Flavin-functionalized gold nanoparticles as an efficient catalyst for aerobic organic transformations

Article abstract of DOI:10.1002/cctc.201402619

Monolayer-protected gold clusters functionalized with synthetic flavins were synthesized and their catalytic activity in aerobic organic transformations investigated. Gold nanoparticles with 5-ethyl-3-(8-thiooctyl)lumiflavinium perchlorate acts as an efficient catalyst for the aerobic oxidation of organic sulfides to the corresponding sulfoxides upon treatment with hydrazine at room temperature and under atmospheric pressure in oxygen. With a catalytic amount of gold nanoparticles with 3-(8-thiooctyl)lumiflavin, diimide reduction of various olefins can be performed with hydrazine at room temperature under atmospheric pressure in air with greater yields of product alkanes than with non-supported 3-methyllumiflavin catalyst under the same conditions. Kinetic studies revealed that the mono-layer-protected gold cluster-catalyzed reactions proceeded faster than those with non-supported catalysts over the full substrate concentration range for the hydrogenation of olefins and at lower substrate concentrations for sulfoxidation. This positive effect was rationalized by assuming a Michaelis-Menten-type mechanism in which the specific inclusion of substrates into the enzyme-like reaction cavities was a key factor in the high efficiency of the supported flavin catalysts.

Full text of DOI:10.1002/cctc.201402619

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DOI: 10.1002/cctc.201402619
Flavin-Functionalized Gold Nanoparticles as an Efficient
Catalyst for Aerobic Organic Transformations
Yasushi Imada, Motonari Ohsaki, Mikiko Noguchi, Takatoshi Maeda, Misa Fujiki,
Soichiro Kawamorita, Naruyoshi Komiya, and Takeshi Naota*[a]
Monolayer-protected gold clusters functionalized with synthet-
ic flavins were synthesized and their catalytic activity in aerobic
organic transformations investigated. Gold nanoparticles with
5-ethyl-3-(8-thiooctyl)lumiflavinium perchlorate acts as an effi-
cient catalyst for the aerobic oxidation of organic sulfides to
the corresponding sulfoxides upon treatment with hydrazine
at room temperature and under atmospheric pressure in
oxygen. With a catalytic amount of gold nanoparticles with 3-
(8-thiooctyl)lumiflavin, diimide reduction of various olefins can
be performed with hydrazine at room temperature under at-
mospheric pressure in air with greater yields of product alka-
nes than with non-supported 3-methyllumiflavin catalyst under
the same conditions. Kinetic studies revealed that the mono-
layer-protected gold cluster-catalyzed reactions proceeded
faster than those with non-supported catalysts over the full
substrate concentration range for the hydrogenation of olefins
and at lower substrate concentrations for sulfoxidation. This
positive effect was rationalized by assuming a Michaelis–
Menten-type mechanism in which the specific inclusion of sub-
strates into the enzyme-like reaction cavities was a key factor
in the high efficiency of the supported flavin catalysts.
Introduction
Simulation of the enzymatic functions of flavin-containing oxi-
dases[1] and oxygenases[2] with synthetic flavin catalysts has
been studied to provide a new synthetic methodology for vari-
ous catalytic oxidative transformations.[3] A typical catalytic
cycle for flavin-catalyzed dehydrogenative and oxygenative
transformation is shown in Scheme 1. The mechanisms of
flavin-containing oxidases and their synthetic flavin simulations
involves dehydrogenation of the substrate (AH2) with oxidized
flavin (Flox), incorporation of molecular oxygen to form hydro-
peroxyflavin (FlOOH),[4] and subsequent elimination of hydro-
gen peroxide (Scheme 1, path a). This redox process allows
a variety of aerobic dehydrogenative transformations for alco-
hols,[5] amines,[6] and thiols[7] with[5a,b,6a] or without[5c,6b,7] visible-
light irradiation. The simulation of oxygenases provides an al-
ternative strategy for catalytic oxidative transformation, where
the reaction proceeds via a similar redox process that includes
the reduction of Flox with AH2, the incorporation of oxygen,
and oxygen transfer to a substrate (S) to regenerate Flox
Scheme 1. Redox process for the catalytic dehydrogenative (a) and oxygena-
tive (b) transformation with flavins and molecular oxygen.
