Microwave-Assisted Catalytic Acetylation of Alcohols by Gold-Nanoparticle-Supported Gadolinium Complex

Article abstract of DOI:10.1080/00397911.2014.977399

A gold nanoparticle (AuNP)-supported gadolinium complex (RS-Au-L-Gd) catalyst was prepared through simple chelation of GdCl₃ to the surface-bound spacer, 1,4,7-tris(carboxymethyl)-10-(11-mercaptoundecyl)-1,4,7,10-tetraazacyclododecane (HSDO3A). This AuNP-supported Gd complex was found to be a highly effective catalyst for the acetylation of various alcohols and phenol in the presence of acetic anhydride. With a loading of 0.4 mol% of RS-Au-L-Gd, the almost complete transformation can be achieved in 60 s under microwave irradiation conditions. This hybrid catalyst was air stable, water soluble, dissolvable in many organic media, and precipitable. It can be readily recycled more than eight times without any significant loss of its catalytic activity. GRAPHICAL ABSTRACT.

Full text of DOI:10.1080/00397911.2014.977399

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Microwave-Assisted Catalytic
Acetylation of Alcohols by Gold-
Nanoparticle–Supported Gadolinium
Complex
Tsao-Ching Changa & Shuchun Joyce Yua
a Department of Chemistry and Biochemistry, National Chung Cheng
University, Ming Hsiung, Chia Yi, Taiwan, China
Accepted author version posted online: 17 Nov 2014.Published
online: 12 Jan 2015.
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To cite this article: Tsao-Ching Chang & Shuchun Joyce Yu (2015) Microwave-Assisted Catalytic
Acetylation of Alcohols by Gold-Nanoparticle–Supported Gadolinium Complex, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
45:5, 661-672, DOI: 10.1080/00397911.2014.977399
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Synthetic Communications1, 45: 661–672, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397911.2014.977399
MICROWAVE-ASSISTED CATALYTIC ACETYLATION OF
ALCOHOLS BY GOLD-NANOPARTICLE–SUPPORTED
GADOLINIUM COMPLEX
Tsao-Ching Chang and Shuchun Joyce Yu
Department of Chemistry and Biochemistry, National Chung Cheng
University, Ming Hsiung, Chia Yi, Taiwan, China
GRAPHICAL ABSTRACT
Abstract A gold nanoparticle (AuNP)–supported gadolinium complex (RS-Au-L-Gd)
catalyst was prepared through simple chelation of GdCl3 to the surface-bound spacer,
1,4,7-tris(carboxymethyl)-10-(11-mercaptoundecyl)-1,4,7,10-tetraazacyclododecane
(HSDO3A). This AuNP-supported Gd complex was found to be a highly effective catalyst
for the acetylation of various alcohols and phenol in the presence of acetic anhydride. With
a loading of 0.4 mol% of RS-Au-L-Gd, the almost complete transformation can be achieved
in 60 s under microwave irradiation conditions. This hybrid catalyst was air stable, water
soluble, dissolvable in many organic media, and precipitable. It can be readily recycled
more than eight times without any significant loss of its catalytic activity.
Keywords Acetylation; gadolinium catalyst; gold nanoparticle supported catalyst
Received May 3, 2014.
Address correspondence to Shuchun Joyce Yu, Department of Chemistry and Biochemistry,
National Chung Cheng University, Ming Hsiung, Chia Yi 621, Taiwan, ROC. E-mail: chejyy@ccu.edu.tw
661

