Role of anions in the synthesis of cyclobutane derivatives via [2 + 2] cycloaddition reaction in the solid state and their isomerization in solution

Article abstract of DOI:10.1021/jo201268p

Trans-3-(4′-Pyridyl)acrylic acid (4-PA) is inert to photodimerization reaction both in solution and solid state. It is made photoreactive by forming salts with various acids. The anions of these salts play a key role in directing the packing of 4-PAH⁺ in the solid state. The anions CF ₃CO₂^-, Cl^-, ClO₄ ^-, and BF₄^- direct the parallel alignments of 4-PAH⁺ in head-totail (HT) fashion and lead to the formation of HT-photodimer. On the other hand, bivalent anion SO₄^2- directs parallel alignment of 4-PAH⁺ in head-to-head (HH) fashion and lead to the formation of HH-photodimer. The details of the anioncontrolled stereoselective syntheses of these two cyclobutane derivatives are presented. Interestingly, both cyclobutane compounds undergo isomerization from rctt-form to rctc-form in solution catalyzed by acid. (Figure presented)

Full text of DOI:10.1021/jo201268p

ARTICLE
pubs.acs.org/joc
Role of Anions in the Synthesis of Cyclobutane Derivatives via [2 + 2]
Cycloaddition Reaction in the Solid State and Their Isomerization in
Solution
Goutam Kumar Kole, Geok Kheng Tan, and Jagadese J. Vittal*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
S Supporting Information
b
ABSTRACT: trans-3-(40-Pyridyl)acrylic acid (4-PA) is inert to
photodimerization reaction both in solution and solid state. It is
made photoreactive by forming salts with various acids. The
anions of these salts play a key role in directing the packing ꢀof
4-PAH+ in the solid state. The anions CF3CO2ꢀ, Clꢀ, ClO4
,
and BF4ꢀ direct the parallel alignments of 4-PAH+ in head-to-
tail (HT) fashion and lead to the formation of HT-photodimer.
2ꢀ
On the other hand, bivalent anion SO4 directs parallel
alignment of 4-PAH+ in head-to-head (HH) fashion and lead
to the formation of HH-photodimer. The details of the anion-
controlled stereoselective syntheses of these two cyclobutane
derivatives are presented. Interestingly, both cyclobutane com-
pounds undergo isomerization from rctt-form to rctc-form in
solution catalyzed by acid.
’ INTRODUCTION
packing that dictates the stereochemistry of the product is very
important.
Designing new molecules with structural diversity and their
stereoselective syntheses are of major importance to organic
chemists.1 Synthesis of complex molecules often requires multi-
ple steps that consume gallons of hazardous solvents to purify the
desired product from a mixture of byproducts. “Green” synthetic
routes have become popular to synthetic chemists, as the use of
hazardous solvents is minimized.2 Similarly, solid-state synthesis
is thus the best method in green chemistry, as it consumes no
solvents and it often leads selectively to only one isomer, i.e., one
product among many possibilities. Moreover, it enables the pro-
ducts to adopt geometries inaccessible in the solution phase and,
as a result, the molecules in the solid state can react, with perfect
control of stereochemistry.3
The synthesis of strained cyclobutane derivatives with desired
functionalities by a [2 + 2] cycloaddition route in the solid state is
still an active area of research because these derivatives can serve
as novel ligands for synthesizing coordination polymers (CPs)/
metalꢀorganic frameworks (MOFs).4 The desired functionality
for cyclobutane can be assigned by judicious choice of the photo-
reactive olefin monomers and controlling their packing in the
solid state. In the classical example, trans-cinnamic acid is packed
in head-to-tail (HT) fashion in its R-polymorph and results in R-
truxillic acid under UV light. On the other hand, β-truxinic acid
can be obtained from another polymorph β-trans-cinnamic acid
where it is aligned in head-to-head (HH) fashion, whereas another
polymorph γ-form is photostable.5 Therefore, the molecular
MacGillivray et al. have explored the design of template-
controlled synthesis of cyclobutane derivatives from photostable
monomers.6 Recently, molecular salts4a,7 have proven to be pro-
mising candidates just like cocrystals6b,c,8 and coordination com-
pounds9 for the same purpose. Yamada7fꢀh et al. have explored
the role of cationꢀπ interactions on the solid-state photodimer-
ization, yet the role of the anions in such interactions was un-
explored. In our previous report7a we have shown how an anion
can influence the packing of trans-3-(40-pyridinium)acrylic acid
(4-PAH+) and therefore the stereoselective syntheses of two
cyclobutane derivatives, viz., 2,4-bis(40-pyridyl)cyclobutane-1,
3-dicarboxylic acid (HT-BPCD) and 3,4-bis(40-pyridyl)cyclo-
butane-1,2-dicarboxylic acid (HH-BPCD), were documented.