(Scheme 1, path b). Various heteroatom compounds, such as
amines,[8a,b] sulfides,[7c,8] cyclic ketones,[9] and arylboronic
acids,[10] undergo aerobic oxygenation to afford the corre-
sponding amine N-oxides, sulfoxides, ketones, and phenols by
this strategy under mild reaction conditions. Simulations of
oxygenases have also been performed with H2O2 as an oxidant
based on the direct oxidation of Flox to FlOOH.[11] These studies
have indicated that synthetic flavins act as efficient catalysts
for the generation of a powerful reducing agent, diimide NH=
NH,[12] from hydrazine through the dehydrogenation and oxy-
genation processes. The principle has been applied to the de-
velopment of a highly green method for the aerobic reduction
of olefins, in which the catalytic hydrogenation of various ole-
fins can be performed under atmospheric pressure in air to
produce water and nitrogen gas as the sole waste products.[13]
[a] Prof. Dr. Y. Imada,+ M. Ohsaki, M. Noguchi, T. Maeda, M. Fujiki,
Dr. S. Kawamorita, Dr. N. Komiya, Prof. Dr. T. Naota
Department of Chemistry
Graduate School of Engineering Science
Osaka University
Toyonaka, Osaka 560-8531 (Japan)
E-mail: naota@chem.es.osaka-u.ac.jp
[+] Present address
Department of Chemical Science and Technology
Institute of Technology and Science
The University of Tokushima
Minamijosanjima, Tokushima 770-8506 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402619.
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As part of our program aimed at new types of catalysts
bearing enzyme-like functions, we are investigating the forma-
tion of novel synthetic flavin catalysts with artificial reaction
cavities for high efficiency and substrate specificity. Association
complexes of neutral flavins with 2,6-bis(dibenzyloxyamino)-
pyridines bearing 3,5-dialkoxy-[13c] and 3,4,5-trialkoxybenzyl
ether[14] dendron units have proven to act as efficient supra-
molecular organocatalysts for the highly substrate-specific
aerobic reduction of olefins. In an attempt to develop a new
enzyme-like catalyst, monolayer-protected gold clusters
(MPC)[15] have been selected as a new platform for next-gener-
ation synthetic flavin catalysts owing to their high potential for
the design and construction of artificial reaction cavities on the
nanoparticle surfaces.
and mechanism for the aerobic transformation of sulfides and
olefins.
catalyst 2
R1SR2
R1SOR2
ð1Þ
ð2Þ
ƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!
O2 ð1 atmÞ, NH2NH2
catalyst 1
R1R2C¼CHR3
R1R2CHCH2R3
ƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!
air ð1 atmÞ, NH2NH2
Gold nanoparticles with 5-ethyl-3-(8-thiooctyl)lumiflavinium
perchlorate (1) and with 3-(8-thiooctyl)lumiflavin (2), the surfa-
ces of which are stabilized with PPh3 and linked to neutral and
cationic lumiflavins through thiooctamethylene spacers, have
been synthesized and characterized by using spectroscopic
and microscopic techniques. Cationic flavin-modified nanopar-
ticles 2 act as an efficient catalyst for the aerobic oxidation of
sulfides to sulfoxides upon treatment with hydrazine at room
temperature under atmospheric pressure in oxygen [Eq. (1)].
By using a catalytic amount of neutral flavin-bearing gold
nanoparticles 1, the diimide reduction of various olefins can be
performed with hydrazine at room temperature under atmos-
pheric pressure in air with much greater yields of product alka-
nes than with non-supported 3-methyllumiflavin (3) as catalyst
under the same conditions [Eq. (2)]. These nanoparticle cata-
lysts have advantages over non-supported 3-methyllumiflavins
3 and 5-ethyl-3-methyllumiflavinium perchlorate (4) owing to
their higher catalytic activity and reusability. The efficiency of
these catalysts has been rationalized by assuming a mechanism
based on the Michaelis–Menten-type equilibrium for the inclu-
sion of the substrates in the artificial cavities generated around
the flavins and the particle surfaces. Herein, we provide details
on the synthesis and characterization of gold nanoparticles
with flavin functionalities, with a focus on the catalytic activity
Results and Discussion
Synthesis and characterization of gold nanoparticles func-
tionalized with neutral and cationic flavins
Gold nanoparticles functionalized with neutral flavin 1 were
prepared by the place-exchange reaction[16] of PPh3-stabilized
gold nanoparticles (7)[17] with bis[8-(3-lumiflavinyl)octyl] disul-
fide (5).[18] The product was purified by using gel permeation
chromatography with CH2Cl2 as an eluent. Cationic flavinium
analogue 2 was prepared by similar treatment with bis(5-ethyl-
lumiflavinium) diperchlorate (6), which was obtained by the re-
ductive ethylation of 5 (Scheme 2).
