Templating photodimerization of trans-cinnamic acid esters with a water-soluble Pd nanocage

Article abstract of DOI:10.1021/jo0617722

A water-soluble octahedral Pd nanocage acting as a reaction vessel templates the photodimerization of substituted trans-cinnamic acid methyl esters in water. Irradiation of the host-guest complexes of trans-cinnamic acid methyl esters with the Pd nanocage resulted in selective formation of a syn head-head dimer in addition to the corresponding cis isomer. These results suggest that the guest molecules are preoriented in a selective fashion with the hydrophilic ester group facing water and the hydrophobic aryl group tucked within the cavity of the host. Such an orientation occurs at the hydrophobic-hydrophilic interface between the nanocage exterior and interior. Weak intermolecular C-H-π and π-π interactions between the host and the guest(s) are likely to be responsible for the lack of mobility of the reactant olefins during their short excited-state lifetime.

Full text of DOI:10.1021/jo0617722

Templating Photodimerization of trans-Cinnamic Acid Esters with a
Water-Soluble Pd Nanocage
S. Karthikeyan and V. Ramamurthy*
Department of Chemistry, UniVersity of Miami, Coral Gables, Florida 33124
murthy1@miami.edu
ReceiVed August 28, 2006
A water-soluble octahedral Pd nanocage acting as a reaction vessel templates the photodimerization of
substituted trans-cinnamic acid methyl esters in water. Irradiation of the host-guest complexes of trans-
cinnamic acid methyl esters with the Pd nanocage resulted in selective formation of a syn head-head
dimer in addition to the corresponding cis isomer. These results suggest that the guest molecules are
preoriented in a selective fashion with the hydrophilic ester group facing water and the hydrophobic aryl
group tucked within the cavity of the host. Such an orientation occurs at the hydrophobic-hydrophilic
interface between the nanocage exterior and interior. Weak intermolecular C-H-π and π-π interactions
between the host and the guest(s) are likely to be responsible for the lack of mobility of the reactant
olefins during their short excited-state lifetime.
Introduction
CdC bonds could in principle give rise to more than 10 isomers;
therefore, a strategy to favor dimerization and to obtain a single
isomer in solution is highly desirable.4 In this context, we first
attempted to template the photodimerization of 3-methyl- and
4-methyl-trans-cinnamic acids through the use of a water-soluble
octahedral Pd nanocage host. In spite of complexation of these
two olefins with the host as evidenced by 1H NMR data, none
of these dimerized in aqueous solution (see Supporting Informa-
tion). At this stage, we are not sure why trans-cinnamic acids
During the past two decades, there has been continued interest
in controlling excited-state processes through the use of well-
defined organized media. The media have included crystals,
solid host-guest complexes, liquid crystals, polymers, micelles,
zeolites, clays, and silica.1 Quite recently, the impact of green
chemistry has led chemists to explore water-soluble supramol-
ecules based on hydrogen bonding, hydrophilic-hydrophobic
interactions, and van der Waals forces as hosts to conduct
chemical reactions.2 Our group is currently investigating water-
soluble hosts such as dendrimers, polymers, calixarenes, cu-
curbiturils, cavitands, and Pd-nanocage as media to conduct
photochemical reactions.3 In the present paper, we describe our
attempts in controlling the stereochemistry of the cyclobutanes
formed during the photodimerization of trans-cinnamic acid
esters. These compounds belonging to a class of R,â-unsaturated
carbonyl compounds when irradiated are known to undergo two
types of reactions, namely, (a) geometric isomerization (uni-
molecular process) and (b) dimerization (bimolecular process).
Geometric isomerization, however, is the main reaction in
isotropic solutions. The photodimerization reactions involving
(1) Ramamurthy, V. Photochemistry in Constrained and Organized
Media; VCH Publishers: New York, 1991.
