Chemistry Reference
In-Depth Information
SCHEME 4.28.
1-Alkyl-2-imidazoyl- and 2-pyrimidyl nitrenium ions.
.
144
The rate
produced by protonation of nitrene
96a
generated by LFP of azide
97a
constants
k
s
and
k
gs-
were determined to be 9.4 s
1
and 3
10
7
M
1
s
1
, respec-
tively.
144
The ion
has a lifetime of over 100ms at neutral pH in the absence of
buffers or other nucleophiles.
144
This ion is about 10
4
-fold longer lived than the carbenium ion
95a
s
þ
value
assigned to the 1-methyl-2-imidazolyl group on the basis of solvolysis rate constants
for S
N
1 reactions involving carbenium ions is
0.82, slightly more negative than
s
þ
for 4-methoxyphenyl.
145
The rate constants for reaction of
.
141
The
98
98
and its 4-methox-
yphenyl analogue
99
with aqueous solvent are within a factor of six of each other, but
k
s
for
is five orders of magnitude smaller than
k
s
for the 4-methoxyphenyl
nitrenium ion
95a
.
113,144
Not all heteroaryl nitrenium ions are stabilized. Takeuchi
and Watanabe apparently generated the 2-pyrimidyl nitrenium ion
64t
by photolysis
in aromatic solvents containing TFA (Scheme 4.28).
146
A series of electrophilic
aromatic substitution products on the aromatic solvents, the reduction product
2-aminopyrimidine, and apparent radical coupling products of the solvents indi-
cated that the ion was quite unselective and that it exhibited both singlet and triplet
reactions. It was concluded that the singlet-triplet gap in
100
100
was sufficiently
small that
3
wasaccessibleandthattherateof intersystem crossing could be
affected by heavy atom containing solvents.
146
Apparently, the pyrimidyl nitro-
gens localize the positive charge on the exocyclic N, destabilizing the singlet ion
relative to the triplet.
146
Novak and coworkers examined the hydrolysis reactions of a series of esters,
101a-l
100
, of hydroxylamines or hydroxamic acids that are derived from carcinogenic
