Chemistry Reference
In-Depth Information
O
O
3
or
47
h
ν
Ph
N
3
39
+
40
+ propiophenone
Ph
N:
C
6
H
6
35
36
R=
N
3
or
N
3
3
47
SCHEME 5.18.
modeling. X-ray structure analysis of
determined that nitrene dimerization did not
occur between the N-atoms on adjacent molecules with the shortest intermolecular
distance but that it rather involved molecules with the most favorable orbital
alignment for azo dimer formation.
The stability of triplet alkyl nitrenes was further highlighted by the observation
that a dimer
46
was formed from the dimerization of two nitrene molecules, rather
than the reaction of a nitrene molecule with an azido precursor. Singh et al. showed
that photolysis of
35
in the presence of an alkyl azide without a built-in sensitizer,
which does not absorb light above 300 nm, did not lead to any new products
(Scheme 5.18).
64
Because no cross-products between the nitrene
35
36
and the
azidoalkanes
36
decayed through reaction with another nitrene intermediate, rather than with an
azidoalkane. Thus, triplet alkyl nitrenes are long-lived intermediates, which are
reactive only toward other triplet species. Schuster et al. have reported similar
findings for phenyl nitrene intermediates.
72
3
and
47
were observed, it can be concluded that the nitrene
5.5.3 Hydrogen Atom Abstraction by Triplet Alkyl Nitrenes
To date, there have been no unambiguous reports on triplet alkyl nitrenes decaying by
abstracting hydrogen atoms. In comparison, triplet nitrenes such as alkoxycarbonyl
nitrenes have been shown to efficiently abstract hydrogen atoms from the sol-
vent.
83,84
Sankaranarayanan et al. used calculations to explore the feasibility of a
triplet alkyl nitrene abstracting hydrogen atoms.
85
The authors demonstrated that the
transition state barriers for H-atom abstraction by triplet nitrenes correlate with the
charge on the nitrogen atoms. More specifically, the ground state of triplet nitrenes
with electron-withdrawing substituents is destabilized more than the corresponding
transition states for H-atom abstraction. Thus, the electron-deficient (i.e., less stable)
triplet nitrenes are more reactive toward H-atom abstraction than stabilized nitrenes
that have electron-donating substituents. The effects of electron-withdrawing sub-
stituents on the ability of triplet alkyl nitrenes to abstract hydrogen atoms can be
highlighted by the isomeric nitrenes
48
and
49
(Fig. 5.3). The triplet nitrene
49
is
8 kcal/mol less stable than
, due to the destabilizing effect of the fluoro substituents
adjacent to the nitrene moiety. The calculated transition state for the abstraction of a
hydrogen atom from methanol by
48
48
was located at 19 kcal/mol above its ground
