Large molecular motions are tolerated in crystals of diamine double salt of trans-chlorocinnamic acids with trans-1,2-diaminocyclohexane

Article abstract of DOI:10.1021/ol050330u

(Chemical Equation Presented) Contrary to the general assumption that photoreactions in crystals may not proceed with large molecular motions, a pedal-like motion prompted by electronic excitation is believed to be involved during the β-dimer formation from the crystals of the diamine double salt of trans-2,4-dichlorocinnamic acid and trans-1,2-diaminocyclohexane.

Full text of DOI:10.1021/ol050330u

ORGANIC
LETTERS
2
005
Vol. 7, No. 10
895-1898
Large Molecular Motions Are Tolerated
in Crystals of Diamine Double Salt of
trans-Chlorocinnamic Acids with
trans-1,2-Diaminocyclohexane
1
†
‡
§
,†
Arunkumar Natarajan, Joel T. Mague, K. Venkatesan, and V. Ramamurthy*
Department of Chemistry, UniVersity of Miami, Coral Cables, Florida 33124,
Department of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118, and
Department of Organic Chemistry, Indian Institute of Science,
Bangalore, India 560 012
murthy1@miami.edu
Received February 16, 2005
ABSTRACT
Contrary to the general assumption that photoreactions in crystals may not proceed with large molecular motions, a pedal-like motion prompted
by electronic excitation is believed to be involved during the
dichlorocinnamic acid and trans-1,2-diaminocyclohexane.
â
-dimer formation from the crystals of the diamine double salt of trans-2,4-
The pioneering work of Schmidt and co-workers that laid
the foundation of solid-state bimolecular reactions has led
to extensive investigation of the [2 + 2] photodimerization
reactions that involve large molecular motions in the crystal-
line state, clear exceptions to the generally held notion that
solid-state reactions involve very little motion. Results of
this study are presented in this Letter.
2
,4
1
of cinnamic acids. trans-Cinnamic acids crystallize in three
polymorphic forms, namely, R, â, and γ, and exhibit distinct
crystal-packing-dependent photochemical behavior. Of the
Using 1,2-trans-diaminocyclohexane as a template, we
have explored the possibility of obtaining chiral δ-truxinate
from trans-cinnamic acids. Whereas the R- and â-dimers
2
several strategies to explore aligning molecules in the
crystalline state, the latest one involves the use of templates
3
to orient guest molecules to favor photodimerization.
(3) (a) Gao, X.; Friscic, T.; MacGillivray, L. R. Angew. Chem., Int. Ed.
003, 43, 232. (b) Friscic, T.; Macgillivray, L. R. Chem. Commun. 2003,
2
During the course of the investigation of template-designed
photodimerization of trans-cinnamic acids, we came across
1306. (c) MacGillivray, L. R.; Reid, J. L.; Ripmeester, J. A.; Papaefstathiou,
G. S. Ind. Eng. Chem. Res. 2002, 41, 4494. (d) MacGillivray, L. R.; Reid,
J. L.; Ripmeester, J. A. J. Am. Chem. Soc. 2000, 122, 7817. (e) Ito, Y.;
Borecka, B.; Trotter, J.; Scheffer, J. R. Tetrahedron Lett. 1995, 36, 6083.
(f) Ito, Y.; Borecka, B.; Olovsson, G.; Trotter, J.; Scheffer, J. R. Tetrahedron
Lett. 1995, 36, 6087. (g) Ito, Y.; Fujita, H. J. Org. Chem. 1996, 61, 5677.
(h) Ito, Y.; Hosomi, H.; Ohba, S. Tetrahedron 2000, 56, 6833. (i) Ito, Y.;
Kitada, T.; Horiguchi, M. Tetrahedron 2003, 59, 7323. (j) Coates, G. W.;
Dunn, A. R.; Henling, L. M.; Ziller, J. W.; Lobkovsky, E. B.; Grubbs, R.
H. J. Am. Chem. Soc. 1998, 120, 3641. (k) Feldman, K. S.; Campbell, R.
F. J. Org. Chem. 1995, 60, 1924. (l) Damen, J.; Neckers, D. C. J. Am.
Chem. Soc. 1980, 102, 3265.
†
University of Miami.
Tulane University.
Indian Institute of Science.
‡
§
(
1) (a) Cohen, M. D.; Schmidt, G. M. J. J. Chem. Soc. 1964, 1996. (b)
Cohen, M. D.; Schmidt, G. M. J.; Sonntag, F. I. J. J. Chem. Soc. 1964,
000. (c) Schmidt, G. M. J. J. Chem. Soc. 1964, 2014. (d) Ramamurthy,
2
V.; Venkatesan, K. Chem. ReV. 1987, 87, 433. (e) Bassani, D. M. CRC
Handbook of Organic Photochemistry and Photobiology, 2nd ed.;
Horspool, W., Lenci, F., Eds.; CRC Press: Boca Raton, FL, 2004; Vol. 3,
pp 20-1. (f) Natarajan, A.; Ramamurthy, V. The Chemistry of Cyclo-
butanes; Rappoport, Z., Liebman, J. F., Eds.; John Wiley & Sons: Ltd., in
press.
(4) (a) Cohen, M. D. Angew. Chem., Int. Ed. Engl. 1975, 14, 386. (b)
Jones, W.; Ramdas, S.; Theocharis, C. R.; Thomas, J. M.; Thomas, N. W.
J. Phys. Chem. A 1981, 85, 2594. (c) Thomas, J. M. Nature 1981, 289,
633.
(2) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647.
1
0.1021/ol050330u CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/16/2005