Gold nanoparticles 1 formed yellow homogeneous colloidal
solutions upon dissolution in halogenated solvents such as
CHCl3 and CH2Cl2, whereas cationic particles 2 were dissolved
exclusively in 2-trifluoroethanol or hexafluoroisopropanol to
afford a clear purple solution. Photographs of colloidal solu-
tions of 1, 2, and non-functionalized nanoparticles 7 are
shown in Figure 1. Functionalization on the gold surface with
flavins on 1 and 2 was confirmed by spectroscopic observa-
tion. UV/Vis spectra of solutions of 1, 2, and 7 exhibited broad
Scheme 2. Synthesis of flavin-functionalized gold nanoparticle 1 and 2.
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Figure 1. Colloidal solutions of a) 1 in CHCl3, b) 2 in CF3CH2OH, and c) 7 in
CF3CO2H.
Figure 3. TEM images and corresponding size distributions of a,c) 1 and
b,d) 2. Samples were prepared from colloidal solutions of 1 in CHCl3 and 2
in CF3CH2OH.
The size and dispersity of nanoparticles 1 and 2 in the colloi-
dal solutions were estimated by using TEM. The results indicat-
ed that the gold nanoparticles were well separated without
the formation of more highly ordered aggregates and had rea-
sonably narrow size distributions (Figure 3). The mean diame-
ters of the gold nanoparticles were estimated to be 1.6ꢁ
0.5 nm for 1 and 2.1ꢁ0.6 nm for 2. Elemental analysis indicat-
ed that the molar ratio of gold/PPh3/flavin was 22.1:2.1:1 for 2
and the flavin contents were determined to be 5.59ꢁ
10ꢀ4 molgꢀ1 for 1 and 1.84ꢁ10ꢀ4 molgꢀ1 for 2.
Figure 2. UV/Vis spectra of solutions of a) 1, 5, and 7 in CHCl3 (5.0ꢁ10ꢀ4 m)
and b) 2, 6, and 7 in CF3CH2OH (5.0ꢁ10ꢀ4 m).
Aerobic oxidation of sulfides
Cationic lumiflavin-functionalized gold nanoparticles 2 acted as
an efficient catalyst for the aerobic oxidation of sulfides under
mild conditions. When a mixture of methyl p-methyl phenyl
sulfide (8) and gold nanoparticle catalyst 2 (0.05 mol% flavin
content) in CF3CH2OH was allowed to react with NH2NH2·H2O
(1 equiv) at 358C for 24 h under atmospheric pressure in O2,
methyl p-methyl phenyl sulfoxide (9) was obtained in 98%
yield [Eq. (3)]. One important advantage of this catalyst over
the conventional non-supported catalyst 4[8] is the high reusa-
bility owing to its thermal and chemical stability. A typical recy-
cling of catalyst 2 for the aerobic oxidation of 8 is shown in
Table 1. After the reaction, the product sulfoxide 9 was ob-
tained quantitatively upon dilution with acetone and subse-
absorption bands characteristic of gold nanoclusters. No signif-
icant plasmon resonance was observed for any of the solutions
at l=520 nm, which indicated that the clusters were less than
2 nm in diameter (Figure 2).[19] Nanoparticles 1 exhibited clear
absorption peaks at l=350 and 450 nm, whereas 2 had ab-
sorption bands at l=410 and 550 nm, which were assigned to
the p–p* band of the lumiflavin moieties, respectively, as were
1
similar bands of 5 and 6. The H NMR spectrum of 1 in CDCl3
showed broad but well-assignable signals of lumiflavin and oc-
tamethylene linker moieties owing to the moderately de-
creased molecular mobility caused by fixation to the nanopar-
ticle surface (Figure S1, Supporting Information). In contrast,
complex 2 was NMR-silent owing to the radical character of
the cationic flavin moiety. The IR spectrum of 2 had intense C=
O and ClꢀO stretching bands characteristic of cationic lumifla-
vin moieties at n˜ =1730 (C=O), 1670 (C=O), and 1110 cmꢀ1
(ClꢀO).
