Chemistry Reference
In-Depth Information
SCHEME 4.16.
Nitrenium ions subjected to semiempirical calculations.
(Scheme 4.16) are ground state singlets with singlet-triplet gaps of 18-43 kcal/mol.
The calculations showed that both the singlet and the triplet states of these ions were
stabilized relative to NH
2
รพ
by electron donation from the aromatic ring, but the
stabilization of the singlet state was considerably larger. Calculated bond lengths for
the singlet state were consistent with considerable contributions from the canonical
structures shown in Scheme 4.16. Calculated free energy profiles for the reaction of
64b
with H
2
O showed that attack at C-4 and C-2 follows relatively low-energy paths
in the gas phase and in aqueous solution, while the transition state for attack at N is
disfavored by greater than 20 kcal/mol in the gas phase and in solution.
102
Attack at
C-3 did not result in a stable adduct at the MNDO level. Experimentally,
38b
generates a yield of 39% of the
para
-product
53
, but only 2% of the
ortho
-product
at 40
C in solutions containing 0.5M Cl
(Scheme 4.12).
18
47b
4.3 ARYL NITRENIUM IONS ABOUT 1990-2010
By the late 1980s, nitrenium ions, or transition states resembling them, were
implicated in the mechanisms of a number of thermal and photochemical reactions
of
N
-arylhydroxylamines, their derivatives, and related compounds. A few stabilized
nitrenium ions (e.g.,
in Scheme 4.17) had been observed,
58,103
but no
reactive aryl nitrenium ion had been observed, and data on the lifetimes and
65
and
66
SCHEME 4.17.
Highly stabilized nitrenium ions.
