Competitive isomerization and dimerization in co-crystals of 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol and sorbic acid: A new look at stereochemical requirements for [2+2] dimerization

Article abstract of DOI:10.1039/b802103a

Competitive [2+2] photodimerization and E→Z isomerization reactions occur in a co-crystal of 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol upon irradiation with 325 nm light. At 90 K both reactions are observed, whereas at 280 K the dimerization reaction is very fast and inhibits isomerization as the nature of the chromophore is affected by the reaction. The temperature dependence of the stereospecificity of the dimerization reaction is related to the large sliding motion required to bring the reacting molecules into juxtaposition. The progress of the reactions has been monitored by photocrystallographic methods. The Royal Society of Chemistry.

Full text of DOI:10.1039/b802103a

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Competitive isomerization and dimerization in co-crystals of
1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol and sorbic acid: a new look
at stereochemical requirements for [2+2] dimerizationw
Shao-Liang Zheng,* Oanh Pham, Christophe M. L. Vande Velde,z
Milan Gembicky and Philip Coppens*
Received (in Cambridge, UK) 6th February 2008, Accepted 28th February 2008
First published as an Advance Article on the web 31st March 2008
DOI: 10.1039/b802103a
Competitive [2+2] photodimerization and E-Z isomerization
reactions occur in a co-crystal of 1,1,6,6-tetraphenyl-2,4-hexa-
diyne-1,6-diol upon irradiation with 325 nm light. At 90 K both
reactions are observed, whereas at 280 K the dimerization
reaction is very fast and inhibits isomerization as the nature of
the chromophore is affected by the reaction. The temperature
Scheme 1 Diagram of (a) Diol and (b) HSA.
dependence of the stereospecificity of the dimerization reaction
is related to the large sliding motion required to bring the
butane derivative, the detailed behavior depending on the
reacting molecules into juxtaposition. The progress of the reac-
temperature.
ꢀ
The compound crystallizes in the triclinic space group P1
tions has been monitored by photocrystallographic methods.
(see Table S1, ESIw). The HSA molecules are linked to the
Diol by hydrogen bonding between the hydroxy groups of the
Photochemical reactions taking place in organic solids have
been studied extensively. The two most common photo-reac-
Diol and carboxyl oxygen atoms of HSA [280 K: Oꢀ ꢀ ꢀO
2.680(2) and 2.734(2) A, see Table S2, ESIw and Fig. 1]. The
Diol molecule occupies a crystallographic inversion center
(1/2, 1/2, 1/2), while the HSA molecules are packed in parallel
pairs related by an inversion center at (1, 1, 1/2). Accordingly,
the double bonds are parallel to each other. As shown in
Fig. 2, the distance between the potentially reactive atoms is
within the range proposed for photodimerization.11 This ar-
rangement produces the centrosymmetric cyclobutane deriva-
tive on reaction.
tions of olefins are geometric isomerization1–4 and dimeriza-
tion, the latter pioneered by Schmidt and coworkers in the
sixties5 and extensively studied since then.6,7 Intermolecular
[2+2] photo-additions can be ‘template-controlled’8 by using
diols like resorcinol,7 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-
diol (‘Diol’, Scheme 1a) and derivatives9,10 in co-crystal
systems, in which case the reactions tend to proceed as
single-crystal to single-crystal reactions (SCSC), without
breakdown of the crystal lattice.
In preceding work we showed that olefin-acids, as well as
their coordination compounds, can be isolated in a supramo-
lecular framework, and the corresponding SCSC E-Z and
Z-E photo-isomerizations can proceed without side reac-
tions.1 We report here on competitive photo-reactions in co-
crystals of sorbic acid (‘HSA’, Scheme 1b) and Diol, with the
composition Diol-2HSA.y On exposure of a single crystal to
325 nm light the sorbic acid molecules undergo either E-Z
isomerization or [2+2] photodimerization to form a cyclo-
A diffractometer-mounted crystal of Diol-2HSA was ex-
posed to 325 nm light from a 48 mW/(0.3 cm2) He/Cd laser at
280 K. After 1 min exposure the volume of the unit cell is
reduced by 2.5% (Table S1, ESIw). A three-dimensional Four-
ier difference map (Fig. 3a)12 clearly shows that the photo-
induced peaks correspond to the dimerization product, the C
atoms being displaced by 0.89 and 1.20 A from the original
carbon plane. Least squares refinement of the data indicates
that dimerization proceeds to completion (Table S1, ESIw).
The C22–C23 bond length in the product is measured as
1.583(2) A (Fig. 2a), and the terminal carbon (C26) remains
in the E-configuration. No other significant photoinduced
peaks are found in the three-dimensional Fourier difference
map when irradiation is continued for up to 10 h, indicating
that at 280 K no isomerization occurs upon 325 nm excitation.
