Chemistry Reference
In-Depth Information
SCHEME 4.1.
Electronic states and acid-base properties of aryl nitrenium ions.
The suggestion in the late 1960s that
is the reactive intermediate responsible
for the carcinogenicity of metabolites of aromatic amines,
12
and the substanti-
ation of that proposal during the 1990s,
13-17
spurred research into the reactions
of
1
in water.
18-31
Many reactive examples of
1
have lifetimes in water in the microsecond range
making it possible to detect and characterize the cations during laser flash
photolysis experiments.
20-26
Because of those long lifetimes, many examples of
1
can be studied in water via
photolysis and solvolysis without interference from ion pairs that dominate in
less polar solvents.
19-29
The acid-base chemistry of
1
1
(Scheme 4.1) is most readily investigated in
water.
22,30
X
bonds under solvolysis conditions, and by photolysis of various precursors.
13-31
Under solvolysis conditions,
Aryl nitrenium ions can be generated in water by heterolytic cleavage of N
is generated as a steady-state intermediate that does
not reach directly detectable concentrations, but it can be studied by indirect
methods.
13,14,18,19,27,29
Laser flash photolysis provides the opportunity to directly
observe the cation.
15-17,20-26,28,30,31
Results from both types of studies are discussed
and compared herein.
Although both singlet (
S
0
) and triplet (
T
1
) states of
1
exist, experiments
19-39
and
calculations
40-47
agree that the singlet is, with few exceptions,
37,38,46,47
the ground
state due to donation of electron density from the
1
-orbitals of the aromatic ring into
the empty
p
-orbital of the presumably planar nitrenium center. The triplet
p
is
accessible via triplet-sensitized photolysis,
33,37,38
but most chemistry that has
been examined is due to singlet
1
. In this chapter, the electronic state of
1
should
be assumed to be the singlet unless stated otherwise.
If
Y
(Scheme 4.1). Water is acidic
enough to protonate many singlet nitrenes, so this reaction provides a route to
ΒΌ
H
,
1
is the conjugate acid of an aryl nitrene
2
1
by
