Highly efficient direct aerobic oxidative esterification of cinnamyl alcohol with alkyl alcohols catalysed by gold nanoparticles incarcerated in a nanoporous polymer matrix: A tool for investigating the role of the polymer host

Article abstract of DOI:10.1002/chem.201303880

The selective aerobic oxidation of cinnamyl alcohol to cinnamaldehyde, as well as direct oxidative esterification of this alcohol with primary and secondary aliphatic alcohols, were achieved with high chemoselectivity by using gold nanoparticles supported in a nanoporous semicrystalline multi-block copolymer matrix, which consisted of syndiotactic polystyrene-co-cis-1,4- polybutadiene. The cascade reaction that leads to the alkyl cinnamates occurs through two oxidation steps: the selective oxidation of cinnamyl alcohol to cinnamaldehyde, followed by oxidation of the hemiacetal that results from the base-catalysed reaction of cinnamaldehyde with an aliphatic alcohol. The rate constants for the two steps were evaluated in the temperature range 10-45 °C. The cinnamyl alcohol oxidation is faster than the oxidative esterification of cinnamaldehyde with methanol, ethanol, 2-propanol, 1-butanol, 1-hexanol or 1-octanol. The rate constants of the latter reaction are pseudo-zero order with respect to the aliphatic alcohol and decrease as the bulkiness of the alcohol is increased. The activation energy (E_a) for the two oxidation steps was calculated for esterification of cinnamyl alcohol with 1-butanol (E_a=57.8±11.5 and 62.7±16.7 kJ mol ^-1 for the first and second step, respectively). The oxidative esterification of cinnamyl alcohol with 2-phenylethanol follows pseudo-first-order kinetics with respect to 2-phenylethanol and is faster than observed for other alcohols because of fast diffusion of the aromatic alcohol in the crystalline phase of the support. The kinetic investigation allowed us to assess the role of the polymer support in the determination of both high activity and selectivity in the title reaction. Golden boost to the esterification: The highly efficient and selective aerobic oxidation of cinnamyl alcohols, catalysed by gold nanoparticles supported in a nanoporous polymer matrix, in the presence of primary and secondary alkyl alcohols afforded alkyl cinnamates (see figure). The active role of the host-polymer matrix was investigated.

Full text of DOI:10.1002/chem.201303880

DOI: 10.1002/chem.201303880
Full Paper
&
Nanoporous Catalysis
Highly Efficient Direct Aerobic Oxidative Esterification of
Cinnamyl Alcohol with Alkyl Alcohols Catalysed by Gold
Nanoparticles Incarcerated in a Nanoporous Polymer Matrix:
A Tool for Investigating the Role of the Polymer Host
Antonio Buonerba, Annarita Noschese, and Alfonso Grassi*[a]
Dedicated to Professor Adolfo Zambelli on the occasion of his 80th birthday
Abstract: The selective aerobic oxidation of cinnamyl alco-
hol to cinnamaldehyde, as well as direct oxidative esterifica-
tion of this alcohol with primary and secondary aliphatic al-
cohols, were achieved with high chemoselectivity by using
gold nanoparticles supported in a nanoporous semicrystal-
line multi-block copolymer matrix, which consisted of syn-
diotactic polystyrene-co-cis-1,4-polybutadiene. The cascade
reaction that leads to the alkyl cinnamates occurs through
two oxidation steps: the selective oxidation of cinnamyl al-
cohol to cinnamaldehyde, followed by oxidation of the hem-
iacetal that results from the base-catalysed reaction of cinna-
maldehyde with an aliphatic alcohol. The rate constants for
the two steps were evaluated in the temperature range 10–
458C. The cinnamyl alcohol oxidation is faster than the oxi-
dative esterification of cinnamaldehyde with methanol, etha-
nol, 2-propanol, 1-butanol, 1-hexanol or 1-octanol. The rate
constants of the latter reaction are pseudo-zero order with
respect to the aliphatic alcohol and decrease as the bulki-
ness of the alcohol is increased. The activation energy (Ea)
for the two oxidation steps was calculated for esterification
of cinnamyl alcohol with 1-butanol (Ea =57.8Æ11.5 and
62.7Æ16.7 kJmolÀ1 for the first and second step, respective-
ly). The oxidative esterification of cinnamyl alcohol with 2-
phenylethanol follows pseudo-first-order kinetics with re-
spect to 2-phenylethanol and is faster than observed for
other alcohols because of fast diffusion of the aromatic alco-
hol in the crystalline phase of the support. The kinetic inves-
tigation allowed us to assess the role of the polymer support
in the determination of both high activity and selectivity in
the title reaction.
wind.[4] Cinnamic esters are obtained from various plant sour-
ces or, alternatively, synthesised when the naturally occurring
compounds are not cheap and/or commercially available.
The conventional synthesis of esters from alcohols, when
the latter are naturally occurring and/or cheaper than the re-
spective acid, involves a two-step reaction: oxidation of the al-
cohol to the corresponding acid, followed by condensation
with an alcohol.[5] The latter step is an equilibrium reaction
that requires the elimination of water by co-distillation or trap-
ping with specific reagents to move the equilibrium toward
the expected reaction products. Alternatively, activated forms
of the carboxylic acids—the acyl halides—must be used, which
leads to irreversible reactions. The latter synthetic approach is,
of course, more convenient but the formation of toxic wastes
(hydrochloric acid and/or other by-products) is undesirable
from the viewpoint of sustainable chemistry. Because of the in-
dustrial interest given to alkyl cinnamates, efforts have been
made to optimise the synthesis under more convenient and
environmentally friendly conditions.
Introduction
Cinnamic aldehydes, acids and esters are widely distributed in
nature and constitute a class of compounds with a broad spec-
trum of pharmaceutical activities, such as anti-inflammatory,
anticonvulsant, antifungal, analgesic and antihypertensive ef-
fects. Moreover, they also find application in the perfumery
and cosmetics industries.[1] For example, methyl caffeate is re-
ported to exhibit antitumour activity against Sarcoma 180, as
well as antimicrobial activity.[2] Methoxy-substituted cinnamates
play an important role in the control of inflammatory diseas-
es.[3] Long-chain cinnamic esters are well-known as sunscreen
agents and are ideally suited for cosmetic applications because
they are not skin irritants and prevent the drying effect of
[a] Dr. A. Buonerba, Dr. A. Noschese, Prof. A. Grassi
Dipartimento di Chimica e Biologia and
NANOMATES Research Centre for NANOMAterials and
nanoTEchnology at Salerno University
Universitꢀ degli Studi di Salerno
Via Giovanni Paolo II
The synthesis of methyl, ethyl, 1-propyl, 1-butyl or longer
alkyl chain cinnamates has recently been reported by using
heteropolyacid catalysts and microwave irradiation.[6] Good
conversion and selectivity were generally observed with strong
irradiation power (e.g. 130 mA, 410 W). For example, in the
84084 Fisciano (SA) (Italy)
Fax: (+39)089969824
E-mail: agrassi@unisa.it
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303880.
