Controlling photochemical geometric isomerization of a stilbene and dimerization of a styrene using a confined reaction cavity in water

Article abstract of DOI:10.1021/ol702322u

Utility of a water-soluble deep cavity cavitand, octa acid, as a reaction medium is illustrated by carrying out photochemical reactions of a stilbene and a styrene included within the octa acid in water. Geometric isomerization of trans-4,4′-dimethyl stilbene is restricted while dimerization of 4-methyl styrene is facilitated within the octa acid cavity. The excited-state chemistry of both systems is different in this medium from that in organic solvents. The change in chemistry is attributed to the supramolecular effects provided by the host cavity.

Full text of DOI:10.1021/ol702322u

ORGANIC
LETTERS
2
007
Vol. 9, No. 24
059-5062
Controlling Photochemical Geometric
Isomerization of a Stilbene and
Dimerization of a Styrene Using a
Confined Reaction Cavity in Water
5
Anand Parthasarathy, Lakshmi S. Kaanumalle, and V. Ramamurthy*
Department of Chemistry, UniVersity of Miami, Coral Gables, Florida 33146
murthy1@miami.edu
Received September 24, 2007
ABSTRACT
Utility of a water-soluble deep cavity cavitand, octa acid, as a reaction medium is illustrated by carrying out photochemical reactions of a
stilbene and a styrene included within the octa acid in water. Geometric isomerization of trans-4,4 -dimethyl stilbene is restricted while dimerization
′
of 4-methyl styrene is facilitated within the octa acid cavity. The excited-state chemistry of both systems is different in this medium from that
in organic solvents. The change in chemistry is attributed to the supramolecular effects provided by the host cavity.
Two most common reactions of olefins are geometric
with dimensions shown in Figure 1 as a reaction cavity for
1
isomerization and dimerization. Geometric isomerization is
2
utilized by proteins to trigger a number of biological events,
whereas dimerization is used by synthetic chemists to build
3
complex molecules having cyclobutane rings. Devising
strategies to perform the above two reactions with high
specificity in water is a worthwhile goal. We have been
exploring with reasonable success the use of a synthetic
4
water-soluble deep cavity cavitand known as octa acid (OA)
(
1) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Sausalito, CA, 1991.
2) (a) Dugave, C., Ed. cis-trans Isomerization in Biochemistry; Wiley-
(
Figure 1. Pictorial representation of OA.
VCH: Weinheim, Germany, 2006. (b) Dugave, C.; Demange, L. Chem.
ReV. 2003, 103, 2475-2532. (c) van der Horst, M. A.; Helingwerf, K. J.
Acc. Chem. Res. 2004, 37, 13-20. (d) Zimmer, M. Chem. ReV. 2002, 102,
7
59-781.
5
photochemical reactions. In this report we demonstrate that
(
3) Griesbeck, A. G., Mattay, J., Eds. Synthetic Organic Photochemistry;
Marcel Dekker Inc.: New York, 2005.
it forms supramolecular complexes with olefins such as
1
0.1021/ol702322u CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/23/2007

stilbenes and styrenes in borate buffer at pH > 8.9 and
controls the excited-state behavior of both 4,4′-dimethylstil-
bene and 4-methylstyrene.
The emission from trans-4,4′-dimethylstilbene (1a; 2 ×
-
5
10
M) in borate buffer was broad, characteristic of
6
-5
aggregates. Remarkably, upon addition of OA (4 × 10
M) the turbid buffer solution containing 1a became transpar-
ent, and the emission characteristic of monomer trans-1a
appeared (Supporting Information, Figure S1). This observa-
tion opened up the possibility of investigating in detail host-
guest complexation, photochemistry and photophysics of
trans and cis isomers of 1a and 4-methylstilbene (1b) and
Scheme 1. Photoisomerization of Stilbene
Figure 2. 1H NMR (500 MHz, 20 mM borate buffer/D
of (i) OA (2 mM, top), (ii) 1:2 complex of trans-1a with OA
middle), and (iii) 1:2 complex of cis-1a with OA (bottom).
