Chemistry Reference
In-Depth Information
aldehyde. Several reports describe the state of the art in asymmetric hydroformylation
with both rhodium and platinum systems [4b,6]. After the discovery of the high enanti-
oselectivity provided by rhodium/diphosphite and rhodium/phosphine-phosphite
systems, with total conversion in aldehydes and high regioselectivities, rhodium systems
became the catalysts of choice for the asymmetric hydroformylation reaction. An impor-
tant breakthrough in this fi eld has been the characterization of intermediates in the
asymmetric hydroformylation of styrene through
in situ
infrared (IR) and HP-NMR
studies. These facts have provided important insights in the understanding of the
reaction [1,6c] .
10.2.2. Mechanism for Rhodium - Catalyzed Hydroformylation
The catalytic cycle for rhodium hydroformylation has been extensively studied mainly
for the RhH(CO)(PPh
3
)
3
catalysis. A general proposal including all the steps of the
reaction is shown in Scheme 10.2 [1] .
O
CO
H
Rh
L
H
Rh
CO
H
Rh
CO
C
L
L
L
L
L
CO
H
2
Rh
CO
Rh
CO
CO
L
OC
L
L
L
C
O
9
2ae
2
e
1
L
CO
CO
H
Rh
L
H
Rh
CO
CH
3
C
2
H
4
OC
L
L
L
CH
2
O
C
H
3c
3t
H
Rh
L
L
L
OC
L
H
2
4ae
4
CH
3
CH
3
CH
3
CH
3
CH
3
CH
2
CH
2
CH
2
CH
2
O
O
O
O
C
R
C
CH
2
Rh
L
C
Rh
L
C
Rh
CO
CO
C
Rh
L
C
Rh
CO
OC
L
L
L
CO
L
CO
L
CO
L
L
OC
L
8ae
8ee
7c
7t
5c
CH
3
CH
3
CO
CH
2
Rh
L
CH
2
Rh
CO
L
L
CO
CO
OC
L
Scheme 10.2.
With PPh
3
as the ligand, a common starting complex is the species RhH(PPh
3
)
3
CO
1
, which, under 1 bar of carbon monoxide, forms the complexes
2ee
and
2ae
, containing
two phosphine ligands in equatorial positions or one in apical and the other ligand in
an equatorial position. Dissociation of either equatorial L or equatorial CO from
1
or
2
leads to the square planar intermediates
3c
and
3t
(never observed), which have phos-
phines in
cis
- or
trans
- confi guration, respectively. Complexes
3
react with ethene to give
