Chemistry Reference
In-Depth Information
8B.7.1.4. Survey of Ligands Used for the Ir-Catalyzed Allylic Substitution
Most
asymmetric Ir-catalyzed allylic substitutions have been carried out with catalysts pre-
pared from [Ir(COD)Cl]2 and a phosphoramidite. Phosphoramidites are usually air
stable and can be conveniently prepared from appropriate secondary amines [297-299].
In addition to phosphoramidites, a number of ligands based on other frameworks have
been used. The ligands that have been most often employed are described in Figure
8B.33. One has to bear in mind always that a ligand can be transformed in the course
of a catalyzed reaction.
Early on, the ligand
Monophos - NMe
2
(
L58
) [287a,300] was employed. It induced
moderate to good results in allylic alkylations. For an unknown reason, it is inactive in
aminations [287b]. The easily available ligand
L59
[300] is usually the fi rst one that is
tried. Arguably,
L60
[290] is generally the best ligand. However, enantiomerically
enriched 1-(2-methoxyphenyl)ethylamine, which is needed for its synthesis, is not yet
commercially available from fi ne chemicals suppliers [301]. Ligands with different aryl
groups, for example, Ar =
o
- (MeO)C
6
H
4
and Ar = Ph, have also been explored [299].
The corresponding ligand is easily available from (
S
) - 1 - phenylethylamine and in fact a
good substitute for
L60
. For certain combinations of reactants,
L61
[300] was found to
be particularly effective. Excellent results have been obtained with ligands
L62
[302a,b]
and
L63
[302b,c], which are simplifi ed versions of
L59 - L61
.
Phosphites (
L64a
[303] ,
L64b
[304] ),
Ph - Pybox
(
L42c
) [305] , and
DIAPHOX
(
L18
)
[306] have also been explored as chiral ligands in Ir-catalyzed allylic alkylations. The
catalysts were prepared from [Ir(COD)Cl]
2
. These ligands have almost exclusively been
used in reactions of
aryl
allylic substrates, which are privileged substrates. The true value
of a catalyst, however, becomes apparent from the results of allylic substitutions at
alkyl
allyl derivatives.
8B.7.1.5. Ir-Complexes of Phosphoramidites as Precatalysts
8B.7.1.5.1. Catalyst Preparation, Reaction Conditions
Ligands can be altered by C- H
activation at aryl (see above) or CH
3
groups in an Ir-catalyzed allylic substitution. Cata-
H
3
C
H
3
C
H
3
C
S
S
S
CH
3
Ar
Ar
Ar
O
O
O
O
a
S
P
N
a
S
PN
a
S
PN
c
-C
12
H
23
PN
c
-C
12
H
23
O
O
O
O
CH
3
Ar
S
H
3
C
Monophos-NMe
2
L59
Ar = Ph
L60
Ar =
o
-(MeO)C
6
H
4
L61
Ar =
L62a
Ar = Ph
L62b
Ar =
o
-(MeO)C
6
H
4
L63a
Ar = Ph
L63b
Ar =
o
-(MeO)C
6
H
4
(
L58
)
α
-naphthyl
Ar
1
N
H
O
P
O
O
O
N
a
S
P
OR
N
Ar
2
O
N
N
NH
Ar
1
DIAPHOX
(
L18
)
Ph
Ph
L64a
R=Ph
L64b
R=(CH
2
)
2
SEt
Ph-PYBOX
(
L4 2c
)
Figure 8B.33.
Chiral ligands employed for Ir-catalyzed allylic substitutions.