2
O) spectra
(
signals observed for the two OA molecules forming the top
and bottom halves of the capsule. The large upfield shift in
the signals due to methyl hydrogens of trans-1a (∆δ with
4
-methylstyrene (3) placed within the OA capsule in water
(Schemes 1 and 2).
respect to CDCl ) -4.65 ppm) suggested them to be located
3
deeper within the tapering poles of the capsule where the
aryl rings provide magnetic shielding. Pictorial representa-
tions of the NOESY data (Figure 3) reveal interactions
Scheme 2. Photodimerization of 4-Methylstyrene
Formation of 1:2 (guest:host) complexes in D
2
O (pD ≈
Figure 3. Pictorial representation of the NOESY correlations of
trans-1a and cis-1a with one-half of OA.
8
.9; borate buffer) by trans and cis isomers of 1a and 1b
1
with OA was established through H NMR titration experi-
ments (Figures S2 and S3 in Supporting Information). The
identical nature of the top and bottom halves of the OA
capsule in the case of 1a was suggested by the single signal
between the methyl group of trans-1a and H
and H of OA (Figure 1 for lettering of hydrogens in OA)
Figure S4 for NOESY data in Suporting Information).
d e f g
, H , H , H ,
h
(
δ ) -2.3 ppm) for both methyl groups of trans-1a
molecule, reflecting symmetrical positioning within the OA
capsule (Figure 2). This was reinforced by the single set of
(
Upfield shift for the methyl group of trans-1b (δ ) -2.1
ppm; ∆δ ) -4.45 ppm) and NOESY data (Figure S5 in
Supporting Information) suggested the methyl group to be
positioned at the narrower ends of the capsule similar to that
of trans-1a.
(
4) Gibb, C. L. D.; Gibb, B. C. J. Am. Chem. Soc. 2004, 126, 11408-
1409.
5) (a) Kaanumalle, L. S.; Gibb, C. L. D.; Gibb, B. C.; Ramamurthy, V.
1
(
J. Am. Chem. Soc. 2004, 126, 14366. (b) Kaanumalle, L. S.; Gibb, C. L.
D.; Gibb, B. C.; Ramamurthy, V. J. Am. Chem. Soc. 2005, 127, 3674-
1
In contrast to the trans isomers, H NMR and NOESY
3
1
675. (c) Kaanumalle, L. S.; Ramamurthy, V. Chem. Commun. 2007, 1062-
data (cis-1a: δ ) -0.7 ppm; cis-1b: δ ) -1.2 ppm; Figures
S6 and S7 in Supporting Information) suggested the methyl
groups of the cis isomers to be located near the spacious
equatorial region of the capsule. Thus, NMR data clearly
indicated cis and trans isomers of 1a and 1b to occupy
different regions of the capsule having different amounts of
064. (d) Kaanumalle, L. S.; Gibb, C. L. D.; Gibb, B. C. Org. Biomol.
Chem. 2007, 5, 236-238. (e) Natarajan, A.; Kaanumalle, L. S.; Jockusch,
S.; Gibb, C. L. D.; Gibb, B. C.; Turro, N. J.; Ramamurthy, V. J. Am. Chem.
Soc. 2007, 129, 4132-4133. (f) Sundaresan, A. K.; Ramamurthy, V. Org.
Lett. 2007, 9, 3575-3578.
(6) Catalan, J.; Zimanyi, L.; Saltiel, J. J. Am. Chem. Soc. 2000, 122,
2
377-2378.
5060
Org. Lett., Vol. 9, No. 24, 2007

free space around the CdC bond of the stilbenes. We
envisioned that the differences in the available free space
around the two isomers would have significant impact on
their excited-state rotational dynamics. With this in mind we
irradiated the OA encapsulated trans and cis isomers of 1a
provided they are smaller in size than stilbenes 1a and 1b.
