Volume-demanding cis-trans isomerization of 1,2-diaryl olefins in the solid state

Article abstract of DOI:10.1021/jo0520644

Volume-demanding cis-trans photoisomerization of the aromatic substituted alkenes 1-3 in the solid state at room temperature and at 50 °C is presented. Alkene 3 did not undergo the cis-trans isomerization in the solid state either at room temperature or a

Full text of DOI:10.1021/jo0520644

Volume-Demanding Cis-Trans Isomerization of 1,2-Diaryl Olefins
in the Solid State
Arunkumar Natarajan,† Joel T. Mague,‡ K. Venkatesan,§ T. Arai,| and V. Ramamurthy*,†
Department of Chemistry, UniVersity of Miami, Coral Cables, Florida 33124, Department of Chemistry,
Tulane UniVersity, New Orleans, Louisiana 70118, Department of Organic Chemistry, Indian Institute of
Science, Bangalore, India 560 012, and Department of Chemistry, UniVersity of Tsukuba, Tuskuba,
Ibaraki 305, Japan
murthy1@miami.edu
ReceiVed October 2, 2005
Volume-demanding cis-trans photoisomerization of the aromatic substituted alkenes 1-3 in the solid
state at room temperature and at 50 °C is presented. Alkene 3 did not undergo the cis-trans isomerization
in the solid state either at room temperature or at 50 °C. The importance of the presence of void space
near the reaction center to facilitate the large volume change during cis-trans photoisomerization is
discussed.
Introduction
photodimerization reaction of a diamine double salt of substi-
tuted cinnamic acids with trans-1,2-diaminocyclohexane in the
solid state that required a large pedal-like rotation of the alkene
chromophore prior to dimerization.4 These examples clearly
suggest that certain amounts of molecular motions are tolerated
within a crystal. We now report that one-way cis-trans
isomerization of aromatic substituted alkenes requires much
larger molecular motions in the solid state compared to the
examples discussed above. These examples emphasize the need
The concept of a reaction cavity was formulated by Cohen
to describe photodimerization reactions in crystals.1 According
to this concept, large distortion of the reaction cavity will not
be tolerated by surrounding molecules and, therefore, molecules
in the crystalline state would not undergo a reaction that re-
quires large motions. However, a few years ago, we and others
presented examples wherein molecules upon excitation undergo
large motions in the crystalline state prior to dimerization.2
Ogawa and co-workers have reported the presence of orienta-
tional disorder in stilbenes and azobenzenes,3a,b and Garcia-
Garibay and co-workers have demonstrated a large crankshaft-
type motion in crystals of the highly congested bis(triaryl-
methyl)peroxide.3c Recently, we reported examples of a [2+2]
(2) (a) Murthy, G. S.; Arjunan, P.; Venkatesan, K.; Ramamurthy, V.
Tetrahedron 1987, 43, 1225. (b) Gnanaguru, K.; Ramasubbu, N.; Venkate-
san, K.; Ramamurthy, V. J. Org. Chem. 1985, 50, 2337. (c) Theocharis, C.
R.; Jones, W.; Thomas, J. M.; Motevalli, M.; Hursthouse, M. B. J. Chem.
Soc., Perkin Trans. 2 1984, 2, 71. (d) Theocharis, C. R.; Clark, A. M.;
Hopkin, S. E.; Jones, P.; Perryman, A. C.; Usanga, F. Mol. Cryst. Liq. Cryst.
1988, 156, 85.
(3) (a) Harada, J.; Ogawa, K.; Tomoda, S. J. Am. Chem. Soc. 1995, 117,
4476. (b) Harada, J.; Ogawa, K. J. Am. Chem. Soc. 2001, 123, 10884. (c)
Khuong, T.-A. V.; Zepeda, G.; Sanrame, C. N.; Dang, H.; Bartberger, M.
D.; Houk, K. N.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2004, 126,
14778.
† University of Miami.
‡ Tulane University.