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Table 1. Aerobic oxidation of 8 with gold nanoparticle catalyst 2.[a]
Table 2. Aerobic reduction with catalysts 1 and 3.[a]
Entry
Olefin
Product
Yield[b] [%]
1
3
1
2
92
91
62
82
Run
Yield[c] [%]
1[a]
2[b]
3[b]
4[b]
98
98
98
97
3
4
5
74
76
41
67
60
28
[a] Reaction conditions: 8 (0.50 m), catalyst 2 (1.0 mol%), NH2NH2·H2O
(5 equiv), CF3CH2OH solvent, T=358C, t=24 h, P=1 atm O2. [b] The cata-
lyst from the previous run was reused instead of a recharged catalyst.
[c] Yield of sulfoxide 9 was based on GLC analysis with diethylene glycol
diethyl ether as internal standard.
6
7
40
34
28
25
quent centrifugal separation of the catalyst. The recovered cat-
alyst 2 could be stored for a long time under an Ar atmos-
phere and reused repeatedly for subsequent runs without loss
of efficiency.
8
9
75
70
47
45
Aerobic reduction of olefins
10
76
38
Neutral flavin-modified nanoparticles 1 exhibited high catalytic
activity in the aerobic reduction of alkenes with NH2NH2 under
atmospheric pressure of O2. The catalytic activities of support-
ed and non-supported flavin catalysts were examined for the
aerobic reduction of styrene with NH2NH2·H2O (4 equiv) in
CHCl3 in air at 258C [Eq. (4)]. The reaction profiles for the re-
duction of styrene with flavin-functionalized nanoparticles 1,
non-supported lumiflavin 3, and a 1:1 mixture of 3 and PPh3-
stabilized gold nanoparticles 7 are shown in Figure S2. Yields
of ethylbenzene were determined periodically by gas–liquid
chromatography (GLC) analysis with an internal standard. Re-
duction with catalyst 1 (4 mol%) gave the product with high
efficiency in 92% yield after 24 h, whereas the yields with cata-
lyst 3 under the same conditions plateaued at 62% owing to
the consumption of hydrazine by the self-redox process de-
picted in Equations (5) and (6). Notably, non-functionalized par-
ticles 7 inhibited the reaction with catalyst 3 considerably,
therefore, the high catalytic activity of 1 was attributed to the
synergy between neutral flavin and PPh3-stabilized
nanoparticles.
11
12
84
85
50
77
13
14
15
86
89
79
66
57
49
[a] Reaction conditions: olefin (6.25ꢁ10ꢀ2 m), NH2NH2·H2O (4.0 equiv),
flavin catalyst 1 or 3 (4.0 mol%), CHCl3 solvent, T=258C, 24 h, in air.
[b] Based on GLC.
Representative results of the aerobic reduction of olefins
with flavin catalysts 1 and 3 in CHCl3 are summarized in
Table 2. Various aromatic and aliphatic olefins were converted
into the corresponding hydrogenated products with high effi-
ciency if the reactions were conducted with NH2NH2·H2O
(4.0 equiv) in the presence of catalyst 1 (4 mol%) at 258C for
24 h in air. The catalytic activity of nanoparticles 1 was higher
than that of the non-supported catalyst 3 in all cases. Ester
and hydroxyl moieties were tolerated by the reaction (en-
tries 14 and 15). Terminal olefins were selectively reduced in
the presence of trisubstituted olefins (entry 15). Although the
conversion of disubstituted olefins was low (entries 5–7), the
½Oꢂ
NH2NH2
HN¼NH
ð5Þ
ð6Þ
ƒƒ!
2 HN¼NH ! NH2NH2 þ N2
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products were obtained in high yields by the addition of sup-
plementary hydrazine (4 equiv) after the reactions terminated.
strate 8, then plateaued after a certain substrate concentration.
