Level 1 – Case 7
40
2. The reaction of 1 with NaBH 4 in deuteromethanol as solvent yielded the
deuteration products 5 and 6 (Scheme 7.3).
(CO) 5 Cr
OEt
Ph
NaBH 4
CD 3 OD
Ph
EtO
D
H
EtO
Ph
H D
1
6
5
Scheme 7.3
Discussion
The isolobal analogy principle states that DE-unsaturated alkoxychromium(0)
carbenes should behave as DE-unsatura t ted esters in processes taking place outside
of th t e coordination sphere of the metal. In consequ q ence, such comp m lexes may be
regarded as a kind of supe u r-esters, with their reactivity enhanced du d e to the presence of a metal in their structures. In this context, the behavior of DE-unsaturated
alkoxychromium(0) carbene comp m lexes and DE-unsaturated organic esters towards the addition of nucleophiles, should be similar. Hydrides are nucleophiles
and, if we consider that comp m lexes 1 should behave like a conj n ugated system, their
reactivity towards NaBH 4 could be interpreted considering the two possible alternative 1,2- or 1,4-addition processes (Scheme 7.4a and b).
Vinyl ether 2 (the major reac
a
tion product), could come from a 1,4-(conjugated)hydride addition, protonation of th t e resulting intermediate at the chromium center
and redu d ctive elimination with loss of th t e metallic fragment. On the oth t er hand,
allyl ether 3 (the t minor reaction product) could be obtained by a similar sequ q ence,
starting from a simp m le 1,2-hydride addition to the carb r ene. The overall reduction
process would be then an examp m le of two parallel reactions, with the 1,4-addition
clearly favored over the 1,2-process. To confirm the validity of these hypotheses it
is necessary to check if they are able to justify the results obtained in the deuteration experiments.
In Schemes 7.4a and b we have represented the expected deuterium-labe a led positions in the final products. When NaBD 4 is emp m loyed, th t e labe a led hydrogen atoms are colored red and in the experiment in CD 3 OD as solvent, they are colored
blue.
Ph
[Cr]
OEt
H
–
[Cr] r
OEt
Ph
H
–
H
+
H
H
[Cr]
OEt
Ph
H H
EtO
1,2-Hydride addition
1
3
Scheme 7.4a
40
2. The reaction of 1 with NaBH 4 in deuteromethanol as solvent yielded the
deuteration products 5 and 6 (Scheme 7.3).
(CO) 5 Cr
OEt
Ph
NaBH 4
CD 3 OD
Ph
EtO
D
H
EtO
Ph
H D
1
6
5
Scheme 7.3
Discussion
The isolobal analogy principle states that DE-unsaturated alkoxychromium(0)
carbenes should behave as DE-unsatura t ted esters in processes taking place outside
of th t e coordination sphere of the metal. In consequ q ence, such comp m lexes may be
regarded as a kind of supe u r-esters, with their reactivity enhanced du d e to the presence of a metal in their structures. In this context, the behavior of DE-unsaturated
alkoxychromium(0) carbene comp m lexes and DE-unsaturated organic esters towards the addition of nucleophiles, should be similar. Hydrides are nucleophiles
and, if we consider that comp m lexes 1 should behave like a conj n ugated system, their
reactivity towards NaBH 4 could be interpreted considering the two possible alternative 1,2- or 1,4-addition processes (Scheme 7.4a and b).
Vinyl ether 2 (the major reac
a
tion product), could come from a 1,4-(conjugated)hydride addition, protonation of th t e resulting intermediate at the chromium center
and redu d ctive elimination with loss of th t e metallic fragment. On the oth t er hand,
allyl ether 3 (the t minor reaction product) could be obtained by a similar sequ q ence,
starting from a simp m le 1,2-hydride addition to the carb r ene. The overall reduction
process would be then an examp m le of two parallel reactions, with the 1,4-addition
clearly favored over the 1,2-process. To confirm the validity of these hypotheses it
is necessary to check if they are able to justify the results obtained in the deuteration experiments.
In Schemes 7.4a and b we have represented the expected deuterium-labe a led positions in the final products. When NaBD 4 is emp m loyed, th t e labe a led hydrogen atoms are colored red and in the experiment in CD 3 OD as solvent, they are colored
blue.
Ph
[Cr]
OEt
H
–
[Cr] r
OEt
Ph
H
–
H
+
H
H
[Cr]
OEt
Ph
H H
EtO
1,2-Hydride addition
1
3
Scheme 7.4a