Ni-Catalyzed direct 1,4-difunctionalization of [60]fullerene with benzyl bromides

Article abstract of DOI:10.1039/c5cc01534k

A new Ni-catalyzed direct 1,4-difunctionalization of [60]fullerene with various benzyl bromides has been developed. The use of a DMSO additive combined with a nickel catalyst is indispensable for the formation of 1,4-dibenzyl fullerenes with a variety of

Full text of DOI:10.1039/c5cc01534k

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Ni-Catalyzed direct 1,4-difunctionalization of
[60]fullerene with benzyl bromides†
Cite this:DOI: 10.1039/c5cc01534k
Weili Si,a Xuan Zhang,a Naoki Asao,a Yoshinori Yamamotoab and Tienan Jin*a
Received 19th February 2015,
Accepted 5th March 2015
DOI: 10.1039/c5cc01534k
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A new Ni-catalyzed direct 1,4-difunctionalization of [60]fullerene
with various benzyl bromides has been developed. The use of a
DMSO additive combined with a nickel catalyst is indispensable for
the formation of 1,4-dibenzyl fullerenes with a variety of functional
groups. The reaction proceeds through the formation of a fullerene
monoradical species.
1,4-Difunctional fullerenes possessing two functional groups in
the 1,4-position of the fullerene core are one of the important
classes of electron-accepting materials for high performance
organic photovoltaics (OPVs).1 1,4-Bisadducts are known to show
a higher LUMO energy level compared to the 1,2-adducts,1b
which is beneficial for obtaining a higher open circuit voltage
Scheme 1 Transition-metal-catalyzed functionalization of C60: our previous
and current studies.
in OPVs.2 In addition, among the 1,4-adducts, the 1,4-dialkyl forming an active alkyl radical species that reacts with C60 selectively
adducts such as 1,4-dibenzyl and disilylmethyl fullerenes exhibit to afford the mono-substituted hydrofullerenes in the presence of
a better solubility and higher LUMO levels than the 1,4-diaryl water.6a Furthermore, we have disclosed an uncommon polar solvent
adducts.1b In this context, a number of synthetic methods for effect on the generation and stabilization of a fullerene monoradical
fullerene 1,4-bisadducts have been reported,3,4 such as the reaction species in the Cu-catalyzed dimerization6c and C–H amination of
of fullerene anions with alkyl halides, addition of nucleophiles to hydrofullerenes.6d Inspired by our previous observations, we focused
fullerene cations, and free radical addition. However, a direct on the transition-metal-catalyzed direct 1,4-difunctionalization of C60
and efficient 1,4-difunctionalization of the pristine [60]fullerene with active bromides to achieve 1,4-bisadducts selectively. Herein,
has been rarely developed,3c particularly by using a transition- we report a novel Ni-catalyzed direct 1,4-difunctionalization of
metal-catalyzed method which has become one of the most C60 with various functional benzyl bromides to afford 1,4-dibenzyl
efficient fullerene functionalization methods.5 Only one example fullerenes in good to high yields. The reaction proceeds through the
of transition-metal-catalyzed 1,4-diallylation has been reported formation of a fullerene monoradical intermediate and the use of
by Itami et al. during the regioselective tetraallylation of C60 DMSO co-solvent is significant for the implementation of the present
using a Pd catalyst.5c
1,4-difunctionalization sufficiently under mild conditions.
Recently, we have been interested in developing new fullerene
On the basis of our previous Co-catalyzed hydrobenzylation
functionalization methods by using cobalt or copper catalysts through of C60,6a we examined the selective 1,4-dibenzylation through the
the formation of a fullerene monoradical species (Scheme 1).6 For optimization of transition metal catalysts and polar cosolvents at
example, we reported that the low-valent cobalt catalyst is capable of room temperature under an argon atmosphere (Table 1). The yields
were determined by HPLC analysis using C70 as an internal standard.