Comparison of the two contrasting reaction profiles in Fig-
ure 4a and b also indicated that nanoparticle catalyst 2 exhib-
ited slightly higher catalytic activity than 4 at [8]0 concentra-
tions below 2.0ꢁ10ꢀ1 m.
Kinetic studies and mechanistic rationale
To verify the Michaelis–Menten-type mechanism, the Mi-
Aerobic oxidation of sulfides
chaelis constant (Km) and maximum reaction velocity (Vmax
)
were estimated to be 2.1ꢁ10ꢀ1 m and 9.4ꢁ10ꢀ6 msꢀ1, respec-
tively, from a Hanes–Woolf plot (Figure 5).[20] The linear de-
pendence (R2 =0.997) of [8]0/kobs on [8]0 indicated that the in-
clusion complex 11, which consisted of FlOOH on nanoparticle
10 and substrate sulfide R1SR2, acted as an active
species in the rate-determining step of the catalytic
process. Therefore, the observed positive and negligi-
ble effects of gold nanoparticle catalyst 2 at lower
and higher concentrations of 8 were attributable to
the acceleration and deceleration of the rate-deter-
mining oxygen-transfer step, respectively. Thus, we
could be fairly certain from these kinetic studies that
the association complex 11 induced sufficient catalyt-
ic activity at lower concentrations owing to the inclu-
sion of sulfide molecules inside the cavities on the
nanoparticle surfaces through p-stacking and hydro-
phobic interactions, whereas the catalytic activity was
degraded at higher sulfide concentrations by satura-
To elucidate the mechanism of the MPC-catalyzed aerobic oxi-
dation, the concentration-dependency of the initial rate for the
aerobic oxidation of 8 at 298 K under atmospheric pressure in
O2 was examined by GLC analysis (Figure 4). The observed ini-
Figure 4. Concentration dependence of [8]0 in the aerobic oxidation of 8 with hydrazine
and catalyst a) 2 or b) 4 in CF3CH2OH. Conditions: [8]0 =1.0ꢁ10ꢀ1–5.0ꢁ10ꢀ1 m,
[NH2NH2·H2O]0 =5.0ꢁ10ꢀ1 m, [flavin]0 =7.0ꢁ10ꢀ4 m, P=1 atm O2, T=298 K.
tion of the effective sulfide concentration around the
cavities.
Aerobic reduction of olefins
tial rate constants kobs for the formation of 9 (d[9]/dt=kobst) by
the aerobic oxidation of 8 (1.0–5.0ꢁ10ꢀ1 m) with NH2NH2·H2O
(5.0ꢁ10ꢀ1 m) and flavin catalysts (7.0ꢁ10ꢀ4 m) were determined
to be 2.9ꢁ10ꢀ6 msꢀ1 ([8]0 =1.0ꢁ10ꢀ1 m), 4.8ꢁ10ꢀ6 msꢀ1 (2.0ꢁ
10ꢀ1 m), 5.4ꢁ10ꢀ6 msꢀ1 (3.0ꢁ10ꢀ1 m), and 6.6ꢁ10ꢀ6 msꢀ1 (5.0ꢁ
10ꢀ1 m) for catalyst 2, and 2.6ꢁ10ꢀ6 msꢀ1 ([8]0 =1.0ꢁ10ꢀ1 m),
4.5ꢁ10ꢀ6 msꢀ1 (2.0ꢁ10ꢀ1 m), 5.7ꢁ10ꢀ6 msꢀ1 (3.0ꢁ10ꢀ1 m), and
7.8ꢁ10ꢀ6 msꢀ1 (5.0ꢁ10ꢀ1 m) for catalyst 4. A high concentra-
tion dependence of [8]0 with catalyst 4 was observed over the
full range of concentrations examined (Figure 4b). This indicat-
ed that oxygen transfer from FlOOH to the sulfide (Scheme 1)
was the rate-determining step for the overall catalytic process
in the reaction with 4. In con-
To elucidate the mechanism for the MPC-catalyzed aerobic re-
duction of olefins, the concentration dependence on the initial
reaction rates with flavin catalysts 1 and 3 was examined for
the hydrogenation of styrene with hydrazine in CHCl3 at 298 K
in air. The changes in the initial rates of styrene hydrogenation
for various hydrazine and styrene concentrations are shown in
Figure 6. Reactions with MPC catalyst 1 generally proceeded
much faster than those with non-supported catalyst 3, which
indicated the higher catalytic activity of 1 over the full range
of concentrations (Figure 6a–d). The clear dependence on
[NH2NH2·H2O]0 and the lack of dependence on [styrene]0 in the
trast, similar treatment with cata-
lyst 2 showed strong depend-
ence at lower concentrations
([8]0 =1.0–2.0ꢁ10ꢀ1 m) and very
weak dependence at higher con-
centrations
(3.0–5.0ꢁ10ꢀ1 m)
(Figure 4a). This rate stagnation,
observed exclusively in the
range of high concentrations of
8, was attributed to a Michaelis–
Menten-type mechanism in the
rate-determining
step
(Scheme 3a), in which the rate
of oxygen transfer increased
Scheme 3. Michaelis–Menten-type mechanism for the proposed rate-determining steps of a) aerobic oxidation of
with the concentration of sub- sulfides and b) aerobic reduction of olefins with hydrazine.
