Synthesis and highly selective bromination of azacalix[4]pyrimidine macrocycles

Article abstract of DOI:10.1021/jo902245q

(Chemical Equation Presented) A number of N-substituted azacalix[4]pyrimidines were synthesized by two methods. While straightforward condensation reaction between 4,6-dichloropyrimidine and 4,6-bis(alkylamino)- pyrimidines gave identically N-substituted

Full text of DOI:10.1021/jo902245q

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Synthesis and Highly Selective Bromination of Azacalix[4]pyrimidine
Macrocycles
Li-Xia Wang,† De-Xian Wang,*,† Zhi-Tang Huang,† and Mei-Xiang Wang*,†,‡
†Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and
Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China and
‡The Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education),
Department of Chemistry, Tsinghua University, Beijing 100084, China
mxwang@iccas.ac.cn; wangmx@mail.tsinghua.edu.cn
Received October 19, 2009
A number of N-substituted azacalix[4]pyrimidines were synthesized by two methods. While
straightforward condensation reaction between 4,6-dichloropyrimidine and 4,6-bis(alkylamino)-
pyrimidines gave identically N-substituted azacalix[4]pyrimidines in low yields, a general and
moderate-to-high yielding 1 þ 3 macrocyclic fragment coupling reaction afforded azacalix[4]-
pyrimidines that contained either the same or different N-substituents. Upon treatment with
N-bromosuccinimide (NBS) under controlled conditions, methylazacalix[4]pyrimidine was selec-
tively brominated at lower rim to produce mono-, di-, and tribrominated azacalix[4]pyrimidines in
good yields. While azacalix[4]pyrimidine derivatives adopted 1,3-alternate conformation in the solid
state, the synthesized macrocycles were fluxional in solution, and the interconversion of various
conformational structures was rapid relative to the NMR time scale.
Introduction
different conjugations between the bridging nitrogen atoms
and their neighboring pyridines, tetramethylazacalix[4]-
pyridine4b has been found to yield cavities of different sizes
to interact with different guest species.4b,c,e,h Besides, the
various electronic effects of the heteroatoms also regulate
the electron density of aromatic rings, producing the cavity
of varied electronic features. The tetramethylazacalix[4]-
pyridine, for instance, acts as a powerful polydentate ligand
As an emerging generation of macrocyclic host molecules
in supramolecular chemistry, heterocalixaromatics1 such as
nitrogen-,1-7 oxygen-,1,8-13 and sulfur-bridged14 calixaro-
matics have been attracting fast growing interest in recent
years. Because heteroatoms can adopt different electronic
configurations and form various degrees of conjugation with
their adjacent aromatic rings, the conformation and the
cavity structures of heterocalixaromatics are fine-tuned by
the bond lengths and bond angles of the bridging hetero-
atoms.1a For example, by the formation of marginally
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DOI: 10.1021/jo902245q
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Published on Web 12/31/2009
J. Org. Chem. 2010, 75, 741–747 741
2009 American Chemical Society

Wang et al.
JOCArticle
and hydrogen bond acceptor to recognize metal cations4c,h
and both aliphatic and aromatic diols,4e respectively.
Furthermore, the azacalixaromatics are readily functiona-
lized not only on the aromatic rings4i,j,10,12 but also on the
bridging positions,4d,j allowing the construction of polyfunc-
tionalized host molecules.
chemistry of novel and functional heterocalixaromatics, we
report herein the highly efficient synthesis and selective
bromination of azacalix[4]pyrimidines.
Pyrimidine is a useful ligand in coordination chemistry. It
has also been used recently by Katz11c and by Dehaen12 for
the construction of oxygen- and sulfur-bridged calix[2]arene-
[2]pyrimidines. To enhance the ability of pyrimidine nitrogen
atoms to coordinate metal ions and to form hydrogen bonding
with hydrogen bond donors, however, it is essential to intro-
duce electron-donating groups such as an amino group onto
4- and 6-positions. We therefore envisioned that azacalix-
[4]pyrimidines would provide a new type of polyfunctionalized
macrocyclic host molecules in supramolecular chemistry.