(2) (a) Whitesides, G. M.; Simanek, E. E.; Mathias, J. P.; Seto, C. T.;
Chin, D. N.; Mammen, M.; Gordon, D. M. Acc. Chem. Res. 1995, 28, 37-
44. (b) Conn, M. M.; Rebek, J., Jr. Chem. ReV. 1997, 97, 1647-1668. (c)
Fujita, M.; Umemoto, K.; Yoshizawa, M.; Fujita, N.; Kusukawa, T.; Biradha,
K. Chem. Commun. 2001, 509-519. (d) Lee, J. W.; Samal, S.; Selvapalam,
N.; Kim, H.-J.; Kim, K. Acc. Chem. Res. 2003, 36, 621-630. (e) Fiedler,
D.; Leung, D. H.; Bergman, R. G.; Raymond, K. N. Acc. Chem. Res. 2005,
38, 349-358. (f) Vriezema, D. M.; Comellas Aragones, M.; Elemans,
J. A. A. W.; Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M. Chem.
ReV. 2005, 105, 1445-1490. (g) Purse, B. W.; Rebek, J., Jr. Proc. Natl.
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10.1021/jo0617722 CCC: $37.00 © 2007 American Chemical Society
Published on Web 12/16/2006
452
J. Org. Chem. 2007, 72, 452-458

Photodimerization of trans-Cinnamic Acid Esters
FIGURE 2. Absorption spectra of 1a, 1c, 1f, and 1k and Pd nanocage
in water.
have found the Pd nanocage capable of encapsulating within
its hydrophobic cavity varied dimensional organic molecules
(e.g., cis-stilbene, adamantane carboxylate, acenaphthalene, 1,3,-
5-tri-tert-butyl benzene, etc.).6,8 The hydrophobic cavity of the
Pd nanocage has been used for templating highly regioselective
homo and hetero [2 + 2] photochemical dimerization of
acenaphthalenes.7 The host has also been used for alkane
oxidation, catalysis, bimolecular recognition, etc.8 We have
recently reported the Pd nanocage templated photodimerization
of coumarin derivatives that suggested that the cavity of the Pd
nanocage is able to hold two coumarin molecules within its
cavity in a fashion that on irradiation leads to the formation of
a syn head-head dimer with >90% selectivity.9 These results
and the challenging possibility of controlling two reaction
pathways (geometric isomerization and dimerization) prompted
us to undertake the present study on the photodimerization of
trans-cinnamic acid esters. Moreover, trans-cinnamic acid esters
absorb at longer wavelengths as compared to the Pd nanocage.
The absorption spectra of the cage and the four water-soluble
cinnamic acid derivatives are shown in Figure 2. The absorption
characteristics of these guest molecules suggest that direct
excitation of the cage-included trans-cinnamic acid ester deriva-
FIGURE 1. Structure of M6L4 octahedral host (Pd nanocage).
in spite of complexing with the nanocage did not dimerize. We
believe that the water-soluble nature of the trans-cinnamic acids
results in the guest spending considerable amount of time in
water leading to geometric isomerization rather than dimeriza-
tion. Currently, we do not have experimental evidence to
confirm this proposition. To overcome this problem, we chose
trans-cinnamic acid methyl esters for the templated dimerization
reactions.
The Pd nanocage (the host compound used in this study)
originally synthesized by Fujita and co-workers is a self-
assembly of six metal ions (M) and four tridendate ligands (L)
resulting in a water-soluble M6L4 octahedral cage (Figure 1).5
This octahedral cage is comprised of four triangular tris(4-
pyridyl)1,3,5-triazine panels connected to each other by the
metal ions (Pd+2) at the corners of the triangle (Figure 1). Four
of the eight faces are open, and the other four faces present
alternatively are covered by the triangular panel. Encapsulation
of guest molecules within the hydrophobic cavity will occur
through the empty faces of the octahedron. Adjacent metal ions
of the cage are 13.1 Å apart, while the distance between metal
ions opposite to each other is 18.7 Å. Eliminating the very
narrow corners where the ligands are coordinated to the metal
ions, we visualize the Pd nanocage to have a free space ∼15 Å
× 10 Å. The detailed investigations of Fujita and co-workers
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(5) Fujita, M.; Oguro, D.; Miyazawa, M.; Oka, H.; Yamaguchi, K.;
Ogura, K. Nature 1995, 378, 469-471.
(6) Kusukawa, T.; Fujita, M. J. Am. Chem. Soc. 2002, 124, 13576-
13582.
(7) (a) Yoshizawa, M.; Takeyama, Y.; Kusukawa, T.; Fujita, M. Angew.
