Chemistry Reference
In-Depth Information
substitution mechanism.
59,60
The formation of these adducts can be correlated with
the carcinogenic activity of arylamines.
61,62
In recent years, the groups of Falvey,
63-68
McClelland,
21-24
and Novak
69-74
have
developed convenient precursors for studying the solution phase chemistry of
nitrenium cations, which has allowed the measurement of their lifetimes, the
determination of their UV-Vis and IR spectra, and the determination of rate
constants for reactions with selected nucleophiles, in aqueous solution. The most
convenient way to generate mono-substituted nitrenium cations, RNH
þ
, was devel-
oped by McClelland et al. who photolyzed aryl azides in water to generate singlet
nitrenes, which are subsequently protonated to form nitrenium cations.
21-24
This
method works particularly well when the singlet nitrene to be intercepted has a
relatively long lifetime (
10 ns) in aprotic solvent at ambient temperatures. In this
manner,
p
-biphenylyl nitrenium cation (
p
-BpNH
þ
), produced by protonation of
singlet
p
-biphenylyl nitrene (
1
p
-BpN), was readily detected by transient UV-Vis
spectroscopy.
21-24
Phillips et al. subsequently studied this nitrenium cation in water
using time-resolved resonance Raman spectroscopy and assigned the spectra with
the aid of DFT calculations.
25
Similarly, Michalak and Platz produced fluorinated
arylnitrenium cations in acidic acetonitrile solution by flash photolysis of the
corresponding aryl azides.
75
The observation of corresponding nitrenium ion spectra for short-lived nitrenes,
such as
o
-BpN, 1-NpN, and 2-NpN, were prevented by the very rapid intramolecular
cyclization of these nitrenes. However, recently we discovered that aryl nitrenes
could be efficiently protonated in formic acid which led us to renewed attempts at
interception of
o
-BpN, 1-NpN, and 2-NpN.
76
Ultrafast spectroscopy is an excellent tool to study proton transfer reactions due to
their extremely rapid reaction rates. There are not many reported studies of proton
transfer rates between solute and solvent, where the solute acts as proton acceptor
and the solvent as proton donor. Apart from singlet nitrenes, this process has been
studied for a series of singlet arylcarbenes
29-32,34,35,77
and bipyridine in the singlet
excited state
78
(hydrogen atom transfer) in alcohols. Although the reaction is
diffusion controlled, the proton transfer rate is not instantaneous (within 100 fs)
and is limited by the time needed for solvent reorganization. Our studies of singlet
nitrenes in various protic solvents is an attempt to understand the solvent parameters
controlling the protonation rate and create a nitrenium cation even in the
presence of very competitive deactivation channels (for instance intramolecular
rearrangements).
>
1.3.1
p
-Biphenylyl Nitrenium Cation
We first studied proton transfer reactions of
p
-biphenylyl azide,
p
-BpN
3
because
both the corresponding nitrene
26,27 1
p
-BpN and the analogous nitrenium cation
21,23
p
-BpNH
þ
are well-characterized. Ultrafast laser flash photolysis (270 nm) of
p
-biphenylyl azide (
p
-BpN
3
) in a mixture of 50% water and 50% acetonitrile
produces the spectra shown in Figure 1.10.
76
A transient absorption band centered
at 350 nm, which is assigned to singlet
p
-biphenylyl nitrene
1
p
-BpN, is formed
