Chemistry Reference
In-Depth Information
Ar
2
P
4a
:
4b
:
4c
:
Ar = Ph
4-MeC
6
H
4
3,5-Me
2
C
6
H
3
Pd
2+
NCMe
NCMe
2SbF
-
(10 mol %)
Ar
2
OH
(CH
2
Cl)
2
r.t., 18 h
CO
2
Et
4a
4b
4c
O
61% ee (82%)
78% ee (88%)
42% ee (87%)
+
H
2
Et
4c
(10 mol %)
84% ee (85%), r.t. (20 h)
88% ee (95%), 60°C (4 h)
(CH
2
Cl)
2
/toluene 1/2
Scheme 8D.6.
of dicationic (
S
)-BINAPs-Pd species with weakly coordinating anions such as SbF
6
−
can
be employed as an effi cient catalyst system for glyoxylate-ene reactions. Signifi cantly,
the dicationic Pd catalyst affords the α-hydroxy esters with high enantioselectivity at
relatively higher reaction temperature (60°C). The medium size of the Tol-BINAP
ligand leads to higher enantioselectivity than those of the sterically less demanding
BINAP and more bulky Xyl-BINAP. The best result of 88% ee is obtained in the reac-
tion using [Pd{(
S
) - tol - binap}(MeCN)
2
](SbF
6
)
2
(
S
) -
4b
as a catalyst and 1,2-dichloroeth-
ane/toluene (1/2 in volume) as a solvent system.
The “ naked ” Pd(diphosphine)(SbF
6
)
2
generated
in situ
from Pd(diphosphine)Cl
2
and
2 equiv of AgSbF
6
is highly catalytically active than the bench-stable acetonitrile complex
4
(Scheme 8D.7) [27]. On the basis of the X-ray structure of the BINAP-PdCl
2
[28] and
SEGPHOS - PdCl
2
where the metal-centered geometry is close to square planar, effective
shielding by the most obtuse dihedral angle of SEGPHOS is the origin of the highest
enantioselectivity [29]. It should be noted that the “naked” Pd complex
5
achieves a high
yield and enantioselectivity even with less reactive mono- and 1,2-di-substituted olefi ns
in the trifl uoropyruvate ene reactions (Table 8D.1). Signifi cantly, the reaction of isobu-
tene with trifl uoropyruvate by using 0.002 mol % of (
S
) - BINAP - Pd
2+
(SbF
6
)
2
5b
proceeds
smoothly to give the ene product in 96% yield with 96% ee even under solvent-free
conditions (entry 10) [29] .
The asymmetric ene reaction of ketone silyl enol ethers is synthetically important as
a short access to optically active alcohols with not only the homo-allylic but also the
remaining silyl enol ether functionality. Mikami et al. examined the extension of one-
directional carbonyl-ene reaction to two-directional keto ester-ene reaction and ene-
aldol reaction. If the silyl enol ether group of the one-directional product sequentially
reacts with the enophile, two-directional carbonyl-ene products would be obtained. The
one-directional product might also react with different aldehydes to give two-directional
ene-aldol products. To investigate the one-directional keto ester-ene reaction, the fol-
lowing two problems arise:
