Chemistry Reference
In-Depth Information
FIGURE 5.5.
Calculated stationary points on the triplet energy surface for
a
-cleavage of
nitrene
38a
. Energies are in kilocalories per mole.
Source
: Adapted from Ref. 97.
O
O
O
h
ν
NaBH
4
CN
N
3
Ar
N
Ar
N
Ar
Crystals
0˚C
H
37 56 57
b: Ar =
-Cl-Ph
c: Ar =
p
-Br-Ph
f: Ar =
p
-Ph-
p
-Ph
p
SCHEME 5.23.
undergo energy transfer to form nitrenes. However, the product studies show that
37g
does not yield any lesser amount of products attributed to
a
-cleavage than
37a
. Thus,
38a,g
a
Muthukrishnan et al. theorized that nitrenes
-cleavage to form
benzoyl and iminyl radicals, which was further supported by calculations (Fig. 5.5).
Mandel et al. investigated the solid-state reactivity of
underwent
and found it yielded
56b,c,f
in a crystal-to-crystal reaction (Scheme 5.23).
98
Thus, solid-state irradiation
of
37b,c,f
to form a benzoyl and azidoal-
kane radical pair. The azidoalkyl radical rearranged to form an iminyl radical.
Because the iminyl and benzoyl radicals were held in close proximity by the crystal
lattice, they combined to form
37b,c,f
selectively yielded
a
-cleavage in
37b
,
c
,
f
. This mechanismwas further supported by trapping
the benzoyl radical formed in the solid-state with oxygen.
56
5.5.5.2 Photoinduced Azido Cleavage in Azidoalkanes.
In 1968, Lewis and
Saunders reported that irradiation of triphenylmethyl azide resulted in the formation
of a triphenylmethyl and azido radical pair in addition to the expected imine product
(Scheme 5.24).
99
The authors used ESR spectroscopy to characterize the triphe-
nylmethyl radical and isotope labeling to verify that the azido and triphenylmethyl
radical primarily decayed by recombination.
Klima et al. also reported that an azide
N bond cleavage to form
the corresponding alkyl and azido radical pair (Scheme 5.25).
77
Photolysis of
58
underwent C
58
primarily yielded
59
, and it was hypothesized that
59
was derived from breaking the
C
N bond to form
60
and an azido radical, which then abstracts a H-atom from