Chem., Int. Ed. 2002, 41, 1347-1349. (b) Yoshizawa, M.; Takeyama, Y.;
Okano, T.; Fujita, M. J. Am. Chem. Soc. 2003, 125, 3243-3247. (c)
Takaoka, K.; Kawano, M.; Ozeki, T.; Fujita, M. Chem. Commun. 2006,
15, 1625-1627.
(3) (a) Kaanumalle, L. S.; Nithyanandhan, J.; Pattabiraman, M.; Jayara-
man, N.; Ramamurthy, V. J. Am. Chem. Soc. 2004, 126, 8999-9006. (b)
Kaanumalle, L. S.; Gibb, C. L. D.; Gibb, B. C.; Ramamurthy, V. J. Am.
Chem. Soc. 2004, 126, 14366-14367. (c) Arumugam, S.; Vutukuri, D. R.;
Thayumanavan, S.; Ramamurthy, V. J. Am. Chem. Soc. 2005, 127, 13200-
13206. (d) Kaanumalle, L. S.; Ramesh, R.; Murthy Maddipatla, V. S. N.;
Nithyanandhan, J.; Jayaraman, N.; Ramamurthy, V. J. Org. Chem. 2005,
70, 5062-5069. (e) Kaanumalle, L. S.; Gibb, C. L. D.; Gibb, B. C.;
Ramamurthy, V. J. Am. Chem. Soc. 2005, 127, 3674-3675. (f) Karthikeyan,
S.; Ramamurthy, V. Tetrahedron Lett. 2005, 46, 4495-4498. (g) Kaliappan,
R.; Kaanumalle, L. S.; Ramamurthy, V. Chem. Commun. 2005, 4056-
4058. (h) Pattbiraman, M.; Natarajan, A.; Kaliappan, R.; Mague, J. T.;
Ramamurthy, V. Chem. Commun. 2005, 4542-4544. (i) Pattabiraman, M.;
Natarajan, A.; Kaanumalle, L. S.; Ramamurthy, V. Org. Lett. 2005, 7, 529-
532. (j) Pattabiraman, M.; Kaanumalle, L. S.; Natarajan, A.; Ramamurthy,
V. Langmuir 2006, 22, 7605-7609. (k) Kaliappan, R.; Kaanumalle, L. S.;
Natarajan, A.; Ramamurthy, V. Photochem. Photobiol. Sci. 2006, 5, 925-
930.
(8) (a) Yoshizawa, M.; Miyagi, S.; Kawano, M.; Ishiguro, K.; Fujita,
M. J. Am. Chem. Soc. 2004, 126, 9172-9173. (b) Yoshizawa, M.; Tamura,
M.; Fujita, M. J. Am. Chem. Soc. 2004, 126, 6846-6847. (c) Tashiro, S.;
Tominaga, M.; Kawano, M.; Therrien, B.; Ozeki, T.; Fujita, M. J. Am. Chem.
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M.; Ohhara, T.; Tanaka, I.; Kurihara, K.; Nimura, N.; Fujita, M. J. Am.
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J. Org. Chem, Vol. 72, No. 2, 2007 453

Karthikeyan and Ramamurthy
SCHEME 1. Photodimerization of trans-Cinnamic Acid Methyl Esters (1a-1l)
tives is possible. The other derivatives used in this study are
expected to have similar absorption spectrum. Also, based on
the absorption spectra, we believe that energy transfer from the
excited singlet state of the host to the guest is likely. Thus, under
our conditions, the trans-cinnamic acid ester derivatives are
placed in reactive S1 either by direct excitation or via an energy
transfer pathway.
