Chemistry Reference
In-Depth Information
remedy was provided by procedure E, that is, application of salt-free reaction conditions.
With this protocol, enantiomeric excess of
>
98% ee can often be reached upon use of
L60
.
The following conclusions concerning alkylations of allylic carbonates can be drawn
(cf. Table 8B.54 ):
(1) Particularly high selectivities and yields are obtained with alkenyl- and aryl-
substituted allylic carbonates. Using ligand
L59
/procedures D or E or ligand
L60
/
procedures B, D, or E, enantiomeric excess
98:2 in
favor of the branched product are typical. Exceptions are allylic substrates with
an
o
- substituted aryl group, for example, R = 2 - (MeO)C
6
H
4
.
(2) Generally, enantioselectivities are high with alkyl - substituted allylic carbonates
as substrates, while regioselectivities vary. Values
>
95% and regioselectivity
≥
90:10 are typical in case of
carbonates with small and rigid substituents (R = Me, cyclohexyl); values in the
range 70:30 to 90:10 are found for reactions with substrates containing fl exible
or sterically very demanding substituents (R =
n
- Pr,
n -
octyl, PhCH
2
CH
2
, (
t
- Bu)
Ph
2
SiOCH
2
).
(3) The scope concerning substrates is broad. Even sensitive allylic boronates have
been used [317]. The example cited in Table 8B.54 proceeded with excellent
regioselectivity and enantiomeric excess of up to 84% ee, even though nonopti-
mal reaction conditions were used. Pd catalysts failed to promote the reaction.
>
Of course, ligands other than phosphoramidites have been probed for allylic alkyl-
ations of malonates. It is a general observation that regioselectivity was high with arylal-
lyl derivatives but low with alkylallyl acetates or carbonates. Examples are
PHOX
ligands (cf. Section 8B.2.3) [286a],
i-
Pr - Pybox
[305b], phosphites of type
L64
[303] , and
DIAPHOX
ligands [306a] .
Naturally,
-keto esters and 2-alkylmalonates have also been probed as pronucleo-
philes (Scheme 8B.80) [299a,315]. Generally, mixtures of epimers (ca. 1:1) are formed
from
β
-keto esters. The allylation products prepared from 2-alkenylmalonates have been
transformed into cyclopentene derivatives by Ru-catalyzed ring closing metathesis
(RCM) [299a,315] .
β
Procedure D:
Procedure B:
O
O
O
O
O
O
O
O
O
O
Me
OMe
MeO
OMe
MeO
OMe
MeO
OMe
OMe
b
/
l
= >99:1,
97% ee (95%)
b
/
l
= 94:6,
95% ee
(dr = 51:49, 79%)
b
/
l
= 96:4,
97% ee (74%)
b
/
l
= 93:7,
92% ee (78%)
b
/
l
= >99:1,
95% ee
(dr = 53:47, 68%)
Scheme 8B.80.
Allylic alkylations: sodium salts of 2-substituted malonates and
β
- keto esters with
cinnamyl methyl carbonate; ligand:
L60
.