§ Indian Institute of Science.
| University of Tsukuba.
(1) (a) Cohen, M. D. Angew. Chem., Int. Ed. Engl. 1975, 14, 386. (b)
Cohen, M. D.; Schmidt, G. M. J.; Sonntag, F. I. J. Chem. Soc. 1964, 2000.
(c) Ramamurthy, V.; Venkatesan, K. Chem. ReV. 1987, 87, 433.
(4) Natarajan, A.; Mague, J. T.; Venkatesan, K.; Ramamurthy, V. Org.
Lett. 2005, 7, 1895.
10.1021/jo0520644 CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/31/2005
J. Org. Chem. 2006, 71, 1055-1059
1055

Natarajan et al.
TABLE 1. Results of Photochemical Studies of Substrates 1-3 in
the Solid State
SCHEME 1
temp
(°C)
duration of
hυ* (hrs)
%
reactanta
producta
medium
conversion
cis-1
cis-1
cis-1
trans-1
trans-1
trans-1
cis-1
trans-2a
trans-2a
trans-2a
cis-2a
trans-2b
trans-2b
trans-2b
trans-2c
trans-2c
trans-2c
trans-3
trans-3
trans-3
benzene
solid
solid
solid
benzene
solid
solid
solid
benzene
solid
solid
benzene
solid
solid
benzene
solid
solid
rt
rt
50
rt
rt
rt
50
rt
rt
rt
50
rt
rt
50
rt
rt
2
18
18
12
2
18
18
12
2
18
18
2
18
18
2
c:t ) 15:85b
25
60
c
trans-1
cis-2a
cis-2a
cis-2a
trans-2a
cis-2b
cis-2b
cis-2b
cis-2c
cis-2c
cis-2c
cis-3
c:t ) 2:98b
55
100
c
c:t ) 0:100b
75
90
c:t ) 0:100b
45
70
c:t ) 0:100b
c
cis-3
cis-3
18
18
c
50
a Melting points: cis-1 ) 100-101 °C, trans-1 ) 131-133 °C; cis-2a
) 102-103 °C, trans-2a ) 156 °C; cis-2b ) 130-131 °C, trans-2b )
193-194 °C; cis-2c ) 141-142 °C, trans-2c ) 219-220 °C; cis-3 ) 133-
135 °C, trans-3 ) 305-306 °C. b Represents ratio at the photostationary
state. c No reaction.
result in cis-trans isomerization even after several hours of
irradiation both at room temperature and at 50 °C (Scheme 1
and Table 1). In contrast to the cis isomers, photolysis of trans-
1, -2a-c, and -3 as crystals did not result in the corresponding
cis isomers either at room temperature or at 50 °C.
for void space near the reacting molecules. Crystalline cis-
olefins, cis-1 and cis-2a-c, upon irradiation undergo one-way
geometric isomerization to yield the corresponding trans isomers
(Scheme 1). However, olefin cis-3 was photostable even after
several hours of photoreaction in the crystalline state. To
understand the factors that control the geometric isomerization
of cis-1 and cis-2a-c and the absence of such a process in cis-
3, we have obtained crystal structures of these olefins. Results
of photochemical and X-ray structural studies are presented in
this report.
