Chemistry Reference
In-Depth Information
with high enantioselectivity. While a peroxyenamine intermediate must take a conforma-
tion, allowing an interaction between σ
OO
−
orbial for epoxide ring
closure, the diamine unit on the peroxyenamine regulates its conformation to make the
epoxide formation diffi cult but accelerates the protonation. As the result, enantio-
enriched peroxides were selectively generated. However, the elevated temperature
weakens the regulation, and the peroxyenamine undergoes the O-O bond cleavage.
Phase transfer catalysis is a powerful methodology for organic synthesis under
aqueous conditions and has been applied to asymmetric epoxidation of
∗
orbital and enamine
π
- unsaturated
carbonyl compounds [88]. Cinchona alkaloid-derived quaternary ammonium salts have
been developed for the asymmetric epoxidation of
α
,
β
- unsaturated ketones by several
research groups [89]. Dimeric cinchona phase transfer catalyst
44
was also identifi ed by
Jew and Park (Scheme 11.53) [90]. With aqueous hydrogen peroxide as oxidant, chal-
cone derivatives underwent epoxidation in high enantiomeric excesses. The addition of
surfactants led to signifi cant improvements not only in yield but also in enantioselectiv-
ity, and Span 20 (sorbitan monolaurate) was found to be most effective. The catalyst
with naphthyl group as the spacer between the cinchona units and the corresponding
monomeric catalysts bearing an arylmethyl group on nitrogen atom displayed no asym-
metric induction, indicating the synergistic operation of the two cinchona units in the
epoxidation event.
α
,
β
44
(1 mol
%)
Span 20 (1 mol
%)
50% KOH (1 equiv.)
O
O
O
+
30% H
2
O
2
(10 equiv.)
Ph
Ph
Ph
Ph
i
Pr
2
O, RT
95%, >99% ee
OMe
OMe
Br
−
Br
−
N
+
N
+
F
OH
HO
N
N
44
Scheme 11.53.
Maruoka and coworkers have identifi ed chiral spiro ammonium salts bearing an
axially chiral binaphthyl unit as the phase transfer catalyst for a wide variety of organic
synthesis [88] They designed a new phase transfer catalyst
45
(X = Br) with dual function
for asymmetric epoxidation of enones using aqueous sodium hypochlorite as oxidant
(Scheme 11.54) [91]. The hydroxyl groups are appropriately placed to recognize and
activate the enone substrate through hydrogen bonding. Indeed, the removal of the
hydroxyl groups considerably retarded the reaction's progress and the enantioselectivity
was diminished. X-ray crystallographic analysis of
45
(X = PF
6
) revealed that the diaryl-
hydroxymethyl groups form chiral pockets close to the nitrogen cation and that the
hexafl uorophosphate anion, which is hypothetically regarded as hypochlorite anion in
the epoxidation, is located in the chiral pocket.