The reaction of C60 with benzyl bromide 1a in the presence of the
CoCl2dppe catalyst and the Mn reductant in 1,2-dichlorobenzene
(ODCB) without using any cosolvent and water did not give any
products (entry 1). However, we were pleased to find that the reaction
a WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University,
Sendai 980-8577, Japan. E-mail: tjin@m.tohoku.ac.jp
b State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
116023, China
proceeded upon adding a small amount of DMSO to the ODCB
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization data. See DOI: 10.1039/c5cc01534k
solution, giving the 1,4-dibenzylation product 2a in 40% yield along
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Table 1 Dibenzylation of C60 with benzyl bromide 1a by using various
transition metal catalysts and cosolventsa,b
Table 2 Ni-catalyzed 1,4-dibenzylation of C60 with various functional
benzyl bromides 1a,b
Entry
Metal
Cosolvent
2a
3a
4a
C60
1
2
3
4
5
6
7
8
CoCl2dppe
CoCl2dppe
CoCl2dppe
CoCl2dppe
CoCl2dppe
CoCl2dppe
CoCl2dppf
CoCl2bpy
NiCl2dppe
NiCl2dppp
NiCl2dppf
NiCl2(PPh3)2
PdCl2dppe
None
DMSO
DMF
CH3CN
THF
MeOH
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
0
40
0
0
0
0
0
22
60 (58)
43
39
36
0
0
0
18
0
0
53
0
0
5
17
13
3
0
5
0
0
0
0
0
5
5
99
0
85
99
99
21
99
36
18
0
9c
10
11
12d
13
13
18
0
0
48
95
0
0
a
Reaction conditions: C60 (0.03 mmol), 1a (3 equiv.), ODCB (4 mL),
DMSO (0.15 mL), metal catalyst (10 mol%), Mn (3 equiv.), rt, 12 h, argon
b
atmosphere. HPLC yields determined using C70 as an internal standard.
c
Isolated yields are provided in parentheses. NiCl2dppe (5 mol%) and Mn
d
(1.5 equiv.) were used for 24 h. Reaction time is 5 days.
a
Reaction conditions: C60 (0.03 mmol), NiCl2dppe (5 mol%), Mn
(1.5 equiv.), active bromide (3.0 equiv.), ODCB (4 mL), DMSO (0.15 mL),
rt, argon atmosphere. Isolated yields are shown.
with dimer 4a in 5% yield (entry 2). It was found that the other
polar cosolvents such as DMF, CH3CN, and THF were inactive
in the formation of 2a (entries 3–5) and the use of a protic
b
solvent MeOH produced the hydrobenzylation product 3a cases, a small amount of 1,2-hydroalkylfullerene 3, fullerene dimer 4
preferentially similar to our previous water-incorporated hydro- or multiadducts were produced. The benzyl bromides 1b and 1c
benzylation reaction (entry 6).6a By using DMSO as a cosolvent, bearing a bromo-substituent at the ortho- or the para-position of the
we further examined other cobalt catalysts such as CoCl2dppf phenyl ring afforded the corresponding 1,4-bisadducts 2b and 2c in
and CoCl2bpy, which were totally inactive or dramatically 70% and 61% yields, respectively, which can be used as important
decreased the yield of 2a (entries 7 and 8). Among the other intermediates in various cross-coupling reactions to construct more
transition metal catalysts such as NiCl2dppe, NiCl2dppp, and complex molecules. The reactions with benzyl bromides 1d–f having
NiCl2dppf having a bidentate phosphine ligand tested, the use methyl, methoxy, and ester functional groups on the phenyl ring
of NiCl2dppe obviously increased the yield of 2a (58%) and produced the corresponding 1,4-bisadducts 2d–f in 70–85% yields
the product selectivity even when using a decreased catalytic regardless of the substitution positions. However, the simple non-
loading of 5 mol% (entries 9–11). The reaction with NiCl2(PPh3)2 substituted benzyl bromide 1g resulted in a relatively lower yield of
having a monodentate phosphine ligand exhibited a lower 2g (55%) owing to the increase in the formation of multiadducts.
