2,6-Bis(porphyrin)-substituted pyrazine: a new class of supramolecular synthon binding to a transition-metal ion and fullerene (C60)

Article abstract of DOI:10.1016/j.tet.2008.10.057

2,6-Disubstituted-3,5-dimethylpyrazines have been synthesized via biased acetal synthesis from symmetric 2,3,5,6-tetrakis(chloromethyl)pyrazine. The pyrazine ligands coordinated to trans-dichloropalladium(II) at the nitrogen whose neighboring carbons were

Full text of DOI:10.1016/j.tet.2008.10.057

Tetrahedron 65 (2009) 282–288
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
2,6-Bis(porphyrin)-substituted pyrazine: a new class of supramolecular synthon
binding to a transition-metal ion and fullerene (C60)
*
Yusaku Eda, Kennosuke Itoh, Yoshio N. Ito, Toshio Kawato
Department of Chemistry, Faculty of Sciences, Kyushu University, 4-2-1 Ropponmatsu, Chuo-ku, Fukuoka 810-8560, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
2,6-Disubstituted-3,5-dimethylpyrazines have been synthesized via biased acetal synthesis from sym-
metric 2,3,5,6-tetrakis(chloromethyl)pyrazine. The pyrazine ligands coordinated to trans-dichloro-
palladium(II) at the nitrogen whose neighboring carbons were connected to less hindered methyl groups.
2,6-Bis(porphyrin)-substituted pyrazine bound C60 to yield 1:1 inclusion complex. The binding of C60
with the pyrazine ligand and its zinc complex was determined by ESI-MS, NMR, and fluorescence
spectroscopic analyses.
Received 4 September 2008
Received in revised form 17 October 2008
Accepted 18 October 2008
Available online 1 November 2008
Keywords:
Pyrazine
Ó 2008 Elsevier Ltd. All rights reserved.
Porphyrin
Fullerene
Pd(II) complex
1. Introduction
employment of a pyrazine derivative as a conjugated bridge that
connects a functional metal ion and fullerene C60. Although syn-
There is much current interest in the supramolecular chemistry
because of the remarkable applications in various fields of science
and technology. Recently many research attempts have been cen-
tered on the design and construction of host systems that are ca-
pable of capturing fullerene C60 as a guest,1–11 since its significant
properties as electron acceptors were recognized.12 Thus, many
publications dealing with structural, spectroscopic, electro-
chemical, and other functional properties of supramolecular sys-
tems possessing C60 have appeared. Among them, disubstituted
aromatic compounds could be arranged as good hosts by in-
troducing certain functional groups for molecular recognition,3–7
because two functional groups faced close to a guest.13,14 As a con-
sequence, 2,6-disubstituted pyridines were expected to be de-
veloped as unique host molecules.14 On the other hand, due to the
coordinative ability of aromatic nitrogen atoms, pyridine-based li-
gands have been widely used in the fields of metal coordination
chemistry; thus, metal complexes of substituted pyridines have
been developed.15 In the case of pyrazine, two nitrogen atoms co-
ordinate to different metal ions to act as N,N0-bidentate bridging
ligands.16 In such a polynuclear metal complex, metal ions can in-
thetic methods of substituted pyrazines have been developed in
the fields of metal coordination chemistry as well as medicinal
chemistry for the skeleton of biologically active sites,17 consider-
able difficulty has been experienced in controlling circumstances
around the two nitrogen atoms of pyrazine by introducing dif-
ferent type of substituents at the 2,6-positions and the 3,5-posi-
tions. In a previous paper, we have reported pyrazine acetal
synthesis by a biased reaction of trans-bis(chloromethyl) sub-
stitutents of pyrazine with sodium alkoxides.18 In this article, we
have extended our investigation using the biased acetal synthesis
from symmetric 2,3,5,6-tetrakis(chloromethyl)pyrazine (1) to
provide a new type of pyrazine ligands with two functional groups
that can be positioned to embrace C60 forming a 1:1 inclusion
complex.
