Chemistry Reference
In-Depth Information
TABLE 12.3. Triplet-Sensitized Irradiation of 2-Naphthoyl Azide (7) in Cyclohexane
Containing Nitrene Trapping Reagents
51
Product Yields (Absolute %)
Isocyanate
8
Amide
9
12a
12b
Amide
10
[7] (mM)
No trapping reagents
0.44
5
63
25
0.66 M
cis
-4-methyl-2-pentene
3.3
2
3
90
ND
6
1.7
Trace
Trace
86
ND
8
0.85
ND
Trace
82
ND
15
0.33
ND
ND
71
ND
23
0.66 M
trans
-4-methyl-2-pentene
5.3
5
6
ND
30
44
1.6
Trace
5
ND
34
66
0.57
ND
Trace
ND
9
74
0.54
ND
ND
ND
6
86
ND, the decay was not detected.
range of olefin concentration. From photolysis of
in a mixture of
trans
- and
cis
-4-
methyl-2-pentenes at different ratio, it was
68
concluded that the rate constant for
forming
cis
-aziridine (
1b
) from
cis
-4-methyl-2-pentene must be at least 230 times
larger than the rate constant for the forming
trans
-aziridine (
14b
) from
trans
-4-
methyl-2-pentene. Lwowksi and coworkers proposed that both singlet and triplet
nitrenes (
1
3b
15b
and
3
3b
) react with olefins, and that dichloromethane solvates and
stabilizes
1
3b
without decreasing its reactivity.
68
The properties of pivaloyl nitrenes
are discussed in more details later in this chapter.
The third very important point to note is that the same type of products were
formed in the direct and triplet-sensitized photolysis of carbonyl azides.
34,51
Schuster
et al. demonstrated
51
that both direct and triplet-sensitized photolysis of
7
generates
the reactions of singlet 2-naphthoyl nitrene (
1
13
products characteristic of
)
(Table 12.3). They used transient absorption spectroscopy to confirm that a triplet
excited state of 2-isopropylthioxantone (ITX), a triplet sensitizer, is quenched by
azide
7
containing ITX with light above 385 nm produces small amount of isocyanate (5%),
due to the direct excitation of
7
with a diffusion-limited rate. Irradiation of a cyclohexane solution of
7
, along with insertion product
9
and amide
10
in a
modest yield (25%).
Schuster et al.
51
have also showed that triplet-sensitized photolysis of
7
in the
presence of
cis
-or
trans
-4-methyl-2-pentene (Table 12.3) gave aziridines (
)
with complete retention of the olefin stereochemistry (Table 12.3). Earlier,
34
similar
results have been obtained for the benzoyl azide (R
12a,b
ΒΌ
Ph,
1a
), the direct- and
acetophenone-sensitized photolysis of
produced the same trapping products and
these products were characteristic of a singlet nitrene
1
3a
1a
.
