Level 3 – Case 39
260
N H
O
IBX
I
N H
O
-H
1
SET
R
R
N
O
II
N
O
R
R

o
 
  
  
 
trig
-
-
5 exo
N
O
Me
R
N
O
CH 2
III
R
2
H
Scheme 39.2
The confirmation of this hypothesis requires resolving two main questions.
1. Which is the origin of the hydrogen atom in the final quenching step? Does it
come from the substrate itself, the IBX, or the solvent?
2. Which step is rate-determining? The single electron transfer (SET) step or the
radical cyclization step?
Discuss whether the mechanism proposed in Scheme 39.2 is fully consistent with
all these experimental results.
E Ex xp pe er ri im me en nt ta al l D Da at ta a
1 1. . I Is Is I Is I Is Is I I o ot to to to to to t top op op pe e L La ab be el li li li li l lin i in in in i i g g E Ex E Ex E Ex Ex E E p
x xp xp xp p
x xpe er ri ri ri r ri ri r rim i im im im i ime en nt ts ts t ts ts ts ts t t
Norbornene-derived anilide 3 was chosen as starting material in the isotope labeling experiments that are collected in Scheme 39.3. When the reaction of 3 with
IBX was performed in deuterated media [THF-d 8
d d /DMSO-d 6 (10/1)], deuterated
6
product 4 was obtained.
Carrying out the reaction of 3 in THF/DMSO-d 6
d d (10/1) led to bicyclic anilide 6
containing no deuterium. The same compound 6 was obtained when labeled anilide 5 was reacted with IBX in THF/DMSO (10/1) (Scheme 39.3). Finally, when
the experiment was carried out with 3 and IBX in neat DMSO, no reaction was
observed. These results lead to a new question: what is the role of THF in the
reaction?