Photolysis of N-nitrosamines in acidic acetonitrile
produces aminium radical cations via protonation of
the initially-formed aminyl radicals. The kinetics of these
species can be monitored by transient UV spectroscopy
via their absorption band which is found at ca. 300 nm in
the case of the piperidinium radical, for example. By
measuring the aminium radicals' lifetimes as a function of the
concentration of added olefin, absolute values for the
bimolecular rate constants for the addition reactions were obtained.
In the case of the piperidinium radical, these
rate constants varied from <1 × 106 M-1
s-1 for acrylonitrile to 1.1 ± 0.1 × 109
M-1 s-1 for
1,1-diphenylethylene
and generally increased with decreasing ionization potential of the
olefin, thus confirming the electrophilic nature of
the piperidinium radical. The rate constants for analogous
reactions of diethylaminium radicals were 1.5−25 times
smaller indicating the importance of steric factors in aminium radical
additions to olefins. The rate constant for the
intramolecular 1,5-addition of the secondary aminium radical cation to
an unactivated double bond is estimated to
be ca. 1 × 106 s-1, but the
intramolecular addition rate constant increases to >1 ×
108 s-1 upon the phenyl
substitution
at the olefinic terminus.