Chemistry Reference
In-Depth Information
i
-Bu
OCO
2
CH(CF
3
)
2
p
-BuC
6
H
4
ZnBr
RhTp(C
2
H
4
)
2,
Et
2
O,
LiBr, dba, 0°C
H
i
-Bu
l
b
b
/
l
= 10:1, 95% ee (90%)
Scheme 8B.69.
Stereoselective allylic alkylation of organozinc compounds.
Ph
O
N
O
P
SEt
O
PPh
2
L
4
L
Figure 8B.31.
Ligands used in the Rh-catalyzed allylic alkylation of malonates.
(
S
,
S
) -
CHIRAPHOS
(
L34
) as a ligand [279]. The branched product was obtained with
good regio- but moderate enantioselectivity (23% ee).
The fi rst asymmetric
C
-allylation was described by Pregosin et al. in 1999 [280]. They
applied the phosphite ligand
L54
as well as phosphinoxazoline
L55
(Fig. 8B.31) in the
asymmetric allylation using cinnamyl - type substrates (Table 8B.49 ).
Ligand
L54
gave rise to a 1:1 mixture of linear and branched product in excellent
yield but low enantioselectivity (entry 1), while ligand
L55
provided nearly exclusively
the linear achiral product (entry 2). Better results were obtained with secondary allylic
substrates (Scheme 8B.70). Although the yields were somewhat lower, high ees of up to
82% were obtained for the minor, proximal regioisomer, while the major one (distal)
showed low selectivity.
A signifi cant improvement came from the group of Hayashi et al. in 2003 (Table
8B.49, entries 3-7) [281]. Based on the assumption that high ees can only be obtained
from racemic allyl acetates if the lifetime of the intermediary (allyl)Rh complexes
(Scheme 8B.67 ) is suffi ciently long to allow equilibration, they run their reactions
under high dilution conditions and with slow addition of the pronucleophile. As base,
Cs
2
CO
3
was used instead of NaH to decrease the amount of malonate nucleophile in
solution. Under these conditions excellent enantio- and regioselectivities could be
obtained (entries 3-6). Only the naphthyl derivative gave rise to a mixture of regioi-
somers (entry 7).
Gong et al. investigated the reaction of cyclic nitroallyl acetates with hard nucleo-
philes such as boronic acids and organozinc reagents [282]. As ligands, they used
BINAP
(
L23a
) (Fig. 8B.32) and derivatives thereof. Good results were obtained with [Rh(OH)
(COD)]
2
and Rh(acac)(C
2
H
4
)
2
as precatalysts. Up to 99% ees could be obtained with
both organometallic reagents (Table 8B.50). The substituted nitroalkenes obtained are