With this in mind we explored the chemistry of 4-methyl-
1
styrene 3 (Scheme 2). H NMR titration experiments revealed
that, as expected, 3 formed 2:2 complexes with OA (Figure
S10 in Supporting Information). Surprisingly, in the case of
3 two complexes with different arrangements of the olefin
1
and 1b and analyzed the products by H NMR (in situ) and
1
by GC (following extraction with CDCl
3
).
were identified by H NMR. Two signals of unequal intensity
Within 15 min of irradiation (λ > 310 nm) trans-1a in
(55:45) at -0.5 and -0.8 ppm (Figure 4) due to the 4-methyl
-
3
hexane (5 × 10 M) established a pseudostationary state
consisting of 18% trans and 76% cis. Upon continued
7
irradiation phenanthrene was the final product. Similar
-
4
excitation of trans-1a@OA
2
capsule ([trans-1a]: 5 × 10
M and [OA]: 10-3 M) only led to slow isomerization and
to establishment of a pseudostationary state in about an hour
consisting of 85% trans and 15% cis isomers (Figure S8 in
Supporting Information for absorption spectra of 1a and
8
OA). Phenanthrene was formed in less that 5% yield even
after 4 h of irradiation. Consistent with slow isomerization
of trans to cis and enhanced fluorescence, the excited singlet
state of trans-1a had a longer lifetime (τ: 1.51 ns) within
OA capsule than in hexane (τ < 0.7 ns) (Figure S9 in
Supporting Information). The specific effect of the capsule
on the trans isomer was confirmed by the fast isomerization
1
Figure 4. H NMR (500 MHz, 20 mM borate buffer/D O)
2
spectrum of 2:2 complex of 3 with OA.
-4
obtained with cis-1a@OA
2
([cis-1a]: 5 × 10 M and [OA]:
-3
10
M) that showed 85% transformation to the trans isomer
within 30 min. Above results unequivocally established the
OA capsule to have a significant impact on the excited singlet
state rotational dynamics of trans- but not of cis-1a. We
believe the extraordinary influence of OA on the isomer-
ization of 1a to result from the “tighter fit” of trans-1a at
the tapered ends of the capsule that provide a barrier for the
relocation of the two methyl groups from the narrower
bottom to broader middle region of the capsule (trans to cis
conversion).
The above model suggested that removal of one methyl
group from 1a should allow isomerization by the flipping
of the unsubstituted phenyl ring that has more free space
around it. Investigations of the excited-state behavior of
trans-4-methylstilbene (1b) that would clarify this speculation
group suggested the existence of two complexes with
different structures. Had the signals been due to methyl group
of two olefin molecules (arranged in an unsymmetrical
fashion) of a single complex, the intensity of the two should
have been equal. Observation of only one set of signals for
the host protons (Figure 4) further suggested that the two
olefin molecules are arranged in a symmetrical fashion within
the capsule in each of these complexes. On the basis of
NOESY data we speculate the two complexes to have
structures A and B shown in Figure 5.
-
4
were pursued. Irradiation of trans-1b in hexane (5 × 10
-
4
-3
M) and in water ([trans-1b]: 5 × 10 M and [OA]): 10
M; pH ≈ 8.9) as OA complexes gave identical mixtures of
8
cis and trans isomers (85% and 15%). Further, the lifetime
of the excited singlet state of trans-1b (τ < 0.7 ns in both
OA and hexane) remained unaffected within the capsule.
These studies unequivocally demonstrated that the excited-
state geometric isomerization of 1b is independent of the
capsule, while that of 1a is dependent. A comparison of the
behavior of 1a and 1b suggested that the influence of OA
on the excited-state behavior of guests is structure specific.
On the basis of the above results we visualized that two
olefin molecules could be included within OA capsule,
Figure 5. Cartoon representation of the encapsulated guest
molecules in OA.