2. Results and discussion
Pivotal starting material, 2,6-bis(diethoxymethyl)-3,5-dimethyl-
pyrazine (2), and its Pd(II) complex, trans-[PdCl2(2)2] (3), were pre-
pared according to the previously reported procedure (Scheme 1).18
In order to confirm the structures of the Pd complex 3 as well as the
diacetal ligand 2, X-ray crystallographic analysis of a single crystal of
3 was carried out in this study (Fig. 1). Two pyrazine ligands in 3
adopted C2 symmetry and coordinated to a Pd(II) ion at the nitrogen
whose neighboring carbons were connected to less hindered methyl
groups. The N–Pd–Cl angles are 89.73(8)ꢀ and 90.27(8)ꢀ and the
Pd–N and Pd–Cl bond lengths are 2.043(2) and 2.2916(11) Å,
teract each other through the p-bonds of pyrazine ligands.
In view of the results, our interest in designing highly func-
tional supramolecular systems has been directed toward the
* Corresponding author. Tel./fax: þ81 92 726 4752.
E-mail address: kawato@chem.rc.kyushu-u.ac.jp (T. Kawato).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.10.057

Y. Eda et al. / Tetrahedron 65 (2009) 282–288
283
OEt
OEt
OEt
N
EtO
EtO
N
Pd
N
N
N
PdCl2(CH3CN)2
CH2Cl2
EtONa
EtOH
Cl
Cl
Cl
Cl
Cl
OEt
OE t
EtO
Cl
N
N
OEt
1
2
OEt
OEt
N
OEt
O
OX
X = H
3
p-TsOH . H2O
benzene
66%
THF
or
tert-BuOH
X = Na
, TFA, Δ
N
H
40%
N
N
O
O
O
O
N
C60
N
H
N
H
4
. C60
N
O
O
HN
NH
O
O
5
4
Scheme 1. Synthesis and reaction of bis(acetal)pyrazines (2 and 4).
respectively, falling in the normal range of those in common Pd(II)
coordination compounds.19
was used without pre-hydrolysis to aldehyde because of synthetic
convenience (Scheme 1), although dipyrromethane derivatives
were generally prepared with aldehyde in pyrrole.20 Acid-catalyzed
condensation of 5 with 5-phenyldipyrromethane and benzalde-
hyde, followed by oxidation with DDQ, did not give desired
Since metal complexation at one side of pyrazine nitrogen was
achieved, inclusion of C60 at another side of the pyrazine ligand was
attempted by introducing proper functional groups at 2,6-positions
of pyrazine. We attended first to the work of Balch et al., who
reported that electron-rich benzyl ether groups interacted with C60
to form supramolecular aggregation.8 Thus, benzyloxybenzyl acetal
compound 4 was chosen for the initial study largely for practical
reasons; benzyloxybenzyl groups were considered to be easily in-
troduced as pyrazine substituents by acetalization. Contrary to our
previous result, however, reaction of 1 with sodium 4-benzyloxy-
benzyl alkoxide did not give intended benzyloxybenzyl acetal but
yielded only intractable black material. The desired acetal 4 could
be obtained by acid-catalyzed transacetalization of 2 with 4-ben-
zyloxybenzyl alcohol as colorless glassy solid (Scheme 1). Such an
acetal exchange reaction of 2 with slightly excess amount of other
alcohols proceeded similarly to give corresponding pyrazine ace-
tals; thus, compound 2 was found to be a convenient starting
material for pyrazine derivatives. We then measured 1H NMR of 4
before and after addition of C60 in toluene-d8. Since their 1H NMR
spectra did not exhibit distinct difference in chemical shifts, asso-
ciation constant for 4 and C60 could not be determined by the 1H
NMR method. Although the possibility of encapsulation of C60
within 4 in solution cannot be rejected from the NMR data alone,
low recognition ability of 4 with C60 is responsible for the high
flexibility and low electronic interaction of benzyloxybenzyl group
with C60. This failure prompted introduction of more electron-do-
nating and rigid substituents for functional groups.
