Chemistry Reference
In-Depth Information
R
COR
1,2-Alkyl
shift
O
COPh
N
N
hν
ClO
4
N
ClO
4
CH
3
CN
ClO
4
50
(R = Ph or H)
51
52
(95%)
MeO
COH
MeO
COPh
H
H
MeOH
MeOH
N
N
53
(93%)
54
(81%)
SCHEME 10.7.
10.3.4 Alkylaryl Nitrenium Ions
Depending on the reaction conditions, alkylaryl nitrenium ions display dual
reactivity and, as a result of the resonance stability afforded by the aryl substituent,
often undergo efficient transformations. Haley's photolysis of l-benzisoxazolium
perchlorates
,
31
which generates singlet nitrenium ions
, offers an illustrative
example of this situation (Scheme 10.7). In aprotic solvents and in the absence of
suitable nucleophiles,
50
51
Ph) behaves as an alkyl nitrenium ion undergoing ring
expansion to yield stable 3-azahomoadamantyl carbenium ion
51
(R
¼
, while in the
presence of nucleophiles, such as methanol, attack occurs at the ring position of
52
51
(R
¼
H) to yield 4-substituted anilines
53
and
54
. Regardless of the reaction medium,
intermediate
51
does not behave as a nitrogen electrophile.
10.3.5 Heteroatom-Stabilized Nitrenium Ions
Arguably, the most valuable nitrenium ions from a synthetic standpoint are those in
which electronic stabilization is afforded by interaction of vacant
p
orbital at the
nitrogen center with the filled nonbonding orbital of an adjoining heteroatom, such as
oxygen or nitrogen. Since the singlet state in this situation is highly favored over the
triplet state, the generation and reaction of these species is often significantly more
efficient than other nitrenium ions. For example, the methoxy nitrenium (
59
) and
1,l-dimethyldiazenium (
) ions, archetypal members of this class, undergo concerted
cycloaddition with alkenes to aziridinium ion adducts in high yield (Scheme 10.8).
In spite the fact that the reaction between electrophiles and alkenes is one of the
most fundamentally important and well-studied processes in organic chemistry, the
56
