Chemistry Reference
In-Depth Information
L59
[8c,302b,c] as well as
L60
[290] have revealed that the absolute confi guration
of the allylation products is controlled by the axial confi guration. Ligands of the
l
series generally gave superior results. In the case of
L59
, the ligand with
u
-
confi guration is inactive [302c].
(b) One of the
N
-arylethyl groups can be replaced by a bulky alkyl or cycloalkyl
group. The best results were obtained with the ligands
L62
and
L63
containing
a cyclododecyl group [302a,b].
(c) Despite the dominating infl uence of the axial chirality (cf. a), good results can
be obtained with ligands containing a biphenyl instead of the binaphthyl unit
(
L63
) [292c,302b] .
(d) Ligands with substituents R
= CH
3
, OCH
3
, or aryl yield catalysts inducing low
selectivity as well as activity [315,316]. This is understandable considering that
the allyl complexes
K5
are highly crowded.
(e) Of great importance is the group X, as demonstrated by excellent result obtained
with ligand
L60
. Coordination of OCH
3
to Ir has been proposed as reason.
However, similar results have been obtained with a corresponding ligand with a
methyl instead of a methoxy group [299]. Thus, it might be the increase of the
steric bulk of X that is important. Increase of the steric bulk of
L
would be
expected to facilitate dissociation of ligand
L
from complex
K3
.
′
8B.7.2. Alkylations with Stabilized Enolates as Nucleophiles
8B.7.2.1. Use of 1,3-Dicarbonyl and Related Compounds as Pronucleophiles
8B.7.2.1.1. Intermolecular Alkylations
Ir - catalyzed allylic substitutions were initially
carried out with dimethyl sodiomalonate and
acetates
as substrates and
L59
as ligand
(procedure B, cf. Section 8B.7.1.5) [287c]. Enantioselectivities were high; however,
regioselectivities were not satisfactory (Scheme 8B.79). A remarkable improvement was
achieved with
L60
as ligand [299a]. Today, we know that
L2
undergoes C- H activation
faster than
L1
; thus,
in situ
C -H activation by malonate appears likely in the case of
L60
.
CH(CO
2
Me)
2
NaCH(CO
2
Me)
2
+
R
OAc
R
R
CH(CO
2
Me)
2
[Ir(COD)Cl]
2
/
L*
/LiCl,
THF, rt,
b
l
L*
R
b
/
l
Ref.
Yield (%)
Ee (%)
L59
Ph
91: 9
98
86
287c
Me
75:25
96
82
287c
i
-Pr
55:45
56
94
287c
L60
Ph
99: 1
79
97
299a
n
-Pr
87:13
87
97
299a
Scheme 8B.79.
Allylic substitutions with allylic acetates.