In this article we shall discuss the effect of anions in a series of
molecular salts of 4-PA in the context of [2 + 2] cycloaddition
reaction and the acid-catalyzed isomerization of these two
cyclobutane derivatives.
’ RESULT AND DISCUSSION
Crystal Structure and Photoreactivity of [4-PAH]Cl H2O
Salt (1). The hydrated molecular salt 1 crystallizes in P1 space
3
group. Various kinds of OꢀH
Cl, NꢀH
O hydrogen
3 3 3
3 3 3
Received: June 20, 2011
Published: July 21, 2011
r
2011 American Chemical Society
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|

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Figure 1. Ladder-like arrangement of discrete parallel pairs of 4-PAH+ cations aligned in HT-fashion by [(Clꢀ)2(H2O)2] aggregates in 1. In this salt,
4-PAH+ cation pairs are aligned parallel in HT-fashion with a distance between the CdC bond pair of 3.80 Å. Water molecules play an important role to
align 4-PAH+ cations in parallel as shown in Figure 1. Both the ground powder and single crystals of this salt undergo quantitative photodimerization
upon UV irradiation for 50 h as followed by 1H NMR spectroscopy.
Figure 2. Discrete parallel pairs of 4-PAH+ cations in HT-fashion in 2.
bonds play constructive roles to stabilize the solid-state archi-
tecture. The compound incorporates a water molecule into the
lattice to minimize repulsive interactions between the Clꢀ ions.
The [(Clꢀ)2(H2O)2] aggregates nicely align the pairs of 4-PAH+
cations to form a hydrogen bonded tape-like structure propagat-
ing along the x-axis. Such [(Clꢀ)2(H2O)2] aggregates commonly
occur in the crystal structures of various inorganic coordination
complexes10,11 and organic materials7f,12,13 where they are found
to aggregate both in planar10,12 and nonplanar11,13 forms. Unlike
Figure 3. The quasi-zwitterionic arrangement of 4-PAs in 3.
in the CF3CO2ꢀ salt,7a the 4-PAH+ cations are aligned parallel as
discrete pairs, and this leads to the arrangement of linear poly-
meric H-bonded chains which run parallel to each other approxi-
mately on the bc-plane.
Crystal Structure and Photoreactivity of [4-PAH]ClO4 Salt
(2). This anhydrous salt 2 crystallizes in P1 space group. The
charge-assisted hydrogen bonding of types NꢀH OꢀCl and
3 3 3
OꢀH OꢀCl plays a constructive role in stabilizing the solid-
3 3 3
ꢀ
state structure of the salt. Each ClO4 anion is involved in
H-bonding with one NꢀH group (charge assisted H-bonding) of
the pyridyl moiety and with one OꢀH of the carboxylic acid
group. The bonding in the salt further involves the electrostatic
interaction with two NꢀH groups. Each pyridyl NꢀH moiety
Figure 4. The quasi-zwitterionic 4-PAs are oriented in slipped stacked
infinite parallel arrangement in HH-fashion.