When the same experiment is carried out at 90 K, a three-
dimensional Fourier difference map plotted after 1 h exposure
shows light-induced peaks corresponding to both dimerization
and isomerization of the C24–C25 bond to the Z-configura-
tion (Fig. 3b). The carbon atoms are displaced by 0.91 and
1.16 A from the original plane through these atoms. The
location of the photoinduced peak representing the terminal
Department of Chemistry, State University of New York at Buffalo,
Buffalo, New York, 14260-3000, USA. E-mail:
chem9994@buffalo.edu, coppens@buffalo.edu; Fax: 1-716-645-6948;
Tel: 1-716-645-6800ext2217
w Electronic supplementary information (ESI) available: CCDC
676953–676960. For crystallographic data in CIF or other electronic
format, Tables S1 and S2, and Fig. S1. See DOI: 10.1039/b802103a
z On leave from: Department of Chemistry, University of Antwerp,
Universiteitsplein 1, 2610 Wilrijk, Belgium. Current address: Labor-
´
atoire de Chimie des Polymeres, Universite Libre de Bruxelles, CP 206/
1 Boulevard du Triomphe, 1050, Brussels, Belgium.
y A methanol solution HSA (3 mL) of (0.05 mmol) was added
dropwise to a stirred methanol solution (5 mL) of Diol (0.05 mmol)
at 50 1C for 30 min. Slow evaporation at room temperature produced
pale yellow crystals after two weeks (yield 82%).
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Fig. 3 The three-dimensional Fourier difference maps of the HSA (a)
at 280 K after 1 min exposure (blue: 4.5; light blue: 3.0; orange: ꢂ3.0;
red: ꢂ4.5 e Aꢂ3) and (b) at 90 K after 1 h exposure (blue: 2.0; light
blue: 1.0; orange: ꢂ1.0; red: ꢂ2.0 e Aꢂ3). The center of symmetry is
indicated by the small circle within the broken purple lines.
Fig. 1 Three-dimensional supramolecular architecture of Diol-2HSA
viewed along the [0, ꢂ1, 1] direction.
the reaction proceeds to 100% conversion on further irradia-
tion (after 20 s exposure).
carbon atom (C26) indicates that dimerization and isomeriza-
tion take place in the same molecule, as the C26 peak is at a
distance of 0.74 A from the image of the C25 atom of the
unreacted molecule. The second significant difference between
the 90 K and the 280 K behavior is that at the former
temperature the population of the Z–Z dimer product is only
5.6(2)% after one hour exposure, while at the higher tempera-
ture the formation of the E–E dimer is very rapid and proceeds
to completion. At 90 K, photoinduced carbon atom peaks
corresponding to the E–E or E–Z dimer are conspicuously
absent, suggesting that dimerization only occurs after isomer-
ization of two adjacent molecules. The conversion at 90 K
reaches a limit of 6.4(2)% at 8 h exposure (Table S1, ESIw and
Fig. 4), accompanied by a decrease in crystal quality. At 280 K
on the other hand, 1000 pulses from a nitrogen laser (l =
337 nm) with an estimated total energy of only 150 mJ on the
crystal are sufficient to produce the E–E dimer with a 8.0(2)%
yield, the isomerization not taking place (Table S1, ESIw), and
The occurrence of isomerization only together with dimer-
ization indicates that the processes are interdependent at lower
temperatures. Examination of the structure shows that the
reacting double bonds are offset by as much as 1.48 A in the
direction parallel to the bonds, so that a large lateral shift of
the molecules is required to reach a geometry conducive for
formation of the essentially square cyclobutane ring. Turow
ska-Tyrk and Feeder and co-workers report that the [2+2]
SCSC dimerization of 2-benzyl-5-benzylidenecyclopentanone
(‘BBCP’) proceeds only at room temperature but not at
100 K,13,14 even though we note that the o4.2 A distance11
criterion is satisfied at both temperatures. Evidently, this
criterion, which has been widely quoted, is insufficient to
Fig. 2 Perspective views showing 50% probability displacement
Fig. 4 Percentage photo-conversion of sorbic acid as a function of
ellipsoids of the reaction of HSA (a) at 280 K, and (b) at 90 K.
time at 90 K.
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competition between different reactions in complex molecular
crystals is more general than currently known.
We would like to thank Prof. David F. Watson for use of his
equipment in the initial photo-irradiation experiments. We are
indebted to a referee for helpful comments. Research sup-
ported by Research Experience for Undergraduate Program of
the National Science Foundation and NSF grant
CHE0236317. CVV would like to thank the University of
Antwerp for a one year grant as a postdoctoral research
assistant.
Notes and references
Scheme 2
1 S.-L. Zheng, C. Vande Velde, M. Messerschmidt, A. Volkov, M.
Gembicky and P. Coppens, Chem.–Eur. J., 2008, 14, 706–714;
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2007, 2735–2737; S.-L. Zheng, M. Messerschmidt and P. Coppens,
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2 R. Arad-Yellin, S. Brunie, B. S. Green, M. Knossow and G.
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3 A. Natarajan, J. T. Mague, K. Venkatesan, T. Arai and V.
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4 Cis–Trans Isomerization in Biochemistry, ed. C. Dugave, Wiley-
VCH, Weinheim, Germany, 2006.
