Chemistry Reference
In-Depth Information
MeO
2
C
CO
2
Me
OAc
NaCH(CO
2
Me)
2
H
3
C
R
H
3
C
R
[Ir(COD)Cl]
2
/P(OPh)
3
THF, rt
R
R
R = H >99% ee
85% ee (91%)
R = CH
3
>
9% ee
71% ee (83%)
Scheme 8B.75.
Ir-catalyzed allylic alkylations with enantiomerically pure allylic substrates [287b].
These results and the conservation of the
Z
- confi guration (cf. Scheme 8B.72) are
proof that intermediates of the Ir-catalyzed reactions are nonsymmetric π - or σ - allyl - Ir
complexes, which undergo
slow
racemization (or epimerization). Analogous Rh-
catalyzed reactions display even higher degrees of stereospecifi city. Evans and Nelson
proposed π / σ - or
enyl
complexes as intermediates [272]. Whether the latter are synonyms
for nonsymmetric π-complexes or distinct species is not clear.
8B.7.1.3. Asymmetric Catalysis—Early Work with PHOX as Ligands
The fi rst
asymmetric Ir-catalyzed allylic substitution was carried out with dimethyl sodiomalonate
and cinnamyl acetate as substrate. The
PHOX
i
- Pr - PHOX
(
14a
) was used as chiral
ligand (Scheme 8B.76) [286]. Although the reaction was comparatively slow, regio- and
enantioselectivities were high. Unfortunately, the PHOX-Ir catalysts turned out not
to be generally suited; in particular, regioselectivities were generally low with alkyl-
substituted substrates.
MeO
2
C
CO
2
Me
OAc
NaCH(CO
2
Me)
2
CO
2
Me
+
R
R
R
[Ir(COD)Cl]
2
/
i
-Pr-PHOX
THF, 65°C
b
l
CO
2
Me
R=Ph
b
/
l
= 95:5, 91% ee (99%)
O
N
PPh
2
i
-Pr
i
-Pr-PHOX
(
14a
)
Scheme 8B.76.
First Ir - catalyzed asymmetric allylic substitution.
Preparation of (π - allyl)(PHOX)Ir
III
complexes was possible with the help of standard
methods. The reaction of one of the complexes with dimethyl sodiomalonate was probed.
Addition of the nucleophile occurred at the central rather than the terminal allylic
carbon to give an iridacyclobutane [296a]. This reaction mode had in fact previously
been encountered by Bergman, Stryker, and others and is found with many types of
(π - allyl)Ir
III
complexes [296b,c] .