result from R and â packing, respectively, formation of δ-
and ꢀ-dimers requires crisscross alignment of the alkene
chromophore. The cycloadditon of 1,1′ and 2,2′ leads to
δ-dimer, and the cycloaddition of 1′,2 and 1,2′ leads to
of the solvent. The stoichiometry of the double salt was
determined using H NMR. X-ray-quality single crystals of
1
3
e
diamine double salts 1 and 2 were obtained by recrystalli-
zation in ethanol and acetonitrile, respectively. The melting
points of salts 1 and 2 are 145-152 and 118-122 °C,
respectively. The crushed crystals of salts 1 and 2 were
sandwiched between Pyrex plates, sealed with Parafilm and
irradiated using a 450 W medium-pressure mercury arc lamp.
Irradiation of the salt at high temperatures (50, 65, and 75
ꢀ
-dimer as depicted in Scheme 1. Irradiation of the diamine
Scheme 1
°
C) was carried out by placing this setup on a hot plate. The
temperature of the hot plate was monitored using a thermo-
couple. The crystals neither melted nor decomposed during
irradiation at these temperatures. The products were isolated
as acids by addition of aqueous HCl and converted to methyl
esters (cis isomers and dimers) by the addition of diazo-
methane.
The trans-2,4-dichlorocinnamic acid packs in the â form
in the crystalline state leading to the mirror symmetric
1
a
â-truxinate. The X-ray structure revealed the packing of
neighboring molecules to possess a short distance of 3.77 Å
(Figure 1).
double salt 1 formed between 1,2-trans-(R,R)-diaminocy-
clohexane and trans-2,4-dichlorocinnamic acid produced the
mirror symmetric â-dimer instead of the anticipated δ- and/
or ꢀ-dimer (Scheme 2). Similar irradiation of diamine double
Scheme 2
Figure 1. Packing of trans-2,4-dichlorocinnamic acid in the â-form
in the crystalline state.
3
e
Contrary to the literature report on the photoreaction of
diamine double salt of trans-cinnamic acid with trans-1,2-
diaminocyclohexane, which gave predominantly the ꢀ-trux-
illate, irradiation of salt 1 yielded â-truxinate as the major
product (49% at 57% conversion) along with the cis isomer
(
6%) and δ-truxinate (2%) (Scheme 2, Table 1). The
Table 1. Time- and Temperature-Dependent Study for the
Formation of Cis vs â-Dimer on Photolysis of Salts 1 and 2
temp
duration of
%
â-truxinate cis δ-dimer
(
°C) irradiation (h) conversion
(%)
(%)
(%)
salt 1
salt 2 formed between 1,2-trans-(()-diaminocyclohexane and
trans-2-chlorocinnamic acid resulted in the cis isomer as the
major product. Both reactions as described below involve
rt
rt
rt
rt
3
12
21
32
8
35
45
57
7
30
38
49
1
4
5
6
1
2
2
5
large molecular motions in the crystalline state.
The diamine double salts 1 and 2 were prepared by mixing
equiv of cinnamic acids and 1 equiv of 1,2-diaminocy-
clohexane in methanol/ether solution followed by evaporation
salt 2
0
rt
10
10
10
10
10
4
19
23
26
34
0
2
3
3
4
4
16
19
22
28
0
1
1
1
2
2
4
0
65
(
5) As the photoreaction of chiral salt 1 produced chiral δ-dimer only in
70
1
-2% yield, the salt 2 was made using (()-trans-1,2-diaminocyclohexane.
1896
Org. Lett., Vol. 7, No. 10, 2005

