Control of the intramolecular [2+2] photocycloaddition in a bis-stilbene macrocycle

Article abstract of DOI:10.1021/jo900443e

(Chemical Equation Presented) The intramolecular [2+2] photocyclization of a bis-stilbene macrocycle was studied. The reaction can be controlled by the insertion and removal of urea protecting groups. Upon UV-irradiation in both the solid state and DMSO s

Full text of DOI:10.1021/jo900443e

SCHEME 1. Synthesis of Bis-Urea Stilbene Macrocycles
Control of the Intramolecular [2+2]
Photocycloaddition in a Bis-Stilbene Macrocycle
Yuewen Xu,† Mark D. Smith,† Jeanette A. Krause,‡ and
Linda S. Shimizu*,†
Department of Chemistry and Biochemistry, UniVersity of
South Carolina, Columbia, South Carolina 29208, and
The Richard C. Elder X-ray Crystallographic Facility,
Department of Chemistry, UniVersity of Cincinnati,
Cincinnati, Ohio 45221
shimizul@mail.chem.sc.edu
ReceiVed February 26, 2009
covalent approaches, which fix two stilbene units within
macrocyclic frameworks,4 have the potential to exert greater
control over the photodimerization of stilbene derivatives. We
report herein that subtle conformational effects can be used to
control the selectivity of an intramolecular [2+2] photoaddition
of stilbenes in solution. This paper describes the synthesis of a
bis-urea macrocycle containing two cis-stilbenes. The presence
or absence of a triazinanone urea protecting group exerts
surprising effects on the reaction of these macrocycles. Upon
UV irradiation both macrocycles appear to undergo a cis-trans
photoisomerization. However, only the unprotected urea mac-
rocycle affords a single product for the subsequent [2+2]
cycloaddition, the cis,trans,cis-cycloadduct in high conversion.
In contrast, UV irradiation of the triazinanone protected urea
macrocycle gives a complex mixture of products. We study the
reasons for this marked difference in selectivity.
Bis-stilbene macrocycle 5a was synthesized from commercial
dimethyl cis-stilbene-4,4′-dicarboxylate 1a (Scheme 1). First,
the diester 1a was reduced to diol 2a. Bromination of diol 2a
with NBS/PPh3 yielded dibromide 3a.5 The dibromide 3a was
cyclized with the triazinanone in NaH/THF to afford the
protected cis-macrocycle 4a.6 The protecting group was removed
with use of diethanolamine in methanol to yield target macro-
cycle 5a.7 The trans-stilbene macrocycle was prepared by a
similar route starting with trans-stilbene 1b.
The intramolecular [2+2] photocyclization of a bis-stilbene
macrocycle was studied. The reaction can be controlled by
the insertion and removal of urea protecting groups. Upon
UV-irradiation in both the solid state and DMSO solution,
the free urea macrocycle undergoes a cis-trans photoi-
somerization that is followed by a [2+2] cycloaddition to
afford a single product in high yield. The presence of the
triazinanone urea protecting groups does not alter the cis-
trans photoisomerization but greatly decreases the selectivity
of the photocycloaddition step.
The [2+2] cycloaddition reaction has been one of the most
efficient and stereoselective methods for synthesizing cyclobu-
tane derivatives in the solid state.1 However, in solution it is
more difficult to control both yield and stereoselectivity of the
process. Olefins such as stilbene typically afford a mixture of
products upon UV-irradiation in solution.2 Supramolecular3 and
We used X-ray crystallography to compare the solid state
structures of compounds 4a and 5a. Single crystals of 4a were
obtained by slow diffusion of hexane into a THF solution of
4a. In the X-ray crystal structure (Figure 1a), the tert-butyls of
the protecting groups are positioned above and below the
macrocyclic plane. The two urea groups are aligned in opposite
directions, which minimizes the dipole interactions. The distance
between the two stilbene double bonds was 13.4 Å, which
should be too distant for an intramolecular [2+2] cycloaddition.
† University of South Carolina.
‡ University of Cincinnati.
(1) (a) Khan, M.; Brunklaus, G.; Enkelmann, V.; Spiess, H. W. J. Am. Chem.
