Heterogeneous photocatalytic C-C coupling: Mechanism of plasmon-mediated reductive dimerization of benzyl bromides by supported gold nanoparticles

Article abstract of DOI:10.1039/c5cy00655d

The use of gold nanoparticles supported on TiO₂ (Au@TiO₂) as photocatalysts was extended to include photoinduced reductive C-C coupling. Surface plasmon excitation of supported AuNPs in the presence of an amine leads to the C-C coupling of a variety of substituted benzyl bromides at room temperature with good yields in a free radical-mediated reaction. The overall efficiency of the C-C coupling is largely dependent on the nature of the amine used.

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Heterogeneous Photocatalytic C-C Coupling:
Mechanism of Plasmon-mediated Reductive
Dimerization of Benzyl Bromides by Supported
Gold Nanoparticles
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Anabel E. Lanterna, Ayda Elhage, Juan C. Scaiano*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The use of gold nanoparticles supported on TiO (Au@TiO
2
2
)
light leads to NP surface electrons that can be exploited in several
ways, including photothermal and photochemical strategies.
Recently, the reduction of nitro compounds has been reported with
1
1, 14
as photocatalysts was extended to include photoinduced
reductive C-C coupling. Surface plasmon excitation of
supported AuNP in the presence of an amine leads to the C-C
coupling for a variety of substituted benzyl bromides at room
catalysts comprising AuNP on different semiconductor supports like
15
2 2 2
TiO , CeO and ZrO .
We report here on the ability of AuNP supported on TiO2 to
temperature with good yields in a free-radical mediated photocatalyze reductive C-C coupling reactions mediated by
reaction. The overall efficiency of the C-C coupling is largely free radicals. Several catalysts were used in the past to
photoassist C-C coupling of benzyl bromide and its derivatives,
through an electron-transfer mechanism using an amine as a
dependent on the nature of the amine used.
9, 16
sacrificial electron donor.
Here we assess supported AuNP
Reactions leading to C-C coupling are key to organic synthesis. The
majority of these reactions are not mediated by free radicals and for
example trans-metalation, has been the subject of many studies and
as a heterogeneous photocatalyst for reductive dehalogenation
reactions using the dimerization of the 4-nitrobenzyl bromide
1
, 2
(
Scheme 1) as a model reaction. We concentrate on mechanistic
has been extensively reviewed. While radical-radical reactions can
readily lead to C-C bond formation, their role in organic synthesis is
not as important as catalytic processes frequently involving
aspects and try to understand what conditions will favour C-C
coupling. We were pleased to observe that supported AuNP
successfully catalyze radical C-C coupling of 4-nitrobenzyl
bromide by plasmon mediated catalysis; further, the catalyst is
easy to separate and reuse (vide infra). It is in fact fascinating
that some systems lead to efficient radical recombination
reactions, even if the active radicals are at steady state
1
palladium centers. This is due to the fact that while the kinetics of
free radical self-reactions frequently approach diffusion control,
radicals under steady state conditions are present only in nanomolar
concentrations and thus, radical-molecule reactions tend to dominate
radical decay pathways, at least in those systems that are not
9, 17,18
3
, 4
nanomolar concentrations.
controlled by the persistent free radical effect (PRE),
although
5
examples controlled by PRE are also known.
NO
2
Organic halides are very important and versatile compounds with
many applications as starting materials, solvents, reagents, and
intermediates in synthetic organic chemistry. Conventional methods
for reducing alkyl, alkenyl and aryl halides bonds consist of metal–
AuNP@TiO
30 nm LED
2
Br
5
2
eq. DIPEA, CH
N2 atmosphere
2 2
Cl
2
O N
2
O N
2
halogen exchange,
a
hydride source or radical reductive
1
6
dehalogenation. Radical reactions are a powerful class of chemical
transformations. However, the formation of radical species to initiate
these reactions has often required the use of stoichiometric amounts
Schemeꢀ1.ꢀReductiveꢀdimerizationꢀofꢀ4-nitrobenzylꢀbromide.ꢀ
The reductive dimerization of 4-nitrobenzyl bromide is easily
obtained by green LED light irradiation of a catalyst suspension
7
of toxic reagents. In this sense, the use of visible-light-mediated
photoredox catalysis to generate radical species under milder
conditions has been extensively studied in the past decade.