of these molecules (1a-1l) are provided in Scheme 1. As
illustrated in Scheme 1, these molecules on irradiation could
undergo unimolecular geometric isomerization or dimerization
leading to the formation of a cis isomer (2a-2l), syn head-
head (3a-3l), syn head-tail (4a-4l), anti head-head (5a-
5l), and anti head-tail (6a-6l) dimers. Despite the presence
of the two reaction pathways, unimolecular geometric isomer-
ization is the major pathway during solution-phase irradiation
of these molecules.4 The 1:2 host/guest (H/G) complexes of
these trans-cinnamic acid esters (1a-1l) with the Pd nanocage
were prepared by stirring required amounts of the two in 0.6
mL of D2O at 60 °C. The exact amounts of host and guests
(1a-1l) used for complexation are provided in the Experimental
Procedures. The clear solution obtained indicative of the guest
molecules’ encapsulation within the hydrophobic cavity of the
Pd nanocage was then filtered, and the complexation was
Results and Discussion
We have investigated the photodimerization of 12 trans-
cinnamic acid esters (Scheme 1). Apart from controlling the
photochemistry of these compounds, our interest in understand-
ing how the substitutions in the cinnamic acid could influence
its encapsulation behavior within the Pd nanocage prompted
us to undertake studies with various derivatives. Methoxy
(OCH3), ethoxy (OCH2CH3), methyl (CH3), and carbomethoxy
(COOCH3) groups were substituted at the ortho, meta, and para
positions of the aromatic ring, and the alkyl groups of the ester
were always maintained as methyl. The photochemical reactions
1
confirmed by H- NMR.
The 1H- NMR spectra of the host-guest complexes recorded
to confirm encapsulation of 1a-1l with the Pd nanocage are
provided in the Supporting Information. The changes in chemi-
454 J. Org. Chem., Vol. 72, No. 2, 2007

Photodimerization of trans-Cinnamic Acid Esters
SCHEME 2. Changes in Chemical Shift (δsolvent - δPd-nanocage) for 1a-1l upon Complexation with Pd Nanocagea
a In all cases, a H/G Ratio of 1:2 was used. The solvent used for comparison is either D2O or CDCl3. The guest molecules 1b, 1g, 1h-1j, and 1l are
insoluble in water; hence, the changes in chemical shift reported here for these guest molecules is with respect to CDCl3.
cal shift due to encapsulation of these guest molecules within
the Pd nanocage are provided in Scheme 2. Because of the
anisotropic ring current effect of the aromatic panels of the Pd
nanocage, the olefin and the aromatic protons of the guest
molecules are shifted upfield by 1.8-2.5 ppm, while the methyl
protons of the ester group -COOCH3 attached to the olefin
showed a marginal shift (∼0.0-0.3 ppm) on encapsulation. The
observed changes in chemical shift suggest that the ester group
faces the water through the empty face of the octahedron,
whereas aromatic and alkene parts of the guest molecule are
cocooned within the hydrophobic cavity of the Pd nanocage.
The possible structure of the host-guest complex based on the
observed upfield chemical shift for the aromatic and alkene
protons is provided in Figure 3. For the sake of clarity, only
one of the two olefins is included in the figure.
The chemical shifts of the aromatic substituents were also
dependent on the type (methoxy (OCH3), ethoxy (OCH2CH3),
methyl (CH3), or carbomethoxy (COOCH3) and position (ortho,
meta, or para) on the aromatic ring. For example, in the case of
methoxy substituted trans-cinnamic acid methyl esters (1e, 1f,
and 1g), para substitution results in 0.5 ppm, while ortho or
meta substitution results in a 1.5 ppm upfield chemical shift
FIGURE 3. Possible orientation of (a) ortho-methoxy-trans-cinnamic-
(Scheme 2) of the methyl protons of the methoxy group. To
obtain insight on what governs the chemical shift of the protons
of the substituents, we calculated the molecular dimensions of
ortho, meta, and para methoxy trans-cinnamic acids methyl
esters (1e-1g) using Chem 3-D ultra, which suggested that the
para derivative (1g) was longer than the ortho and meta
derivatives (1e and 1f); 1g was 12.7 Å and 1e and 1f were ∼11.0
Å, respectively.10 Probably due to the molecular length differ-
acid methyl ester (1e), meta-methoxy-trans-cinnamic-acid methyl ester
(1f), and (c) para-methoxy-trans-cinnamic-acid methyl ester (1g) within
Pd nanocage. For clarity, only one guest molecule is shown in the figure.
