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Remarkably, the bond distances between the TS moieties and
the thiourea hydrogen bond donor also decrease with increasing
solvation power (this is also true for the complexes of thiourea and
epoxide{). That is, the interactions are indeed amplified in water
and the corresponding barrier is the lowest overall. While water
stabilizes polar transition states, the additional rate enhancement
with 1 is likely to be due to its inclusion into the hydrophobic
hydration cavity (Fig. 1). The stabilization of the TS with an
individual water molecule is also significant (TS?H2O, Fig. 3, right)
but considerably less than the bulk water effect. Dynamic
modeling of the hydrophobic effect would be highly desirable.
Further evidence for this effect is provided by the 20–40%
decrease in the yields (Table 3) when the reactions are carried out
in D2O instead of H2O. D2O has a ca. 20% higher viscosity that
makes mixing more difficult and reduces the hydrophobic effect.14
This work was supported by the Deutsche Forschungs-
gemeinschaft (SPP1179).
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