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case of butyl cinnamate, a cinnamic acid conversion of
92.8 mol% was obtained after irradiation of the reaction mix-
ture for 3 h, catalysed by SO42À-La2O3-ZrO2-HZSM-5. Although
this method seems feasible, the increase of the alkyl cinnamate
output and the use of this technology in industry is not
straightforward.
of CA and these are mainly focused on methyl cinnamate syn-
thesis.[24] Recently we reported the efficient and selective aero-
bic oxidation of alcohols catalysed by AuNPs[25] incarcerated in
a nanoporous semi-crystalline polymer matrix that consisted of
multi-block copolymers[26] of syndiotactic polystyrene-co-1,4-
cis-polybutadiene (AuNPs-sPSB). Primary and secondary benzyl
or allyl alcohols were readily oxidised to the corresponding al-
dehydes, whereas alkyl alcohols were not oxidised at all under
the same experimental conditions. Although primary and sec-
ondary alkyl alcohols are known to be less reactive in this reac-
tion, the observed selectivity appeared unprecedented, as did
the specific properties of the polymeric support employed.
These results prompted us to investigate the oxidation and
oxidative esterification of CA catalysed by AuNPs-sPSB; we also
tried to obtain information on the reaction mechanism.
Supported gold nanoparticles (AuNPs) are emerging as
a powerful tool in catalysis. AuNPs were found to be active in
several chemical transformations,[7] which include the oxidation
of carbon monoxide,[8] alkanes,[9] alcohols,[10] polyols and alde-
hydes, the epoxidation of alkenes,[11] the reduction of a,b-unsa-
turated carbonyl compounds[12] and nitroarenes,[13] as well as
the direct synthesis of hydrogen peroxide[14] and acetylene hy-
drochlorination.[15] Probably one of the most interesting appli-
cation of AuNPs is in tandem catalysis, in which cascade reac-
tions are carried out in a single reactor, thus the efficiency of
the chemical synthesis, in terms of energy consumption and
waste formation, is increased. The efficient synthesis of
esters[16] and amides[17] has been successfully achieved by one-
pot aerobic oxidative esterification of alcohols or by oxidative
coupling of alcohols with amines, respectively. A variety of ma-
terials have been explored to efficiently support the AuNPs,
such as carbon[7e] or inorganic oxides, such as alumina, titania[7j]
and ceria,[18] or polymers [e.g. poly(vinyl alcohol),[19] poly(N-
vinyl-2-pyrrolidone)],[20] dendrimers (poly(amido amine);
PAMAM),[21] cationic ion-exchange resins,[22] and random co-
polymers of styrene and its para-substituted derivatives.[23]
Oxidation of cinnamyl alcohol (CA) to cinnamaldehyde has
generally been carried out as a benchmark test for the per-
formance of AuNP-supported catalysts in the selective oxida-
tion of alcohols to aldehydes. In principle, a complex mixture
of products can be obtained as a result of oxidation, dehydro-
genation, hydrogenolysis and decarbonylation pathways
(Scheme 1).[24] Few reports deal with the oxidative esterification
Results and Discussion
Synthesis of the AuNPs-sPSB catalyst
The AuNPs-sPSB catalyst (gold content=2%w/w) was synthe-
sised by the method previously reported,[25] by using the
multi-block copolymer sPSB with a high concentration in sty-
rene (93%w/w) to enhance the selectivity properties of the
polymeric support and keep the butadiene concentration high
enough to yield good swelling of the polymer matrix. The
powder wide-angle X-ray diffraction (WAXD) pattern of the
AuNPs-sPSB catalyst is shown in Figure 1. The region 4–308
provides information about the crystalline phase of the poly-
meric support,[27] whereas the peak at 388, due to the <111 >
plane of the fcc crystal lattice of gold, allows evaluation of the
average dimension of the nanoparticles from the Scherrer
equation.[28] The as-synthesised catalyst presents the syndiotac-
tic polystyrene (sPS) polymer phase in the d-crystalline form
and comprises THF molecules clathrated in the nanopores of
Figure 1. WAXD patterns of the AuNPs-sPSB catalyst after thermal treatment
at 1708C for a) 5 h and b) after a catalytic cycle.
Scheme 1. Reaction pathways for oxidation/reduction of cinnamyl alcohol.
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the crystalline phase[27] and AuNPs with an average dimension
of about 4.4 nm (Figures S1 and S2 in the Supporting Informa-
tion). The annealing of AuNPs-sPSB at 1708C for 5 h to activate
the AuNPs[25] converts the crystalline phase of the polymer
matrix from d to b form and produces a slight increase of the
AuNPs dimension to 7.4 nm (Figure 1a).
The transmission electron microscopy (TEM) confirmed the
average diameter of the AuNPs and reveals their optimal dis-
persion in the polymer matrix (Figure 2). From inspection of
the selected-area electron diffraction (SAED) micrograph (Fig-
ure 2c; Figure S3 in the Supporting Information) it is possible
to recognise two patterns: the diffuse halo attributed to the
unoriented AuNPs and the spotted pattern due to the sPS
crystalline phase. Attempts to obtain a narrower distribution of
the AuNPs by variation of the experimental conditions of the
synthetic process were unfruitful. To obtain smaller AuNPs,
gold precursors different from HAuCl4 are likely necessary.
Figure 2. a) TEM micrograph of annealed AuNPs-sPSB catalyst (1708C for
5 h), b) the AuNPs size distribution, c) SAED micrograph.
CA oxidation is quantitative after 1.5 h with a selectivity of
99% (Table 1, entry 2; Figure 3), whereas use of KOH (20 equiv)
led to the same result after 4 h (Table 1, entry 3). When the
temperature was decreased to 10 or 258C, longer reaction
times were necessary to obtain high conversion and selectivity
to cinnamaldehyde (Table 1, entries 4 and 5; Figure 3). The in-
crease of the reaction temperature to 458C shortens the reac-
tion time and leaves selectivity totally unaffected (99%;
Table 1, entry 6). The kinetic plots of CA oxidation to cinnamal-
Oxidation of CA to cinnamaldehyde catalysed by
AuNPs-sPSB
The aerobic oxidation of CA catalysed by AuNPs-sPSB was pre-
liminarily investigated under the experimental conditions pre-
viously reported [358C, 1:1 v/v water/chloroform, PO2 =1 bar,
KOH (1 equiv)].[25] After 2 h, the alcohol conversion is 97% with
a selectivity of 97% for cinnamaldehyde (Table 1, entry 1; Fig-
ure S5 in the Supporting Information). With KOH (6 equiv), the
Table 1. Aerobic oxidation of cinnamyl alcohol catalysed by AuNPs-sPSB.