Among supramolecular systems studied, porphyrin-based
building blocks have been proved to be especially successful for
1,4–7,9–11
holding C60
.
We thus examined covalent 2,6-difunctionali-
zation of pyrazine with porphyrin to search C60 binding behavior
(Scheme 2). For the synthesis of 2,6-bis(2,20-dipyrromethyl)-3,5-
dimethylpyrazine (5) as a precursor of porphyrin, diethyl acetal 2
Figure 1. X-ray structure of Pd(II) complex 3.

284
Y. Eda et al. / Tetrahedron 65 (2009) 282–288
Ph
1. TFA
CH3CN
N
N
N
HO
HO
b
a
b
a
Ph
Ph
Ph
Ph
HN
HN
N
2. DDQ
2.0%
C60
N
M
N
M
N
M
N
M
c
c
Ph
5 +
N
N
N
N
N
c
c
N
N
N
N
N
N
N
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
7 : M = 2H
Zn(OAc)2
81%
CHCl3, MeOH
8 : M = Zn(II)
Scheme 2. Synthesis of bis(porphyrin)pyrazines (7 and 8) and their adducts with C60
.
porphyrin 7 but yielded polymer or polymer pigment. The prepa-
ration of porphyrins bearing one nitrogen heterocycle has been
achieved successfully by bimolecular condensation of a dipyrro-
methane and a dipyrromethane-dicarbinol.21 Therefore, dipyrro-
methane-dicarbinol 6 was prepared according to the literature and
was employed for the synthesis of 7 (Scheme 2). Structure of the
product 7 was determined by elemental analyses and 1H NMR data.
ESI-MS spectrum of a mixture of resultant 7 and C60 exhibited the
molecular ionic peak due to the adduct at 1902.4938 ([7þC60þH]þ),
which provided an evidence for the formation of a 1:1 complex of
compound 7 and C60. In order to confirm the coordination site of 7
to a Pd(II) ion, NMR spectra were measured after stepwise addition
of PdCl2(CH3CN)2 to 7 in CDCl3. As a result, the downfield shift
of 7 exhibited considerable upfield shifts and the other b-pyrrole
protons b, c exhibited relatively small degree of upfield shifts
(Fig. 2), while the protons of methyl groups attached to the pyrazine
ring exhibited downfield shift. It is known that meso-aryl protons
shift to a lower magnetic field, whereas
b-pyrrole protons and
pyrrole NH protons shift to higher magnetic fields as a result of
4,9
interaction with C60
.
These experimental results show that
b-pyrrole proton a is close to C60, while the other pyrrole protons
are far from C60. Therefore, the complexation of C60 was considered
to take place in the expected porphyrin–porphyrin cleft of 7
(Scheme 2). Such a conventional 1:1-type C60 binding was de-
termined by Job’s plot for change in 1H NMR chemical shift due to
b
-pyrrole proton a by binding of C60 with 7 (Fig. 3). Measurement of
NMR data due to the -pyrrole proton a was also suitable for the
determination of binding constant Ka, because the chemical shift
change was largest in all -pyrrole protons, thus, experimental er-
(from
d¼2.64 to 3.67 ppm) for the peak due to the methyl groups
b
attached to the pyrazine ring was observed, while the peak of the
NH protons was intact, indicating that a Pd(II) ion was not inserted
into the porphyrin ring. These observations suggested that 7 co-
ordinated to a Pd(II) ion at the 4-nitrogen atom whose neighboring
carbons were connected to less hindered methyl groups. For further
study about the functionality of the porphyrin-substituents, we
treated 7 with zinc acetate to yield porphyrin–zinc complex 8.
