrate
scope was demonstrated with 15 examples and yields of 43-
99% in batch and 84-99% in flow.
NPs/O2 in continuous flow reactor: The detailed flow reactor setup is
presented in Supporting information. The tube-in-tube heated mixer
reactor was connected to an oxygen cylinder (p(O2)= 2 bars). The flow
device was flushed with toluene enriched with oxygen, and the
temperature was adjusted to 80 °C. Once a constant temperature of the
solvent (flow rate 0.444 mL/min) was reached, a solution of the alcohol
dissolved in the corresponding solvent (50-500 mM) and the peroxide
initiator (10 mol%) was pumped through the system (residence time 45-
90 min). The collected fractions corresponding to steady state regime
were evaporated in vacuo and the crude product was purified via silica-
gel chromatography to yield the corresponding aldehyde when necessary.
The products were characterized by comparison with authentic samples
or literature data. Products 1b, 2b, 3b, 4b, 5b, 6b, 7b, 8b, 9b, 12b, 13b,
14b, 15b are commercially available. 3-(3,4-Dimethoxyphenyl)-prop-2-
enal (10b).[19] 1H NMR (200 MHz, CDCl3) δ (ppm) 9.58 (d, J=8 Hz, 1H),
7.34 (d, J=16 Hz, 1H), 7.06-7.11 (m, 2H), 6.82 (d, J=8 Hz, 1H), 6.47-6.58
(m, 1H), 3.85 (s, 6H). 13C NMR (50 MHz, CDCl3) δ (ppm) 193.4, 152.7,
151.8, 149.2, 126.9, 126.5, 123.3, 111.0, 109.7, 55.9, 55.8. MS (EI, 70
eV) m/z 161.1 (100), 192.1 (86), 77.0 (57). 3-(4-(benzyloxy)-3-
methoxyphenyl)-prop-2-enal (11b).[20] 1H NMR (200 MHz, CDCl3) δ (ppm)
9.58 (d, J=6 Hz, 1H), 7.19-7.38 (m, 6H), 6.99-7.04 (m, 2H), 6.83 (d, J=8
Hz, 1H), 6.47-6.58 (m, 1H), 5.14 (s, 2H), 3.85 (s, 3H). 13C NMR (50 MHz,
CDCl3) δ (ppm) 193.5, 152.7, 151.1, 149.9, 136.3, 128.6, 128.1, 127.3,
127.2, 126.8, 123.2, 113.4, 110.4, 70.8, 56.0. MS (EI, 70 eV) m/z 91.1
Experimental Section
General procedure for benzylic and allylic alcohols oxidation by Au
NPs/O2 in batch reactor: to a dried schlenk tube under O2 atmosphere,
catalyst (0.08 mmol of Au, 0.08 equiv.), tert-butyl hydroperoxide solution
in decane (0.1 mmol, 0.1 equiv.), anhydrous solvent (2.0 mL, 0.5 M) and
the substrate (1 mmol, 1.0 equiv.) were added. The solution was kept
under a vigorous stirring. After completion of the reaction monitored by
TLC or GC/TCD detector, DCM was added (10 mL) and the mixture
filtered through a pad of celite, further eluted with DCM (5 mL). The
organic phase was washed with brine, further extracted with more DCM
(3x5 mL), prior being concentrated in vacuo. When necessary, the crude
residue was purified by column chromatography over silica gel using a
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10.1002/cctc.201700179
ChemCatChem
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Acknowledgements
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This work was supported by the CNRS, the University Nice
Sophia Antipolis, and the ANR program CD2I (Nanocausys,
grant 12-CDII-0010-02). The French MESR and the Embassy of
France in Germany are acknowledged for fellowships to PDG
and NE, respectively. We are grateful to Ms Marion Alberola for
ICP-AES experiments.
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