Compared to conventional calixarenes,15 one of the other
salient features of heterocalixaromatics is the incorporation
of various heteroaromatic rings. The combination of brid-
ging heteroatoms and heteroaromatic rings has resulted
indeed in a few useful macrocyclic hosts in supramolecular
chemistry. Due to the conjugative electron-donating effect of
the bridging nitrogen atoms, azacalix[n]pyridines, for in-
stance, are able to strongly and selectively complex with
metal ions4c,h and neutral molecular guests4e including full-
erenes,4a,b,f,g while oxacalix[2]arene[2]triazines provide a
unique electron-deficient cleft to accommodate a halide
anion.16 On the other hand, NH-bridged calix[2]arene-
[2]triazines10a and oxacalix[2]arene[2]pyrazine17 have been
found to undergo, respectively, hydrogen-bond-directed and
silver-coordination-driven molecular self-assembly. As a
continuation of our interest in exploring supramolecular
Results and Discussion
We initially attempted the synthesis of methylazacalix-
[4]pyrimidine 3a from a straightforward cyclic condensa-
tion reaction between 4,6-dichloropyrimidine 1 and 4,6-bis-
(methylamino)pyrimidine 2a (Table S1 in the Supporting
Information). In the presence of sodium hydride as a base,
the reaction of 1 and 2a in a mixture of 1,4-dioxane and
toluene solution proceeded smoothly to give desired
methylazacalix[4]pyrimidine 3a and a mixture of inseparable
oligomers. As summarized in Table 1, a large excess amount
of sodium hydride was necessary to effect the condensation
reaction (entries 1 and 2, Table 1), and use of too many folds
of base, however, did not further improve the chemical yield
of the macrocyclic product (entries 3 and 4, Table 1).
Although the formation of azacalix[4]pyrimidine did not
require reaction in a highly dilute concentration, a substrate
concentration around 0.067 M (2 mmol substrate in 30 mL of
solvent) appeared beneficial (entries 2, 5, and 6, Table 1). A
mixture of 1,4-dioxane and toluene (1:1) as the solvent
worked better than a single organic solvent such as 1,4-
dioxane and toluene (entries 2, 7, and 8, Table 1).
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Under the optimized conditions, macrocyclic condensa-
tion reaction between 1 and other 4,6-bis(amino)pyrimidines
TABLE 1. Synthesis of 3a from a Straightforward Macrocyclic Con-
densation Reaction between 1 and 2a
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(b) Katz, J. L.; Selby, K. J.; Conry, R. R. Org. Lett. 2005, 7, 3505. (c) Katz, J.
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Chem., Int. Ed. 2008, 47, 7485.
NaH
(equiv)
1,4-dioxane/
toluene (v/v)
substrate
concn (mM)
entry
3a (%)a
1
2
3
4
5
6
7
8
6.3
8.3
10.4
12.5
8.3
8.3
8.3
8.3
1:1
1:1
1:1
1:1
1:1
1:1
1:0
0:1
66.7
66.7
66.7
66.7
133.4
33.3
66.7
66.7
22
35
33
34
26
27
32
0b
aIsolated yield. bOnly dimer was obtained in 29%.
(17) Ma, M.-L.; Li, X.-Y.; Wen, K. J. Am. Chem. Soc. 2009, 131, 8338.
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SCHEME 1. Synthesis of Azacalix[4]pyrimidines 3 from Reac-
tion between 1 and 2
TABLE 2. Synthesisof 3a from a 3 þ 1 Macrocyclic Fragment Coupling
Reaction of 2a with 4a
base
(equiv)
THF/ substrate
toluene (v/v) concn (mM)
time
(h)
3a
(%)a
entry
1
2
3
4
5
6
7
8
9
10
11
12
13
K2CO3 (3.5)
KOBut (3.5)
NaOH (3.5)
NaOBut(3.5)
NaH (3.5)
NaH (7.5)
NaH (7.5)
NaH (7.5)
NaH (7.5)
NaH (7.5)
NaH (7.5)
NaH (7.5)
NaH (7.5)
1:0
1:0
1:0
1:0
1:0
1:0
1:0
1:1
1:5
1:5
1:8
5:1
0:1
66.7
66.7
66.7
66.7
66.7
66.7
5.6
5.6
5.6
2.8
5.6
24
24
24
24
6
6
6
6
6
6
8
6
72
0
0
5
SCHEME 2. Synthesis of Dichlorinated Linear Trimers 4
26
51
40
34
54
70
52
56
52
14
5.6
5.6
aIsolated yield.
