Chemistry Reference
In-Depth Information
TABLE 8B.34. The Fiaud Test with M o ( CO )
6
as Catalyst
H
H
H
Retension
X
x
Mo(CO)
6
Toluene,
Δ
NaCH(CO
2
Me)
2
H
H
H
H
Inve rsion
Mo
H
CO
2
Me
In ve rsion
MeO
2
C
H
H
X
H
n
Conversion (%) After
Entry
Substrate
X
1 h
2 h
4 h
1
x
OAc
19
46
96
2
n
O A c
3
6
<
2
x
a
3
OCONMe
2
41
71
100
4
n
OCONMe
2
1
2
4
5
x
OCOCF
3
8
12
25
6
n
OCOCF
3
100
7
b
n
OCOCF
3
84
97
100
a
No base required for deprotonation.
b
Reaction without Mo catalyst.
leaving group. To prove this option, the acetate was replaced by a better electron-
donating carbamate group and a more electron-withdrawing trifl uoroacetyl group.
Indeed, the
exo
-carbamate showed an increased reactivity (entry 3), although carba-
mates are moderate leaving groups, while the isomeric
endo
- carbamate was nearly inert
(entry 4). On the other hand, trifl uoroacetate is known to be a good leaving group with
a non-nucleophilic carbonyl group. Whereas the
exo
- trifl uoroacetate has been found to
react three times slower than the
exo
-acetate (entry 5), an extreme acceleration was
observed for the reaction of the
endo
- trifl uoroacetate (entry 6), but this substrate also
reacted without a catalyst (entry 7).
The difference between Pd and Mo can be rationalized as follows: The Pd-catalyzed
reaction starts with the coordination of the Pd to the allylic double bond, followed by
cleavage of the leaving group, as a result of back donating [227]. In contrast, Mo is
assumed to associate fi rst with the Lewis-basic carbonyl group of the leaving group,
followed by coordination to the C= C bond.
If the double retention mechanism proposed by Ko
č
ovský et al. is still operating with
a chiral ligand attached to the Mo, then the nucleophile approaching from the
syn
- face
(probably via fi rst coordinating to the metal) should experience a direct interaction with
the chiral ligand [228]. This might result in a highly asymmetric induction. Therefore,
