Chemistry Reference
In-Depth Information
O
O
h
v
C
6
H
12
/C
6
H
10
N
N
3
8 + 9 +
11
7
SCHEME 12.5.
Photolysis of 2-naphthoyl
azide
(
7
)
in cyclohexane
containing
cyclohexene.
51
capture by trapping agents. Instead, the isocyanate could be formed from the
carbonyl azide excited state. This hypothesis was further supported by recent
ultrafast spectroscopic studies
36,37
that are discussed in more details later in this
chapter.
The second interesting point to note is that carbonyl nitrenes add to olefins
stereospesifically.
6,24,51,68
For instance, photolysis of
in excess of
cis
-or
trans
-4-
methyl-2-pentene gives modest yields of
N
-benzoylaziridines (Scheme 12.6), and the
addition is stereoselective; less than 2% of the isomeric
N
-benzoylaziridines is
formed.
24b,68
Photolysis of 2-naphthoyl azide
1a
in cyclohexane solution containing
either
cis
-or
trans
-4-methyl-2-pentene (0.2M) gives the corresponding aziridines
(
7
98%, within the experimental detection limit) of
the olefin stereochemistry (Table 12.2).
51
Note that benzoyl nitrene, generated
photochemically from
12a,b
) with complete retention (
>
1a
, was also stereospecifically inserted into the tertiary
H bonds of
cis
- and
trans
-1,4-dimethylcyclohexane.
50
Based on the Skell-Woodworth hypothesis developed for carbenes,
69
it could be
concluded that aziridines and C
C
H insertion products are formed in the reaction of
singlet carbonyl nitrenes.
24,51,68
Unfortunately, these experiments do not give
information about ground state multiplicity of carbonyl nitrenes. It could be that
either singlet state is the ground state of the carbonyl nitrenes or singlet nitrenes relax
very slowly to a triplet state. However, when concentration of
cis
-4-methyl-2-
pentene was reduced to only 0.01M, the addition of 2-naphthoyl nitrene remains
stereospecific (Table 12.2).
51
Thus, extending the 2-naphthoyl nitrene (
) lifetime
by diluting the concentration of the trapping agent does not lead to relaxation of a
putative excited singlet nitrene to its lower energy triplet state.
51
It should be noted that situation with the pivaloyl azide
13
1b
is more complicated.
Photolysis of
in neat
cis
-4-methyl-2-pentene yields the
cis
-
N
-pivaloyl-2-iso-
propyl-3-methylaziridine (
1b
14b
, 31%) with high stereoselectivity (the yield of
15b
is
O
O
H
R
2
R
1
N
3
N
h
v
+
cis
: R
1
= CH
3
, R
2
= H
trans
: R
1
= H, R
2
= CH
3
R
2
R
1
7
12a,b
SCHEME 12.6.
Photolysis of 2-naphthoyl azide
7
in the presence of
cis
-or
trans
-4-methyl-2-
pentene.
51
