Graphene oxide and rose bengal: Oxidative C-H functionalisation of tertiary amines using visible light

Article abstract of DOI:10.1039/c1gc15865a

Visible light induced oxidative C-H functionalisation of tertiary amines catalysed by the combination of graphene oxide and Rose Bengal was developed. This reaction avoids the use of stoichiometric amounts of peroxy compounds as terminal oxidants. This re

Full text of DOI:10.1039/c1gc15865a

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Graphene oxide and Rose Bengal: oxidative C–H functionalisation of
tertiary amines using visible light†
Yuanhang Pan,a Shuai Wang,c Choon Wee Kee,a Emilie Dubuisson,a Yuanyong Yang,a Kian Ping Loha and
Choon-Hong Tan*a,b
Received 18th July 2011, Accepted 20th September 2011
DOI: 10.1039/c1gc15865a
Visible light induced oxidative C–H functionalisation of
tertiary amines catalysed by the combination of graphene
oxide and Rose Bengal was developed. This reaction avoids
the use of stoichiometric amounts of peroxy compounds
as terminal oxidants. This reaction is useful for tri-alkyl
amines including chiral tertiary amines. Both cyanide and
trifluoromethyl nucleophiles were shown to participate in
this reaction, providing a-cyano- and a-trifluoromethylated
Fig. 1 Ruthenium bipyridyl complex and Rose Bengal.
tertiary amines.
used in dye-sensitized solar cells, are considered to be cheaper
and easier to modify relative to metal-based photosensitizers,9
thus they are attractive alternatives as photoredox catalysts.
The viability of organic dyes as photoredox catalysts has been
demonstrated by several groups.10 For example, Zeitler et al.
reported eosin Y catalysed dehalogenation and enantioselective
a-alkylation using a combination of photoredox catalyst and
organocatalyst.10a Fukuzumi reported a selective aerobic bromi-
nation catalysed by 9-mesityl-10-methylacridinium perchlorate
(Acr+–Mes).10b We have also demonstrated that Rose Bengal
(RB, Fig. 1) was able to photocatalyse a-oxyamination of
1,3-dicarbonyl compounds and 2,2,6,6-tetramethylpiperidine-1-
oxyl (TEMPO) using visible light.10c
Although GO has been reported as a photocatalyst for
hydrogen production from water under UV irradiation,11 the
potential application of GO in synthetic photochemistry has
not been explored. Therefore we wish to report the combination
of GO and RB which works in synergy to efficiently catalyse
the a-functionalisation of tertiary amines in the presence of
visible light. The oxidative a-functionalisation of tertiary amines
through highly reactive iminium intermediates has been reported
with transition metals as catalysts and peroxo-compounds
as terminal oxidants.12 Recently, the groups of Stephenson,7c
Rueping13 and Ko¨nig14 shown that this type reaction can be
photocatalysed with Ru(bpy)3Cl2 or Eosin Y using various
nucleophiles. We embark on our investigation with a-cyanation
of tertiary amines (Table 1),15 an important reaction which can
lead to a mild synthetic approach towards amino acids and
provide an alternative method to the Strecker reaction.‡
Graphene oxide (GO), a two-dimensional carbon sheet, is
traditionally used as a precursor to prepare graphene. Its
unique physical and chemical properties have attracted the
attention of chemists due to potential applications in plastic
electronics, optical materials, solar cells and biosensors,1 but
its potential as a catalyst in organic transformation remains
relatively unexplored.2,3 The feasibility and potential of GO as
catalyst were demonstrated by the seminal work of Bielawski
and co-workers on the use of GO for the oxidation of al-
cohols and hydration of alkynes.3a Subsequently, RGO was
reported to catalyse the hydrogenation of nitrobenzene at
room temperature.3b The use of GO and reduced graphene
oxide (RGO) as “carbocatalyst” in organic transformations is
a nascent area and should lead to exciting discoveries.
On a different note, the use of visible light in organic synthesis
has attracted the attention of various synthetic organic chemists
recently.4 The groups of MacMillan,5 Yoon,6 and Stephenson7
have showed the ability of metal-based photosensitizers, such
as Ru(bpy)3Cl2 (tris(2,2¢-bipyridine)-ruthenium(II) chloride)
(Fig. 1), as photoredox catalysts for organic transformations
under visible light irradiation.8 Organic dyes, which are often
aDepartment of Chemistry, National University of Singapore, 3 Science
Drive 3, Singapore, 117543. E-mail: chmtanch@nus.edu.sg; Fax: (+65)
6779 1691; Tel: (+65) 6516 2845
bKey Laboratory of Natural Medicine and Immuno-Engineering of
Henan Province, Henan University, Jinming Campus, Kaifeng, Henan,
475004, P.R. China
cSchool of Chemistry & Chemical Engineering, Huazhong University of
Science and Technology, Wuhan, Hebei, 430074, P.R. China
We chose green Light Emitting Diodes (LEDs) as the light
source for this study, as RB has a strong absorption wavelength
at around 549 nm; a-cyanation product 2a was observed with
† Electronic supplementary information (ESI) available: General proce-
dures for oxidative C–H functionalisation reactions, spectroscopic data.