ences and the orientation of the substituents, the protons of
methoxy group in the ortho or meta position become situated
close to one of the aromatic panels of the host, interacting
strongly with the aromatic groups of the Pd nanocage, whereas
J. Org. Chem, Vol. 72, No. 2, 2007 455

Karthikeyan and Ramamurthy
To gain additional insight on the structure of the host-guest
complex and to predict the stereochemistry of the dimer that
could be obtained from the Pd nanocage templated photodimer-
ization of trans-cinnamic acid esters, modeling studies on the
host-guest complex were carried out. Our modeling studies
are based on NMR complexation results as well as the possible
weak intermolecular interactions of the guest molecule encap-
sulated within the host.11 The four possible arrangements of two
guest molecules (1g) within the host leading to the formation
of the syn head-head (3g), syn head-tail (4g), anti head-head
(5g), and anti head-tail (6g) dimers are provided in Figure
5a-d. Of these four arrangements, due to the reasons outlined
below, the ones leading to the formation of the syn head-head
(Figure 5a) and syn head-tail (Figure 5b) dimers will be
preferred within the Pd nanocage more than their anti counter-
parts. In these cases, the hydrophobic aromatic and alkene parts
of the guest reside within the octahedral cavity of the Pd
nanocage, while the hydrophilic ester group is exposed to the
aqueous exterior through the empty face of the octahedron.
Stabilization by hydrophobic effects of such an arrangement of
guest molecules within the Pd nanocage will be augmented by
weak intermolecular forces such as C-H-π and π-π interac-
tions between the host and the guest(s).11 The observed upfield
shift of the protons of aromatic and olefin chromophores (1.8-
2.5 ppm) is consistent with this model. On the contrary, the
formation of anti dimers would require the hydrophobically
unfavorable exposure of one of the aromatic groups to water
(Figure 5c,d). Alternatively, the two reactive CdC bonds would
be far away to interact in the excited state. Moreover, due to
the positioning of the aromatic groups in water, stabilization of
these complexes from weak intermolecular forces (C-H-π and
π-π) between the host and the guest(s) would be considerably
lower when compared to the corresponding syn arrangements.
On the basis of these arguments, the existence of cinnamic acids
in an anti orientation within the Pd nanocage seems less likely.
FIGURE 4. 1H NMR (D2O) spectra showing the changes in chemical
shift for the protons of 1a on addition of (i) 0, (ii) 0.09, (iii) 0.18, (iv)
0.27, (v) 0.36, (vi) 0.45, and (vii) 0.54 equiv of Pd nanocage. [1a] )
2.37 mM.
the methoxy group in the para position lacks such effective
interaction. Our speculated mode of encapsulation of ortho,
meta, and para methoxy trans-cinnamic acid methyl esters
(1e-1g) within the Pd nanocage, based on observations from
the 1H NMR studies, is provided in Figure 3a-c. From Figure
3a,b, one can see that the encapsulation of the ortho and meta
substituted derivatives within the Pd nanocage leads to orienta-
tion of the methyl protons of the methoxy group toward one of
the aromatic panels, whereas for the para substituted derivative,
the methyl protons of the methoxy group are not oriented toward
the aromatic panels of the host.
The arrangements leading to the formation of two syn dimers
(head-head and head-tail) are equally favorable within the Pd
nanocage except for the fact that in the latter case, the two
CdC bonds are oriented in a criss-cross fashion. The former
would be easily formed through an allowed [2 + 2] supra-
supra addition without much change in the positions of various
atoms of the olefins. On the other hand, the latter would have
to undergo considerable motion to yield either syn head-tail
or anti head-head dimers. One of the olefins in the criss-
crossed oriented pairs will have to rotate by about ∼90° to be
able to form bonds at both ends. We believe that within the
nanocage, large motions required for formation of these dimers
from not-so-ideally oriented olefin pairs would be resisted by
the cage. On this basis, we predict that the dimer of the syn
head-head stereochemistry would be the most favored one from
the Pd nanocage templated photodimerization of trans-cinnamic
acid esters.
To obtain additional information on the nature of the host-
1
guest complex, the H- NMR titration of the guest molecule
(1a) was performed with the host (Figure 4). To the guest (1a)
dissolved in water ([1a] ) 2.37 mM), increasing amounts of
the host (1.1 × 10-4 to 1.33 × 10-3 M) were added. After each
addition of the host, the solution was sonicated for 10 min, and
1
the H- NMR spectrum was recorded. Addition of increasing
amounts of the host to the guest molecule in D2O led to an
upfield shift of protons of aromatic and olefinic hydrogens of
the guest molecule. The methyl protons of COOCH3 attached
to the olefin as indicated before underwent a marginal shift.
