Palladium-Catalyzed Cyclization: Regioselectivity and Structure of Arene-Fused C60 Derivatives

Article abstract of DOI:10.1021/jacs.7b09459

The palladium-catalyzed cyclization on the fullerene C₆₀ cage has been achieved using several aryl halides and C₆₀. This reaction was found to be accelerated by the addition of pivalic acid, which can be rationally explained by the c

Full text of DOI:10.1021/jacs.7b09459

Article
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX-XXX
pubs.acs.org/JACS
Palladium-Catalyzed Cyclization: Regioselectivity and Structure of
Arene-Fused C Derivatives
60
Yoshifumi Hashikawa, Michihisa Murata, Atsushi Wakamiya, and Yasujiro Murata*
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
*
S Supporting Information
ABSTRACT: The palladium-catalyzed cyclization on the full-
erene C60 cage has been achieved using several aryl halides and
C . This reaction was found to be accelerated by the addition of
60
pivalic acid, which can be rationally explained by the computa-
tional study based on the concerted metalation−deprotonation
mechanism. We also demonstrated the regioselective π-
functionalization using prefunctionalized designed molecules
possessing the same substructure on the C60 cage. The single
crystal X-ray analysis and electrostatic potential map revealed that
the orientation of entrapped H O inside the naphthalene-fused
2
open-cage C derivative is electrostatically demanded due to the
60
naphthalene-fusion and construction of the opening.
INTRODUCTION
■
The multifunctionalization of fullerenes has received attention
over the past decades and a great deal of efforts has been done
for both elucidation on the nature of multiple adducts of C60
1
and applications in electronic devices and biomedical field. So
far, several methods have been established for syntheses of
multiple functionalized fullerenes. The representative isolable
multiple adducts of C60 are shown in Figure 1. The most
conventional way is a direct sequential multiaddition that
provides highly functionalized C derivatives including
6
0
2
2
3
fulleroids and η -metal complexes. To introduce functional
groups at the desired positions, a sophisticated way based on
the tethering control has been proposed. The addition pattern
is likely to depend on the length of the linker of the tether
4
molecule. However, these methods are mostly uncontrollable
and makes isolation to be difficult.
Even though only several examples of the successful stepwise
5
multiaddition of C60 were reported, there still remains an
elusive subject to control the addition degree and pattern that
should determine the electronic nature of multiple function-
alized fullerenes. In addition, functionalization and skeletal
modification applying these methods have been mostly limited
to pristine fullerenes C60 and C70 whereas the further
functionalization of prefunctionalized and/or open-cage C60
derivatives is still under development. As to open-cage C60
derivatives, in general, the reaction takes place on their rims of
Figure 1. Representative examples and concepts for synthesizing
isolable multiadducts of C60 and Pd-catalyzed regioselective function-
alization reported herein (this work).
6
the opening, which enables us to create a larger opening. The
high reactivity of the opening is governed by the degree of
coefficients of the HOMO and LUMO. Owing to this7
electronic nature, it is possible to expand the π-conjugation
whereas functionalization of the fullerene skeleton excluding
the rim of the opening is not likely to have occurred.
Received: September 5, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b09459
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Table 1. Reaction Conditions for Synthesis of Naphthalene-Fused C60 (1)
a
a
entry
X
temp. (°C)
Pd. cat. (mol %)
conversion (%)
1
(%)
1
2
3
Br
Br
Br
Br
Cl
Cl
Cl
Cl
180
180
180
180
180
180
200
200
PdCl2
Pd (dba) ·CHCl
3
(6)
(3)
(6)
(6)
(6)
(6)
(6)
(6)
3
14
25
46
12
40
78
85
1
9
2
3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
20
29
1
b
4
5
c
6
24
c
7
bc
,
51 (52)
35
8
a
b
c
Estimated by HPLC analyses. The value in parentheses is isolated yield. Conducted in the presence of pivalic acid (30 mol %) as an additive. 1-
Chloronaphthalene (1-ClNp) was used as a solvent instead of o-dichlorobenzene (ODCB).