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4874 J. Org. Chem. 2009, 74, 4874–4877
10.1021/jo900443e CCC: $40.75  2009 American Chemical Society
Published on Web 05/22/2009

FIGURE 2. Single-crystal X-ray structure confirmed assignment of
6, which crystallized as a bis(DMSO) solvate complex.
FIGURE 1. Views from the X-ray structures: (a) triazinanone protected
stilbene macrocycle 4a and (b) bis-urea stilbene macrocycle 5a.
1
cycloadduct 6 in high yield (Scheme 2). The H NMR spectra
SCHEME 2. Proposed Reaction Pathway for 5a
in DMSO-d6 shows the cyclobutane CH’s at 4.5 ppm, consistent
with literature reports for the stereochemistry of the cyclobutane
ring in 6.4,13 This product does not have the expected stereo-
chemistry for the [2+2] photoaddition of the cis-stilbene (5a)
starting material. Photoisomerization of the double bonds of one
of the cis-stilbene units to give a mixed cis,trans-intermediate
and subsequent [2+2] photocyclization would also give the
wrong stereochemistry. Indeed, the formation of product 6 is
most easily rationalized by the intramolecular syn addition of
the trans,trans-intermediate 5b (Scheme 2).
Single crystals of 6 were obtained from a DMSO-d6 solution
(2 mg/0.5 mL). The X-ray structure in Figure 2 provides clear
confirmation of the cis,trans,cis arrangement of the cyclobutane
ring, which is usually characterized by the chemical shift of
the cyclobutane hydrogens alone.4,13 Crystallization of this
compound is aided by the urea groups and the molecule
crystallized as a bis(DMSO) solvate complex. The regiochem-
istry of the cyclobutane results in close contacts between the
two cis phenyl rings, which show centroid-centroid distances
of 3.66 Å.
Single crystals of bis-urea 5a (Figure 1b) were obtained upon
slow cooling of a MeOH/H2O solution from 85 °C to room
temperature overnight. Similar to 4a, the two urea groups are
oriented oppositely.8 The distance between the two double bonds
within the macrocycle was again quite large (12.9 Å). Therefore,
we did not expect to observe the intramolecular [2+2] reaction
in either case, and we were interested to see if two macrocycles
could be coupled via a [2+2] photoaddition to give dimers.
Macrocycle 5a self-assembles in the crystal into an offset
structure through typical urea-urea hydrogen bonds with
NH· · ·O distances of 2.88 to 2.89 Å. This offset assembly brings
the stilbene double bonds of adjacent macrocycles closer
resulting in several contacts at ∼4.9 Å.
Stilbene and its derivatives are known to undergo three types
of photochemical conversions, including cis-trans isomeriza-
tion,9,10 photodimerization or cross-photocycloaddition with
alkenes and arenes,11 and photochemical conversion to phenan-
threnes.12 Given the large separation of the reactive double
bonds in the solid state structures of 4a and 5a, the intramo-
lecular cycloadditions were expected to be disfavored over
intermolecular reactions.13 We first investigated the photochem-
istry of macrocycles 4a and 5a in solution (DMSO-d6) as a
control. Our hypothesis was that these compounds should show
unselective reactions to afford multiple products. Indeed, when
the triazinanone protected urea 4a was irradiated under UV light
at room temperature with a Hanovia 450 W medium pressure
mercury arc lamp, a complex mixture of products was observed.
In contrast, similar UV-irradiation of the unprotected urea 5a
(7.58 mM in DMSO-d6) gave a single product the cis,trans,cis-
To gain insight into the mechanism of this transformation,
the reaction was monitored by UV-vis spectroscopy. During
the first hour of UV-irradiation, the absorption maxima of the
solution of 5a (2.5 × 10-5 M in CHCl3) shifted from λmax
)
282 nm (ε ) 50 000) to λmax ) 289 nm and the intensity of this
band increased (Figure 3a). Further UV-irradiation resulted in
a continued red shift and the slow decay of this band (Figure
3b).