H-bond to one ClO4ꢀ anion and further electrostatically inter-
acts with two ClO4ꢀ anions. All these interactions result in the
overall structure of this three-dimensional salt. The alignment of
the 4-PAH+ cations are again found to be parallel in HT-fashion,
H-bonding, as shown in Figure 3, play an important role to con-
struct the solid-state architecture. The quasi-zwitterionic 4-PAs
are arranged on a two-dimensional plane, forming strong inter-
and the distance of separation between the isolated CdC bond
pairs is 3.91 Å (Figure 2). Both the ground powder and single
crystals undergo 100% photodimerization under UV light for
30 h as observed with 1H NMR spectroscopy.
molecular NꢀH O hydrogen bonding, and the PF6ꢀ anions
3 3 3
sit at the middle, forming weaker CꢀH F bonds, as shown in
3 3 3
Figure 3. The axial F atoms of PF6ꢀ anions have similar weaker
Crystal Structure and Photoreactivity of [4-PA][4-PAH]PF6
Salt (3). The anhydrous salt of compostion C16H15F6N2O4P (3)
was crystallized in monoclinic P21/n space group. The composi-
tion implies that there are two 4-PAH+ cations for one PF6ꢀ anion,
and this means that one of the 4-PA molecules is a zwitterion.
Unlike the other salts, it is a 2:1 salt, comprising one 4-PAH+, one
4-PA zwitterion, and one PF6ꢀ anion. Each 4-PA molecule has a
cationic part, viz., the protonated pyridyl group, but the proton is
disordered between 4-PAH+ and 4-PA and it is difficult to
distinguish CO2ꢀ from CO2H. Therefore, they have been treated
as quasi-zwitterions here. Various types of weak and strong
CꢀH F interaction with 4-PAs of nearby layers.
3 3 3
A detailed scrutiny of the solid-state structure of this salt
reveals that the quasi-zwitterionic 4-PAs are oriented in slipped
stacked infinite parallel arrangement in HH-fashion. The dis-
tance of separation is 5.10 Å (Figure 4), which is the same as the
length of the a-axis. The salt was photostable when irradiated
under UV light for 60 h, because the distance between the two
nearest CdC bonds was too long to undergo photodimerization.
This long distance can be rationalized from the cationꢀcation
repulsion which is not properly balanced by the monovalent
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Figure 5. Crystal structure of rctt-HH-BPCD and the mode of H-bonding.
Table 1. List of All the Salts and Their Photoreactivity
acids used to form salts
photoreactivity
nature of alignment
details of alignment
head-to-tail, 3.80 Å
% of conversion
HCl (1)
photoreactive
photoreactive
photostable
parallel; discrete pairs
parallel; discrete pairs
infinitely parallel
100
HClO4 (2)
HPF6 (3)
head-to-tail, 3.909 Å
head-to-head, 5.096 Å
head-to-tail, 3.766 Å
head-to-head, 3.672ꢀ3.754 Å
ꢀ
head-to-tail, <4.2 Å
ꢀ
100
0
CF3CO2H (4)
H2SO4 (5)
HNO3 (6)
HBF4 (7)
photoreactive
photoreactive
photostable
infinitely parallel
1007a
100 upon grinding7a
infinitely parallel
predicted to be nonparallel
predicted to be parallel
predicted to be nonparallel
0
photoreactive
photostable
100
0
H3PO4 (8)
PF6ꢀ anion, unlike that in the (SO42ꢀ)(HSO4ꢀ) salt7a where the
Scheme 1. Structures of the rctc-Isomers after Isomerization
cationꢀcation repulsion is stabilized by the bivalent sulfate anion.