Entry[a]
T
[8C]
KOH/CA
Solvent[b]
t
[h]
Conversion[c,d]
[%]
Products
Yield[d]
[%]
Selectivity[d]
[%]
1
2
3
4
5
6
7
8
35
35
35
10
25
45
35
35
1
6
H2O/CHCl3
2
97
>99
>99
>99
98
cinnamaldehyde
3-phenyl-1-propanol
cinnamaldehyde
3-phenyl-1-propanol
cinnamaldehyde
3-phenyl-1-propanol
cinnamaldehyde
3-phenyl-1-propanol
cinnamaldehyde
3-phenyl-1-propanol
cinnamaldehyde
3-phenyl-1-propanol
cinnamaldehyde
3-phenyl-1-propanol
7-phenylhepta-4,6-dien-3-one
cinnamaldehyde
94
3.0
98
1.5
92
1.9
95
5.0
94
4.0
96
1.3
18
2.0
31
97
3.0
99
1.0
92
1.9
95
5.0
96
4.0
99
1.0
91
9.0
69
H2O/CHCl3
1.5
20
6
H2O/CHCl3
4
H2O/CHCl3
8
6
H2O/CHCl3
5
6
H2O/CHCl3
0.75
97
6
H2O/ethyl acetate
H2O/2-butanone
24
20
6
8
44
3.6
3.4
52
4.0
84
8.7
8.3
93
7.0
93
3-phenyl-1-propanol
cinnamaldehyde
3-phenyl-1-propanol
cinnamaldehyde
9
35
35
6
6
H2O/cyclohexanone
H2O/toluene
6
3
56
90
10
3-phenyl-1-propanol
0.9
1.4
[a] Reaction conditions: CA (0.51 mmol), AuNPs-sPSB (200 mg, Au=2%w/w), O2 (1 atm), anisole as an internal standard (0.51 mmol). [b] v/v=1:1, total
volume=6 mL. [c] Conversion of CA. [d] Determined by GC-MS analysis.
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cerated in a cross-linked atactic polystyrene matrix because of
slow diffusion of the reagent through the amorphous polymer
phase.[10f] Our findings for CA oxidation confirmed this hypoth-
esis. Alcohols with aromatic substituents behave similarly to ar-
omatic compounds (e.g. toluene, 4-nitrophenol, azobenzene,
benzaldehyde), which are known to establish strong interac-
tions with the cavities of the e crystalline form and lead to
well-characterized co-crystalline compounds.[27] Moreover, it
was recently demonstrated that sPS clathrates in chloroform
undergo rapid guest-exchange processes, which promotes fast
diffusion of the guest molecules in the nanochannels of the e-
crystalline form.[29] Thus, chloroform plays the following pecu-
liar roles: 1) swelling of the polymer matrix; 2) in situ formation
of the crystalline nanoporous e phase; 3) fast diffusion of the
reagents through the nanochannels of the polymer support.
From a sustainable point of view, chloroform is an unpleasant
solvent. However in these oxidation reactions it is used in very
small amounts, for polymer swelling and to ensure biphasic
catalysis. In principle, the reagents and products can be ex-
tracted from the organic layer by treatment with immiscible
polar solvents and the AuNPs-sPSB catalyst, swollen in chloro-
form, can be reused without the requirement for additional
chlorinated solvents. Actually, it was previously shown that the
AuNPs-sPSB catalyst can be used at least six times without ob-
servation of a decrease in the activity or selectivity of the alco-
hol oxidation.[25]
Figure 3. Cinnamyl alcohol oxidation to cinnamaldehyde in the temperature
range 10–458C.
dehyde in the temperature range 10–458C are displayed in
Figure 3. The rate constants, calculated assuming first-order ki-
netics with respect to CA, are in the range k=1.08ꢁ10À4
Æ
7.1ꢁ10À6–1.3ꢁ10À3 Æ1.3ꢁ10À4 sÀ1. The corresponding Arrhe-
nius plot gives an activation energy (Ea) value of 57.8Æ
11.5 kJmolÀ1 (Figure S14 in the Supporting Information).
Alternative solvents to chloroform, to be mixed with water
for efficient oxidation catalysis, were investigated. Ethyl acetate
produced a dramatic decrease in the catalytic activity (Table 1,
entry 7), probably a result of the reduced swelling of the crys-
talline polymer support. Reaction in 2-butanone led to the for-
mation of 7-phenylhepta-4,6-dien-3-one—the result of the
base-catalysed aldol reaction of 2-butanone with cinnamalde-
hyde—in low yields (Table 1, entry 8). Use of cyclohexanone
gave a CA conversion of 56% with a selectivity of 93% for cin-
namaldehyde in 6 h (Table 1, entry 9), whereas toluene pro-
duced an alcohol conversion of 90% with a selectivity of 93%
for cinnamaldehyde (Table 1, entry 10). The aromatic solvent
efficiently swells the crystalline polymer support to ensure
good accessibility of the substrates to the AuNPs incarcerated
in the polymer phase. Nonetheless, chloroform remained the
solvent of choice.
It is also noteworthy that 3-phenyl-1-propanol was found as
a minor product in all of the CA oxidation runs. This com-
pound could result from hydrogen addition to the olefinic
carbon–carbon double bond of CA (Scheme 1). This piece of
experimental evidence supports the formation of a AuÀH[23b]
intermediate species and the possibility of intermolecular hy-
drogen borrowing[30] or transfer[31] from the alcohol to be oxi-
dised to other substrates.
Figure 3 displays the kinetic profiles of the CA oxidation in
the temperature range 10–458C. As previously observed for
the oxidation of benzyl alcohol catalysed by AuNPs-sPSB,[25]
the reaction exhibits pseudo-first-order kinetics with respect to
the CA. This confirms that the access of CA to the AuNPs is not
controlled by a diffusive process through the amorphous
phase of the polymer, rather through the nanoporous crystal-
line phase. The rate constants for the CA oxidation were mea-
sured in the temperature range 10–458C and the correspond-
ing Arrhenius plot (Figure S14 in the Supporting Information)
gives Ea =57.8Æ11.5 kJmolÀ1 for this reaction. This value is in
The peculiar properties of chloroform as the reaction solvent
were investigated next. The diffuse-reflectance FTIR (DRIFT)
analysis combined with WAXD investigation of the catalyst
soon after an oxidation run in water/chloroform showed that
the polymer support is crystalline in the nanoporous e-crystal-
line phase of sPS (Figure 1b; Figure S4 in the Supporting Infor-
mation) and the crystallisation process that leads to this crys-
talline polystyrenic phase is unexpectedly fast under the cata-
lytic conditions. We have previously shown that the efficient
aerobic oxidation of both benzyl alcohol and 1-phenylethanol
catalysed by AuNPs-sPSB in chloroform/water follows first-
order kinetics with respect to the alcohol;[25] this result is simi-
lar to that observed for alcohol oxidation catalysed by AuNPs
supported on metal oxides. We attributed this kinetic behav-
iour to the fast diffusion of the substrates into the nanochan-
nels of the e-crystalline polymer phase; in fact, zero-order ki-
netics with respect to alcohol were observed with AuNPs incar-
good agreement with that found for the oxidation of p-methyl
[18b]
benzyl alcohol catalysed by AuNPs-Ce/O2
and higher than
that for oxidation of activated p-hydroxybenzyl alcohol cata-
lysed by AuNPs incarcerated in poly(N-vinyl-2-pyrrolidone).[20b]
It is notable that in the both cases a specific activating role of
the support was claimed,[10c,18b,20] whereas the AuNPs-sPSB con-
sists only of a hydrocarbon polymer support.