662
T.-C. CHANG AND S. J. YU
INTRODUCTION
The preparation of complex and multifunctional organic molecules often
demands protection and deprotection procedures to allow the survival of reactive
functional groups before their transformation into the target molecules. Acetylation
of alcohols is an important and routinely utilized protecting methods in organic
chemistry.[1,2] Since acetyl is one of the most common protecting groups used for
the hydroxyl functionality because of its high stability[3] in acidic conditions and ease
of removal by mild alkaline hydrolysis.[4] Many effective catalysts have been reported
for acetylation of alcohols, such as organic acids and bases,[1b,5] simple metal salts,[6]
metal triflates,[7] and others.[8] However, some of these catalysts are not fully
satisfactory because of their toxicity, long reaction times, harsh reaction conditions,
poor yields, time-consuming work-up procedures, and the production of corrosive
chemical wastes. Furthermore, most of the metal triflates are relatively expensive
and some need to be handled under critically anhydrous conditions for best results.
To solve this problem, the strategy of immobilizing homogeneous transition-metal
complexes to insoluble supports to prepare recoverable catalysts has been extensively
applied in sustainable chemistry. This type of hybrid catalyst should be in principle
congregating the advantages from both heterogeneous and homogeneous systems.
A polymer-supported gadolinium triflate has been recently reported to be an
effective heterogeneous catalyst for acetylation of alcohols.[7e] This is a promising
demonstration of a recyclable catalytic system. However, in many other cases,
supported catalysts suffer a series of problems due to heterogenization and
high costs.[9] This has led to alternative liquid-phase methodologies to restore
homogeneous reaction conditions. Hence soluble supports[10] have been used to
obtain detailed structures of the active catalysts, and much more fundamental
knowledge about the catalyst modification, degradation, and at the interface would
then become comprehensible.
Over the past few years, we have productively synthesized a series of gold-
nanoparticle-supported metal complexes. Gold nanoparticles (AuNPs) are some of
the most studied metal particles in the field of nanocatalysis. Alkanethiolate-
protected AuNPs have been shown to present excellent solid surfaces similar to
the (1 1 1) surface of bulk gold.[11] These AuNPs also behave like soluble molecules,
exhibiting both dissolvability and precipitability.[12] Therefore, alkanethiolate-
protected AuNPs are good candidates as supports for metal-complex catalysts, with
the metal-nanoparticle catalyst combining the advantages of both supported and
homogeneous catalysts. Several recent reports by our group and others have demon-
strated the successful use of AuNP-supported metal-complex catalysts for various
organic transformations.[13]
In extension of our continuing studies on gold-nanoparticle-supported
catalysts for sustainable and efficient synthetic procedures,[13a–c] it is found that
AuNP-supported Gd complex is an air-stable, water-soluble, mild, and efficient
recyclable hybrid catalyst for the acetylation of various alcohols and phenol
(Scheme 1). In addition, the rates of these catalytic acetylation reactions can be
further accelerated under microwave (MW) irradiation conditions. Also, this
AuNP-supported hybrid catalyst can be successively reused many times and its
catalytic activity remains nearly unchanged even after eight cycles.

ALCOHOL ACETYLATION BY AuNP-SUPPORTED Gd COMPLEX
663
Scheme 1. AuNP-supported and molecular Gd complex–catalyzed acetylation of alcohols.
RESULTS AND DISCUSSION
Our design for the preparation of AuNP-supported Gd-complex catalyst is
illustrated in Fig.1. Immobilization of complexes with use of covalent tethering tech-
niques is, at present, the most favorable approach to design stable hybrid catalysts.
[10a]
The compound [1,4,7-tris(carboxymethyl)-10-(11-mercaptoundecyl)-1,4,7,10-
tetraazacyclododecane (DO3ASH, 1) was synthesized to be used as the tethering
linker because the thiol end and the DO3A end have been demonstrated to readily
bind surface Au and Gd3þ [14]
respectively. The AuNPs surfaces were functionalized
,
with use of the place-exchange method[15] by treating 155 mg of dodecanethiolate-
covered AuNPs[15c,16] (Au-SR, 2.9 ꢀ 0.9 nm) with various quantities of 1 (100–150 mg)
Figure 1. Synthesis of Gd-functionalized AuNPs, RS-Au-L-Gd (3).

664
T.-C. CHANG AND S. J. YU
Table 1. Molecular Gd-DO3A (4)-catalyzed acetylation of alcohols
NMR yieldc (isolated yield)d (%)
Thermal
Entry
1a
Substrate
Product
(60°C / 4 h)
MW (300 W / 60 s)
93e
65e
CH3(CH2)6OH
2a
3a
CH3(CH2)11OH
96 (95)
96e
68 (65)
64e
4a
99 (98)
92 (90)
61 (60)
51 (47)
5a
6a
7b
94 (90)
78 (75)
49 (46)
41 (37)
36 (32)
8b
76 (75)
86 (85)
72 (70)
9b
14e
13e
22e
10b
11a
55e
(Continued)