Description of the Molecular Salts of 4-PA with CF3CO2H,
H2SO4, HNO3, HBF4, and H3PO4. The solid-state structures and
photoreactivity of salts of 4-PA with CF3CO2H (4) and H2SO4
(5) were discussed previously in relation to the stereoselec-
tive syntheses of the two photodimers HT- and HH-BPCD.7a
The quantitative conversion of 5 upon grinding in air with an
associated structural transformation due to the absorption of extra
water molecules was also documented by us.7a The structural
descriptions of these two salts are briefly presented in Supporting
Information. 4-PA was further reacted with HNO3, HBF4, and
H3PO4, and the resulting compounds were characterized and
tested for their photoreactivity. The composition of nitrate,
tetrafluoroborate, and biphosphate salts were determined from
elemental analysis and thermogravimetric analysis as C8H8N2O5
(6), C8H8NO2BF4 (7), and C8H10NO6P (8), respectively, in the
absence of suitable single crystals. Upon irradiation under UV
light, 7 underwent quantitative photodimerization in the solid
state in HT-fashion, whereas 6 and 8 did not result in any
photodimerization. From this observation, we rationalize that in
7, 4-PAH+ cations are stacked parallel in HT-fashion in the solid
state where the distance of separation is 3.5ꢀ4.2 Å. The parallel
orientation can either be infinite parallel or discrete pairs. The
orientation of 4-PAH+ cations in 6 and 8 can be expected to be
nonparallel or the distance is beyond Schmidt’s distance limit.
Crystal Structure of HH-BPCD. The crystal structure of HT-
BPCD was described previously.7a Now we shall describe the
structure of HH-BPCD. By neutralizing the ground powder of 5,
diffraction quality single crystals were obtained after 2ꢀ3 days
upon evaporating the aqueous solution slowly. The compound
of composition C16H14N2O4 (9) crystallizes in P1 space group,
and the stereochemistry has been proven to be rctt as expected.
series of salts of 4-PA, we shall now compare these salts in terms
of structural diversity and photoreactivity. We have seen the HT-
parallel orientation of 4-PAH+ cations in 1, 2, 4, and 7 and HH-
parallel orientation in 5 which underwent quantitative photo-
dimerization. Among them, discrete parallel pairs are found in 1
and 2, whereas in 4 the arrangement is infinite. Again, in 1, the
discrete pairs arrange along ladder-like one-dimensional chains
aligned by [(Clꢀ)2(H2O)2] clusters; on the contrary, in 2 it occurs
randomly. The differences in mode of packing in 1 and 2 can be
rationalized from the sizes and shapes of a single point charge
Clꢀ anion with the tetrahedral ClO4ꢀ anion. In 3, the 4-PAH+
cations are found to stack parallel in HH-fashion but with a
The solid-state structure is stabilized by OꢀH N hydrogen
3 3 3
bonding between the carboxylic acid and pyridyl groups to form a
one-dimensional-chain-like structure as shown in Figure 5.
Comparative Study of the Molecular Salts of 4-PA. Having
discussed the solid-state structure and photoreactivity of this
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Figure 6. The isomerization of rctt-HT-BPCD to rctc-HT-BPCD: two doublet of doublet (dd) peaks for the cyclobutane protons converted to three
triplet peaks (1:2:1). The 1H NMR spectrum as evidence for the isolation of rctc-HH-BPCD is shown in Supporting Information.
longer distance of 5.096 Å. On the other hand, in 5, the 4-PAH+
cations stack infinitely parallel with partial crisscross orientation
in HH-fashion with distances amenable to photodimerization.
Single crystals of this salt underwent 66% photodimerization; the
same salt showed quantitative conversion upon grinding in a mortar,
which allowed the absorption of an extra molecule of water,
triggering the structural rearrangement. The presence of bivalent
sulfate anions in 5 can stabilize the possible cationꢀcation repulsion,
but the monovalent PF6ꢀ anion is inadequate to balance the cationꢀ
cation repulsion in 3; therefore, the distance is longer in 3. H3PO4
was chosen to achieve HH-orientation of 4-PAH+ by the pre-
sence of bivalent HPO42ꢀ anion, but it has resulted in a 1:1 salt
with monovalent H2PO4ꢀ; therefore, HH-parallel orientation of
4-PAH+ has not been obtained in 8.