In lieu of these approaches, modification of the electronic
structure of substrates is expected to become a useful
methodology toward the regiocontrolled functionalization of
C and its derivatives because the electronic factor contributes
Scheme 1. Reaction Scope of the Palladium-Catalyzed
Cyclization on the C60 Cage
60
to the selectivity. In this context, we have developed a
palladium-catalyzed functionalization of C60 and theoretically
examined the reaction mechanism. Furthermore, we demon-
strated that an open-cage C60 derivative and its structural
analogue (i.e., 1,4-substituted C60 derivative) effectively under-
go cyclization to give the sole product. Endohedral derivatives
1
were also able to be synthesized by this method. Using H
NMR signals of entrapped H O as a magnetic probe, we
2
studied the effects of naphthalene-fusion on the shielding
environment inside the fullerene cages. As to the naphthalene-
fused open-cage C60 derivative, we got single crystals for
endohedral one. Both the X-ray analysis and electrostatic
potential map suggested that the orientation of entrapped H O
2
is conclusively controlled by the electrostatic effects of the
naphthalene-fusion and construction of the opening.
RESULTS AND DISCUSSION
■
Palladium-Catalyzed Cyclization on the C60 Cage.
Recently, transition-metal-mediated functionalization of C has
60
received growing attention because of the potential feasibility of
8
−16
high conversion and yield.
For example, the successful
syntheses of chiral C60 derivatives using asymmetric catalysts
such as Cu(II) or Ag(I) salt have been reported by Martin and
1
1d,e
co-workers.
In most cases, however, a stoichiometric
1
0a,11b,c,12−16
amount of transition metal reagents is required.
Most recently, several methods for structural modification of
C60 using rhodium- or palladium-catalyst have been devel-
8
,9
oped. In particular, the research groups of Wang and Itami
have been independently made a remarkable contribution to
9
c−m
Figure 2. Representation of the HOMO on the cages of (a) 1,4-(n-
this research field.
As the simplest arene-fused C60
Bu) C (5) and (b) open-cage C60 derivative 6′ calculated at the
derivatives, there is a report on the synthesis of naphthalene-
2
60
17
B3LYP/6-31G(d) level of theory (orbitals inside the cages are omitted
for clarity. The t-butyl groups in 6 were replaced with the methyl
groups in 6′).
fused C60 derivative 1 containing one pentagonal ring.
According to this report, 1 was prepared from C60 and 1,8-
diiodonaphthalene under the pyrolytic conditions and the yield
lies in the range of 1−14%. Initially, we examined the efficient
B
DOI: 10.1021/jacs.7b09459
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Scheme 2. Synthesis of 1,4-(n-Bu) C60 (5)
2
Scheme 3. Gram-Scale Synthesis of 8 and Conversion into 6
1
Figure 3. Structures of endohedral derivatives and H NMR chemical
shifts (500 MHz, r.t.) of entrapped H O for H O@C , H O@1,
2
2
60
2
H O@8, H O@6, and H O@10.
2
2
2
CHCl , and Pd(OAc) albeit in low yields (entries 1−3). It is
3
2
noteworthy to mention that the use of pivalic acid as an
18
additive (30 mol %) without changing the temperature and
reaction time has a significant role in the improvement of the
conversion up to 46% (entry 4). However, the yield of 1 still
remained low (29%) due to the inevitable formation of multiple
adducts. When 1 equiv of 1-chloronaphthalene was used
instead of 1-bromonaphthalene, the conversion was decreased
to 12% from 25% (entry 5). This indicates that the oxidative
addition of the C−X bond to the Pd center is accelerated in X =
Br. In the case where 1-chloronaphthalene was used as a
solvent, the conversion was drastically improved to 40% and
desired 1 was formed in 24% yield (entry 6). Increasing the
temperature up to 200 °C resulted in the good conversion
Scheme 4. Regioselective Naphthalene-Fusion
(
78%) and yield (52%) (entry 7). Under the similar conditions
to entry 7, addition of pivalic acid again reproduced a higher
conversion (85%) with inevitable formation of multiadducts as
seen in entry 4 (entry 8).