Association of 7 and C60 in solution was first investigated by a 1H
NMR titration method (270 MHz, 296.5 K, toluene-d8). With in-
creasing concentrations of C60 in a mixture of 7 and C60 in toluene-
b
ror of the determined Ka was smallest for the experiment. The as-
sociation constant Ka was determined to be 830ꢁ50 Mꢂ1 by
nonlinear curve-fitting of the NMR data with Ka and dmax treated as
parameters (Fig. 4). Moreover, in the 13C NMR spectra, the signal of
C60 shifted from 143.2 to 142.1 ppm due to the porphyrin ring
current effect. This tendency in C60- and porphyrin-induced
chemical shift change in 13C NMR spectra is consistent with those of
recent reports on porphyrin–C60 systems.4,5,10 Complexation of 8
d8, b-pyrrole proton a near pyrazine ring (Scheme 2) and NH proton
Figure 2. Partial 1H NMR spectra (270 MHz, 296.5 K, toluene-d8) of 7 with C60. As-
signments of peaks are shown in Scheme 2.
Figure 3. Job’s plot for 1H NMR chemical shift change of
binding of C60 with 7 (270 MHz, 296.5 K, toluene-d8); [7]tþ[C60]t¼2.14 mM.
b-pyrrole proton a due to the

Y. Eda et al. / Tetrahedron 65 (2009) 282–288
285
Figure 6. Fluorescence spectral changes for
7
(3.27
mM) by addition of C60
Figure 4. Change in the chemical shift of b-pyrrole proton a in 7 by addition of C60 in
toluene-d8 at 296.5 K. The solid line is the theoretical isotherm obtained by nonlinear
curve-fitting to the experimental data.
(0–6.20ꢃ10ꢂ4 M) in toluene at 298 K. Excitation wavelength¼550 nm at the second
Q-band of the UV–vis spectra.
with C60 also takes place in solution as evidenced by changes in the
chemical shifts of the protons of 8 by addition of C60. Such
a chemical shift change for 8 was smaller than that of 7. By a similar
NMR titration method for 7, the association constant Ka of 8 with
diundecyl-21H,23H-porphyrin-5-yl)benzene: DP).6 As for the
Q-absorption bands, 7 showed one major absorbance at 516 nm
and three minor bands at 550, 591, and 648 nm. Zinc complex 8 also
exhibited two split Soret absorption bands at 420 and 434 nm, but
small red shifts and decrease of the intensity of both bands were
observed (Fig. 5). Little changes in UV–vis spectra for 7 and 8 by
addition of C60 were observed. These results suggest that there is
little charge transfer interaction between C60 and 7 or 8.
C60 was determined to be 550ꢁ180 Mꢂ1
.
Ground state absorption spectrum of 7 displayed two split Soret
absorption bands at 417 and 430 nm due to the presence of two
porphyrin moieties (Fig. 5). Similar phenomenon of the splitting of
Soret absorption band had been reported for an analogous
bis(porphyrin)-aromatic compound, in which 3,5-dimethylpyr-
azine ring of 7 was replaced by a phenyl ring (1,3-bis(10,20-
Binding of C60 with 7 and 8 was then determined by fluores-
cence spectroscopy. The fluorescence spectra of 7 in toluene
(3.27 mM) exhibited maxima at 650 nm upon excitation at 550 nm.
It was found that the fluorescence of porphyrin upon excitation at
Q-absorption band diminished with C60 in toluene (Fig. 6). Stern–
Volmer plot for the fluorescence titration of 7 with C60 was found to
curve upward (Fig. 7). Such a nonlinear curvature suggests that the
excited state of porphyrin in 7 is quenched in both static and dy-
namic processes. In other words, the fluorescence of 7 in the
complex is interpreted as a result of partial quench by an intra-
11
ensemble electron transfer process involving a complexed C60
.
Therefore, the association constant Ka for 7 and C60 was determined
by the use of Benesi-Hildebrand equation. A straight line was
obtained by plotting F0/(F0ꢂF) versus 1/[C60], where F0 and F are the
fluorescence intensity at 650 nm of 7 without and with C60, re-
spectively, indicating the 1:1 stoichiometry of the complex of 7 and
C60 (Fig. 8). The association constant Ka was calculated from ratio
for the intercept versus slope to be 1680ꢁ120 Mꢂ1. This value was
broadly in agreement with that of its phenyl analogue DP,6
1975ꢁ105 Mꢂ1, which was determined by Stern–Volmer method.
