Chemistry Reference
In-Depth Information
FVT of tetrazolo[5,1-
c
]pyrimidines and tetrazolo[1,5-
a
]pyrimidines affords 1-
cyanoimidazoles and 1-cyanopyrazoles, respectively (Scheme 8.14).
18
Ring contrac-
tion of aryl- and heteroaryl nitrenes may be a concerted process in some cases,
22
but
evidence for complex mechanisms involving ring-opening and ring closure is accu-
mulating.
22,23,37
Several examples of formal ring contraction reactions are described
in the following section.
8.4 RING OPENING
While the reactions described so far may be regarded as typical, they not universal.
The ring expansions may be partly or completely superseded by ring-opening
reactions, leading to ylides, diradicals, or nitrenes, all of which may be observed
directly by matrix isolation ESR and/or IR and UV-Vis spectroscopies in favorable
cases. Furthermore, the ring-opened species may be precursors of the ring contrac-
tion products. We distinguish two types of ring opening, Type I and Type II,
Scheme 8.15.
37
8.4.1 Type I Ring Opening to Nitrile Ylides
Not only do 2-pyridyl carbene
3
and phenyl nitrene
1
interconvert via 1-azacyclo-
heptatetraene
2
; all the pyridyl carbenes (
88
,
90
, and
3
) interconvert and eventually
Y
X
Y
Y
X
X
Z
N
N
N
Z
Z
Y
X
Z
X, Y, Z = CH or N
N
Type I ring opening
—
ylidic
C
NH
N
N
N
CN
C
CN
N
N
CN
Type II ring opening
—
diradicaloid or nitrenoid
CN
SCHEME 8.15.