Oxidative esterification of CA with aliphatic alcohols
Recently, the reaction mechanism of aerobic oxidation of alco-
hols and the oxidative esterification of alcohols catalysed by
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AuNPs have been extensively investigated.[10,16,18b,23,24,32] The re-
action pathway of the latter includes alcohol oxidation to give
aldehyde, which in turn reacts with the same or another alco-
hol molecule to yield the corresponding hemiacetal, which is
finally oxidised to the ester. The reaction requires a strong
Brønsted base as a co-catalyst, especially when the AuNPs are
supported/incarcerated in organic polymers. The AuNPs-sPSB
catalyst reached the highest activity for benzyl alcohol oxida-
tion with a KOH/alcohol molar ratio of 1; activity decreases at
higher molar ratio.[25] Oxidative esterification of CA with 1-buta-
nol was, thus, preliminarily carried out with variance of the
KOH concentration to search for the best reaction conditions;
the main results are given in Table 2.
CA conversion was quantitative after 1.5 h. Further increase of
KOH (12 equiv) resulted in a decrease of catalytic activity,
which confirmed that the concentration of the base is critical
to the catalytic performance of the polymer-incarcerated
AuNPs. To date the role of base is not completely understood.
In oxidation reactions catalysed by polymer-incarcerated
AuNPs, the hydroxyl species, coordinated to the nanoparticle
surface, would promote deprotonation of the alcohol and
favour its coordination to the gold nanoparticle surface, fol-
lowed by b-hydrogen transfer to gold. The results we obtained
for the CA oxidative esterification suggest a more complex role
of this co-catalyst. The oxidative esterification of CA with differ-
ent primary and secondary alcohols, namely methanol, etha-
nol, 2-propanol, 1-hexanol, 1-octanol and 2-phenylethanol was
carried out by using KOH (6 equiv) at 358C to gain information
on the selectivity of the nanoporous host–polymer matrix.
Under these mild conditions, esterification of CA with short
alkyl chain alcohols (methanol and ethanol) proceeds rapidly
to afford methyl cinnamate
When the concentration of KOH was increased from 1 to
12 equivalents (Table 2, entries 1–4; Figures S6 and S7 in the
Supporting Information), butyl cinnamate reached the highest
yield (48%) at KOH/CA molar ratio of 6 after 3 h (Table 2,
entry 3; Figure S7 in the Supporting Information), whereas the
(90%, 4 h; Table 2, entry 5; Fig-
Table 2. Synthesis of alkyl cinnamates by aerobic oxidative esterification of cinnamyl alcohol and alkyl alco-
ure S8 in the Supporting Infor-
mation) and ethyl cinnamate
(94%, 5 h; Table 2, entry 6; Fig-
ure 4a) in high yields, whereas
the esterification of bulkier alco-
hols, 2-propanol, 1-hexanol and
1-octanol (Table 2, entries 7–9) is
slower. These findings suggest
that the bulkiness of the alkyl
chain is a critical parameter in
the diffusion toward the gold
catalyst through the polymer
support. Surprisingly, oxidative
esterification of 2-phenylethanol
is particularly efficient (Table 2,
entry 20; Figure 4b), probably
a result of the high affinity of
this aromatic alcohol with the
polystyrene matrix. It is well
known that aromatic com-
pounds establish strong interac-
tions with the cavities of the d-
and e-crystalline forms of sPS.[27]
The kinetic profiles of the reac-
tions of CA with ethanol and 2-
phenylethanol are compared
(Figure 4). CA is readily oxidised
to cinnamaldehyde after 1.5–2 h,
then the reaction of the alde-
hyde with the alkyl alcohol
begins and leads to the hemia-
cetal, which is definitively oxi-
dised to the alkyl cinnamate.
The reaction with ethanol is
quantitative after 5 h, whereas
that of 2-phenylethanol reaches
a conversion of 44% of cinnamyl
hols.
Entry[a]
1
Alkyl alcohol
1-butanol
KOH/CA
1
t
[h]
Conversion[b,c]
[%]
Products
Yield[c]
[%]
3
3
3
3
4
5
5
5
5
5
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
butyl cinnamate
cinnamaldehyde
3-phenyl-1-propanol
butyl cinnamate
cinnamaldehyde
3-phenyl-1-propanol
butyl cinnamate
cinnamaldehyde
3-phenyl-1-propanol
butyl cinnamate
cinnamaldehyde
3-phenyl-1-propanol
methyl cinnamate
cinnamaldehyde
3-phenyl-1-propanol
ethyl cinnamate
29
69
2
2
1-butanol
1-butanol
1-butanol
methanol
ethanol
3
28
69
3.0
48
46
6.0
44
48
8.0
90
8.1
1.9
94
0
6.0
9.2
86
4.8
12
85
3.0
11
3
6
4
12
6
5
6
6
cinnamaldehyde
3-phenyl-1-propanol
isopropyl cinnamate
cinnamaldehyde
3-phenyl-1-propanol
hexyl cinnamate
cinnamaldehyde
3-phenyl-1-propanol
octyl cinnamate
7
2-propanol
1-hexanol
1-octanol
2-phenylethanol
6
8
6
9
6
cinnamaldehyde
86
3.0
44
50
2.0
3-phenyl-1-propanol
2-phenylethyl cinnamate
cinnamaldehyde
10
6
3-phenyl-1-propanol
[a] Reaction conditions: CA (0.51 mmol), AuNPs-sPSB (200 mg, Au=2%w/w), alkyl alcohol (5.1 mmol), 358C, O2
(1 atm), H2O/CHCl3 (v/v=1:1, total volume=6 mL), anisole as an internal standard (0.51 mmol). [b] Conversion
of CA. [c] Determined by GC-MS analysis.
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Notably, the one-pot synthesis of alkyl cinnamates catalysed
by AuNPs-sPSB exhibits one of the highest reaction rates and
selectivities, under mild conditions, reported to date in the lit-
erature.[16] The turnover frequency (TOF) calculated for the first
oxidation step at 358C is 21.2 hÀ1, whereas TOF=5.6 hÀ1 for
the esterification of CA to methyl cinnamate (Table 2, entry 5).
To the best of our knowledge the highest TOF reported for the
oxidative esterification of CA to methyl cinnamate is 7.4 hÀ1 at
1008C.[16b] In the oxidative esterification of CA with alkyl alco-
hols catalysed by AuNPs-sPSB we found that the oxidation of
CA to cinnamaldehyde proceeds faster than the oxidative cou-
pling of the latter with alkyl alcohols.
We investigated the esterification of cinnamaldehyde with 1-
butanol catalysed by AuNPs-sPSB to shed light on the kinetics
of this cascade reaction in the temperature range 10–458C. To
avoid overlap of the kinetic curves due to the two consecutive
reactions, esterification of cinnamaldehyde with 1-butanol was
also performed independently (Table 4 and Figure 5). The rate
Table 4. Esterification of cinnamaldehyde with 1-butanol.