With respect to stereoselective synthesis, the two cyclobutane
isomers, viz., HH- and HT-BPCD, were synthesized in quantitative
yield. The corresponding selective preorganization of 4-PAH+
cations before UV irradiation was achieved just by changing the
acid for salt formation, namely by tuning the anion present in the
salts. Now, one question may arise: whether these alignments are
predictable. From Table 1, only the HH-dimer was produced from
the sulfateꢀbisulfate salt, and the other monovalent anions produced
either HT-dimer or photostable salts. Hence, one can assume that
the HT-parallel orientation is quite common and thermodynami-
cally morestable becauseofthecationꢀπ interaction.7fꢀh,14 On the
other hand, the HH-parallel orientation is quite unstable because of
the cationꢀcation repulsion that needs to be stabilized by some bi-
or trivalent anions. The observed longer distance between 4-PAH+
cations in 3 shows some evidence of the cationꢀcation repulsion
where PF6ꢀ anion is inadequate to balance the repulsive force.
Isomerization of HT- and HH-BPCD. Isomerization of cyclo-
butane derivatives containing pyridyl groups was reported pre-
viously by Horner and H€unig.15 Recently, we also studied the
acid-catalyzed isomerization of rctt-tpcb [tpcb =1,2,3,4-tetrakis-
(40-pyridyl)cyclobutane] to rtct-tpcb quantitatively via rcct-tpcb
where rtct isomer is the most stable.4c,16 1,2,3,4-Tetrakis(40-carbox-
yphenyl)cyclobutane, another cyclobutane derivative reported by
us,4a did not show any such isomerization, neither at acidic nor
at alkaline pH, which shows that free pyridyl bases are respon-
sible for such isomerization. The coordination polymer of tpcb,
[Zn2(CF3CO2)2(μ-CH3CO2)2(rctt-tpcb)]9d did not result in
any such isomerization in the presence of CF3CO2H, which
reflects that the mechanism of this isomerization involves pro-
tonation of the uncoordinated pyridyl groups at acidic pH.
HH- and HT-BPCD, the two new cyclobutane derivatives, were
interestingly found to undergo isomerization in acidic solution,
which was faster upon heating. If we consider the geometry of
rctt-HT- and HH-BPCD molecules, flipping of either pyridyl
group can result in isomerization. If both pyridyl groups flip to
the other side of the cyclobutane ring for HH-BPCD, then the
resulting isomer would be rccc which is not stable. Therefore, the
structures of the resulting isomers are expected be rctc as shown
in Scheme 1. We can see from the structures that the rctc-isomer
product is less stable than the rctt-isomers, as three substituents
are on the same side of the cyclobutane ring, resulting in steric
hindrance. On the other hand, the charge repulsion between the
adjacent pyridinium cations in rctt-HH-BPCD may be the driving
force for the isomerization, but this isomerization was not quantita-
tive and about 90% conversion was observed in 1H NMR spectral
studies. Although, to our disappointment, we could not grow bigger
single crystals for X-ray diffraction data collection, the isolation of
rctc-HT-BPCD was confirmed by the 1H NMR spectrum of the
ꢀ
recrystallized salt with CF3CO2 (Figure 6). Two doublet of
doublet (dd) peaks for rctt-cyclobutane have converted to three
triplets for rctc-cyclobutane.
’ CONCLUSION
The photoreactivity of molecular salts of 4-PA with several
strong acids have been discussed in the context of crystal packing,
and the anion-controlled stereoselective syntheses of two cyclo-
butane derivatives, viz., HT-and HH-BPCD, are described. We
anticipate that bivalent anions such as sulfate have a crucial role
to stabilize the cationꢀcation repulsion in the HH-orientation.
