Chemistry Reference
In-Depth Information
N
N
S
( )
n
TfOH
and
TFA
CCl
4
0°C
S
S
N
3
228;
= 6
229;
n
= 8
n
230
(32% from
228
)
231
(16.5% from
229
)
SCHEME 10.36.
230
231
substitution to form the 17- and 19-membered ring thiamacrocycles
and
,
respectively (Scheme 10.36).
84
10.6.3 Cyclization to Carbazoles, Indoles, and Related N-Heterocycles
The cyclization of diaryl nitrenium ions represents an appealing strategy with which
to access both carbazole and indole ring systems in spite of the tendency of these
electron-rich heterocycles to undergo nitrenium ion-induced polymerization
(Scheme 10.37). Employing laser flash photolysis (LFP) and computational model-
ing (DFT), Falvey has examined the mechanism of this process and concluded that
232
-
electron-5-atom electrocyclization which is coupled to the transfer of the C2 proton
to an external base.
29
Unfortunately, in the presence of external nucleophiles, intermolecular intercep-
tion of diaryl nitrenium ions appears to be favored over ring closure and indeed,
while DeRosa,
85
Shudo,
86
and Falvey
87
have reported the generation of these species
through the heterolysis and homolysis 1,1-arylhydrazine derivatives, in each case,
undergoes conversion to
234
, the 4
H
-tautomer of carbazole, via a concerted 4
p
B:
Me
H
H
2
BF
4
-
hν
Me
N
Me
N
N
N
Ph
Ph
Nuc
234
(observed)
233
232
Electrocyclization
H
H
-H
+
Ph
Ph
N
Nuc
N
N
H
Nuc
N
H
235
236
237
238
SCHEME 10.37.
