H. Kawanami, Y. Ikushima / Tetrahedron Letters 43 (2002) 3841–3844
3843
(a)
(c)
(b)
gas
gas
supercritical
phase
(co-solvent
+scCO2)
less liquid
(co-solvent
+CO2)
polar liquid
(co-solvent)
0 MPa
8.0 MPa
11.8 MPa
increase in pressure
Figure 2. A schematic diagram of the phase behavior of the co-solvent/CO2 mixture in a reaction vessel at 40°C.
In conclusion, we have demonstrated that carbon diox-
ide fixation by 2-methyl aziridine in the presence of
iodine was effectively converted into 4-methyl-2-oxazo-
lidinone without regioisomers under supercritical condi-
tions, and the maximum yield was obtained at the near
critical pressure of 11.8 MPa.
reaching a maximum yield at the near-critical pressure
of 11.8 MPa. In order to explain the significant increase
in yield in the near critical region, CO2 density was also
shown in Fig. 1.9 It can be found that the increase in
CO2 density with pressure shows relatively good agree-
ment with the increase in yield of 2a (Fig. 1). In the
case of the reaction of 2a, increasing the mole fraction
of CO2 by increasing density of CO2 under constant
volume can contribute to a shift in the equilibrium
(Path A in Scheme 2) in favor of the generation of 3,
then leading to an increase in the yield of 2a by the
irreversibly nucleophilic attack of 3 to 1a% in CO2. At
the near critical pressure of 11.8 MPa, we can see
higher yields than those expected from the relationship
between the yields and the CO2 density, which cannot
be understood by only an increase in the formation of
3. To clarify the unusual increase in yield, visual obser-
vation was carried out using a high-pressure view cell.
Above 10.2 MPa, gas–liquid binary phase had disap-
peared to form a homogeneous fluid phase which is
regarded as the supercritical state. It is likely that a
significant increase in local concentration16,17 of reac-
tant 1a around 3 would be very large, around 11.8
MPa, resulting in the outstanding increase in yield as
seen in Fig. 1.
References
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In Table 1, a decrease in the yield of 2b with increasing
pressure was further observed (runs 8–11).18 Since the
reaction took place in the liquid or supercritical phase,
this decrease could be explained by the variation of
CO2 content in each phase which occurred during the
increase in pressure.19 When the CO2 is introduced as
shown in Fig. 2, the liquid phase changes from the
polar phase containing only the co-solvent to a less
polar one made up of CO2 and the co-solvent (a to b)
and to a further less polar solvent (b to c), because the
content of only non-polar CO2 in the liquid phase is
increased with increasing pressure.19 In the less polar
phase such as b and c, the formation of 1,3-dipole 5
with a transient benzylic carbocation and a negative
charged nitrogen would be prevented, because the ion-
ization reaction which proceeds as SN1 is known to be
decelerated.1d,20 Thus, the carbon dioxide fixation
through path B would be more unfavorable in scCO2.
Consequently, under the supercritical conditions the
reaction of CO2 with 1a would be more favored than
that with 1b.
5. For review and examples, see: Dyen, M. E.; Swern, D.
Chem. Rev. 1967, 67, 197–246.
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10. The typical experimental procedure is as follows: propyl-
ene imine (3.0 mmol), iodine (0.1 mmol) and acetonitrile
(1.0 mL) were charged into a 50 cm3 reactor at 40°C and
CO2 was introduced into the reactor. In the case of the
scCO2, liquid CO2 was subsequently charged into the
reactor using a high-pressure liquid pump and com-