Chemistry Letters 2002
861
although well-compressed scCO2 at pressures >15 MPa appears
to behave like a liquid solvent.
In the near-critical density region, the local density of CO2
around the solute molecules is much higher than the bulk
density,20{22 as a result of so-called ‘‘solvent clustering.’’ Under
the condition of density fluctuation, it is deduced that the entropy
gain and/or required energy for transferring CO2 molecules from
the clusterized exciplex to the much more disordered bulk during
the enantiodifferentiating process are large, affording the very
large ÁÁVz values in the present system. At the higher pressures,
the difference between the local density and the bulk density is not
considerable, and the entropy of the CO2 molecules around the
solutes is close to that of the bulk solvent. Therefore, lower
sensitivity of ee at higher pressure region is well compatible with
the results of the high-pressure study in conventional organic
solvents,7 where a hydrostatic pressure of up to several hundreds
of MPa is required to achieve an appreciable change in ee. The
effect of the sensitizer structure on the ÁÁVz might be also
explained by the difference of the microenvironmental structures
and/or the reaction mechanism for each reaction.
Figure 1. Pressure dependence of the relative rate constant (kS=kR),
or the product ee, in the enantiodifferentiating photoisomerization of
1Z sensitized by 2a (l), 2b (n) and 2c (s) in scCO2 at 45 ꢀC.
In this first asymmetric photosensitization in SCF we
elucidated the critical control of product ee and even the
switching of product chirality can be attained in scCO2 with an
appropriate sensitizer through a small change of pressure
particularly in the low density region near the critical density.
Furthermore, the entropy is much more effective on the chirality
control in scCO2 near the critical density rather than in liquid
phases or highly compressed scCO2.
to a switching of the product chirality from R to S near the reduced
density of 1. In each case, the ee approaches an apparent plateau at
higher pressures to give the ultimate ee.
To analyze the pressure effect in scCO2 more quantitatively,
the differential activation volume (ÁÁVz ¼ ÁVzS À ÁVz
)
R
S-R
for the formation of (S)-and ( R)-1E was evaluated from the ee
changes shown in Figure 1. Based on the transition state theory,
the pressure effect on kS=kR at a given temperature (T) is given by
eq 1.7
This work was partly supported by the Sasakawa Scientific
Research Grant from The Japan Science Society.
References and Notes
1
2
3
H. Rau, Chem. Rev., 83, 535 (1983).
Y. Inoue, Chem. Rev., 92, 741 (1992).
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(1989).
Y. Inoue, N. Yamasaki, T. Yokoyama, and A. Tai, J. Org. Chem., 57, 1332 (1992).
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(1997).
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S. Asaoka, T. Kitazawa, T. Wada, and Y. Inoue, J. Am. Chem. Soc., 121, 8486
(1999).
½@ lnðkS=kRÞ=@P ¼ ÀÁÁVzS-R=RT
ð1Þ
T
Integration of eq 1, assuming a constant activation volume over
the entire pressure range employed, gives eq 2:
4
lnðkS=kRÞ ¼ ÀðÁÁVzS-R=RTÞP þ C
where C is the integration constant equal to lnðkS=kRÞP¼0
5
6
ð2Þ
.
From the slopes of the plots in Figure 1, distinctly different
ÁÁVz values were obtained for 2a–c in the near-critical and
7
8
9
S-R
higher pressure regions; the results are listed in Table 1, along
with those obtained in pentane. The absolute ÁÁVzS-R values for
the near-critical region are far greater than those obtained for the
same photoisomerizations performed in pentane (À5:6 to
þ3:5 cm3/mol).7 These striking differences in ÁÁVz clearly
indicate that the transition-state structure and/or enantiodiffer-
entiation mechanism involved in near-critical CO2 and conven-
tional organic solvent are completely different from each other,
10 Y. Inoue, T. Wada, S. Asaoka, H. Sato, and J.-P. Pete, Chem. Commun., 2000, 251.
11 Y. Inoue, H. Ikeda, M. Kaneda, T. Sumimura, S. R. L. Everitt, and T. Wada, J. Am.
Chem. Soc., 122, 406 (2000).
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13 A. Baiker, Chem. Rev., 99, 453 (1999).
14 ‘‘Supercritical Fluid Science and Technology,’’ ACS Symposium Series 406, ed.
by K. P. Johnston and M. L. Pennenger, American Chemical Society, Washington,
D.C. (1989).
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ed. by T. J. Bruno and J. F. Ely, CRC Press, Boca Raton (1991).
16 T. Clifford, ‘‘Fundamentals of Supercritical Fluids,’’ Oxford University Press Inc.,
New York (1999).
Table 1. Differential activation volumes (ÁÁVzS-R/cm3 molÀ1) for the
enantiodifferentiating photoisomerization of 1Zsensitized by 2a–c in scCO2
and in pentane
17 R. Eberhardt, S. Lobbecke, B. Neidhart, and C. Reichardt, Liebigs Ann./Recl.,
1997, 1195.
18 N. Yamasaki, Y. Inoue, T. Yokoyama, A. Tai, A. Ishida, and S. Takamuku, J. Am.
Chem. Soc., 113, 1933 (1991).
¨
in scCO2
in pentane
19 The GC analysis was performed on a Shimadzu GD17A with a Supelco ꢁ-Dex 225
column (30 mm ꢁ 0:25 mm i.d.) at 80 ꢀC. The temperature of the injection port
was kept below 150 ꢀC to avoid the thermal E-Z isomerization of 1E.
20 C. A. Eckert, D. H. Ziger, K. P. Johnston, and S. Kim, Fluid Phase Equilib., 14,
167 (1983).
21 C. A. Eckert, D. H. Ziger, K. P. Johnston, and S. Kim, J. Phys. Chem., 90, 2738
(1986).
22 S. C. Tucker, Chem. Rev., 99, 391 (1999).
Sensitizer
P < 11 MPa
P > 15 MPa
P ¼ 0:1{400 MPa
2a
2b
2c
45
À160
À274
1.0
À7:7
À3:71a
À2:08
À1:22
À12:1
aReference 7.