These absorption bands correspond to the π f π* transition
of the stilbene double bond. The observed red shift is consistent
with the isomerization of the cis-stilbene to the trans-isomer
(intermediate 5b). Indeed, the parent cis-stilbene displays a π
f π* transition at λmax ) 282 nm (ε ) 12 300) while trans-
stilbene shows this transition at longer wavelength (λmax ) 295
nm) and has a larger extinction coefficient (ε ) 25 000).14
Product 6 lacks the stilbene double bond and has no absorption
in this region. We synthesized 5b (Scheme 1) and found that it
displayed a similar absorption band at 292 nm (Figure 3b, inset).
It is likely that the slow steps of this process are the photoi-
somerizations, first to a mixed cis,trans-intermediate, which
does not appear to cyclize given the stereochemistry of the
observed product. Further photoisomerization likely affords the
trans,trans-intermediate 5b. Under UV-irradiation, pure 5b
undergoes a [2+2] cycloaddition to form product 6 exclusively.
The reaction of 5a was also monitored by using 1H NMR, which
indicated that after 2 h of UV-irradiation the major resonances
correspond to starting material 5a and product 6. Also visible
in low concentration were resonances that were consistent with
the formation of 5b (Supporting Information).
(8) Shimizu, L. S.; Hughes, A. D.; Smith, M. D.; Davis, M. J.; Zhang, B. P.;
zur Loye, H.-C.; Shimizu, K. D. J. Am. Chem. Soc. 2003, 125, 14972.
(9) Waldeck, D. H. Chem. ReV. 1991, 91, 415.
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88, 2336. (b) Saltiel, J. J. Am. Chem. Soc. 1968, 90, 6394. (c) Anger, I.; Sandros,
K.; Sundahl, M.; Wennerstro¨m, O. J. Phys. Chem. 1993, 97, 1920.
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R. J. J. Org. Chem. 1980, 45, 948. (c) Caldwell, R. A.; Mizuno, K.; Hansen,
P. E.; Vo, L. P.; Frentrup, M.; Ho, C. D. J. Am. Chem. Soc. 1981, 103, 7263.
(d) Tolbert, L. M.; Ali, M. B. J. Am. Chem. Soc. 1982, 104, 1742. (e) Mizuno,
K.; Maeda, H.; Inoue, Y.; Sugimoto, A.; Vo, L. P.; Caldwell, R. A. Tetrahedron
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(12) (a) Wood, C. S.; Mallory, F. B. J. Org. Chem. 1964, 29, 3373. (b) Doyle,
T. D.; Filipescu, N.; Benson, W. R.; Banes, D. J. Am. Chem. Soc. 1970, 92,
6371. (c) Cuppen, Th., J. H. M.; Laarhoven, W. H. J. Am. Chem. Soc. 1972, 94,
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(13) (a) Jon, S. Y.; Ko, Y. H.; Park, S. H.; Kim, H. J.; Kim, K. Chem.
Commun. 2001, 1938. (b) Rao, K. S. S. P.; Hubig, S. M.; Moorthy, J. N.; Kochi,
J. K. J. Org. Chem. 1999, 64, 8098. (c) Schmidt, G. M. J. Pure Appl. Chem.
1971, 27, 647–678.
(14) Graselli, J. G.; Ritchey, W. M. Atlas of Spectral Data and Physical
Constants; CRC Press: Cleveland, OH, 1975; 399
J. Org. Chem. Vol. 74, No. 13, 2009 4875

FIGURE 4. Models of the trans-stilbene intermediates were constructed
with Spartan.15 Monte Carlo searches of the conformer distribution at
ground state with Molecular Mechanics (MMFF) suggest these are the
lowest energy structures of (a) 4b and (b) 5b.
afforded 6 exclusively in solution and in the solid state. In
contrast, both the protected macrocycles (4a and 4b) produced
a mixture of products upon UV-irradiation.
The differences in product selectivity of the photodimerization
reaction of protected 4a versus unprotected 5a may be explained
as the diminishing of distances between two double bonds. In
both cases the observed shifts in the UV-absorption spectra and
1H NMR data suggested that photoisomerization (4a and 5a)
proceeded to give the trans,trans-isomers (4b and 5b). Since
neither of these trans,trans-intermediates have yet afforded
crystals, we used Spartan to model their ground state conforma-
tions.15 The models suggest that the trans-intermediate 5b adopts
a shorter distance between stilbene double bonds than the
corresponding trans-intermediate in protected 4b. Although
neither of these lowest energy structures (Figure 4) have the
olefins preorganized at the <4.2 Å preferred for [2+2]
cycloaddition,13c the alkenes are significantly closer in 5b (4.6
Å vs. 6.1 Å). In solution under UV-irradiation, both structures
should be able to access a number of higher energy conforma-
tions; however, the triazinanone ring in 4b introduces further
rigidity and enforces a more linear arrangement of the urea,
which effectively pushes the alkenes further apart. In contrast,
5b is more flexible and may access additional conformations
that decrease the distance between the alkenes. Indeed, experi-
mental evidence suggests that 5b adopts a conformation that
enables the selective formation of 6.