Analogous fluorescence quenching was observed for 8 with C60
in toluene (Fig. 9). Since Stern–Volmer plot at 596 nm for 8 was
curved upward as in the case of 7, the association constant Ka for 8
and C60 was determined similarly by the Benesi–Hildebrand
equation to be 1500ꢁ120 Mꢂ1
.
The association constant of C60 with Zn–porphyrin complex 8
shows decreased binding energies relative to uncomplexed
porphyrin of 7. This result is consistent with the propensity in
metallo-porphyrins reported in the literature.7,10 The electron-rich
porphyrin nitrogen atoms are effective for the bond-stabilization by
Figure 5. UV–vis absorption spectra of 7 (2.62 mM) and 8 (4.51 mM) in toluene at
298 K.

286
Y. Eda et al. / Tetrahedron 65 (2009) 282–288
Figure 9. Fluorescence spectral changes of
8 (2.26 mM) by addition of C60 (0–
Figure 7. Stern–Volmer plot for the fluorescence quenching at 650 nm of 7 with C60 in
toluene.
6.20ꢃ10ꢂ4 M) in toluene at 298 K. Excitation wavelength¼551 nm at the first Q-band
of the UV–vis spectra.
the dispersive forces associated with
p–p interactions between
porphyrin and C60. The 1H NMR titration method offered much
smaller values for the association constant Ka than the fluorescence
spectral titration for 7 and 8. Since the binding constants obtained
by two different methods do not agree within the Estimated
Standard Deviations (ESD), the two methods may be measuring
different reactions occurred under different concentrations of the
host and C60. The discrepancies may be related to the changes in the
fluorescence shown in Figure 9. The relative intensity of the two
bands at 596 and 645 nm changes during the titration, though
exact reason is not clear as things stand now. Because of the low
solubility of C60 and its complex in toluene, we could not improve
the experimental conditions of NMR titration, in which the amount
of the toluene solution of guest C60 was too much for that of the
initial host solution. As a result, low values by NMR titration
method might not exhibit correct Ka values. The Ka value of 7 by
fluorescence spectral method is slightly smaller than the value for
DP.6 The difference in the Ka values is ascribed to the distinction of
experimental methods rather than the negative effect of pyrazine
nitrogen. This feature encourages the use of bis(porphyrin)pyrazine
as a synthon of functional supramolecular systems with metal ions.
3. Conclusion
In this study, pyrazine derivatives could be arranged as a good
host for C60 by introducing porphyrin rings at the 2,6-positions. On
the other hand, 3,5-dimethylpyrazine-2,6-diacetal was found to
coordinate Pd(II) ion at 4-nitrogen whose neighboring carbons
were connected to less hindered methyl groups. Accordingly, cir-
cumstances around the two nitrogens of pyrazine were controlled
by the introduction of proper substituents at proper positions. From
these results, starting 2,6-pyrazine diacetals have been proved to be
versatile in the synthesis of highly functionalized ligands. Such
disproportioned pyrazines should presage interesting supramo-
lecular systems.
4. Experimental
4.1. General comments
All melting points (mp) were measured by a YAMATO model
MP-21 melting point apparatus or a Yanaco model MP-500V micro
melting point apparatus and were uncorrected. UV–vis spectra
were measured by a SHIMADZU UV-2200 spectrophotometer.
Fluorescence spectra were measured by a HITACHI F-2500 spec-
trophotometer. 1H and 13C NMR spectra were recorded on a JEOL
GSX-270 FT NMR spectrometer with CDCl3 or C6D5CD3 as solvent
and TMS as internal standard (
d
¼0 ppm). Pyrazine ring is written in
the abbreviation Pz for the peak assignment. ESI-MS spectra were
determined on a Bruker Daltonics MicroTOF-ks1focus ESI-TOF-MS
spectrometer. Elemental analyses were performed by the Service
Figure 8. Benesi–Hildebrand plot for the quenching at 650 nm of 7 with C60 in
toluene.

