Chemistry Reference
In-Depth Information
CH
2
PhCH
3
Ph
NH
3
+
21
PhCH
3
Ph
Ph
NH
2
NH
2
NH
2
PhCH
3
h
ν
1
NH
2
PhCH
3
TFA
2 h
Ph
N
NH
2
Ph
19
ClO
4
22
(Mixture of isomers)
18
ISC
CH
2
h
ν
PhCH
3
PhCH
2
ISC
Ph
3
NH
2
NH
3
Ph
20
23
SCHEME 10.3.
intermediate
20
participates in H-atom transfer to generate bibenzyl (
23
) (in the case
of toluene) or triphenylmethanol (in the case of triphenylmethane).
10.3.2 Alkyl Nitrenium Ions
The chemical reactivity of alkyl nitrenium ions, such as those generated from the
Ag(0)-assisted solvolysis
N
-chloramines
24
is often complex and results from the
small triplet-singlet (T
1
-S
0
) energy gap in these species, which leads to intersystem
crossing and the accompanying involvement of two distinct reaction manifolds
(Scheme 10.4). Although triplet ground-state alkyl nitrenium ions
display useful
reactivity characteristic of
N
-centered radicals, such as participation in 5-
exo
-
tet
cyclizations to form pyrrolidine derivatives
28
, these highly reactive intermediates
are also subject to hydrogen-atom abstraction to form amines
32
31
, by way of the
corresponding radical cations
30
. In common with carbocations, singlet alkyl
nitrenium ions, such as
, undergo 1,2-alkyl and 1,2-hydride shifts leading to
the formation of resonance-stabilized iminium ions
25
. More usefully, these divalent
electrophiles, which are isoelectronic with carbenes, also undergo concerted [2
26
þ
1]
cycloaddition with alkenes to form aziridinium ions. In the case of
25
, the generation
and solvolysis of bridged aziridinium ion
29
leads to the formation of amino alcohols
33
and
34
, the products of alkene
anti
-aminohydroxylation.
10.3.3 Aryl Nitrenium Ions
In light of the iminocyclohexadienyl cation-like nature of aryl nitrenium ions (
)
and the fact that the majority of the charge density in these species resides at the
14
