Level 2 – Case 29
196
should be less stable due to the steric repulsion between the eclipsing phenyl
groups. In this case, the C-Br bonds are weaker and the participation in the equilibrium of a stabilized benzylic carbocation 19 should be more favored. The ring
opening-free rotation process would lead to the more stable intermediate 17 from
which trans-stilbene is formed.
In Summary
As we have seen, the study of the stereochemistry of the debromination reaction is
the key choosing between the two mechanistic pathways. Both proposals could
justify the kinetic data (rate law, nucleophilicity of the telluride, effects of solvent
polarity) however, only Mechanism 2 could satisfactorily explain the stereoselectivity in all cases. The intermediacy of a bromonium ion and the role of the telluride as a scavenger of the Br
+ seem to be the best option with all the data in hand.
A Ad dd di it ti io on na al l C Co om mm me en nt ts s
Suggest some experiments that could bring additional evidences in support of the
bromonium ion. Comment other alternatives to the intermediacy of free bromonium ions in the reaction.
This problem is based on the work by Butcher TS, Zhou F, Detty MR (1998) J.
Org. Chem. 63:169-176.
S Su ub bj je ec ct ts s o of f R Re ev vi is si io on n
S N 2. Elimination reactions. Bromonium ion intermediates. Neighboring group participation.
196
should be less stable due to the steric repulsion between the eclipsing phenyl
groups. In this case, the C-Br bonds are weaker and the participation in the equilibrium of a stabilized benzylic carbocation 19 should be more favored. The ring
opening-free rotation process would lead to the more stable intermediate 17 from
which trans-stilbene is formed.
In Summary
As we have seen, the study of the stereochemistry of the debromination reaction is
the key choosing between the two mechanistic pathways. Both proposals could
justify the kinetic data (rate law, nucleophilicity of the telluride, effects of solvent
polarity) however, only Mechanism 2 could satisfactorily explain the stereoselectivity in all cases. The intermediacy of a bromonium ion and the role of the telluride as a scavenger of the Br
+ seem to be the best option with all the data in hand.
A Ad dd di it ti io on na al l C Co om mm me en nt ts s
Suggest some experiments that could bring additional evidences in support of the
bromonium ion. Comment other alternatives to the intermediacy of free bromonium ions in the reaction.
This problem is based on the work by Butcher TS, Zhou F, Detty MR (1998) J.
Org. Chem. 63:169-176.
S Su ub bj je ec ct ts s o of f R Re ev vi is si io on n
S N 2. Elimination reactions. Bromonium ion intermediates. Neighboring group participation.