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tions lower than 1.0ꢁ10ꢀ1 m (Figure 6c). These profiles indicat-
ed that diimide reduction with styrene also contributed to
rate-determination in the reaction with catalyst 1 over a partic-
ular range of [styrene]0 concentrations, owing probably to the
significant acceleration of diimide formation with hydrazine.
Thus, the higher catalytic activity of catalyst 1 was ascribed to
Michaelis–Menten-type mechanism (Scheme 3b), in which
equilibrated incorporation of hydrazine into the cavities of 13
acted as a key step in reaction acceleration. The resulting in-
crease in the effective concentration of hydrazine upon its in-
clusion into the cavities led to an acceleration of the nucleo-
philic attack by incorporated hydrazine on the oxidized form
of the neutral flavin (14), which made the step much faster
than the nucleophilic addition of liberated hydrazine to non-
supported 3.
Figure 5. Hanes–Woolf plot of [8]0/kobs versus [8]0 for gold nanoparticle 2-
catalyzed aerobic oxidation of 8 with NH2NH2 in CF3CH2OH. Conditions:
[8]0 =1.0ꢁ10ꢀ1–5.0ꢁ10ꢀ1 m, [NH2NH2·H2O]0 =5.0ꢁ10ꢀ1 m, [flavin]0 in
2=7.0ꢁ10ꢀ4 m, P=1 atm O2, T=298 K.
A significant change in the concentration dependence of
[styrene]0 was observed in the reaction catalyzed by 1, in
which the rate increased steeply at lower concentrations of
reactions with catalyst 3 (Figure 6b and d) indicated that nu-
cleophilic addition of hydrazine to the oxidized form of the
neutral flavin was the rate-determining step for the overall cat-
alytic process, as described in our previous report.[13c] In the re-
actions with catalyst 1, a strong concentration dependency on
[NH2NH2·H2O]0 was observed over the full range of concentra-
tions (Figure 6a), with dependency on [styrene]0 at concentra-
styrene but reached a limit at concentrations over 1.0ꢁ10ꢀ1
m
(Figure 6c). This rate stagnation could also be explained
through the Michaelis–Menten mechanism (Scheme 3b). At
higher styrene concentrations, the inclusion of styrene into the
cavities occurred to a greater extent by p-stacking interactions
between the aromatic moieties of coordinated PPh3 and sty-
rene, which interfered with the inclusion of hydrazine
and subsequent diimide formation. This process
would extinguish the contribution of the diimide re-
duction step to the rate-determination of the overall
catalytic process.
Conclusions
Gold nanoparticles functionalized by neutral and cat-
ionic lumiflavins (1 and 2, respectively) act as highly
efficient catalysts for the aerobic oxidation of sulfides
and the aerobic reduction of olefins at room temper-
ature under atmospheric pressure in oxygen or air.
The nanoparticle catalysts have advantages over
non-supported lumiflavin catalysts in terms of their
activities and reusability. Kinetic studies based on the
Michaelis–Menten equation reveal that the equilibrat-
ed formation of the association complexes in specific
cavities generated at particle surfaces with the long-
carbon-chain-linked flavin is a key factor in enhancing
catalytic activities.
