NaBH 4 Reduction of DE-Unsaturated Chromium Carbene Complexes 45
[Cr]
Ph
OEt
H
Ð
Ph
EtO
H H
H
H
[Cr]
Ph
OEt
H
Ð
Ph
[Cr]
EtO
H
H H
H
2H
+
. . .
EtO
H
H
H
Ph
[Cr]
EtO
Ð
Ð
H
Ð
Ph
[Cr]
Ð
10 (H position deuterated by NaBD 4 )
11 (H position deuterated by CD 3 OD)
9
12
13
14
3
Scheme 7.7
Additional Comments
An alternative interpretation to the mechanism proposed in Scheme 7.7, could be
the protonation of allenyl intermediate 13 before the addition of a new hydride, to
form free allene 15. The subsequent reduction of this compound would also lead
to allyl ethers 3 in a pathway fully compatible with the observed deuteration pattern. To exclude this possibility, the authors of the original work prepared allene
15 from 1-ethoxy-3-phenyl-2-propyne and BuLi, (formulate the reaction!) and
treated it with NaBH 4 in EtOH under the same conditions employed for compound
9. The allene 15 was recovered unchanged after several hours of reaction (Scheme
7.8).
. . .
EtO
H
Ph
[Cr]
Ð
H
+
. . .
EtO
H
Ph
H
NaBH 4
EtO
Ph
13
15
EtOH, -20
o C
3
Scheme 7.8
This problem is based on the work by Gómez-Gallego M, Mancheño MJ, Ramírez
P, Piñar C, Sierra MA (2000) Tetrahedron 56:4893-4905.
Subjects of Revision
Isotopic labeling experiments. Fisher carbenes: structure and reactivity.
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