Chemistry Reference
In-Depth Information
Ph
O
O
sugar
O
60 °C
O
O
OA ll
16 h
OBn
MO
2
C
O
re
face
99 %
facial selectivity: 97/3
M = (-)-menthyl
MO
2
C
Scheme 1
O
OH
O
OH
OSiMe
3
Ph
Ph
+
+
PhCHO
anti
sy n
Scheme 2
O
+
COMe
+
COMe
endo
exo
activation parameters (kJ
.
mol
-1
) at 25 °C
Δ
G
Δ
H
−ΤΔ
S
water
methanol
80
89.8
38
38
42
51.8
Scheme 3
reactions (Scheme 2) as a result of their negative volume of activation.
9
Stereoselectivity was reversed in water affording syn adducts as the major
isomers. This main feature was attributed to the fact that syn transition
state occupies a smaller volume than the anti one.
In order to prove the origin of the reactivity in water for reactions with
negative volume of activation, Lubineau measured the thermodynamic
activation parameters of a Diels-Alder reaction (Scheme 3) as a model of a
reaction with high negative volume of activation (DV
6¼
=
30 cm
3
mol
1
).
The whole acceleration comes from a favorable change of entropy, showing
clearly the implication of the hydrophobic effect.
10
In the cycloaddition
between cyclopentadiene and methyl vinyl ketone, the acceleration in
water is mainly caused by destabilisation of the initial state relative to the
organic solvent.
11
The slight stabilisation of the transition state which
was observed results from enforced hydrophobic interactions, the term
''enforced'' being used by Engberts to distinguish the hydrophobic
bonding of the reactants during the activation process from hydrophobic
interactions not dictated by the activation process.
12
Following these mechanistic studies, Lubineau was clearly convinced
that it was not necessary to work with compounds having a good
solubility in aqueous phase. As the soluble reactants molecules disappear
to give the product, new molecules go again through the solution.
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