Entry[a]
T
[8C]
t
[h]
Yield[b]
[%]
Selectivity[b]
[%]
Figure 4. Kinetic profiles of the oxidative esterification of cinnamyl alcohol
with: a) ethanol (Table 2, entry 6); b) 2-phenylethanol (Table 2, entry 10).
1
2
3
4
10
25
35
45
6
7
6
6
6.7
12
>99
89
>99
>99
>99
>99
ester in the same time. The rate constants for this reaction
with the quoted alkyl alcohols are given in Table 3. The reac-
tion proceeds faster with short-chain aliphatic alcohols, where-
as small rate constants were measured for 2-propanol, 1-hexa-
nol and 1-octanol. Moreover, the kinetics for the oxidative
esterification of cinnamaldehyde is pseudo-zero order for ali-
phatic alcohols and pseudo-first order for 2-phenylethanol
(Table 3), which confirms the different mechanism for access of
the oxidation substrates to the catalytic sites.
[a] Reaction conditions: cinnamaldehyde (0.51 mmol), 1-butanol
(5.1 mmol), AuNPs-sPSB (200 mg, Au=2%w/w), O2 (1 atm), H2O/CHCl3
(v/v=1:1, total volume=6 mL), KOH (3.06 mmol, 6 equiv); anisole as an
internal standard (0.51 mmol). [b] Determined by GC-MS analysis.
constant (k) of 4.09ꢁ10À6 molLÀ1 sÀ1 calculated for the latter
reaction at 358C (Table 4, entry 3; Figure S9 in the Supporting
Information) well compares to that found for the second step
of the oxidative esterification of cinnamyl alcohol with 1-buta-
nol (k=4.93ꢁ10À6 molLÀ1 sÀ1; Table 3, entry 3; Figure S7 in the
Supporting Information). The Arrhenius plot gives Ea =62.7Æ
16.7 kJmolÀ1 (Figure S15 in the Supporting Information) for
this oxidation reaction.
Table 3. Rate constants for the oxidative esterification of cinnamaldehyde
with alkyl alcohols.[a]
Entry
Alkyl alcohol[b,c]
kobs [molLÀ1 sÀ1
]
Evaluated from
Table 2, entry x
1
2
3
4
5
6
7
methanol
ethanol
1-butanol
2-propanol
1-hexanol
1-octanol
2-phenylethanol[d]
4.63ꢁ10À6
4.65ꢁ10À6
4.93ꢁ10À6
5.38ꢁ10À7
6.12ꢁ10À7
1.14ꢁ10À6
4.35ꢁ10À5 sÀ1
5
6
3
7
8
9
10
Notably, 3-phenyl-1-propanol, found in traces in CA oxida-
tion (Tables 1 and 2), is not observed in the oxidative esterifica-
tion of cinnamaldehyde with 1-butanol, which suggests that
this compound is formed in the first reaction step.
Oxidative esterification of p-substituted cinnamyl alcohols
[a] Evaluated from the second reaction step of the cinnamyl alcohol oxi-
dation. [b] Reaction conditions: cinnamyl alcohol (0.51 mmol), AuNPs-
sPSB (200 mg, Au=2%w/w), alkyl alcohol (5.1 mmol), KOH (3.06 mmol),
358C, O2 (1 atm), H2O/CHCl3 (v/v=1:1, total volume=6 mL), anisole as an
internal standard (0.51 mmol). [c] Pseudo-zero-order reaction. [d] Pseudo-
first-order reaction.
Alkyl cinnamates with hydroxy, halide or alkoxy substituents
on the aromatic ring are important chemicals in the pharma-
ceutical and cosmetic industries.[1] To test the validity of the
synthetic protocol described, oxidative esterification of p-meth-
Chem. Eur. J. 2014, 20, 5478 – 5486
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entries3 and 4; Figures S12 and S13 in the Supporting Infor-
mation), likely as a result of the activating effect of the elec-
tron-releasing substituent. This supports the formation of an
ionic intermediate species with a positive charge on the carbi-
nol carbon atom to be oxidised.[18b] The overall oxidative
esterification of p-methoxy- and p-chloro-cinnamyl alcohol
with methanol and 1-butanol is slower than the analogous re-
action of CA. Also, in this case, the reaction is faster for metha-
nol and seems to be activated by electron-withdrawing
groups, which increase the electrophilicity of the carbonyl
carbon atom of the intermediate aldehyde and lead to faster
formation of the hemiacetal.[33]
Conclusion
Figure 5. Oxidative esterification of cinnamaldehyde with 1-butanol in the
temperature range 10–458C (Table 5).
Tandem catalysis under sustainable conditions is a topic of in-
creasing interest for the one-pot synthesis of alkyl esters by al-
cohol oxidation. The efficient and selective aerobic oxidation
of cinnamyl alcohol and para-substituted derivatives, such as
p-chloro- and p-methoxy-cinnamyl alcohol, to the correspond-
ing aldehydes has been successfully achieved under mild con-
ditions, catalysed by AuNPs supported in a semi-crystalline
nanoporous polymer matrix. Moreover, the cinnamyl alcohols
are readily oxidised to alkyl cinnamates with high selectivity in
presence of primary or secondary alkyl alcohols. The rate con-
stants for the two reaction steps were measured in
oxy- and p-chloro- cinnamyl alcohol with methanol and 1-buta-
nol was investigated under the experimental conditions of the
CA oxidation (Table 5). Quantitative conversion for the oxida-
tion of p-substituted cinnamyl alcohols to aldehydes was ach-
ieved after 2 h. However, p-methoxy cinnamyl alcohol (Table 5,
entries1 and 2; Figures S10 and S11 in the Supporting Informa-
tion) is oxidised faster than p-chlorocinnamyl alcohol (Table 5,
the temperature range 10–458C and the activation
Table 5. Oxidative esterification of p-substituted cinnamyl alcohols with methanol
and 1-butanol.
energy for cinnamyl alcohol oxidation and oxidative
esterification of cinnamaldehyde with 1-butanol were
calculated from the corresponding Arrhenius plots. It
was shown that the first reaction step (Ea =57.8Æ
11.5 kJmolÀ1), namely the oxidation of CA to cinna-
maldehyde, is faster than the second reaction step
(Ea =62.7Æ16.7 kJmolÀ1) that involves oxidation of
the hemiacetal to the ester.
The kinetic studies allowed confirmation of the
active role played by the nanoporous polymer matrix
in the determination of both the activity and selectiv-
ity of the reaction. Actually, the kinetics of CA oxida-
tion to cinnamaldehyde is first order with respect to
the alcohol as result of fast migration of this reagent
through the nanoporous polymer matrix; the second
step is diffusion controlled for those alcohols, namely
long-chain alkyl alcohols, which are not prone to per-
meate the polymer matrix. The latter reaction rates
decrease as the steric hindrance of the alcohol in-
creases (methanolꢀethanolꢀ1-butanol>2-propanol
ꢀ1-hexano l>1-octanol). On the contrary, the oxida-
tive esterification of CA with 2-phenylethanol is
pseudo-first order in both steps as a result of fast dif-
fusion of the aromatic alcohols through the polystyr-
ene matrix. These findings support the hypothesis
that the oxidative esterification reaction occurs
through two consecutive steps and not in a concerted
way on the surface of the AuNPs.