Synthesis of π-Functionalized C60 Derivatives. To
explore the scope of this palladium-catalyzed reaction, we
conducted the reaction of C with several aryl bromides under
60
the optimized conditions (Scheme 1). For these reactions, we
replaced the solvent from 1-halonaphthalene to 1-methylnaph-
thalene to avoid the fusion of naphthalene rings.
In the similar way to the case of 1-halonaphthalene, we
conducted the Pd-catalyzed reaction of C60 with a stoichio-
metric amount of 1-bromo-4-n-butylnaphthalene. Compound 2
possessing a n-butyl group on the fused naphthalene ring was
formed and isolated in 46% yield. By replacing the aryl bromide
with 1-bromo-7-tert-butylpyrene, corresponding pyrene-fused
and facile synthesis of 1 from C and 1-halonaphthalene in the
presence of palladium catalyst.
6
0
The results are summarized in Table 1. The reaction
wasmonitored by HPLC (high performance liquid chromatog-
raphy) equipped with the Buckyprep column. Naphthalene-
fused C60 derivative 1 appeared at 5.93 min, which is
distinguishable from the peak for C60 at 7.43 min. After
chromatographic purification using silica gel, the resultant
compound (1) was characterized by APCI (atmospheric
C (3) was obtained in 48% yield. Even when the aryl bromide
60
with an electron-deficient core was used, this reaction can work
well to provide fused derivative 4 with keeping the moderate
yield (45%).
1
pressure chemical ionization) mass and H NMR analyses.
To the best of our knowledge, this is the first metal-catalyzed
direct arene-fusion using pristine C60 and aryl halides.
Molecular Design toward the Regioselective Func-
tionalization of C . Next, we moved our attention to
60
The conversion of C60 and 1-bromonaphthalene into 1 was
found to be slightly increased in the order of PdCl , Pd (dba) ·
regioselective functionalization of C60 by utilizing the Pd-
catalyzed reaction. For the regiocontrol, the design of the
2
2
3
C
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Table 2. Electrochemical Half-Wave Potentials and
Calculated LUMO Levels of 2−6, 8−10, C60 and PC BM
a
61
1
red
2
red
3
b
compd.
Eox (V)
E
(V)
E
(V)
E
(V) LUMO (eV)
red
2
−
+0.918
−1.156
−1.145
−1.093
−1.199
−1.247
−1.234
−1.318
−1.369
−1.093
−1.17
−1.543
−1.530
−1.503
−1.609
−1.627
−1.623
−1.697
−1.733
−1.482
−1.57
−2.066
−2.047
−1.932
−2.117
−2.139
−2.150
−2.382
−2.463
−1.947
−2.09
−3.10
−3.11
−3.35
−3.07
−2.88
−2.93
−2.90
−2.76
−3.23
−3.09
3
4
5
6
8
9
1
−
−
+1.037
c
c
+1.071
−
+0.901
c
0
d
C60
PC BM
−
−
e
61
a
Conditions: 0.50 mM samples with 0.10 M n-Bu N·BF in ODCB at
4
4
−1
a scan rate of 100 mV s . The values were calibrated with the
oxidation potential of ferrocene/ferrocenium. Calculated at the
b
B3LYP/6-31G(d) level of theory. t-Butyl groups in 6, 8, and 10 were
c
d
e
replaced with methyl groups for calculations. E . ref 12c. ref 28.
pa
Figure 4. (a) Single crystal X-ray structure of H O@10 and (b,c)
2
packing structures with short contacts of π−π interaction and intra/
intermolecular CH/π interaction. Thermal ellipsoids are shown in 50%
probability.
Figure 6. UV−vis absorption spectra of (a) C , 5, and 9 and (b) 8, 6,
60
and 10 for 50 μM solutions in ODCB. The dotted curves are enlarged
views (by 10-fold) for each spectrum.
Figure 5. Electrostatic potential map of 10, calculated at the MP2/6-
31G(d,p)//M06-2X/6-31G(d,p) level of theory.