activity, resulting in 2a in 36% yield together with the recovered Likewise, 40-(bromomethyl)-[1,10-biphenyl]-2-carbonitrile (1h) was also
C60 in 48% yield at longer reaction times (5 days) (entry 12). a good benzylic substrate to give the corresponding 1,4-bisadduct 2h
Other transition metal catalysts such as PdCl2dppe were tested in good yield. The naphthyl-incorporated substrate 1i having a
to be inactive in the present reaction (entry 13). It is noted that, bromine substituent at the a-position of the naphthalene moiety
in the absence of transition metal catalysts or without Mn was also compatible with the present conditions to afford the
reductant under otherwise identical conditions, almost no pro- 1,4-bisadduct 2i in 65% yield, in which the a-bromo-substituent
ducts were formed and the use of anhydrous DMSO is crucial for remained intact. It was noted that other active dibromides such
improving the selectivity of 2a.
as allyl bromide and cinnamyl bromide showed low yields and
Under the optimized conditions, the scope of the Ni-catalyzed selectivities, and aromatic dibromides such as bromobenzene
1,4-difunctionalization of [60]fullerene with active alkyl bromides and 2-bromonaphthalene were totally inactive.
was investigated (Table 2). All of the reactions were monitored by
In order to understand the reaction pathways, we examined the
TLC and HPLC analysis, and the corresponding 1,4-dialkylfullerenes reactions using the fullerene dimer 4a and 1,2-hydrobenzylated
2 were isolated by using silica gel column chromatography. In some fullerene 3a, respectively. The reaction of 4a with benzyl bromide 1a
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In conclusion, we have described a new and efficient
Ni-catalyzed direct 1,4-difunctionalization of C60 with benzyl
bromides. A variety of functional groups were tolerated, afford-
ing the fullerene 1,4-bisadducts in good to high yields. The
use of DMSO as a cosolvent combined with the NiCl2dppe
catalyst enables the reaction to proceed under mild conditions
using a catalytic process through the formation of a fullerene
monoradical.
This work was supported by a Scientific Research (B) award
from the Japan Society for Promotion of Science (JSPS) (No.
25288043) and a World Premier International Research Center
Initiative (WPI), MEXT, Japan. W.S. acknowledges the support
of the China Scholarship Council (CSC).
Scheme 2 Control experiments: (a) reaction of dimer 4a with bromide
1a, (b) reaction of hydrofullerene 3a with bromide 1a. The HPLC yields are
shown, which were determined using C60 as an internal standard.
Notes and references
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under the standard Ni-catalyzed conditions gave the corresponding
1,4-bisadduct 2a in 51% yield together with 3a in 27% yield
(Scheme 2a). In addition, the reaction of 3a with 1a produced 3a
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reaction may involve the formation of radical species (ESI,†
Scheme S1). Similar to our previous cobalt catalyst–Mn systems,6a
the reduction of Ni(II) by the Mn reductant followed by the reaction
of the resulting Ni(0) complex with benzyl bromide forms the
benzyl radical species and the Ni(I) complex.7 The addition of the
resulting benzyl radical to C60 forms the fullerene monoradical
(Scheme 1), which might be reversible with the (ArCH2)C60–Ni(II)
complex or with the fullerene dimer 4. Subsequently, the coupling
of the fullerene monoradical with the benzyl radical forms the
1,4-bisadduct 2 and the reduction of the Ni(I) complex by Mn
regenerates the active Ni(0) complex. We assume that the hydro-
fullerene 3 is formed by the hydrolysis of the (ArCH2)C60–Ni(II)
complex with a small amount of H2O in the reaction system,
which is proposed to be reversible. Although the unique role of
the DMSO polar solvent remains unclear at this stage, we assume
that DMSO may facilitate the single electron transfer process and
stabilize both fullerene and benzyl radicals.
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