Entry[a]
1
R
Alkyl alcohol Conversion[b] Products
[%]
Yield[c]
[%]
ÀOMe methanol
>99
methyl p-methoxycinnamate
p-methoxycinnamaldehyde
21
76
3-(p-methoxyphenyl)1-propanol 0.7
2
ÀOMe 1-butanol
>99
butyl p-methoxycinnamate
p-methoxycinnamaldehyde
6.2
91
3-(p-methoxyphenyl)1-propanol 0.7
3
4
ÀCl
ÀCl
methanol
1-butanol
>99
>99
methyl p-chlorocinnamate
p-chlorocinnamaldehyde
butyl p-chlorocinnamate
p-chlorocinnamaldehyde
52
48
26
74
[a] Reaction conditions: p-substituted CA (0.51 mmol), alkyl alcohol (5.1 mmol),
AuNPs-sPSB (200 mg, Au=2%w/w), O2 (1 atm), H2O/CHCl3, (v/v=1:1, total volume=
6 mL); 358C, KOH (3.06 mmol), 6 h, anisole as an internal standard (0.51 mmol).
[b] Conversion of p-substituted CA determined by GC-MS analysis. [c] Determined by
GC-MS.
Thus, the oxidative esterification reaction catalysed
by AuNPs-sPSB was extended to p-substituted cin-
Chem. Eur. J. 2014, 20, 5478 – 5486
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5484
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
1H NMR spectroscopy. At the end of the reaction, the polymer was
coagulated in plenty of acetonitrile. The catalyst was recovered by
filtration and the filtrate analysed by GC-MS.
namyl alcohols of potential pharmaceutical interest to support
the general validity of the method.
Experimental Section
General procedure for direct esterification of alcohols
catalysed by AuNPs-sPSB (Table 2, entry 3)
Materials and instrumentation
A round-bottomed two-necked flask (50 mL) equipped with a mag-
netic stirrer bar was charged, in the order noted, with H2O (3 mL),
KOH (172 mg, 3.06 mmol), anisole (55 mL, 0.51 mmol), cinnamyl al-
cohol (66 mL, 0.51 mmol), 1-butanol (470 mL, 5.1 mmol), CHCl3
(3 mL) and catalyst (200 mg). The mixture was stirred at 358C
under atmospheric pressure of O2. Aliquots of the reaction mixture
were sampled at the desired reaction time and treated with plenty
of acetonitrile. The polymer was separated by filtration and the fil-
HAuCl4·3H2O (Sigma–Aldrich), sodium triethylborohydride (1.0m in
THF; Sigma–Aldrich), cinnamyl alcohol (97%; Fluka), cinnamalde-
hyde (98%; Carlo Erba), p-methoxycinnamaldehyde (96%; SAFC), p-
chlorocinnamaldehyde (96%; Aldrich), methanol (HPLC grade;
Sigma), ethanol (99.8%; Fluka), isopropyl alcohol (99.8%; Sigma–Al-
drich), 1-butanol (98%; Labscan), 1-hexanol (98%; Sigma–Aldrich),
1-octanol (>99%; Sigma–Aldrich), sodium borohydride (98%;
Sigma–Aldrich), hydrochloric acid (37%; Sigma–Aldrich), potassium
hydroxide (85%; Sigma–Aldrich), anisole (99%; Sigma–Aldrich),
water (HPLC grade; Pancreac), chloroform (HPLC grade; Romil),
acetonitrile (HPLC grade; Sigma–Aldrich), ethyl acetate (ꢁ99.5%;
Sigma–Aldrich), methyl ethyl ketone (99.5%; Carlo Erba) and cyclo-
hexanone (ꢁ99.5%; Sigma–Aldrich) were used as received. p-Meth-
oxycinnamyl alcohol and p-chlorocinnamyl alcohol were synthes-
ised by reduction of the corresponding aldehydes by literature
methods.[34] Oxygen was supplied by Rivoira and used as received.
Deuterated solvents were purchased from Euriso-Top or Sigma–Al-
drich and used as received. The AuIII standard solution (Carlo Erba;
1.000Æ0.002 gLÀ1 in aqueous HCl solution, 2%w/w) for the AAS
and ICP-OES measurements was used as received. The AuNPs-sPSB
catalyst (Au=2%w/w) was synthesised and characterised as previ-
ously reported.[25,26]
1
trate was analysed by GC-MS and H NMR spectroscopy. At the end
of the reaction, the polymer was coagulated in plenty of acetoni-
trile. The catalyst was recovered by filtration and the filtrate ana-
lysed by GC-MS.
General procedure for direct esterification of cinnam-
aldehyde catalysed by AuNPs-sPSB (Table 4, entry 3)
A round-bottomed two-necked flask (50 mL) equipped with a mag-
netic stirrer bar was charged, in the order noted, with H2O (3 mL),
KOH (172 mg, 3.06 mmol), anisole (55 mL, 0.51 mmol), freshly dis-
tilled cinnamaldehyde (66 mL, 0.51 mmol), 1-butanol (470 mL,
5.1 mmol), CHCl3 (3 mL) and catalyst (200 mg). The mixture was
stirred at 358C under atmospheric pressure of O2. Aliquots of the
reaction mixture were sampled at the desired reaction time and
treated with plenty of acetonitrile. The polymer was separated by
NMR spectra were recorded with a Bruker AVANCE 400 spectrome-
1
ter (400 MHz for H and 100 MHz for 13C). WAXD patterns were ob-
1
filtration and the filtrate was analysed by GC-MS and H NMR spec-
tained in reflection mode with an automatic Bruker D8 powder dif-
fractometer and nickel-filtered CuKa radiation. TEM analysis was car-
ried out with a Tecnai 20 (FEI) microscope operating at 200 kV. The
specimens for TEM analysis were sonicated in 2-propanol, then
transferred (10 mL) onto a copper grid covered with a lacey carbon
film supplied from Assing. The size-distribution analysis of the
AuNPs was performed with the software Photoshop CS5 Extended.
Atomic adsorption spectroscopy (AAS) and inductively coupled
plasma optical emission spectrometry (ICP-OES) analysis of the
AuNPs-sPSB catalyst were performed on a PerkinElmer AAnalyst
100 spectrophotometer equipped with a Au hollow cathode lamp
(PerkinElmer) and a PerkinElmer Optima 7000 DV spectrometer, re-
spectively. DRIFTS measurements were performed with a Bruker
Vertex 70 spectrometer. The catalyst solutions to be analysed were
obtained as previously reported.[25] GC analyses were performed
with a GC-MS 7890A/5975C chromatograph from Agilent Technolo-
gies, equipped with a HP-Innowax column (polyethylene glycol,
30 m, 0.25 mm ID) or an Optima 17MS column (1:1 diphenylpolysil-
oxane/dimethylpolysiloxane, 30 m, 0.25 mm ID), a mass-selective
detector and a FID detector.
troscopy. At the end of reaction, the polymer was coagulated in
plenty of acetonitrile. The catalyst was recovered by filtration and
the filtrate analysed by GC-MS.