See DOI: 10.1039/c1gc15865a
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Table 1 a-Cyanation of tertiary amine in the presence of RB and GOa
Table 2 a-Cyanation of a series of cyclic tertiary aminesa
Entry
Carbon catalyst
Yield (%)b
Initial rate
t (h) Yield (%)b (10-4 mmol min-1)c
Entry
1
2
1
2
3
4
none
60
97
65
56
96
94 (63)
10
0
94
50 wt% GOc
50 wt% graphite
50 wt% activated carbon
200 wt% GO
50 wt% GO
30
72
80
97 (60)
99 (45)
90 (63)
6.3 (3.2)
5.4 (4.1)
2.2 (0.4)
5
6d
7e
8f
9g
2
3
50 wt% GO
50 wt% GO
50 wt% GO
a Reaction was performed using 0.05 mmol of 1 and 0.125 mmol of
TMSCN in 0.5 ml of CH3CN. b Isolated yield. c GO was prepared
according to the reported method.16 d Ru(bpy)3Cl2 was used instead
of RB. Number in bracket is the yield without GO. e Reaction was
performed without RB. f Reaction was performed in dark. g Recycled
GO was used.
4
30
97 (62)
8.0 (4.2)
60% yield when N-aryl-tetrahydroisoquinoline 1a and TMSCN
were subjected to green LEDs irradiation using 5 mol% of RB
(Table 1, entry 1). Following the report of Bielawski and co-
workers,3a 200 wt% of GO was added to the reaction system
and full conversion with 96% yield was observed within the
same reaction time (Table 1, entry 5). To our delight, when the
amount of GO was reduced to 50 wt%, the yield was not affected
to any observable extent (Table, entry 2). Natural graphite and
activated carbon showed no effect on the reaction (Table 1,
entries 3 and 4). The addition of GO to Ru(bpy)3Cl2 catalysed
reaction also resulted in an increased in reaction rate and yield of
the reaction (Table 1, entry 6). Interestingly, GO can be recycled
without the loss of the activity (Table 1, entry 9). Both light
and photoredox catalyst were proved to be essential for this
reaction. No desired product was observed when the reaction
was conducted in the dark and only trace amount of product was
detected when the reaction was performed without RB (Table 1,
entries 7 and 8).
a Reaction was performed using 0.1 mmol of 1 and 0.25 mmol of
TMSCN in 1.0 ml of CH3CN b Isolated yield; number in bracket is
the yield without GO. c Initial rate was determined by 1H NMR analysis
during the first 45 min using pentachlorobenzene as internal standard.
to the iminium intermediate.17 However, when 4-methyl-N,N-
dimethylaniline was subjected to the reaction condition that
we have just developed, good yields were obtained (Scheme
2). The superiority of the combination of RB and GO was
obvious; not only was the yield improved, chemoselectivity was
also improved. While mono-cyanation product was observed as
the only product when GO was used, a mixture of mono- and
bis-cyanation products was observed in the absence of GO.
With the established conditions, we evaluated the performance
of different N-aryl-tetrahydroisoquinolines with TMSCN in the
presence of 5 mol% RB and 50 wt% GO (Table 2). Similar
observations were made; the presence of GO increase the
reaction rate and also improve the overall yield of the reactions.
Electron donating N-protecting groups allow the reaction to
be completed in a shorter time (Table 2, entries 1 and 4).
This is ascribed to the increased stabilization of the iminium
intermediate in these substrates. Moderate yield was achieved
when KCN was used, which improved significantly when GO
was added (Scheme 1).
Scheme 2 a-Cyanation of 4-methyl-N,N-dimethylaniline in presence
of RB and GO.
Tri-alkyl amines are more challenging substrates for a-CH
functionalisation reactions. We were pleased to find that a-
cyanation occurs with chiral amine 1f under the optimized
conditions to provide adduct 2g selectively (Scheme 3). We ob-
served that reaction rates increased when an oxygen atmosphere
was introduced to the reaction via a balloon, as compared to
just exposing the reaction to air. No bis-cyanation product
was observed. The a-cyanation of (S)-nicotine was intriguing
as it occurs selectively at the 5¢-position rather than the N-
methyl group to give adduct 2h. The stereoelectronic preference
of this reaction would require further investigation to be fully
understood.
Scheme 1 a-Cyanation between tertiary amines and KCN.
Linear amines usually exhibit reduced reactivity due to
the absence of stabilization provided by the benzylic position
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intermediate, which could be easily trapped by nucleophile to
release the final product under mild conditions. The nucleophiles
investigated in this report were cyanide and trifluoromethyl
anion.
Alternatively, RB is well known for the generation of singlet
oxygen in polar aprotic solvents by energy tranfer from the triplet
RB* to ground-state triplet oxygen (pathway b). The resulting
singlet oxygen is able to oxidize tertiary amines. This process is
supported by Ferroud’s report using the pre-synthesised singlet
oxygen source.21 However, 65% yield of 2a was obtained when
Fluorescin (a sensitizer that does not produce singlet oxygen)
was used instead of RB in the optimized conditions without GO
(see ESI†). On the other hand, TPP/DCM system, a well known
singlet oxygen generation system was also tested in the reaction
between N-aryl-tetrahydroisoquinolines 1a and TMSCN. 100%
conversion with only 19% yield of 2a was obtained after 30 h.22
Therefore, singlet oxygen is not crucial and thus reduction of
RB* may occur predominantly on triplet oxygen.23
Scheme 3 a-Cyanation of tri-alkyl amines in presence of RB and GO.
There is a strong demand for versatile fluorine-containing
building blocks.18 The preparation of trifluoromethylated com-
pounds is particularly important as this moiety is present in
many pharmaceuticals.19 Using 1a as the iminium precursor and
TMSCF3, trifluoromethylated N-aryl-tetrahydroisoquinoline
product 3a was obtained using RB/GO catalytic system (Scheme
4).20 A significant amount of a by-product, trifluoromethylated
N-aryl-dihydroisoquinoline was obtained even with GO.
The role of GO in this reaction is not clear at this stage.
Controlled experiment revealed that GO can be recycled without
the loss of its activity (Table 1, entry 9) and itself alone does
not catalyse this reaction (Table 1, entry 7). These observations
ruled out the possibility that GO showed significant oxidation
activity in these reactions. On the other hand, hydrophobic
graphite and activated carbon had no effect on the reaction
rate enhancement (Table 1, entries 3 and 4). The stabilization of
iminium intermediate by slightly acidic GO and the intrinsic high
surface area of GO may participate in the rate enhancement.
UV/Vis absorption and fluorescence quenching study (see ESI†)
revealed that there was no strong p–p interaction between RB
and GO, which was reasonable due to the electrostatic repulsion
between the RB anion and the negatively charged functional
groups of GO.
In summary, we have developed a cooperative catalyst system
based on an organic dye and GO, and applied them successfully
in photocatalysing oxidative C–H functionalisation of tertiary
amines. These reactions were found to proceed smoothly under
mild conditions and afford the desired products in good to
excellent yields. Adding of GO as co-catalyst typically resulted
in an increased in reaction rate and also the yield of products
obtained. Our methodology avoids the use of metal catalyst
and stoichiometric amount of peroxy-compounds as terminal
oxidant. Cheap and readily available organic dye is used and
air is used as the oxidant. To the best of our knowledge, this is
the first example of using GO to facilitate the synthesis of small
molecular organic compounds under visible light irradiation.
Detail mechanistic study and other applications of GO in
synthetic organic chemistry are ongoing in our laboratory.
Scheme 4 a-Trifluoromethylation between 1a and TMSCF3 in presence
of RB and GO.
Mechanistically, we proposed that the highly reactive iminium
intermediate was formed by the oxidation of the C–H bond
adjacent to the nitrogen (Fig. 2). RB accepts a photon from the
visible light source to populate the excited state RB* and remove
one electron from the nitrogen atom via a single electron transfer
(SET) process.7c,10a,13a The photoredox cycle is accomplished
by re-oxidizing the radical anion RB∑- to the ground state
RB by molecular oxygen (pathway a). The relatively acidic
tertiary amine radical cation is deprotonated by a strongly basic
∑
species, superoxide anion O2 -. The strongly reductive species
generated, the a-aminated carbon radical is further oxidized by
a second SET process resulting in the highly reactive iminium
Acknowledgements
This work was supported by ARF grants (R-143-000-461-112)
and a scholarship (to Pan, Y.) from the National University of
Singapore
Notes and references
‡ Representative procedure for a-cyanation between N-aryl-
tetrahydroisoquinoline 1a and TMSCN catalysed by the combination
of Rose Bengal and GO: 1a (23.9 mg, 0.1 mmol, 1.0 equiv.) and GO
Fig. 2 Proposed mechanism of a-functionalisation of tertiary amines
catalysed by Rose Bengal using visible light.
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(12.0 mg, 50 wt%) were added to a solution of RB (5.0 mg, 0.005 mmol,
5 mol%) in 1.0 ml CH3CN followed by adding TMSCN (31.3 ml,
0.25 mmol, 2.5 equiv.) slowly. The reaction mixture was stirred under
green LED irradiation at room temperature. After 30 hours, the solvent
was removed in vacuo and the crude product was directly loaded onto
a short silica gel column. Flash chromatography was performed using
gradient elution with hexane/EA mixtures (40/1–10/1 ratio). After
removing solvent, product 2a (25.5 mg) was obtained as pale yellow
solid in 97% yield.
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singlet oxygen will be very high. Its lifetime in acetonitrile is also not
high. The reaction may occur predominantly on electron-transfer
process rather than singlet oxygen process.
3344 | Green Chem., 2011, 13, 3341–3344
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The Royal Society of Chemistry 2011
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