Chemistry Reference
In-Depth Information
Ph
Ph
Ph
Ph
h
ν
N
Ph
N
3
.
Ph
Ph
Ph
Ph
SCHEME 5.24.
O
N
3
O
.
O
h
ν
.
PhC
H
3
+
Ph
Ph
N
3
Ph
Ph
Ph
Ph
58 60 59
(MeSi)
3
SiH
O
Ph
Ph
61
SCHEME 5.25.
60
to form
59
. Additionally, photolysis of
58
in the presence of a H-atom donor such
as (CH
3
Si)
3
SiH trapped
60
to form
61
.
Laser flash photolysis of
58
detected the radical
60
(
l
max
310 nm) directly and
verified that
N bond cleavage. Klima et al. also spectroscopically
confirmed that the azido radical abstracts a hydrogen atom to form hydrazoic acid.
The authors used density-functional theory calculations to explain why
58
undergoes C
58
reacts
35
so differently compared to
(Fig. 5.6). Calculations showed that T
1K
and T
2K
in
58
are almost degenerate and are thus in equilibrium with each other. The
calculated transition state barrier for cleaving the C
Nbondin
58
is 2 kcal/mol
above the T
1K
of
and
35
58
, whereas this transition state barrier is
11 kcal/mol larger for
35
, which explains why
35
does not undergo C
N bond cleavage efficiently.
Thus, the C
is facilitated by the resonance stabilization
provided by the phenyl group. The calculated bond dissociation energy for
breaking the C
N cleavage of
58
N bond in
58
is 55 kcal/mol, which is considerably lower
than the energy of T
1K
for
Nbond
photochemically. In comparison, the calculated bond dissociation energy for the
C
58
, and thus it is feasible to break this C
is 71 kcal/mol, which is comparable to the energy of its T
1K
and
T
2K
that makes its cleavage less likely.
Nbondin
35
5.5.5.3 Intramolecular Hydrogen Atom Abstraction in Azidoalkanes.
Muthuk-
rishnan et al. showed that photolysis of
g
-azidobutyrophenone
62
yields a 1,4 ketyl
) via intramolecular H-atom abstraction (Scheme 5.26).
100
The 1,4 ketyl
biradical expelled a nitrogen molecule to form 1,5 ketyl iminyl biradical
biradical (
63
64
, which
decayed by ring closure to form
. The 1,5 ketyl iminyl biradicals were character-
ized by transient spectroscopy. In argon-saturated solutions, the biradicals have their
65
