Chemistry Reference
In-Depth Information
TABLE 8B.52. Asymmetric Rh - Catalyzed Allylations Using a
meso
- Dicarbonate
OCO
2
Et
Ar
ArB(OH)
2
[Rh(COD)OH]
2
/
L
,
THF, Cs
2
CO
3
, 50°C
+
Ar
OCO
2
Et
OCO
2
Et
OCO
2
Et
p
d
Entry
Ar
L
Yield (%)
p
/
d
p
(% ee)
d
(% ee)
1
Ph
(
S
) -
BINAP
(
L23a
)
45
a
65:35
76
72
2
P h
L56
60
a
50:50
90
72
3
P h
L57
70
a
95:5
90
>
98
4
4 - MeOOC - C
6
H
4
L57
95
95:5
90
>
98
5
4 - CF
3
- C
6
H
4
L57
86
92:8
88
n.d.
6
2 - Naphthyl
L57
78
95:5
90
n.d.
a
Conversion.
n.d., not determined.
In 2006, Lautens et al. also reported on the enantio-, regio-, and diastereoselective
rhodium-catalyzed desymmetrization of cyclic
meso
-allyl dicarbonates, also with boronic
acids as hard nucleophiles (Table 8B.52) [284]. These
meso
- substrates are more chal-
lenging than the oxabicycles, because the allylic displacement can take place via an S
N
2
or S
N
2
pathway, and the regioselectivity of the reaction has to be considered. Several
ligands were screened under this point of view. Initial studies were carried out with
phenylboronic acid as pronucleophile and (
S
) -
BINAP
(
L23a
) as ligand. The 1,2-
trans
-
substituted product was the major one, in a 2:1 ratio over the 1,4-substituted product.
Importantly, none of the
cis
isomers were obtained, and the ees of the product were
moderate (entry 1). Unfortunately, the reaction was relatively slow and therefore the
conditions were optimized by varying the chiral ligands. Biarylbisphosphines proved to
be the most effective class of ligands. Excellent enantioselectivities were obtained with
ligand
L56
, although without any regioselectivity (entry 2). By far, the best results were
obtained with ligand
L57
, which induced both excellent regio- and enantioselectivity;
conversion was acceptable (entry 3). With this ligand, a range of further arylboronic
acids were investigated, and similar results were obtained (entries 4-6).
′
8B.7. IRIDIUM - CATALYZED ENANTIOSELECTIVE
ALLYLIC ALKYLATIONS
The Ir-catalyzed allylic substitution was introduced by Takeuchi et al. in 1997 [285].
The fi rst asymmetric variant followed in the same year [286]. Two years later, phos-
phoramidites were identifi ed as particularly suitable ligands [287a]. Since then, the fi eld
has developed fast. Hartwig et al. added important contributions concerning allylic
amination [288] and reaction mechanism [289]. Alexakis, Polet, and others introduced
