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suggested that triplet nitrenes have potential as building blocks for high-spin
assemblies. The reactivity of triplet and singlet nitrene intermediates is also affected
by the substituent on the nitrogen atom because the nitrenes are electron deficient;
therefore, electron-donating substituents stabilize them, whereas electron-withdraw-
ing groups destabilize them. In this chapter, we focus specifically on the characteri-
zation and reactivity of triplet alkyl nitrenes, as singlet alkyl nitrenes have not yet
been reported. Thus, in this chapter, we mainly review the solution and solid-state
reactivity of triplet alkyl nitrenes, with a bias toward the authors' own work.
5.2 DIRECT PHOTOLYSIS AND THERMAL ACTIVATION
OF AZIDOALKANES
In general, direct irradiation or thermal activation of alkyl azides in solution results in
the corresponding imine products with the loss of a nitrogen molecule.
19,20
The imine
products can be formed from singlet alkyl nitrene intermediates (Scheme 5.1);
however, there are no reports of products being isolated from the decomposition of
azidoalkanes, which can be attributed to the trapping of singlet alkyl nitrene inter-
mediates in significant amounts.
20,21
Thus, it was theorized that the rearrangement of
azidoalkanes does not take place through a singlet nitrene, but rather in a concerted
manner from a singlet excited state of the azidoalkanes.
22
Furthermore, this has led to
the assumption that singlet alkyl nitrenes are not a minimum on the potential energy
surface and are thus not intermediates with finite lifetimes.
23,24
More recent theoretical
calculations support the theory that singlet alkyl nitrenes exist as reactive intermediates
even though they must be short-lived and are thus not easily trapped in bimolecular
reactions. To date, they have not been detected by spectroscopy.
7,25
Irradiation of 2-azido-2-methylpropane and azidomethane (
1a
1b
,
) in cryogenic
2
matrices did not yield triplet alkyl nitrenes but resulted in
as the only product
(Scheme 5.2).
26-28
These experimental results did not allow for the determination
of whether these imines were formed via rearrangement of singlet alkyl nitrenes or
R
R
h
or
ν
Δ
R
N
3
N
R
- N
2
Alkyl azide
Imine product
R
R
N:
Singlet alkyl nitrene
SCHEME 5.1.