Figure 2a for 5 (R = n-Bu). This stands in contrast to C60 in
which the HOMO delocalizes on the whole of the skeleton. In
terms of bond lengths, 1,4-dialkylated C60 has a structural
feature of acenaphtylene. Thus, we expected that the
construction of the similar structural motif on the C60 cage
induces the localized HOMO with large coefficients. On the
electronic structure of substrates is of particular importance.
After consideration of potential substrates by association with
DFT calculations (B3LYP/6-31G(d)), we found that 1,4-
19
dialkylated C60 (R C ) has large coefficients of the HOMO
2
60
that is dominantly localized on bonds a and b as shown in
D
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2
9
Figure 7. Plausible reaction pathway for the formation of naphthalene-fused C60 (1) via CMD mechanism, calculated at the M06-2X level of theory
with basis sets of SDD for Pd and Br and 6-31G(d,p) for the rest. Trimethylphosphine was used for calculations to be simplified instead of
tricyclohexylphosphine. Gibbs energies (at 298 K) are given in kcal/mol.
basis of this expectation, we designed open-cage C60 derivative
bearing the similar molecular orbitals to 5 (Figure 2b). It is
of 8. The reduction of 8 by BH ·THF gave a mixture of 1,2-/
1,4-hydrogenated 8 in <10% yield. In contrast to these
3
6
envisaged that open-cage C60 derivative 6 would undergo the
regiocontrolled functionalization on bond a, but not on the rim
of the opening.
reductants, sodium dithionite Na S O under phase transfer
conditions at 100 °C was found to be effective for the
2
2
4
23
exclusive formation of isomerically pure 6 in 73% yield
−
1
,4-Di-n-butyl[60]fullerene (5) was synthesized by the
(Scheme 3). In this reaction, HSO is considered to be a
2
23
reaction of C60 with n-butyl magnesium chloride followed by
quenching with n-butyl iodide (Scheme 2). In this reaction, 1,4-
adduct 5 was preferably formed without considerable amount
of 1,2-substituted isomer due to the steric contribution of the
reactive species that reacts with 8 in 1,2-fashion and then
protonation led to the formation of 1,2-hydrogenated derivative
of 8, followed by thermal transformation into 6.
Regioselective Naphthalene-Fusion on the Prefunc-
tionalized C60 Derivatives. We applied the Pd-catalyzed
reaction to 5 and 6. Under the neat conditions using 1-
chloronaphthalene (1-ClNp), only the sole product was
obtained from 5 in 35% yield (Scheme 4). The APCI MS,
20
two n-butyl groups.
In order to synthesize designed molecule 6, we prepared 8
with an eight-membered-ring opening. As we have already
reported, open-cage C60 derivative 8 can be obtained by one-
21
1
13
step reaction starting from C . However, separation of 8
H NMR, and C NMR spectra determined the structure to be
6
0
from the reaction mixture is inconvenient for a gram-scale
9 in which a naphthalene ring is fused on bond a. Even though
the HOMO also localizes on bond b, the reaction took place
only on bond a probably because of the steric effects from the
n-butyl groups. 1-Bromonaphthalene (1-BrNp) also worked
well to furnish 9 in 50% yield. In the case of open-cage C60
derivatives, the reactions usually take place on the rim of the
opening. However, naphthalene-fused compound 10 was
formed from 6 and 1-ClNp under 200 °C for 3 h, albeit in
6% yield together with 11% of 8. In this reaction, the formation
of any other naphthalene-fused derivatives of 6 was not
observed. By increasing the temperature at 220 °C for 1 h, 10
was isolated in 17% yield while the conversion into 8
predominantly occurred (43%) (Scheme 4). The low yield of
reaction because of the similar polarity of C and 8. To obtain
6
0
8
in a large scale, more polar 7 having two keto groups was first
synthesized from C60 (52%) through the one-pot process
Scheme 3). After easy separation of 7 by silica gel column
(
chromatography, 7 was further treated with an excessive
amount of tri(isopropyl)phosphite P(OiPr) , which underwent
3
21,22
intramolecular carbonyl coupling
gram-scale.