Discussion
In 1964, Schmidt et al. reported that several crystalline cis-
cinnamic acids upon irradiation yielded the corresponding trans
isomers.6 The authors suggested that the isomerization occurred
through a mechanism involving a metastable cyclobutane
intermediate. On the basis of the photobehavior of trans-2,4-
dichlorocinnamic acid salt with trans-1,2-diaminocyclohexane,
we suggested that the above mechanism might not be general.4
In the present study, except for cis-3, all other substrates (cis-1
and cis-2a-c) were photoreactive in the solid state and gave
the corresponding trans isomers (Scheme 1, Table 1). Irradiation
of cis-2a as crystals for 18 h at 50 °C was remarkable in leading
conversion to 100% trans-2a. Perusal of Table 1 reveals that
the yield of the trans isomer for the same duration of irradiation
increased upon increasing the temperature to 50 °C. The greater
percentage of conversion at 50 °C as compared to that over the
same time period at room temperature suggests that the
molecules have more rotational freedom at the higher temper-
ature. This could be due to an expansion of the crystal lattice
that increases the intermolecular distance enough to permit a
more facile rotation of the phenyl moiety. Alternatively, the
elevated temperature could cause a phase change or destruction
Results
Crushed crystals (5 mg) of cis- or trans-1-3 were sandwiched
between Pyrex plates, sealed with Parafilm, and irradiated using
a 450 W medium-pressure mercury lamp. The wavelengths of
irradiation were selected using appropriate filters (>350 nm,
Corning-0-52 cutoff filter for cis-1 and cis-2a-c; and >425
nm, Corning-3-73 cutoff filter for cis-3). For high-temperature
irradiations, the above setup was placed directly on a thermo-
statically controlled hot plate and the temperature was monitored
using a thermocouple. The crystals neither melted nor decom-
posed during irradiation at these temperatures. Solid-state
irradiation of substrate cis-1 for 18 h at room temperature led
to 25% geometric photoisomerization yielding trans-1. Photo-
reaction of this substrate at an elevated temperature (50 °C) for
18 h of irradiation increased the rate of the isomerization
process, giving 60% trans-1. Solution photobehavior of 1-3,
investigated previously, has shown that cis-1 and cis-2a isomer-
ize from both cis and trans isomers, whereas cis-2b, cis-2c, and
cis-3 proceed only from cis to trans.5 Photolysis of cis-2a-c in
the solid state also led to one-way cis-trans isomerization with
an increased rate of geometric isomerization when irradiated at
50 °C. On the other hand, photolysis of cis-3 as a solid did not
(5) (a) Arai, T.; Tokumaru, K. In AdVances in Photochemistry; Neckers,
D. C., Volman, D. H., Bunau, G. V., Eds.; John Wiley & Sons: New York,
1995; Vol. 20, pp 1-27. (b) Arai, T.; Tokumaru, K. Chem. ReV. 1993, 93,
23. (c) Furuuchi, H.; Kuriyama, Y.; Arai, T.; Sakuragi, H.; Tokumaru, K.
Bull. Chem. Soc. Jpn. 1991, 64, 1601. (d) Kikuchi, Y.; Okamoto, H.; Arai,
T.; Tokumaru, K. Chem. Lett. 1993, 833. (e) Arai, T.; Takahashi, O. J.
Chem. Soc., Chem. Comm 1995, 1837. (f) Karatsu, T.; Kitamura, A.; Zeng,
H.; Arai, T.; Sakuragi, H.; Tokumaru, K. Bull. Chem. Soc. Jpn. 1995, 68,
920. (g) Arai, T.; Hozumi, Y.; Takahashi, O.; Fujimori, K. J. Photochem.
Photobiol., A: Chemistry 1997, 104, 85.
(6) Bregman, J.; Osaki, K.; Schmidt, G. M. J.; Sonntag, F. I. J. Chem.
Soc. 1964, 2021.
1056 J. Org. Chem., Vol. 71, No. 3, 2006

Cis-Trans Isomerization of 1,2-Diaryl Olefins in the Solid State
FIGURE 2. Packing diagram of cis-2b showing the empty channel
along the a-axis.
FIGURE 1. Packing diagram of cis-1 showing the cavity along the
c-axis.
the distances between neighboring olefinic double bonds are
also larger (>5 Å) than the proposed maximum distance for
formation of dimer intermediates.