Acknowledgements
Financial support from Ministero dell’Istruzione dell’Universitꢂ
e della Ricerca (MIUR) (FARB-2012) and POR Campania FSE
2007-2013, “Sviluppo di reti di eccellenza tra Universitꢂ - Centri
di Ricerca - Imprese”, Asse IV, “Materiali e strutture intelligenti”
(MASTRI), CUP B25B09000010007. The authors thank Dr. Patri-
zia Iannece and Dr. Tonino Caruso for technical assistance with
GC analysis and Dr. Maria Sarno for the TEM acquisitions.
Keywords: esterification · gold · nanoparticles · oxidation ·
supported catalysts
[1] P. Sharma, J. Chem. Pharm. Res. 2011, 3, 403–423.
[2] N. H. Nam, Y. J. You, Y. D. Kim, H. Hong, H. M. Kim, Y. Z. Ann, Bioorg.
Med. Chem. Lett. 2001, 11, 1173.
[3] S. Kumar, P. Arya, C. Mukherjee, B. K. Singh, N. Singh, V. S. Prasad, A. K.
Ghose, Biochemistry 2005, 44, 15944.
[4] A. Alexander, R. K. Choudhary, U.S. Patent No. 5527947, 1996.
[5] a) F. A. Carey, R. J. Sundberg, Advanced Organic Chemistry 5th ed. Part B:
Reactions and Synthesis, Springer, Heidelberg, 2007, p. 252; b) J. Otera,
J. Nishikido, Esterification: Methods, Reactions, and Applications, Wiley-
VCH, Weinheim, 2010, pp. 3–157.
General procedure for oxidation of cinnamylic alcohols
catalysed by AuNPs-sPSB (Table 1, entry 2)
A round-bottom two-necked flask (50 mL) equipped with a magnet-
ic stirrer bar was charged, in the order noted, with H2O (3 mL),
KOH (172 mg, 3.06 mmol), anisole (55 mL, 0.51 mmol), cinnamyl al-
cohol (66 mL, 0.51 mmol), CHCl3 (3 mL) and catalyst (200 mg). The
mixture was stirred at 358C under atmospheric pressure of O2. Ali-
quots of the reaction mixture were sampled at the desired reaction
time and treated with plenty of acetonitrile. The polymer was sepa-
rated by filtration and the filtrate was analysed by GC-MS and
[6] L. Shu, Y. Hongjun, Eur. Chem. Bull. 2013, 2, 76–77.
[7] a) Y. Zhang, X. Cui, F. Shi, Y. Deng, Chem. Rev. 2012, 112, 2467–2505;
b) M. Stratakis, H. Garcia, Chem. Rev. 2012, 112, 4469–4506; c) A. Corma,
Chem. Eur. J. 2014, 20, 5478 – 5486
www.chemeurj.org
5485
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
H. Garcia, Chem. Soc. Rev. 2008, 37, 2096–2126; d) S. B. Kalidindi, B. R.
Jagirdar, ChemSusChem 2012, 5, 65–75; e) C. Della Pina, E. Falletta, M.
Rossi, Chem. Soc. Rev. 2012, 41, 350–369; f) T. Mallat, A. Baiker, Annu.
Rev. Chem. Biomol. Eng. 2012, 3, 11–28; g) J. M. Campelo, D. Luna, R.
Luque, J. M. Marinas, A. A. Romero, ChemSusChem 2009, 2, 18–45; h) P.
Zhao, N. Li, D. Astruc, Coord. Chem. Rev. 2013, 257, 638–665; i) M.
Sankar, N. Dimitratos, P. J. Miedziak, P. P. Wells, C. J. Kiely, G. J. Hutchings,
Chem. Soc. Rev. 2012, 41, 8099–8139; j) M. Chen, D. W. Goodman,
Chem. Soc. Rev. 2008, 37, 1860–1870; k) Modern Heterogeneous Oxida-
tion Catalysis. Design, Reactions and Characterization (Ed.: N. Mizuno),
Wiley-VCH, Weinheim, 2009.
[19] C. L. Bianchi, P. Canton, N. Dimitratos, F. Porta, L. Prati, Catal. Today
2005, 102–103, 203–212.
[20] a) H. Tsunoyama, H. Sakurai, Y. Negishi, T. Tsukuda, J. Am. Chem. Soc.
2005, 127, 9374–9375; b) H. Tsunoyama, N. Ichikuni, H. Sakurai, T. Tsu-
kuda, J. Am. Chem. Soc. 2009, 131, 7086–7093.
[21] D. Astruc, F. Lu, d J. R. Aranzaes, Angew. Chem. 2005, 117, 8062–8083;
Angew. Chem. Int. Ed. 2005, 44, 7852–7872.
[22] F. Shi, Q. Zhang, Y. Ma, Y. He, Y. Deng, J. Am. Chem. Soc. 2005, 127,
4182–4183.
[23] a) H. Miyamura, R. Matsubara, Y. Miyazaki, S. Kobayashi, Angew. Chem.
2007, 119, 4229–4232; Angew. Chem. Int. Ed. 2007, 46, 4151–4154;
b) M. Conte, H. Miyamura, S. Kobayashi, V. Chechik, J. Am. Chem. Soc.
2009, 131, 7189–7196.
[8] M. Haruta, T. Kobayashi, H. Sano, N. Yamada, Chem. Lett. 1987, 16, 405–
408.
[9] L. Kesavan, R. Tiruvalam, M. H. A. Rahim, M. I. bin Saiman, D. I. Enache,
R. L. Jenkins, N. Dimitratos, J. A. Lopez-Sanchez, S. H. Taylor, D. W.
Knight, C. J. Kiely, G. J. Hutchings, Science 2011, 331, 195–199.
[10] a) S. E. Davis, M. S. Ide, R. J. Davis, Green Chem. 2013, 15, 17–45; b) C.
Parmeggiani, F. Cardona, Green Chem. 2012, 14, 547–564; c) T. Tsukuda,
H. Tsunoyama, H. Sakurai, Chem. Asian J. 2011, 6, 736–748; d) C. P.
Vinod, K. Wilson, A. F. Lee, J. Chem. Technol. Biotechnol. 2011, 86, 161–
171; e) T. Mallat, A. Baiker, Chem. Rev. 2004, 104, 3037–3058; f) C. Luc-
chesi, T. Inasaki, H. Miyamura, R. Matsubara, S. Kobayashi, Adv. Synth.
Catal. 2008, 350, 1996–2000.
[24] a) C. Keresszegi, T. Bꢆrgi, T. Mallat, A. Baiker, J. Catal. 2002, 211, 244–
251; b) N. Dimitratos, A. Villa, D. Wang, F. Porta, D. Su, L. Prati, J. Catal.
