Chemistry Reference
In-Depth Information
In a similar manner to our BIPHEP-Ru and -Pd complexes, the enantiopure BIPHEP-
Pt complex is also reported by Gagné et al. (Scheme 8D.15) [42]. Racemic BIPHEP-
Pt(CO
3
) with (
S
)-BINOL gives a 1:1 diastereomer mixture BIPHEP-Pt/(
S
) - BINOL
complexes
11
. The (
S
)/(
S
) -
11
can be seen in a 96:4 ratio by
tropo
- inversion at 92 - 122 ° C.
The (
R
)/(
S
) -
11
isomer can be converted to a 95:5 ratio of the (
S
)/(
S
) -
11
major isomer
in pyridine at lower temperature (40°C). The thermodynamically less favorable (
R
)/
(
S
) -
11
isomer can be obtained by recrystallization. The treatment of (
S
)/(
S
) -
11
(95:5)
with conc. HCl followed by recrystallization leads to the enantiopure (
S
) -
12
with reten-
tion of confi guration in 63% yield. Similarly, use of conc. HCl or TfOH for diastereopure
(
R
)/(
S
) -
11
leads to the enantiopure (
R
) -
12
or (
R
) -
13
complexes, respectively.
Ph
2
P
Ph
2
P
O
O
Pt
Pt
O
O
Ph
2
Ph
2
(
R
)/(
S
)-
11
(
S
)/(
S
)-
11
: (
R
)/(
S
)-
11
= 95:5
1) HCl
2) Recry
1) HCl
2) Recry
TfOH
Ph
2
P
Ph
2
P
Ph
2
P
Cl
Cl
OTf
Pt
Pt
Pt
Ph
2
Cl
Ph
2
Cl
Ph
2
OTf
(
R
)-
12
(
S
)-
12
(
R
)-
13
Scheme 8D.15.
The enantiopure dicationic BIPHEP-Pt complexes, thus purifi ed, act as chiral Lewis
acids (Scheme 8D.16). In the carbonyl-ene reaction catalyzed by the dicationic species
generated from BIPHEP - PtCl
2
12
and AgSbF
6
, the (
S
) - and (
R
) - ene products are
obtained with 72% ee (99% conv. at r.t.) and 70% ee (90% conv. at room temperature)
from (
R
) - and (
S
) -
12
, respectively.
O
Pt cat. (2 mol %)
OH
+
AgSbF
6
(4 mol %)
H
2
Et
CO
2
Et
*
CH
2
Cl
2
, 0°C
(
R
)-
12
70% ee (
S
)
(
S
)-
12
72% ee (
R
)
Scheme 8D.16.