3
-3
Irradiation (>290 nm) of 3 included within OA ([3]: 10
-
3
M and [OA]: 10 M) gave two dimers 4 and 5 in the ratio
(
7) (a) Mallory, F. B.; Mallory, C. W. In Organic Reactions; Dauben,
W. G., Ed.; John Wiley & Sons, Inc.: New York, 1984; Vol. 30, pp 1-455.
b) Saltiel, J.; D’Agostino, J.; Megarity, E. D.; Metts, L.; Neuberger, K.
45:55 (Scheme 2), identical to the ratio of the two complexes
(
present in solution (Figure S11 in Supporting Information
R.; Wrighton, M.; Zafiriou, O. C. In Organic Photochemistry; Chapman,
O. L., Ed.; Marcel Dekker, Inc.: New York, 1973; Vol. 3, pp 1-113.
9
for absorption spectra of 3 and OA). This suggested that
(
8) Stilbene absorbs at a slightly longer wavelength than OA. At the
irradiation wavelength, stilbene alone absorbs the light, and we believe that
the reaction occurs from the excited singlet state of stilbene.
(9) 4-Methylstyrene absorbs at a slightly shorter wavelength than OA.
At the irradiation wavelength, OA alone absorbs the light.
Org. Lett., Vol. 9, No. 24, 2007
5061

the two complexes reacted independently and did not
interconvert during the course of irradiation. It is important
to note that the dimers formed within the OA capsule are
different from the ones obtained in organic solvents by direct
irradiation (6) and triplet sensitization (7). On the basis of
known singlet and triplet energies and electron donor-
acceptor properties of 3 and OA we believe that the dimeri-
zation within the capsule was prompted by triplet sensitiza-
tion of 3 by OA.11 Formation of dimer 7 in solution from
the triplet state has been rationalized on the basis of
involvement of a most stable 1,4-dibenzylic diradical as an
intermediate (Scheme 3).10 Formation of dimer 4 instead of
crystals, zeolites, solid host-guest complexes, and organic
1
3
glasses. As far as we are aware, no predictable model has
emerged from these studies. However, it is common knowl-
edge that exquisite control on the site of isomerization of
retinal in rhodopsin and bacteriorhodopsin results from the
restrictions placed on the guest (reacting molecule) by the
10
2
host cavity. Results reported here on two systems bear
similarity to these protein-controlled isomerization processes
and suggest that the observed specificity is system dependent
and understandable on the basis of host-guest interactions
and free space within the reaction cavity. During the last
three decades photodimerization of olefins has been exten-
sively investigated in various organized assemblies with the
aim of orienting olefins toward a single dimer.1
3c,14,15
The
ability of the OA capsule to preorganize olefins toward
dimers that are not formed in organic solvents adds a new
dimension.
Scheme 3. Proposed Mechanism for the Formation of 4
1
6
The results presented above establish that a careful choice
of the host-guest assembly based on host-guest inter-
action(s) and available free space could lead to a predictable
photochemical outcome. Ability to generate dimeric products,
within water-soluble hosts, that are not obtained in conven-
tional solution medium opens up new opportunities in this
area. We are currently exploring homo- and heterodimer-
ization of olefins within the OA capsule. Further work needed
for a greater comprehension of the observed selectivity and
full exploitation of OA as a photochemical reaction cavity
is underway in our laboratory.
7
within OA suggests that, within the restricted space of a
capsule, stability of the diradical intermediate is not the con-
trolling factor. Most likely, dimer 5 is formed from complex
A (Figure 5) in which the olefins are pre-organized to form
a 1,3-dimer. Dimer 4 that is formed as one of the products
within OA is known to be a product of electron-transfer
Acknowledgment. We thank the National Science Foun-
dation, U.S.A., for generous financial support (CHE-0213042
and CHE-0531802).
1
2
sensitization of 3 by 1,4-dicyanobenzene in acetonitrile.
Its formation within the capsule from the triplet state is
Supporting Information Available: Experimental details
and additional NMR spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
surprising, and we believe that it resulted from complex B
(Figure 5) via a previously unknown diradical rearrangement
process (Scheme 2). Further work is underway to confirm
this suggestion.
