Chemistry Reference
In-Depth Information
TABLE 8B.65. C u - Catalyzed Allylic Alkylations of Vinylaluminium Reagents
R
1
R
1
Al
i
-Bu
2
R
OPO(OEt)
2
1 mol % CuCl
2
•2 H
2
O
R
0.5 mol % (Ag-
L82
)
2
THF,
−
15°C, 2-6 h
Entry
R
R
1
Yield (%)
ee (%)
1
Ph
84
92
n -
Hex
2
2 - NO
2
Ph
94
96
n -
Hex
3
1 - Naphth
88
91
n -
Hex
4
Ph
91
91
c -
Hex
5
Ph
3 - Cl - Pr
86
92
6
Ph
4 -
t -
BuO - Bu
90
89
Very recently, Hoveyda et al. used the same ligand system also for the allylation of
vinylaluminum reagents [373], easily obtained by hydroalumination of alkynes. The S
N
2
′
product was formed nearly exclusively with a wide range of substrates in high yield and
enantioselectivity (Table 8B.65 ).
8B.8.2.2.4. Kinetic Resolutions of Vinyl Epoxides
Vinyl epoxides are excellent sub-
strates for transition metal-catalyzed allylic alkylations. Most reactions were carried out
under Pd catalysis [374], but chiral organocopper reagents can also be used for the
regioselective ring opening of these allylic substrates. If racemic epoxides are reacted
with 0.5 equiv of the copper reagent, kinetic resolution can occur. Pineschi, Feringa, and
others were the fi rst reporting on such kinetic epoxide openings with dialkyl zinc in the
presence of chiral binaphthol-based phosphorus amidate (
R
,
R
,
aS
) -
L59
[375] . The cor-
responding allylic alcohols were obtained with moderate to high regioselectivities.
Both pathways, the direct S
N
2 (
α
-product) and the conjugate opening S
N
2
′
addition
(
-product), seem to proceed with complete
anti
- selectivity. While the enantiomeric
excess with the fi ve-membered-ring substrate was moderate (Table 8B.12, entries 1 and
2), high selectivities were obtained with the larger six- and seven-membered substrates
(entries 3 - 5).
Equey and Alexakis performed the same experiments with commercially available
Me
3
Al and ligand (
S
,
S
,
aS
) -
L59
[376]. Comparable enantiomeric excesses were observed
(entries 6 and 7), but the regioselectivities were lower compared with the experiments
with the analogue zinc reagent (except for the six-membered ring). Best results were
obtained with the ferrocene-based ligands (
R
,
pS
) -
L40c
/
L40d
(Fig. 8B.43 ) [377] . They
allowed the highly selective addition of a wide range of Grignard reagents to the cyclo-
hexenyl substrate (entries 8 - 12) (Table 8B.66 ).
γ
8B.8.2.2.5. Desymmetrization of meso - Substrates
Based on their good results obtained
in the ring opening of vinyl epoxides, Pineschi et al. also investigated the ring opening
of
meso
-epoxides such as the cyclooctatetraene monoepoxide (Scheme 8B.102), which
