Level 1 – Case 7
42
H
OEt O
Ph
H
Ph
EtO
D
H
Ph
EtO
D
4
6
2
expected labeled position with NaBD 4
expected labeled position with CD 3 OD
product obtained with CD 3 OD!
product obtained with NaBD 4 !
Figure 7.2
What else could happen after an initial 1,2-hydride addition to the carbene carbon?
We have already mentioned that a rearrangement during the process is possible,
e.g., a migration of the metallic fragment Cr(CO) 5 to the conjugated position (a
1,3-migration).
The question is: Why are we considering this particular rearrangement and not
any other? The reason is simple. We must remember that the chromium is not pret
sent in the final products, it is replaced by a hydrogen by protonation and reductive elimination. The former position of the metal will be labeled in the final products in the experiments carried out in deuteromethanol. Looking at the structure of
labeled compound 6 in Fig. 7.2, only a 1,3-migration of the metallic fragment
could explain the presence of a deuterium atom at the benzylic position.
In Scheme 7.5 we have represented this alternative. The first step would be the
addition of the hydride to the Cr=C bond leading to intermediate 7 from which the
formation of allyl ether 3 is immediate. Another possible evolution for 7 could be
the 1,3-migration of the Cr(CO) 5 moiety that would lead to complex 8 from which
vinyl ether 2, the major reaction product, can be easily obtained. To check if both
pathways are consistent with the deuteration experiments, we have represented in
Scheme 7.5 the expected labeled positions. The incorporation of deuterium from
the reagent (H in red) takes place during the hydride addition step, whereas in the
CD 3 OD experiment, the deuterium transfer occurs during the protonation step (H
in blue).
The mechanism proposed in Scheme 7.5 explains the results obtained in the
deuteration experiments. Only the allylic position is deuterated in the minor reaction product 3 and the vinylic hydrogen of 2 would be replaced by deuterium in
the experiment with NaBD 4 , but the allylic position would be labeled with
CD 3 OD.
42
H
OEt O
Ph
H
Ph
EtO
D
H
Ph
EtO
D
4
6
2
expected labeled position with NaBD 4
expected labeled position with CD 3 OD
product obtained with CD 3 OD!
product obtained with NaBD 4 !
Figure 7.2
What else could happen after an initial 1,2-hydride addition to the carbene carbon?
We have already mentioned that a rearrangement during the process is possible,
e.g., a migration of the metallic fragment Cr(CO) 5 to the conjugated position (a
1,3-migration).
The question is: Why are we considering this particular rearrangement and not
any other? The reason is simple. We must remember that the chromium is not pret
sent in the final products, it is replaced by a hydrogen by protonation and reductive elimination. The former position of the metal will be labeled in the final products in the experiments carried out in deuteromethanol. Looking at the structure of
labeled compound 6 in Fig. 7.2, only a 1,3-migration of the metallic fragment
could explain the presence of a deuterium atom at the benzylic position.
In Scheme 7.5 we have represented this alternative. The first step would be the
addition of the hydride to the Cr=C bond leading to intermediate 7 from which the
formation of allyl ether 3 is immediate. Another possible evolution for 7 could be
the 1,3-migration of the Cr(CO) 5 moiety that would lead to complex 8 from which
vinyl ether 2, the major reaction product, can be easily obtained. To check if both
pathways are consistent with the deuteration experiments, we have represented in
Scheme 7.5 the expected labeled positions. The incorporation of deuterium from
the reagent (H in red) takes place during the hydride addition step, whereas in the
CD 3 OD experiment, the deuterium transfer occurs during the protonation step (H
in blue).
The mechanism proposed in Scheme 7.5 explains the results obtained in the
deuteration experiments. Only the allylic position is deuterated in the minor reaction product 3 and the vinylic hydrogen of 2 would be replaced by deuterium in
the experiment with NaBD 4 , but the allylic position would be labeled with
CD 3 OD.