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is increased (and consequently, the frontier orbital gap is increased), the switch froman
open-shell singlet state nitrene to a closed shell singlet-state nitrene along the electron-
donating continuumgenerally occurs before the nitrene becomes a singlet ground-state
species. Methoxynitrene is an example of a nitrene in this intermediate position. The
methoxy group is a sufficiently strong
-donor to increase the orbital energy gap to
disfavor the open-shell singlet state, rendering the closed-shell singlet state to be lower
in energy. But the energy gap is not so large that the energetic penalty of placing an
electron in this higher energy orbital exceeds the compensatory exchange energy of the
triplet state. For this nitrene, the lowest-energy singlet state is closed-shell, but the
triplet state is still the preferred ground state. Other nitrenes with a triplet ground state
but a lowest-energy singlet state that is closed-shell include hydroxylnitrene,
14
phosphorylnitrenes,
23,24
and certain carbonylnitrenes.
16,25-28
p
2.2.6 Electronic Structure of Carbonylnitrenes
While much of the discussion thus far has focused on how
-donating substituents
attached to the nitrene center breaks the degeneracy of the frontier
p
-orbitals, these
observations can be applied generally to any type of substituent that interacts
differentially with the orthogonal
p
-orbitals on the nitrene center. Carbonylnitrenes
represent a good example. Carbonyl groups are
p
-acceptors, but in carbonylnitrenes,
there is a partial bonding interaction between the carbonyl oxygen and the nitrene
center.
29,30
Consequently, these “nitrenes” have significant zwitterionic azirine
character as depicted in Figure 2.3. This bonding interaction significantly separates
the frontier orbitals and leads to singlet nitrenes that are closed-shell. In contrast to
p
p
-donor-substituted nitrenes, it is the in-plane
p
-orbital on the nitrene center that is
raised in energy rather than the out-of-plane
p
-orbital. Moreover, for these species,
the singlet and triplet states tend to be nearly degenerate, and both ground-state
singlet and ground-state triplet carbonylnitrenes have been reported.
25,29,31
A similar
interaction between the oxygen on phosphorylnitrenes
24,32
is thought to interact with
the nitrene center for these species.
2.2.7 Comparison of the Molecular Structure Between
Singlet and Triplet Phenylnitrene
Phenylnitrene is a particularly important species from an application's stand-
point since derivatives of phenylnitrene find use in a variety of biological and
+
O
O
R
R
N
N
-
Carbonylnitrene
FIGURE 2.3.
Singlet carbonylnitrenes have significant zwitterionic character.
