Discontinuous pressure effect upon enantiodifferentiating photosensitized isomerization of cyclooctene

Article abstract of DOI:10.1039/b202699f

A hydrostatic pressure of up to 750 MPa induced discontinuous changes in the enantiomeric excess of the (E)-isomer obtained in the enantiodifferentiating photoisomerization of (Z)-cyclooctene and (Z,Z)-cycloocta-1,5-diene, sensitized by chiral benzene-1,2,4,5-tetracarboxylates; indicating a switching of the enantiodifferentiation mechanism, which is attributable to dramatic conformational changes of chiral alkoxycarbonyl auxiliaries at a specific pressure.

Full text of DOI:10.1039/b202699f

Discontinuous pressure effect upon enantiodifferentiating photosensitized
isomerization of cyclooctene
Masayuki Kaneda,a Sadayuki Asaoka,a Haruhiko Ikeda,a Tadashi Mori,a Takehiko Wadaa and
Yoshihisa Inoue*ab
a Department of Molecular Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan.
E-mail: inoue@chem.eng.osaka-u.ac.jp; Fax: 81 6 6879 7923; Tel: 81 6 6879 7921
b Entropy Control Project, ICORP, JST, 4-6-3 Kamishinden, Toyonaka 560-0085, Japan
Received (in Cambridge, UK) 19th March 2002, Accepted 25th April 2002
First published as an Advance Article on the web 15th May 2002
A hydrostatic pressure of up to 750 MPa induced dis-
continuous changes in the enantiomeric excess of the (E)-
isomer obtained in the enantiodifferentiating photoisomer-
ization of (Z)-cyclooctene and (Z,Z)-cycloocta-1,5-diene,
sensitized by chiral benzene-1,2,4,5-tetracarboxylates; in-
dicating a switching of the enantiodifferentiation mecha-
nism, which is attributable to dramatic conformational
changes of chiral alkoxycarbonyl auxiliaries at a specific
pressure.
1,5-diene (2ZZ), at much higher pressures for further
elucidation of the pressure effect upon asymmetric photo-
sensitization.
A pentane solution (11 cm3) of 1Z or 2ZZ (5 mmol dm23),
containing 1 mmol dm23 of (2)-1-methylheptyl, (2)-menthyl
or (2)-bornyl benzene-1,2,4,5-tetracarboxylate (3a–c) as chiral
sensitizer (Scheme 1), was irradiated at l > 250 nm under an
applied pressure of up to 750 MPa in a vessel equipped with a
sapphire window for UV irradiation (Hikari Koatsu Co.,
Hiroshima, Japan), by using a 250 W ultra-high pressure
mercury lamp (Ushio UI-501C) fitted with a Vycor filter and a
cylindrical quartz vessel filled with water. The irradiated
solution was retrieved from the vessel and subjected to capillary
GC analyses over a PEG-20M column (20 m 3 0.25 mmF) for
determination of the E/Z ratio and over a chiral column (Supelco
b-DEX 225; 30 m 3 0.25 mmF) for ee determination (error <
0.5% ee) after selective extraction of the produced (E)-isomer,
1E or 2EZ, with aqueous silver nitrate solution as described
previously.8 The results are summarized in Table 1 and Fig.
1.
Asymmetric photosensitization is an attractive alternative to
conventional asymmetric synthesis using chiral catalysts or
