Chemistry Reference
In-Depth Information
TABLE 8B.30. Allylic Alkylations Using
In Situ
- Formed Catalyst
Ar
OCO
2
Me
MeO
2
C
CO
2
Me
l
NaCH(CO
2
Me)
2
CO
2
Me
or
+
Ar
(C
2
H
5
CN)
3
Mo(CO)
3,
L43a
,THF
Ar
CO
2
Me
OCO
2
Me
b
l
Ar
b
Product
Entry
Substrate
Ar
T ( ° C)
t (h)
Yield (%)
b
:
l
ee (%)
1
l
Ph
rt
3 h
70
49:1
99 (
S
)
2
l
Ph
65
3 h
88
32:1
99
(S
)
3
b
Ph
rt
3 h
61
32:1
97 (
S
)
4
b
Ph
65
3 h
70
13:1
92 (
S
)
5
b
2 - Thienyl
65
2 h
78
19:1
88 (
S
)
6
b
2 - Pyridyl
65
2 h
69
8:1
96 (
S
)
7
b
1 - Naphthyl
65
2 h
82
99:1
87 (
S
)
MoL*
Ar
Ar
MoL*
Figure 8B.23.
Possible (
π
- allyl)Mo complexes.
formation of one of the two possible π-allyl complexes (Fig. 8B.23), or if these two
complexes are in a fast equilibrium, and the enantiodifferentiation results from a pre-
ferred attack on one of the complexes, the same reaction was also carried out with
racemic branched substrates.
If the equilibrium between the two allyl complexes is fast compared with the nucleo-
philic attack, one should get nearly the same result as with the linear substrate. On the
other hand, if the equilibration is slow, one should expect more or less racemic products.
In the reactions of the phenylallyl carbonates, the selectivities obtained with the branched
were slightly lower compared with the linear substrates (entries 3 and 4), indicating that
the equilibration is signifi cantly faster than the nucleophilic attack. Other aryl- and
hetaryl-substituted derivatives gave similar results (entries 5-7). The cycloheptatrienyl
complex (C
7
H
8
)Mo(CO)
3
can replace the (EtCN)
3
Mo(CO
3
) catalyst giving very similar
results. The effectiveness of these precatalysts presumably derives from the lability of
the EtCN- or C
7
H
8
-ligands, thus allowing a facile substitution with the chiral ligand [212].
But both complexes are air sensitive and are made from commercially available Mo(CO)
6
,
which is air stable. Therefore, for applications in large scale, Mo(CO)
6
is a much more
attractive catalyst.