increased yields of the â-truxinate and the cis isomer upon
irradiation of 1 at elevated (50, 65, 75 °C) temperatures
suggest a faster reaction.
Scheme 3
To investigate the origin of the major product, â-truxinate,
the crystal structure of salt 1 was determined. The X-ray
crystal structure analysis of the diamine double salt 1 revealed
the reactive olefins to be crisscrossed and the distances
between the ends of the alkenes to be unequal (3.39 and
4.55 Å, Figure 2). â-Dimer formation requires both the
Figure 2. (a) Crystal structure of diamine double salt 1 showing
alkene distances. (b) Crystal structure of salt 1 viewed along the
“
a” axis showing a crisscross alignment of olefins.
At least four possible mechanisms could be visualized for
the formation of â-dimer: (a) Defect sites where the two
olefins are prealigned in favor of the â-dimer function as
the loci of dimerization. (b) Addition occurs by a concerted
supra-supra process in the excited state. (c) Excitation
prompts rotation of one olefin to yield the arrangement shown
in Scheme 3a. This excited pair (excimer) dimerizes in a
concerted fashion. (d) The [2 + 2]-photodimerization is a
stepwise process, initiated by the covalent bond formation
between the olefinic carbons at the shorter ends (3.39 Å) to
form a biradical intermediate (Scheme 3b). Rotation of one
of the 2,4-dichloro ethyl benzyl radicals followed by ring
closure would lead to the â-truxinate. We believe that our
results cannot be accommodated by mechanisms a and b.
Unless defects of the same type are continuously generated,
a high yield of a single dimer is unlikely. Concerted addition
without rotation would not lead to the â-dimer. Also, the
two reacting ends are not of equal distance (Figure 2a). Such
an arrangement would not favor simultaneous addition at
both ends. At this stage, we are unable to distinguish between
the possibilities c and d.
reactive olefins to be aligned in a parallel fashion. Therefore,
a conversion of the crisscrossed olefins to a parallel alignment
must have occurred prior to dimer formation.
We speculate that such a converison is prompted by a
pedal-like motion of one of the olefins of the salt prior to
6
dimer formation (Scheme 3a). This type of motion has been
reported previously during the photoreaction of the hydrogen
bonded complex between trans-cinnamamide and phthalic
7
acid in the crystalline state. The formation of the cis isomer
(from trans-cis isomerization) along with the â-dimer
suggests a “loose” packing that would allow a large degree
of motion of the reacting olefin. The possibility of large
crankshaft-type motion in crystals of highly congested bis-
(
triarylmethyl)peroxide has been reported by Garcia-Garibay
8
and co-workers. The possibility of dynamic disorder through
C-C rotations in the crystals of trans-stilbene has been noted
9
by Ogawa and co-workers. Thus, rotations of parts of
molecules in crystals, although uncommon, are not unknown.
(6) (a) Ogawa, K.; Sano, T.; Yoshimura, S.; Takeuchi, Y.; Toriumi, K.
J. Am. Chem. Soc. 1992, 114, 1041. (b) Harada, J.; Ogawa, K.; Tomoda, S.
J. Am. Chem. Soc. 1995, 117, 4476. (c) Harada, J.; Uekusa, H.; Ohashi, Y.
J. Am. Chem. Soc. 1999, 121, 5809.
(
7) Ohba, S.; Hosomi, H.; Ito, Y. J. Am. Chem. Soc. 2001, 123, 6349.
(9) (a) Ogawa, K.; Sano, T.; Yoshimura, Y.; Takeuchi, Y.; Toriumi, K.
J. Am. Chem. Soc. 1992, 114, 1041. (b) Harada, J.; Ogawa, K.; Tomoda, S.
J. Am. Chem. Soc. 1995, 117, 4476. (c) Harada, J.; Ogawa, K.; Tomoda, S.
Acta Crystallogr. 1997, B53, 662.
(8) 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,
4778.
1
Org. Lett., Vol. 7, No. 10, 2005
1897

