Chemistry Reference
In-Depth Information
2.3 THE CENTRAL ROLE OF THE SINGLET NITRENE
Most nitrenes studied to date have been found to have triplet ground states. With few
exceptions, however, all thermal methods to generate nitrenes, as well as the vast
majority of direct photochemical methods for generating nitrenes, initially place the
nascent nitrene on the higher-energy singlet surface. Consequently, it is possible for
triplet ground-state nitrenes to decay though singlet nitrene channels. Indeed, since
singlet nitrenes are a highly reactive species, it is frequently the case that the rates for
singlet nitrene reactions (unimolecular rearrangement, bimolecular trapping) exceed
the rate of intersystem crossing to form the ground-state triplet nitrene.
The result of this initial population of the nitrene onto the singlet surface is that the
singlet nitrene is bequeathed a singular importance in the chemistry of nitrenes: the
singlet nitrene is the branching point for all subsequent reactions, and the nature of
this species governs which downstream processes and decay pathways will be
favored (Scheme 2.2). Because the singlet nitrene partitions the decay channels and
directs the formation of products, the discussion in this section focuses on how the
properties of
singlet
nitrenes—such as the structural features, substituents, and
electronic configuration—exert powerful influences on the rates of the diverse singlet
nitrene branching pathways. Emphasis is placed on how theoretical considerations
and computational methods have been used as powerful tools to aid in deciphering
these complex processes. We also provide rough qualitative theoretical frameworks
that describe the reactivity and properties of singlet nitrenes. To focus this section,
we restrict discussion to arylnitrenes, which have received most of the experimental
and computational attention to date (Scheme 2.3).
R
N
1
N
3
1*
-N
2
hv
ISC
N
3
N
3
N
N
R
R
R
R
Singlet nitrene
Singlet excited
azide
Triplet nitrene
R
Azoarene
H
+
n
n
N
NH
+
NH
R
R
Arylnitrenium
ion
Benzazirine
NUC
R
C-H insertion product
NUC
NH
2
NUC
N
N
NUC
R
R
R
Dehydroazepine
SCHEME 2.2.
A typical reaction diagram of photolysis of an aryl azide showing the singlet
nitrene as the key branching point.