enzymes.1–3 In a series of studies on the enantiodifferentiating
photosensitized isomerizations of several cycloalkenes,4–12 we
demonstrated that good-to-high enantiomeric excesses (ee’s) of
up to 77%, and interestingly an inversion of the product
chirality, or the relative rate of (S)- and (R)-product formation
(kS/kR), can be obtained by solely changing the temperature4–12
and solvent.9,12 Such unprecedented switching phenomena have
been accounted for in terms of the entropy factors operating in
the enantiodifferentiating process within the intervening ex-
ciplex.13
Although the pressure effect upon (asymmetric) photo-
chemistry has not been extensively explored, particularly at
high pressures,14 we have demonstrated that similar switching
of product chirality can be induced by applying pressure.8 Thus,
the enantiodifferentiating photoisomerization of (Z)-cyclooc-
tene (1Z) afforded antipodal products at ambient and high
pressures. The results appeared to be well described by the
linear relationship between pressure (P) and ln(kS/kR):
Unexpectedly, the product ee did not display a straightfor-
ward pressure dependence. As can be seen from Fig. 1 the ln(kS/
kR) value obtained for each sensitizer/substrate combination is
Table 1 Enantiodifferentiating photosensitization of 1Z and 2ZZ sensitized
by chiral benzene-1,2,4,5-tetracarboxylates 3a–c at various pressures in
pentane at 25 °C
SubstrateSens* P/MPa
% Conv % Yield E/Z ratio % ee
ref.
a
a
a
c
c
c
a
a
a
a
a
c
c
c
c
c
a
a
a
c
c
c
c
c
c
c
c
c
1Z
3a
0.1
31
37
15
22
26
9.7
5.5
4.8
13
18
21
24
0.22
0.35
0.46
0.048
0.034
0.055
0.21
0.32
0.36
23.8
0.0
+2.1
+4.3
+4.4
ln(kS/kR) = 2(DDV‡S-R/RT)P + C
100
200
300
400
485
0.1
100
200
300
400
400
450
500
525
650
0.1
100
200
300
400
500
650
750
0.1
(1)
43
where DDV‡S-R represents the differential activation volume, R
the gas constant and C the DDV‡S-R value at P = 0.8 However,
further attempts to examine the effect of even higher pressures,
in an attempt to enhance the product’s ee, were not possible
owing to instrument limitations.
In the present study we employed a newly developed high-
pressure vessel, and examined the enantiodifferentiating photo-
isomerizations of 1Z and a new substrate, (Z,Z)-cycloocta-
b
b
b
+4.4
3b
38
45
211.2
23.6
+5.7
41
b
0.020
0.050
0.146
+12.4
+17.9
+19.2
+19.9
+18.9
+21.0
+23.1
+14.2
+14.3
+13.0
+10.3
+7.1
+5.4
+5.2
+4.8
24.4
20.7
+2.3
b
23
12.4
15.3
12.3
9.0
0.105
b
b
b
b
b
6.7
7.0
19
20
23
7.1
6.6
3.2
7.0
4.4
3.6
3.9
5.0
3.5
0.055
0.052
0.31
0.34
0.43
0.065
0.045
0.021
0.035
0.034
0.044
0.047
0.059
0.043
3c
3a
39
41
46
b
b
b
b
b
2ZZ
16.4
17.1
15.9
16.7
100
200
300
+4.2
Scheme 1 Enantiodifferentiating photoisomerization of 1Z and 2ZZ
sensitized by chiral sensitizer 3a–c.
a Reference 8. b Value not determined. c This work.
1272
CHEM. COMMUN., 2002, 1272–1273
This journal is © The Royal Society of Chemistry 2002