In summary, the immobilization of two cis-stilbenes within
a bis-urea macrocycle facilitated the cis-trans photoisomer-
ization and subsequent highly selective [2+2] photocycloaddi-
tion to yield a single cyclobutane adduct quantitatively. This is
in contrast to the photochemistry of the triazinanone protected
macrocycle, which also underwent the cis-trans photoisomer-
ization but does not show any selectivity in the subsequent [2+2]
cycloaddition. We are currently investigating the feasibility of
using carbonates instead of ureas to see if they exhibit similar
selectivity. Carbonates would be synthetically useful as they
are readily hydrolyzed and could provide a handle for further
structural elaboration.
FIGURE 3. The UV-visible absorption spectra of 5a (2.5 × 10-5
M
in CHCl3) was monitored over the course of the reaction: (a) 0 min to
1 h and (b) 1 to 10 h. Inset: Absorption spectra of the trans-stilbene
5b (2.5 × 10-5 M in CHCl3), which was synthesized independently.
In comparison, the presence of the triazinanone protecting
group did not appear to alter the cis-trans photoisomerization
step but greatly diminished the selectivity of the subsequent
[2+2] photocycloaddition. Similar to the bis-urea 5a, the
triazinanone protected macrocycle 4a showed a red shift from
284 to 292 nm after 2 h, consistent with its photoisomerization
to a trans-stilbene 4b. This band subsequently decayed as the
double bond further reacts to give a mixture of cyclobutane
isomers. The product distribution from 4a was analyzed by using
LC-MS, which showed six products with m/z 723 (Supporting
Information). This molecular weight could correlate with the
starting materials, intermediates, or the [2+2] cycloaddition
products as well as doubly charged intermolecular dimers. Closer
analysis indicated that only one of these products corresponded
to a doubly charged intermolecular dimer and that the major
products are due to intramolecular cycloadditions. Given that
our UV-vis studies suggest that 4a undergoes a cis-trans
photoisomerization followed by loss of the stilbene double bond,
we assume that the major products are a mixture of cyclobutane
stereoisomers, which were not further characterized. Experi-
mentally, it appears that the presence of the triazinanone
protecting group adversely affected the selectivity of the [2+2]
photocycloaddition.
Experimental Section
cis-Stilbene Diol 2a. A solution of LAH (0.274 g, 6.87 mmol)
was stirred in 30 mL of THF at 0 °C in a 250 mL round-bottomed
flask fitted with an addition funnel. The addition funnel was charged
with dimethyl cis-stilbene-4,4′-dicarboxylate 1a (0.925 g, 3.12
mmol) in 50 mL of THF. The mixture was added dropwise to the
LAH solution and the reaction was stirred for 6 h. The reaction
was quenched by 0.3 mL of H2O, 0.6 mL of 1 N NaOH, and then
0.9 mL of H2O. The mixture was filtered to remove precipitate,
dried over Na2SO4, and reduced in vacuo. A white solid was
We next investigated if this cis-trans photoisomerization and
subsequent [2+2] cycloaddition also occurred in the solid state.
We predicted that the reaction would favor intermolecular [2+2]
photodimers instead of 6, because the photoisomerization of
the stilbene requires large spatial displacements of atoms. A
crystalline sample of 5a (4 mg) was placed between two Pyrex
microscope slides. Surprisingly, irradiation of 5a at room
temperature for 14 h afforded 6 as the only product in high
conversion (85%). Although the reaction was slightly slower
in the solid state, it appears that crystalline constraints had no
influence on the products observed. The free ureas (5a and 5b)
1
obtained (0.71 g, 95%). Mp 116-119 °C; H NMR (300 MHz,
(15) Spartan 04 for Macintosh, v. 1.1.1; Wavefunction, Inc., Irvine, CA, 2007.