5 M. D. Cohen and G. M. J. Schmidt, J. Chem. Soc., 1964,
1996–2000; J. Bregman, K. Osaki, G. M. J. Schmidt and F. I.
Scheffer, J. Chem. Soc., 1964, 2021–2030.
6 Y. Ito, B. Borecka, M. Trotter and J. R. Scheffer, Tetrahedron
Lett., 1995, 36, 6083–6086; Y. Ito, B. Borecka, G. Olovsson, M.
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and references cited therein.
8 See reviews: T. Friscic and L. R. MacGillivray, Z. Kristallogr.,
2005, 220, 351–363; D.-K. Bucar, G. S. Papaefstathiou, T. D.
Hamilton, Q. L. Chu, I. G. Georgiev and L. R. MacGillivray, Eur.
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and K. Hirotsu, Angew. Chem., Int. Ed., 1999, 38, 3523–3525; K.
Tanaka, F. Toda, E. Mochizuki, N. Yasui, Y. Kai, I. Miyahara
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assure a significant yield of the reaction at lower temperatures.
The mutual offset of the double bonds quantified by the
CQCꢀ ꢀ ꢀC(QC) angle (a in Scheme 2) is a measure of the
lateral shift required, and directly related to the orbital-over-
lap requirement for [2+2] addition described by Desiraju and
Kearsly,15 and summarized by Natarajan and Ramamurthy.7
The larger the deviation of this angle from an ideal 901, the
larger the lateral shift required for ring formation. The a angle
is 65.71 in the current structure and 66.91 in the BBCP crystals.
Significantly, we find that for a-cinnamic acid, with an a angle
of 86.01 (Fig. S1, ESIw),16 the dimerization does proceed at
both 280 K and 90 K.17
We conclude that at 90 K, the excited state molecules do not
have sufficient freedom of motion to dimerize rapidly, thus
allowing isomerization to occur. We hypothesize that as the
volume-demanding isomerization proceeds in the centrosym-
metric crystal lattice, the disturbance of the lattice favors
isomerization of a neighboring molecule. At the same time
the distance between the double bonds in the adjacent mole-
cules decreases, so that a second excitation can lead to
dimerization of the pair of molecules, even at low temperature.
The induced strain in the lattice is evident from the deteriora-
tion of crystal quality on prolonged irradiation, as observed
for the E-Z isomerization of tiglic acid.1 This may interfere
with further progress of the reaction.
10 T. Lavy, Y. Sheynin and M. Kaftory, Eur. J. Org. Chem., 2004,
4802–4808; T. Lavy, Y. Sheynin and M. Kaftory, CrystEngComm,
2007, 9, 123–127.
The suppression of the isomerization on rapid dimerization
at 280 K may be attributed to the modification of the
chromophore on dimerization such that excitation by
325 nm light is no longer possible.
11 G. M. J. Schmidt, Pure Appl. Chem., 1971, 27, 647–678; V.
Ramamurthy and K. Venkatesan, Chem. Rev., 1987, 87, 433–481.
12 A. Volkov, P. Macchi, L. J. Farrugia, C. Gatti, P. Mallinson, T.
Richter and T. Koritsanszky, XD2006—A Computer Program
Package for Multipole Refinement, Topological Analysis of Charge
Densities and Evaluation of Intermolecular Energies from Experi-
mental and Theoretical Structure Factors, 2006; http://
xd.chem.buffalo.edu/.
13 I. Turowska-Tyrk, Chem. Phys. Lett., 2002, 361, 115–120; I.
Turowska-Tyrk, J. Phys. Org. Chem., 2004, 17, 837–847.
14 N. Feeder and K. Honda, Mol. Cryst. Liq. Cryst., 1998, 313,
327–334; K. Honda, F. Nakanishi and N. Feeder, J. Am. Chem.
Soc., 1999, 121, 8246–8250.
15 S. K. Kearsly and G. R. Desiraju, Proc. R. Soc. London, Ser. A,
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16 V. Enkelmann, G. Wegner, K. Novak and K. B. Wagener, J. Am.
Chem. Soc., 1993, 115, 10390–10391; I. Abdelmoty, V. Buchholz,
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In summary, single crystals of the title compound show
temperature-dependent competition between isomerization
and dimerization, which leads to the occurrence of both
phenomena at a small yield at 90 K, but to a very rapid
dimerization without isomerization at 280 K. The large mole-
cular shift needed for dimerization to take place in the crystals
of the title compound and the observed reactivity of
a-cinnamic acid at 90 K point to the importance of the offset
of the double bonds in the [2+2] dimerization process.
Whereas temperature-dependence of the [2+2] cycloaddition
reaction has been described before,14,18 this is to our knowl-
edge the first example of temperature-dependent stereospeci-
ficity of an organic solid state reaction. We expect that the
example presented here is by no means unique and that
17 S.-L. Zheng and P. Coppens, unpublished results.
18 M. S. K. Dhurjati, J. A. R. P. Sarma and G. R. Desiraju, J. Chem.
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