TABLE 2. End-to-End Alkene-Alkene Distances Obtained from
Crystal Structures
Having ruled out a metastable dimer as an intermediate during
geometric isomerization, we examined the crystal structures of
cis-1 and -2 for clues to other possible mechanisms for
isomerization in the crystalline state. The presence of void space
in the crystal lattice was scrutinized by examining the molecular
packing along the a-, b-, and c-axes and parallel to the ab, ac,
and bc faces. As shown in Figure 1, an empty channel along
the c-axis is evident in the case of cis-1. Because the presence
of an empty channel is adjacent to the phenyl side of the
molecule, we believe that the rotation of this side of the alkene
is more likely than rotation of the side with the bulkier
fluoroanthenyl moiety. Our reasoning is supported by the
absence of short-contact atoms with the phenyl moiety along
the top face of the molecule (Table 2). The few short contacts
near the phenyl moiety are not on the plane in which the rotation
of the double bond is expected to occur (see Supporting
Information).
a,b
short contacts for R1, R2
adjacent CdC distances
compound
(Å)
top face
bottom face
cis-1
6.77
no, yes
no, yes
no, yes
no, yes
yes, yes
yes, yes
yes, yes
no, yes
yes, yes
yes, yes
cis-2a
cis-2b
cis-2c
cis-3
5.3, 5.2c
8.1
7.57
4.25, 4.27, and 4.9, 5.2d
a cis-1: R1 ) phenyl, R2 ) 8-fluoranthyl. cis-2a-c: R1 ) phenyl,
p-cyanophenyl, and p-nitrophenyl, R2 ) pyrene. cis-3: R1dR2 ) diazulene.
b Short contact is defined as the intermolecular distance by which two atoms
are separated by less than the sum of van der Waals radii (C-C, C-H,
and H-H are 3.4, 2.9, and 2.4 Å, respectively). c Two alkene ends are
nonequivalent. d Two independent pairs with nonequivalent ends.
of crystallinity which would lead to a relaxation of the
intermolecular contacts to the phenyl group.
Drawings generated through the MERCURY7 and PLATON8
programs, packing diagrams, short contacts, and the cavity plots
of the molecules (cis-1, cis-2a-c, and cis-3) are shown in
Figures 1-5. Torsion angles between the aromatic rings and
the double bonds for all substrates are presented in the
Supporting Information. The crystal structure of cis-1 shows
neighboring molecules stacked along the c-axis (Figure 1) with
the alkene chromophores separated by 6.77 Å (Table 2). As
this distance is significantly greater than that of 4.2 Å postulated
by Schmidt as appropriate for the [2+2] formation of the
metastable dimer, it appears that isomerization of cis-1 to trans-1
is very unlikely to proceed via a dimer pathway. A similar
situation exists for the other reactive olefins, cis-2a-c, where
The reactivity during photoreaction of cis-1-styryl pyrenes
cis-2a-c differed, as shown in Table 1. Although solid-state
irradiation of cis-2b for 18 h at room temperature led to 75%
conversion to trans-2b, irradiation of cis-2a and -2c gave only
55 and 45% conversion to corresponding trans isomers,
respectively. On the basis of the presence of a void space
adjacent to the phenyl moiety along the a-axis on examining
the crystal structure of cis-2b (Figure 2), we anticipate that the
phenyl side of the CdC bond of cis-2b will undergo rotation
upon excitation. Analysis of crystal structures of cis-2a and -2c
reveals the presence of void space near the phenyl moiety
(Figure 3, Table 2). In the case of substrate cis-2a, the adjacent
alkene molecules are aligned in a criss-cross fashion. In such
an arrangement, rotation of the alkene along with the phenyl
moiety can occur in the direction represented by the arrows in
Figure 3a. Analysis of packing diagrams generated using the
PLATON program8 showed the presence of cavities in the
crystal lattices of cis-2a and cis-2c. The presence of a cavity
(7) Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M. K.; Macrae,
C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr. 2001, B58,
815.
(8) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 2005.
J. Org. Chem, Vol. 71, No. 3, 2006 1057

Natarajan et al.
FIGURE 3. (a) Packing diagram of cis-2a along the c-axis. (b) Packing diagram of cis-2c along the c-axis.