4876 J. Org. Chem. Vol. 74, No. 13, 2009

DMSO-d6) δ 4.44 (d, J ) 6.0 Hz, 4H), 5.15 (t, J ) 6.0 Hz, 2H),
6.57 (s, 2H), 7.18 (s, 8H); 13C NMR (75 MHz, DMSO-d6) δ 63.3,
127.1, 128.9, 130.3, 135.9, 142.3; MS (EI) m/z 179, 193, 221, 240
(M+); HRMS (EI) calcd for C16H16O2 240.1150, found 240.1147.
cis-Stilbene Dibromide 3a. The diol 2a (0.50 g, 2.08 mmol)
was stirred in 40 mL of THF and PPh3 (1.20 g, 4.58 mmol) and
NBS (0.815 g, 4.58 mmol) were added. The reaction mixture was
stirred overnight and then was quenched with saturated sodium
bicarbonate (3 mL) and diluted with H2O (40 mL). The solution
was reduced to half its volume in vacuo and was extracted with
CH2Cl2 (3 × 30 mL). The organic layer was separated then dried
over MgSO4, and the volatiles were removed under vacuum. The
crude mixture was loaded onto a silica gel and eluted with 1:6
EtOAc/hexanes to afford the dibromide 3a (0.59 g, 78%). Mp
108-112 °C; 1H NMR (300 MHz, CDCl3) δ 4.47 (s, 4H), 6.58 (s,
2H), 7.20-7.28 (m, 8H); 13C NMR (75 MHz, CDCl3) δ 33.6, 129.3,
129.5, 130.4, 136.9, 137.5; MS (EI) m/z 89, 103, 206, 287, 366
(M+), 370 (M+4); HRMS (EI) calcd for C16H14Br2 363.9462, found
363.9464.
4,4′-cis-Bis(urea)stilbenophane (5a). Compound 4a (0.078 g,
0.108 mmol) was dissolved in MeOH (15 mL) and a solution of
20% diethanol amine in H2O (5 mL, pH ∼2, previously adjusted
with conc. HCl) was added. The mixture was heated at 85 °C
overnight. A milky precipitate formed, which was filtered and
washed with H2O (5 mL) and MeOH (5 mL) to afford the
macrocycle 5a in 90% yield. Mp 340-342 °C dec; 1H NMR (400
MHz, DMSO-d6) δ 4.18 (d, J ) 6.0 Hz, 8 H), 6.39 (t, J ) 6.0 Hz,
4 H), 6.56 (s, 4 H), 7.10-7.16 (m, 16 H); 13C NMR (100 MHz,
DMSO-d6) δ 42.7, 127.1, 128.4, 129.6, 135.3, 139.6, 157.9; MS
(EI) m/z 129, 226, 254, 264, 290, 528 (M+); HRMS (EI) calcd for
C34H32N4O2 528.2525, found 528.2520.
4,4′-trans-Bis(urea)stilbenophane (5b). Mp 318-320 °C dec;
1H NMR (400 MHz, DMSO-d6) δ 4.10 (s, 8 H), 6.46 (t, J ) 6.8
Hz, 4 H), 7.06 (d, J ) 8.0 Hz, 8 H), 7.07 (s, 4 H), 7.34 (d, J ) 8.0
Hz, 8 H); 13C NMR (100 MHz, DMSO-d6) δ 43.2, 126.0, 127.4,
127.8, 135.3, 140.6, 159.4; MS (EI) m/z 115, 178, 191, 220, 248,
264, 290, 528 (M+); HRMS (EI) calcd for C34H32N4O2 528.2525,
found 528.2520.
