Chemistry Reference
In-Depth Information
R
R
N
NN
N
NH
N
N
N
R
N
55
53
: R = H
54
: R = Me
56
Figure 2.7
Structures of polycyclic polyamines 53-56
even alkali metal solutions in ammonia and amines, so the pK
a
values for protonated cages
remains unknown [40]. A common feature for outside protonated adamanzanes is the strong
basicity of the free amines. In several cases monoprotonated adamanzanes have a pK
a
between 13 and 15 units (in water), while the pK
a
of N,N
0
-dimethyl[2
3
.3
2
.2
1
]adamanzane is
measured to be 24.9 (acetonitrile), acting as a superbasic proton sponge. The remarkably
increased basicity is explained by a hydrogen bonding stabilization of the monoprotonated
species between the two bridgehead nitrogen atoms. When the distance between the two
bridgehead nitrogen atoms becomes larger, it results in a weakening of the hydrogen
bonding stabilization and a concomitant decrease in the base strength, as supported by the
B3LYP/6-31
G**//HF/6-31G** calculations [41].
A number of other medium ring di- and polyamines have been found to have enhanced
basicities. For instance, Lehn
รพ
s [1.1.1]cryptand 59 has an estimated pK
a
inwater of 17.8 [42]
for the internal protonation, while the externally protonation has a much smaller pK
a
value.
The rates of the proton transfer in and out of the cavity are very slow, the internally
protonated species cannot be deprotonated unless the cage is destroyed, making it a
thermodynamically very strong and kinetically extremely slow base. Variations on the
Me
N
N
NN
H
+
O
O
N
N
O
N
N
N
N
Me
58
59
57
O
N
N
O
HN
Me
Me
N
N
NH
O
N
N
Me
Me
O
O
N
N
O
62
60
61
Figure 2.8
Structures of polycyclic amines 57-62
