Chemistry Reference
In-Depth Information
NH
2
HN
CH
2
CO
2
CH
2
CH
2
N
CH
3
N
N
NO
2
S
2
C
O
N
S
O
N
S
O
O
N
3
O
OH
O
OH
O
2
N
+
N
3
ICH
2
CO
2
CH
2
CH
2
N
CH
3
NAMA-I
SCHEME 3.20.
3.4 GENERAL CONSIDERATIONS
The foregoing discussion has been limited to 4-amino-3-nitrophenyl azides, since
this configuration of substituents is definitely known to afford nitrenium ions
exclusively, and these nitrenium ion collapse to form covalently bound derivatives.
52
The electron-donating capacity of the
para
-amino group can be attenuated by amide
formation, but nitrenium ion formation and collapse still occurs effectively as shown
in Scheme 3.21 (V. Voskresenska and R.M. Wilson, unpublished work).
24
Other
para
-electron-donating substituents might also afford very basic nitrenes that
effectively abstract protons to form nitrenium ions. Thus,
para
-thioethers
112
and
ethers
113
have both been used successfully as PAL agents that might proceed through
nitrenium ion species.
46
However, the question of mechanistic bifurcation between
nitrene ring expansion and proton abstraction chemistry as a function of electron-
donating capacity of the
para
-substituents has not been adequately studied.
The presence of the nitro group is also an important component in the generation
of effective PAL species, since this powerful electron-withdrawing group makes the
resulting nitrenium ion much more susceptible to the Michael addition that ulti-
mately produces the cross-linking covalent bond, Scheme 3.9. McClelland's work
has shown that without this type of Michael-facilitating group, nitrenium ions tend to
undergo hydrolysis to the quinone, which does not produce a cross-linking covalent
bond, see Scheme 3.8.
24,46
. Even so aryl azides lacking such a Michael facilitating
nitro group have been used with success in PAL studies.
113,114
Thus, a nitro group
N
3
NH
2
OCH(CH
3
)
2
h
ν
48%
HOCH(CH
3
)
2
NO
2
HNCO
2
Et
NO
2
HNCO
2
Et
SCHEME 3.21.
