Chemistry Reference
In-Depth Information
TABLE 10.1. h - Catalyzed Asymmetric Hydroformylation of Styrene Using Diphosphites
10l, 10m, 16l, and 16m
a
Entry
Ligand
TOF
b
%2 - PP
c
% e e
d
1
10l
28
95
38 (
S
)
2
10 m
17
88
69 (
S
)
3
16l
4
91
23 (
S
)
4
16 m
45
94
40 (
R
)
5
e
10 m
11
92
86 (
S
)
a
[Rh(acac)(CO)
2
] = 0.02 mmol; ligand/Rh = 2.2; substrate/Rh = 1000; Toluene = 20 mL; P
H2/CO
= 10 bar. T = 25 ° C.
b
TOF in mol styrene × mol Rh
− 1
× h
− 1
determined after 1-h reaction time.
c
Regioselectivity for 2 - phenylpropanal.
d
Enantiomeric excess.
e
T = 15 ° C.
The infl uence of the backbone substituent was studied by comparing ligands
10
-
13
(Fig. 10.1). Surprisingly, ligands
11
, which have a more sterically hindered phenyl group,
provided lower enantioselectivities than ligands
10
[11a] did.
A possible cooperative effect between the different chiral centers was studied by
using ligands
10l
-
o
and
16l
-
o
. Initially, van Leeuwen and coworkers studied the coopera-
tive effect between the chiral ligand bridge and the axially chiral binaphthyl phosphite
moieties by comparing ligands
10l
,
10m
,
16l
, and
16m
. The hydroformylation results
clearly indicate a cooperative effect that leads to a matched combination for ligand
10m
with (
S
ax
, 2
R
, 4
R
,
S
ax
) confi gurations (ee's up to 86%) (Table 10.1) [9b]. Later, Bakos
and coworkers, with ligands
10n
,
10o
,
16n
, and
16o
, found a similar cooperative effect
between the chiral ligand bridge and the chiral phosphite moiety [11b]. However, the
matched combination afforded poorer results (ee's up to 17%) than those obtained with
bulky biaryl phosphite ligands
10b, 10d
and
10m
(ee's up to 90%) due to the lower steric
bulk of the phosphite moieties
n
and
o
(
vide supra
) .
Interestingly, the hydroformylation results obtained with ligands
10b
and
10d
, which
have conformationally fl exible, axially chiral biphenyl moieties, are similar to those
obtained with ligand
10m
. This indicates that diphosphite ligands containing the confor-
mationally fl exible, axially chiral biphenyl moieties predominantly exist as a single
atropoisomer in the [RhH(CO)
2
(diphosphite)] complexes when the right bulky substitu-
ents in the
ortho
positions are present
(vide infra
) [9b]. It is, therefore, not necessary to
use expensive conformationally rigid binaphthyl moieties to reduce the degrees of
freedom of the system.
To investigate whether a relationship exists between the solution structures of the
hydridorhodium diphosphite species [RhH(CO)
2
(diphosphite)] [12] and catalytic per-
formance, van Leeuwen and coworkers extensively studied the rhodium-diphosphite
complexes formed under hydroformylation conditions by HP-NMR techniques. It is well
known that these complexes have a TBP structure. Two isomeric structures of these
complexes, one containing the diphosphite coordinated in a bis-equatorial (
ee
) fashion
and one containing the diphosphite in an equatorial-axial (
ea
) fashion, are possible (Fig.
10.2 ).
Studies using diphosphite ligands
10
and
16
indicated that the stability and catalytic
performance of the [RhH(CO)
2
(diphosphite)] species depend strongly on the confi gura-
