Chemistry Reference
In-Depth Information
CH
3
h
v
(direct)
PhCH
3
NH
3
+ PhCh
2
+
(adducts also form)
1
NH
2
+
N
NH
2
+
Ph
12
h
v
triplet
sensitizer
PhCH
3
PhCH
2
3
NH
2
+
NH
3
+
+ PhCH
2
PhCH
2
CH
2
Ph
FIGURE 6.6.
Generation of NH
2
þ
through photolysis of an
N
-aminopyridinum ion.
16
The chemical reactions of NH
2
þ
were examined experimentally by Takeuchi
et al.
15
In these studies, NH
2
þ
was generated through photolysis of
N
-amino(2,4,6-
triphenylpyridinium) ions in mixtures of TFA and toluene or benzene. Addition of
NH
2
þ
to the arenes was discovered, along with the formation of dimeric products,
including bibenzyl and benzyl toluene isomers. A subsequent study by Srivastava
et al.
16
elucidated the relationship between the products from these reactions and the
spin states of the intermediate nitrenium ion.
These findings are summarized in Figure 6.6. Photolysis of the precursor
12
generates the singlet state of NH
2
þ
. The latter either undergoes intersystem crossing
to the lower energy triplet state or reacts with the solvent and/or additives. For
example, with methanol, the major singlet product is
O
-methylhydroxylamine. With
toluene, singlet products arise from electrophilic aromatic substitution (toluidine
isomers) and hydride abstraction from benzylic C
H bonds, creating NH
3
and
carbenium ions. Triplet NH
2
þ
abstracts benzylic C
H bonds creating radicals.
Support for these assignments came from two experiments. Addition of a
nonreactive diluent, hexafluorobenzene, was hypothesized to reduce the rates of
the singlet reactions with the toluene trap, thus allowing a greater fraction of the
singlet NH
2
þ
ions to relax to the lower energy triplet. This in turn would lead to
increased yields of the radical products. Results of the experiment confirmed this
hypothesis. Yields of bibenzyl increased at the expense of benzyltoluenes. In a
second experiment, the NH
2
þ
was generated through triplet state energy transfer
(sensitization) to the photoprecursor. This process generates the excited triplet state
of the latter, which in turn was expected to form triplet NH
2
þ
in a spin-conserving
bond breaking reaction. Again product analysis confirmed this hypothesis. Enhanced
yields of the radical products were detected.
The effect of halogen substitution was examined by Gobbi and Frenking
17
as well
as Gonzales et al.
18
The halogens act as
p
donors and
s
acceptors and thus they act to
raise the energy of the
p
orbital relative to the
E
st
in favor
of the singlet state. The dihalo-species are all clearly ground state singlets. The
mono-halo nitrenium ions have smaller gaps. Generally, the fluoro, being the
smallest halogen, allows for more acute bond angles and thus shows the greatest
relative stabilization of the singlet. The larger halogens require wider bond angles
and thus have
s
(Table 6.1). This shifts
D
E
st
values that move more in favor of the triplet.
D