SCHEME 3. Synthesis of Azacalix[4]pyrimidines 3 from Reac-
tion between 2 and 4
2b-e was performed. Unfortunately, the desired products
3b-d were obtained in very low yields (7-22%). As an
extreme case, reaction of 4,6-dichloropyrimidine 1 with 4,6-
bis(para-methoxybenzylamino)pyrimidine 2e did not give
any macrocyclic product 3e at all (Scheme 1).
In order to improve the synthetic efficiency and also to
establish the method for the preparation of differently N-
substituted azacalix[4]pyrimidine derivatives, we then stu-
died the 1 þ 3 macrocyclic cross-coupling reaction between
2a and dichlorinated linear trimer 4a. The latter reactant 4a
was conveniently prepared from the aromatic nucleophilic
substitution reaction of 2a with 4 equiv of 1 (Scheme 2). The
optimization results for the reaction of 2a with 4a are
tabulated in Table 2. It was obvious that bases such as
K2CO3, KOBut, NaOH, and NaOBut did not promote the
reaction effectively, giving either no product or a very low
yield of the product (entries 1-4, Table 2). Fortunately, the
use of NaH as a base afforded desired macrocyclic product
3a. The chemical yield of 3a was, however, governed strongly
by the solvent used. In tetrahydrofuran (THF), for example,
the chemical yield of 3a ranged from 34 to 51% (entries 5-7,
Table 2), while a very low yield (14%) of 3a was observed in
toluene (entry 13, Table 2). In a mixture of THF and toluene
with the volume ratio of 1:5 to 5:1, good yields (52-70%) of
the product were obtained (entries 8-12, Table 2). It was also
notable that the substrate concentration affected the effi-
ciency of the formation of the product (entries 6, 9, and 10,
Table 2). The highest chemical yield (70%) of 3a was
obtained when the substrate concentration of 5.6 mM was
employed (entry 9, Table 2).
derivatives 2a-e with other linear trimers 4b-e, which were
prepared from 2a-e and 4,6-dichloropyrimidine 1 (Scheme 2),
afforded the corresponding azacalix[4]pyrimidine derivatives
3b-g (Scheme 3). Almost all reactions gave moderate to good
chemical yields. In the case of N-PMB-substituted azacalix-
[4]pyrimidine 3e, a slightly low chemical yield (22%) was
obtained owing to the cleavage of para-methoxybenzyl group(s)
from the reactants and product under the reaction conditions.
This was evidenced by the isolation of para-methoxybenzalde-
hyde from the reaction mixture. It was worth noting that the
1 þ 3 fragment coupling approach, which was different from the
direct condensation reaction of 1 with 2 (Scheme 1), allowed us
to prepare azacalix[4]pyrimidines bearing different substituents
on the nitrogen bridges. For example, azacalix[4]pyrimidines 3f
and 3g, which contained methyl and allyl, and methyl and PMB,
Applying the optimized conditions for the synthesis of 3a
(entry 9, Table 2), the reaction of 4,6-bis(amino)pyrimidine
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Wang et al.
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FIGURE 1. X-ray crystal structure of 3b: (a) top view and (b) side view. All hydrogen atoms and the disorders of C(18), C(19), and C(20) are
˚
omitted for clarity. Selected bond lengths (A): C(2)-N(3) 1.409, N(3)-C(5) 1.382, C(7)-N(6) 1.405, N(6)-C(9) 1.379, C-
˚
(11)-N(9) 1.413, N(9)-C(13) 1.377, C(4)-N(12) 1.369, N(12)-C(15) 1.407. Selected interatomic distances (A): C(1)-C(10) 8.288; C(6)-C-
(14) 8.191.
FIGURE 2. X-ray crystal structure of 3e: (a) top view and (b) side view. All hydrogen atoms and the disorders of 3e are omitted for clarity.
˚
Selected bond lengths (A): C(1)-N(1) 1.415, N(1)-C(3) 1.374, C(1A)-N(1A) 1.415, N(1A)-C(3C) 1.374, C(1C)-N(1C) 1.415, N(1C)-C(3B)
˚
1.374, C(1B)-N(1B) 1.415, N(1B)-C(3) 1.374. Selected interatomic distances (A): C(4)-C(4C) 7.931; C(4A)-C(4B) 7.931.
respectively, were synthesized in good yields from the reaction of
2a with 4c and with 4e (Scheme 3).
fluxional in solution, and the rates of interconversion of
various conformational structures might be very rapid re-
lative to the NMR time scale. The high conformational
mobility of these macrocycles was most likely due to the
lack of steric hindrance and intramolecular hydrogen bonds,
both being key factors in stabilizing conformational struc-
tures of conventional calix[n]arenes in solution. The stability
gained from the conjugation effect of the linking nitrogen
atoms with their adjacent aromatic rings seems insufficient
to prevent the rotation of aromatic rings around the meta-
meta axes or through the annulus.
