Chemistry Reference
In-Depth Information
Using the electron-withdrawing benzonitriles as the labile, stabilizing ligands, the
isolated bench - stable MeO - BIPHEP - Pd catalyst
6
have been developed, and it has been
shown that complex
6b
is more catalytically active compared with the
in situ
- generated
catalyst
5c
for the asymmetric ene reaction of ethyl glyoxylate (
5c
, 55% conv.;
6b
, 70%
conv.) (Scheme 8D.12 ) [34] .
O
Pd cat. (2 mol %)
OH
+
H
2
Et
CH
2
Cl
2
, 0
°
C, 5 h
CO
2
Et
6a
6b
5c
(50 ± 10% conv.)
81% ee (70 ± 10% conv.)
81% ee (55% conv.)
Ph
2
P
Ph
2
P
Pd
2+
NCAr
NCAr
2SbF
-
MeO
MeO
MeO
MeO
Pd
2+
2SbF
-
Ph
2
Ph
2
5c
6a
: Ar = C
6
H
5
6b
: Ar = 3,5-CF
3
C
6
H
3
Scheme 8D.12.
MeO - BIPHEPs - Pt complexes
7
with an OTf
−
as a counter anion are found to act as
a catalyst for a similar asymmetric glyoxylate-ene reaction (Scheme 8D.13) [35]. Enan-
tioselectivity up to 85% ee is achieved by addition of achiral acidic phenol that facilitates
the reaction by trapping the OTf
−
anion and traces of water.
t
BuOH and water is,
however, completely inactive. As the diphosphine electronic effects, it is indicated that
7c - d
bearing more electron-rich ligands increase both reactivity and enantioselectivity.
This method provides another solution to the problem of detrimental counter anion and
water coordination to electrophilic cationic metal complexes.
A further advanced asymmetric catalysis can be highlighted by the use of chirally
fl exible
tropos
(originated from
tropos
in Greek, meaning turn) [36] ligands. In 1998,
we, for the fi rst time, employed
tropos
BIPHEP [37] ligands for the catalytic asymmetric
hydrogenation with the Ru catalysts [38]. The
tropos
BIPHEP ligand can also be used
in dicationic Pd(II) complexes. A combination of chiral metal complexes derived from
tropos
BIPHEP ligand and the chiral DABN (2,2′ - diamino - 1,1 ′ - binaphthyl) as a chiral
activator [39] can also be used in a similar but more advantageous manner to
atropos
BINAP ligand [40]. With 1.0 equiv of (
R
)-DABN, complexation of both enantiomers of
racemic BIPHEP-Pd takes place to afford a 1:1 ratio of a diastereomeric mixture of (
R
)/
(
R
) -
9
and (
S
)/(
R
) -
9
. The diastereomeric mixture of
9
exhibits
tropo
- inversion of the
BIPHEP-Pd portion at 80°C to afford favorable (
R
)/(
R
) -
9
exclusively. In addition, the
novel tetra(phosphanyl)triphenyl (TETRAPHOS) ligand is designed to exhibit nonpla-
nar helicity upon metal complexation [41]. The single diastereomer TETRAPHOS-Pd/
DABN complex
7
is also quantitatively obtained through
tropo
- inversion under the
heating conditions.
