Chemistry Reference
In-Depth Information
Et
Et
5 mol % Rh
Et
+
*
TfOH (10 mol %)
O
O
O
(CH
2
Cl)
2
56% ee (74%)
59% ee (84%)
85% ee (70%)
96% ee (83%)
91% ee (91%)
99% ee (92%)
-
-
(23%)
(5%)
(3%)
-
31
31
32
32
33
33
rt, 12.5 h
rt, 30 min
rt, 5 min
5°C, 3 h
rt, 15 min
5°C, 3 h
Ph
2
P
N
Rh
+
*
N
Ph
2
Ar
2
P
N
N
Rh
+
(
R
)-DABN
*
=
*
2SbF
-
PPh
2
Ar
2
N
N
Ph
2
P
SbF
-
Rh
+
N
31
:
Ar = Ph
32
: Ar = 3,5-Me
2
-C
6
H
4
N
33
*
Scheme 8D.51.
yield and enantioselectivity (Scheme 8D.51) [80]. Additionally, the use of complex
33
bearing tetraphos ligand instead of BIPHEP leads to higher catalytic activity and enan-
tioselectivity [81]. In the tetraphos case, the time-dependent enantioselectivity is not
observed at all, even at room temperature during the reaction time. Diamine-free tet-
raphos-Rh complex is thus more rigid than BIPHEP-Rh complex, but surprisingly more
fl exible than the BIPHEP-Rh complex upon complexation with DABN.
8D.6.2. Chiral Pd Catalysts
In 2001, Mikami et al. reported highly enantioselective ene-cyclization of 1,6-enyne to
give tetrahydrofuran derivatives bearing quaternary chiral center via two different cata-
lytic systems (Scheme 8D.52) [82]. One is the combination of neutral complex of
Pd(OCOCF
3
)
2
and SEGPHOS ligand in d
6
-benzene, and the other is the combination
of cationic complex of [Pd(MeCN)
4
](BF
4
)
4
and DM-SEGPHOS in DMSO. The
C
2
-
symmetric bidentate chiral phosphine ligands such as SEGPHOSs are so effective in
quantitatively giving the enantiomerically pure cyclo-product in both systems.
A highly enantioselective spiro cyclization catalyzed by cationic chiral Pd complexes
is reported with a new
PN
-ligand bearing achiral oxazoline unit substituted by sterically
demanding
gem
- dialkyl groups (Scheme 8D.53 ) [83] . In Pd - catalyzed spiro cyclizations,
PP
-ligand such as SEGPHOS and BINAP provides the primary spiro products along
with olefi n migration, leading to secondary olefi n regioisomer in low enantioselectivities.
Our (a
S
) -
PN
- ligand
doubly
substituted by methyl groups provides the spiro products
with higher enantiomeric excess (total 96% ee) in excellent yield.