To functionalize the resulting azacalix[4]pyrimidine de-
rivatives, bromination reaction was investigated prelimi-
narily. Being substituted by two amino groups, the
pyrimidine ring in macrocycle 3a was ready to undergo
regiospecific electrophilic bromination reaction at the
5-position, giving the lower-rim brominated products
(Scheme 4). To our delight, the degree of bromination
was nicely and conveniently controlled by varying the ratio
of halogenating reagents NBS and by controlling the
reaction temperature (Table 3). For example, the use of
1.2 equiv of NBS in chloroform at room temperature gave
monobrominated product 5 in a moderate yield (entry 1,
Table 3). The use of acetic acid as the solvent instead of
The structures of all products were established on the basis
of spectroscopic data, microananlysis (see Supporting
Information), and single-crystal X-ray diffraction analysis
(Supporting Information, Table S1). The conformational
structures of azacalix[4]pyrimidines warrant addressing. As
illustrated in Figures 1 and 2, azacalix[4]pyrimidines 3b and
3e adopted twisted 1,3-alternate conformation in the solid
state. Judging from the bond lengths and angles, we found
that each of the bridging nitrogen atoms was sp2 electron-
configured and formed conjugation with one of its neighbor-
ing pyrimidine rings. Due to the different steric effect bet-
ween n-butyl and para-methoxybenzyl, the cavity of 3b and
3e was varied. The upper-rim distance of 3b was in the range
˚
˚
of 8.191 to 8.288 A, while that of 3e was 7.931 A (see captions
of Figures 1 and 2). It was also interesting to note that two
molecules of 3e self-assembled into a spherical dimeric
structure, which further allied into channels in the solid state
(Figure 3).
It was important to note that all macrocyclic products 3
gave very simple 1H and 13C NMR spectra (see Supporting
Information), indicating symmetrical structure in solution.
It was most likely that azacalix[4]pyrimidines 3 are very
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Wang et al.
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FIGURE 3. Spherical dimeric structure of 3e along the c-axis (a) and b-axis (b), and a packing diagram of 3e along the c-axis (c).
SCHEME 4. Selective Bromination of Azacalix[4]pyrimidine 3a
TABLE 3. Selective Bromination of Azacalix[4]pyrimidine 3a
entry NBS (equiv) solvent
T (°C) time (h) Product (%)a
CHCl3
CH3CO2H rt
product in 85% yield in 4 h (entry 4, Table 3). Further
increase of the amount of NBS used and of the reac-
tion temperature gave rise to the formation of tribromi-
nated product. This was exemplified by the isolation of
tribrominated product 7 as the major product when 3a was
refluxed with 6 equiv of NBS in acetic acid for 20 h (entry 5,
Table 3). It should be noted that no fully brominated
product was observed under all forcing conditions tried.
The selective formation of 1,3-dibrominated product 6 and
no formation of fully brominated product implied that
the steric factor may play a decisive role in the selec-
tive electrophilic bromination reaction. As revealed by
single-crystal X-ray diffraction analysis (Table S1, Sup-
porting Information), while dibromo-substituted azacalix-
[4]pyrimidine product 6 adopted a highly symmetrical
1,3-alternate conformation (Figure 4), slightly twisted
1
2
3
4
5
1.2
1.2
2.5
2.5
6.0
rt
70
24
12
12
4
5 (41)
5 (85)b
6 (84)
6 (85)c
7 (58)d
CHCl3
CH3CO2H 70
CH3CO2H reflux
20
aIsolated yield. bA trace amount of 6 was observed. cA trace amount
of 7 was observed. dA trace amount of 5 and 6 was observed.
chloroform facilitated the reaction and improved the che-
mical yield of 5 to 85% (entry 2, Table 3). When 2.5 equiv of
NBS was employed and the reaction carried out in chloro-
form at 70 °C, dibrominated product 6 was isolated as the
major product in 84% (entry 3, Table 3). Change of solvent
from chloroform to acetic acid resulted in the rapid for-
mation of 1,3-dibrominated product 6 as nearly the sole
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Wang et al.
JOCArticle
FIGURE 4. X-ray crystal structure of 6: (a) top view and (b) side view. All hydrogen atoms are omitted for clarity.
FIGURE 5. X-ray crystal structure of 7: (a) top view and (b) side view. All hydrogen atoms are omitted for clarity.
1,3-alternate conformational structures were observed for
product 7 in the solid state (Figure 5).
room temperature and a small amount of water was slowly
added. The solvents were removed under reduced pressure, and
the residue was dissolved in CH2Cl2 (200 mL). The organic
solution was washed with brine (3 ꢀ 50 mL) and dried over with
anhydrous MgSO4. After removal of solvent, the residue was
chromatographed on a silica gel column (100-200) with a
mixture of petroleum ether and ethyl acetate as the mobile
phase to give pure products 3a-d.
General Procedure for the Synthesis of Azacalix[4]pyrimidines
3a-g from 2a-e and 4a-e by 1 þ 3 Fragment Coupling Reaction.
To a solution of 2a-e (1 mmol) in dry THF (30 mL) at room
temperature was added NaH (180 mg, 7.5 mmol) slowly, and
the mixture was heated to reflux. After 12 h, the dry toluene
(150 mL) and 4a-e (1 mmol) were added to the mixture, and the
reaction mixture was refluxed for another several hours,
then cooled to room temperature and a small amount of water
was slowly added. The solvents were removed under reduced
pressure, and the residue was dissolved in CH2Cl2 (200 mL).
The organic solution was washed with brine (3 ꢀ 50 mL) and
dried over with anhydrous MgSO4. After removal of solvent, the
residue was chromatographed on a silica gel column (100-200)
with a mixture of petroleum ether and ethyl acetate as the mobile
phase to give pure products 3a-g.
Conclusion
In summary, we have developed two synthetic approaches to
azacalix[4]pyrimidines. While macrocyclic condensation reac-
tion between 4,6-dichloropyrimidine and 4,6-bis(alkylamino)-
pyrimidines generally suffered from low yields, an efficient 1þ3
macrocyclic fragment coupling reaction afforded azacalix-
[4]pyrimidines in the yield ranging from 22 to 70%. Being
substituted by two amino groups, the pyrimidine ring in the
macrocycle was ready to undergo regiospecific electrophilic
bromination reaction at the 5-position. Simply by controlling
the reaction temperature and the ratio of NBS, selective low-rim
bromination reaction was accomplished, producing mono-, di-,
and tribrominated products in good yields. The easy availability
of azacalix[4]pyrimidines would render these novel multi-nitro-
gen-containing macrocycles useful in molecular recognition and
supramolecular assembly. The selectively brominated azacalix-
[4]pyrimidine derivatives, on the other hand, would provide
unique building blocks for the construction of high-level molec-
ular architectures by taking advantage of flourishing metal-
catalyzed cross-coupling reaction protocols.
Selective Monobromination of 3a To Prepare Compound 5. A
mixture of 3a (107 mg, 0.25 mmol) and NBS (53.4 mg, 0.3 mmol)
in HOAc (or CHCl3) (10 mL) at room temperature was stirred.
After 12 h, the solvent was removed under reduced pressure, and
the residue was dissolved in dichloromethane (50 mL). The
organic solution was washed with aqueous KOH solution
(2 M) and dried over with anhydrous MgSO4. After removal of
solvent, the residue was chromatographed on a silica gel column
(200-300) with a mixture of dichloromethane and acetone as the
mobile phase to give pure 5 (108 mg, 85%) as a white solid. If the
solvent was CHCl3, the reaction rate was very slow. After 24 h, the
pure product was 52 mg, 41%: mp >300 °C; IR (KBr) ν 1589,
Experimental Section
General Procedure for the Synthesis of Azacalix[4]pyrimidines
3a-d from a Straightforward Macrocyclic Condensation Reac-
tion between 1 and 2a-d. To a solution of 2a-d (2 mmol) in dry
solvents (30 mL) at room temperature was added NaH (200 mg,
8.3 mmol) slowly, and the mixture was heated to reflux. After
12 h, 1 (2 mmol) was added to the mixture slowly and the
reaction mixture was refluxed for another 24 h, then cooled to
1526, 1481, 1219 cm-1 1H NMR (300 MHz, CDCl3) δ 8.81
;
746 J. Org. Chem. Vol. 75, No. 3, 2010

Wang et al.