Additionally, catalysis by supported gold nanoparticles in organic
in
CH2Cl2
under
nitrogen
atmosphere,
using
8, 9-11
diisopropylethylamine (DIPEA) as sacrificial electron donor.
The reaction approached completion after irradiation for 5 h,
showing the formation of the dimer as the main product and
minor yields of of p-nitrotoluene, the only by-product. Because
of the milder conditions used in this reaction, any traces of the
1
2
transformations has become popular in the last few years.
Reactions using photoinduced methods and supported AuNP are
promising due to the strong visible absorption of the AuNP surface
1
3
plasmon band (SPB), plus the robustness, easy separation and
potential recyclability associated with heterogeneous
19
amino product were found. As it was expected, in the
presence of oxygen, nitrobenzaldehyde is obtained almost
nanocomposites. Excitation of the plasmon band in AuNP by visible
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20
quantitatively. The reductive dehalogenation reaction was studied laboratory using the TiO2 recovered from the commercial
under the screening conditions summarized in Table 1, monitoring catalyst. See SI) shows no reaction product under the same
the conversion by HPLC. Solvent effects on the role of electron irradiation conditions (Table 2, entries xii and xiii). In addition,
2
donors were evaluated using commercial supported AuNP@TiO as it is well known that the injection of electrons from Au to TiO2
26
a photochemical catalyst under 532 nm LED irradiation. A survey of upon visible light irradiation can be easily achieved. In order
solvents revealed the reaction appears to work best in halogenated to determine the role of the TiO2, we also performed the
2 2 2 2
solvents such as CH Cl (or ClCH CH Cl, not shown). In addition, reaction using small AuNP (average size ca. 2.5 nm) supported
results strongly suggest that the reductive dimerization of 4- on an inert material (Fenton treated diamond nanoparticles -
nitrobenzyl bromide only takes place in the presence of an electron AuNP@HO-DNP- see SI). In this case, neither the support nor
donor (ED). In this sense, amines are the most suitable sacrificial the AuNP@HO-DNP showed photocatalytic activity under the
electron donors for this reaction (Table 1). Three different amines same irradiation conditions (Table 2, entries x and xi).
1
6
27
were used and as expected for the weaker electron donor, the Tanaka, and more recently Álvarez-Griera, suggested that the
secondary amine (diisopropylamine – DIPA) showed the lowest product 2 may be formed by the radical−radical dimerization of 1, or
conversion from reactant. Our group has recently demonstrated that alternatively by a second reduction of 1 into the benzyl carbanion,
the substrate-reduction is shared by the transition metal which may react with benzyl bromide via a nucleophilic S
N
2
photocatalysts and the α-aminoalkyl radicals in photoredox catalysis mechanism to give 2. In order to better understand the mechanism of
2
1
of a reductive cyclization reaction. We can extend this concept for the reaction, the same experiment was performed in the presence of
the C-C coupling reaction studied here. It is well known that (2,2,6,6-tetramethyl-piperidin-1-yl)-oxyl (TEMPO) (Table 2, entries
2
8, 29
deprotonation of an amine-derived radical cation leads to an α- viii and ix). This very well known radical-trapping agent
clearly
2
2
23
aminoalkyl radical, which is a strong reducing agent. For reduces the yield of the reaction. In fact, the trapping adduct
instance, the oxidation potential in ACN (against SCE) for the (Scheme 2) was detected by H NMR (see SI) proving the reaction
enough to reduce the 4- takes place under radical coupling rather than by a nucleophilic
nitrobencylbromide (-0.86 V). However, diisopropylethylamine substitution. These adducts are well known as they have been widely
1
2
4
radical from Et
3
N
is -1.12 V,
2
0
2
9, 30
(
DIPEA) exhibited a significant increase in yield for almost the same used as polymerization initiators.
conversion than triethylamine (TEA), and it was selected for our
standard reduction protocol.
Table 2. Yields (%) of 2 obtained by the reductive dimerization of 1 under
different conditions using 2 eq. of DIPEA as electron donor in CH Cl
2
2
.
Table 1. Yields (%) of 2 obtained by the reductive dimerization of 1 under
#
i
Time (h)
5
Catalyst
Filter
% Yield
83
different conditions using supported AuNP@TiO
nm LED irradiation.
