Chemistry Reference
In-Depth Information
R
R'
O
O
O
O
O
O
O
O
P
P
P
P
P
P
P
P
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
(2
R
,4
R
)
14
(2
R
,3
R
)
15
(2
S
,5
S
)
16
(2
S
,4
S
)
10
R= R'= Me
11
R= R'= Ph
12
R= Me, R'= H
13
R= Ph, R'= H
R
R
R'
O
O
O
O
O
O
=
O
O
O
O
R
R
R'
(
R
/
S
)
ax
a
R=
t
-Bu; R'=
t
-Bu
g
R= SiMe
3
n
o
(
R
/
S
)
ax
b
R=
t
-Bu; R'= OMe
h
R= SiEt
3
(
R
/
S
)
ax
c
R= H; R'= H
i
R= Si
t
-BuMe
2
(
R
)
ax
d
R= SiMe
3
;R'=H
j
R= H
(
S
)
ax
e
R= SiEt
3
;R'=H
k
R= H
(
R
)
ax
f
R= Si
t
-BuMe
2
;R'=H
l
R= SiMe
3
(
S
)
ax
m
R= SiMe
3
Figure 10.1.
Diphosphite ligands
10
-
16
.
2,4-diol (Fig. 10.1) [9a]. Inspired by these excellent results, other research groups have
studied several modifi cations in these types of ligand (Fig. 10.1) [9b,11]. They mainly
studied the infl uence of the bridge length, the phosphite moieties, and the possibility of
a cooperative effect between chiral centers on the performance of the catalysts.
The infl uence of the bridge length was studied with diphosphite ligands based on
(2
R
,4
R
) - pentane - 2,4 - diol (ligands
10a
and
10b)
, (2
R
,3
R
) - butane - 2,3 - diol (ligands
14a
and
14b)
, and (2
S
,5
S
) - hexane - 2,5 - diol (ligands
15a
and
15b)
. In general, ligands
10
,
which have three carbon atoms in the bridge, provided higher enantioselectivities than
ligands
14
and
15
, which have two and four carbon atoms in the bridge [11a].
The effect of different phosphite moieties was studied with ligands
10a
-
o
. In general,
sterically hindered phosphite moieties are necessary for high enantioselectivities [9b,11b].
Thus, ligands
10c
,
10j
,
10k
,
10n
, and
10o
show low asymmetric induction (ee's up to
20%). Also, the results of using ligands
10a - i
indicated that varying the
ortho
and
para
substituents on the biphenyl and binaphthyl phosphite moieties has a great effect on
asymmetric induction. The optimal combination is therefore obtained with ligands
10b
and
10d
(ee's up to 90% at 20 bar of syn gas and at 25°C).