Also, the absence of two independent signals, one due to the
complexed and the other due to the uncomplexed guest
molecule, suggests a fast exchange between these two types of
guest molecules in the NMR time scale. The observed minimal
shift beyond a host/guest ratio of 1:2.4 suggested maximum
complexation of the guest molecule within the Pd nanocage.
The exact stoichiometry of the host-guest complex is currently
unknown. For our irradiation studies, generally the 1:2 host/
guest ratio was used, although in one case, we had used a higher
H/G ratio.
To verify our prediction, the host-guest complexes formed
between Pd nanocage and guests (1a-1l) were irradiated with
a 450 W medium-pressure Hg arc lamp for 3 h. The samples
were then extracted by stirring with CDCl3 for 1 h and analyzed
1
by H NMR for the products that indicated the formation of a
syn head-head dimer and the corresponding cis isomer for guest
(11) (a) Nishio, M.; Hirota, M.; Umezawa, Y. The CH/π Interaction
EVidence, Nature, and Consequences; Wiley-VCH Press: New York, 1998.
(b) Castellano, R. K.; Diederich, F.; Meyer, E. A. Angew. Chem., Intl. Ed.
2003, 42, 1210-1250.
(10) The molecular dimensions were calculated using Chem 3-D ultra
software with a MM2 force field.
456 J. Org. Chem., Vol. 72, No. 2, 2007

Photodimerization of trans-Cinnamic Acid Esters
FIGURE 5. Possible orientations 1g within Pd nanocage leading to the formation of (a) syn head-head dimer, (b) syn head-tail dimer, (c) anti
head-head dimmer, and (d) anti head-tail dimer. The dimensions of the dimers were calculated using Chem 3-D ultra software using the MM2
force field.
TABLE 1. Photodimerization of 1a-1l within the Pd Nanocagea
dimer formed with the 1H NMR chemical shifts of cyclobutane
protons reported in the literature.3j,4 The percent conversion for
3 h of irradiation was between 30 and 40. To ascertain that
dimerization that is reported here occurs mainly within the cage
and not from the olefins in an aqueous phase, we conducted
irradiation of 1a at two host/guest ratios (1:4 and 1:2). The host-
guest complexes were prepared, irradiated, extracted, and
analyzed by 1H NMR. Analysis by 1H NMR indicated no
substantial difference in the dimer distribution between the
previous two ratios. For H/G ratio of 1:2, we observed the
formation of 54% of 2a and 46% of dimers (3a/6a, 80:20). For
a H/G ratio of 1:4, we observed 50% of 2a and 50% of dimers
(3a.6a, 76:24). To ensure that the photoreactions were uniform
and that the observed products were not due to secondary
photoreactions, we conducted irradiation of the host-guest
complex (H/G, 1:2) for different durations (1, 1.5, and 3 h).
Extraction of samples followed by 1H NMR analyses indicated
40% (1 h), 50% (1.5 h), and 54% (3 h) of 2a and 60% (3a/6a,
75:25), 50% (3a/6a, 76:24), and 46% (3a/6a, 80:20) of dimers.
Importantly, the ratio of the two dimers remained nearly the
same, suggesting that the dimer 6a is also a primary photo-
product. The results obtained here are interesting given the fact
that these guest molecules on solution irradiation undergo mainly
geometric isomerization. The cavity of the Pd nanocage is
unequivocally able to accommodate and bring two guest
molecules in close proximity, leading to the formation of the
syn head-head dimer as the major product.