ALCOHOL ACETYLATION BY AuNP-SUPPORTED Gd COMPLEX
Table 1. Continued
665
NMR yieldc (isolated yield)d (%)
Thermal
Entry
12b
Substrate
Product
(60°C / 4 h)
MW (300 W / 60 s)
0
18e
Note. Reaction conditions: solvent ¼ DMF/H2O ¼ 4:1.
aAlcohol ¼ 2 mmol, acetic anhydride ¼ 6 mmol, catalyst 4 ¼ 0.01 mmol (0.5 mol%).
bAlcohol ¼ 0.5 mmol, acetic anhydride ¼ 5 mmol, catalyst 4 ¼ 0.01 mmol (2 mol %).
cNMR yield was determined by 1H NMR spectroscopy analysis. The NMR signals of the a-CH2
protons of the starting ROH and the a-CH2 protons of acetate products were integrated against the OCH3
protons (d ¼ 3.74 ppm) of a 0.045 M internal standard 4-iodoanisole.
dIsolated yields were obtained either by column chromatography on silica gel (EA/hexane, 1/70) or by
the extraction with CH2Cl2 (3 mL). The organic layer was then washed with DI water (3 × 5 mL), dried
with Na2SO4, and evaporated under vacuum to afford the desired acetate products.
eOnly the NMR yield was given, due to either the low conversion or the high volatility of the product.
in CHCl3 at ambient temperature for 24 h. The resulting mixed thiolates-covered
AuNPs, RS-Au-L (2), have diameters of 2.5–3.6 nm. Because these surface functio-
nalized AuNPs typically obtained under our reaction conditions are soluble in many
1
organic solvents we could therefore use simple solution H NMR spectroscopy to
determine the average thiolate coverage and the amount of spacer linker loading.
Molar ratios of L/RS ranging from 1:0.4 to 1:0.6, where RS ¼ dodecanethiolate
=
and L DO3ASH (see the Supplemental Material), can be obtained under our reac-
tion conditions. Further treatment of 2 with GdCl3 would make the molecular
Gd-complex tethered onto AuNPs via direct binding with surface DO3ASH. The
resulting Gd-functionalized AuNPs, RS-Au-L-Gd (3), have average core sizes of
3.8–5.3 nm with the Gd loading levels of 0.35–0.37 mmol/g, which were determined
by transmission electron microscopy (TEM) and inductively coupled plasma–atomic
emission spectrometry (ICP-AES), respectively (see the Supplemental Material).
The AuNP-supported form was also confirmed by energy-dispersive x-ray spec-
troscopy (EDX) and FT-IR spectroscopy. Meanwhile, as for the control study,
the molecular Gd-complex (Gd-DO3A, 4) was prepared according to the reported
procedures.[17]
As for sustainability concern, MW radiation has been an alternative to conven-
tional heating in organic synthesis for transforming electromagnetic energy into heat
=
though highly dielectric media such as dimethylsulfoxide (DMSO), ROH (R Me,
Et, and Pr), and ethylene glycol for very rapidly heating.[18] However, because of the
nature of most organic substrates, many reactions preferably proceed in less polar
solvent systems which have substantially low dielectric constants.[19] In the current
study, we have introduced AuNP not only as the catalyst support but also as a high
MW-absorbing material that will couple MW effectively and get heated to high
temperatures[20] very rapidly. Therefore, even with a heterogeneous system in a

666
T.-C. CHANG AND S. J. YU
Table 2. MW-assisted acetylation of alcohols and phenol catalyzed by RS-Au-L-Gd (3)
NMR yielda (isolated yield)b (%)
Cat. 3
Entry
1
Substrate
Product
No cat.
CH3(CH2)6OH
6
>99c
2
3
CH3(CH2)11OH
8
8
>99 (96)
>99c
4
5
0
4
98 (94)
91 (90)
6
7
5
9
99 (98)
99 (97)
8
4
96 (93)
99 (96)
9
3
4
10
80 (77)
(Continued)