to afford 8 (69%) in a
Subsequently, we tried hydrogenation of 8 under several
2−
conditions. First, we generated dianion 8 by CH SNa in
3
CH CN but the quenching with sat. NH Cl aq. resulted in the
3
4
formation of multihydrogenated compounds and 48% recovery
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electron ring currents is weaker than that for H O@1. This is
2
because of the scission of two closest [5,6]-bonds with retaining
the 60π electron system. It has been proven that the cleavage of
the [5,6]-bond is less sensitive to change of the magnetic
3
environment inside the fullerene cages, as seen in He@
26
2
27
C CH and H O@C NR. The reduction of H O@8 into
60
2
60
2
H O@6 results in the destruction of the 60π electron system,
2
which drastically reflects the shielding environment (δ −8.23
ppm for H O@6) probably due to the change in diamagnetic
2
ring currents extending around the entire fullerene sphere in
addition to further weakening the paramagnetic ring current. In
addition, introduction of the fused-naphthalene ring on the
cage of H O@6 induces the enhanced shielding and the proton
2
signal of entrapped H O appeared at higher-field region (δ
2
−
10.22 ppm for H O@10).
Single Crystal X-ray Analysis. The single crystals of
2
H O@10 that are suitable for X-ray analysis were obtained
2
from CS solution by slow evaporation. The solid-state and
2
packing structures of H O@10 are shown in Figure 4.
2
The crystal structure of H O@10 demonstrated that two
2
molecules of CS was cocrystallized and the naphthalene ring
2
was fused on bond a. From the packing structure of H O@10
2
(
Figures 4b), the short contacts between adjacent molecules
were observed. The short contacts arise from the π−π
interaction with the distance of 3.204−3.398 Å, which was
also seen in the crystal structures of 7 and 8 (see Supporting
Information). We also found the unique short contacts among
fused-naphthalene rings and t-butyl groups on the pyridyl
Figure 8. Plausible reaction pathway for the formation of naphthalene-
fused C60 (1) via the Heck-type mechanism. Calculation methods
are the same as described in Figure 7.
30
moieties in H O@10 (Figure 4c). The closest CH/π distances
2
between the H-atom in the t-butyl group and the mean plane of
the naphthalene ring are 2.701−3.127 Å.
1
0 arises from the side reaction, i.e., thermal dehydrogenation
The position of entrapped H O was determined at the final
from the opening of 6. To prevent this side-reaction, 1-BrNp
was employed at lower temperature (180 °C). As the result, the
yield of 10 was improved up to 53% without thermal
dehydrogenation of 6.
It should be noted that the reaction of 8 (structurally similar
to 6) and 1-halonaphthalene (1-XNp) in the presence of the
palladium catalyst resulted in the formation of a mixture of
inseparable multiadducts because the HOMO is distributed on
the whole of the skeleton (Figure S57). This indicates that the
Pd-catalyzed cyclization can be regiocontrolled by the proper
modification of electronic structures such as 5 and 6.
2
stage of the X-ray analysis. After placing the O(1)-atom of
entrapped H O, there still remained a low Q peak inside the
2
fullerene cage, which was assignable to the H(1)-atom. Another
H-atom (i.e., H(2)) was placed and restrained using DFIX
instructions during the refinements. Without DFIX instruc-
tions, the position of H(2) cannot be determined. This results
suggest that the entrapped water molecule is located at the
center of the cage with the H(1)−O(1) bond facing to the
opening. To unveil the origin of the unique orientation of
entrapped H O, we calculated the electrostatic potential map
2
Preparation of Endohedral Derivatives and Effects of
Naphthalene-Fusion on Shielding Environment inside
the Fullerene Cages. To study the effects of naphthalene-
fusion on the magnetic environment inside the fullerene cages,
naphthalene-fused derivatives would become model com-
pounds when a water molecule is incarcerated inside their
cavities. It is because the incarcerated water molecule could be
potentially used as a magnetic probe to evaluate the shielding
for 10 at the MP2/6-31G(d,p)//M06-2X/6-31G(d,p) level of
theory (Figure 5). Interestingly, inside the cage of 10, the
positive potential is observed under the bond where the
naphthalene ring is fused. In contrast, negative potential is
located under the double bond in the opening. These positive
and negative electrostatic potentials should contribute to the
orientation of entrapped H
restriction of the rotational motion of entrapped H
rationally explain the orientation of entrapped H
single crystal X-ray structure of H O@10.