Y. Eda et al. / Tetrahedron 65 (2009) 282–288
287
Center of the Elementary Analysis of Organic Compounds affiliated
to the Faculty of Sciences in Kyushu University. Silica gel (MERCK,
Silica gel 60) and alumina (MERCK, Aluminum oxide 90 active
neutral) were used for column chromatography. Silica gel (Wako,
Wakogel FC-40) was used for flash column chromatography and
medium pressure liquid column chromatography. Benzene and
pyrrole were freshly distilled at atmospheric pressure prior to use.
DDQ was recrystallized from chloroform. All other reagents were
obtained from commercial suppliers and used without further
purification. 2,6-Bis(diethoxymethyl)-3,5-dimethylpyrazine (2),18
trans-dichlorobis[2,6-bis(diethoxymethyl)-3,5-dimethylpyrazine]
palladium(II) (3),18 and dipyrromethane-dicarbinol 621 were
prepared according to the literature.
CH2Cl2–methanol to afford 7 (8.20 mg, 2.0%) as a purple solid. Mp
>300 ꢀC; 1H NMR (270 MHz, CDCl3):
¼ꢂ2.71 (br s, 4H, NH), 2.64 (s,
6H, Pz–CH3), 7.70–7.81 (m,18H, PhH), 8.07–8.21 (m, 8H, PhH), 8.30–
8.31 (m, 4H, PhH), 8.82 (s, 8H,
d
b
-pyrrole), 9.00 (d, J¼4.9 Hz, 4H,
-pyrrole); UV–vis (toluene):
b
-pyrrole), 9.12 ppm (d, J¼4.9 Hz, 4H,
b
lmax¼417 ( ¼454,000), 430 (416,000), 516 (36,200), 550 (14,300),
3
591 (10,600), 648 nm (6670 dm3 molꢂ1 cmꢂ1); ESI-MS obsd
1181.4754, calcd 1181.4762 ([MþH]þ, M¼C82H56N10). Anal. Calcd for
C82H56N10$1/2H2O: C, 82.74; H, 4.83; N, 11.77%. Found: C, 82.55; H,
4.70; N, 11.78%.
4.2.4. Zn(II) complex of 2,6-bis(10,15,20-triphenylporphyrin-5-yl)-
3,5-dimethylpyrazine (8)
A saturated solution of zinc acetate in methanol (0.1 ml) was
4.2. Materials
added to a solution of 7 (3.90 mg, 3.3 mmol) in chloroform (4 ml)
and the mixture was stirred overnight at room temperature. The
solvent was removed by evaporation and the residue was dissolved
in chloroform (4 ml). The solution was washed with water (3ꢃ4 ml)
and dried over anhydrous Na2SO4. After filtration, the filtrate was
concentrated in vacuo to afford 8 (3.50 mg, 81%) as a purple solid.
4.2.1. 2,6-Bis[di(4-benzyloxybenzyloxy)methyl]-3,5-dimethyl-
pyrazine (4)
A mixture of 2 (318 mg, 1.0 mmol), 4-benzyloxybenzyl alcohol
(946 mg, 4.4 mmol), and p-toluenesulfonic acid monohydrate
(9 mg, 0.05 mmol) in benzene (40 ml) was refluxed for 18 h, during
which period resultant ethanol was removed by a Dean–Stark trap
with molecular sieve MS 5 Å. The resulting mixture was neutralized
with triethylamine and the solvent was removed by evaporation.
The yellow residue was purified by medium pressure liquid column
chromatography on silica gel, eluting with ethyl acetate–n-hexane
mixture (1:4 v/v) to afford 4 (663 mg, 66%) as a colorless glassy
1H NMR (270 MHz, C6D5CD3):
18H, PhH), 8.09–8.28 (m, 12H, PhH), 8.92 (s, 8H,
J¼4.6 Hz, 4H, -pyrrole), 9.36 ppm (d, J¼4.9 Hz, 4H,
d
¼2.94 (s, 6H, Pz–CH3), 7.45–7.59 (m,
-pyrrole), 9.17 (d,
-pyrrole); UV–
b
b
b
vis (toluene): lmax¼420 ( ¼313,000), 434 (262,000), 551 (26,800),
3
590 nm (5990 dm3 molꢂ1 cmꢂ1).