ALCOHOL ACETYLATION BY AuNP-SUPPORTED Gd COMPLEX
Table 2. Continued
667
NMR yielda (isolated yield)b (%)
Cat. 3
Entry
11
Substrate
Product
No cat.
3
75c
12
0
52 (48)
Note. Reaction conditions: alcohol ¼ 0.5 mmol, acetic anhydride ¼ 5 mmol, catalyst 3 ¼ 2 × 10ꢁ3 mmol
(0.4 mol%), solvent ¼ DMF (80 µL).
aNMR yield was determined by 1H NMR spectroscopy analysis. The NMR signals of the a-CH2
protons of the starting ROH, the a-CH2 protons of acetate products were integrated against the OCH3
protons (d ¼ 3.74 ppm) of a 0.045 M internal standard 4-iodoanisole.
bIsolated yields were obtained either by column chromatography on silica gel (EA/hexane, 1/70) or by
the extraction with CH2Cl2 (3 mL). The organic layer was then washed with DI water (3 × 5 mL), dried
with Na2SO4, and evaporated under vacuum to afford the desired acetate products.
cOnly the NMR yield was given due to the high volatility of the product.
low-MW-absorbing organic solvent, these AuNP-supported metal complex catalysts
can still provide compatible catalytic activity to that of the corresponding homo-
geneous system.[13a] As shown in both Tables 1 and 2, the MW heating method pro-
vided significant rate acceleration as compared to the conventional thermal heating
method. With a catalyst loading of 0.5–2 mol%, the MW-assisted, 4-catalyzed reac-
tions of primary alcohols and phenol offered 72–99% yields with final temperatures
of 120–140°C in 60s (entries 1–10, Table 1), whereas the same catalyses gave much
worse yields after 4 h under thermal conditions. Even with the less reactive, secondary
alcohols such a s L-menthol, the reaction under MW gave 18% yield in 60s, while the
same mixture at 60°C for 4 h provided 0% conversion (entry 12, Table 1).
However, the Gd complex immobilized at the interphase, in a form of RS-Au-
L-Gd (3), was found to be a very active catalyst for acetylation of alcohols and
phenol under MW irradiation conditions. As shown in Table 2, with a catalyst
loading of 0.4 mol%, the MW-assisted, 3-catalyzed acetylation of primary alchols
and phenol were carried out heterogeneously in dimethylformamide (DMF)[21] to
offer 91–99% yields of acetate products in 60 s (entries 1–9, Table 2). Even for bulky
alcohols and less reactive alcohols, the AuNP-supported 3 (0.4 mol%) still offered
49–80% yields of products in 60 s, while the homogeneous reactions gave 18–72%
yields when using molecular 4 (0.5–2 mol%) as the catalyst (entries 10–12 in Tables
1 and 2). Furthermore, similar MW-assisted reactions without the presence of Gd
catalysts were carried out for the control study. It was found that the product
conversions of 0–9% (Table 2) with final temperatures of 75–94 °C were obtained,
while the MW-assisted, RS-Au-L-Gd (3)–catalyzed reactions gave 49–99% yields

668
T.-C. CHANG AND S. J. YU
Table 3. Recycling and reuse of catalyst RS-Au-L-Gd (3) in the acetylation of phenola
Catalyst
Cycle (% NMR yieldb)
2
3
4
5
6
7
8
1
3
99
99
99
99
99
99
99
94
aReaction conditions: phenol (0.5 mmol) in DMF (80 µL), acetic anhydride ¼ 5 mmol, catalyst
3 ¼ 2 × 10ꢁ3 mmol (0.4 mol%).
bDetermined by 1H NMR spectroscopy analysis.
Note. The NMR signals of the a-CH2 protons of the starting ROH, the a-CH2 protons of acetate
products were integrated against the OCH3 protons (d ¼ 3.74 ppm) of a 0.045 M internal standard 4-
iodoanisole.
of acetate products and final temperatures of 110–130 °C. These results suggested
that AuNPs-containing reactions mixtures are indeed absorbing MW much more
effectively to provide higher temperatures.[22]
Because the AuNP surface-bound Gd-complex catalysts in our study can be
heterogeneously suspended in most organic solvents, they can be easily separated
and quantitatively recovered from the reaction mixture through simple filtration.
A series of catalyst recycling experiments were readily accomplished in the system
containing 0.5 mol of phenol, 5 mmol of acetic anhydride, and 2 × 10ꢁ3 mmol of
RS-Au-L-Gd (3) (0.4 mol%) in DMF (80 µL) under 300 W MW radiation for 60 s.
As shown in Table 3, the catalytic system can be effectively recycled with reactivity
remaining at ≥94% yield throughout eight cycles.
CONCLUSION
In summary, we have developed a hybrid catalyst system by immobilizing
molecular gadolinium(III) complex onto the surface of AuNPs. The resulting
colloidal particles are air stable, water soluble, dissolvable in many organic media,
and precipitable. We have also demonstrated a greener and highly efficient method-
ology for the acetylation of various alcohols and phenols by using this AuNP-bound
interphase Gd catalyst under MW irradiation conditions. As compared to their
corresponding unbound free form the AuNP-bound Gd complex was shown to be
a more effective catalyst and provides significant reactivity enhancement. In
addition, this hybrid catalyst can be successfully recycled more than eight times
without any significant loss of its catalytic activity.
As demonstrated herein, hybrid catalysts of this type combine the advantages
from both heterogeneous and homogeneous systems. It is expected that many
sustainable and applicable catalysts of this form can be made by tethering a wide
range of metal complexes or even organocatalysts to a variety of metal nanosurfaces,