O, concomitantly suggesting the
O. This can
O in the
2
2
24
and/or electrostatic effects. Thus, we prepared a series of
2
21
endohedral derivatives from H O@C60 or H O@8.
2
2
2
The structures and chemical shifts of proton signals of
Electrochemical and Photophysical Properties. The
cyclic voltammetries and UV−vis absorption measurements
were conducted for 2−6 and 8−10 (Table 2 and Figure 6). For
each compound, three reversible reduction waves were
observed and the first reduction potentials are mostly
proportional to the values of calculated LUMO levels
(B3LYP/6-31G(d)) (Table 2). By the naphthalene-fusion,
the reduction potentials of 2, 9, and 10 are cathordically shifted
entrapped H O are displayed in Figure 3. By naphthalene-
2
fusion on the C60 cage, the chemical shift of entrapped H O
2
was found to be upfield-shifted by Δδ 2.86 ppm from δ −4.64
(
H O@C ) to −7.50 ppm (H O@1). The enhanced shielding
2
60
2
1
of H resonance of H O is substantially due to the vanishing of
2
the interrupted paramagnetic π-electron ring currents in the
25
two pentagonal rings highlighted with blue. Even though
open-cage derivative H O@8 also loses the pentagonal ring
by 0.063, 0.119, and 0.122 eV with reference to those for C , 5,
2
60
currents at the two positions, the water signal appeared at δ
and 6, respectively. Pyrene-fused derivative 3 exhibited a quasi-
−6.09 ppm, indicating that the effect of paramagnetic π-
reversible oxidation wave at +0.918 eV. The DFT calculations
F
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suggested that the HOMO and LUMO are localized on the
pyrene core and the C60 cage, respectively. Thus, the
stableradical cation and radical anion species are considered
whereas there is less considerable effect of acetate anion (39.7
kcal/mol) arising from palladium diacetate as a palladium(II)
source. The structure of INT2 only for t-butyl derivative has
remarkable distortion compared to other possible intermediates
•
+
•−
to be (t-butylpyrene) (C ) and (t-butylpyrene)(C ) ,
6
0
60
−
−
respectively. Whereas 3 contains a pyrene core in its structure,
the characteristic emission of pyrene is quenched by the
effective energy transfer from the pyrene core to the C60 cage.
In the case of 4, the cathordic shift by the naphthalene fusion
was canceled out to show the same reduction potential as C60
because of the introduction of an imide moiety and thus it has a
good solubility in common solvents in contrast to C .
possessing a carbonate (HCO ) or acetate (CH CO ) ligand.
3 3 2
This indicates that INT2 (R = t-butyl) is enough reactive to
readily pass through TS2 in the presence of a pivalate ligand.
These results could finely reproduce the experimental
observation that the addition of pivalic acid resulted in higher
conversion of C60 (Table 1, entries 4 and 8).
We also considered the alternative pathway via the Heck-type
mechanism (Figure 8). Even though the intermediate INT3′ in
the Heck-type mechanism is calculated to be more stable (13.4
kcal/mol) than that in the CMD mechanism (31.4 kcal/mol),
the activation energy required for the intramolecular C60
insertion (TS2′) is rather high (46.0 kcal/mol) when compared
to the TS2 value in the CMD mechanism (39.9 kcal/mol). This
is probably because that the formation of the four-membered
transition state on the C60 cage in the Heck-type TS is less
favored by the structural disturbance. From these points of
view, we concluded that the Heck-type mechanism can be
excluded for the rational explanation of this reaction.
60
Because the first reduction potentials for fused derivatives 2−
4
(
lie within the range between corresponding potentials of C
6
0
1
2c
28
−1.09 eV)
and PC BM (−1.17 eV), these have a
6
1
potential application into n-type materials for solar cells.