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1
On the other hand, the HT-orientation is stable due to possible
cationꢀπ interaction between two olefins forming the pair. The
acid-catalyzed isomerizations of these two cyclobutane deriva-
tives from rctt- to rctc- forms are also addressed. We again have
proved that the utilization of weak but directional supramolecular
interactions can be a powerful tool for covalent synthesis. All the
functional cyclobutane derivatives are potential ligands for mak-
ing cocrystals and CPs/MOFs. Further studies will be published
in a separate contribution.
[4-PA][4-PAH]PF6 (3). H NMR (300 MHz, DMSO-d6, 298 K):
δH = 8.73 (d, 4H), 7.90 (d, 4H), 7.62 (d, 2H), 6.89 (d, 2H). 13C NMR
(75 MHz, DMSO-d6, 298 K): δC = 166.7, 147.5, 144.8, 140.2, 126.0,
123.2. FT-IR (KBr, cmꢀ1): 3112, 2787, 1705, 1633, 1607, 1496, 1395,
1348, 1312, 1244, 1214, 1091, 991, 961, 826, 745, 686, 558, 524. Analysis
found (%): C, 43.54; H, 3.40; N, 6.50. C16H15F6N2O4P requires: C,
43.26; H, 3.40; N, 6.31.
Crystal data for 3 at 223 K: C16H15F6N2O4P, M = 444.27, monoclinic,
space group P21/n, a = 5.0962(3) Å, b= 15.5577(10) Å, c= 11.4752(8) Å, β=
95.944(1)°, V = 904.9(1) Å3, Z = 2, Dcalcd = 1.630 g cmꢀ3, μ = 0.239 mmꢀ1
,
3
R1 = 0.0464, wR2 = 0.1221 and Goof = 1.047 [for 1908 data I > 2σ(I)].
’ EXPERIMENTAL SECTION
1
[4-PAH]NO3 (6). H NMR (300 MHz, D2O, 298 K): δH = 8.72
(d, 2H), 8.14 (d, 2H), 7.70 (d, 1H), 6.92 (d, 1H). 13C NMR (75 MHz,
DMSO-d6, 298 K): δC = 166.5, 150.6, 142.9, 138.5, 129.7, 125.3. FT-IR
(KBr, cmꢀ1): 3112, 2773, 2557, 1704, 1632, 1585, 1496, 1381, 1289,
1232, 1186, 1091, 994, 960, 852, 809, 740, 681, 579, 528, 498. Analysis
found (%): C, 45.10; H, 3.89; N, 13.21. C8H8N2O5 requires C, 45.29; H,
3.80; N, 13.20. No solvent loss was observed in TGA experiment.
Decomposition temperature, ∼210 °C.
General Methods. All materials were purchased from various
commercial sources and were used without further purification. All
solvents used were of analytical reagent grade. All the crystals were
grown from water. All the strong acids were added dropwise to react with
4-PA (acting as a base) to form molecular salts in water. The resulting
clear aqueous solutions were allowed to evaporate slowly to produce
single crystals of the corresponding molecular salts within a few days.
The yields for such crystallizations are in the range of 90ꢀ95%. Neutral
cyclobutane derivatives were separated from their salts by neutralizing
with dilute aqueous NaOH solution from aqueous media.
1
[4-PAH]BF4 (7). H NMR (300 MHz, D2O, 298 K): δH = 8.70
(d, 2H), 8.13 (d, 2H), 7.62 (d, 1H), 6.93 (d, 1H). 13C NMR (75 MHz,
DMSO-d6, 298 K): δC = 166.6, 150.7, 142.9, 138.6, 129.8, 125.4. FT-IR
(KBr, cmꢀ1): 3174, 3114, 2774, 2558, 1704, 1632, 1585, 1496, 1395,
1290, 1233, 1187, 1142, 1086, 993, 940, 851, 808, 740, 682, 627, 578,
528, 497. Analysis found (%): C, 40.06; H, 3.34; N, 5.73. C8H8NO2BF4
requires C, 40.55; H, 3.40; N, 5.91. No solvent loss was observed in the
TGA experiment. Decomposition temperature, ∼205 °C.
