Chemistry Reference
In-Depth Information
The enantioselective and regioselective iridium-catalyzed allylic etherifi cation was
studied by Hartwig and others [18]. The iridium complex of Feringa's type of phos-
phoramidate ligands catalyzed the allylic etherifi cation as well as allylic amination. The
early work was the etherifi cation with phenolates as nucleophiles [18a]. Recently, the
direct allylic etherifi cation with aliphatic alcohols in the presence of an alkali metal base
was achieved (Scheme 5.15) [18c]. The reaction of
50
with benzyl alcohol gave the
branched allylation product
51
in 68% yield and 93% ee. For high conversions, the use
of highly powdered K
3
PO
4
was important. Additionally, a catalytic amount of an alkyne
additive was used to suppress olefi n isomerization that forms vinyl ether side products.
[Ir(COD)Cl]
2
, Ligand
12
O
Bn
K
3
PO
4
Ph
OAc
+
BnOH
Ph
Ph
Me
51
(68%, 93% ee)
50
Toluene
Regioselectivity (99:1)
Scheme 5.15.
Takemoto, Miyabe, and others found that oximes and hydroxylamines with an
N
-
electron-withdrawing substituent, known as hydroxamic acids, act as reactive oxygen
nucleophiles [19]. The iridium complex of pybox
20
catalyzed the reaction of phosphate
18
with oxime
52
to form the branched product
53
with good enantioselectivity (Scheme
5.16 ) [11] . The asymmetric allylic etherifi cation with hydroxylamines was also studied
[19b]. Carreira and others reported the enantioselective and regioselective iridium-
catalyzed allylic etherifi cation using potassium silanolates as oxygen nucleophiles and
the iridium-catalyzed kinetic resolution of secondary allylic carbonates using a new class
of chiral [2.2.2]-bicyclooctadiene ligands for iridium [20a,b]. Kimura and Uozumi
reported the new P-chiral phosphorodiamidite ligands for iridium-catalyzed allylic ether-
ifi cation [20c] .
N h
O
[Ir(COD)Cl]
2
OP(O)(OEt)
2
Ph
Ligand
20
+
N
HO
Ba(OH)
2
·H
2
O
CH
2
Cl
2
52
18
53
(83%, 90% ee)
Regioselectivity (94:6)
Scheme 5.16.
The regio- and enantioselective ruthenium-catalyzed allylic etherifi cation was studied
[21]. Sasai and others reported the highly regio- and enantioselective
O
- allylation cata-
lyzed by a planar-chiral cyclopentadienyl ruthenium complex
56
(Scheme 5.17 ) [21b] .
The reaction of
54
with
o
-cresol was effectively catalyzed by ruthenium catalyst
56
in
the presence of K
2
CO
3
to give the branched ether
55
in 92% yield and 95% ee, along
with a trace amount of the linear ether.
