Chemistry Reference
In-Depth Information
temperature than without the DPPF ligand (75% ee). It is noted that the choice of
the counter anion is critical to obtain high yield and enantioselectivity. The perchlorate
counterpart affords lower yield and enantioselectivity than those obtained with the
hexafl uoroantimonate complex. Unfortunately, the Pd and Pt counterparts give only
low enantioselectivity (7 - 44% ee).
8D.3.3. Chiral Cu and Sc Catalysts
Evans and others reported that chiral
C
2
-symmetric bis(oxazolinyl) (box) Cu(II) com-
plexes
17
as Lewis acid catalysts effectively facilitate the glyoxylate-ene reactions
(Scheme 8D.21 ) [47] . The bis(aqua) complex
17a
, Cu((
S
,
S
) -
t
Bu - box)(H
2
O)
2
(SbF
6
)
2
, is
readily prepared as an air- and water-stable catalyst, with only a slight decrease in the
reaction rate relative to the anhydrous complex,
18a
. The
17a
- catalyzed ene reaction
affords not only the high level of enantioselectivity but also excellent control of regiose-
lectivity in the glyoxylate-ene reactions with various 1,1-disubstituted olefi ns. The anhy-
drous Cu((
S
,
S
) - Ph - box)(OTf)
2
complex
18b
is also an effective catalyst but provides the
opposite sense of asymmetric induction. The reversal in the enantioselectivity of the ene
products depending on the oxazoline ring substituent (e.g.,
t -
Bu vs. Ph) can be rational-
ized by the tetrahedral and square-planar intermediates to account for the absolute
confi gurations of the products.
It should be noted that the anhydrous box Cu(II) complex
18a
achieves high yield
and enantioslectivity even with less reactive monosubstituted and 1,2-disubstituted
olefi ns, in contrast to BINOL-Ti-catalyzed glyoxylate-ene reactions, which does not
provide any ene product with these olefi ns (Scheme 8D.22 ) [42] .
Me
Me
Me
Me
2+
2+
O
O
O
O
2SbF
-
2X
-
N
N
N
N
Cu
Cu
t
Bu
R
t
Bu
R
H
2
OOH
2
17a
R =
t
Bu, X = SbF
6
:
R = Ph, X = OTf:
18a
18b
O
H
17a
(1 mol %)
R
CO
2
Et
CH
2
Cl
2
, 0-25
°
C
O
93-98% ee (62-97%)
+
R
H
CO
2
Et
OH
18b
(10 mol %)
R = Me, Ph, CH
2
OBn
CH
2
OTBDPS
R
CO
2
Et
CH
2
Cl
2
, 0-25
C
°
76-99% ee (85-99%)
Scheme 8D.21.