2
as photocatalyst and 532
AuNP@TiO2
--
a
ii
22
AuNP@TiO
--
2
2
Dark
ND
25
#
ED
Solvent
Time (h)
% Yield
iii
iv
v
5
--
i
ii
none
i-PrOH
Glycerol
DIPEA
DIPEA
DIPEA
TEA
CH
i-PrOH
MeOH/CH
Cyclohexane
CH CN
2
Cl
2
22
24
24
5
ND
ND
ND
4
b
5
TiO
2
--
21
5
5
AuNP@TiO
--
500 nm
69
*
iii
iv
v
2 2
Cl
vi
vii
viii
ix
x
500 nm
ND
ND
NQ
ND
10
b
5
TiO
2
500 nm
3
5
16
c
5
AuNP@TiO
--
2
--
--
--
--
--
vi
vii
viii
CH
CH
CH
2
2
2
Cl
Cl
Cl
2
2
2
5
83
c
5
5
20
5
AuNP@HO-DNP
HO-DNP
DIPA
5
20
xi
xii
xiii
5
26
d
*
5
AuNP@TiO
AuNP@TiO
2
33
2 2
ND= values under Limit of Detection. MeOH/CH Cl = 1:2
d
5
2
500 nm
ND
a
b
NQ = values under the limit of quantitation. Heating at 39 °C. TiO
etching of AuNP on the commercial Au@TiO
of TEMPO. Homemade.
2
obtained by the
Once the best conditions for the reaction were found, the photo-
catalytic activity of supported AuNP was tested. Under dark
conditions and heating up the solvent boiling point, no traces of
product were found (Table 2, entry ii). This indicates light is
required to drive the reaction. In order to determine whether the
plasmon excitation of AuNPs was involved in the photo-catalytic
cycle, blank experiments were carried out in the presence and in the
c
2
used (See SI). Addition of 2 equivalents
d
CH2
NO2
fast
N
O
N O
O N
2
2
absence of TiO used as a catalyst. Both reactions showed noticeable
3
4
5
formation of 2 (Table 2, entries iii and iv). However, when a long
pass cut-off filter (500 nm) is used no photochemical reactions take
place without the AuNP (Table 2, entry vii), while good yields are
Scheme 2. Reaction between TEMPO (3) and the p-nitrobenzyl radical (4) gives
the trapping adduct (5).
2
obtained with AuNP@TiO (Table 2, entry v); this suggests that the
The C-C centered radical coupling is the less favored of the
conventional methods used for reducing alkyl, alkenyl and aryl
short wavelength light contamination present in LED sources (see
SI, Figure S5) was contributing to the reaction. These results show
that the reaction is taking place under plasmon-mediated
photocatalysis.
6
, 10, 31
halides bonds.
interact with the AuNP surfaces,
However, it is well known that free radicals can
3
2, 33
and in the present system, the
radical-radical dimerization could be favoured by the presence of
AuNP. A plausible mechanism (Scheme 3) involves the injection of
Further, it has been shown that the AuNP size can affect its
catalytic activity. In our case, the use of AuNP@TiO2 catalyst
with a mean diameter particle ca. 6 nm (synthesized in our
25
2
the photoexcited electrons at the AuNP into the TiO conduction
3
4
band (CB), and the injected electrons can be transferred to the alkyl
2
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halide. This is followed by halide anion release and formation of a
3
5
1
00
carbon-centered radical. In this mechanism, the AuNP is returned
to its original oxidation state by the sacrificial electron donor. As
mentioned above, the α-aminoalkyl radical formed by deprotonation
of the sacrificial electron donor can also act as a strong reducing
expt 1
expt 2
X
8
6
4
2
0
0
0
0
0
X = average
2
1, 23
X
agent
and generate benzyl radicals in the same process (Scheme
3
bottom). Interestingly in the case of nanodiamonds as support (see
Table 2), the material is unable to accept the electron from gold and
as a result AuNP do not improve the catalytic performance upon
plasmon excitation.
X
1
2
3
Times used
Figureꢀ1.ꢀPercentꢀreusabilityꢀforꢀtheꢀcatalystꢀstudiedꢀforꢀtheꢀdimerizationꢀofꢀ4-
nitrobenzylꢀbromideꢀunderꢀoptimizedꢀreactionꢀconditionsꢀforꢀtwoꢀindependentꢀ
experiments.ꢀTheꢀXꢀinꢀꢀgraphꢀshowsꢀaverageꢀvalues.ꢀ
Table 3. Reduction of different substituted benzyl bromides using four 530
nm LEDs working at 3A.