least in the ground state, by applying the pressure beyond a
certain limit. Subsequent photoexcitation of the resulting more
compact conformer, or analogous conformational changes in
the excited state, can lead to a distinctly different exciplex
conformation and therefore induce the discontinuous changes
observed in the ee at higher pressures.
Inspection of the electronic spectra of 3a under pressure
showed only a slight bathochromic peak shift and absorbance
enhancement, which are too small to confidently discuss the
spectroscopic discontinuity. Circular dichroism (CD) spectros-
copy, however, would be more sensitive to such conformational
changes, but the sapphire window used does not allow CD
spectral examination due to birefringence; probably, a diamond
window would be more suitable for such measurements.
The unprecedented discontinuous pressure dependence of the
ee found in the present study can be useful as an additional tool
for controlling the product chirality and ee, wherever such
conformational switching is induced by pressure. Further
studies to elucidate the detailed mechanism, as well as CD
spectral examination at high pressure, are currently in pro-
gress.
We thank Dr Guy A. Hembury for assistance in the
preparation of this manuscript.
Fig. 1 Pressure dependence of the ee of 1E (solid line) or 2EZ (broken line)
obtained upon enantiodifferentiating photoisomerization of 1Z or 2ZZ
sensitized by 3a (5,2), 3b (-) and 3c (:).
Notes and references
1 H. Rau, Chem. Rev., 1983, 83, 535.
2 Y. Inoue, Chem. Rev., 1992, 92, 741.
not a simple linear function of P over the entire pressure range
employed, but instead, in each case, affords a bent plot
approximated by two or three straight lines. These results
clearly indicate that the DDV‡ value (eqn. 1) and therefore the
sensitizing species and enantiodifferentiation mechanism differ
in each pressure range, even though the same chiral sensitizer is
used. In view of the discontinuity of the ln(kS/kR)-versus-P plot,
the pressure-induced change in viscosity or diffusion rate
constant, each of which is a continuous variable, does not
appear to be responsible for this unusual behavior.
3 S. R. L. Everitt and Y. Inoue, in Organic Molecular Photochemistry, ed.
V. Ramamurthy and K. Schanze, Marcel Dekker, New York, 1999, p.
71.
4 Y. Inoue, T. Yokoyama, N. Yamasaki and A. Tai, J. Am. Chem. Soc.,
1989, 111, 6480.
5 Y. Inoue, N. Yamasaki, T. Yokoyama and A. Tai, J. Org. Chem., 1992,
57, 1332.
6 H. Tsuneishi, T. Hakushi, A. Tai and Y. Inoue, J. Chem. Soc., Perkin
Trans. 2, 1995, 2057.
7 Y. Inoue, H. Tsuneishi, T. Hakushi and A. Tai, J. Am. Chem. Soc., 1997,
119, 472.
In this context, it should be noted that the bending positions
observed in Fig. 1 are common to all the chiral sensitizers and
substrates employed, occurring at 200 and ca. 400 MPa. Hence,
the structural variations in the chiral auxiliaries or the substrate
structure cannot account for the discontinuous changes of
product ee. It is, however, the benzenetetracarboxylate moiety,
shared by all of the sensitizers, which appears most responsible.
In this light, it is found that the ester moiety of the alkyl
benzoate starts to fold into a more compact structure at the
transition pressures, as revealed by observing a discontinuous
change in the C = O stretching vibration by means of high-
pressure Raman spectroscopy.15 It is likely that, in the present
system, a similar conformational change occurs in the ester
moieties of each chiral benzenetetracarboxylate sensitizer, at
8 Y. Inoue, E. Matsushima and T. Wada, J. Am. Chem. Soc., 1998, 120,
10687.
9 R. Hoffmann and Y. Inoue, J. Am. Chem. Soc., 1999, 121, 10702.
10 S. Asaoka, H. Horiguchi, T. Wada and Y. Inoue, J. Chem. Soc., Perkin
Trans. 2, 2000, 737.
11 S. Asaoka, M. Ooi, P. Jiang, T. Wada and Y. Inoue, J. Chem. Soc.,
Perkin Trans. 2, 2000, 77.
12 Y. Inoue, H. Ikeda, M. Kaneda, T. Sumimura, S. R. L. Everitt and C.
Wada, J. Am. Chem. Soc., 2000, 122, 406.
13 Y. Inoue, T. Wada, S. Asaoka, H. Sato and J.-P. Pete, Chem. Commun.,
2000, 251.
14 R. van Eldik, T. Asano and W. J. le Noble, Chem. Rev., 1989, 89,
549.
15 V. L. Slager, H. C. Chang, Y. J. Kim and J. Jonas, J. Phys. Chem. B.,
1997, 101, 9774.
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