Experimental Section
General
NMR spectra were obtained on a Jeol JNM-GSX270 and
a Varian Unity-Inova 500 spectrometer. IR spectra were
obtained on a Bruker Equinox 55/S spectrometer. Mass
spectra were obtained on a Jeol JMS-DX303 mass spec-
Figure 6. Concentration dependence of a,b) [NH2NH2·H2O]0 and c,d) [styrene]0 in the
aerobic reduction of styrene with hydrazine and catalysts a,c) 1 or b,d) 3 in CHCl3 at
298 K. Conditions: a) [styrene]0 =6.3ꢁ10ꢀ2 m, [NH2NH2·H2O]0 =1.0–2.5ꢁ10ꢀ1 m, [flavin]0 in
1=2.5ꢁ10ꢀ3 m, air (1 atm); b) [styrene]0 =6.3ꢁ10ꢀ2 m, [NH2NH2·H2O]0 =1.0–2.5ꢁ10ꢀ1 m,
[3]0 =2.5ꢁ10ꢀ3 m, air (1 atm); c) [styrene]0 =4.0ꢁ10ꢀ2–2.5ꢁ10ꢀ1 m, [NH2NH2·H2O]0 =
2.5ꢁ10ꢀ1 m, [flavin]0 in 1=2.5ꢁ10ꢀ3 m, air (1 atm); d) [styrene]0 =4.0ꢁ10ꢀ2–2.5ꢁ10ꢀ1 m,
[NH2NH2·H2O]0 =2.5ꢁ10ꢀ1 m, [3]0 =2.5ꢁ10ꢀ3 m, air (1 atm).
trometer. Elemental analyses were performed on
a
PerkinElmer Series II CHNS/O analyzer 2400. UV/Vis spec-
tra were obtained on a Shimadzu Multispec1500 spec-
trometer. Gas chromatography analyses were conducted
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on a Shimadzu GC-18 A chromatograph by using a DB-1 glass ca-
pillary column (0.25 mmꢁ30 m) with injection and column temper-
atures of 300 and 100–2508C, respectively. 3,[8b] 4,[8b] 5,[18] and 7[17]
were prepared according to literature procedures. Spectroscopic
data for hydrogenated products, the 1H NMR spectrum of 1, and
a TEM image and size distribution of 7 are shown in the Support-
ing Information.
tion mixture stirred at RT for 6 h. After evaporation of the solvent,
the resulting black residue was washed successively with CH2Cl2
and CH3CN to give 2 (10 mg). IR (KBr): n˜ =1730 (C=O), 1670 (C=O),
1110 cmꢀ1 (ClꢀO); UV/Vis (CF3CH2OH): l=416 (e=1600), 530 nm
(e=1200); elemental analysis calcd (%) for [Au22.2(PPh3)2.1Fl]x:
C 13.4, H 1.17, N 1.03; found C 13.39, H 1.32, N 1.03.
Aerobic oxidation of methyl p-methyl phenyl sulfide (8)
Bis[8-(3-lumiflavinyl)octyl] disulfide (5)
with catalyst 2
Yellow powder; m.p. 193–2008C; 1H NMR (270 MHz, CDCl3): d=
1.25–1.43 (m, 16H, CH2), 1.62–1.76 (m, 8H, CH2), 2.45 (s, 6H, C7CH3),
2.55 (s, 6H, C8CH3), 2.65 (t, J=7.4 Hz, 4H, SCH2), 4.10 (t, J=7.4 Hz,
4H, NCH2), 4.11 (s, 6H, NCH3), 7.41 (s, 2H, C9H), 8.04 ppm (s, 2H,
C6H); 13C NMR (68 MHz, CDCl3): d=19.6, 21.6, 26.9, 27.8, 28.6, 29.2,
29.3, 32.0, 39.2, 42.1, 115.1, 131.6, 132.6, 134.5, 134.8, 136.5, 147.5,
148.9, 155.5, 159.6 ppm; IR (KBr): n˜ =2926, 2853, 1709, 1652, 1587,
1553, 1461, 1337, 1268, 1194, 1017, 804, 769, 732 cmꢀ1; high-
resolution MS (fast atom bombardment): calcd for C42H55N8O4S2
[M+H+] 799.3788, found 799.3821.