Particularly, 2 and 3 possess reduction potentials, which are
comparable to that of PC BM. Furthermore, 1,4-substituted
6
1
derivative 5, open-cage derivatives 6 and 8, and fused
derivatives 9 and 10 have the higher-lying LUMO levels,
which are expected to endow the higher open-circuit voltage
28
(VOC) in solar cells.
The UV−vis absorption spectra of C , 5, 6, and 8−10 were
6
0
recorded using 50 μM sample solutions in ODCB (Figure 6).
In terms of the absorption property, 8 shows a close
resemblance to C , which is the 60π electron system. The
SUMMARY
■
6
0
In conclusion, we have developed a palladium-catalyzed
cyclization for the concise syntheses of arene-fused C60
derivatives. Using this method, we synthesized naphthalene-,
pyrene-, and naphthalimide-fused C60 derivatives (1−4). For
the regioselective functionalization of C , we designed and
color of these solutions is purple. By the modification of the
electronic structures in C60 by 1,4-addition, the 58π electron
system should be constructed in 5. The similar spectral view
and color (yellow) was again observed for 6 retaining the
original 58π electronic configuration. The fusion of the
naphthalene ring influenced absorption intensities of 9 and
60
synthesized prefunctionalized C60 derivatives 5 and 6. These
compounds have the similar electronic structures, which were
also supported by UV−vis absorption spectra. By applying the
palladium-catalyzed reaction to 5 and 6, regio-controlled
products 9 and 10 were obtained in each reaction. We also
prepared endohedral derivatives such as H O@1 and H O@10,
10 bearing 56π electrons on their body, which showed an
intense yellow color with increasing extinction coefficients in
the measured range of 300−800 nm except around 330 nm.
Mechanistic Study for the Palladium-Catalyzed Cyc-
lization of the Naphthalene Ring on the C60 Cage. To get
deep insights into the reaction mechanism of Pd-catalyzed
2
2
1
which indicated that the H NMR signals of entrapped H O
2
highly depend on their structural features with π-electron ring
currents. The combined study based on the single-crystal X-ray
analysis and DFT calculations revealed that the orientation of
cyclization of a naphthalene ring on the C cage, we performed
6
0
DFT calculations at the M06-2X level of theory with basis sets
of SDD for Pd and Br and 6-31G(d,p) for the rest.
Trimethylphosphine was used instead of tricyclohexylphos-
phine for calculations to be simplified. During the calculations,
we noticed that there are two possible pathways, that is, the
entrapped H O inside 10 is restrained by the electrostatic factor
2
originated from the naphthalene-fusion and construction of the
opening. The reduction potentials for arene-fused derivatives
were also verified by cyclic voltammetries, implying the
potential application into solar cells due to high electron-
accepting abilities. Finally, to elucidate the reaction mechanism
for this palladium-catalyzed cyclization, we performed the DFT
calculations along with the CMD and Heck-type mechanisms.
We experimentally observed that the addition of pivalic acid
promotes the conversion of C into 1. It should be mentioned
2
9
concerted metalation−deprotonation (CMD) mechanism
30
(
Figure 7) and the Heck-type mechanism (Figure 8). At
the first stage in these mechanisms, palladium(0) undergoes
oxidative addition into the C−Br bond of 1-bromonaphthalene
and then carbopalladation of thus resultant palladium(II)
complex (INT 1) against C , which forms INT2. In the case of
6
0
60
the CMD mechanism, INT2 is further converted into
palladacycle INT3 through a six-membered transition state
TS2 with transferring hydrogen. After conformational change
of INT3, the C−C bond formation in INT4 affords product 1
through TS3, which leads to regeneration of palladium (0). The
DFT calculations based on the CMD mechanism suggest that
the metalation−deprotonation process (TS2) is most likely to
that the CMD mechanism is preferable for the rational
explanation of this reaction and the pivalate ligand has a
significant role for the enhanced reactivity. This reaction
demonstrated herein is postulated to have a potential versatility
to synthesize π-functionalized C60 derivatives and create a new
class of structural motifs in the nanocarbon chemistry.