The NMR spectra were recorded with a 300 MHz FT-NMR spectro-
meter with TMS as internal reference. ESI-MS spectra were recorded in
negative ion mode from aqueous solution (<50 μg mLꢀ1) by the syringe-
3
pump method. For thermogravimetric analyses (TGA), the samples
were heated at a constant rate of 5 °C minꢀ1 from room temperature and
the atmosphere was maintained with a continuous flow of nitrogen.
UV Irradiation. The UV irradiation experiments were conducted by
using a radiation of wavelength 350 nm and an approximate intensity of
1.75 mW cmꢀ2. About 20 mg of sample (finely powdered sample or
single crystals) was packed gently between two pyrex glass slides, and
the UV irradiation was completed by flipping the packed glass slide pairs
for each sample at half of their irradiation time interval to ensure uniform
irradiation.
[4-PAH]H2PO4 (8). 1H NMR (300 MHz, D2O, 298 K): δH = 8.72
(d, 2H), 8.15 (d, 2H), 7.69 (d, 1H), 6.93 (d, 1H). 13C NMR (75 MHz,
DMSO-d6, 298 K): δC = 167.1, 149.0, 143.4, 140.9, 125.3, 122.98. FT-IR
(KBr, cmꢀ1): 3112, 2787, 2114, 1706, 1634, 1607, 1497, 1396, 1348, 1312,
1245, 1215, 1092, 991, 962, 827, 745, 687, 558, 524. After UV irradiation,
1H NMR (300 MHz, DMSO-d6, 298 K): δH = 8.91 (d, 4H), 8.07 (d,
4H), 4.64 (dd, 2H), 4.19 (dd, 2H). Analysis found (%): C, 38.46; H,
3.89; N, 6.02. C8H10NO6P requires C, 38.88; H, 4.08; N, 5.67. No
solvent loss was observed in the TGA experiment. Decomposition
temperature, ∼230 °C.
1
[4-PAH]Cl H2O (1). H NMR (300 MHz, DMSO-d6, 298 K):
3
δH = 8.90 (d, 2H), 8.24 (d, 2H), 7.73 (d, 1H), 7.07(d, 1H). 13C NMR
(75 MHz, DMSO-d6, 298 K): δC = 166.3, 150.1, 142.5, 138.4, 129.4,
125.1. FT-IR (KBr, cmꢀ1): 3174, 3114, 2773, 2557, 1704, 1632, 1584,
1494, 1396, 1290, 1233, 1187, 1064, 994, 941,852, 808, 740, 682, 528,
497. After UV irradiation, 1H NMR (300 MHz, DMSO-d6, 298 K): δH =
8.90 (d, 4H), 8.05 (d, 4H), 4.64 (dd, 2H), 4.19 (dd, 2H). Analysis found
(%): C, 47.28; H, 4.64; N, 6.76. C8H10ClNO3 requires: C, 47.19; H,
4.95; N, 6.88. The calculated and observed water losses in the TGA
experiment are 8.8% and 8.7%. Decomposition temperature, ∼180 °C.
Crystal data for 1 at 223 K: C8H10ClNO3, M = 203.62, triclinic, space
group P1, a = 6.8763(4) Å, b = 8.0599(5) Å, c = 9.6919(5) Å, R =
69.981(1)°, β = 73.382(1)°, γ = 68.996(1)°, V = 462.92(5) Å 3, Z = 2,
Dcalcd = 1.461 g.cmꢀ3, μ = 0.386 mmꢀ1, R1 = 0.0330, wR 2 = 0.0928, and
Goof = 1.145 [for 2062 data I > 2σ( I)].