OL702322U
It is quite natural for the reader to wonder how OA capsule
is different from other organized assemblies in which
geometric isomerization and dimerization of olefins have
been performed. Geometric isomerization of olefins have
been investigated in a variety of solid matrices such as
(
13) (a) Zheng, S-L; Messerschimidt, M.; Coppens, P. Chem. Commun.
2
007, 2735-2737. (b) Arad-Yellin, R.; Brunie, S.; Green, B.; Knossow,
M.; Tsoucaris, G. J. Am. Chem. Soc. 1979, 101, 7529-7537. (c) Rama-
murthy, V., Ed. Photochemistry in Organized and Constrained Media;
VCH: New York, 1991. (d) Duveneck, G. L.; Sitzmann, E. V.; Eisenthal,
K. B.; Turro, N. J. J. Phys. Chem. 1989, 93, 7166-7170. (e) Natarajan,
A.; Mague, J. T.; Venkatesan, K.; Arai, T.; Ramamurthy, V. J. Org. Chem.
2006, 71, 1055-1059. (f) Moorthy, J. N.; Venkatakrishnan, P.; Savitha,
G.; Weiss, R. G. Photochem. Photobiol. Sci. 2006, 5, 903-913. (g) Yang,
L-Y; Harigai, M.; Imamoto, Y.; Katoka, M.; Ho, T-I.; Federova, O.;
Shevyakov, S.; Liu, R. S. H. Photochem. Photobiol. Sci. 2006, 5, 874-
882.
(
10) (a) Lewis, F. D.; Kojima, M. J. Am. Chem. Soc. 1988, 110, 8660-
8
8
2
664. (b) Lewis, F. D.; Kojima, M. J. Am. Chem. Soc. 1988, 110, 8664-
670. (c) Nozaki, H.; Otani, I.; Noyori, R.; Kawanisi, M. Tetrahedron 1968,
4, 2183-2192. (d) Kojima, M.; Sakuragi, H.; Tokumaru, K. Bull. Chem.
Soc. Jpn. 1989, 3863-3868.
(
11) On the basis of the ability of OA capsule to sensitize the di-π-
(14) Bassani, D. M. In CRC Handbook of Organic Photochemistry and
Photobiology, 2nd ed.; Horspool, W., Lenci, F., Eds.; CRC Press: Boca
Raton, FL, 2003; pp 20-1-20-20.
methane rearrangement of benzonorbornadiene and dibenzobarralene, we
believe that it has triplet energy close to that of acetophenone (∼73 kcal/
mol).
(15) (a) Takaoka, K.; Kawano, M.; Ozeki, T.; Fujita, M. Chem. Commun.
2006, 1625-1627. (b) Yoshizawa, M.; Takeyama, Y.; Okano, T.; Fujita,
M. J. Am. Chem. Soc. 2003, 125, 3243-3247. (c) Yoshizawa, M.;
Takeyama, Y.; Kusukawa, T.; Fujita, M. Angew. Chem., Int. Ed. 2002, 41,
1347-1349.
(12) (a) Asanuma, T.; Gotoh, T.; Tsuchida, A.; Yamamoto, M.; Nishijima,
Y. J. Chem. Soc., Chem. Commun. 1977, 485-486. (b) Tojo, S.; Toki, S.;
Takamuku, S. J. Org. Chem. 1991, 56, 6240-6243. (c) Schepp, N. P.;
Johnston, L. J. J. Am. Chem. Soc. 1994, 116, 6895-6903. (d) Asanuma,
T.; Yamamoto, M.; Nishijima, Y. J. Chem. Soc., Chem. Commun. 1975,
(16) Nishioka, Y.; Yamaguchi, T.; Yoshizawa, M.; Fujita, M. J. Am.
Chem. Soc. 2007, 129, 7000-7001.
6
08-609.
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Org. Lett., Vol. 9, No. 24, 2007