Guest
% cis (2a-2l)
% dimers
1a
1b
1c
1d
1e
1f
1g
1h
1i
54
78
37
55
66
58
60
79
43
49
77
40
46 (3a/6a, 80:20)
22 (3b)
63 (3c)
45 (3d)
44 (3e/6e, 76:24)
42 (3f)
40 (3g)
21 (3h)
57 (3i)
1j
1k
1l
51 (3j)
23 (3k)b
60 (3l)
a The conversion was around 30-40% for 3 h irradiation. The H/G ratio
of 1:2 was used in all the cases. The percent of the products formed was
determined by integration of 1H NMR signals. b The NMR analysis of the
extracted sample indicated a peak at δ ) 4.9 ppm along with the cyclobutane
protons of a syn head-head dimer. The splitting of the peak at δ ) 4.9
ppm resembled that of the cyclobutane protons of a dimer. The exact
1
stereochemistry of this dimer is currently unknown. H NMR integration
indicated that this uncharacterirzed dimer and the syn head-head dimer
are formed in equal amounts.
molecules 1b-1d, 1f-1j, and 1l. For guest molecules 1a and
1e, the syn head-head dimer and anti head-tail dimers were
formed in the ratio 80:20 and 76:24. In both cases, the syn
head-head dimer was the major isomer formed. The results
obtained on irradiation of substrates (1a-1l) as a host-guest
complex with the Pd nanocage are provided in Table 1. The
stereochemistry of the dimer formed was confirmed by compar-
ing the 1H NMR chemical shifts of cyclobutane protons of the
The molecular dimensions of the syn head-head, syn head-
tail, anti head-head, and anti head-tail dimers formed from
1g were calculated to obtain additional support to our predic-
J. Org. Chem, Vol. 72, No. 2, 2007 457

Karthikeyan and Ramamurthy
tion.10 Molecular dimension data of the dimers show that the
pair of syn dimers (head-head: height 12.4 Å, width 10.1 Å
and head-tail: height 11.2 Å, width 12.8 Å; Figure 4a,b) are
more compact than their corresponding anti dimers (head-
head: height 14.2 Å, width 9.9 Å and head-tail: height 15.9
Å, width 12.9 Å; Figure 4c,d). On the basis of these molecular
dimensions and the free space available within the Pd nanocage
(15 Å × 10 Å), the accommodation of olefins leading to both
the anti dimers within the cavity of the Pd nanocage can easily
be found to be difficult. Of the two syn dimers (head-head
and head-tail), the former is slightly more compact, while the
latter is slightly larger to fit the Pd nanocage. Also as stated
previously for reasons of considerable motion of the double
bonds, the syn head-tail dimer formation within the Pd
nanocage seems to be a less favorable process. In our studies,
probably for the previous reasons, the Pd nanocage templated
irradiation of trans-cinnamic acid esters (1a-1l) resulted in the
formation of only syn head-head dimer.
In conclusion, we have used a self-assembled water-soluble
octahedral host, namely, the Pd nanocage for conducting
photodimerization of trans-cinnamic acid esters in water. The
investigation reported in this paper complements the work of
Fujita and co-workers on the photodimerization of acenaphth-
ylenes within the Pd nanocage and our own on the photodimer-
ization of coumarins.7,9 One of the main advantages of this host
(Pd nanocage) is its commercial availablility.15 Considering the
fact that some of the water-soluble supramolecular hosts reported
in the literature require tedious, multistep syntheses and
purification procedures, use of this commercially available host
is a major advantage.16 The main drawback in using the Pd
nanocage as a reaction nanovessel for conducting photochemical
studies is the similar absorption characteristics of both this host
and most organic molecules.7 Hence, it is essential to make a
judicious choice of the substrates that would be photoactive
when encapsulated and irradiated as a host-guest complex with
the Pd nanocage. With the previous encouraging results with
the Pd nanocage as a reaction vessel, we are currently
investigating its utility in other photochemical reactions.
Summary
de Mayo and co-workers were the first ones to explore the
use of weak interactions to control the regiochemistry of the
photocycloaddition of enones in solution.12 They were able to
achieve modest control on the process using a micellar medium.
The molecules were aligned at the interface between the
hydrophobic interior and the hydrophilic exterior of a micelle.
Similar attempts by Whitten and co-workers to obtain selective
dimers from stilbazoles in conventional micellar media were
not successful.13 In contrast to normal micelles, when they
employed a reverse micelle with much smaller size than the
reaction medium, a syn HH dimer was obtained from stilba-
zole.14 Whitten and co-workers speculate that the well-ordered
and more constrained water pool in reverse micelles is able to
provide a better orientation than conventional micelles. The
previous results of de Mayo and Whitten’s groups suggest that
the hydrophobic-hydrophilic interface could be used to orient
molecules but that the reaction space must be more rigid and
less dynamic than a conventional micelle. Using the principles
formulated by these pioneers, we have achieved superior control
during the photocycloaddition of olefins through the rationale
of confined spaces offered by cucurbiturils, calixarenes, and a
Pd nanocage instead of a micelle.
