Chemistry Reference
In-Depth Information
π
-allyl intermediate, before the nucleophilic attack occurs, minimizing the memory
effect.
8B.4.1.4. Scope and Limitations
In general, the asymmetric Mo-catalyzed allylic
alkylation provides the required products with good regio- and enantioselectivity for a
wide range of allylic substrates and nucleophiles. Thus, it is an interesting tool for asym-
metric synthesis.
8B.4.1.4.1. Variation of the Allylic Substrate
In most cases, allylic carbonates were
applied as substrates, but the corresponding acetates and phosphates can be used as well.
Moberg et al. investigated the reaction of several substituted cinnamyl-like substrates
and their regioisomers under microwave irradiation and under conventional heating
conditions (Table 8B.36). The different substituents had a signifi cant infl uence on the
regioselectivity of the reaction: The more electron donating the substituent, the higher
the ratio of branched product (entries 1-3). In general, the linear allylic substrates
showed a higher regioselectivity than the corresponding branched ones (entries 1 and 2
vs. 4 and 5) [215b]. Bulkier, 2-substitutetd malonates also afforded the products with
good regio - and enantioselectivity (entries 6 - 9) [211] .
Interestingly, no reaction was observed with some branched and cyclic substrates
(Fig. 8B.27 ) [228] .
Trost et al. investigated reactions of dienyl and polyenyl substrates in the presence
of ligands
L43a
and
L43h
[234]. These substrates can be more complicated than normal
allylic substrates, because in principle, three different products can be expected (Scheme
8B.55 ).
TABLE 8B.36. Allylic Alkylation Using Different Aryl - Substituted Allylic Carbonates
R
X
Ar
MeO
2
C
CO
2
Me
l
NaCR(CO
2
Me)
2
CO
2
Me
+
Ar
or
R
Mo(CO)
6
(4 mol %),
L43a
Ar
CO
2
Me
X
b
l
Ar
b
Product
Entry
Substrate
Ar
X
R
T ( ° C)
Yield (%)
b
:
l
ee (%)
165
a
1
l
4 - Cl - Ph
OCO
2
Me
H
51
32:1
96
2
l
4 - CF
3
- Ph
OCO
2
Me
H
165
a
50
11:1
99
3
l
4 - MeO - Ph
OCO
2
Me
H
165
a
59
51:1
98
4
b
4 - Cl - Ph
OCO
2
Me
H
165
a
78
26:1
96
5
b
4 - CF
3
- Ph
OCO
2
Me
H
165
a
48
10:1
98
6
l
Ph
OCO
2
Me
Me
60
67
24:1
98
7
l
2 - Furyl
OCO
2
Me
Me
60
71
32:1
97
8
b
2 - Furyl
OAc
Me
60
65
32:1
87
9
b
2 - Furyl
OAc
All
rt
50
99:1
98
a
Under microwave irradiation.