As noted in Table 1, the dimer is accompanied by small
amounts of the cis isomer. On the basis of the study of
several cis-cinnamic acid derivatives, Schmidt and co-
workers have suggested cis-trans isomerization in the solid
1
0
state to involve an unstable cyclobutane intermediate. As
seen in Figure 3, the ratio of the cis isomer and cyclobutane
Figure 4. Crystal structure of salt 2 showing three closely lying
cinnamate molecules hydrogen bonded to the trans-1,2-diaminocy-
clohexane moiety; only the closely lying cinnamate pair alkene-
alkene distances are shown.
irradiation of salts 1 and 2 must be due to the loss of long-
range order in the crystal when trans-cis isomerization
occurs. Results presented above show that diamine double
salts can assemble two olefins in the crystalline state. To
probe whether they can do the same in solution, we are
currently pursuing solution photochemistry of these salts.
The basic solid-state chemistry principle, namely, the
Figure 3. Temperature dependence of the cis isomer and cyclobu-
tane from salt of 1. The salt was photolyzed for the same duration
(20 h) at all temperatures.
“
topochemical postulate”, was formulated on the assumption
varies slightly with the temperature. Had the formation of
the cyclobutane intermediate, as suggested previously, been
the rate-determining step and had the dimer failed to
thermally revert, one would expect the ratio of the dimer to
the cis isomer to be independent of the temperature.
that molecules present in the crystals do not undergo large
degrees of motion upon excitation. The two examples
provided in this report suggest exceptions to the well
accepted topochemical postulate. We and others had shown
previously that for photodimerization, the olefinic bonds need
not to be parallel (within 4.2 Å) to one another in a crystal.11
In 7-methoxy coumarin, an “in plane” motion allowed the
crisscrossed olefinic bonds to become parallel. In one of
the examples presented here, much larger motion is needed
to achieve a parallel arrangement so that dimerization could
occur. Photochemistry has evolved from such exceptions to
established postulates. It remains to be seen if these excep-
tions have any wide implications for the field.
The photobehavior of the diamine salt of o-chlorocinnamic
acid and (()-trans-1,2-diaminocyclo-hexane 2 was quite
different from that of salt 1. Photolysis of salt 2 gave the cis
isomer as the major product (26%), and both â- (7%) and
δ-dimers (3%) were formed as minor products for an overall
conversion of 36% (Table 1). Photolysis of salt 2 at elevated
temperatures (40, 65, 75 °C) led to an increased yield of the
cis isomer and the â-truxinate (Table 1). The above results
suggest that the o-chloro cinnamate, unlike the 2,4-dichlo-
rocinnamate, does not orient the cinnamate groups effectively
for photodimerization. The lack of ordered packing of the
cinnamate groups of trans-1,2-diaminocyclohexane salt of
o-chloro-cinnamic acid (2), as evident from the crystal
structure, explains the poor dimerization yield (Figure 4).
The diaminocyclohexane dication is hydrogen bonded to
three nearest cinnamate molecules of which two are arranged
in a crisscross fashion with alkene-alkene end distances of
1
1b
Acknowledgment. V.R. thanks the NSF for financial
support (CHE-0212042). J.T.M. thanks the Louisiana Board
of regents through the Louisiana Educational Quality Support
Fund for purchase of the Bruker APEX diffractometer.
Supporting Information Available: Crystal structure
coordinates and tables of positional and thermal parameters,
bond lengths, and bond angles; H NMR data of salts and
dimers. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
4.15 and 4.7 Å. The third cinnamate molecule is placed
farther away, with alkene-alkene end distances of >5 Å.
On excitation of 2, due to the presence of vacant space
around the molecule, the cinnamate ion can undergo geo-
metric isomerization in the excited state, leading to formation
of the cis isomer. The absence of mixed dimers from cis-
and trans-olefins or homodimers from the cis-olefins during
OL050330U
(11) (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. (e) Leiserowitz, L.; Schmidt, G. M. J. Acta Crystallogr.
1965, 18, 1058.
(10) Bregman, J.; Osaki, K.; Schmidt, G. M. J.; Sonntag, F. I. J. Chem.
Soc. 1964, 2021.
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Org. Lett., Vol. 7, No. 10, 2005