HH-BPCD (9). 1H NMR (300 MHz, DMSO-d6, 298 K): δH = 12.84
(s, 2H), 8.28 (d, 4H), 7.09 (d, 4H), 4.28 (d, 2H), 3.90 (d, 2H). 13C NMR
(75 MHz, DMSO-d6, 298 K): δC = 173.4, 149.0, 147.6, 123.1, 43.5, 41.5.
FT-IR (KBr, cmꢀ1): 3055, 2918, 2483, 1730, 1609, 1556, 1422, 1368,
1330, 1275, 1195, 1122, 1069, 1012, 964, 923, 903, 852, 838, 817, 752,
707, 650, 625, 555, 522, 442. Analysis found (%): C, 64.40; H, 4.77; N,
9.28. C16H14N2O4 requires C, 64.42; H, 4.73; N, 9.39.
Crystal data for 9 at 223 K: C16H14N2O4, M = 298.29, monoclinic, space
group Cc, a = 14.723(2) Å, b = 9.6367(14) Å, c = 10.5485(16) Å, β =
118.305(3)°, V = 1317.7(3) Å3, Z = 4, Dcalcd = 1.504 g cmꢀ3, μ = 0.110
3
mmꢀ1,R1 = 0.1405, wR2 = 0.3440, and Goof = 1.666 [for 1939 data I>2σ(I)].
’ ASSOCIATED CONTENT
[4-PAH]ClO4 (2). 1H NMR (300 MHz, DMSO-d6, 298 K):
δH = 8.90 (d, 2H), 8.25 (d, 2H), 7.73 (d, 1H), 7.07(d, 1H). 13C NMR
(75 MHz, DMSO-d6, 298 K): δC = 166.4, 150.5, 142.8, 138.4, 129.6,
125.1. FT-IR (KBr, cmꢀ1): 3114, 2774, 1704, 1632, 1585, 1496, 1395,
1290, 1233, 1187, 1142, 1086, 993, 940, 851, 808, 740, 682, 627, 578,
528, 497. After UV irradiation, 1H NMR (300 MHz, DMSO-d6, 298 K):
δH = 8.91 (d, 4H), 8.07 (d, 4H), 4.65 (dd, 2H), 4.19 (dd, 2H). Analysis
found (%): C, 38.09; H, 3.32; N, 5.53. C8H8ClNO6 requires: C, 38.50; H,
3.23; N, 5.61. TGA was not carried out for this perchlorate sample.
Crystal data for 2 at 223 K: C8H8ClNO6, M = 249.60, triclinic, space
group P1, a = 5.0430(3) Å, b = 8.4238(6) Å, c = 12.5243(9) Å, R =
105.625(1)°, β = 100.485(1)°, γ = 95.560(1)°, V = 497.81(6) Å 3, Z = 2,
Dcalcd = 1.665 g.cmꢀ3, μ = 0.398 mmꢀ1, R1 = 0.0339, wR 2 = 0.0936, and
Goof = 1.065 [for 2188 data I > 2σ( I)].
S
Supporting Information. Additional structural diagrams,
b
1H and 13C NMR spectra, and TGA and other characterizations.
This material is available free of charge via the Internet at http://
pubs.acs.org. CCDC 829609-829612 contains the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: chmjjv@nus.edu.sg. Fax: +65 6779 1691. Tel: +65 6516
2975.
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dx.doi.org/10.1021/jo201268p |J. Org. Chem. 2011, 76, 7860–7865

The Journal of Organic Chemistry
ARTICLE
’ ACKNOWLEDGMENT
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We thank the Ministry of Education for their financial support
through grant no. R 143-000-439-112. We thank Prof. Lip Lin
Koh and Ms. Yimain Hong for the X-ray data collection.
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’ NOTE ADDED AFTER ASAP PUBLICATION
Several corrections were made in the Results and Discussion
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