4,4′-cis-Bis(triazinanone)stilbenophane (4a). Triazinanone (0.504
g, 1.38 mmol) was weighed into an oven-dried 100 mL round-
bottomed flask under N2 (g) and dissolved in 20 mL of THF. Next,
NaH (0.165 g, 4.13 mmol) was added and the reaction was heated
at 80 °C for 30 min. The solution was cooled, 3a (0.504 g, 1.38
mmol in 20 mL of THF) was added, and the mixture was heated
at 80 °C overnight. The reaction was quenched with H2O (30 mL)
and reduced to half its original volume under vacuum. The
remaining aqueous solution was extracted with CH2Cl2 (3 × 30
mL). The organic layer was separated, dried over Na2SO4, and
evaporated. The residue was purified by column chromatography
(silica gel, eluent ) 1:2 acetone:hexanes) to afford a white
(4,4′,4′′,4′′′-((1R,2R,3S,4S)-Cyclobutane-1,2,3,4-tetrayl)tetra-
kis(benzene-4,1-diyl))tetrakis(methylene)bisurea (6). A DMSO-
d6 (1 mL) solution of macrocycle 5a (4 mg) was irradiated with a
450 W Hannovia high pressure mercury vapor lamp at room
temperature. Product 6 was obtained quantitatively. 1H NMR (400
MHz, DMSO-d6) δ 3.79 (d, J ) 6.8 Hz, 8H), 4.48 (s, 4H), 5.91 (t,
J ) 6.4 Hz, 4H), 6.66 (dd, J ) 8.4, 1.6 Hz, 4H), 6.76 (dd, J ) 8.4,
1.6 Hz, 4H), 6.86 (d, J ) 8.4 Hz, 4 H), 7.05 (d, J ) 8.4 Hz, 4 H);
13C NMR (100 MHz, DMSO-d6) δ 44.4, 45.1, 126.4, 127.8, 128.0,
130.2, 138.2, 138.6, 160.2; MS (EI) m/z 178, 204, 219, 230, 249,
260, 290, 528 (M+).
1
precipitate (0.11 g, 12%). Mp 328-330 °C dec; H NMR (400
ACKNOWLEDGMENT:. The authors gratefully acknowl-
edge support in part for this work from the NSF (CHE-0718171)
and from the Petroleum Research Fund (44682). Synchrotron
data were collected through the SCrALS (Service Crystal-
lography at Advanced Light Source) project at Beamline 11.3.1
at the ALS, Lawrence Berkeley National Laboratory. The ALS
is supported by the U.S. Department of Energy, Office of Energy
Sciences Materials Sciences Division, under contract DE-AC02-
05CH11231.
MHz, DMSO-d6) δ 0.75 (s, 18 H), 4.20 (s, 8 H), 4.30 (s, 8 H),
6.59 (s, 4 H), 7.13 (d, J ) 8.0 Hz, 8 H), 7.24 (d, J ) 8.1 Hz, 8H);
13C NMR (100 MHz, DMSO-d6) δ 27.8, 46.7, 53.8, 60.5, 128.4,
129.0, 129.7, 135.9, 137.5, 155.5; MS (EI) m/z 191, 206, 234, 248,
263, 290, 389, 552, 580, 665, 722 (M+); HRMS (EI) calcd for
C46H54N6O2 722.4308, found 722.4312.
4,4′-trans-Bis(triazinanone)stilbenophane (4b). Mp 305-307
°C dec; 1H NMR (400 MHz, CDCl3) δ 1.25 (s, 9 H), 1.26 (s, 9 H),
3.49 (t, J ) 14.8 Hz, 4 H), 4.10-4.20 (m, 4 H), 4.22 (s, 4H),
5.38-5.46 (m, 4H), 7.00 (s, 2H), 7.02 (s, 2H), 7.13 (d, J ) 8.0
Hz, 4 H), 7.19 (d, J ) 8.0 Hz, 4 H), 7.29 (d, J ) 8.0 Hz, 4 H),
7.32 (d, J ) 8.0 Hz, 4 H); 13C NMR (100 MHz, CDCl3) δ 28.80,
28.82, 49.2, 49.3, 54.29, 54.31, 62.5, 62.8, 126.4, 126.5, 128.3,
128.6, 128.7, 136.28, 136.3, 137.3, 157.4, 157.4; MS (EI) m/z 161,
178, 206, 234, 248, 290, 722 (M+); HRMS (EI) calcd for
C46H54N6O2 722.4308, found 722.4322.
Supporting Information Available: NMR spectra for new
compounds, X-ray reports, and CIF files of 4a, 5a, and 6. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO900443E
J. Org. Chem. Vol. 74, No. 13, 2009 4877