4.3. X-ray crystallography of Pd(II) complex 3
solid. 1H NMR (270 MHz, CDCl3):
d
¼2.62 (s, 6H, Pz–CH3), 4.51 (d,
J¼11.5 Hz, 4H, OCH2–Ph–), 4.66 (d, J¼11.5 Hz, 4H, OCH2–Ph–), 5.01
A single crystal suitable for X-ray diffraction was obtained by
recrystallization from a mixture of dichloromethane and n-hexane.
The crystal was mounted at the end of a glass fiber. X-ray diffraction
experiments were performed at 293 K on a Rigaku RAXIS RAPID X-
(s, 8H, Ph–CH2O–), 5.63 (s, 2H, Pz–CH), 6.87–6.99 (m, 8H, PhH),
7.20–7.42 ppm (m, 28H, PhH); 13C NMR (67.8 MHz, CDCl3):
d¼158.5,
151.8, 146.1, 136.9, 129.9, 129.6, 128.6, 127.9, 127.4, 114.7, 102.7, 70.0,
69.1, 21.0 ppm. Anal. Calcd for C64H60N2O8: C, 78.03; H, 6.14; N,
2.84%. Found: C, 77.89; H, 6.18; N, 2.68%.
ray diffractometer with graphite monochromated Mo Ka radiation
(
l
¼0.71075 Å) and a rotating anode generator. Absorption correc-
tions were applied (absorption coefficient
m
¼0.617 mmꢂ1) to the
4.2.2. 2,6-Bis(2,20-dipyrromethyl)-3,5-dimethylpyrazine (5)
A mixture of 2 (1.25 g, 4.0 mmol) and pyrrole (7.13 ml) was
degassed with bubbling of nitrogen for 10 min. TFA (30 ml) was
raw intensity data. Of the 10,166 reflections, which were collected,
4736 were independent (Rint¼0.027); equivalent reflections were
merged. Data analysis was carried out with the CrystalstructureÔ
crystallographic software package. The structure was solved by
direct methods using SIR92 and refined by a full matrix least-
squares procedure on F2 with SHELXL-97. The refinement con-
injected and the solution was stirred at 50 ꢀC for 3 h until the
starting aldehyde disappeared. The solution was then diluted with
36 ml of chloroform and was washed with 0.1 M NaOH aq (1ꢃ8 ml,
1ꢃ10 ml, 1ꢃ12 ml) and water (3ꢃ12 ml). The organic layer was
dried over anhydrous sodium sulfate and filtered. The filtrate was
distilled under reduced pressure to remove remaining pyrrole. The
residue was purified by flash column chromatography on silica gel,
eluting with ethyl acetate–n-hexane–triethylamine mixture
(50:50:1 v/v), followed by crystallization from ethanol to afford 5
(639 mg, 40%) as light brown needles. Mp 180.2–181.0 ꢀC; 1H NMR
verged at R1¼0.046, wR2¼0.122, with I>2.0
s(I). The structure of
molecule was obtained by ORTEP3 included in the Crystal-
structureÔ software.
Crystal data. C32H56Cl2N4O8Pd, M¼802.11, triclinic, space group
P-1 (#2), a¼7.6227(8) Å, b¼11.4031(9) Å, c¼12.0607(10) Å,
a
¼
92.648(2)ꢀ,
b
¼90.933(4)ꢀ,
g
¼91.881(4)ꢀ, V¼1046.49(16) Å3, Z¼1,
calculated density Dcalcd¼1.273 g cmꢂ3
0.25ꢃ0.2 mm.
