Chemistry Reference
In-Depth Information
cooperative effect between the chiral sugar backbone stereocenters (C-3 and C-5) and
the axial chiral binaphthyl phosphite moieties. Accordingly, ligands
20
and
21
provide
better enantioselectivities than ligands
19
and
22
; (iii) the absolute confi guration of the
product is governed by the confi guration at the stereogenic center C-3. Accordingly,
ligands
17
,
20
, and
22
with
S
confi guration at C-3 gave (
S
) - 2 - phenylpropanal, while
ligands
18
,
19
, and
21
, with
R
confi guration at C-3 gave (
R
) - 2 - phenylpropanal. These
results overcome the problem presented by carbohydrate-derived ligands as they are
available in only one enantiomeric form; and (iv) as observed with the previously men-
tioned ligands
10a
-
d
, there is an infl uence on the substituents in the biaryl phosphite
moieties. Thus, ligands
20b
,
d
and
21b
,
d
, with either methoxy substituents or trimethylsi-
lyl groups, always produced the best enantioselectivities.
The characterization of the rhodium complexes formed under hydroformylation
conditions by NMR techniques and
in situ
IR spectroscopy showed that there is a rela-
tionship between the structure of the [RhH(CO)
2
(P - P)] (P - P =
17
-
22
) species and their
enantio-discriminating performance. In general, enantioselectivities were highest with
ligands with a strong bis-equatorial (
ee
) coordination preference (Fig. 10.2), while the
equilibrium of species with bis-equatorial (
ee
) and equatorial - axial (
ea
) coordination
modes (Fig. 10.2) considerably reduced the ee's [9d,e].
Through all these years, several authors have developed new diphosphite ligands with
biaryl, spiro, pyranoside, mannitol, and macrocyclic backbones (Fig. 10.4) for asymmet-
ric hydroformylation of vinyl arenes with low to moderate success (ee's from 16% to
76%) [15] .
Phosphine-Phosphite Ligands
The fi rst report on asymmetric hydroformylation using
phosphite-phosphine ligands was carried out by Takaya and coworkers in 1993 [16]. With
the aim to combine the effectiveness of the BINOL chemistry for asymmetric catalysis
and the effectiveness of the phosphite moiety for asymmetric hydroformylation, they
developed the (
R
,
S
) - BINAPHOS ligand
33
. This turned out to be a very effi cient ligand
(Fig. 10.5 ).
In the last few years, a wide range of structural variations has been reported. In this
context, in 1997, Nozaki and coworkers used ligands
33
-
35
and found that the sense of
enantioselectivity is governed by the confi guration of the binaphthyl bridge, whereas the
enantiomeric excess depends strongly on the confi guration of both binaphthyl moieties
(Fig. 10.5) [10a]. Enantioselectivity is therefore higher when the confi gurations of the
two binaphthyl moieties are opposite (i.e., diastereoisomers
R
,
S
or
S
,
R
). Similar trends
were observed with ligands
36
and
37
, which have a chiral biphenyl bridge.
To further understand the role of the chirality at the bridge and the axial chirality at
the phosphite moiety in transferring the chiral information to the product outcome,
ligands
38
and
39
were studied (Fig. 10.6) [10a]. Ligand
38
, which has an (
R
) - binapthyl
in the bridge, provides an ee of 83% (
R
). This value is close to the (
R
,
S
) - BINAPHOS
value (94% (
R
) ee). This suggests that, in the formation of the Rh-complex, the binaph-
thyl bridge controls the conformation of the biphenyl phosphite moiety. Likewise, ligand
39
provides an ee of 69% (
S
), which suggests that the binaphthyl phosphite moiety also
controls the conformation of the biphenyl bridge upon the coordination to rhodium.
However, the control by the binaphthyl bridge is more effi cient than that of the binaph-
thyl phosphite moiety.
Next, the effect of several substituents in the phosphine moiety has been extensively
studied by Nozaki's group (Fig. 10.7). Their results indicate that both regio- and enantio-
selectivities can be increased by suitable choice of the aryl phosphine group. The best