JOCArticle
(s, 1H), 8.76 (d, J = 0.9 Hz, 1H), 8.72 (d, J = 0.8 Hz, 2H), 7.04 (d,
J = 0.9 Hz, 1H), 6.19 (d, J = 0.9 Hz, 2H), 3.63 (s, 6H), 3.56 (s,
6H); 13C NMR (75 MHz, CDCl3) δ 162.64, 162.59, 162.1, 161.8,
160.1, 159.3, 158.1, 107.6, 101.5, 96.1, 37.1, 35.9; MS (EI) m/z (%)
508 [M þ 2]þ (38), 506 [M]þ (40), 427 [M - Br]þ (100). Anal.
Calcd for C20H19N12Br: C, 47.35; H, 3.77; N, 33.13. Found: C,
46.95; H, 3.56; N, 32.83.
Selective Tribromination of 3a To Prepare Compound 7. A
mixture of 3a (107 mg, 0.25 mmol) and NBS (267 mg, 1.5
mmol) in HOAc (10 mL) was refluxed for 20 h. After workup
as that for 6, product 7 was obtained (96 mg, 58%) as a white
solid: mp >300 °C; IR (KBr) ν 1587, 1556, 1510, 1473, 1426
cm-1 1H NMR (300 MHz, CDCl3) δ 8.77 (s, 1H), 8.75
;
(s, 2H), 8.71 (s, 1H), 5.86 (d, J = 0.8 Hz, 1H), 3.67 (s, 6H),
3.60 (s, 6H); 13C NMR (75 MHz, CDCl3) δ 164.3, 163.1,
162.48, 162.45, 158.6, 157.1, 106.4, 100.4, 96.5, 39.1, 38.1;
MS (EI) m/z (%) 668 [M þ 6]þ (6), 666 [M þ 4]þ (20), 664 [M þ
2]þ (21), 662 [M]þ (7), 587 [M - Br þ 4]þ (37), 585 [M - Br þ
2]þ (100), 583 [M - Br]þ (43), 505 [M - 2Br þ 2]þ (8), 503 [M -
Selective Dibromination of 3a To Prepare Compound 6. A
mixture of 3a (107 mg, 0.25 mmol) and NBS (111 mg, 0.625
mmol) in HOAc (or CHCl3) (10 mL) was heated to 70 °C. After
4 h, the solvent was removed under reduced pressure, and the
residue was dissolved in dichloromethane (50 mL). The organic
solution was washed with aqueous KOH solution (2 M) and
dried over with anhydrous MgSO4. After removal of solvent, the
residue was chromatographed on a silica gel column (200-300)
with a mixture of petroleum ether and ethyl acetate as the mobile
phase to give pure 6 (125 mg, 85%) as a white solid. If the solvent
was CHCl3, the reaction rate was much slower. After 12 h, the
pure product (123 mg, 84%) was obtained: mp 287-288 °C; IR
(KBr) ν 1588, 1546, 1475, 1427, 1117 cm-1; 1H NMR (300 MHz,
CDCl3) δ 8.83 (s, 2H), 8.70 (s, 2H), 5.25 (s, 2H), 3.57 (s, 12H);
13C NMR (75 MHz, CDCl3) δ 163.3, 162.0, 158.9, 158.4, 111.5,
93.3, 36.9; MS (EI) m/z (%) 588 [M þ 4]þ (44), 586 [M þ 2]þ
(100), 584 [M]þ (46), 507 [M - Br þ 2]þ (43), 505 [M - Br]þ (45),
425 [M - 2Br]þ (32). Anal. Calcd for C20H18N12Br2: C, 40.97;
H, 3.09; N, 28.67. Found: C, 40.86; H, 3.11; N, 28.21.
2Br]þ (5), 425 [M - 3Br]þ (12). Anal. Calcd for C20H19
-
N12Br3: C, 36.11; H, 2.58; N, 25.27. Found: C, 36.26; H,
2.65; N, 24.98.
Acknowledgment. We thank the Natural Science Foundation
of China, Ministry of Science and Technology (2007CB808005),
and Chinese Academy of Sciences for financial support.
Supporting Information Available: Experimental details,
full characterization of products, 1H and 13C NMR of the
products, X-ray molecular structure of 3b, 3e, 6, and 7 (CIFs).
This material is available free of charge via the Internet at http://
pubs.acs.org.
J. Org. Chem. Vol. 75, No. 3, 2010 747