,
crystal size 0.25ꢃ
(270 MHz, CDCl3):
d
¼2.62 (s, 6H, Pz–CH3), 5.66 (s, 2H, meso-H),
5.86–5.88 (m, 4H), 6.12 (q, J¼2.2 Hz, 4H), 6.58–6.63 (m, 4H),
8.22 ppm (br s, 4H, NH). Anal. Calcd for C64H60N2O8: C, 72.70; H,
6.10; N, 21.20%. Found: C, 72.68; H, 6.13; N, 21.13%.
Full crystallographic data for the structure of 3 in this paper have
been deposited with the Cambridge Crystallographic Data Centre
under deposition number 691723. Copy of the data can be obtained,
free of charge, on application to CCDC,12 Union Road, Cambridge CB2
1EZ, UK (fax:þ44(0)1223336033ore-mail:deposit@ccdc.cam.ac.uk).
4.2.3. 2,6-Bis(10,15,20-triphenylporphyrin-5-yl)-3,5-dimethyl-
pyrazine (7)
To a mixture of 5 (142 mg, 0.36 mmol) and the dipyrromethane-
dicarbinol 6 (0.72 mmol) in acetonitrile (280 ml) was added TFA
(0.63 ml, 8.5 mmol) under protection from light. After 10 min, DDQ
(483 mg, 2.1 mmol) was added to the solution, which was stirred
for 1 h at room temperature. After addition of triethylamine
(1.17 ml, 8.4 mmol), the entire reaction mixture was filtered
through a pad of alumina (3ꢃ10 cm) and eluted with CH2Cl2 until
the elutant was no longer dark. The resulting porphyrin solution
was concentrated in vacuo and the residue was purified by column
chromatography on silica gel, eluting with CH2Cl2 and then CH2Cl2–
ethyl acetate mixture (99:1 v/v), followed by recrystallization from
4.4. 1H NMR titration experiment of 7 with C60 in toluene-d8:
general procedure
A solution of 7 in toluene-d8 (600 ml, 1.34 mM) was taken in an
NMR sample tube and 1H NMR spectrum was recorded after each
addition of 40, 80, 120, 160, 200, 240, 320, 400, 480, 560, 640, 800,
960, 1120, and 1280
Association constant Ka was evaluated from the change (Dd) in the
chemical shifts of the -pyrrole proton a of 7 applying a nonlinear
curve-fitting method using the equation shown below:
ml of a solution of C60 in toluene-d8 (2.14 mM).
b

288
Y. Eda et al. / Tetrahedron 65 (2009) 282–288
4. Kubo, Y.; Sugasaki, A.; Ikeda, M.; Sugiyasu, K.; Sonoda, K.; Ikeda, A.; Takeuchi,
M.; Shinkai, S. Org. Lett. 2002, 4, 925–928.
2
Dd
[
Ddmaxð½C60ꢄtD½7ꢄtD1=KaLðð½C60ꢄtD½7ꢄtD1=KaÞ L4½C60ꢄt
ꢃ½7ꢄtÞ Þ=ð2½7ꢄtÞ
5. Wu, Z.-Q.; Shao, X.-B.; Li, C.; Hou, J.-L.; Wang, K.; Jiang, X.-K.; Li, Z.-T. J. Am.
Chem. Soc. 2005, 127, 17460–17468.
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1=2
where [C60]t and [7]t are total concentrations of C60 and 7, respec-
tively; Ddmax is Dd at 100% complexation.
8. Balch, A. L.; Catalano, V. J.; Lee, J. W.; Olmstead, M. M. J. Am. Chem. Soc. 1992, 114,
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Acknowledgements
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This work was supported by the Grant-in-Aid for Scientific Re-
search (No. 20550125 and No. 17029049 on Priority Areas (417))
from the Ministry of Education, Culture, Sports, Science and Tech-
nology (MEXT) of Japan. We thank Professor Tsutomu Katsuki of
Kyushu University for his kind support throughout the measure-
ment of ESI-MS spectra and X-ray crystallography. We also thank
Dr. Kiichi Amimoto of Hiroshima University for his advices and
stimulating discussion for X-ray analysis.
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