ALCOHOL ACETYLATION BY AuNP-SUPPORTED Gd COMPLEX
669
and they may be useful for promoting many types of chemical reactions in a greener
fashion.
EXPERIMENTAL
All chemicals were used as supplied without further purification. 1H NMR spectra
were recorded in CDCl3 and methanol-d4 on a 400-MHz spectrometer (Bruker DPX-
400), and coupling constants are reported in hertz. Chemical shifts are given in parts
per million (ppm) relative to TMS. All MW irradiation experiments described herein
were performed with a single-mode Discover Labmate System from CEM Corporation,
using either a standard cylindrical Pyrex vessel for sealed vessel processing (capacity
10mL) or a 10-mL Pyrex round-bottomed flask for open-vessel reflux chemistry.
Synthesis of AuNP-Supported Gd Complex (RS-Au-L-Gd, 3)[23]
Aqueous NaOH (1 M, 2 mL) was added to a solution of AuNPs (2, 167 mg)
and GdCl3 · 6H2O (154 mg, 0.20 mmol) in H2O (3 mL) to maintained its pH at 6–
8. After stirring for 24 h at room temperature, the reaction mixture was filtered
and the filtrate was evaporated to dryness under reduced pressure. The residue
was dissolved in minimal H2O (1 mL) and acetone (15 mL) was added to allow the
precipitation of RS-Au-L-Gd (3), yield ¼ 80 mg.
General Procedures for the (4)-Catalyzed Acetylation of Alcohols
Under Thermal Conditions
A mixture of molecular 4 (1.0 × 10ꢁ2 mmol of Gd), alcohol (2.0 mmol for liquid
substrate and 0.5 mmol for solid substrate), and acetic anhydride (1.5 mmol for
liquid substrate and 5 mmol for solid substrate) in DMF/H2O (400 µL, DMF/H2O
¼ 4:1) was allow to stir for 4 h at 60 °C. After the reaction, the resulting mixture
1
was used directly for H NMR analysis for the analytic yield.
MW-Assisted RS-Au-L-Gd (3)–Catalyzed Acetylation of Alcohols
A mixture of colloidal AuNP-surface-bound Gd catalyst RS-Au-L-Gd (3)
(2.0 × 10ꢁ3 mmol Gd), alcohol (0.5 mmol), and acetic anhydride (5 mmol) in DMF
(80 µL) was placed in the reaction vessel under 300 W MW (CEM single-mode Dis-
cover Labmate System) for 60 s. After the reaction, the resulting mixture was used
1
directly for H NMR analysis for the analytic yield.
MW-Assisted, (4)-Catalyzed Acetylation of Alcohols
A mixture of molecular 4 (1.0 × 10ꢁ2 mmol Gd), alcohol (2.0 mmol for liquid
substrate and 0.5 mmol for solid substrate), and acetic anhydride (1.5 mmol for
liquid substrate and 5 mmol for solid substrate) in DMF (400 µL) was placed
in the reaction vessel under 300 W MW for 60 s. After the reaction, the resulting
1
mixture was used directly for H NMR analysis for the analytic yield.

670
T.-C. CHANG AND S. J. YU
FUNDING
This work was supported by the Ministry of Science and Technology of Taiwan
[Grant Number 100-2113-M-194-005-MY2].
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
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