Chemistry Reference
In-Depth Information
TABLE 10.3. Effect of Heavy Atom Solvents on the Solvolysis of Compound 84
Ag
+
+
+
N
MeO
N
Cl
N
CH
3
OH
NH
Cl
84
85
86
87
Yield (%)
Yield of S
0
Products (%)
Yield of T
1
Products (%)
S
0
/T
1
Solvent
85
86
87
CH
3
OH-C
6
H
12
56
10
8
74
66
8.2
CH
3
OH-
p
-Br
2
C
6
H
4
33
10
25
68
43
1.7
CH
3
OH-CCl
4
13
<
1
59
73
13
0.22
CH
3
OH-CHCl
3
4
<
1
63
67
4
0.06
1
CH
3
OH-CHBr
3
<<
1
45
46
1
0.02
CH
3
OH-CH
3
OH
59
20
7
86
79
11.3
crossing to the triplet state. In this event, the nitrenium ion could undergo intra-
molecular hydrogen atom abstraction with the solvent to generate
. If this spin
inversion could be promoted and the yield of the amine increased, Gassman
concluded that this would provide convincing evidence for the formation of a
discrete nitrenium ion intermediate. It is well-known that solvents such as bromo-
form and chloroform, which contain heavy atoms, can promote spin inversion
through coupling of spin and orbital angular momenta.
42
Gassman therefore
investigated the effect of methanol-heavy atom solvent mixtures upon the solvolysis
of azabicyclo[2.2.1]heptane
83
(Table 10.3).
It is apparent from Table 10.3 that halogenated solvents have a dramatic influence
on the product distribution of this reaction, the most dramatic effect being observed
when bromoform was employed as a co-solvent. In this case, a 500-fold change in
product ratio, favoring the formation of amine
84
was observed. Since, in the pre-
sence of heavy-atom solvents, the triplet-state nitrenium ion appeared to be the
predominant reactive species, Gassman argued that a discrete singlet-state nitrenium
ion must be present prior to spin inversion to form the triplet species.
87
10.5.2 Cyclization of Unsaturated N-Chloramines
(The Nitrenium Ion Question)
Although Gassman's pioneering work stimulated the field of nitrenium ion che-
mistry,
2g
it was not immune to scrutiny by others working in the area, especially
his proposed mechanism for the so-called
-route to heterocycles.
43,44
In two
papers, in 1968
43
and 1970,
44
Gassman reported the cyclization of 4,5-unsaturated
N
-chloramines as a novel route to pyrrolidines. For example, treatment of a refluxing
methanolic solution of
88
with silver nitrate led to the formation of azabicyclo[3.2.1]
p