FIGURE 4. Cavity plot generated using the PLATON program for (a) cis-2a and (b) cis-2c molecules.
with a volume of 10.08 Å3, between two adjacent pyrene
moieties in cis-2a, increases the packing distance between two
adjacent cis molecules (Figure 4a). Similarly, in the case of cis-
2c, the reaction cavity (each cavity volume is 5.9 Å3) is located
adjacent to the alkene and phenyl moieties, thereby creating
void space near the reaction center (Figure 4b). Stereoscopic
views of cis-1, -2a-c (see Supporting Information) are useful
in visualizing the presence of void space around the reaction
center.
isomerization in the crystalline state (Figure 5). The crystal
structure of cis-3 reveals that there are eight molecules in the
unit cell (Z ) 8). It also reveals the presence of many atoms
within the short contacts on both the top and the bottom faces
of the azulene moieties of the alkene (Table 2). Analysis of the
crystal lattice along all directions showed no void space near
the reaction site. We suggest that the absence of free space and
the presence of too many short contacts with nearby atoms near
the aryl group endow photostability to 3c.
During our investigation of the one-way geometric isomer-
ization process in the solid state, we noted that cis-3 (Scheme
1) did not undergo cis-trans isomerization in the solid state.
The crystal structure of cis-3 clearly supports the model that
free volume is required for the occurrence of geometric
The conventional model of photochemical geometric isomer-
ization is torsional relaxation of the double bond which involves
a one-bond flip process (OBF), i.e., turning over one-half of
the molecule. This is a three-dimensional process requiring the
presence of a large free volume within a reaction cavity. In the
1058 J. Org. Chem., Vol. 71, No. 3, 2006

Cis-Trans Isomerization of 1,2-Diaryl Olefins in the Solid State
examples provided above, this is exactly what happens. Cohen
defines a reaction cavity as the space occupied by the reactant
molecules plus the empty space surrounding it. In the crystalline
state, unlike in biological media, the reaction cavity is rigid and
the space needed for a given reaction must be built in.11
Therefore, in loosely packed crystals, the occurrence of geo-
metric isomerization requiring large motions should not be a
surprise.12
Experimental Section
The synthesis and characterization of 1-3 have been reported
previously.5
Photoreaction and Analysis Procedure. Crystals 1-3 (∼5 mg),
either as ground single crystals or in polycrystalline form (powder),
were sandwiched between two Pyrex plates and spread to cover a
surface area of 2-3 cm2. The plates were sealed with Parafilm on
all the sides before the irradiation process. After the irradiation,
FIGURE 5. Crystal structure of cis-3 showing short contacts.
1
the solid was dissolved in 2 mL of CDCl3 and analyzed using H
absence of a large globular (three-dimensional) free volume,
this process would be prevented in a crystal by the walls of the
reaction cavity. An alternative mechanism known as the “hula-
twist” is being currently considered as a possibility in a
constrained medium.9 The hula-twist process (HT), unlike
conventional one-bond processes, requires less volume change
during cis-trans conversion. The hula-twist is more of a two-
dimensional rather than a three-dimensional process. During the
cis-trans conversion via a one-bond flip mechanism, one-half
of the molecule undergoes a 180° flip; i.e., half of the molecule
rises from the molecular plane and sweeps a 180° motion before
it rests on the same plane in a different geometry. On the other
hand, a hula-twist process which involves a simultaneous
rotation of two adjacent bonds (a single and a double bond) or
a 180° translocation of one C-H unit results in cis-trans
isomerization. In the hula-twist process, only one C-H unit of
the molecule rises above the molecular plane during cis-trans
conversion. Therefore, the volume demand on this process is
much less than that during the one-bond flip. The point to be
noted is that, between the one-bond flip and the hula-twist, the
latter would be preferred under conditions where the reaction
cavity has only a limited free volume. Although the suggestion
has been made that the geometric isomerization within a crystal
may occur via a hula-twist process, concrete evidence in favor
of this is still lacking.10 The occurrence of geometric isomer-
ization in crystalline cis-1 and -2a-c in this study can be
understood on the basis of the conventional one-bond flip on
the phenyl side of the alkenes. However, the possibility of the
hula-twist mechanism could not be fully ruled out. Molecules
in crystalline cis-3 must be so tightly packed so that neither the
one-bond flip nor the hula-twist is able to generate the trans
isomer.