‡
be a rate-determining step with the energy barrier (ΔG , 298
ASSOCIATED CONTENT
■
K) of 39.9 kcal/mol in which the carbonate anion arising from
the stoichiometric carbonate base acts as a ligand. Interestingly,
addition of pivalic acid as a cocatalyst is predicted to have a
significant impact to a decrease of energy barrier for cleaving
the C−H bond of the naphthalene moiety (TS2, 34.5 kcal/
mol) by 5.4 kcal/mol compared to that for carbonate anion
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b09459.
Detailed experimental procedures, characterization data,
and computational results (PDF)
G
DOI: 10.1021/jacs.7b09459
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(
7) (a) Iwamatsu, S.-i.; Uozaki, T.; Kobayashi, K.; Re, S.; Nagase, S.;
Data for C H N OS (CIF)
9
4
36
2
4
Murata, S. J. Am. Chem. Soc. 2004, 126, 2668−2669. (b) Iwamatsu, S.-
Data for C H N (CIF)
4
8
21
i.; Murata, S. Tetrahedron Lett. 2004, 45, 6391−6394.
Data for C82.75H N O S (CIF)
26
2
2 1.50
(
8) Rh: (a) Becker, L.; Evans, T. P.; Bada, J. L. J. Org. Chem. 1993,
Data for C H N O (CIF)
8
5
29
2
2
58, 7630−7631. (b) Gonzalez, R.; Knight, B. W.; Wudl, F.; Semones,
M. A.; Padwa, A. J. Org. Chem. 1994, 59, 7949−7951. (c) Duczek, W.;
Radeck, W.; Niclas, H.-J.; Ramm, M.; Costisella, B. Tetrahedron Lett.
AUTHOR INFORMATION
Corresponding Author
yasujiro@scl.kyoto-u.ac.jp
■
1
997, 38, 6651−6654. (d) Nair, V.; Sethumadhavan, D.; Sheela, K. C.;
Eigendorf, G. K. Tetrahedron Lett. 1999, 40, 5087−5090. (e) Muraoka,
T.; Asaji, H.; Yamamoto, Y.; Matsuda, I.; Itoh, K. Chem. Commun.
2000, 199−200. (f) Nair, V.; Sethumadhavan, D.; Sheela, K. C.; Nair,
S. M.; Eigendorf, G. K. Tetrahedron 2002, 58, 3009−3013. (g) Nambo,
M.; Noyori, R.; Itami, K. J. Am. Chem. Soc. 2007, 129, 8080−8081.
*
ORCID
Michihisa Murata: 0000-0002-5479-2993
Atsushi Wakamiya: 0000-0003-1430-0947
Yasujiro Murata: 0000-0003-0287-0299
(
h) Nambo, M.; Segawa, Y.; Wakamiya, A.; Itami, K. Chem. - Asian J.
2
011, 6, 590−598.
Notes
(9) Pd: (a) Shiu, L.-L.; Lin, T.-I.; Peng, S.-M.; Her, G.-R.; Ju, D.-D.;
Lin, S.-K.; Hwang, J.-H.; Mou, C.-Y.; Luh, T.-Y. J. Chem. Soc., Chem.
Commun. 1994, 647−648. (b) Shen, C. K. F.; Chien, K.-M.; Liu, T.-Y.;
Lin, T.-I.; Her, G.-R.; Luh, T.-Y. Tetrahedron Lett. 1995, 36, 5383−
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
5
384. (c) Mori, S.; Nambo, M.; Chi, L.-C.; Bouffard, J.; Itami, K. Org.
Lett. 2008, 10, 4609−4612. (d) Nambo, M.; Itami, K. Chem. - Eur. J.
009, 15, 4760−4764. (e) Zhu, B.; Wang, G.-W. J. Org. Chem. 2009,
74, 4426−4428. (f) Zhu, B.; Wang, G.-W. Org. Lett. 2009, 11, 4334−
337. (g) Chuang, S.-C.; Rajeshkumar, V.; Cheng, C.-A.; Deng, J.-C.;
Financial support was partially provided by the JSPS
KAKENHI Grant Number JP15J09892, JP15H00993,
JP17H05152, JP17H06119, and JP17H06789.
2
4
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