2006, 244, 113–121; c) L. J. Durndell, C. M. A. Parlett, N. S. Hondow, K.
Wilson, A. F. Lee, Nanoscale 2013, 5, 5412–5419.
[25] A. Buonerba, C. Cuomo, S. Ortega Sꢇnchez, P. Canton, A. Grassi, Chem.
Eur. J. 2012, 18, 709–715.
[26] a) A. Buonerba, M. Fienga, S. Milione, C. Cuomo, A. Grassi, A. Proto, C.
Capacchione, Macromolecules 2013, 46, 8449–8457; b) A. Buonerba, V.
Speranza, A. Grassi, Macromolecules 2013, 46, 778–784; c) A. Buonerba,
C. Cuomo, V. Speranza, A. Grassi, Macromolecules 2010, 43, 367–374;
d) C. Cuomo, M. C. Serra, M. Gonzalez Maupoey, A. Grassi, Macromole-
cules 2007, 40, 7089–7097; e) M. Caprio, M. C. Serra, D. E. Bowen, A.
Grassi, Macromolecules 2002, 35, 9315–9322; f) N. Galdi, A. Buonerba, P.
Oliva, L. Oliva, Polym. Chem. 2014, DOI: 10.1039/C3PY01703F.
[27] a) Syndiotactic Polystyrene, Characterization, Processing, and Application
(Ed.: J. Schellenberg), John Wiley & Sons, Hoboken, New Jersey, 2010;
b) G. Milano, G. Guerra, Prog. Mater. Sci. 2009, 54, 68–88; c) E. B. Gowd,
K. Tashiro, C. Ramesh, Prog. Polym. Sci. 2009, 34, 280–315; d) G. Guerra,
C. Daniel, P. Rizzo, O. Tarallo, J. Polym. Sci. Part B 2012, 50, 305–322;
e) M. Malanga, Adv. Mater. 2000, 12, 1869–1872; f) C. Capacchione, A.
De Roma, A. Buonerba, V. Speranza, S. Milione, A. Grassi, Macromol.
Chem. Phys. 2013, 214, 1990–1997; g) P. Rizzo, C. Daniel, A. De Girolamo
Del Mauro, G. Guerra, Chem. Mater. 2007, 19, 3864–3866.
[28] Metal Nanoclusters in Catalysis and Material Science: The Issue of Size
Control (Eds.: B. Corain, G. Schmid, N. Toshima), Elsevier, Amsterdam,
2008, pp. 131–132.
[11] J. Huang, T. Akita, J. Faye, T. Fujitani, T. Takei, M. Haruta, Angew. Chem.
2009, 121, 8002–8006; Angew. Chem. Int. Ed. 2009, 48, 7862–7866.
[12] Y. Zhu, H. Qian, B. A. Drake, R. Jin, Angew. Chem. 2010, 122, 1317–1320;
Angew. Chem. Int. Ed. 2010, 49, 1295–1298.
[13] A. Corma, P. Serna, Science 2006, 313, 332–334.
[14] a) J. K. Edwards, B. Solsona, E. N. Ntainjua, A. F. Carley, A. A. Herzing, C. J.
Kiely, G. J. Hutchings, Science 2009, 323, 1037–1041; b) S. J. Freakley, M.
Piccinini, J. K. Edwards, E. N. Ntainjua, J. A. Moulijn, G. J. Hutchings, ACS
Catal. 2013, 3, 487–501.
[15] M. Conte, C. J. Davies, D. J. Morgan, T. E. Davies, D. J. Elias, A. F. Carley, P.
Johnston, G. J. Hutchings, J. Catal. 2013, 297, 128–136.
[16] a) T. Ishida, M. Nagaoka, T. Akita, M. Haruta, Chem. Eur. J. 2008, 14,
8456–8460; b) R. Bernini, S. Cacchi, G. Fabrizi, S. Niembro, A. Prastaro,
A. Shafir, A. Vallribera, ChemSusChem 2009, 2, 1036–1040; c) H. Miya-
mura, T. Yasukawa, S. Kobayashi, Green Chem. 2010, 12, 776–778; d) A.
Wittstock, V. Zielasek, J. Biener, C. M. Friend, M. Bꢃumer, Science 2010,
327, 319–322; e) B. Xu, R. J. Madix, C. M. Friend, J. Am. Chem. Soc. 2010,
132, 16571–16580; f) B. Xu, J. Haubrich, C. G. Freyschlag, R. J. Madix,
C. M. Friend, Chem. Sci. 2010, 1, 310–314; g) F.-Z. Su, J. Ni, H. Sun, Y.
Cao, H.-Y. He, K.-N. Fan, Chem. Eur. J. 2008, 14, 7131–7135; h) P. Liu, C.
Li, E. J. M. Hensen, Chem. Eur. J. 2012, 18, 12122–12129; i) R. L. Oliveira,
P. K. Kiyohara, L. M. Rossi, Green Chem. 2009, 11, 1366–1370; j) C. Mars-
den, E. Taarning, D. Hansen, L. Johansen, S. K. Klitgaard, K. Egeblad,
C. H. Christensen, Green Chem. 2008, 10, 168–170; k) B. Xu, X. Liu, J.
Haubrich, C. M. Friend, Nat. Chem. 2010, 2, 61–65.
[29] F. Kaneko, T. Tsuchida, Polymer 2013, 54, 760–765.
[30] S. Bꢃhn, S. Imm, L. Neubert, M. Zhang, H. Neumann, M. Beller, Chem-
CatChem 2011, 3, 1853–1864.
[31] J. M. Mayer, Acc. Chem. Res. 2011, 44, 36–46.
[32] C.-R. Chang, X.-F. Yang, B. Long, J. Li, ACS Catal. 2013, 3, 1693–1699.
[33] a) R. Bruckner in Advanced Organic Chemistry: Reaction Mechanisms,
Academic Press, San Diego, 2002, p. 280; b) J. Clayden, N. Greeves, S.
Warren, P. Wothers in Organic Chemistry, Oxford University Press, 2001,
pp. 143–147.
[17] J.-F. Soulꢄ, H. Miyamura, S. Kobayashi, J. Am. Chem. Soc. 2011, 133,
18550–18553.
[34] N. Usami, K. Kitahara, S. Ishikura, M. Nagano, S. Sakai, A. Hara, Eur. J. Bio-
chem. 2001, 268, 5755–5763.
[18] a) A. Abad, P. Concepcion, A. Corma, H. Garcꢅa, Angew. Chem. 2005, 117,
4134–4137; Angew. Chem. Int. Ed. 2005, 44, 4066–4069; b) A. Abad, A.
Corma, H. Garcꢅa, Chem. Eur. J. 2008, 14, 212–222; c) Y. Pꢄrez, C. Aprile,
A. Corma, H. Garcꢅa, Catal. Lett. 2010, 134, 204–209.
Received: October 3, 2013
Published online on March 18, 2014
Chem. Eur. J. 2014, 20, 5478 – 5486
www.chemeurj.org
5486
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