NMR. The melting points of the substrates were determined and
are uncorrected. Irradiations were performed using a 450 W
medium-pressure mercury arc lamp in a water-cooled immersion
well. Photoreactions were conducted using appropriate filters (350
nm, Corning-0-52 cutoff filter for cis-1 and cis-2a-c; and 425 nm,
Corning-3-73 cutoff filter for cis-3).
Crystal Structure Determination. Full spheres of data were
collected using 606 scans in ω (0.3° per scan) at æ ) 0, 120, and
240°.13a,b The raw data were reduced to F2 values using the SAINT+
software,13c and global refinements of unit-cell parameters employ-
ing approximately 2000-5000 reflections chosen from the full data
sets were performed. Multiple measurements of equivalent reflec-
tions provided the basis for empirical absorption corrections as well
as for corrections for any crystal deterioration during the data
collection (SADABS13d). All structures were completed by suc-
cessive cycles of full-matrix, least-squares refinement followed by
the calculation of difference maps. All hydrogen atoms attached to
carbon were placed in calculated positions with isotropic displace-
ment parameters 20% larger than those of the attached atoms and
allowed to ride. All computations associated with structure solution,
refinement, and presentation were performed with the SHELXTL13b
package. The cavity plots were obtained using the PLATON8
program. For crystal structure visualization, MERCURY (CSD
software)7 was used.
Acknowledgment. V.R. thanks the NSF (CHE-0212042) for
financial support, and J.T.M. thanks the Louisiana Board of
Regents through the Louisiana Educational Quality Support
Fund (Grant No. LEQSF (2003-2003)-ENH-TR-67) for the
purchase of the diffractometer.
Supporting Information Available: Dihedral angles, packing
diagrams generated through the Mercury program showing the short
contacts for 1-3, and the stereoscopic view of the packing diagrams
of 1-3. This material is available free of charge via the Internet at
http://pubs.acs.org.
Conclusion
JO0520644
There is a general belief that reactions that require large
motions would not occur in the solid state. This is based on the
assumption that molecules are densely packed in the crystalline
state. If for any reason the packing is such that it leaves void
space near the reacting molecules, this space could be utilized
during the transformation of a reactant to a product. In the
(11) (a) Ramamurthy, V.; Weiss, R. G.; Hammond, G. S. AdV. Photo-
chem. 1993, 18, 67. (b) Weiss, R. G.; Ramamurthy, V.; Hammond, G. S.
Acc. Chem. Res. 1993, 26, 530. (c) Liu, R. S. H.; Hammond, G. S. Acc.
Chem. Res. 2005, 38, 396.
(12) For a summary of photoreactions of olefins in crystals, see:
Natarajan, A.; Ramamurthy, V. In The Chemistry of Cyclobutanes; Rapport,
Z., Liebman, J. F., Eds.; John Wiley: New York, 2005; pp 807-872.
(13) (a) SMART, version 5.625; Bruker-AXS: Madison, WI, 2000. (b)
SHELXTL, version 6.10; Bruker-AXS: Madison, WI, 2000. (c) SAINT+,
version 7.03; Bruker-AXS: Madison, WI, 2004. (d) Sheldrick, G. M.
SADABS, version 2.05; University of Gottingen: Germany, 2002.
(9) (a) Liu, R. S. H.; Hammond, G. S. Chem.-Eur. J. 2001, 7, 2. (b)
Liu, R. S. H.; Hammond, G. S. Acc. Chem. Res. 2001, 34, 555.
(10) Kaupp, G. Photochem. Photobiol. 2002, 76, 590.
J. Org. Chem, Vol. 71, No. 3, 2006 1059