A
solution of 8 (4.5 mL, 0.033 mmol), NH2NH2·H2O (1.7 mL,
0.033 mmol), and 2 (0.1 mg, 0.018 mmol) in trifluoroethanol (67 mL)
was stirred at 358C for 24 h under atmospheric pressure in O2. The
yield of methyl p-methyl sulfoxide (9) was determined to be 98%
by GLC analysis with diethyleneglycol diethyl ether as internal stan-
dard. Sulfoxide 9 was isolated and characterized by spectroscopic
methods. Colorless solid; m.p. 52–548C (Ref. [21]: 50–548C). IR
(KBr): n˜ =3041, 2994, 2919, 1654, 1597, 1495, 1448, 1410, 1302,
1179, 1147, 1090, 1039, 1017, 956, 810, 687 cmꢀ1; H NMR (CDCl3,
1
500 MHz): d=2.42 (s, 3H), 2.71 (s, 3H), 7.33 (dm, J=8.5 Hz, 2H),
7.54 ppm (dm, J=8.5 Hz, 2H); 13C NMR (CDCl3, 125 MHz): d=21.3,
44.0, 123.5, 130.0, 141.5, 142.6 ppm; high-resolution MS (electron
ionization): calcd for C8H10SO [M+] 154.0452, found 154.0439.
Preparation of bis(5-ethyllumiflavinium) diperchlorate (6)
A mixture of 5 (0.200 g, 0.250 mmol), Na2S2O4 (0.348 g, 2.00 mmol),
NaBH3CN (0.314 g, 5.00 mmol), CH3CHO (2.20 g, 49.9 mmol), and
CH3CO2H (150 mL) in CHONMe2 (12 mL) was stirred for 2 h at 608C
under Ar. After cooling to RT, the reaction was quenched by the
addition of Na2S2O4 (50 mL) in aqueous solution and the mixture
extracted with CH2Cl2 (50 mL). The organic layer was washed with
brine (50 mL) and the solvent removed under reduced pressure.
An aqueous solution mixture of 2m HClO4 (5.4 mL), NaNO2
(0.301 g, 4.36 mmol), NaClO4 (0.680 g, 5.55 mmol), and CHCl3
(1.0 mL) was added to the resulting brown residue, and the mix-
ture stirred for 5 min at RT. The resulting precipitate was collected
by filtration and washed successively with cool H2O (5 mL) and di-
ethyl ether (5 mL). The crude product was purified by reprecipita-
tion with CH3CN and diethyl ether to give 6 (0.130 g, 49%) as
a violet powder. IR (KBr): n˜ =2928, 2854, 1731, 1661, 1557, 1455,
1361, 1130, 757, 627 cmꢀ1; high-resolution MS (fast atom bombard-
ment): calcd for C46H65N8O4S2 [M+H+ꢀ2ClO4ꢀ] 857.4570, found
857.4574.
Aerobic reduction of olefins with catalyst 1
General procedure: A mixture of olefin (0.025 mmol), NH2NH2·H2O
(5.0 mL, 0.10 mmol), and 1 (1.80 mg, 1.0 mmol flavin) in CHCl3
(0.4 mL) was stirred at 258C for 24 h in air. The yields of hydrogen-
ated products were determined by GLC analysis with an internal
standard. The results are shown in Table 2. Products were isolated
and characterized by spectroscopic methods, as shown in the
Supporting Information.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan.
Keywords: flavins · gold · nanoparticles · oxidation · reduction
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Received: August 7, 2014
Published online on && &&, 0000
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FULL PAPERS
Y. Imada, M. Ohsaki, M. Noguchi,
T. Maeda, M. Fujiki, S. Kawamorita,
N. Komiya, T. Naota*
&& – &&
Flavin-Functionalized Gold
Nanoparticles as an Efficient Catalyst
for Aerobic Organic Transformations
A gold in aerobics: Gold nanoparticles
functionalized by neutral and cationic
lumiflavins act as highly efficient cata-
lysts for the aerobic oxidation of sulfides
and the aerobic reduction of olefins at
room temperature under atmospheric
pressure in oxygen or air. The nanoparti-
cle catalysts have advantages over
non-supported lumiflavin catalysts in
terms of their activities and reusability.
MPC=Monolayer-protected gold clus-
ters. R1, R2, R3 =aryl and alkyl groups.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 9
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9
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These are not the final page numbers!