Experimental Procedures
Complexation of 1a-1l with Pd Nanocage and Irradiation
Studies. Required amounts of the guest (G) (1a: 2.1 mg (21 ×
10-3 M), 1b/1c/1d: 1.7 mg (16.0 × 10-3 M), 1e: 1.9 mg (16.4 ×
10-3 M), 1f/1g: 1.2 mg (10.4 × 10-3 M), 1h/1i/1j: 1.1 mg (8.8 ×
10-3 M), and 1k/1l: 1.2 mg (9.09 × 10-3 M) were stirred with 10
mg (5.5 × 10-3 M for 1a) or 20 mg (11 × 10-3 M for 1a), 15 mg
(8.3 × 10-3 M, for 1b/1c/1d/1e), 9 mg (5.0 × 10-3 M, for 1f/1g),
8 mg (4.45 × 10-3 M, for 1h/1i/1j), and 10 mg (5.5 × 10-3 M, for
1k/1l) of a Pd nanocage in 0.6 mL of D2O in a hot water bath for
5 h to obtain a clear solution of the inclusion complex with a host/
1
guest ratio of 1:2 or 1:4 (for 2a). The H NMR was recorded to
confirm the complexation. The solution was then irradiated for 3 h
using a 450 W medium-pressure Hg lamp. The products were
extracted by stirring with CDCl3 dried over anhydrous sodium
1
sulfate and analyzed by H NMR for the product formation. The
conversion to the product was determined by NMR integration of
peaks of the starting material (alkene protons) and that of the dimer
formed (cyclobutane protons).
NMR Titration Studies. Known amounts of (2-3 mg) of 1a
were stirred with 1 mL of D2O for 12 h. The solution was then
filtered with filter paper of fine porosity, and the H NMR was
recorded. To this solution, the host was added sequentially (0.2
mg) and sonicated for 10 min before recording the H NMR until
1
1
the amount of added host was 2.4 mg. The H/G ratio was
determined by integration of the peaks corresponding to the host
and the guest.
(12) (a) Berenjian, N.; de Mayo, P.; Sturgeon, M.-E.; Sydnes, L. K.;
Weedon, A. C. Can. J. Chem. 1982, 60, 425-436. (b) Lee, K.-H.; de Mayo,
P. J. Chem. Soc., Chem. Commun. 1979, 493-495. (c) de Mayo, P.; Sydnes,
L. K. Chem. Commun. 1980, 994-995.
Acknowledgment. V.R. is grateful to the National Science
Foundation (CHE-0213042) for financial support.
(13) Quina, F. H.; Whitten, D. G. J. Am. Chem. Soc. 1977, 99, 877-
883.
(14) Takagi, K.; Suddaby, B. R.; Vadas, S. L.; Backer, C. A.; Whitten,
D. G. J. Am. Chem. Soc. 1986, 108, 7865-7867.
(15) Wako International. Product name: Palladium Nanocage and Product
no.: 160-20471.
(16) (a) Shinkai, S.; Mori, S.; Koreishi, H.; Tsubaki, T.; Manabe, O.
J. Am. Chem. Soc. 1986, 108, 2409-2416. (b) Yoon, J.; Cram, D. J. Chem.
Commun. 1997, 497-498. (c) Gui, X.; Sherman, J. C. Chem. Commun.
2001, 2680-2681. (d) Day, A.; Arnold, A. P.; Blanch, R. J.; Snushall, B.
J. Org. Chem. 2001, 66, 8094-8100.
Supporting Information Available: 1H NMR spectra of the
host-guest complexes of 1a-1l and 3-methyl- and 4-methyl-trans-
cinnamic acids, syntheses and characterization of the guest mol-
ecules (1b-1l), and 1H NMR characterization of the dimers. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
JO0617722
458 J. Org. Chem., Vol. 72, No. 2, 2007