Level 1 – Case 2
10
N
H
SPh
SPh
Cl
N
SPh
H
SPh
N
SPh
SPh
6
11
7
Scheme 2.8
It is more likely to assume that if sulfo f f nyl chloride is present in the medium,
the quaternization of the nitrogen in 6 is possible, leading to a N-s N N ulfenyl intermediate 12 in the first step of the reaction. Now, the electrophilicity of the C=N bond
is increased and a nucleophilic attack by the SPh group would form the episulfonium species 13 that by ring opening would lead to N- N N phenylsulfe f f nyl 2,3-bissulfide 14. Removal of the SPh group u in 14 dur d ing the aqu
t
eous workup k k would
yield the fi f f nal bis-sulfide 7 (Scheme 2.9).
H 2 O
N
SPh
H
SPh
SPh
N
SPh
SPh
SPh
Cl
13
14
N
SPh
SPh
6
PhSCl
N
SPh
SPh
12
SPh Cl
N
H
SPh
SPh
7
Scheme 2.9
In Summary
Sulfenylation of indole with sulfenyl chlorides occurs initially at the 3-position. In
excess of the reagent, the second sulfide group is introduced at the 2-position by
initial formation of an indolenium 3,3-bis-sulfide intermediate like 9, followed by
migration of one of the sulfide groups to the 2-position through an episulfonium
species 10.