Chemistry Reference
In-Depth Information
ring expansion, azirene formation, reaction with neighboring groups, ring opening to
ylides, diradicals or nitrenes, and ring contraction.
8.2 RING EXPANSION
(Hetero)aromatic nitrenes and carbenes undergo ring expansion to seven-membered
ring ketenimines, carbodiimides, and allenes, that is, (aza)cycloheptatetraenes. Thus,
phenyl nitrene
, generated from phenyl azide, undergoes photochemical
1
1
and
thermal
2
ring expansion to 1-azacyclohepta-1,2,4,6-tetraene
(Scheme 8.1), which
is readily observed by IR spectroscopy, when the photochemical reactions are
performed in low-temperature matrices.
1
The ground state triplet nitrene
2
is itself
directly observable by ESR, IR, and UV spectroscopy, and its photolysis to
azacycloheptatetraene
1
is likely to take place via a reactive excited state.
1b
Femtosecond time-resolved IR spectroscopy in solution has revealed the direct
conversion of vibrationally excited phenyl nitrene
2
on a
picosecond timescale.
3
It is usually assumed that it is the open-shell singlet nitrene
(S
1
) that undergoes ring expansion,
4
but in a recent investigation of a different
reaction, the protonation of 4-diethylamino-3-nitrophenyl nitrene on the nitrene-N to
produce the nitrenium ion, it was concluded that reaction takes place in the higher-
energy closed-shell nitrene (S
2
).
5
The bent ketenimine function in
1
to azacycloheptatetraene
2
gives rise to a strong absorption at 1891 cm
1
in
the IR spectrum,
1,6
and
15
N labeling causes a shift to 1876 cm
1
.
6
Several substituted
azacycloheptatetraenes have been generated analogously.
6
Intermolecular trapping
of azacycloheptatetraenes with nucleophiles in solution is a well-known means of
synthesizing 2-substituted 3
H
-azepines.
7
The interconversion with 2-pyridyl carbene
3
2
is described in the following.
The singlet nitrene can be intercepted by an
o
-phenyl group in the photolysis of
o
-biphenylyl azide
4
, leading to carbazole
9
via isocarbazole
8
, but the azacyclo-
heptatetraene
is still formed (1868 cm
1
by time-resolved IR spectroscopy in
solution
8
and about 1900 cm
1
by matrix-isolation IR spectroscopy) (Scheme 8.2).
9,10
The assumed azirene intermediate
7
is long
lived under matrix-isolation conditions and is directly observable by its UV-Vis
absorptions at 340 and 450-550 nm. The ring expansion of the nitrene to
6
was not observed. The triplet nitrene
5
7
is reversible
on photolysis at 254 nm; therefore,
7
serves as a reservoir for carbazole formation via
its reversion to the nitrene
5
. The azacycloheptatetraene
7
can be trapped with amines
N
3
CH
N
N
N
1
2
3
SCHEME 8.1.