R1
R2
AuNP@TiO2
530 nm LED, 5h
Br
+
2
eq. DIPEA, CH2Cl2
N2 atmosphere
R1
R2
R1
R2
R2
R1
Ar-X
Ar-Ar
Ar-H
%
Ar-X
Conv
% Yield
of Ar-Ar
% Yield
of Ar-H
Ar-Ar/Ar-H
ratio
#
R
1
R
2
i
OCH
H
3
H
83
67
20
3.3
Schemeꢀ 3.ꢀ Top:ꢀ Proposedꢀ mechanismꢀ forꢀ theꢀ plasmon-mediatedꢀ reductiveꢀ
ii
iii
iv
H
H
H
29
28
97
80
74
ND
ND
ND
>>
>>
>>
ꢀ
dimerizationꢀ ofꢀ 4-nitro-benzylbromideꢀ byꢀ supportedꢀ AuNP.ꢀ R =ꢀ p-nitrophenyl.ꢀ
CH
Ph
3
Bottom:ꢀReductionꢀofꢀRCH Xꢀbyꢀα-aminoalkylꢀradicals.ꢀ
2
100
v
NO
NO
H
2
2
H
H
100
81
51
73
12
15
14
6.9
3.4
The scope of the reaction was studied with different substituted
benzyl bromide compounds all under the same conditions. As can be
a
vi
72
seen in table 3, electron-withdrawing substituents either para or vii
NO
2
100
5.3
1
a
ortho position, are prompted to react faster than electron-donor Quantification by H NMR using caffeine as an external standard. Working
substituents. Electron-donor or non-substituted benzyl bromide current: 1.6 A or about half the dose as in entry v. ND: values under Limit of
Detection.
derivatives frequently need longer irradiation times to aim complete
conversion (See table S1). In addition, using irradiation at lower
intensity leads to both less conversion and a reduced Ar-Ar/Ar-H
ratio (Table 3, entry vi), reflecting the light-intensity dependence on
ꢀ
Conclusions
the benzyl radical formation. In accordance with the PRE, the The use of supported AuNP catalysts was successfully extended to a
radical-molecule cross-reaction becomes more important when the C-C coupling reaction. To the best of our knowledge, this is the first
3
6
local radical concentration diminishes.
example where supported AuNP catalyst is used in this type of
demonstrated good catalytic
The supported AuNP used within this work were recovered and reactions. AuNP supported on TiO
2
reused in the dimerization of 4-nitrobenzyl bromide to test the activity on the reductive dimerization of benzyl bromides. We also
potential recyclability of the catalyst. Figure 1 presents the percent confirmed the C-C coupling occurs via radical coupling rather than
yield of the reaction after 5 h of LED irradiation over three turnover by nucleophilic substitution (S 2). It is in fact surprising to see
N
cycles. The catalyst is clearly reusable for this purpose, although radicals in very low concentrations (usually nanomolar) undergo
some variability is observed; this is not uncommon with self-reaction, rather than radical-molecule reactions. For this to occur
3
7
nanostructured catalysts.
The decrease on the yield of the reaction can be related to the
growth of the particles (from 2.5 nm to 4.1 nm, see SI) and is high resonance stabilization, as is the case for benzylic radicals.
several criteria must be met:
•
The radicals need to have low reactivity, which generally means
3
2
reasonable after the catalyst has been reused three times.
Reversible association with AuNP (as shown for benzyl radical)
may contribute to attenuated reactivity.
•
Low solvent reactivity, and this is likely the reason that the
reaction seems to require halogenated solvents.
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-1 -1
•
Low concentrations and modest reactivity for any other free
radical termination is typically around 2 x 10 M s , gives an
average radical concentration about 30 nM.
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Acknowledgement
We thank the Natural Sciences and Engineering Research Council of Canada,
the Canada Research Chairs program and the Canadian Foundation for
Innovation for generous support.
Notes and references
Department of Chemistry and Centre for Catalysis Research and
Innovation, University of Ottawa, 10 Marie Curie, Ottawa, ON K1N 6N5,
Canada.
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